Nucor Education and Research Center (NERC) 2013

Transcription

Nucor Education and Research Center (NERC) 2013-2014
Technical Reports
Submitted by:
Dr. Heshmat A. Aglan
Director, NERC
College of Engineering
Tuskegee University, AL 36088
Submitted to:
Nucor Steel Corporation
May 2014
Summary
This report consists of research results generated by the TU-NERC providing
solution for some industry problems related to steel manufacturing. These research
opportunities expose undergraduate engineering students to the latest trends in
steel technology.
The 2013-2014 research projects are listed below. Individual student reports are
also included.
Project Title
1.
2.
3.
4.
5.
6.
Cooling Rate Effects on Yield
Strength on Long Products.
The Effects of Corrosion from
HCl Pickling Solution Vapor on
Uncoated Steels.
Fracture Toughness of a 100 ksi
Advanced High Strength Steel
Produced with High and Low
Nb and TMP Processing.
Long Heating Cycle Intercritical
Partitioning to Produce Duplex
Microstructures.
Quantification of MacroInclusion Distribution in Sheet
Steel Samples Using
UT and Thermal Scanning
Techniques.
Effects of Cooling Rates on
Intercritically Partitioned Dual
Phase and Armor Steels Using a
Cooling Simulator.
Student Assigned
Nucor
Division
Nucor Mentor
Allyson Lattimore
Marion, OH
Dr. Ignatius
Okafor
Christian Eddy
Hickman, AR
Dr. Ron O’Malley/
Kunle Oguntunde
David Alexander
Decatur, AL
Dr. Ron O’Malley
Matthew Stewart
Decatur, AL
Dr. Ron O’Malley
Richard Ellis
Decatur, AL
Dr. Ron O’Malley
Xavier Bland
Decatur, AL
Dr. Ron O’Malley/
Abhilash Dash
Technical Report
on
Cooling Rate Effects on Yield Strength on Long Products
Submitted by:
Allyson J. Lattimore
Sophomore, Mechanical Engineering
Submitted to:
Dr. Heshmat Aglan
Nucor Education and Research Center (NERC)
May 2014
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ACKNOWLEDGEMENTS
This research was sponsored by the Nucor Corporation through the Tuskegee University
Nucor – Education and Research Center (NERC). The technical guidance and support of the
Tuskegee University Research Team was very influential and greatly appreciated. Also, the
priceless counsel, encouragement, and generosity of the Nucor Corporation team members is
appreciated.
Tuskegee University Research Team
Nucor Corporation Team
Dr. Heshmat Aglan
Dr. Ignatius Okafor
Mr. Kaushal Rao
Mr. Curtis Kelly
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TABLE OF CONTENTS
I. Introduction ............................................................................................................................... 4
Abstract ......................................................................................................................... 4
Assignment Overview .................................................................................................... 5
II. Literature Review ..................................................................................................................... 7
III. Experimental ............................................................................................................................15
IV. Results and Discussion .......................................................................................................... 19
V. Conclusion ............................................................................................................................. 30
VI. References ...............................................................................................................................31
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I. INTRODUCTION
Abstract
One of the most common physical property related customer complaints in the long
product industry is differences between mill certificate reported yield strength and what the
customer gets upon verification. Often, barring equipment malfunction, no one is wrong because
the properties were determined at different times or different conditions. Indeed different results
have come for the same products in the same mill. In such cases the mill wright repeats the test.
Consistency of results within and outside the plant is an essential requirement for customer
satisfaction and retention. A number of inconsistencies reported in the mill have been explained
or blamed on chemistry variation within the bar. Some have been explained by referring to bar
finishing temperatures. The purpose of this work is to study the role of bar cooling rate in this
issue.
Mechanical properties of steels are directly related to their microstructure following heat
treatment, which is normally performed to obtain a specific microstructure and set of mechanical
properties. Recently, in industry, more studies have been conducted to examine the cooling rate
effects on mechanical properties and microstructure of professionally manufactured steel.
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Project Overview
Mentors: Drs. H. Aglan (Tuskegee University) & Ignatius Okafor (NSMAR)
Title: Cooling Rate Effects on Yield Strength of Long Products
Objective: To expose the students to the relationship of microstructure, grain size and cooling
rate to physical properties of steel.
Background: Literature review must be performed on the effects of temperature and grain size
on the mechanical properties of steels. The effect of holding times of a piece of steel in furnace
and grain growth shall be understood and documented. The effect of the resulting microstructure
on mechanical properties shall be studied and documented. The effect of cooling rate on grain
growth shall be noted and understood.
Work and Tasks: NSMAR A36 products shall be used for this study.
Task 1: Background and literature review
• General overview of steel, types, and composition
• Relation between heat treatment and grain size
• Effect of microstructure on mechanical properties
• Relation between cooling rate and grain growth
Task 2: Sample Preparation
• Machine 6 to 10 sample pieces of about 6 to 10 inches from a round bar
• Cut into dog bone shape with shank of min. 2 inches and 1⁄3” round diameter;
widest part will be between 1 and 1.5 inches.
Task 3: Pre-Experiment Analysis
• Microstructural Examination
- Mold, grind, and polish samples before etching with 98% Ethanol
- Examine grain size and microstructure phases
• Mechanical Testing
- Vickers Microhardness Testing
- Tensile Testing
Task 4: Heat Treatment and Cooling
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• Wrap samples in Tantalum sheet and hold in muffled furnace for 5 minutes at
980°C
• Transfer to vertical air cooling chamber with airflow anemometer attached
• K-type thermocouple is spot welded to sample then connected to Omega TC08 data logger on a computer to collect data from the readings
• Preset fan speed on vertical air cooling chamber
• Sample with thermocouple is annealed for 5 minutes at 920°C then cooled
• Leave sample in chamber until cooled to ambient temperature
• Repeat experiment for different fan speeds
Task 5: Post-Experiment Analysis
• Microstructural Examination
- Mold, grind, and polish samples before etching with 98% Ethanol
- Examine microstructure and record grain size at each cooling rate
• Mechanical Testing
- Vickers Microhardness Testing
- Tensile and Yield Strength Testing
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II. LITERATURE REVIEW
1.0 Steel
1.1 Steel
Steel is a combination of iron mixed with carbon and various other elements. The levels
of carbon in a batch of steel determine its chemical and physical properties, as well as how it is
used. Steels are separated into three main categories based on the carbon content as follows: mild
steel has less than 0.25% of carbon, medium steel has between 0.25% and 0.45% of carbon, and
high carbon steel contains between 0.45% and 1.50% of carbon.
1.2 Properties of Steel [1, 2]
There are some properties and characteristics that are consistent throughout all types of
steel. As the amount of carbon in the steel increases, so does the strength and hardness resulting
in the reduction of the ductility and malleability. Also, plain carbon steel can only be
strengthened to a certain pressure before the toughness begins to decrease. Changing the
temperature affects various properties of the steel. For example, low the temperature lowers the
steels impact resistance, while increasing the temperature raises the chances for oxidation to take
place. Overall, the properties of the steel are determined by the various processes it undergoes,
resulting in numerous possible physical and chemical properties of steel.
1.3 Types of Steel
Due to the numerous combinations of carbon levels and various metals, the possibilities
are almost limitless when creating different types of steel. Some of the most common types of
steel are: high carbon, chromium steel, stainless steel, high speed, and nickel-chromium steel.
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High Carbon Steels
High carbon steels, a combination of iron and carbon, are one the most commonly used forms of
steels. Due to it being softer than most steels, it is easily sharpened and therefore used in making
wood cutting tools.
Chromium Steel
Chromium steel has high levels of chromium and is generally corrosion resistant. Also, this form
of steel is very strong and most commonly used in the production of automobile and plane parts.
Stainless Steel
Stainless steel, similar to chromium steel, is the most corrosion resistant and most commonly
used form. Comprised of carbon and 11% chromium and nickel, stainless steel is used in a wide
variety of products including watches ad surgery tools.
High Speed Steels
High speed steel contains tungsten, cobalt, molybdenum, or chromium to create one the
toughest forms of steel. Due to the high speed method used to create it, this steel has the ability
to cut other metals making it ideal to use in drills, tools, and power saws.
1.4 Heat Treatment of Steel [1]
When examining the composition and chemical/physical properties of steel, it is
important to understand how various treatments affect these attributes. One of the most
commonly used applications is the heat treatment of steel. By holding the steel at a certain
temperature then cooling it using different techniques, the operator is able to modify the
composition to specifically meet the application needs of a batch of steel. Heat treatment allows
the alteration of metal, specifically steel, to fit the criteria set forth for a specific application.
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2.0 Components of Steel
2.1 Prior Austenite Grain Size
The austenite grain size is the dimension of the grains of steel observed when the steel is
heated to 1700°F and transforms from ferrite to austenite. In heat treated steels, the mechanical
and chemical properties of the samples are greatly influenced by the size of the prior austenitic,
or parent, grain size. Formed during the high temperature holding stage, it is generally difficult to
measure depending on the alloy and microstructure.
Determining the boundaries of the prior austenite grain size is important due to the fact
that they are necessary to measure the size of the austenite grain size (AGS). Conditioning of the
prior AGS allows the transformation of the microstructure [3]. By controlling these boundaries,
one can create the desired final microstructure and composition of the steel. During the heat
treatment process, the boundaries of the prior AGS tend to contain most of the ferrite particles.
Etching has been determined as the best method to reveal the prior austenite grain
boundary. The measurements found are generally compared to ASTM standards and/or by linear
intercept analysis. According to ASTM Standard E1382-97(2010), measurement of the grain size
is made with a semiautomatic digitizing tablet or by automatic image analysis using a
microscopic image of the grain size. If the grain boundaries are clearly shown, these test methods
are applicable to any type of grain structure and size.
2.2 Mechanical Properties
Tensile Strength
Each sample of steel is composed of its unique combination of iron, carbon, and other
alloying elements. Depending on the elements used to create the steel affects various properties
of the steel’s performance in the field. One of the affected properties is the tensile strength of the
steel. Tensile strength is defined as the amount of stretching stress a material can withstand
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before breaking or failing [4]. This strength is analyzed with consideration of the sample size, the
amount of applied force, and the composition of the material. Once placed into a tensile machine,
the steel is stretched until a point where permanent deformation is created, known as the yield
strength. After exceeding the yield strength, the sample continues to be pulled until it breaks at
the point known as the ultimate tensile strength of the steel. The tensile strength of steel is very
pertinent when considering which applications the steel should be used for.
Hardness
In various applications of steel, the material must be able to withstand puncture and
abrasions. When this is the case, the hardness of steel must be examined and ensured to be within
the required measurements. By definition, hardness is the material’s resistance to indentation and
scratches [5]. Its value is the calculated ratio of the forced applied to the overall surface area of
the tested section. To collect more specific data, some experiments use micro-hardness to
analyze the steel. Micro-hardness testing is defined as an examination technique to measure the
hardness of the microstructural components of a metal [6]. To complete hardness testing, there
are various methods including the Rockwell, Vickers, Knoop, and Brinell methods. For the sake
of this experiment, the Vickers method will be implemented. This method uses an indenter that
has a square based diamond pyramid with an angle of 136° (see Figure 1). In order to insure the
test produces accurate results, the surface must be smooth and perpendicular to the indenter [7].
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Figure 1. Schematic of Vickers Hardness Indenter [8]
The advantages of using the Vickers method versus the previously mentions methods are
numerous including the ability to produce highly accurate readings without the need for multiple
indenters. Although the equipment needed to complete this testing is bulkier and more
expensive, the vast and high caliber of results make the investment worthwhile. In addition to the
accurate results, this method can be applied to a range of surfaces and materials and still produce
the easily interpreted impression as any other method [9].
3.0 Heat Treatment Effects
When examining the composition and chemical/mechanical properties of steel, it is
important to understand how various treatments affect these attributes. One of the most
commonly used applications is the heat treatment of steel. A crucial aspect of heat treatment is
the holding time of the steel in the furnace. By holding the steel at a certain temperature for a
specific period of time, the operator is able to modify the composition to specifically meet the
application needs of the steel. Heat treatment allows the alteration of metal, specifically steel, to
fit the criteria set forth for a specific application.
3.1 Effects on Yield Strength
Exposing steel samples to various temperatures during heat treatment affects various
properties of the steel such as the ductility. In various studies, there is a somewhat linear relation
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between the temperature and yield stress of steel. By holding the steel at very high temperatures,
there is more time for martensite and faster cooling allows the grain size to decrease. This
microstructural change and grain shrinkage cause an increase in the yield strength and hardness
of the steel.
4.0 Cooling Rate Effects
When subjecting steel to heat treatment, the cooling rate and method greatly affects
various aspects of the material. There are various techniques used to cool steel including water
quenching, air cooling, and furnace cooling. Cooling the steel allows for manipulation of certain
material properties such as the hardness, grain growth, and yield strength. By managing these
properties, manufacturers can make the steel more efficient for certain applications. In long
products, the steel is cooled at a continuous rate in a controlled chamber. Executing the cooling
with this method causes a homogeneous microstructure and high strength.
4.1 Microstructure
When a steel sample undergoes cooling from the high temperatures of heat treatment, the
austenite is transformed into more complex structures such as martensite and pearlite. The rate at
which cooling takes places becomes critical as it determines which phases and crystallization
structures form within the steel. Additionally, any alteration in the grain structure will, in turn,
affect the mechanical properties of the steel as they are dependent on certain characteristics of
the microstructure. As the steel cools, the carbon begins to diffuse throughout and therefore
increases the amount of pearlite formed. A slower cooling rate allows more time for the particles
to migrate, resulting in the formation of non-uniform structures and a larger grin size [10]. On
the contrary, an increased cooling rate provides a more refined grain structure and subsequently
more visibility of the martensite phase.
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Yankovskii et al found that cooling at a rate between 30-40°/s causes the formation of an
expanded ferrite-pearlite structure which improves the mechanical properties of steel [11]. With
an increased cooling rate, the carbon has less time to diffuse and more ferrite-pearlite is formed
which increases the strength of the steel. A faster cooling rate is favorable due to the limited time
for grain migration and increased control on phase formation.
4.2 Hardness
After steel has undergone heat treatment, the moments immediately following are crucial
when examining the microstructure and material properties. During the heating process, the
microstructure grains are transformed from one phase to another (e.g., martensite to pearlite).
Also, the size and features of the steel may cause different areas of the sample to cool at different
rates. As a result, the sample could have various hardness values in the different regions. The
rate of cooling will eliminate the possibility of non-uniform microstructure and hardness. Studies
have shown the most efficient method of cooling is to do so with a rate between 0.5 and 20°C/s
[12]. Additionally, the hardness was greatly increased for higher cooling rates above 10°C/s.
In the aforementioned experiment with AISI 1020/1040/1060 steel [13], the
microhardness was shown to increase at a trend directly proportional to the cooling rate and
carbon content. Also, as the pearlite and martensite percentages increased so did the
microhardness. The martensite phase forms at a greater speed and under high temperatures which
attributes to the strengthening characteristics of the phase. Generally speaking, the hardness will
increase with rapid cooling due to the refined state of the microstructural grains. This confirms
that there is a definite relationship between the cooling rate and hardness of steel.
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4.3 Yield Strength
The final mechanical properties of steel are greatly affected by the microstructure
acquired during heat treatment. To maintain these properties, the cooling rate becomes a critical
variable in the manufacturing process. When cooling the steel at rates between 25 to 80°C/s, selftempered martensite is formed and the steel exhibits significantly higher strength values [14].
Multiple studies have shown that following furnace heat treating, air cooling at high cooling
rates will produce the greatest strength in the steel. The general relation is the higher the cooling
rate, the higher the yield strength will be following heat treatment.
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III. EXPERIMENTAL
3.1 Materials and Equipment
1. Nucor Decatur A36 Chemistry Long Product Steel
2. Sampla Dry Keeper
3. General® LCD Digital Anemometer
4. Clark LM-100 Micro-Hardness Tester
5. Buehler SimpliMet® 1000 Automatic Mounting Press
6. Buehler EcoMet® 250 Grinder-Polisher
7. MTS 810 Material Test System with Hydraulic Wedge Grip
8. Fisher Scientific Isotemp® Programmable Muffled Furnace
3.2 Experimental Procedure
Sample Preparation
In order to set up the experiment, dog-bone shaped samples were cut from long product
steel at Nucor Decatur. The samples were approximately six and one half inches long with a one
inch diameter on the ends. The shank of the sample is about two inches long with a diameter of
approximately three-quarters of an inch. This sample geometry was chosen to mimic the original
shape of the steel sent to the customer. This is important due to the inconsistent properties being
reported by the customer versus that of the manufacturing post. Also, the ends were machined to
have flattened ends for easier gripping during heat treatment. Additionally, this shape eliminates
the need for additional machining to perform tensile testing.
Figure 2. Experimental Sample.
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After the samples were machined, each sample was set in a muffled furnace set to a
temperature of 920°C. The samples were held at this temperature for five minutes to ensure
complete homogenization of the steel and to ensure the sample was above the annealing
temperature. Once the samples were heat treated, they were then transferred to the cooling
method of choice. After tensile testing for each testing method, a piece of the sample was cut
from the center for further examination.
After taking the hardness readings, the samples were molded using the Buehler SimpliMet®
1000 Automatic Mounting Press with PhenoCure Resin Powder. Following the molding, the samples
underwent a grinding and polishing process using the Buehler EcoMet® 250 Grinder-Polisher.
During the first stage, grinding papers of 120, 180, 240, 320, 400, and 600 grit, respectively, were
used with water at a pressure of 7 lbs. Next, the samples were polished using an ultra pad, trident
pad, then polishing cloth with a 9, 3, and 0.05µm polishing solutions respectively. Lastly each
sample was lightly etched using a solution of Nital composed of 2% nitric acid and 98% ethanol.
Cooling Methods
The samples were originally tested by fan cooling using a vertical cooling chamber. For
the fan cooling, eight different samples were used; one for each fan speed setting on the fan
varying from low to high. During the cooling of each sample, a K-type thermocouple was
attached to the center of the sample to record the temperature change throughout the experiment.
Prior to the experiment, a sample at room temperature was placed in the cooling chamber to
measure the fan speeds and resultant temperatures. These measurements were done using a
General® LCD Digital Anemometer. The fan speeds varied from 2.73 m/s to 12.47 m/s and
produced an average temperature of 30°C.
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Table 1. Experimental Fan Speeds and Temperature.
Speed Setting
Average Speed (m/s)
Average Temperature (°C)
1
2.73
29.33
2
3.77
28.53
3
5.13
28.17
4
6.57
27.83
5
8.03
27.80
6
9.80
28.30
7
12.20
29.97
8
12.47
30.63
The fan cooling method produced various speeds but generally the same temperature so
additional samples were examined using different cooling methods including air cooled, furnace
cooled, and water quenched. The purpose of expanding the project to include these alternative
methods was to ensure that various cooling rates were examined opposed to various fan speeds.
For each of the additional cooling methods, a K-type thermocouple was attached to the center of
the sample to record the temperature data during testing.
During each of the cooling method experiments, the sample was placed in a muffle
furnace and held at 920°C for five minutes to ensure homogenization. For air cooling, once the
sample was held it was transferred to a stand and left to cool until room temperature was reached
(~25°C). For the furnace cooled sample, after heat treatment, the furnace was turned off and the
sample was left to cool until ambient temperature was reached. Lastly, the water quenched
sample was heat treated then transferred into a bowl of room temperature water. The findings
from each of these experiments will be discussed in the “Results” section of this report.
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Microstructural Analysis and Micro-Hardness
Microstructural analysis was performed to examine and analyze the grain size of the A36
steel. Using the PaxCam 5, the microstructure of the samples was examined and recorded. After
taking numerous pictures of the grains at 5X magnification, the grain size changes were
confirmed visually. Using the Clark LM-100 Micro-Hardness Reader with a PaxCam 5
attachment, the microstructure and hardness was examined. For each sample, either two or three
points were examined to find the average hardness values. Between these points, the hardness
was recorded for the ferrite region and a region that could be bainite, pearlite, or some
combination.
Tensile Testing
Using the MTS 810 Material Test System with hydraulic wedge grip, the tensile test was
performed on the experimental (heat treated and cooled steel) samples. The gauges of the
samples, about 2.55 inches long with a thickness of 0.30 inches, were each stretched in the
positive and negative vertical directions simultaneously until the steel reached a point of
irreversible formation and breakage.
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IV. RESULTS AND DISCUSSION
4.1 Cooling Rates
The basis of this experiment is to examine various cooling rates and create a correlation
between the rates and the mechanical properties of the steel. To ensure a wide range of cooling
rates could be examined, various cooling methods were implemented and evaluated. During the
testing of the steel, a K type thermocouple was attached to each sample to record any changes in
temperature at any time within the testing. From the data provided by the thermocouple, the
cooling rate of each method was determined and yielded the following results (Figures 3 to 11).
Fan Cooled
Speed 1- Slow Speed
Fan Cooled Speed 1
1000
900
Temperature (°C)
800
700
Cooling Rate:
600
10.86375 °C/minute
500
0.1810625 °C/second
400
300
200
100
0
0
1000
2000
3000
4000
5000
Time (centiseconds)
Figure 3. Cooling plot for fan cooled at speed 1.
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6000
7000
Speed 4 – Medium Speed
Fan Cooled Speed 4
1000
900
Temperature (°C)
800
700
Cooling Rate:
600
500
29.2733 °C/minute
400
0.4878889 °C/second
300
200
100
0
0
500
1000
1500
2000
Time (centiseconds)
2500
3000
Speed 5 – Medium Speed
Fan Cooled Speed 5
1000
900
Temperature (°C)
800
700
Cooling Rates:
600
500
23.2 °C/minute
400
0.386667 °C/second
300
200
100
0
0
500
1000
1500
2000
2500
3000
3500
4000
Time (centiseconds)
20
4500
5000
Speed 6 – Medium High Speed
Fan Cooled Speed 6
1000
900
Temperature (°C)
800
700
Cooling Rate:
600
500
27.01818 °C/minute
400
0.450303 °C/second
300
200
100
0
0
500
1000
1500
2000
2500
3000
3500
Time (centisecond)
Speed 7 – High Speed
Fan Cooled Speed 7
1000
900
Temperature (°C)
800
700
600
Cooling Rate:
500
29.1633 °C/minute
400
0.486056 °C/second
300
200
100
0
0
500
1000
1500
2000
Time (centisecond)
21
2500
3000
Speed 8 – High Speed
Temperature (°C)
Fan Cooled Speed 8
1000
900
800
700
600
500
400
300
200
100
0
Cooling Rate:
40.2634 °C/minute
0.67106 °C/second
0
500
1000
1500
Time (centiseconds)
2000
2500
Furnace Cooled
Furnace Cooled
1000
900
Temperature (°C)
800
700
600
Cooling Rate:
500
0.629026 °C/minute
400
300
0.0104837 °C/second
200
100
0
0
5000
10000
15000
20000
25000
Time (centiseconds)
Figure 9. Cooling plot for furnace cooled steel.
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30000
35000
Air Cooled
Temperature (°C)
Air Cooled
1000
900
800
700
600
500
400
300
200
100
0
Cooling Rate:
8.76289 °C/minute
0.146048 °C/second
0
1000
2000
3000
4000
Time (centisecond)
5000
6000
7000
1200
1400
Figure 10. Cooling plot for air cooled steel.
Water Quenched
Water Quenched
1000
900
Temperature (°C)
800
700
600
Cooling Rate:
500
439 °C/minute
400
300
7.31667 °C/second
200
100
0
0
200
400
600
800
Time (centisecond)
1000
Figure 11. Cooling plot for water quenched cooled steel.
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4.2 Microstructure and Hardness Testing
After conducting the tensile testing, the micro-hardness values were gathered at
approximately 2 points on the sample. Each sample has a hardness reading for the ferrite region and
the secondary microstructure region, which seemed to be bainite or a bainite-pearlite combination.
After collecting the data, the results show an increase in hardness for the faster cooled samples. Some
of the values are significantly higher than others but an average value was recorded. The points that
are higher could be due to a pearlitic presence in that region. This information supports the claim that
micro-hardness increases at a direct proportional rate as the cooling rate. Also, while examining the
micro-hardness, the microstructure of the samples was examined to determine any changes in the
grain size. As predicted, the faster cooling rates produced a more defined and smaller grain size.
Fan Cooled
Once examined with the microscope, the micro-hardness was measured in multiple areas
on the sample. The average ferrite micro-hardness was 254 HV and the bainite region average is
356 HV amongst the samples. In comparison, the average overall hardness was 305.063 HV. As
the speeds increase, the grain boundaries decrease and are more refined (Figure 12).
Speed 1
228/392 - 310 HV
Speed 2
261/334 – 297.5 HV
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Speed 3
256/349 – 302.5 HV
Speed 4
220/365 – 292.5 HV
Speed 5
261/313 – 287 HV
Speed 6
276/320 - 349 HV
Speed 7
Speed 8
251/341 – 296 HV
282/432 – 357 HV
Figure 12. Microstructure images and micro-hardness value (Ferrite/Bainite – Average).
25
Air Cooled
After heat treatment, a sample was removed from the furnace and left to cool without any
effects to simulate a slow cooling rate. Once examined with the microscope, the micro-hardness
was measured in multiple areas on the sample. For samples with the slower cooling rate, a lower
hardness is observed, as expected (Figure 13). The ferrite micro-hardness was 220 HV and the
bainite region was 402 HV amongst the samples. In comparison, the average overall hardness
was 311 HV.
Air Cooled
220/402 – 311 HV
Furnace Cooled
Leaving a sample in the furnace, untouched, allows the sample to endure the slowest
cooling rate possible in the experiment. Once examined with the microscope, the micro-hardness
was measured in multiple areas on the sample (Figure 14). Having the slowest cooling rate, a
lower hardness is expected since the particles have more room to migrate. The ferrite microhardness was 246 HV and the bainite region was 357 HV amongst the samples. The average
overall hardness was 301.5 HV.
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Furnace Cooled
246/357 – 301.5 HV
Water Quenched
To examine to effects of the faster cooling rates, one sample was quenched in water to
create a faster cooling rate. Once examined with the microscope, the micro-hardness was
measured in multiple areas on the sample. The ferrite micro-hardness was 374 HV and the
bainite region was 402 HV amongst the samples. The average overall hardness was 388 HV.
Water Quenched
374/402 – 388 HV
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4.3 Mechanical Testing
Using the MTS 810 Material Test System with hydraulic wedge grip, the tensile test was
performed on the experimental fan cooled, air cooled, furnace cooled, and water quenched samples.
When observing the strength of the steel (Figure 16), the samples with the slower speeds such as air
cooling, furnace cooling, and fan cooling speeds one through four showed a higher strain percentage
(about 18-20%) than the faster cooling rates (fan speeds 5 to 8). Additionally, the tensile strength was
significantly lower than that of the faster cooling (in the range of 650 MPa). In the samples with the
faster cooling rates such as water quenching and fan speeds five through eight, there was a lesser
strain percentage (about 15%) and higher tensile strength (about 750-780 MPa). The reasoning
behind this would be that as the steel cools, the particles migrate and the grains have time to expand.
As a result, the slower cooling rates allow the particles to migrate causing more ferrite to be present.
In the faster cooling rates, the grains are more defined and there is more bainite/martensite present.
The refined grains combined with the bainite/martensite presence increases the strength but decreases
the ductility. With the slower cooling rates, and subsequently smaller grain, the steel becomes more
ductile but has less strength causing a higher stress level but lower strain percentage, respectively
(Figures 16).
28
900
800
700
Stress, MPa
600
500
400
300
Air cooled
Speed # 1
Speed # 3
Speed # 5
Speed # 7
200
100
Furnace Cooled
Speed # 2
Speed # 4
Speed # 6
Speed # 8
0
0
5
10
Strain, %
15
20
25
Figure 16. Tensile strengths of heat treated samples at various cooling rates of fan speed and
conventional cooling processes.
29
V. CONCLUSION
This experiment is a study conducted to expose the relationship of microstructure, grain
size and cooling rate to physical properties of steel. The samples, having undergone heat
treatment and various cooling methods, were examined through the microstructure, microhardness, and tensile strength. Prior to heat treatment, the steel showed varying physical
properties from manufacturing to application.
During the experiment, the steel was heated to 920° C in a muffle furnace and then either
water quenched, fan, air, or furnace cooled. Once examined, the samples with from the fan
cooling rates were exposed to generally the same air temperature from the fan. Since the goal
was to examine various cooling rates, additional trials were conducted. In those trials the steel
was heated to 920 °C then either air cooled, furnace cooled, or water quenched in an attempt for
better results. After preparation and etching, the microstructure of the steel from the slower rates
showed a larger grain size. After this revelation, to execute the project goal, mechanical testing
was conducted on the samples. As a result of the faster cooling rates, the steel showed greater
strength and smaller, more refined grains. Reasoning for this could be that the bainite increases
strength and increasing the cooling rate subsequently increases the bainite levels affecting the
strength.
Overall, this experiment strongly suggests that by using a faster cooling rate by methods
such as water quenching and high speed fan cooling a stronger steel could be produced. In doing
so, the hardness of the steel is slightly altered but on the contrary, the tensile strength of the steel
increased.
30
V. REFERENCES
1. Bramfitt, B. and Benscoter, A. (2002). Metallographer's guide: Practices and procedures
for irons and steels. Materials Park, OH: ASM International.
2. Cai, X., Garratt-Reed, A. J., and Owen, W. S. (1985). The development of some dualphase steel structures from different starting microstructures. Metallurgical Transactions
A, 16(4), 543-557.
3. Bae, Y., Sang Lee, J., Choi, J., Choo, W. and Hong, S. (2004). Effects of austenite
conditioning on austenite/ferrite phase transformation of HSLA steel. Materials
Transactions, 45(1), 137-142.
4. “Tensile Strength.” Encyclopedia Britannica. Encyclopedia Britannica Online Academic
Edition. Encyclopedia Britannica Inc., 2013. Web. 04 October 2013.
<http://www.britannica.com/EBchecked/topic/587505/tensile-strength>.
5. Kumar, S. R. Satish, and A. R. Santha Kumar. "Mechanical Properties of Steel." Trans.
Array Design of Steel Structures. Indian Institute of Technology Madras, Print.
6. http://www.scotforge.com/sf_glossary.htm#m
7. http://www.matweb.com/reference/vickers-hardness.aspx
8. Séblin. B., Jahazeeah. Y,, Sujeebun. S,, Manohar, , and Wong Ky. B (n.d.). uom.ac.mu.
Retrieved from
http://www.uom.ac.mu/faculties/foe/mped/Students_Corner/notes/EnggMaterials/steelbkl
et.pdf
9. Krauss, G., and Grossman, M. A. (1980). Principle of heat treatments in steel. American
Society for Metals.
10. http://www.calce.umd.edu/TSFA/Hardness_ad_.htm
31
11. MMM0347 report
12. Okafor Report 2/13/14
13. http://d-scholarship.pitt.edu/6290/
14. http://enpub.fulton.asu.edu/chawla/papers/Cooling_rate_tensile_Ochoa.pdf
15. http://www.instron.us/wa/applications/test_types/hardness/vickers.aspx
16. Yankovskii, V.M., Beilinova, T.A., Vasiliev, E.L., Khallas, I.S. (1979). Properties of
tanks made of carbon steel after heat treatment, Metal Science and Heat Treatment,
21(8): 586-589.
32
Technical Report
on
The Effects of Corrosion from HCl Pickling Solution Vapor on
Uncoated Steels
Submitted by:
Christian Eddy
Freshman, Mechanical Engineering
Submitted to:
Dr. Heshmat Aglan
Nucor Education &Research Center (NERC)
May, 2014
1
ACKNOWLEDGEMENTS
The research and results presented in this report are possible through contributions
from the following people and Nucor. This work was sponsored by the Nucor Corporation
through the Tuskegee University Nucor – Education and Research Center (NERC). The
technical guidance and support of the Tuskegee University Research Team is greatly
acknowledged. The valuable advice and encouragement rendered by the Nucor Corporation
Team is also appreciated.
Dr. H. Aglan
Mr. Kaushal Rao
Mr. Kunle Oguntunde
Dr. R. O’Malley
Mr. Curtis Kelly
2
TABLE OF CONTENTS
Abstract
……………………………………
4
Project Overview
…………………………………….
5
1.0 Background
…………………………………….
6
2.0 Materials and Experimental Procedures
………………………………….....
12
3.0 Results and Discussion
…………………………………….
17
4.0 Conclusion
…………………………………….
23
5.0 References
…………………………………….
24
3
NERC 2013-14 Research Project
Evaluating the Effects of Corrosion from HCl Pickling Solution Vapor on Coated and
Uncoated Steels
Student: Christian Eddy (Mech. Engr, Freshman)
Nucor Mentors: Kunle Oguntunde - Nucor Hickaman / Ron O’Malley – Nucor Decatur
[email protected]; [email protected]
Abstract
Pickling lines are used to remove oxide scale from as-hot rolled steels to produce a clean steel
surface for painting or further processing such as cold rolling and/or galvanizing. The pickling
process employs a hot HCl bath to remove the oxides. Unfortunately vapors from the pickling
process can also promote corrosion of the surrounding pickle line structural components, such as
beams and roof structures. This project proposes to measure the influence of different steels and
coatings on the rate of corrosion in the presence of HCl vapor in a controlled environment.
4
NERC 2013-14 Project Overview
Student’s Name: Christian Eddy
Mentors: Drs. H. Aglan (Tuskegee University) & Kunle Oguntunde (NSHIC) / Ron O’Malley
(NSDEC)
Title: Evaluating the Effects of Corrosion from HCl Pickling Solution Vapor on Coated and
Uncoated Steels
Objective: To investigate the influence of the HCL pickling solution vapor on different hot
rolled coated and uncoated steel samples.
Background: When steel is cooled after hot rolling is performed, the oxygen in atmosphere
chemically reacts with the hot surface iron on the steel and forms a compound normally referred
to as scale. Pickling or descaling is removal of heavy, tightly adhering oxide films resulting from
hot forming processes, thermal operations, or welding using a reagent essentially an acid (HCl,
HNO3, H2So4, etc). However, the scale removal through the pickling process by hydrochloric
acid (HCl) depends on both the scale structure and the conditions of the pickling bath
(temperature, acid concentration, and dissolved iron concentration). Moreover, the HCL vapor
from these pickling lines may affect other parts surrounding that may lead to corrosion of the
exposed parts. This research emphasizes on the effects of HCL acid pickling vapors on different
types of steels, both coated and uncoated.
Proposed Work and Tasks: Nucor Steel Hickman samples will be supplied for this project.
•
Task 1: Literature review
o General overview of the steels, their types and composition.
o Basic concepts such as pickling, cleaning and their procedures will be understood.
o Industrial cleaning and descaling procedures of the hot rolled steels will be
studied.
• Task 2: Sample preparation
o Steel samples will be identified for different chemistries and will be inspected to
find the scaling and other contamination.
o The identified samples will be coated with different coatings.
• Task 3: HCl Vapor Exposure
o Both the coated and uncoated sampled of different hot rolled steel samples will
be exposed to HCL vapor in a controlled environment.
o The sample exposure times and conditions will be recorded.
• Task 4: Visual & Microscopic Evaluation
o Both visual and microscopic examination will be made on both coated and non
coated samples to evaluate the effect of acid vapor over time.
o The results will be correlated to the effectiveness of their chemistries and coatings
applied
• Task 6: Reporting the Results
o Results from all the studies will be reported.
5
1. Background
1.1 Steel
Steel is an alloy of the element, iron and carbon. It exists as a gray, blue, or
bluish-gray color and other elements may be incorporated into the steel to change the
mechanical properties of the steel. Steel is used as in many architectural and
engineering projects such as bridges, buildings, or even sculptures because of its
strength, durability and hardness [1].Steel has an initial composition of .5-1.5%
carbon, but more is added if the mechanical properties are to be changed [1].
Steel Making Processes:
•
Basic Oxygen Furnace Process: used in most modern steel making plants because of
the speed. This process is roughly ten times faster than the open hearth furnace
process and accounts for more than 60% of steel output. In this process high-purity
oxygen is blown into the molten pig iron. The high-purity oxygen lowers the carbon,
silicon, manganese, and phosphorous levels and leaves the molten iron free to add
other various elements to change the mechanical properties of the steel. Fluxeschemical cleaning agents- are also added to further reduce the sulfur and phosphorous
levels [2].
•
Open Hearth Furnace Process: Steels produced during the open hearth furnace
process are made by heating pig iron to extreme temperatures, about 1600 degrees F
(871 degrees C). Impurities, such as carbon, are oxidized then float to the top and
form slag. The composition of the steel allows the steel to have different grades. The
6
amount of a specific element is instrumental in the different characteristics of steel.
For example, the more carbon in the steel, the harder and more brittle the steel will
be, but too much carbon could be detrimental, so when enough carbon has been
oxidized, carbon steel will form [3].
•
Bessemer Process: involves making steel from pig iron by blowing air throughout
the molten iron in a Bessemer converter to oxidize the impurities. The heat released
from the oxidation process keeps the iron at a molten stage. When the air is being
distributed throughout the molten iron, the impurities bind with the oxygen and float
to the top to form slag as carbon monoxide is burned off [4].
•
Electric Arc Furnace Process: is a process where scrap steel is melted and refined to
produce new steel. The process occurs in a furnace with ceramic heat resistant lining.
The lid is opened in order for the steel to be placed into the furnace, and then
electrodes are lowered into the scrap and are energized to create an electrical current
through the steel to the electrode at the bottom of the furnace. This electrode is neutral
[5].
1.2 – Alloying Elements
•
Carbon is the basic element in the steel making process, along with iron. It is
instrumental in the amount of strength and hardness a specific grade of steel has. But
too much or too little carbon composition can be detrimental.
•
Chromium provides resistance to the corrosion of the steel because it directly resists
oxidation. The more chromium added to the steel the more resistance to corrosion the
steel has. Chromium also improves the hardenability and strength of the steel while
7
improving resistance to wear and abrasion, when heated to high temperatures. About
11% of chromium is added to steel in the stainless steel making process, and the
corrosion resistance is much greater that steels with lesser chromium content.
•
Nickel increases the strength of the steel by improving the ferritic and pearlitic
structure of the steel. This causes the strength of the steel to increase and also the
toughness and hardenability to increase in low alloy steels. Nickel also helps to
reduce to cracking and distortion that is caused by quenching after heat treatment.
Nickel is also responsible for some of the corrosion resistance properties in steel,
mostly in stainless steels. Roughly 8% of nickel is added to stainless steel with a high
chromium content to produce the most heat and corrosion resistant steels [6].
•
Copper when introduced into the steel, reduces the carbon. The lesser amount of
carbon in the steel, the more “weldability” the steel has because there is a reduction in
heating requirements. There is also a heightened corrosion resistance because it is
partially resistant to sulfuric acid [7].
•
Molybdenum adds resistance to pitting corrosion. When the acid attacks in the
crevices, this element is provided as a resistant to that attack [8].
1.3 – High Strength Low Alloy (Cu-HSLA) Steels:
These steels are often referred to as weathering steels because they have improved
atmospheric-corrosion resistance which accounts for their better resistance toward factors
that may cause corrosion than other uncoated steels. The copper content in the Cu-HSLA
steels contributes to the corrosion resistance of the steels. Though High Strength Low
Alloy (Cu-HSLA) steels have resistance to atmospheric corrosion, there are still some
8
substances that may be present in the atmosphere that still affect the steel tremendously.
One substance is HCL, which is generally unaffected by the steel’s properties [9].
1.4 – Acid Pickling:
In the steel making process, there are usually oxides (impurities) that build up and
float to the top of the molten steel. These impurities or mill scale are present from earlier
processes such as hot rolling by furnace and cooling by water and air. In order for the
steel to be ready to undergo further processing, the steel must go through the picking
process, so after the steel is hardened it is submerged in an acid bath, mostly hydrochloric
acid (HCL) in 3 different wt% values- 5, 10, and 15wt%, to remove such impurities by
converting iron oxide into a soluble salt (Figure 1). The rate and effectiveness of the
cleansing process is based on the composition of the acid bath and the time or duration
the steel is submerged in the bath. At the end of the baths, the steel is rinsed with water to
remove the acid from the surface of the steel preventing further surface removal.
Figure 1: Pickling Process
When these factors are not balanced, there can be detrimental effects to the steel rendering
the steel unable to undergo further processing, so steel manufacturers make it their concern
to create a balanced pickling process for the steel to prevent detrimental processes called
under pickling and over pickling [10].
9
• Under pickling is the process where the acid bath has an acid wt% lower than the
desired level meaning the bath is more diluted. This phenomenon is causes the steel to
have remaining oxide residing on the surface of the steel because that acid was not
effective enough or too diluted to remove all of the impurities in the first round,
therefore the steel must undergo another round in the steel bath.
• Over pickling is caused by an extended amount of time under the acid bath and also
when the acid bath has a more acidic composition. This caused more cracking and
brittlement in the affected steel because there is an increased amount of etchingcausing the shiny surface of a piece of smoothed steel, being submerged in nital ( a
mixture of 2 to 5% nitric acid with methyl alcohol), to become a dull color. HCL exists
as a gas, but can also be a liquid. It causes many problems in the steel industry because
when it comes in contact with bare steel. Because of this detrimental effect, the steel
must be downgraded. The affected steel is dipped in a solution or bath of strong acids
called pickle liquor. Once the scale and impurities are removed, excess exposure the
bath could cause the pickle liquor to have detrimental effects on the steel, such as
cracking and brittleness. In the field, corrosion occurs when steel is exposed to these
strong acids [11].
1.5 – Corrosion
When a substance undergoes a process of surface degradation, that process is called
corrosion. This process occurs because the substance is in an environment that is reactive
to the material’s surface. Metals corrode because they are in a chemically unstable
environment. Because of this phenomenon, most metals, with the exception of precious
10
metals (gold, silver, copper, etc.) are found in nature as ores, therefore they must be
converted into their metallic state. These metals tend to want to change revert to a
chemical state that is more stable so metals must have a passive film to prevent or slow
down this process. Some metals have their own passive films, such as most precious
metals, but others must be converted to a more chemically stable substance by other
means.
11
2. Materials and Experimental Procedures
2.1 – Materials and Composition
There are four grades of steels that I will be using in this experiment. All four of these
grades are High- Strength Low alloy steels with a percentage of copper in each: 0.34% Cu in
205YP2, 0.5% Cu in GA0057, 0.21% Cu in GA0021, and 0.1% Cu in 1008Y6 (Table 1).
These steels are considered weathering steels because of their resistance to corrosion. The
samples will be placed in a heated glass desiccator bowl suspended over 50% hydrochloric
acid on a clay plate. The clay plate is drilled with holes to allow the vapor to seep through
and corrode the samples. Not only does copper add to the corrosion resistance of the steel,
but also there are three other apparent elements present- molybdenum, nickel, and chromium
(Table 2).
Table 1: Qualifications of samples before and after testing
12
Table 2: Composition of elements in each grade of CU- HSLA steels
2.2 – Experimental
2.2.1 – Sample Preparation
In setting up the experiment, 4 samples of each grade of steel (16 total) were placed in
a holed clay plate suspended over a HCL solution. The HCL solution sat on a hot plate
heated to 100 F0. The HCL was a 1:1 ratio solution with water because the HCL acid alone
was too strong and was not close to industry standards. The mass and length calculations
were taken before corrosion and after corrosion to determine the mass loss (Table 1; Figure
3).
13
Figure 3: Parent Samples before Testing
Immediately following the calculations, four samples of each grade of steels were
placed in a rectangular pattern in a glass desiccator bowl. The samples sat on a holed clay
plate to allow the HCL vapor to seep through the holes and corrode the samples (Figure 4).
The bowl was then placed on a hot plate and set to 100 degrees Fahrenheit to allow the HCL
to begin to evaporate and formulate vapor. After proper placement of the hotplate, bowl, and
samples, the top was sealed to trap the vapor, therefore speeding up the process (Figure 4).
The samples were kept in the glass desiccator bowl for 2 months and one sample was taken
out weekly to capture the visual observations.
Figure 4: Experimental Set-up and Glass Desiccator Bowl
14
2.2.2– EIS Testing
When there is an alternating current (AC) present, the impedance (Z) is the total
opposition to that current measured in ohms. In order to calculate the impedance (Z), you
must take the square root of the sum of the inductive reactance squared (XL) and the
resistance squared I (Figure 5) [11] . EIS is typically used for coated steels but it can be used
for testing corrosion. This process is not generally used to test the resistance to corrosion but
more so the amount of oxide residing on the surface. First, the impedance, or total opposition
to the AC current, is taken of the parent sample of each grade. This provides information on
what the sample should be like. The closer the measurements of the parent’s impedance and
the corroded sample’s impedance, the more corrosive resistant the grade of steel is[10].
Figure 5: Calculating the Impedance
When setting up for EIS, there must first be an electrical current established. Salt
water was used as the conductor for the alternating current needed for the cell (Figure 6). The
cell had probes oriented all over its surface and one for ground. The electrical current was be
fed into the cell (filled with 5% salt water) and then through the sample to the electrode.
15
When the AC goes through the sample, the electrons pick up any opposition to the current ,
therefore, lowering the impedance [10].
Figure 6: EIS Set-up
16
3. Results and Discussion
3.1 – Visual Observations
Over the course of the experiment, there were many changes in physical appearance
of the samples. The proposed changes were validated through the EIS and corrosion mass
loss index experiments (shown below) and many of the samples were alike in comparison
regarding the oxide layers on the surface of the samples.
Figure 7: Parent Sample/ Week 1
In week one there were generally no physical changes (Figure 7) . There was a small
layer of oxide formation on the surface of the samples characterized by a slight browning.
This change occurred after a few days of exposure to the HCl vapor.
After 3 weeks there was a considerable amount of orange oxide formation on the
surface of the samples. This was the first stage of pickling and it was also the typical rust that
is seen on old cars, nuts, and bolts. The surface of the samples began to retain the moisture
and the orange oxide began to darken. After a month, there was a formation of black oxide
17
(Fe3O4) or magnetite. This black iron oxide (Fe3O4) is ferromagnetic and is more rare than
other iron oxides [13]. On the final day after 2 months, the black iron oxide (Fe3O4) and the
red oxide (Fe2O3) were completely soluble and had begun to rise from the sample and peel
off, just as mill scale would in the pickling line [14]. Nevertheless, the visual observations
were too close in comparison to effectively determine which grade was the most resistant to
corrosion, so Electrochemical Impedance (EIS) was measured (Figure 8).
Figure 8: Progress of visual observations (Week 1- 2 Months)
3.2. – Corrosion Damage Mass Loss in (ASTM – G1):
When calculating the mass loss and corrosion damage, there is a formula that is used.
This formula is comprised of a constant (K), dependent, and independent variable. First, the
sample needed to be weighed before and after cleaning. Next, the length and width are taken
to determine the surface area of each sample. In Figure 9 there is a complete demonstration
on what Corrosion Rate is comprised of. The calculations show that 2.33 is the least g/m2*h
18
of all the samples because its value is halved due to its size in relation to all of the other
grades.
Figure 9: ASTM – G1 and Mass Loss calculations
3.3 –EIS Measurements
Figures 10- 13 represent the ohms versus frequency curves for all the different grades
tested for impedence measurements. Two samples that underwent corrosion protection for 45
days under picklng environment were compared with its parent (control) counterparts. As
mentioned earlier, the higher the impedance, the better the corrosion resistance of the steel
because there is less opposition to the AC current. If there is a greater opposition to the
current, the impedance will be low as there is very little current being received at the other
end of the electrode.
19
Figure 10 represents the impedence results for the sample grade1008Y6. The parent
sample (blue curve) has a higher impedance (in order of 105-107) when compared to their
samples exposed in pickling environments (103). Both corroded samples showed similar
impedance curves representative of the same level of corrosion during the time of exposure.
Figure 10: EIS Results for 1008Y6
Figure 11 shows the impedence measurements for the steel gade GA0021. Both parent and
the 45 day exposed samples showed similar impedence measurements (in the order of 103).
However, the samples exposed to pickling have corroded and is clearly seen in the visual
observation from the earlier section .
Figure 11: EIS Results for GA0021
20
Figure 12 represents the impedence graph for steel grade 205YP2. The parent sample, the
control, shows a greater impedence to the current in the order of 105 – 107. The samples that
were exposed to pickling environment however showed a very little impedence when
compared to its parent (in the order of 103-104). Out of two samples in the pickling
environment, the first sample (corrosion-1) showed a little higher impedence measurement
compared to second sample (corrosion-2). This might be due to the sample surface
irregularitities in obtaining the current and is not significant.
Figure 12: EIS Results for 205YP2
The impedence measurements for the steel sample grade GA0057 is shown in Figure 13. The
control sample (parent) showed a higher impedence when compard to the samples exposed to
pickling environments. The samples exposed in pickling environments showed two different
impedence measurements; sample 1 (corrosion-1) showed a slightly lower impedence value
(103) compared to second sample (corrosion-2), which has value of an order 104. This may be
due to the excessive corrosive film on the sample and surface irregularities to obtain a proper
signal and is not signifcant on a log scale.
21
Figure 13: EIS Results for GA0057
Figure 14 shows the impedence values of all the steel grades tested for 45 days of exposure
in pickling environment. All the curves show a moderate impedence values (103-104) for that
period of exposure in pickling environment. However, the curve for the steel grade 205YP2
shows consistantly a higher impedance (Z) measurements with greater corrosion resistance
compared to other steel grades. This can be again rationalized from the chemical composition
of the steel with moderaly high amounts of copper, chrome and nickel contents that give this
steel relatively greater corrosion resistance.
Figure 14: All Sample Collaborative EIS analysis
22
4. Conclusion
Pickling environments in the steel plants continuously degrade the structural steel on
daily basis. Visual observation revealed sample corrosion degradation due to hot HCl vapors.
All the four Cu-HSLA grades were corroded for a long period of exposure to HCl vapors.
Corrosion damage assesment using ASTM G-1 revealed a greater mass loss index for the
sample grade 1008Y6 with minimal amounts of copper (0.1%). The least was shown for the
sample grade 205YP2 with a 0.34 wt% Cu. Impedence (Z) was calculated for all the samples
exposed to the corrosive environment. The results show that Cu- HSLA steel grade- 205YP2
showed greater impedence and hence has the most corrosive resistant properties. Moderately
greater amounts of copper and chromium present in this grade steel helped improve the
corrosion properties in these HSLA steels.
23
5. References
[1] file:///C:/Users/cobguest/Downloads/200908291713284218.pdf
[2] http://www.californiasteel.com/pdf/desc-pickling.pdf
[3] http://www.ndted.org/EducationResources/CommunityCollege/EddyCurrents/Physics/impedance
.htm
[4] file:///C:/Users/cobguest.AD3/Downloads/App_Note_AC-1.pdf
[5] http://www.worldsteel.org/faq/about-steel.html
[6]http://www.steel.org/en/Making%20Steel/How%20Its%20Made/Processes/Processes
%20Info/The%20Basic%20Oxygen%20Steelmaking%20Process.aspx
[7] http://steel.keytometals.com/articles/art2.htm
[8] http://www.ssina.com/overview/alloyelements_intro.html
[9] http://www.aws.org/conferences/abstracts/08_F.pdf
[10]http://www.asminternational.org/documents/10192/3466171/06117_Chapter%203B.
pdf/a764507a-3499-4d23-b348-5536d31c0ba2
[11] http://www.californiasteel.com/pdf/desc-pickling.pdf
[12] http://arxiv.org/ftp/arxiv/papers/1207/1207.0911.pdf
24
Technical Report
on
Fracture Toughness of a 100 ksi Advanced High Strength Steel Produced with
High and Low Nb and TMP Processing
Submitted by:
David Alexander IV
Junior, Mechanical Engineering
Submitted to:
Dr. Heshmat Aglan
May 2014
2
ACKNOWLEDGEMENTS
This research was sponsored by the Nucor Corporation through the Tuskegee University
Nucor – Education and Research Center (NERC). The technical guidance and mentoring of the
Tuskegee University Research Team was very nurturing and appreciated Also, the opportunity,
welcoming persona of the Nucor Corporation team members is appreciated.
Dr. Heshmat Aglan
Dr. Ronald O’Malley
Mr. Kaushal Rao
Dr. Aldinton Allie
Mr. Curtis Kelly
3
TABLE OF CONTENTS
I. Abstract .................................................................................................................... 4
II. Project Overview ........................................................................................................ 5
III. Literature Review ..................................................................................................... 6
IV. Experimental Process ............................................................................................ 31
V. Results and Discussion ........................................................................................... 34
VI. Conclusion .............................................................................................................. 47
VII. References……………………………………………………………………………......48
4
I.
ABSTRACT
Ultra High Strength (UTS) steels with excellent formability and low cost can be
produced today using titanium additions to promote fine grain structure and TiC
precipitation strengthening. Unfortunately, these steels exhibit high notch sensitivity in
cyclic fatigue and poor low temperature impact toughness, which limits their use in
many applications. When combined with Nb additions, the notch sensitivity and impact
toughness of these Ti based high strength low alloy (HSLA) stels improve significantly.
This project explores the fracture toughness of the steel to earlier lower Nb versions of
the steel that have been tested previously.
Proposed Work and Tasks: Nucor Steel Hickman samples will be supplied for project.
•
•
•
•
•
Literature review was performed on the topics
o Carbon steels, types
o Alloying in steels, microstructural phases, and heat treatment studies.
o Iron carbide phase diagram, CCT diagram.
o 100 ksi AHSS steels, mechanical properties of 100 ksi steels
o Fracture toughness KIC
o Fatigue properties on 100 ksi steel
Task 1: Sample preparation
o Steel samples of different processing conditions and chemistries will be
identified and samples will be cut according to ASTM standards
Task 2: Mechanical properties evaluation
o Mechanical properties including tensile strength, hardness, and fracture
toughness (KIC) will be evaluated for both low and high Nb HSLA steels
using MTS testing machine
o Fatigue crack propagation studies will be evaluated on these steels
Task 3: Microstructural evaluation
o Samples will be cut and microstructure is evaluated to determine the
phases present for both low and high Nb HSLA steel grades
Task 4: Reporting the results
o Results from all the studies will be reported
5
II.
PROJECT OVERVIEW
Mentors: Drs. H. Aglan (Tuskegee University) & Ron O’Malley (NSDEC)
Title: Fracture Toughness of a 100 ksi Advanced High Strength Steel Produced with
High and Low Nb Contents and TMP Processing.
Objective: To evaluate the fracture toughness (K1C) of an advanced high strength low
alloy (HSLA) steel with varying Nb contents and processing conditions.
Background: High Strength Hot Rolled Steel Strip uses precipitation strengthening
microalloying elements (niobium, vanadium and/or titanium, each up to 0.1%), which
form fine carbides and nitrides. These are called High Strength Low Alloy (HSLA)
steels. After cold rolling and annealing dispersion strengthening effect of these
elements is usually lost through particle coarsening; nevertheless, the resulting fine
grained Nb alloyed HSLA steels have attractive combinations of strength and
formability. Typical compositions: 0.05-0.1 % C, 0.25-1.2 % Mn, 0.01-0.05% Nb, 0.010.04% Si for Nb alloyed HSLA grades. The Nb addition moreover increases notch
sensitivity and impact toughness of these grade steels. This research mainly focuses on
evaluation of mechanical properties of both high and low Nb grade HSLA steel with 100
ksi tensile strength and its microstructural properties.
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III.
LITERATURE REVIEW
STEEL MAKING PROCESSES [1, 2, 3]
Basic Oxygen Process and Electric Arc Furnace are the two modern ways of making
steel. Basic Oxygen Process is when pure oxygen is blown into a bath of molten blast
furnace iron and scrap. The oxygen commences a series of forceful exothermic
reactions, as well as the oxidation of such impurities such as carbon, silicon,
phosphorus, and manganese. Lime and fluorspar are added to combine with the
impurities and for slag. After samples have been taken to check the chemical
composition of the steel, the furnace is slanted to allow the slag, which is suspended on
the surface of the molten steel, to be dispensed. The furnace is then slanted in the other
direction and the molten steel dispensed into a ladle, where it either
undergoes secondary steelmaking or is transported to the caster. (Figure 1)
Figure 1: Basic Oxygen Process
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Electric Arc Furnace Process uses a lid containing electrodes, which are lowered into
the furnace. An electric current is passed through the electrodes to form an arc. The
heat produced by this arc liquefies the scrap. The electricity required for this process is
sufficient to power a town with a population of 100,000. During the melting process,
other metals are added to the steel to give it the essential chemical composition. The
modern electric arc furnace on average makes 150 tons in each melt, which takes
around 90 minutes. (Figure 2)
Figure 2: Electric Arc Furnace
STEEL [4]
Steel is an alloy consisting mostly of iron with a carbon content ranging from mild
carbon content to high carbon content.
Mild Carbon Content- less than 0.25% of carbon
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Medium Carbon Content- ranges from 0.25% - 0.45% of carbon
High Carbon Content- ranges from 0.45% - 1.5% of carbon.
STEEL TYPES [5, 6, 7, 8]
There are boundless possibilities when it comes to creating types of steel due to the fact
that varying the carbon levels of the metals changes the properties of the steel.
Boron Steel- is used in the manufacture of a modern vehicle safety cell. The steel is an
element of the martesinic family of steels. It has a very high strength to weight ratio;
hence it is ideal for the modern vehicle designer who wishes to improve the strength of
the safety cell on a vehicle, while also keeping the shell weight to a minimum. The
difficulty with this martesinic steel is that it is severely affected by heat. Overheating
changes the molecular structure of the steel and it loses its strength.
Carbon Steel- is a strong hardened steel that derives its physical properties from the
presence of carbon and is used in hand tools and kitchen utensils. Carbon content may
range from less than 0.015% to slightly more than 2%. Adding this tiny amount of
carbon produces a material that exhibits great strength, hardness, and other valuable
mechanical properties.
Manganese Steel- Manganese is added to steel to improve hot working properties
and increase strength, toughness and hardenability. Manganese is an austenite forming
element.
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Chromium Steel- has increased hardenability. In addition, it brings resistance to
corrosion and oxidation, high temperature strength and abrasion resistance. Straight
chromium steels can be brittle.
Nickel Steel- is responsible for a great toughness and high strength at both high and
low temperatures. Nickel also improves resistance to oxidation and corrosion.
Molybdenum Steel- increases hardenability and helps maintain a specified
hardenability. It also increases high temperature tensile and creep strengths. These
grades are generally heat treated to specified properties.
Chromium - Nickel - Molybdenum Steel – is an extensively used deep hardening
steel. It possesses notable ductility and toughness. With its high alloy content consistent
hardness is formed by heat treatment in moderately heavy sections. Its high exhaustion
strength makes it supreme for highly stressed parts.
Alloying Elements and Their Effects on Steel [7, 8]
Carbon- The basic metal, iron, is alloyed with carbon to make steel and has the effect
of increasing the hardness and strength by heat treatment; the addition of carbon
enables a wide range of hardness and strength.
Manganese is added to steel to improve hot working properties and increase strength,
toughness and hardenability.
Chromium is added to the steel to increase resistance to oxidation. Chromium can
increase the response to heat treatment, thus improving hardenability and strength.
10
Nickel increases strength and hardness without sacrificing ductility and toughness. It
also increases resistance to corrosion and scaling at elevated temperatures when
introduced in suitable quantities. It increases toughness at low temperatures when
added in smaller amounts to alloy steels.
Molybdenum increases strength, hardness, hardenability and toughness, as well as
creep resistance and strength at elevated temperatures. It improves machinability and
resistance to corrosion and it intensifies the effects of other alloying elements. In hotwork steels, it increases red-hardness properties.
Titanium- The main use of titanium as an alloying element in steel is for carbide
stabilization. It combines with carbon to form titanium carbides, which are quite stable
and hard to dissolve in steel, this tends to minimize the occurrence of inter-granular
corrosion.
Phosphorus increases strength and hardness and improves machinability. However, it
adds marked brittleness or cold-shortness to steel.
Sulphur Improves machinability in free-cutting steels; however, without sufficient
manganese, it produces brittleness at red heat. It decreases weldability, impact
toughness, and ductility.
Selenium is added to improve machinability.
Niobium is added to steel in order to stabilize carbon and, as such, performs in the
same way as described for titanium. Niobium also has the effect of strengthening steels
and alloys for high temperature service.
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Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as
in the case of nickel, an austenite forming element. Yield strength is greatly improved
when nitrogen is added to austenitic stainless steels.
Silicon deoxidizes and degasifies. It increases tensile and yield strength, hardness,
forgeability and magnetic permeability.
Cobalt Increases strength and hardness and permits higher quenching temperatures. It
also intensifies the individual effects of other major elements in more complex steels.
Copper is normally present in stainless steels as a residual element. However, it is
added to a few alloys to produce precipitation hardening properties.
Tungsten increases strength, hardness and toughness. Tungsten steels have superior
hot-working and greater cutting efficiency at elevated temperatures.
Vanadium increases strength, hardness and resistance to shock impact. It retards
grain growth, permitting higher quenching temperatures. It also enhances the red
hardness properties of high speed metal cutting tools and intensifies the individual
effects of the other major elements
Aluminum deoxidizes and degasifies. It retards grain growth and is used to control
austenitic grain size.
Lead - While not strictly an alloying element, lead is added to improve machining
characteristics. It is almost completely insoluble in steel, and minute lead particles, well
dispersed, reduce friction where the cutting edge contacts the work. The addition of lead
also improves chip-breaking formations.
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HEAT TREATMENT TECHNIQUES [9, 10, 11]
Heat Treatment is the controlled heating and cooling of metals to modify their
physical and mechanical properties without altering the product shape (Figure 3). Heat
treatment is sometimes done inadvertently due to manufacturing processes that either
heat or cool the metal such as welding or forming. Heat Treatment is often connected
with escalating the strength of material, but it can also be used to alter certain
manufacturability objectives such as advance machining, advance formability, reestablish ductility after a cold working operation. Thus it is a very facilitating
manufacturing process that can not only help other manufacturing process, but can also
advance product performance by growing strength or other desirable characteristics.
Steels are principally suitable for heat treatment, since they resort well to heat treatment
and the commercial use of steels surpasses that of any other material. Steels are heat
treated in order to soften, order, or modify.
Figure 3: Heat Treatment Processes
Annealing – Annealing is the commonest of all the heat treatment processes. Every
piece of metal has been annealed at least once and some parts many times in the
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process of getting from raw material to part. There are two main reasons for annealing.
The first is to soften it and remove stress. The second is to homogenize the structure.
Every time a piece of metal is worked it accumulates stress and gets harder. The harder
it gets, the more difficult it is to work again. In the annealing process the metal is
heated, held at temperature for a time, and then slowly cooled.
Stress Free Annealing is used to lessen residual stresses in large castings, welded
parts and cold-formed parts. Such parts tend to have stresses due to thermal cycling or
work hardening. Parts are heated to temperatures of up to 600 - 650 ºC, held for an
extensive moment in time, typically about 1 hour or more, and then slowly cooled in still
air.
Soft Annealing - Hardened materials or materials rich in carbon, typically above 0.9%,
have a bad free-cutting machinability and cannot be cold-formed without difficulty. In
order to improve machinability of the material, they are soft-annealed by heating to
temperatures between 650°C and 750°C. It is also achievable to work with varying
heating and insignificant cooling-down processes around 723°C, also keeping the
temperature range unvarying for 3 to 4 hours in dependence on the nature of material
and material thickness, finally a slow cooling down.
Normalizing is the process of elevating the temperature to over 600 º C, fully into the
austenite range. The material is held at this temperature to fully convert the structure
into austenite; it is then removed from the furnace and cooled at room temperature
under natural convection. This results in a grain configuration of fine pearlite with
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excess of ferrite or cementite. The resulting material is soft, with the extent of softness
depending on the ambient environment of cooling.
Hardening is done to increase the strength and wear properties of steel. One of the
pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient
carbon content, then the steel can be directly hardened. Three working steps are
required for hardening: heating of the steel to a hardening temperature over 723°C ,
holding the temperature according to the size and grade of the steel, and a sudden cool
down of the steel being at hardening temperature. The holding time up to hardening
temperature is dependent on the size and grade of the steel. Small and difficult-to-form
parts only entail short holding times of a few minutes extent. When increasing the size
of the parts and for high carbon content, a longer holding time is required.
Quenching – Steels which can be hardened without special preparations are hardened
by this process. In this case, the steel is heated to hardening temperature and rapidly
cooled down one time. As a result, the material is very hard and frail and it can show
serious internal stresses; in case of fault-finding conditions, the steel can deform or
break.
Interrupted Hardening - By this process, steels are treated that are especially sensitive
to break and distortion. The steel is quenched only for a short time in water until hissing
is finished. After having been heated up to hardening temperature, the steel is kept in
mild heated oil until temperature balance. Only then it is further cooled down in air. A
complimentary alternative is therefore the technique where the steel powerfully
quenched and then is suspended into a hot bath at 200°C until temperature balance; by
15
this, stresses occurring during the cooling-down process and the danger of break
formation are successfully avoided.
Hot Quenching - By this process, steel of problematical shapes is treated. After being
heated to hardening temperature, the steel is cooled down in a hot bath at temperatures
between 180°C and 500°C until the temperature balance according to the grade of
steel. Then the steel is cooled to ambient temperature, resulting in a steel that shows
only minor internal stresses. Salt melting baths are preferably used as hot baths. The
temperature of the bath must be resulting from the grade of steel.
Tempering is the heat treatment process that improves the ductility and toughness of
steel. In steel, the martensite phase is formed when excess carbon is trapped in the
austenitic lath and rapidly cooled. This untempered martensite must be heated below
the lower critical temperature of the steel to allow the carbon to disperse out of the
body-centered tetragonal structure, producing a more ductile and stable body-centered
structure. Since strength and toughness come at the expenditure of each other in steel,
tempering is a critical heat treatment process that can establish the balance of the two
properties with cautious temperature and time control. When tempering of steel,
material is done over extensive moments of time in order to toughen and amplify the
number of precipitates, it is called aging.
Tempering from inside - The steel is abruptly quenched after being hardened so that
only the outer layer is cold. The lingering heat infiltrates from inside. The steel is cooled
down after reaching the tempering temperature.
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Tempering from outside - The cold steel is gradually heated by means of proper heat
sources. It is cooled down after reaching the tempering temperature between 200°C
and 500°C.
IRON CARBIDE DIAGRAM [12, 13]
Stable iron-graphite and metastable iron-cementite are the two iron carbon equilibrium
diagrams. The stable condition usually takes a very long time to develop. The
metastable is of more interest. Iron carbide (Fe-Fe C) is called cementite because it is
3
hard.
Phases found on Iron Carbide Diagram
•
Liquid solution of iron and carbon
•
Ferrite, a ductile but not very strong interstitial solid solution of carbon in Fe (bcc)
α
(Figure 4)
•
Austenite, an interstitial solid solution of carbon in Fe (fcc) (Figure 5)
•
Cementite, a hard and brittle compound with metallic properties
•
Perlite, a structure that consists of alternate layers of ferrite and cementite (Perlite
γ
is formed from austenite at eutectoid temperature 727°C upon slow cooling.)
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Figure 4: Body Centered Cubic and Face Centered Cubic
Steel groups according to Carbon content
•
Hypoeutectoid steels containing less than 0.76% carbon content
•
Eutectoid steel with carbon content about 0.76%
•
Hypereutectoid steels from 0.76% up to 2% carbon content
Continuous Cooling Transformation Diagram [14]
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Continuous Cooling Transformation diagrams are concluded by gauging some
physical properties during continuous cooling. Normally these properties are specific
volume and magnetic permeability. However, the majority of the work has been done
through specific volume change by dilatometric method. This method is
supplemented by metallography and hardness measurement. Cooling data is plotted
as temperature versus time. Dilation is recorded against temperature. Phase
transformation is indicated by any change in the slope. The dilation data can be
roughly calculated to determine a fraction of transformation (Figure 5).
Figure 5: Continuous Cooling Transformation
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MICROSTRUCTURAL PHASES [13]
The microstructure depends on the composition, or in other words the carbon content
and heat treatment. The phases depending on composition are alpha ferrite, gamma
austenite, delta ferrite, and cementite or iron carbide. Carbon is an impurity located in
the small spaces between structures in iron. Carbon forms a solid solution with the
alpha, gamma, and delta phases of Iron. The solubility in alpha ferrite, a body centered
cubic is limited due to the small interstitial spaces. Alternatively, solubility in austenite, a
face centered cubic containing larger interstitial space, is considerably higher than
ferrite. Cementite is very hard and brittle and is used to strengthen steels. Mechanical
properties are dependent on microstructure, that is, how ferrite and cementite are
mixed. Microstructure phases also have magnetic properties depending on composition.
Alpha ferrite is magnetic below 768 °C; austenite is non-magnetic.
MECHANICAL PROPERTIES OF CARBON STEEL [15]
Steel derives its mechanical properties from a combination of chemical composition,
heat treatment, and manufacturing processes. The main essential element of steel is
iron. The accumulation of very small quantities of other elements can have a marked
effect upon the properties of the steel. Mechanical properties of steel are the reactions
of the material to certain types of external forces. The mechanical properties include
strength, toughness, and ductility.
Strength is the ability of the steel to withstand a force before fracturing (tensile
strength), permanent deformation (yield strength), or high velocity impact (impact
strength).
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Toughness is the total amount of energy a material can absorb before fracture.
Ductility is a measure of the degree to which a material can strain or elongate between
the onset of yield and eventual fracture under tensile loading.
FRACTURE TOUGHNESS [16]
Fracture toughness is an indication of the amount of stress required to transmit a
preexisting flaw. It is a very significant material property since the occurrence of flaws is
not entirely preventable in the processing, fabrication, or service of steel. Flaws may
appear as cracks, voids, metallurgical inclusions, weld defects, design discontinuities, or
some combination thereof. Since engineers can never be totally sure that the steel is
flaw free, it is general practice to presume that a flaw of some chosen size will be
present in some number of components and use the linear elastic fracture mechanics
(LEFM) approach to design critical components. This approach uses the flaw size and
features, component geometry, loading conditions and the material property called
fracture toughness to evaluate the ability of a component containing a flaw to resist
fracture. A limitation called the stress-intensity factor (K) is used to determine the
fracture toughness of steel. The stress intensity factor is a function of loading, crack
size, and structural geometry. The stress intensity factor may be represented by the
following equation:
ADVANCED HIGH STRENGTH STEELS (AHSS) [17, 18, 19]
Advanced High Strength Steel is strengthened by microstructure change during
phase transformation. Over the years, advanced high-strength steels (AHSS) have
been explored thoroughly for seeking the superior mixture of high strength and sufficient
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toughness, because of the heightened concern on reducing weight of steel components
in order to conserve energy and raw materials as well as improve environment
protection, particularly in automobile industry. Some sorts of AHSS, such as dual-phase
(DP) steels and transformation-induced plasticity (TRIP) steels have been developed,
but the strength of the above AHSS with the carbon content of 0.05–0.2 wt% is staying
within a range of 500–1000 MPa. Other AHSS are complex phase (CP), ferritic-bainitic
(FB), stretch-flangeable (SF), hot formed and twinning-induced plasticity (TWIP).
Dual Phase (DP) Steel - The microstructure of DP steel consists of a soft ferrite matrix
and discreet hard martensitic islands, as shown below in Figure 6.
Figure 6: DP microstructure schematic
The ferrite is continuous for many grades up to DP780, but as volume fractions of
martensite exceed 50 percent, the ferrite may become discontinuous. The mixture of
hard and soft phases produces an excellent strength-ductility balance, with strength
increasing with increasing amount of martensite. The basic chemical composition of DP
steel is C and Mn; sometimes some Cr and Mo are added to enhance hardenability. DP
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steel is also called partial martensite steel when the martensite volume fraction
surpasses 20% or more.
Complex phase steel (CP) - CP steels have a mixed microstructure with a
ferrite/bainite matrix containing bits of martensite, retained austenite, and pearlite as
show below in Figure 7.
Figure 7: CP microstructure
Grain refinement is critical to acquiring the preferred properties from CP steel; deferred
recrystallization is often employed to develop very small grains for a very fine
microstructure. Microalloying elements such as titanium or niobium may also be
precipitated. The fine, complex microstructure gives CP steel high yield strength and
high elongation at tensile strengths similar to DP steels. CP can have good edge
stretchability. Additionally, CP steels have good wear characteristics and fatigue
strength and they may be bake hardened. With the hard phases like martensite and
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bainite and some help from precipitation hardening, the strength of CP steel ranges
from 800 to 1000MPa. CP steel has several automotive applications, particularly in body
structure, suspension, and chassis components. The high yield strength and elongation
enables high energy absorption, also making it a high-quality option for crash safety
components, such as fender beams, door impact beams, and reinforcements for Bpillar, etc. BMW has used CP in several components to improve rear crash safety.
According to ThyssenKrupp Steel, replacing conventional microalloyed steel with CP in
B-pillar reinforcement can double its strength.
Martensite steel (MS) - In MS, nearly all austenite is converted to martensite. The
resulting martensitic matrix contains a small amount of very fine ferrite and/or bainite
phases. This structure in general forms during a quick quench following hot-rolling,
annealing, or a post-forming heat treatment. Increasing the carbon content increases
strength and hardness. The resulting structure is mostly plate-like martensite, as in
Figure 8.
Figure 8: Tempered martensite microstructure
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Careful combinations of silicon, chromium, manganese, boron, nickel, molybdenum,
and/or vanadium can increase hardenability. The resulting martensitic steel is best
identified for its extremely high strength. MS has fairly low elongation, but post-quench
tempering can improve ductility, allowing for adequate formability considering its
extreme strength. Frequently used where high strength is vital, MS steel is usually roll
formed and may be bake hardened and electrogalvanized for applications requiring
corrosion resistance, but heat-treating MS decreases its strength.
Mn-B steel, also known as hot-stamping and die-quenched steel, contains mainly
manganese and boron, so it has excellent hardenability. Hot stamping process consists
of heating blanks to austenization, then press forming while the blanks are still red hot
and soft, and lastly, the formed parts are quenched to hard phases like martensite
within the die. The total processing time takes about 15 to 25 seconds.
Transformation Induced Plasticity (TRIP) - Like CP grades, TRIP benefits from a
multi-phase microstructure with a soft ferrite matrix embedded with hard phases. The
matrix contains a high amount of retained austenite, plus some martensite and bainite,
as shown in the schematic of Figure 9.
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Figure9: Schematic of a typical TRIP microstructure
TRIP has a high carbon content to stabilize the meta-stable austenite below
surrounding temperatures. Silicon and/or aluminum are often incorporated to catalyze
the ferrite/bainite formation while restraining carbide formation in this region. TRIP steel
established its name for its exclusive performance during plastic strain: in addition to the
distribution of hard phases, the austenite transforms to martensite. This transformation
allows the high hardening rate to undergo at very high strain levels, consequently
“Transformation-Induced Plasticity.” The amount of strain essential to commence this
transformation may be managed by modifying the stability of the austenite by controlling
its carbon content, size, morphology or alloy content. With less stability, the
transformation begins almost as soon as deformation emerges. With more stability, the
austenitic transformation to martensite is deferred until higher levels of strain are
reached, typically further than those of the forming process. In highly stabilized TRIP
steel automotive parts, this delay can allow austenite to linger until a crash event
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transforms it to martensite. Other factors also affect the transformation, including the
detailed situation of deformation, such as the strain rate, the mode of deformation, the
temperature, and the object causing the deformation. When the austenite-martensite
transformation occurs, the resulting structure is toughened by the hard martensite.
Deformation can continue through very high strain levels, as shown below in Figure 10.
Figure 10: True stress-strain diagram for TRIP grades compared to mild steel
100 KSI ADVANCED HIGH STRENGTH STEEL [18]
Traditionally high strength steels have a single phase ferritic (pure iron) structure. In
contrast, AHSS are primarily steels with a microstructure containing a phase other than
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ferrite, for example martensite, bainite, austensite, and/or retained austensite in
quantities sufficient to produce unique mechanical properties. For example 100 ksi
AHSS can be martensic steel produced by hot forming. The steel blank is heated above
850 °C, formed at that temperature and then is quenched in the die. At the forming
temperature, the steel has excellent stretchability. Quenching immediately after forming
produces the martensic structure without any spring back issues. Martensic steel can be
applied in wide range of ways for example agriculture, automotive components and
structures, aviation components and structures, bridge structures gas and oil
production/pipelines heat exchangers, medical devices, oil sands, petrochemical and
process piping, renewable energy, sports equipment, and train/rail cars and equipment.
HIGH STRENGTH LOW ALLOY (HSLA) 80 ksi [20]
HSLA 80 ksi- The alloy design of microalloyed steel is a V+N+Mo based HSLA steel.
Nitrogen, either present as an inherited remainder in steels or enhanced through
nitrogen additions is used to support the structure of nitrogen−rich vanadium
carbonitride V(C, N) precipitates for the purpose of precipitation strengthening. This
technique has presented the most capable approach for the manufacture of 80 ksi
HSLA steels. The chemistry of vanadium-nitrogen based HSLA steels with a
molybdenum addition is controlled in the range of 0.050% to 0.130%, nitrogen is
intentionally added and controlled in the range of 0.0190% to 0.0220%, and vanadium is
added and controlled in the range of 0.120% to 0.140%. For such steels, the
strengthening mechanism predominantly involves vanadium carbonitride, vanadium
nitride and molybdenum carbide in the fine transformed ferrite grains.
FATIGUE PROPERTIES OF HSLA STEEL[21, 22]
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Fatigue strength is defined as the number of cycles of stress that steel can withstand
before failure occurs. Although processing plays a critical role in fatigue performance,
the endurance confines of the material tested can be distinguished generally by the
alloying method used in advancing steel. Fatigue properties are enhanced by a variety
of methods used to improve ultimate tensile strength, yield strength, and therefore
increasing fracture toughness. The fracture toughness in ductile materials is regulated
by the distribution of plastic strains, depending primarily on the yield stress, which
controls the onset of plasticity, and the strain hardening characteristic, which confines
the extent of strain through which the plasticity is distributed uniformly. Examples that
improve fatigue property variables are pre-strain method, alloying method, sintering
temperature, and sintering density.
VARIANCE IN FRACTURE TOUGHNESS
Fracture behavior is affected notably by temperature, loading rate, stress level, and flaw
size, as well as by plate thickness or constraint, and joint symmetry.
Temperature is directly proportional to ductility and fracture toughness. Tests are used
to evaluate the toughness of steel. The most commonly used is the Charpy V-notch
test, which explicitly evaluates notch toughness, that is, the resistance to fracture in the
presence of a notch. In this test, a small square with a specified- size V-shaped notch at
its mid-length is simply supported at its ends as a beam and fracture by an impact force
from a swinging pendulum. The amount of energy required fracturing the sample or the
form of the fracture surface is determined over a range of temperatures. The
appearance of the fracture surface is usually articulated as the percentage of the
surface that appears to have fractured by shear. A shear fracture is specified by a dull
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or fibrous appearance. A shiny or crystalline appearance is linked with a cleavage
fracture. A shear fracture indicates ductile performance while a cleavage fracture
indicates brittle fracture. The data obtained from a Charpy test are used to plot curves of
energy or percentage of shear fracture as a function of temperature. The temperature
near the bottom of the energy-temperature curve, at which is a selected low value of
energy is absorbed, often 15 ft-lb, is called the ductility transition temperature. These
transition temperatures serve as a rating of the resistance of diverse steels to brittle
fracture. The lower the transition temperature, the greater is the notch toughness. In
general, the notch toughness of most structural steel amplifies with increasing
temperature and decreasing loading rate. The effect of temperature is well known and
has led to the transition temperature approach to designing to avoid fracture.
Loading rate is equally important, not only in designing to avoid fracture, but in the
understanding the satisfactory performance of many accessible structures built from
materials that have low impact toughness values at their service temperatures. The
toughness of most structural steels tested at a constant loading rate undergoes a
significant boost with increasing temperature. Thus, the general effect of a slow loading
rate, compared with impact loading rates, is to shift the fracture toughness curve to
lower temperatures, regardless of the test specimens used. Because of this shift,
increasing the loading rate can decrease the fracture toughness value at a particular
temperature for steels having yield strengths less than 140 ksi. The change in fracture
toughness values or loading rates varying from slow bend to dynamic rates is
particularly important for those structural applications that are loaded slowly, such as
bridges.
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The effect of increasing plate thickness is to uphold a more severe state of tress,
namely plane strain. A triaxial state of stress occurs at the tip of a sharp discontinuity in
a thick plate and this reduces the apparent ductility of the material to a lower bound
value. Conversely, the evident fracture toughness of materials can increase with
decreasing plate thickness, as a result of the relaxation of the lateral constraint in the
vicinity of the notch tip. This evident increase in toughness is controlled exclusively by
the thickness of the plate, even though the natural metallurgical properties of the
material remain unchanged. Thus, the minimum toughness of a particular material
occurs on a specimen thickness large enough so that the state of stress is plane strain.
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IV.
EXPERIMENTAL PROCESS
After the steel samples are cut, a mount for each steel sample must be made using
the SimpliMet® 1000, as shown in Figure 11, oriented to display the longitudinal and
the transversal sides of the cut steel samples.
Figure 11: SimpliMet®
Next the steel must be polished, which is the first step of the process of preparing the
samples for microstructure analysis using the AutoMet 250 (Figure 12). Six grades of
sandpaper (120, 180, 240,320, 400, 600), ranging from coarse to fine, were used. Then
three polishing pads (ultrapad, trident, micro-cloth), also ranging from coarse to fine,
were used.
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Figure 12: AutoMet 250
The second and final step of the process before microstructure analysis is the etching of
the samples. Etching samples with 2% Nital reveals the microstructure in steel. The 2%
Nital consists of 2% nitric acid (HNO3) and 98% ethanol (C2H5OH).
Using the Rockwell-Type Hardness Tester CLC-200R we performed tests on 10 points
for two samples (GD89-1075, GD89-1175) and performed tests on 7 points on the third
sample (GA0089). The unit used to measure the loads was HBW ( H- hardness, BBrinell, W- wolfram, also known as tungsten carbide) which comes from the Brinell
Scale:
where:
F = applied force (N) D = diameter of indenture (mm) d = diameter of indentation (mm)
33
MECHANICAL TESTING
The thickness, width, and gage length of the samples were measured using a
caliper in preparation for the tensile and fracture toughness testing. The material testing
system (MTS) was used to test tensile strength of the GA0089, GD0089-1075, and
GD0089-1175 grades of steel. The tensile graphs were used to find yield strength at the
2% strain point. The samples were cut into a dogbone shape as shown in Figure 13
below.
FIGURE 13: Tensile testing example
For the fracture toughness the MTS was used to test the resistance of the GA0089,
GD0089-1075, and GD0089-1175 with a notch. A caliper was also used to measure the
notch distance.
34
V.
RESULTS and DISCUSSION
MICROSTRUCTURE
The paxcam 5 software and the GX51 Olympus optical microscope were used to take
snapshots of the microstructures of GA0089, GD0089-1075, and GD0089-1175; these
consisted mostly of ferrite, bainite, and some martensite (Figure 14).
GA0089
35
GD0089-1075
GD0089-1175
Figure 14: Microstructure of grades GA0089, GD0089-1075, and GD0089-1175
36
MICROHARDNESS
To test the microhardness of the samples the LM-100 hardness testing machine was
used. Multiple indentions were made using different loads pertaining to the
microstructure in the selected area. Due to the sensitivity of the hardness tester and to
refrain from damaging the sample, the loads were modified according to microstructure
because they vary in levels of hardness. Indentations were taken on two prevalent
phases of all the microstructures ferrite and bainite. The microhardness of the grades
GA0089, GD0089-1075, and GD0089-1175 is shown in Figure 15.
GA0089
G
GD0089-1075
37
GD0089-1175
Figure 15: Ferrite phase on the left, Bainite phase on the right of grades GA0089,
GD0089-1075 GD0089-1175
The hardness values were taken on the samples of steel. The variation in values was
determined by the phase on wnich the indentation took place. Overall the values of the
hardness testing were low (Table 1), which was expected.
38
TABLE 1: Sample hardness values with average values of grades GA0089, GD00891075, and GD0089-1175
HARDNESS
SAMPLE
AVG HBW
(HBW)
GA89
206.2
229.7
230.0
235.3
237.3
232.8
229.45
234.9
GD89-1075
234.3
233.5
221.2
225.2
246.3
235.9
233.571
238.6
GD89-1175
236.4
233.6
233.6
237.3
235.3
235.7
238.9
235.828
39
The microhardness values were higher compared to the hardness because of the
indentations at the bainite phase raised the hardness values. Bainite is a harder phase
than ferrite; therefore it results in a higher hardness reading (Table 2). Also. noting from
the microstructure, the ferrite phase expanded as the processing temperature got
higher. The hardness values from the GD0089-1075 and GD0089-1175 decreased as
the niobium content and processing temperature increased from 0.064% wt in GD0089
processed at 1075 F to 0.065% wt in GD0089 processed at 1175 F.
TABLE 2: Microhardness values of grades GA0089, GD0089-1075, and GD00891175
MATERIAL
PHASE 1
(FERRITE)
PHASE 2
(BAINITE)
TOTAL HARDNESS
(HBW)
GA0089
243
304
274
GD00891075
331
408
370
GD00891175
318
380
349
The mechanical properties of tensile strength, yield strength, and fracture toughness of
the GD0089 increased as the niobium content and processing temperatrure increased
(Table 3).
40
TABLE 3: Average data of mechanical properties of grades GA0089, GD0089-1075,
and GD0089-1175
SAMPLE
TENSILE (MPa)
YIELD (MPa)
b (mm)
FRACTURE K1
GA0089
782.77
540
6.46
142.464
GD0089-1075
771.59
520
6.427
141.66
GD0089-1175
843.055
560
6.423
148.71
In the GA0089 tensile graph (Figure 16) the samples could withstand similar amounts of
stress of around 720 MPa, but on the strain % axis the first sample of GA0089 stretched
about 33% while the second and third sample only stretched under 30%. The variance
in the strain of the samples is a result of the variances in the thicknesses of the different
samples. In the GA0089 Fracture toughness graph the samples exhibited similar
amounts of stress and strain
41
900
800
700
stress, MPa
600
500
400
300
GA0089_1
GA0089_2
GA0089_3
200
100
0
0
5
10
15
20
strain, %
25
30
35
(a)
450
400
350
Stress, MPa
300
250
200
GA0089_1
150
GA0089_2
100
GA0089_3
50
0
0
1
2 Strain, % 3
(b)
4
5
42
GA0089 AVG FRACTURE
500
450
400
Stress, MPa
350
300
250
200
150
100
GA0089 AVG FRACTURE
50
0
0
2
4
Strain, %
6
8
(c)
FIGURE 16: Tensile graph(a) and fracture toughness graph(b) of GA0089 on the
stress and strain curve with average fracture toughness graph(c)
In the GD0089-1075 tensile graph (Figure 17) on the stress and strain curve the two
samples withstood approximately the same stress at around 745 MPa, but the on the
strain % axis the first sample stretched to abut 25 % strain while the second sample
stretched right above 20% strain. In the GD0089-1075 fracture toughness graph on the
stress and strain curve the second sample withstood approximately 420 MPa while the
first and third sample withstood under 400 MPa of stress, but on the strain % axis the
samples of grade GD0089-1075 stretched in between 4% and 5% strain.
43
900
800
700
stress, MPa
600
500
GD0089-1075_1
400
GD0089-1075_2
300
200
100
0
0
5
10
15 %
strain,
20
25
30
(a)
450
400
350
stress, MPa
300
250
GD0089-1075_1
200
GD0089-1075_2
150
GD0089-1075_3
100
50
0
0
1
2
strain, %
(b)
3
4
5
44
GD0089-1075 AVG FRACTURE
450
400
Stress, MPa
350
300
250
200
150
100
50
0
0
1
2
Strain, %
3
4
5
(c)
FIGURE 17: GD0089-1075 tensile graph(a) and fracture toughness graph (b) on a
stress and strain curve with average fracture toughness graph (c)
In the GD0089-1175 graph of the data the tensile strength of the samples are nearly
identical, withstanding a stress of approximately 820 MPa and stretching nearly 25%
strain, but there was a variation in fracture toughness on the graph (Figure 18). The
variation stems from the change in thickness and notch length from the three samples
of GD0089-1175.
45
900
800
700
stress, MPa
600
500
GD0089-1175_1
400
GD0089-1175_2
300
200
100
0
0
5
10
15
strain, %
20
25
30
(a)
500
450
400
stress, MPa
350
300
250
GD0089-1175_1
200
GD0089-1175_2
150
GD0089-1175_3
100
50
0
0
1
2
strain, %
(b)
3
4
5
46
450
400
Stress, MPa
350
300
250
200
150
100
50
0
0
1
2
Strain, %
3
4
5
(c)
FIGURE 19 : GD0089-1175 Tensile strength(a) and fracture toughness graph(b) on
stress and strain curve with average fracture toughness graph (c).
47
VI.
CONCLUSION
This experiment was conducted to evaluate the fracture toughness of 100 ksi high
strength low allow steel with varying niobium contents and processing conditions from
Nucor Decatur. The samples cut from the larger piece of steel were evaluated by the
microstructure, microhardness, hardness, tensile strength, yield strength, and fracture
toughness.
The niobium contents increased from 0.033% wt to 0.065% wt and the processing
temperature increased from 1075 F to 1175 F. As a result there as an increase in
fracture toughness. The heat increase caused grain expansion and a decrease in
hardness. As a result there was an increase in fracture toughness and a decrease in
sample hardness. This information supports both statements that niobium content and
processing temperature are directly proportional to fracture toughness and niobium
content and processing temperature are inversely proportional to sample hardness.
48
VII. REFERENCES
1. The Editors of Encyclopædia Britannica. "Basic Oxygen Process (BOP)
(metallurgy)." Encyclopedia Britannica Online. Encyclopedia Britannica, n.d.
Web. 06 May 2014. <http://www.britannica.com/EBchecked/topic/54973/basicoxygen-process-BOP>.
2. "The Electric Arc Furnace." : Step by Step : EEF. N.p., n.d. Web. 06 May 2014.
<http://www.eef.org.uk/uksteel/About-the-industry/How-steel-is-made/step-bystep/The-electric-arc-furnace.htm>.
3. "Steel Production." www.carbonandgraphite.org. N.p., n.d. Web. 6 May 2014.
<http://www.carbonandgraphite.org/pdf/steel_production.pdf>.
4. "About Steel." World Steel Association -. N.p., n.d. Web. 06 May 2014.
<http://www.worldsteel.org/faq/about-steel.html>.
5. "What Is Boron Steel." Www.basicwelding.org. N.p., n.d. Web. 6 May 2014.
<http%3A%2F%2Fwww.basicwelding.org%2F2012%2F10%2F31%2Fwhat-isboron-steel%2F>.
6. "Carbon Steel." Merriam-Webster. Merriam-Webster, n.d. Web. 06 May 2014.
<http://www.merriam-webster.com/dictionary/carbon%2520steel>.
7. "Effects of Alloying Elements in Steel." Effects of Alloying Elements in Steel. N.p.,
n.d. Web. 06 May 2014. <http://www.chasealloys.co.uk/steel/alloying-elementsin-steel/>.
8. "Metals." Www.skyshop.com.au. N.p., n.d. Web. 5 May 2014.
<http://www.skyshop.com.au/METALS.pdf>.
9. "Tempering, Annealing, Hardening." Www.giz.de/Themen/en/dokumente/en-
metalwork-annealing-hardening-tempering-for-trainees.pdf. N.p., n.d. Web. 6
May 2014. <www.giz.de/Themen/en/dokumente/en-metalwork-annealinghardening-tempering-for-trainees.pdf>.
10. "Heat Treatment."
Http://heattreatment.linde.com/International/Web/LG/HT/like35lght.nsf/repository
byalias/apps_annealing/$file/Annealing.pdf. N.p., n.d. Web. 6 May 2014.
<http://heattreatment.linde.com/International/Web/LG/HT/like35lght.nsf/repository
byalias/apps_annealing/$file/Annealing.pdf>.
49
11. "Annealing, Hardening, Tempering - Course: Working Techniques of Heat
Treatment of Steel. Trainees' Handbook of Lessons: 5. Hardening." Annealing,
Hardening, Tempering - Course: Working Techniques of Heat Treatment of
Steel. Trainees' Handbook of Lessons: 5. Hardening. N.p., n.d. Web. 06 May
2014. <http://collections.infocollections.org/ukedu/ru/d/Jgtz077ce/7.html>.
12. "The Iron Carbide Diagram." Http://www.pg.gda.pl/~kkrzyszt/Topic%209.pdf.
N.p., n.d. Web. 6 May 2014. <http://www.pg.gda.pl/~kkrzyszt/Topic%209.pdf>.
13. "Iron-Carbon Phase Diagram." Http://web.utk.edu/~prack/MSE%20300/FeC.pdf.
N.p., n.d. Web. 6 May 2014. <http://web.utk.edu/~prack/MSE%20300/FeC.pdf>.
14. "Continuous Cooling Transformation Diagram."
Http://www.msm.cam.ac.uk/phase-trans/2012/Manna/Part3.pdf. N.p., n.d. Web. 6
May 2014. <http://www.msm.cam.ac.uk/phase-trans/2012/Manna/Part3.pdf>.
15. "Steel Material Properties." Steelconstruction.info. N.p., n.d. Web. 06 May 2014.
<http://www.steelconstruction.info/Steel_material_properties#Yield_strength>.
16. "Fracture Toughness." Fracture Toughness. N.p., n.d. Web. 06 May 2014.
<http://www.ndted.org/EducationResources/CommunityCollege/Materials/Mechanical/FractureTo
ughness.htm>.
17. Xiaodong, Zhu, Ma Zhaohul, and Wang Li. "Advanced High Strength Steel for
Auto-making." Http://www.baosteel.com/english_n/e07technical_n/021702e.pdf.
N.p., n.d. Web. 6 May 2014.
<http://www.baosteel.com/english_n/e07technical_n/021702e.pdf>.
18. Keeler, Stuart. "The Science of Forming." Metal Forming Magazine 1 Apr. 2009:
n. pag. Web.
19. Chiang, L. K. "Development and Production of HSLA." Development and
Production of HSLA. N.p., n.d. Web. 6 May 2014.
<http%3A%2F%2Fwww.steel.org%2Fen%2Fsitecore%2Fcontent%2FGlobal%2F
Document%2520Types%2FNews%2F2012%2F~%2Fmedia%2FFiles%2FAutost
eel%2FResearch%2FAHSS%2F>.
20. Sivaprasad, S., S. Tarafder, V. R. Ranganath, and K. K. Ray. "Effects of Pre-
strain on Fracture Toughness of HSLA." Materials Science and Engineering
284.1-2 (2000): 195-201. Science Direct. May 2000. Web. 6 May 2014.
<http://www.sciencedirect.com/science/article/pii/S0921509300007395>.
50
21. Ravi, S., V. Balasubramanian, S. Babu, and S. Nemat Nasser. "Assessment of
Some Factors Influencing the Fatigue Life of Strength Mis-matched HSLA Steel
Weldments." Assessment of Some Factors Influencing the Fatigue Life of
Strength Mis-matched HSLA Steel Weldments. Elsevier Science, Apr. 2004.
Web. 06 May 2014.
<http://www.sciencedirect.com/science/article/pii/S0261306903001973>.
22. Brockenbrough, Roger L. "PROPERTIES OF STRUCTURAL STEELS AND
EFFECTS OF STEELMAKING AND FABRICATION." Mhprofessional.com. N.p.,
n.d. Web. 6 May 2014.
<http://www.mhprofessional.com/downloads/products/0071666664/0071666664_
ch01.pdf>.
Technical Report
on
Long Heating Cycle Intercritical Partitioning to Produce Duplex
Microstructures
Submitted by:
Matthew K. Stewart
Submitted to:
Dr. Heshmat Aglan
May, 2014
1
Acknowledgements
This project is jointly funded by the Nucor Steel Corporation, LLC through Tuskegee University
Nucor – Education and Research Center (NERC) and NSF/PIRE through Purdue University,
West Lafayette, IN. The technical guidance and support of the Tuskegee University Research
Team is greatly acknowledged. The invaluable advice and encouragement rendered by the Nucor
team is also appreciated.
Dr. Heshmat Aglan
Dr. Ronald J. O’Malley
Mr. Kaushal Rao
2
TABLE OF CONTENTS
A. Abstract ......................................................................................................................................4
B. Overview.....................................................................................................................................5
1.0. Introduction and Literature Review..........................................................................................6
2.0. Materials and Testing..............................................................................................................27
3.0. Results and Discussion...........................................................................................................30
4.0. Conclusion..............................................................................................................................38
5.0 References................................................................................................................................39
3
Intercritical Partitioning of Mn in High Mn Steel to Produce Duplex Microstructure
Matthew K. Stewart, M.E. (Soph.)
A new class of advanced high strength, high formability, cost competive steels is under
development. One promising route for the manufacture of these “Generation 3” (Gen 3) steels is
through the development of a metastable dual phase austenite-ferrite microstructure’ this phase is
produced by intercritical annealing and compositional partitioning of slower diffusing species
(Mn, B, etc.) using long annealing cycles. In conventional intercritical annealing, only fast
diffusing species such as carbon are partitioned; fast cooling is required to avoid back diffusion
of carbon. If slower diffusing species can be successfully partitioned, lower cooling rates might
allow these steels to be produced in conventional batch annealing processes’ this would allow
production of these Gen 3 steels in conventional steel processing facilities.
4
Student’s Name: Matthew K. Stewart
Mentors: Drs. H. Aglan (Tuskegee University) & Ronald J. O’Malley (NSDEC)
Title: Long Heating Cycle Intercritical Partitioning of to Produce Duplex Microstructure.
Objective: To investigate the feasibility of manganese partitioning in a 0.14% C - 2% Mn steel
using simulated batch annealing to stabilize austenite and produce generation 3 advanced high
strength steels.
Background: New automotive design require newer advanced high strength steels (AHSS) with
improved properties optimized for safety and fuel efficiency. Two generations of AHSS, mainly
first and second generation steels, were developed, including transformation induced plasticity
(TRIP), dual phase (DP), and twinning- induced plasticity steel (TWIP) that exhibited greater
combinations of strength and ductility. However, these steels found to be expensive and limited
in the commercial automotive industry. Efforts are being made to develop a new family “third
generation” steel with properties between first and two generation steel that addresses the issues
of each. Our current research focuses on manganese enrichment of austenite during longer
intercritical annealing, thus stabilizing austenite to room temperature. The resulting
microstructure is a transformation-induced plasticity (TRIP) steel with varying retained austenite
content.
Proposed Work and Tasks: Nucor Steel Decatur 2% Mn cold rolled steel samples will be
supplied for this project.
• Task 1: Literature review
• Techniques involved in the feasibility of the partitioning of Mn from the High Mn steel
will be presented.
• Task 2: Heat treatment
• Samples will be annealed at several intercritical temperatures (ac3) and held for various
times, then cooled in air.
• High Ti bags will be used to protect the steels from the oxidation and scaling.
• Steels will be inter-critically (batch) annealed for at least 3 days.
• Task 3: Microstructural Evaluation
• Samples will be cut and microstructure is evaluated to determine the amount of the Mn
partitioned.
• Duplex microstructure will be identified and the phases will be presented.
• Task 4: Mechanical propertied Evaluation
• Mechanical properties including tensile strength, hardness and fracture toughness (KIC)
will be evaluated for the intercritically annealed samples
• Results from all the studies will be reported.
5
1.1 Introduction and Literature Review of Steel
Dating back to the late 1850s, Henry Bessemer, the inventor of steel, developed an
effective solution in which he used oxygen to reduce the carbon in the contents of iron.
Analyzing the results of steel, he discovered that steel was an alloy that contains iron and carbon
to contribute to the strength of the material. Undergoing the Bessemer process, steel is melted
down and blown with oxygen to release carbon dioxide, producing a more pure material of iron.
In addition, carbon is used most commonly for alloying iron to act as a hardening agent to steel.
While on the other hand iron acts as a binding agent between itself and carbon in the creation of
steel. Lastly, iron by itself is a relatively soft metal; therefore, it does not hold a good edge over
opposing materials. However, once adding the element of carbon it begins to harden the iron,
creating the material of steel [1].
1.2 Steel making process
1. Iron making – Placing coal, fluxes, and iron ore into a furnace will produce molten
iron, better known as ‘hot metal’ due to its impurities that make it a brittle material.
2. Primary Steelmaking – Primary component produces two processes that affect ‘hot
metal; known as Basic Oxygen Steelmaking (BOS) process where the steel is 75%
liquidized hot metal from the furnace. Later, oxygen is blown into the BOS to reduce
the carbon content. Second, is Electric Arc Furnace Steelmaking (EAF) where an
electrically heating furnace makes steel from scrap metal only.
3. Secondary Steelmaking – Is the finalization of the two steps shown above. In the
secondary step treatment of the steel is brought from adjusting the steel composition.
6
Followed by adding and/or removing certain element to reassure the steel meets its
requirements of the consumer and producer.
4. Continuous Casting – Melted steel is casted into a cooling system that solidifies the
steel to create strands of flat products, beams, and wires.
5. Primary foaming – Remaining steel is formulized into different shapes by first being
applied to hot rolling that creates surface quality. They are usually divided into flat
products, long products, and specialty products.
6. Manufacturing - Finally, the steel is given its shape from techniques such as drilling,
welding, coating, heat treatment, and surface treatment [2].
1.3 Types of Carbon Steel
Carbon steel is steel which contains the primary element of carbon, ranging from 0.122.0%. Due to other elements being too small to affect properties, carbon steel is a combination of
two primary elements: carbon and iron. When producing high amounts of carbon for carbon steel
the alloy can be hardened to increase strength, durability, and impact resistance. Due to the
amount of alloy in carbon steel, four basic types of carbon have been discovered and displayed
as such:
Low Carbon Steel
Being the primary of all carbon steels, low carbon steel has an alloying element made up
of a relatively low amount of carbon. Low carbon steel manages to hold 0.05% - 0.25% of
carbon with an addition of 0.4% manganese; therefore, causing the Low Carbon Steel to be fairly
cheap, easy to shape, and contains the less amount of surface hardness.
7
Medium Carbon Steel
Medium carbon steel is carbon steel that contains between 0.29% - 0.55% of carbon
creating a surface that is more than double the strength of the Lower Carbon Steel. With 0.60%1.65% of manganese including in this process the Medium Carbon Steel has the ability to be
classified as a strong and durable product. This particular type of steel provides great balance
between strength and ductility, and is very common in many different toes of steel parts.
High Carbon Steel
High Carbon Steel is a metal alloy containing high amounts of carbon ranging between
0.55% - 0.99% that creates a very strong object that has the ability to hold shape memory, very
well due to its 0.30% - 0.90% manganese. This Carbon is mainly produced for sprigs and wire
due to its ability to take the form of any shape.
Very High Carbon Steel
Production of very high carbon steel creates a brittle service of the steel, while
additionally requiring special handling due to its 0.96% - 2.1% of carbon that makes it the
strongest carbon amongst the four. These steels are used for special purposes such as knives,
tools, axles or punches. When there is more than 1.2% carbon content, the material was made by
powder metallurgy. Any other steels that have higher carbon contents than ultra high carbon
steels are cast iron, shown in Figure 1.
8
Figure 1. Stress Strain Curves showing properties of steels with different carbon content
1.4 Alloyed Elements in Carbon Steels
Alloy steel is steel that is united with a broad variety of elements in order to better assure
the improvement of its mechanical properties. Alloying elements have the ability of achieving
certain properties in a particular material. For example, having been divided into two groups,
low-alloy steels and high-alloy steel possess different alloying elements that produces the power
of creating a different material from each. When producing low-alloy steel at less than 5%,
producer wants the resulting product to increase strength or hardenability. On the other hand,
production of high-alloy steel leads to a percentage of over 5% in order to achieve special
properties, such as corrosion resistance or extreme temperature stability. Lastly, in the
production of steel various alloying element are produced including: [3]
•
Carbon the basic metal, iron, is alloyed with carbon to make steel and has the effect of
increasing the hardness and strength by heat treatment.
9
•
Manganese is added to steel to improve hot working properties and increase strength,
toughness and hardenability. Manganese, like nickel, is an austenite forming element
and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic
stainless steels.
•
Chromium is added to the steel to increase resistance to oxidation. This resistance
increases as more chromium is added. Stainless Steel has approximately 11%
chromium and a very marked degree of general corrosion resistance when compared
with steels with a lower percentage of chromium. When added to low alloy steels,
chromium can increase the response to heat treatment, thus improving hardenability
and strength.
•
Nickel is added in large amounts, over about 8%, to high chromium stainless steel to
form the most important class of corrosion and heat resistant steels. These are the
austenitic stainless steels, typified by 18-8, where the tendency of nickel to form
austenite is responsible for a great toughness and high strength at both high and low
temperatures. Nickel also improves resistance to oxidation and corrosion. It increases
toughness at low temperatures when added in smaller amounts to alloy steels.
•
Molybdenum when added to chromium-nickel austenitic steels, improves resistance to
corrosion especially by chlorides and sulfur chemicals. When added to low alloy steels,
molybdenum improves high temperature strengths and hardness. When added to
chromium steels it greatly diminishes the tendency of steels to decay in service or in
heat treatment.
10
•
Titanium the main use of titanium as an alloying element in steel is for carbide
stabilization. It combines with carbon to for titanium carbides, which are quite stable
and hard to dissolve in steel, this tends to minimize the occurrence of inter-granular
corrosion.
•
Phosphorus is usually added with sulfur to improve machinability in low alloy steels,
phosphorus, in small amounts, aids strength and corrosion resistance. Experimental
work shows that phosphorus present in austenitic stainless steels increases strength.
Phosphorus additions are known to increase the tendency to cracking during welding.
•
Sulfur When added in small amounts sulfur improves machinability but does not cause
hot shortness. Hot shortness is reduced by the addition of manganese, which combines
with the sulfur to form manganese sulfide. As manganese sulfide has a higher melting
point than iron sulfide, which would form if manganese were not present the weak
spots at the grain boundaries are greatly reduced during hot working.
•
Selenium is a mineral found in soil and is added to steel to improve machinability.
•
Niobium is added to steel in order to stabilize carbon, and as such performs in the same
way as described for titanium. Niobium also has the effect of strengthening steels and
alloys for high temperature service.
•
Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as
in the case of nickel, an austenite forming element. Yield strength is greatly improved
when nitrogen is added to austenitic stainless steels.
11
•
Silicon is used as a deoxidizing agent in the melting of steel; as a result, most steels
contain a small percentage of silicon. Silicon contributes to hardening of the ferrite
phase in steels and for this reason silicon killed steels are somewhat harder and stiffer
than aluminum killed steels.
•
Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear
reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt
restriction, usually approximately 0.2% maximum. This problem is emphasized
because there is residual cobalt content in the nickel used in producing these steels.
•
Tantalum is chemically similar to niobium and has similar effects.
•
Copper is normally present in stainless steels as a residual element. However it is
added to a few alloys to produce precipitation hardening properties.
1.5 Iron- Iron Carbide Diagram
An Iron-Iron Carbide Diagram is a graphical representation that demonstrates the
microstructure within materials as a function of material composition and material temperature.
In addition, the Iron-Iron Carbide Diagram shows combinations of carbon in a solid solution of
iron, which contains up to 6.67% of carbon. Carbon content 6.67% corresponds to the fixed
composition of the iron carbide Fe3C. Three regions namely eutectoid E, the hypereutectoid A,
and the hypereutectoid B are seen from the figure 3. The right side of the pure iron line is carbon
in combination with various forms of iron called alpha iron (ferrite), gamma iron (austenite), and
delta iron. Phase diagram is a useful tool to determine the number and types of phases, the wt%
of each phase, and the composition of each phase for a given T and composition of the system.
12
On the other hand, Phase Diagrams are useful contributions when helping us understands thermal
history of a material and the effects on that particular material property (Figure 2).
•
Eutectoid Composition describes the phase transformation of one solid into two
different solids. In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C,
723°C. The phase just above the eutectoid temperature for plain carbon steels is known as
austenite or gamma. We consider this phase to be cooled through the eutectoid
temperature (723°C).
•
Hypoeutectoid Alloys: These are alloys with a composition between 0.022 and
0.76wt%C. These cool from the single phase austenite region, through a two phase ferrite
+ austenite region into the ferrite + cementite region. As the material cools through the
two phase regions the ferrite grains grow in size. Upon crossing the eutectoid line, the
remaining austenite converts to pearlite. The lower the carbon content, the more ferrite
and less pearlite due to the fact that the 2 phase region will contain more austenite and
less gamma.
•
Hypereutectoid Alloys: Alloys with carbon content between 0.76 and 2.14 wt%. These
cool from the single phase austenite region, through a two phase cementite + austenite
region into the ferrite + cementite region. As the material cools through the 2 phase
region cementite grains grow in size. Upon crossing the eutectoid line, the remaining
austenite converts to pearlite. The result is a microstructure with grains of cementite
mixed with grains of pearlite.
13
Figure 2. Iron-Iron carbide phase diagram
Examination of phase diagrams leads interpreters to find out what phases were presented, the
composition of the phases, and the percentage of fractions of the phases. Initially, the phases
present locates the temperature-composition point on the graph to see which phases are present.
Second, leads viewers to a single phase present, meaning the composition of the phase is simply
the overall composition of the alloy element. The second phase regions draw a horizontal line to
intersect the liquids and solids lines at the given temperature. The composition of the liquid
portion of the two phase region is determined by the composition at the point where the tie line
intersects the liquids line. On the other hand, the composition of the solid portion of the two
phase region is determined by the composition at the point where the tie line intersects the solid
line. Lastly, phase diagrams determine the percentage of each phase, which is computed as the
ratio of the length of the tie line from the overall composition to the opposite phase boundary
divided by the overall length of the tie line [4]. The Binary System Diagram presents viewers
with the limits of carbon in iron along with the temperature changing. Most importantly, this
14
diagram presents us with the ability to view the Iron-Iron carbide phase diagram that only
extends to 6.70wt% carbon in order to produce quality steel.
1.6 Different Phases in Steel
A phase is a combination of materials different from another part, mainly caused by the
difference between the structures. Contributions of each different structure form an interface
around surrounding phase; while on the other hand, some structures were created into crystal
form when exposed to substantial amounts of temperatures. [5]
•
Austenite is a metallic, non-magnetic solid solution of carbon and iron that exists in steel
above the critical temperature of 1333°F (723°C). Its face-centered cubic (FCC) structure
allows it to hold a high proportion of carbon in solution. As it cools, this structure either
breaks down into a mixture of ferrite and cementite, or undergoes a slight lattice
distortion known as martensite transformation. The addition of certain other metals, such
as manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment
of low-alloy steels.
•
Ferrite is a body-centered cubic (BCC) formed of iron, in which a very small amount (a
maximum of 0.02% at 1333°F / 723°C) of carbon is dissolved. Ferrite is the component
which gives steel and cast iron their magnetic properties, and is the classic example of a
ferromagnetic material. This is also the reason that tool steel becomes non-magnetic
above the hardening temperature. Plain carbon steels with up to about 0.2 wt% C consist
mostly of ferrite; therefore, explaining why steel becomes a non-magnetic above a
specified hardening temperature.
15
•
Pearlite is a lamellar structure consisting of alternating bands of ferrite and cementite. In
which they form a distinct layers or bands in slowly cooled carbon steels. In addition, the
pearlite exists in equilibrium in carbon steels at normal temperatures.
•
Cementite is iron carbide with the formula Fe3C, and an orthorhombic crystal structure.
It is a hard, brittle material, essentially a ceramic in its pure form. It forms directly from
the melt in the case of white cast iron. In carbon steel, it either forms from austenite
during cooling or from martensite during tempering.
•
Martensite is a body-centered tetragonal form of iron in which some carbon is dissolved.
Martensite forms during quenching, when the face centered cubic lattice of austenite is
distorted into the body centered tetragonal structure without the loss of its contained
carbon atoms into cementite and ferrite.
•
Ledeburite is the eutectic of the iron-carbon system, the constituents being cementite and
austenite at high temperatures; cooling decomposes the austenite to ferrite and cementite.
•
Lath Martensite is formed when there is low carbon content present when the steel is in
the austenite phase. This phase is associated with high toughness and ductility but low in
strength. Lath martensite has grains called laths. Laths have smaller packet sizes resulting
in more impact energy absorption.
•
Plate Martensite is formed when there is high carbon present while the metal is in the
austenite phase. It has much higher strength than lath martensite but may be very brittle
and not ductile [6].
•
Bainite is a combination of ferrite and cementite. It has intermediate hardness and good
toughness. It forms as needles or plates. Once transformed, it cannot be changed back
16
without reheating to austenite. Bainite is a combination of fine carbon needles in a ferrite
matrix and is formed when austenite is cooled at a slower rate than what is needed for
martensite (Figure 3).
Figure 3. Fe-Fe3C diagram with different phases of steel
1.7 Heat Treatments
Heat treating is an industrial process created to significantly alter the physical, and in
most cases the chemical properties of a material. The properties of steel are impacted mainly on
the percentage of carbon. A different percentage of carbon leads to phases of pearlite and ferrite
pro-eutectoid for the hypo-eutectoid. As opposed to hyper-eutectoid steels which in fact contains,
pearlite and cementite pro-eutectoid. The process of heat treatment illustrates to users how an
object has the ability of hardening and softening by using a heating or cooling methods at
extreme temperatures. Undergoing the process of heat treatment of metal processing leads to a
variety of techniques, displayed as such [7]:
17
•
Annealing is the treatment of a metal or alloy by heating to a
predetermined temperature, holding for a certain time, and then cooling to room
temperature to improve ductility and reduce brittleness. The process of annealing is
used variously to soften, relieve internal stresses, and improve machinability while
also obtaining particular mechanical and physical properties.
•
Case hardening is a simple method of steel hardening that uses techniques for steels
with low carbon content. Carbon is added directly onto the surface of the steel,
therefore the inner core is left untouched keeping the processes properties such as
flexibility and relatively soft.
•
Precipitation Strengthening is the techniques where heat is applied to a malleable
material, such as a metal alloy, in order to strengthen the material. This technique
hardens the alloy creating solid impurities, called precipitates, which stop the
movement of dislocations in the crystal lattice structure.
•
Tempering is used to increase the toughness of iron-based alloys by heating it to high
temperatures, though below the melting point, and then cooled in air. Additionally,
tempered steel becomes rusted and as a result the steel beings to crack, corrosion
begin, and eventually complete structural failure.
•
Quenching is the most common method of hardening steel. Quenching is the rapid
cooling of a work piece to obtain certain material properties. The primary material
property expected to obtain is pure martensite. It consists of heating the austenizing
temperature and cooling it fast enough to avoid the formation of ferrite, pearlite, or
bainite.
18
•
Austenization this heat treatment consists of heating the metal to a very high
temperature in order to obtain an austenite structure. The austenizing temperature and
time of austenization differ with composition, but specifically with carbon content.
Generally, the hardness with temperature will reach a peak and then drop. Although,
the hardness will vary with time at a slower rate.
•
Normalizing is also an internal stress reliever for metals that have been machined,
forged, or welded. These steels are harder and stronger than annealed steel and much
tougher than steel of any other condition. This process may be done before hardening
in order to obtain desired hardness. The method is done by heating the steel to a
specified temperature which is much higher than the hardening procedure, letting the
metal soak until the heating is uniform throughout the material, and then air cooling
the metal.
•
Hardening is done by heating the metal to the desired temperature and rapidly
putting the heated metal into the quenching solution thereby cooling the metal. This
process increases strength and hardness while making the metal more brittle.
When producing steel through heat treatment the mechanical properties of the alloy are
altered such as the hardness, strength, toughness, ductility, and elasticity. Due to martensite
causing deformation within the crystals the diffused mechanism changes cause the creation of
alloy. Once being placed in the heat treatment center the atoms of the steel being to expand
making the object less stable. This gives users the ability to create tools of steel, unable to be
produced before heat treatment. In the production of steel, reaching the crystal length structure of
austenite with ferrite and cementite, helps manipulates the steel to be formed into shapes while
also preventing corrosion. Once heated to the desired temperature, users are able to form steel in
19
the desired shape. After applying the process of shaping steel into the desired shape, combining
all crystal structures within the object is done by reheating the object. After reheating the object
one last time to lose up the atoms and crystal structures the steel is placed on a magnet for testing
to be classified as Austenitic Steel. Later, the steel item is placed between 2 of 3 different
cooling methods. First, cooling the steel in sand begins to limit the chance of creating martensite,
therefore relieving the steel of intense deformation. Within minutes the steel is quenched in oil to
compact all molecules together, therefore quickly finishing the cooling process. Lastly, the steel
is recrystallization in a process called tempering to toughen the steel. Tempering causes for the
steel item to be reheated to a temperature below the critical temperature. Tempering causes the
steel to retrieve ferrite and cementite structures. After the process of tempering has been
complete, sanding of the steel object is needed in order to eliminate the oil from the steel
structure. Finally, once the object has been tempered, it has become tough and hard enough to be
sharpened, polished, and finalized.
1.8 Microstructural Phases of Carbon Steel
Microstructural components consist of the different basic structures of phases which
appear within one microstructure. Selection of proper materials depends largely upon the alloy
composition, obtained through heat treatment and processing. Microstructural examination is
able to be obtained through the process of different heat treatments, producing the solutions of
acid and chemicals that better obtains a highly polished surface [8]. Microstructural factors and
various compositions that affect strength also separate out the strength factors of ferrite and
pearlite from the end product. Etching alloy elements consist of three basic methods;
20
•
Immersion is used in order to produce your desired structure, while also reaching the
highly specified microstructural features projected.
•
Swabbing The general purpose of swabbing is for the etchants, while being
conducted by cotton saturated with the reagent. Additionally, users must hold the
specimen with tongs using one hand and swab with cotton, held with togs, in the other
hand. However, for best results, cosmetic cotton puffs can contain impurity fragments
that may recover the surface, or swabbing, and rinsed with running water. The
specimen is then rinsed with ethanol and blown dry with warm air to produce a
quality product.
•
Electrolysis is the method which uses a direct electric to drive an otherwise nonspontaneous chemical reaction. While also producing a current in order to preparedly
separate elements from their nature occurring source
1.9 Mechanical Properties of Carbon Steel
In most cases, objects are being affected by a force that effects its material formation as a
result of applied loads, time, temperature, and other conditions, leading examiners to discover
how a material reacts when it is subjected to some type of force that attempts to reform the steel.
Mechanical Properties are governed by the basic concepts of elasticity, plasticity, and toughness.
•
Hardness is the resistance to localized deformation. This deformation is in the form
of plastic deformation which includes penetration, indentation, scratching, cutting,
and bending. Hardness is not a property of a material, but is the combination of
properties from yield strength, work hardening, true tensile strength, modulus, and
other factors.
21
•
Brinell hardness test is used as a desktop machine that applies a specific load to a
sphere of known diameter. The hardness number is found by dividing the load by the
measured surface area of the indentation left on the test surface. Brinell tests are
frequently used to find the hardness of forgings and castings that have a coarse grain
structure and cannot be read by the Rockwell or Vickers test. Brinell values are test
force independent as long as the ball size and test force relationship is the same [9].
•
Ductility is a measure of how much a material deforms plastically before fracture.
•
Toughness is the ability of a metal to deform plastically and absorb energy before
fracture. A material’s toughness depends on both ductility and strength. The factors
affected by toughness of a material including: strain rate, temperature, and notch
effect. A metal may have high toughness to withstand a static load but would fail
under dynamic or impact loads.
•
Impact toughness of a material is determined from Charpy and Izod tests. The two
tests use different specimens and methods of holding, yet both use the pendulumtesting machine. Impact toughness is determined by measuring the energy absorbed in
fracture, being obtaining by the difference between the height of the pendulum before
and after the swing and multiplying by the weight of the pendulum.
•
Notch toughness is the ability for a material to absorb energy when there is a flaw in
the material. When a flaw such as a notch or crack is present the material will have a
lower toughness value. When a load is placed on the material, it produces multi –
axial stresses adjacent to the flaw. The material develops plastic strains near the crack
tip or notch crevice. The amount of plastic deformation is restricted by the
surrounding material which remains elastic. As a material is prevented from
22
deforming elastically, that is when brittle fractures occur. However, tests are usually
done either with dynamic or static loads, but in reality manufactured components will
have to withstand both static and dynamic loads [10].
•
Hardenability is the ability of an alloy to be hardened by forming martensite by a
heat treatment method. In addition, it measures the rate that hardness reduces with
distance into the interior of steel. Usually, alloying elements retard the formation of
softer microstructures and allow the higher hardness structures to be produced at
lower temperatures [11].
•
Jominy test is a standard procedure to determine the hardenability of steel. The test
begins by heating a cylindrical specimen at austenizing temperature until the austenite
phase has formed. Then, the specimen is removed from the furnace and the bottom of
the steel is quenched using a jet of water with constant flow rate and temperature. The
cooling rate of the steel is at a maximum at the bottom and decreases as the distance
from the quenched end increases. After being cooled to room temperature, the steel is
ground flat and hardness values are taken every 1/16 of an inch along the ground flat.
The hardness values would show that the quenched end has the maximum hardness.
However, since the cooling rate decreases with distance from the bottom, the hardness
will also decrease with the distance from the bottom. The hardenability is then
determined by the depth of hardening. An alloy with high hardenability will retain
large hardness values for large depths in the material [12].
•
Stress strain curve is the relationship for stress and strain that a material
demonstrates under loads. Each material has its own unique relationship of stress
23
which is found by observing the deformation and resulting strain when the material is
under tensile and compressive loads. The significance of these curves is that it reveals
properties of the material such as the modulus of elasticity which is the slope of the
curve for the elastic region. These materials will deform plastically and stretch,
causing strain, as more tensile load is applied. There are certain points on the curves
that signal and help a person predict how a material will behave. Usually, all curves
will begin with a straight linear line, what occurs after that straight line is what makes
the difference between brittle and ductile materials.
1.10 Generations of Steel
Over the development of steel, different classifications have created generation one steel
and on the other hand generation two steel. With the demand for a more quality product of steel,
producers have been forced to create a third generation type of steel that is a combination of both
generation one and two. High Strength Steels have tensile strength in the range of 280-650 MPa,
which causes formability to decrease as tensile strength beings to increase. In addition, there is
Advanced High Strength Steel, which in fact is actually broken down into two different
categories; listed as such:
•
Multiphase Steel has tensile strength between the ranges of 500 to 1000 MPa that
causes the object to have more formability. One example of multiphase steel is
Transformation Induced Plasticity which is a high-strength steel that is typically
used in the production of automotives. This transformation allows for enhanced
strength and ductility.
•
Ultra High Strength Steels is a steel foil designed for use in applications
requiring high strength, hardness and corrosion resistance. Ultra High Strength
24
Steels features a nano-scale microstructure which provides twice the strength of
most commercial metals used in thin foil applications. In addition, these features
show exceptionally high strength and hardness properties, high resistance to
corrosion and usable ductility for forming shapes. Lastly, this steel is used in order
to design woven fabrics, braids, corrugated profiles, fibers, laminates and foil slit
to various widths.
With these great accomplishments, consumers are constantly searching for new and
remodeled products. Following the guide lines of generation one and two steels, producers have
created a production line of generation three type steel. Third generation of advanced highstrength steels are created for the automotive industry, which in fact contains a high volume
fraction of fine-grained ferrite, carbide-free bainite, martensite and retained austenite ( Figure 5).
The level of strength and ductility is highly dependent on the fraction and mechanical stability of
austenitic phase (Figure 4).
Figure 4: The different generations of steels and the tensile strength and elongations produced
[8].
25
1.11 Intercritical Partitioning of High Mn Steel
In the experiment of producing Duplex Microstructures, the chemical compositions of
steel needs to have 0.14% C and 2% Mn, due to these elements of carbon and manganese being
strong austenite stabilizers. Partitioning manganese in steel allows for austenization to occur
throughout the sample, making the overall sample of steel more ductile. The demand to develop
new automotive designs to better obtain fuel efficiency and safety has led to a heating method to
maintain thickness while also being light in weight. Currently, “first generation” steels are based
on ferritic microstructures with an addition of low-temperature transformation products to
increase strength. On the other hand, austenitic steels, including stainless steels and recently
developed twinning-induced plasticity steels, exhibit excellent combinations of strength and
ductility and constitute a group of “second generation” steel. This is a new family of steel with
properties between the first and second generation steels. New family steel better known as
“third generation” steel will have increased amounts of retained austenite with controlled
stability against strain-induced transformation to martensite. Approaching the development of
microstructures of interest leads developers to use lean alloys between 5 to 8 wt pct Mn with an
addition of intercritical annealing in the ferrite-austenite region, enriching austenite in Mn once
being placed in room temperature in order to cool properly.
26
2.0 Materials and Testing
Materials provided by Nucor Decatur were used to properly modify samples of steel to
obtain the desired material. Described in Table 1 is the composition of the steel that was
examined.
Table I. Composition of Experimental Mn-TRIP Steel (Weight Percent) DP590R
Mn
C
Si
Cr
Ni
Mo
P
Al
N
S
2.1050
0.1558
0.1250
0.0470
0.0410
0.0140
0.0083
0.0230
0.0700
0.0004
In a last year’s testing, stainless steel foil was used in order to protect the steel from
oxidizing during the annealing process. Understanding that titanium acts as an oxygen getter,
tests were performed by placing the steel in bags with scraps of titanium in order to increase the
strength of the steel. The contribution of the titanium scraps was intended to reduce residual
stress developed during fabrication and, produces combination of ductility, machinability, and
dimensional of structural stability.
Equipment
•
Buehler SimpliMet 1000 Automatic Mounting Press
•
Buehler EcoMet 250 Automatic Grinder/Polisher
•
MTS 810 Servo Hydraulic Material Testing System
•
Clark Hardness Tester (Rockwell Type Tester)
•
Fisher Scientific Furnace (1st Case)
27
•
ThermCraft Furnace (2nd Case)
•
GX51 Olympus Microscope
Procedures
1.
Set furnace for the following temperatures 682o C, 710o C and 743oC (1260oF, 1310oF,
and 1370oF, respectively)
2. While waiting for the temperature of the furnace to rise, wrap the samples in stainless steel foil
to keep the samples from oxidizing while going through the batch annealing process.
3. Once the set value temperature is reached, place the samples in the furnace (when opening the
furnace the temperature tends to drop so it is best to wait for the temperature to reach what it has
been set to). Start the timer when the samples are all in the furnace.
4. Take out a sample at different holding times (1, 3, and 5 day(s) here) and leave it to air cool at
ambient temperature.
5. Repeat steps 1-4 for every temperature being done.
6. After the samples have air cooled they are engraved to distinguish the temperature annealed
and the number of days annealed.
7. Excessive scaling is then removed from the samples by sanding (various grits). Samples were
then cleaned and washed with acetone; lastly, paper towels were used to get any excess residue
off the sample.
8. Thus cleaned samples were separated for the mechanical testing and microstructural analysis.
28
Figure 5 shows the thermodynamic equilibrium phase diagram for this steel (DP590R). This is a
relation between wt% phase and temperature dependence on the percentage of the fractions
formed. Three intercritical temperatures were chosen based on the enrichment of austenite in the
steel namely 1260oF, 1310oF and 1370oF. The amount of austenite enriched at these
temperatures depends on the maximum amounts of carbon and managanese partitioned out from
the steel. At the same time, the ferrite fraction is kept in a way that it adds the toughness to the
steel. Austenite moreover increases the hardenability of the steel when stabilized at room
temperature giving the steel both toughness and hardness quotients to form a generation 3 AHSS
steel.
Figure 5. Equilibrium diagram obtained from JMATPRO for DP590R steel
29
3.0 Results and Discussion
The Tensile Testing and Yield Strength: Figures 6 through 9 describes the results that were
gathered from the testing. The ASTM E8 standards, helped us understand if the overall length
(L), grips (B), radius of the fillet (R), width (of gauge) (W), gauge length (G), width of the grips
(C), and length of reduced section (A) were at the correct length or measurements (Table II).
Figure 6 shows the stress vs strain for the 1210oF samples held at 1, 3 and 5 days intercritically.
The tensile strength was higher (about 490 MPa) for the sample held intercritically for one day
with about 44% elongation. As the holding time increased from 1 day to three days, the tensile
strength reduced by about 25% (365 MPa) however the elongation increased by about 14%. This
may be due to the enlarged grain size due to longer holding times. For the sample held for 5 days
intercritically, the strength however increased by about 10% (about 400 MPa) and elongation
increased by about 13% (53%). This optimal strength and toughness values are as a result of
manganese being partitioned and maximum enrichment of austenite in this steel at that
intercritical temperature. These measurements are in accordance with the volume fractions and
the microstructure explained in sections below.
Table II. Tensile Specimen Measurements
30
625.
1260F_1day
1260F_1day
1260_3d
500.
375.
250.
125.
0.
0.
15.
30.
45.
60.
Figure 6: The tensile test and elongation for the 1260oF sample set at 1, 3, and 5 days holding.
Figure 7 shows the stress versus strain curves for the samples intercritically annealed at 1310oF
for 1,3 and 5 days. From this figure, the curve for the sample held for 1 day show a maximum
tensile strength of about 800 MPa while the elongation (%strain) is about 18%. For the sample
with three days holding, the tensile strength reduced by about 150% (350 Mpa) when compared
to the sample held for one day. However, the elongation increased by about 250%, from 18% for
one day to 44% for three days holding. This can be attributed to the increased grain size of the
austenite and more amounts of ferrite phase formation in its microstructure. It may also be
rationalized as the manganese being partitioned out and thus decrease in the tensile strength of
the steel. The sample held for five long days however showed an increase in its strength from
300 MPa for three days of holding to 400 MPa for five days holding resulting in maximum
portioning of manganese where the maximum enrichment of austenite occurred giving strength
to the steel. However, the elongation doesn’t change much when compared to the three day held
sample and this might be due to the decarburization of the sample while held for such a long
period of intercritical annealing (Table III).
31
1000.
1310_1day
800.
1310F
1310_3day
600.
1310_5day
400.
200.
0.
0.
12.5
25.
37.5
50.
Figure 7: The tensile test and elongation for the samples annealed at 1310oF for the various
times.
Figure 8 shows the tensile stress versus strain for the sample intercritically annealed at 1370oF
for one, three and five days of holding. The sample held intercritically for one day shows a
greater tensile strength of about 875 MPa with a 25% elongation. The three day held sample as
expected show a decrease in tensile strength of about 410 MPa (210%) with an increased
elongation of about 36% from 25% for one day holding. This decrease in strength can be
attributed to manganese partitioning from the steel that can reduce the strength of the steel. At
the same time, increase in ferrite content increased the elongation of this steel.The five day
intercritically held sample however showed an increase tensile strength when compared to three
day sample. This can be rationalized as maximum enrichment of austenite that gives the steel the
strength properties by completely partitioning the manganese from the steel. The elongation
however decreased unexpectedly from 36% for three days to 20% for five days sample (Table
III). This may be due to following reasons; the sample have decarburized due to lack of proper
wrapping of the sample with the steel with titanium chips. A course of material is lost due to
32
decarburization and has reduced the elongation portion for this steel. This was evident on the
sample tested with a lot of scaling on the sample.
900.
1370_1day
675.
1370-_5day
1370_3day
450.
225.
0.
0.
10.
20.
30.
40.
Figure 8: The tensile test and elongation results for the samples annealed at 1370oF for the
various times.
Figure 9 shows the stress versus strain for both parent sample and in as quenched condition. The
parent sample showed strength of about 1000 MPa and very little elongation (about 7%). The as
quenched sample showed a decrease in strength when compared to parent counterparts. The
strength reduced by about 50% however, the elongation increased from 7% to 43%, a 600%
increase in elongation (Table III). This can be attributed to the microstructural phase change to
austenite and ferrite which would add the strength and elongation respectively in the intercritical
range.
33
Figure 9: The difference in the yield strength and ultimate tensile strength of the parent and the
sample quenched after one day of annealing at 1260°F.
Table III. Stress Strain values for the samples tested intercritically
Temp.
1260oF
1310oF
1370oF
Time
Stress (MPa)
Strain (%)
Parent
976
6.4
AsQuenched
521
42.70
1 day
478.81
43.30
3 days
341.90
49.02
5 days
407.35
59.39
1 day
763.79
26.70
3 days
340.21
56.89
5 days
410.23
42.78
1 day
826.66
23.57
3 days
352.20
36.67
5 days
423.52
23.57
34
MircroHardness: Table IV lists the average microhardness values obtained for three different
intercritical annealing temperatures at various holding times (1,3 and 5 days). A decreased
microhardness trend is observed for all the intercritical temperatures from one day to three days
holding. This may be attributed to the longer holding where the grain size increases and softens.
Other reason is more decarburization and lost in carbon content from the steel that could
sacrifice the hardness of the steel. However, for the five day intercritical holding, the hardness
increased when compared to the three days and this increase is not very high. The possible
reason could be that there is maximum enrichment of austenite for very long holding (5 days)
that might have improved the strength by a bit and have stabilized.
Table IV: Micro indentations for samples heated at 1260, 1310 and 1370 F for one day holding
No.
1 day
3 days
5 days
1260 °F
175
130
158
1310 °F
195
144
164
1370 °F
171
132
155
Microstructure Content: The evaluation of the microstructure was done on PaxCam 5 with the
Olympus GX51 microscope (Figures 10 through 12). The parent sample showed a microstructure
of ferrite and pearlite and small amounts of bainite. During cooling after intercritical annealing,
austenite transformation occurred; this created microstructures composed of ferrite and austenite
35
with minuscule amounts of martensite in all the samples of 1260oF, 1310oF, and 1370oF for
various holding times.
(a)
(b)
(c)
Figure 10. Micrographs of intercritically annealed steels at 1260oF for (a) one, (b) three and (c)
five days.
(a)
(b)
(c)
five days.
(a)
(b)
(c)
five days.
36
Table V shows the volume fraction calculations for these microstructural phases; since
martensite is in very negligible amounts, fractions of ferrite and austenite were studied. A trend
is seen from this table for all the intercritical temperatures (1260F, 1310F and 1370F). As the
holding increased from one day to three days, the ferrite content increased in all the intercritical
temperatures tested. Ferrite, a softer phase increased by a considerable amount that led to a
decreased hardness content and reduced strength properties to the steel. This may have happened
such that the managanese is in the partitionable stage and enrichment of austenite and increased
grain size of ferrite. For the five day held samples at different intercritical annealing
temperatures, the austenite content is seen to increase slightly and a slight decrease in ferrite
content is also seen. This can be assumed that the complete enrichment of austenite happened at
a longer holding times and stability is occurred as a result. The ferrite content however, did not
reduce by a significant amount and seems to give the steel the elongation coefficient for the third
generation AHSS steel.
Table V. Volume fraction studies for all the samples intercritically annealed
Time:
Phases
1260oF
1310oF
1370oF
1 day
Ferrite
25.61%
74.39%
34.74%
65.26%
33.89%
66.11%
21.68%
78.32%
29.49%
70.51%
28.44%
71.56%
7.40%
92.60%
24.53%
75.47%
23.89%
76.11%
3 days
5 days
Austenite
Ferrite
Austenite
Ferrite
Austenite
37
Conclusions
•
Austenite was successfully enriched by partitioning the manganese on these samples
(DP590R) using intercritical batch annealing process.
•
Microstructure showed an optimal austenite content for the longer holding times (5 days).
•
Duplex microstructure (a mixture of ferrite and austenite) was formed on all the samples
tested intercritically.
•
Mechanical testing revealed better strength and toughness properties for the samples with
longer hold times for all the temperatures chosen.
•
These results were correlated and are in agreement with their microstructural
counterparts.
•
The toughness values nonetheless fall in the region of 3rd generation AHSS classification,
however, the strength needs to be improved on these samples.
•
Alloying additions, proper heat treatment processing conditions should enrich the
maximum austenite content which is a crucial factor to increase the strength to these
steels.
38
References:
1. Ron O’Malley’s “Metallurgy Lectures on Steel Making Processes,”
2. http://www.uwplatt.edu/~mirth/me3040ch9.htm
3. http://academic.uprm.edu/pcaceres/Courses/MetalEng/MENG-6B.pdf
4. http://www.cartech.com/techarticles.aspx?id=1450
5. http://quizlet.com/20699470/new
6. http://www.ndted.org/EducationResources/CommunityCollege/Materials/Structure/metallic_structures.ht
m
7. http://www.tech.plym.ac.uk/sme/interactive_resources/tutorials/failureanalysis/Undercarr
iage_Leg/Steel_Metallurgy_Ohio-State.pdf
8. http://www2.bakersfieldcollege.edu/mrozell/documents/Engr%20B45/jominy.pdf
9. http://vacaero.com/Metallography-with-George-Vander-Voort/Metallography-with-GeorgeVander-Voort/martensite-and-retained-austenite.html
39
Technical Report
on
Quantification of Macro-Inclusion Distribution in Sheet Steel Samples Using
UT and Thermal Scanning Techniques
Submitted by:
Richard Ellis
Submitted to:
Dr. Heshmet Aglan
May 2014
1
ACKNOWLEDGEMENTS
This work was sponsored by the Nucor Corporation through the Tuskegee University NucorEducation and Research Center (NERC). The technical guidance and support of the Tuskegee
University Research Team is greatly acknowledged and appreciated. The invaluable advice,
encouragement, and generosity rendered by the Nucor Corporation Team are also greatly
appreciated.
Tuskegee University Research Team/ Nucor Corporation Team
Dr. Heshmat Aglan
Mr. Kaushal Rao
Dr. Ronald O’Malley
The Entire Nucor Corporation Team
2
TABLE OF CONTENTS
Abstract………………………………………………………………………………………… .3
Project Overview………...…………………………………………………………………… …4
Background and Literature………………………………………………………………………5
Experimental…………………………………………………………………………………… 26
Results ….………………………………………………………………………………………. 28
Conclusion ………………………...…………………………………………………………… 39
References………………………………………………………………………………………. 40
3
NERC 2013-14 Research Project
Quantification of Macro-Inclusion Distribution in Sheet Steel Samples Using UT and
Thermal Scanning Techniques
Student: Richard Ellis (Mech. Engr, Junior)
Nucor Mentor: Ron O’Malley – Nucor Decatur
[email protected]
Abstract
Oxides are present as inclusions in all steels as a direct consequence of the “killing” process that
is used in steelmaking to tie up dissolved oxygen. Normally, these inclusions are small (1-8
microns) and benign and have little influence of the properties of the steel. In some cases,
however, these inclusions can cluster to form large inclusions (>100 microns) or exogenous
contaminants, such as eroded refractory components, can become entrapped in the steel. These
large inclusions are normally rare, but can be extremely detrimental in many steel applications.
Detection of these large rare inclusions is difficult. This project explores some methods for
detecting these larger inclusions on larger steel samples using non-destructive testing methods.
4
Student’s Name: Richard Ellis
Mentors: Drs. H. Aglan (Tuskegee University) & Ron O’Malley (NSDEC)
Title: Quantification of Macro-Inclusion Distribution in Sheet Steel Samples Using UT and
Thermal Scanning Techniques
Objective: To investigate different types of inclusion distribution in the steel samples using nondestructive evaluation (NDE) techniques.
Background: Inclusions, which are inescapable components of all steels, play an important role
with respect to their effects on steel properties. Impact strength, ductility and fatigue strength are
most sensitive to the detrimental effects. These micro and macro inclusions are embedded in the
steel during the continuous castings of steels slabs. When this cast is set to rolling operations,
high risk of void formation between matrix and hard inclusions is possible. Bigger inclusions or
macro inclusions heavily deteriorate the properties of the steel including surface finish,
drawability, etc. Several factors such as number, location, size and geometry play a significant
importance in determining the properties of the final product. In this project, different inclusion
types including indigenous, and exogenous will be verified from the steel samples using NDE
techniques
Proposed Work and Tasks: Nucor Steel Decatur samples will be supplied for this project.
•
o Inclusions, their types and formation will be reviewed.
o Different plastic deformation operations and their influence on the inclusions will
be studied.
• Task 2: Sample preparation
o Steel samples will be cleaned for the non-destructive evaluation (NDE).
o Calibrations will be performed on NDE equipments (both thermal wave imaging
and ultrasound techniques)
• Task 3: Testing
o Steel samples will be tested for inclusions types and are categorized based on
their size.
o Samples will be marked at locations and are quantized.
• Task 4: SEM and XRD evaluation
o Scanning electron microscopy (SEM) and X-ray studies will be performed on the
locations cut and mounted from these locations.
o Results from all the studies will be reported.
5
Background and Literature
1.0 Steel and its Processes
1.1 Steel and Steel Making Processes
Steel is an alloy of iron and carbon mixed with small amounts of various other elements such as
silicon, phosphorus, sulfur and oxygen. The levels of carbon in a batch of steel determine its
chemical and physical properties. There are two main methods used to make steel: basic oxygen
furnaces (BOF) and electric arc furnaces (EAF). Basic oxygen furnaces (Figure 1) are used as a
high speed method of steelmaking. Oxygen of high purity is blown through an oxygen lance at
high velocities onto the surface of a bath that contains steel scrap and molten pig iron within a
vessel with a basic lining. The molten iron that is made from the blast furnace goes through a
desulphurization phase to reduce the sulfur content in the steel. Electric arc furnaces (Figure 2)
are high-temperature furnaces that use high-voltage electric arcs to make steel. The furnace is
loaded, the lid is lowered and clamped tight, and the electrodes are lowered into the scrap. When
power is fed to the furnace, the electricity jumps into the steel from the two energized electrodes
and travels through the steel to the neutral electrode connected to ground. The direct and radiant
heat from the electric arcs melts the steel scrap. At the conclusion of this process, iron and steel
scraps are recycled into new steel products. Steel industries are able to produce more specialized
steels by using EAF processes rather than BOF processes.
6
Figure 1 Basic Oxygen Furnace
Figure 2 Electric Arc Furnace
1.2 Chemical Composition in Carbon Steels
Carbon steels usually are iron with less than 1 percent carbon, plus small amounts of manganese,
phosphorus, sulfur, and silicon. Carbon steels can be subdivided into four different groups based
on the carbon content of the steels: low (mild), medium, high, and very high. Low or mildcarbon steels contain less than 0.30 percent carbon and are the most commonly used grades.
Medium-carbon steels contain from 0.30 percent to 0.45 percent carbon. High-carbon steels
contain from 0.45 percent to 0.75 percent carbon and are challenging to weld. Very high-carbon
steels contain up to 1.50 percent carbon and are used for hard steel products. The general trend
for carbon steels is that increased carbon means increased hardness and increased tensile strength
but decreased ductility and increased difficulty machining. In addition, plain carbon steels can
only be strengthened to a certain point before the toughness begins to decrease considerably.
1.3 Alloying Elements
Carbon is the most important constituent of steel. It raises tensile strength, hardness, and
resistance to wear and abrasion. It lowers ductility, toughness and machinability. Carbon
produces the properties in steel that give it strength. As carbon content increases there is a
7
corresponding increase in tensile strength and hardness. Additionally, as carbon content
increases, steel becomes increasingly responsive to heat treatment.
Chromium increases tensile strength, hardness, hardenability, toughness, resistance to wear and
abrasion, resistance to corrosion, and scaling at elevated temperatures. When used in large
quantities, it possesses a remarkable resistance to oxidation and corrosion. Used in conjunction
with other alloys, chromium is one of the popular alloying elements.
Cobalt increases strength and hardness and permits higher quenching temperatures and increases
the red hardness of high speed steel. Cobalt adds much life to a tool by its ability to maintain
hardness and cutting ability when it is heated to a dull red during a machine operation. It also
intensifies the individual effects of other major elements in more complex steels.
Copper In significant amounts is detrimental to hot-working steels. Copper negatively affects
forge welding, but does not seriously affect arc or oxyacetylene welding. Copper can be
detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when
present in amounts exceeding 0.20%. Weathering steels are sold having greater than 0.20%
copper.
Manganese is a deoxidizer and degasifier and reacts with sulfur to improve forgeability. It
increases tensile strength, hardness, hardenability and resistance to wear. It decreases tendency
toward scaling and distortion. It increases the rate of carbon-penetration in carburizing.
Manganese is next to carbon in its importance in steel making because of its ability to resist hot
shortness or the tendency to tear while being forged or rolled.
Molybdenum increases strength, hardness, hardenability, and toughness, as well as creep
resistance and strength at elevated temperatures. It improves machinability and resistance to
8
corrosion and it intensifies the effects of other alloying elements. In hot-work steels and high
speed steels, it increases red-hardness properties.
Nickel increases strength and hardness without sacrificing ductility and toughness. It also
increases resistance to corrosion and scaling at elevated temperatures when introduced in suitable
quantities in high-chromium (stainless) steels. Steels with nickel usually have more impact
resistance than steels where nickel is absent.
Phosphorus increases strength and hardness and improves machinability. However, it adds
marked brittleness or cold-shortness to steel. Due to this fact, phosphorus is seldom deliberately
added to steel. It is rather carried as a residual or incidental element.
Silicon is the most common deoxidizer and degasifier. It increases tensile and yield strength,
hardness, forgeability and magnetic permeability. In amounts up to 1% it has marked
strengthening and toughening effect. In higher amounts, it produces electrical resistance and
gives high magnetic properties.
Sulfur improves machinability in free-cutting steels, but without sufficient manganese it
produces brittleness at red heat. It decreases weldability, impact toughness and ductility. Sulfur
is usually found in all steels and is considered a residual element. Sulfur is considered the basic
element for free machining steels but is however detrimental to the hot forming properties.
Titanium is used as stabilizing elements in stainless steels. Each has a high affinity for carbon
and forms carbides, which are uniformly dispersed throughout the steel. Thus, localized
precipitation of carbides at grain boundaries is prevented. Titanium is also added to low carbon
sheets to make them more suitable for porcelain enameling.
Tungsten increases strength, wear resistance, hardness and toughness. Tungsten steels have
superior hot-working and greater cutting efficiency at elevated temperatures. It promotes red
9
hardness and hot strength in addition to producing dense grain and keen cutting edge. These
properties make tungsten steels very useful for hot working applications such as cutting tools
when the steel is hot enough to be low red in color.
Vanadium increases strength, hardness, wear resistance and resistance to shock impact. It
retards grain growth, permitting higher quenching temperatures. It also enhances the redhardness properties of high-speed metal cutting tools. Vanadium helps steels resist softening at
elevated temperatures and seems to resist shock better than steels without it.
1.4 Heat Treatment Techniques
Annealing is a part of the softening process. It is used variously to soften, relieve internal
stresses, improve machinability, and to develop particular mechanical and physical properties.
When annealing, the metal is heated, held at a specific temperature for a time, and then slowly
cooled. If the condition of the surface does not matter or cleaning takes place later in the form of
casting, then annealing can be done in air. If the surface finish does matter, then a protective
atmosphere is used. By heating between 160 to 325 degrees Fahrenheit above the transformation
temperature, the steel becomes easier to bend, cut and modify. If full annealing is being applied,
the steel must be cooled slowly to ensure pearlite is formed. If process annealing is being
applied, any cooling rate at a condition below the lower transformation temperature may be used
causing the formation of the same crystal structures and hardness. Samples are annealed to
enhance mechanical properties, broaden possible sample applications, eliminate the effects of
cold working, increase uniformity, and to relieve stress.
Normalizing is also a part of the softening process. It is used to soften and relieve internal stress
after cold work and to refine the grain size and metallurgical structure. It may also be used to
10
break up the dendrite cast structure of castings to improve the machinability and future heat
treatment response or to mitigate banding in rolled steel. Normalizing homogenizes the steel to
create a more uniform composition throughout the sample. The process begins with heating the
steel. Once heated, the steel undergoes a cycle that transforms the ferrite crystal structures to
austenite. This is followed by a form of cooling in still or slightly agitated air. The normalizing
process is similar to the annealing process but with a faster cooling rate.
Surface Hardening/Quenching is part of the hardening process. Hardening is the process by
which heat treatments are employed to harden an alloy. Quenching is done to produce
martensite by transforming austenite, which achieves hardness and allows the steel to cool
quickly. This entire hardening process takes place after the annealing and stress relief process.
The purpose of the hardening process is to create a weather resistant surface, and to aid in the
preservation of a tough interior that gives resistance to impact related breakage. Carburizing,
carbonitriding, and nitriding are three treatments of the hardening process. Carburizing is the
absorption and dispersion of carbon into solid alloys by heating the samples to extremely high
temperatures. Carbonitriding is a hardening method where the samples are heated to very high
temperatures in a gaseous environment allowing the collection of carbon and nitrogen in the
surface. Nitriding consists of holding the samples at a temperature below the lower
transformation level in an atmosphere full of nitrogen.
Tempering is also a part of the hardening process. After quenching the steel is hard, brittle, and
internally stressed. Before use, it is usually necessary to reduce these stresses and increase
toughness by tempering. There will also be a reduction in hardness and the selection of
tempering temperature dictates the final properties. As a rule of thumb, within the tempering
range for a particular steel, the higher the tempering temperature, the lower the final hardness but
11
the greater the toughness. Tempering is a process in which hardened steel is heated to a
temperature lower than its lower transformation temperature and cooled at a constant rate. There
exist two types of tempering: martempering and austempering. Martempering involves heating
samples to a specific temperature and then the cooling process is delayed to a level out the
temperature throughout the sample. The results of martempering include steel that has a
martensite microstructure tending to be very brittle. Austempering is when the steel is heated to
a temperature just below the needed temperature for the formation of pearlite and then quenched
in a container with a constant temperature. The result of austempering samples has increased
ductility, toughness, and hardness which lead to saving money and energy.
2.0 Microstructural Phases of Steel
Austenite Phase- Austenite was originally used to describe an iron-carbon alloy, in which the
iron was in the face-centered-cubic (gamma-iron) form (Figure 3). It is now a term used for all
iron alloys with a basis of gamma-iron. Austenite begins to form when the steel is heated above
its lower critical point. In order to fully austenize the steel, it must be heated to a temperature
higher than that of the higher critical temperature. The steel will have a face-centered cubic
crystal structure and will contain no more than 2.03 percent of carbon once this process is
completed. Austenite in iron-carbon alloys is generally only evident above 723°C, and below
1500°C, depending on carbon content. However, it can be retained to room temperature by alloy
additions such as nickel or manganese.
12
Figure 3 Micrograph of Austenite
Bainite Phase- The bainite process is a combination of ferrite and cementite in steel and is
formed when the steel is immediately cooled rapidly followed by gradual final cooling. The
bainite phase consists of two forms off bainite known as upper and lower bainite. Upper bainite
generally forms at temperatures between 550 and 400°C. There are several proposed formation
mechanisms, based on the carbon content and transformation temperature of the steel, resulting
in slightly different morphologies. Low carbon steels exhibit fine bainitic laths, nucleated by a
shear mechanism at the austenite grain boundaries. As the carbon content increases, the
cementite filaments become more continuous, and at high carbon contents, the bainitic ferrite
laths are finer with the cementite stringers more numerous and more continuous. The structure
can appear more like pearlite, and is termed 'feathery' bainite. Lower bainite generally forms at
temperatures between 400 and 250°C, although the precise changeover temperature between
upper and lower bainite depends on the carbon content of the steel. The transformation nucleates,
like upper bainite, by partial shear. The lower temperature of this transformation does not allow
the diffusion of carbon to occur so readily, so iron carbides are formed at approximately 50-60°
to the longitudinal axis of the main lath, contiguously with the bainitic ferrite. With low levels
of carbon, the carbide may precipitate as discrete particles, following the path of the
ferrite/austenite interface (Figure 4).
13
Figure 4 Micrograph of Bainite
Cementite Phase- The cementite phase is also referred to as the iron carbide phase. When
compared to the other microstructural phases of steel, this phase is very hard and consists of
approximately 6.7 percent of carbon. In order to produce a harder or even tougher sample, the
steel must be cooled faster after the heat treatment so that more of the thin layers of cementite are
produced (Figure 5).
Figure 5 Micrograph of Cementite
Ferrite Phase- Ferrite was a term originally used for iron-carbon alloys, in which the iron was in
the body-centered cubic (alpha- or delta-iron) morphology, but is now used for the constituent in
iron alloys, which contains iron in the alpha- or delta-iron form. Alpha ferrite forms by the slow
cooling of austenite, with the associated rejection of carbon by diffusion. This can begin within
a temperature range of 900°C to 723°C, and alpha-ferrite is evident to room temperature. Delta
ferrite is the high temperature form of iron, formed on cooling low carbon concentrations in ironcarbon alloys from the liquid state before transforming to austenite. In highly alloyed steels,
14
delta ferrite can be retained to room temperature. The existence of ferrite decreases the hardness,
tensile strength, as well as other mechanical properties (Figure 6).
Figure 6 Micrograph of Ferrite
Martensite Phase- Martensite is formed in steels when the cooling rate from austenite is
sufficiently fast. It is a very hard constituent, due to the carbon which is trapped in solid
solution. Unlike decomposition to ferrite and pearlite, the transformation to martensite does not
involve atom diffusion, but rather occurs by a sudden diffusionless shear process. The term is
not limited to steels, but can be applied to any constituent formed by a shear process which does
not involve atom diffusion or composition change. The martensite transformation normally
occurs in a temperature range that can be defined precisely for the given steel. The
transformation begins at a martensite start temperature and continues during further cooling until
the martensite finish temperature is reached (Figure 7).
Figure 7 Micrograph of Martensite
15
Pearlite Phase- Pearlite is a grain that has a structure that resembles that of a fingerprint
containing exactly .77 percent of carbon. Pearlite is usually formed during the slow cooling of
iron alloys, and can begin at a temperature of 1150°C to 723°C, depending on the composition of
the alloy. It is usually a lamellar or alternate plate combination of ferrite and cementite. It is
formed by eutectoid decomposition of austenite upon cooling by diffusion of C atoms. When
ferrite and cementite grow contiguously, C precipitating as Fe3C between laths of ferrite at the
advancing interface leaves parallel laths of Fe and Fe3C, which is pearlite. Due to its levels of
cementite and carbon, the hardness and ductility of a sample will be greatly increased (Figure 8).
Figure 8 Micrograph of Pearlite
3.0 Mechanical Properties in Carbon Steels
Hardness is defined as the resistance of a solid matter to permanent shape change when a force
is applied. The values that are ascribed to hardness are due to a complex combination of
deformation and elastic behavior. Hardness values are roughly proportional to the strength of a
metal and can give an idea of the wear properties of a material. There are various methods to
measure hardness including the methods of Vickers, Brinell, Rockwell, Rebound, Electronic
Rebound, Microhardness, and Scratch. Of these methods, Brinell and Rockwell are the most
common. Brinell is a method that uses large loads, up to 30,000 kg, on a rough polished surface
and gives impressions from 2 to 6 mm. Rockwell is a method that forces a pointed probe into the
16
surface and measures the increase in penetration when the load is increased from one level, often
a minor load, to the next, often a major load. The penetration is in tens of micrometers and if the
sample deforms or moves, significant errors may arise. All of the hardness methods mentioned
above deform the surface and if the surface is on uniform or there are variations in hardness
through the material or an indent is too close to an edge or other impression, then inaccuracies
occur. In order to reduce errors when hardness values are used to estimate ultimate strength,
make sure the material is not austenitic or cold worked.
Toughness is defined as the ability of a material to absorb energy and plastically deform without
fracturing. It can also be defined as the amount of energy per volume that a material can absorb
before rupturing. Toughness is a function of both strength and ductility and varies with
temperature. Materials such as steel can change from being tough to brittle as temperature is
decreased. Detailed toughness tests use specimens with starter cracks, and measure the energy
per unit area as the crack grows. Simple toughness tests use specimens of fixed size with a
machine notch and measure the energy needed to break that specimen.
Strength is defined as the ability to withstand an applied stress without failure. There are three
categories in which strength is discussed: ultimate tensile strength, yield strength, and elastic
limit. The ultimate tensile strength is the maximum stress that a material can withstand when
subjected to a force before fracturing. The ultimate tensile strength is useful for the purposes of
specifying a material and for quality control purposes. The yield strength is the point at which
material exceeds the elastic limit and cannot return to its original form. The yield strength is
obtained by an offset method is commonly used for engineering purposes since it avoids the
practical difficulties of measuring the elastic limit or proportional limit. The elastic limit is the
point on the stress- strain curve where the material becomes permanently deformed before
17
removing the load. Elastic limit is the greatest stress the material can withstand without any
measurable permanent strain remaining on the complete release of load. It is determined by
using tedious incremental loading-unloading test procedure.
Impact Strength is the ability of a material to withstand a high velocity impact. Impact strength
is measured by allowing a pendulum to strike a grooved machined tested piece and measuring
the energy absorbed in the break. The absorbed energy typically decreases at lower
temperatures. If the absorbed energy is greater than 27 joules, it is generally considered
satisfactory. The test is usually done at different temperatures to test the steel’s resistance to
elements. This allows for the metal energy consumption rates to be measured.
4.0 Phase Diagrams
A phase diagram is a graphical representation of the equilibrium relations among phases,
typically as a function of one or more intensive variables such as chemical composition,
temperature, pressure, and the activity of a chemical component. Phase diagrams are seen as a
guideline for improving existing applications. They typically show the relationship between the
various phases that appear in the system under the equilibrium conditions. Phase diagrams are
resourceful in four major areas: developing new alloys for specific applications, fabricating
alloys to create useful configurations, designing and controlling heat treatment procedures, and
solving problems that arise in specific alloys in their commercial applications an improving their
reliability.
4.1 The Iron – Iron Carbide Phase Diagram
18
Figure 9 Iron-Iron Carbide Phase Diagram
Steels, in their simplest form, are alloys of iron and carbon. This particular diagram displays
these alloys and the different phases the steel will go through as it relates to temperature. The
different phases that appear in the iron-iron carbide phase diagram include ferrite, austenite,
cementite, and iron carbide liquid solution (Figure 9). As the percentage of carbon increases,
different phases are shown in steel varied by temperature. For example, if a 0.8% carbon steel
sample is heated to approximately 700 degrees Celsius, austenite will form. Again when a steel
sample with 2.5% carbon composition is steady at about 420 degrees Celsius, it is a mixture of
austenite and cementite. However, when the sample is heated and stopped at 1150 degrees
Celsius, the cementite no longer is present but the mixture is austenite and liquid. It is so
important to understand the relationship between carbon and iron as experiments and data are
recorded. By understanding the relationships displayed in the diagrams, a standard fixed
temperature can be relied on for particular phases so that during the process the expected things
that will happen can be noted.
19
4.2 Continuous Cooling Transformation Diagrams (CCT)
Figure 10 Continuous Cooling Transformation Diagram
The Continuous Cooling Transformation diagram (Figure 10) measures the rate of
transformation as a function of time for a continuously decreasing temperature. Basically, a
sample is austenitized and then cooled at a predetermined rate and the degree of transformation
is measured. By increasing or decreasing the amount of carbon content, the CCT diagram will
be affected. For example, if there is an increase in carbon content, the curve of the CCT diagram
will be shifted to the right corresponding to an increase in hardenability and an increase in the
ease of forming martensite. Also, an increase in carbon content decreases the martensite start
temperature.
5.0 Non Destructive Evaluation Techniques (NDE)
Non destructive evaluation is a group of analysis techniques that are used in science and industry
to evaluate the properties of a material, component, or system without causing damage to the
object that is being inspected. The advantages of using non destructive evaluation techniques
includes that it does not permanently alter the article being inspected, it is one of the best suitable
20
methods in analyzing the effects of aging in mechanical or civil structures, and it is a highly
valuable-technique that can save both money and time in product evaluation, troubleshooting,
and research. There are different types of NDE methods that are used, which include ultrasonic,
magnetic particle, liquid penetration, and eddy current testing.
5.1 Ultrasonic Testing
In ultrasonic testing, acoustic waves are transmitted through the component or structure in order
to get an acoustic imaging of the item. The penetration of the sound waves creates these images.
The frequencies used for ultrasonic testing are many times higher than the limit of human
hearing usually in a range between 500 KHz to 20 MHz. Ultrasonic data can be collected and
displayed in a number of different formats. The most common of these formats are the A-scan,
B-scan, and the C-scan presentations. A-scan presentation displays the amount of received
ultrasonic energy as a function of time. The relative amount of received energy is plotted along
the vertical axis and the elapsed time is displayed along the horizontal axis. Relative
discontinuity size can be estimated by comparing the signal amplitude obtained from an
unknown reflector to that from a known reflector. The reflector depth can be determined by the
position of the signal on the horizontal sweep. B-scan presentation is a profile view of a test
specimen. The travel time of the sound energy is displayed along the vertical axis and the linear
position of the transducer is displayed along the horizontal axis. The B-scan is typically
produced by establishing a trigger gate on the A-scan that when the signal intensity is great
enough to trigger the gate, a point is produced on the B-scan. C-scan presentation provides a
plan-type view of the location and size of test specimen features. These presentations are
produced with an automated data acquisition system and provide an image of the features that
reflect and scatter sound within and on the surfaces of the test piece. C-scan uses high frequency
21
sound energy to conduct examinations and record measurements of wall thickness. The
advantages of using a C-scan presentation is that it provides a permanent record of the location
and thickness readings, only one surface need to accessible, often portable and highly automated
operation, and contains various views of the defects.
5.2 Magnetic Particle Testing
Magnetic particle testing is accomplished by inducing a magnetic field in a ferromagnetic
material and then dusting the surface with iron particles. The surface will produce magnetic
poles and distort the magnetic field in such a way that the iron particles are attracted and
concentrated making defects on the surface of the material visible. The types of discontinuities
that will be noted through magnetic particle testing include cracks, seams, laps, voids, or flaws.
5.3 Liquid Penetration Testing
Liquid penetration testing is most often used to detect cracks and voids open to the surface on
nonporous metallic materials. The sensitivity rating of penetrants allows them to be used on
casting and machined surfaces. If there are any mini cracks on the surface of the inspect
specimen, the liquid used in the liquid penetration testing will penetrate through the micro crack
exposing the surface to ultraviolet light showing up as luminescent.
5.4 Eddy Current Testing
Eddy current test uses electromagnetic induction to detect flaws in conductive materials. The
eddy current test consists of a circular coil which is placed on the test surface. The alternating
current in the coil generates changing magnetic field which interacts with the conductive test
surface and generates eddy current. The change in eddy current flow and a corresponding
22
change in the phase and amplitude are measured against known values. Eddy current testing
detects very small cracks in or near the surface of the material.
5.5 Thermal Wave Imaging
In thermal wave imaging, thermal imagers are used. Thermal imagers are instruments that create
pictures of heat rather than light. They measure radiated IR energy and convert the data to
corresponding maps of temperatures. Thermal wave imaging works by having a special lens that
focuses the infrared light emitted by all of the objects in view. The focused light is scanned by a
phased array of infrared-detector elements. The detector elements create a very detailed
temperature pattern called a thermogram. It only takes about one-thirtieth of a second for the
detector array to obtain the temperature information to make the thermogram. This information
is obtained from several thousand points in the field of view of the detector array. The
thermogram created by the detector elements is translated into electric impulses. The impulses
are sent to a signal-processing unit which is a circuit board with a dedicated chip that translates
the information from the elements into data for the display. The signal-processing unit sends the
information to the display, where it appears as various colors depending on the intensity of the
infrared emission. The combination of all the impulses from all of the elements creates the
image.
6.0 Inclusions
Inclusions in steel are any impurities present in the steel that are not incorporated into the
molecular structure of the alloy itself. They can be chemical compounds or bits of foreign
matter, usually nonmetallic in nature. Inclusions in steels are often measured in micrometers and
make up a tiny portion of the steel as a whole, usually around 0.03%. These inclusions are
23
classified as two types in steel: indigenous and exogenous inclusions. It is very important to
identify inclusions in steel because even a very small number of such impurities can significantly
affect the quality of the steel in various ways including reducing strength, flexibility, ability to
hold a weld, and resistance to corrosion.
6.1 Indigenous Inclusions
Indigenous inclusions, also known as endogenous inclusions, are compounds or impurities
formed within the steel making process. They are the result of the reaction of substances
dissolved in the molten steel. Indigenous inclusions are unavoidable to some extent because of
naturally occurring impurities in the various components of a steel alloy. The materials will
react with each other forming non-metallic compounds such as oxides and sulfides during the
manufacturing process. These indigenous inclusions are nothing more than deoxidation
products or precipitated inclusions during cooling and solidification of steel. Deoxidation
products will include alumina inclusions and silica inclusions. Alumina inclusions are dendrite
when formed in a high oxygen environment. Cluster-type alumina inclusions from deoxidation
or reoxidation are typical of aluminum killed steels. Alumina inclusions can easily form three
dimensional clusters through collision and aggregation due to their high interfacial energy.
These alumina inclusions take on various shapes ranging from the shape of a flower plate to the
shape of coral. Silica inclusions are generated by the reaction between the dissolved oxygen and
the added aluminum. Silica inclusions are generally spherical. These inclusions can also form
into clusters. Precipitated inclusions form during cooling and solidification of the steel. While
cooling takes place, the solubility of oxygen, nitrogen, and sulfur decreases while the
concentration of each of the elements in the liquid increases. When this takes place, inclusions
such as alumina and silica began to precipitate. Sulfides form interdendritically during
24
solidification and often nucleate on the oxides already present in the liquid steel. These
inclusions are normally small being less than or equal to ten micro meters. These precipitated
inclusions such as AIN inclusions can be in various shapes such as plate-like,feathery, and
branched rod-like shapes. The shapes of these inclusions are formed both during and after
solidification of the matrix.
6.2 Exogenous Inclusions
Exogenous inclusions are bits of foreign substances. These inclusions arise primarily from the
incidental chemical reoxidation and mechanical interaction of liquid steel with its surroundings.
They can be almost anything from bits of slag to pieces of equipment that may have flaked off
into steel during the casting process. In machining, these inclusions produce chatter, causing pits
and gouges on the surface of machine sections, frequent breakage, as well as excessive tool wear.
Exogenous inclusions usually have the following characteristics in common including the large
size, compound composition, irregular shape, sporadic distribution in the steel, not welldispersed as small inclusions, and are more deleterious to steel properties than small inclusions.
Exogenous inclusions can come from four different sources including reoxidation, slag
entrainment, erosion/corrosion of lining refractory, and chemical reactions. Exogenous
inclusions from reoxidation are the most common form of large macro-inclusions. Air is the
most common source of reoxidation. During the reoxidation involving air, elements like Al, Ca,
Si, etc, are oxidized and their products develop into non-metallic inclusions. Another
reoxidation source comes from silicon dioxide, iron oxide, and manganese oxide in the slag or
lining refractories. Inclusions in this type of reoxidation grow as they near the slag or lining
interface leading to large alumina inclusions with variable composition. This affects the
exogenous inclusions in two ways. One way the exogenous inclusion is affected is that the
25
reaction can erode and uneven the surface of the lining which changes he fluid flow pattern
lining walls and can induce further accelerated breakup of the lining. The other way would be
that the large exogenous inclusion can act as a heterogeneous nucleus for new precipitates.
Exogenous inclusions from slag entrainment produce particles suspended in the steel as slag
inclusions. Slag inclusions contain large amounts of calcium oxide and magnesium oxide and
are generally liquid at the temperature of molten steel. For continuous casting process of steel,
there are five factors that affect slag entrainment into the molten steel. The first factor is the
transfer operations from ladle to tundish and from tundish to mold. The next factor would be
vortexing at the top surface of molten steel. The third factor is emulsification and slag
entrainment at the top surface especially under gas stirring. The final two factors include
turbulence at the meniscus in the mold and slag properties such as interfacial tension and slag
viscosity. Exogenous inclusions from erosion/corrosion of lining refractory are also common
sources of large exogenous inclusions. These inclusions include well block sand, loose dirt,
broken refractory brickwork, and ceramic lining particles and are typically solid having a large
and irregular shape. Exogenous inclusions from chemical reactions produce oxides from
inclusion modification when calcium treatment is improperly performed.
26
EXPERIMENTAL
1.0 Materials and Equipment
1. Sheet steel samples from Nucor Decatur
2. Engraver, Caliper, Ruler, Markers, Sand paper, Acetone, Ultrasonic couplant, and spray
paint
3. Echo Therm for thermal wave imaging
4. Olympus Epoch 1000i for C-scan UT testing
5. Buehler SimpliMet 1000 for mounting and polishing
6. EcoMet 250 for grinding
7. Scanning Electron Microscope (SEM)
2.0 Experimental Process
2.1 Thermal Wave Imaging
1. Marked samples with engraver
2. Sanded samples with sandpaper
3. Cleaned samples with acetone
4. Spray-painted the samples
5. Let the samples dry overnight.
* Attempted to test the samples by using the Echo Therm for thermal wave imaging but the
equipment was not capable of detecting inclusions at the given resolution.
1. Marked the samples with engraver
2. Cleaned the samples with acetone
27
3. Precisely numbered and alphabetized a grid on the samples
4. Applied ultrasonic couplant to the grided samples
5. Used the Olympus Epoch 1000i to discover cracks or defects within the samples
6. Noted location of suggested cracks and defects
7. Polished and ground the samples using the EcoMet 250
8. Mounted the samples using the Buehler SimpliMet 1000
9. Inspected suggested cracks and defects using SEM
28
RESULTS AND DISSCUSSION
Before the samples were tested, the Olympus Epoch 1000i had to be calibrated to
determine the right frequency to receive the data. Once these things were completed, a standard
was set of what the graph of the calibrated Olympus Epoch 1000i should look like. Based off of
this graph, we were able to determine if cracks exist within the steel and if these cracks were
macro or micro inclusions. Below are some example pictures of how the graph would look if the
image is calibrated for no defect, a small defect or micro inclusion or for a big defect or macro
inclusion (Figure 11).
(a)
(b)
(c)
Figure 11. Ultrasound NDE test showing different peaks (a) backwall (b) no peak (c) small peak
indicative of defect
After being able to determine whether the graph showed no defect, small defect, or a
large defect, we were then able to evaluate each sample to find the locations of the defects within
the steel sample. In order to note the exact locations of the detected defects, grids that were
alphabetized and numbered were made on the samples. Below the grid that was made on each
sample can be seen (Figure 12).
29
(a)
(b)
(c)
Figure 12. Grid drawn on the samples for NDE analysis on sample 1-#3471-1 OP(3),2-#279-2
(3)and 3- #3471-1 OP(4) respectively.
A rough drawing of the sample with the grid was made. Once a defect was found within a
particular square of the grid, the location at which the defect was found was noted on the rough
drawing. This was done for every single square on the grid for every single sample. After
recording all of the defect locations, markings were made on the actual steel sample to note the
location of the defects. Below are the results of this process (Figures 13, 14 and 15).
30
1
2
3
4
5
A
B
7
*
*
8
9
1
0
1
1
1
2
1
3
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7
1
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9
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*
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2
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*
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3
1
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3
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3
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3
8
*
*
*
3
3
*
*
D
2
1
*
*
C
E
6
*
*
*
*
*
F
G
*
H
I
*
*
*
*
*
J
Figure 13. Figures showing the location of defects on the sample piece for sample 1 (#3471-1 OP
3
9
4
0
31
1
2
A
3
4
*
5
6
*
7
8
9
1
0
*
*
C
*
*
*
E
1
2
*
B
D
1
1
*
1
3
1
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5
*
*
*
*
1
6
1
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1
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9
2
0
*
*
2
1
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2
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*
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3
9
*
*
*
*
*
*
*
3
2
*
*
*
*
F
*
*
*
*
*
*
G
*
*
H
*
I
J
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
Figure 14. Figures showing the location of defects on the sample piece for the sample 2 (#279-2)
4
0
32
1
2
3
4
5
6
7
A
B
8
9
1
0
1
1
*
*
1
2
1
4
1
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6
*
1
7
1
8
1
9
2
0
2
1
*
2
2
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2
6
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9
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0
3
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*
*
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*
E
*
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G
*
3
7
3
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3
9
4
0
*
*
*
*
*
*
*
*
*
3
6
*
*
C
F
1
3
*
*
*
*
*
*
*
*
*
H
I
J
*
*
*
*
Figure 15. Figures showing the location of defects on the sample piece for the sample 3 (#3471-1
OP(4))
33
After noting all of the locations of the defects, it was important to quantify the data that
was just recorded. From quantifying the data, it was possible to detect the total number
inclusions, the number of macro inclusions, and the number of micro inclusions in each sample.
The data can easily be seen in the generated table below (Table 1).
Table 1. Quantification analysis on the samples for micro and micro analysis
# of macro
inclusions
# of micro
inclusions
Total
Sample 1 #3471-1 OP-(3)
1
21
22
Sample 2 #279-2 (3)
29
31
60
Sample 3 #3471-1 OP (4)
3
39
42
Total
33
91
124
Classification
*25% gain < micro inclusion < 40% gain
*macro inclusion ≥ 40% gain
After recording the data above, the next step was to find the defects. The previously recorded
data was used to calculate the depth at which the defect was located. The sample was first cut
into pieces where the Olympus Epoch 100i detected defects within the steel. Once the sample
was cut, the cut pieces of the sample was mounted and later ground off to the detected
dimensions that were calculated. Below are pictures of how the samples were first cut and then
mounted (Figure 16).
34
(a)
(b)
Figure 16. (a) Samples cut out from the plate for further testing (destructive analysis) and (b)
sample mounted on the bakelite
After mounting the cut pieces of the sample, the mounted piece was ground down to the depth
detected by the Olympus Epoch 1000i. Precaution was taken when grinding the sample because
if the sample is not ground enough the inclusion will not be discoverable and if the sample is
ground too much the inclusion is no longer present. Once the correct depth was reached,
although small, the inclusion was visible to the naked eye. The inclusion was analyzed using a
scanning electron microscope (SEM). Below are the pictures of the defect to the naked eye and
the pictures of the inclusion under the SEM (Figures 17 and 18).
Figure 17. Polished sample showing defect location on the sample
35
(a)
(b)
(c)
Figure 18. SEM images showing the polished area with the defects at different magnifications (a)
75X (b)400X and (c)2.5kX
To determine whether this inclusion was exogenous or indigenous, the sample was
analyzed by SEM/XRD. While doing this analysis, three spectra of the inclusion were taken.
Each spectrum of the inclusion contained different percentages of elements. The results of the
three spectra are shown below.
36
SEM/XRD Analysis Spectrum 1:
Element
Weight%
OK
Al K
Si K
SK
Ca K
Fe L
34.91
1.58
1.89
1.61
5.58
54.44
14.07
1.07
1.34
1.30
5.63
76.59
Totals 100.00
Atomic%
37
Element
Weight%
OK
Al K
Si K
Ca K
Fe L
Mo L
52.79
1.07
1.31
0.07
43.96
0.80
24.52
0.84
1.07
0.08
71.27
2.23
Totals 100.00
Atomic%
38
Element
Weight%
OK
Al K
Si K
Ca K
Fe L
22.45
0.03
3.00
0.49
74.04
7.81
0.02
1.83
0.42
89.92
Totals 100.00
Atomic%
39
CONCLUSION
In order to complete the non destructive evaluation technique using thermal wave
imaging, we must have equipment that can detect the inclusions at a higher resolution. From the
results that were obtained during the ultrasonic testing using the Olympus Epoch 1000i on the
sheet steel samples, these are the conclusions that were made. The inclusions that were found
may have been added to the steel during the casting operations. Also, these inclusions that were
found formed non metallic compounds, suggesting that the inclusions are indigenous.
40
REFERENCES
1. http://ccc.illinois.edu/pdf%20files/publications/03_mexico_nov_inclusion_review_v5a_u
pdated.pdf
2. http://dictionary.reference.com/browse/basic+oxygen+furnace
3. http://steamshed.com/annealing%20process.html
4. http://web.utk.edu/~prack/MSE%20300/FeC.pdf
5. http://www.academia.edu/1709192/Nondestructive_Testing_Techniques_in_Engineering
6. http://www.appliedprocess.com/process
7. http://www.aviationpros.com/article/10387910/thermal-imaging-an-ndt-technology-thatis-evolving-rapidly
8. http://www.azom.com/article.aspx?ArticleID=2598
9. http://www.azom.com/article.aspx?ArticleID=543
10. http://www.bladehq.com/cat--Steel-Types--332
11. http://www.bodycote.com/services/heat-treatment/case-hardening-with-subsequenthardening-operation/carbonitriding.aspx
12. http://www.bodycote.com/services/heat-treatment/harden-and-temper/martemperingmarquenching.aspx
13. http://www.chasealloys.co.uk/steel/alloying-elements-in-steel/
14. http://www.diehlsteel.com/technical-information/effects-of-common-alloying-elementsin-steel.html
15. http://www.ehow.com/info_12147204_electric-arc-furnace-work.html
16. http://www.engineeringtoolbox.com/ndt-non-destructive-testing-d_314.html
41
17. http://www.lmats.com.au/resource-centre/ndt-non-destructive-testing/ndt-eddy-currenttest-et.html
18. http://www.metlabheattreat.com/carburizing.html
19. http://www.ndted.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm
20. http://www.olympus-ims.com/en/knowledge/ultrasound/applications/ultrasonic-faq/
21. http://www.pacmet.com/index.php?h=basicheattreat
22. http://www.petersonsteel.com/wp-content/uploads/2011/03/Elements.pdf
23. http://www.smt.sandvik.com/en/products/strip-steel/strip-products/knife-steel/hardeningguide/the-hardening-procedure/
24. http://www.thefabricator.com/article/metalsmaterials/carbon-content-steel-classificationsand-alloy-steels
25. http://www.treatallmetals.com/nitrid.htm
26. http://www.ttsa.com.au/what-is-thermal-imaging/how-does-thermal-imaging-work.html
27. http://www.twi.co.uk/technical-knowledge/faqs/material-faqs/faq-what-are-themicrostructural-constituents-austenite-martensite-bainite-pearlite-and-ferrite/
28. http://www.wisegeek.com/what-are-inclusions-in-steel.htm
29. http://www.wlfuller.com/html/steel_types.html
30. http://www.worldsteel.org/faq/about-steel.html
31. http://www-materials.eng.cam.ac.uk/mpsite/properties/non-IE/toughness.html
Technical Report
on
Effects of Cooling Rates on Intercritically Partitioned Dual Phase and Armor
Steels Using a Cooling Simulator
Submitted by:
Xavier Bland
Junior, Mechanical Engineering
Submitted to:
Dr. Heshmat Aglan
May 2014
1
ACKNOWLEDGEMENTS
This work was sponsored by the Nucor Corporation through the Tuskegee University
Nucor- Education and Research Center (NERC). The technical guidance and support of
Tuskegee University Research Team is greatly acknowledged. The invaluable advice and
encouragement rendered by the Nucor Corporation team is also appreciated.
Tuskegee University Team:
Dr. Heshmat Aglan
Mr. Kaushal Rao
Dr. Ron O’Malley
Dr. Aldinton Allie
Dr. Ignatius Okafor
2
TABLE OF CONTENTS
TABLE OF CONTENTS
A. Overview.....................................................................................................................................4
B. Abstract.......................................................................................................................................6
1.0. Introduction and Literature Review..........................................................................................7
2.0. Background …………................................................................................................................22
3.0. Materials and Experimental....................................................................................................24
4.0. Results……………………………………………………………………………………………………………………………29
4.0. Conclusion..............................................................................................................................35
5.0 References................................................................................................................................36
3
Students’ Name: Xavier Bland
Mentors: Drs. H. Aglan (Tuskegee University) & Abhilash Dash/ Ron O’Malley/ Aldinton Allie
Title: Effects of Cooling Rate on Intercritically Partitioned Dual Phase and Armor Steels Using
a Cooling Simulator.
Objective: To investigate the effects of varying cooling rates on intercritically partitioned dual
phase steels and armor steels using a forced air cooling device to simulate the cooling rates for
the galvanizing line.
Background: Dual phase steels, a new class of high strength low alloy steel, are used in weight
saving applications in the automobile industry for greater fuel economy due to its formability.
On the other hand, armor plate steels are used in military applications, especially to defend
structures or vehicles from sniper fire or any weapon that emits high velocity projectiles. These
steels are produced either by continuous annealing/box annealing in intercritical range (DP
steels) or heat treatment (annealing at very high temperatures (armor steels), respectively. DP
steels produced in galvanizing lines employ gas cooling methods to achieve DP microstructure
(martensite + ferrite). On the other hand, for armor plate steels, traditionally employed heating
and quenching techniques improve the mechanical and ballistic performance. Additionally,
slower cooling rates can be employed in supplement to quenching processes and still achieve the
required hardness and impact properties achieved through bainitic and/or auto tempered
martensitic microstructures. This research emphasizes employing various cooling rates using a
cooling simulator and studies the microstructural and mechanical properties of the resulting
steels.
Proposed Work and Tasks: Nucor Steel Decatur armor plate steel samples will be supplied for
this project.
•
•
o Different phases associated with the steels and their microstructure.
o Galvanization processes and their techniques will be understood.
o Current trend of cooling systems in the galvanizing lines and the
advancements in the cooling systems will be reviewed.
Task 2: Sample preparation
o Steel samples with required dimensions each from different class (DP and
Armor) will be cut for laboratory heat treatment.
o Different cooling rates will be derived from the as designed cooling
chamber.
4
•
•
•
o Numerous cooling trials will be performed on dummy samples to verify
the cooling rates, sample thickness effects, etc.
Task 3: Cooling simulation
o Once the cooling system is calibrated, samples from both class (DP and
Armor) steels will be heat treated to their required heat treatable range
based on the austenization temperature ranges.
o Samples will then be cooled at different cooling rates.
o Numerous tests will be performed for consistency of cooling cycle.
Task 4: Microstructural Evaluation
o The microstructure of thus cooled samples at different cooling rates will
be identified and verified with their traditional counterparts.
Task 5: Reporting the Results
o The cooling rate effects on the microstructure of the different classes of
the DP and armor plate steels will be reported.
5
Effects of Cooling Rate on Intercritically Partioned Dual Phase and Armor Steels using a
Cooling Simulator
Xavier Bland, M.E. (Junior)
Abstract
Dual Phase (DP) steels are a class of advance high strength steels that derive their strength and
formability through the generation of a mixed microstructure of hard martensite and soft ferrite.
This microstructure is achieved by heating the steel into a region of 2-phase stability (austenite
and ferrite) and then rapidly cooling the steel to convert the austenite in the microstructure to a
hard martensite phase. At Nucor, DP steels are produced on their galvanizing lines where the
cooling can only be accomplished by gas cooling. As a result, Nucor needs to rely on expensive
alloying elements to ensure that martensite is formed at the slower cooling rates encountered in
the gas cooling systems. Otherwise, the austenite will transform back to ferrite on cooling and a
two phase microstructure is not achieved. This project explores the effects of varying cooling
rates on the microstructure and properties achieved in various DP steel chemistries using a forced
air cooling device to simulate the cooling rates observed in a commercial galvanizing line.
On the other hand, for the armor plate steels, this project builds on several previous projects on
armor plate to examine the effects of cooling rate on the hardness and impact toughness of
several steels used in armor plate manufacture. Traditionally, quench and temper processes are
used to balance hardness, toughness, ballistic impact performance of these steels. This project
explores the used of softer cooling to promote bainitic and/ or auto tempered martensitic
microstructures to explore the suitability of these process routings for armor plate production.
6
1.0 Introduction
1.1 Carbon Steels: Categories and Composition
When increasing the carbon of any steel, the hardness is also increased but the ductility is
reduced. Higher carbon content can mean a lower melting point, as well as a reduction in weld
ability. The amount of carbon each steel has can be classified into carbon ranges.
•
Low Carbon Steel:
Steels with 0.05% - 0.3% carbon content are considered low/mild carbon steels. These steels are
malleable and ductile, easily welded and low in cost, but are very low in strength. Some
products made with these steel are, but not limited to: Chains, pipes, wires, nails, and tin
machine parts.
•
Medium Carbon Steel:
Steels with 0.30% - 0.59% carbon content are considered medium carbon steels. This steel has a
balanced ductility, toughness and strength, and good wear resistance. A few uses for this steel
would be axles, crankshafts, heat treated machine parts, large parts, forging and automotive
components.
•
High Carbon Steel:
Steels with 0.6 % -0.99% carbon content are considered high carbon steels. High carbon steels
have high strength, wear resistance, hardness, moderate ductility, but rust easily. Applications of
these steels include screw drivers, hammers, wrenches, band saws, and rolling mills.
•
Ultra High Carbon Steel:
7
Steels with 1.0% - 2.0% carbon content are considered ultra-high carbon steels. These steels
have a great hardness and strength. Ultra-steel is used for special purposes like (non-industrial
purposes) Knives, axles or punches.
1.2 Alloying Elements
Elements in Steel
By definition, steel is a combination of iron and carbon. Various other elements are alloyed
with steel to improve physical properties, such as resistance to corrosion or toughness.
•
Manganese: Like nickel, manganese is added to steel to improve hot working properties
and increase strength, toughness, and hardenability. It increases hot shortness when
mixed with sulphur. It decreases tendency toward scaling and distortion.
•
Chromium: It is added to steel to increase resistance to oxidation. The resistance
increases as more chromium is added. Stainless steel has a high content of chromium and
has high resistance to corrosion.
•
Nickel: It is added in large amounts (over 8%) to high chromium stainless steel to form
the most important class of corrosion and heat resistant steels. In addition to improvint
resistance to oxidation and corrosion, nickel increases strength and hardness without
effecting ductility and toughness.
•
Molybdenum: When added to chromium- nickel austenitic steels, molybdenum raises
resistance to pitting corrosion by chloride and sulfur chemicals. When added to low alloy
steels, molybdenum improves high temperature strengths and hardness. Added to
chromium steels, it impressively reduces the tendency of steels to decay in service or in
heat treatment.
8
•
Titanium: The main use of titanium as an alloying element in steel is to minimize the
occurrence of inter-granular corrosion.
•
Phosphorus: It is usually added with sulfur to improve machinability of low alloy steels.
Added in small amounts phosphorus aids strength and corrosion resistance. Also,
phosphorus is known to increase the tendency to cracking during welding. It adds marked
brittleness or cold-shortness to steel.
•
Sulfur: It improves machinability but does not cause hot shortness. Without manganese
it produces brittleness at red hot. It decreases weld ability, impact toughness and ductility.
•
Selenium: It improves machinability.
•
Niobium (Columbium): It is added to stabilize carbon. Niobium also has the effect of
strengthening steels and alloys for high temperature service.
•
Nitrogen: When added to austenitic stainless steels, the yield strength is greatly
improved.
•
Silicon: used as a deoxidizing agent in melting steel, added in small amounts. When
added, silicon contributes to hardening of steels in the ferritic phase, and is harder and
stiffer.
•
Cobalt: It increases strength and hardness but permits higher quenching temperatures. Iy
also increases the red hardness of speed steel.
•
Tantalum: It is used in stabilizing stainless steel’s elements.
•
Copper: Presented in stainless steels as a residual element, it is added to a few alloys to
produce precipitation hardening properties. Copper can be detrimental to surface quality
and can have serious negative effects with forge welding, but does not affect arc or
oxyacetylene welding.
9
•
Tungsten: It increases strength, wear resistance, hardness and toughness. Tungsten steels
have excellent hot working and greater efficiency at elevated temperatures.
•
Vanadium: It increases strength, hardness, wear-resistance and resistance to shock
impact. It holds back grain growth, permitting higher quenching temperatures.
1.3 Heat Treatment Techniques
The purpose of heat treating steel is to bring about a desired change in the mechanical
properties of a metal, usually hardness, yield strength, and impact resistance. There are five basic
heat treating processes: Hardening, case hardening, annealing, normalizing, and tempering. Each
of these processes brings out various results in a metal, but all them use three basic steps:
Heating, soaking, and cooling. Heating is the first step that is required in heat-treating process.
Slow heating is primarily important in the heating cycle, for if one section of steel is heated
faster than the other, it may result in distortion or cracking. The rate at which the metal may be
heated depends on several factors, for example, the heat conductivity of the steel. Steel that
conducts heat freely may be heated at a faster rate than one in which heat is not absorbed as
rapidly throughout. Once the steel is heated to the proper temperature, it must remain at that
temperature until the entire part has been evenly heated throughout and the change has time to
take place. Holding the steel at this temperature is called soaking. The more mass the steel has,
the longer it must soak. The third step, after the steel has been properly soaked, is to cool it. All
changes that happen during this whole process are predictable; for that reason many metals can
be made to reproduce to specifics in order to increase their hardness, toughness, ductility, tensile
strength and so forth.
10
•
Hardening:
Ferrous metals are usually hardened by heating the metal to a set temperature until its
carbon is dissolved, then cooling it rapidly in a quenching solution such as oil, water, or
brine. When doing this process, the strength and hardness are increased, but also the
metal is made more brittle.
•
Annealing:
Metals are annealed by heating it to set temperature for a required time, then cooling it
back to room temperature. Annealing is used it reduce residual stresses, improve
toughness, induce softness, alter ductility, and/or refine the grain structure.
•
Normalizing:
This is similar to the annealing process but carried out to avoid extreme softness in the
material. Normalized metals are stronger than annealed steels and much tougher in this
process than any other condition. This is achieved by heating the metals to a temperature
above the set temperature and cooling in still air at room temperature.
•
Tempering:
Tempering is a process applied to steel to relieve the strains made during the hardening
process. This involves the heating the hardened steel to temperature lower than the
hardening temperatures, holding it at the set temperature for an acceptable period, and
then cooling to room temperature.
•
Case Hardening: Case hardening is a model heat treatment for steel that requires a tough
core and wear-resistant surface. During the process, the chemical composition of the
surface layer is altered during the treatment by adding carbon, nitrogen or both. For this
process the steel (either straight carbon steel or low-carbon steel) are heated to a set
11
temperature with the presence of a material (solid, liquid or gas), which then decomposes
and deposits more carbon into the surface of a steel. Then, cooled rapidly, the outer
surface becomes hard, leaving the inside to be soft but tough. The objective in case
hardening is to produce a hard case over a tough core.
1.4 Mechanical Properties
•
Strength: This is the ability of a material to resist external loading. For metals the
common measure of strength is the yield strength, which is the minimum stress that
produces permanent plastic deformation. Strength can be measured in tensile,
compressive, shear or torsional.
•
Elasticity: This is the highest stress at which all deformation strains are fully
recoverable. Elasticity is a tensile property of steel.
•
Hardness: This is the resistance to deformation in the form of plastic deformation which
includes penetration, indentation, scratching, cutting, and bending. Hardness is used
because it is a quick and nondestructive test when done in low stress areas of the metals.
There are many methods used to determine the hardness of a material.
•
Ductility: This is a measure of how much a material deforms plastically before fracture.
•
Toughness: This is the ability of a metal to deform plastically and absorb energy before
fracture. A material’s toughness depends on both ductility and strength. One way to
measure toughness is to obtain the area under the stress strain curve. The value obtained
has units of energy per volume. A metal may have high toughness to withstand a static
load but would fail under dynamic or impact loads. Ductility and toughness decrease with
increasing rate of loading. As temperature is decreased the ductility and toughness also
decrease.
12
•
Impact toughness: This is determined from Charpy and Izod tests; the tests use different
specimens and methods of holding, but use the same pendulum-testing machine. For both
tests the material is broken by a single impact event. A stop pointer is used to give a
reading of the distance the pendulum swings back up after the breaking. The impact
toughness is determined by measuring the energy absorbed. Impact toughness is greatly
affected by the temperature. Therefore these tests are usually repeated numerous times at
different temperatures.
•
Hardenability: This is the ability of an alloy to be hardened by forming from a heat
treatment method. Hardenability is a measure of the rate that hardness reduces with
distance into the interior of steel. High hardenability means that the steel will harden
throughout the surface and interior. The alloy composition of steel affects the
hardenability of a material.
•
Brinell hardness test: This is the oldest of the hardness test methods. It uses a desktop
machine that applies a specific load to a sphere of known diameter. The hardness number
is found by dividing the load (kg) by the measured surface area of the indentation left on
the test surface. Many materials can be tested by this method by simply changing the load
and the indenter ball size. Brinell tests are frequently used to find the hardness of forgings
and castings that have a coarse grain structure and cannot be read by the Rockwell or
Vickers test. For metals, the Brinell hardness number ranges from BHN 50 to BHN 750.
•
Jominy test: This procedure is used to determine the hardenability of steel. Everything
in each test is kept constant except alloy composition. The test begins by heating a
cylindrical specimen at austenizing temperature until the austenite phase has formed.
Then, the sample is removed from the furnace and the bottom of the steel is quenched
13
using a jet of water with constant flow rate and temperature. The cooling rate of the steel
is at a maximum at the bottom and decreases as the distance from the quenched end
increases. After being cooled to room temperature, the steel is ground flat and hardness
values are taken every 1/16 of an inch along the ground flat. The hardness values would
show that the quenched end has the maximum hardness. However, since the cooling rate
decreases with distance from the bottom, the hardness will also decrease with the distance
from the bottom. The hardenability is then determined by the depth of hardening. An
alloy with high hardenability will retain large hardness values for large depths in the
material.
1.5 Different Phases of Carbon Steel
•
Ferrite: It has a body center cubic crystal structure and dissolves small amounts of
carbon. At room temperature this is the most stable form of iron. Ferrite is a solid
solution and is capable of containing up to 0.008 percent of carbon at 70 degrees
Fahrenheit.
•
Pearlite: This is a combination of ferrite and cementite. It contains around 88% ferrite
and 12% cementite. Pearlite grain structures resemble human fingerprints. Steel with
exactly 0.77 percent carbon consists of uniform pearlite at room temperature. This is
what austenite transforms into when cooled down slowly. Pearlite consists of two phases:
iron and iron carbide. It is also known to make steels more ductile.
•
Cementite (Iron Carbide): This is a chemical compound of iron and carbon. By weight,
it is 6.67% carbon and 93.3% iron. It is meta stable and forms before graphite, given the
right conditions. It can combine with ferrite to form pearlite. It is essentially a ceramic in
its purest form.
14
•
Bainite: This is a combination of ferrite and cementite in ferrous metals that is harder
than pearlite. Bainite contains needlelike grain structures, and it requires an initial rapid
cooling followed by gradual cooling. Once transformed, it cannot be changed back
without reheating to austenite.
•
Austenite (gamma iron): This is the phase at which solid steel recrystallizes and has a
face-centered cubic crystal structure. Austenite steel holds a greater amount of dissolved
carbon and exhibits increased formability. When austenite is cooled it can be become
supersaturated and it undergoes phase transformations as it seeks equilibrium. Iron atoms
are located in the lattice and the carbons of atoms are located in interstitial positions.
•
Martensite: This phase of steel consists of a distorted, body-centered crystal structure.
Martensite is very hard and brittle and needle like structure. Martensite is a
supersaturated solution of carbon in iron. Due to the high lattice distortion, martensite has
high residual stresses. The high lattice distortion induces high hardness and strength to
the steel. However, ductility is lost (martensite is too brittle) and a post heat treatment is
necessary.
•
Lath Martensite: this forms when a low carbon content is present and in the austenite
phase. This phase has high toughness and ductility but low strength. Lath martensite has
grains called laths, which have smaller packer sizes, resulting in more impact energy
concentration.
•
Plate Martensite: This phase orms when there is high carbon present in the austenite
phase. It has much higher strength than lath martensite but is brittle and not ductile.
15
•
Figure 1: Different phases of steel are displayed with carbon content vs. temperature.
Simply showing what can happen to the internal structure of the steel.
1.6 Atomic Structure of Steel:
•
Hexagonal Close Packed Structure: (HCP) has three layers of atoms; on the top and
bottom layer are six atoms arranged in the shape of a hexagon while the other atoms are
located in the center of the hexagon. The middle layer has a triangle arrangement with
three atoms sitting on the edges of the top and bottom layers. There are twelve atoms and
some elements that have this structure include beryllium, cadmium, magnesium, titanium,
zinc, and zirconium.
16
•
Face Centered Cubic Structure: (FCC) is the crystal structure that contains one atom
in the center of the six sides of a cube and one atom in each corner of the cube. Austenite
has an FCC crystal structure.
•
Body Centered Cubic Structure: (BCC) is the crystal structure that contains an atom in
the center and one atom in each corner of a cube. Ferrite has a BCC crystal structure.
(A)
(B)
Figure 2: (A) displays a body centered cubic structure. (B) displays a face centered cubic
structure.
1.7 Cooling Processes
•
Hot Dipped Galvanization:
Surface preparation is the most important step in the galvanization process.
Inadequate surface preparation can result in coating fails with zinc not reacting
with an unclean steel surface. This will be apparent when the steel is withdrawn
from the zinc bath. Preparation for the galvanizing process consists of three steps:
17
First the steel is freed of any weld slag by a blasting process. Then, a hot alkali
solution, mild acidic bath, or biological cleaning bath removes contaminants such
as dirt, paint markings, grease, and oil from the surface. After dipped, the steel is
rinsed in a bath of fresh water. This step is called the Degreasing/ Caustic
Cleaning.
Next, the steel is dipped again in a dilute solution of heated sulfuric acid or
ambient hydrochloric, which removes rust from the steel surface. The steel is then
rinsed again to prevent cross contamination. This step is called Pickling.
Figure 3: Shown above is the Hot Dipped Galvanization line process.
The final preparation step for the galvanizing process is Fluxing. This process
serves two purposes; it removes any remaining oxides and deposits a protective
layer on the steel to prevent any further oxides from forming prior to immersion
in the molten zinc.
18
After the surface preparation the steel is then placed in a zinc bath for a period
that is determined by the thickness. The zinc is maintained at a temperature of at
least 815 -850 F. Then, the sample is finally removed and allowed to be cooled by
either being quenched in water or air cooled.
•
Sherardizing Process:
Unlike the Hot-Dip process there is no pretreatment process required for
diffusion. Any contamination or oxidation products such as mill scale are
removed by shot blasting. The products are heated in batches together with zinc
powder in closed rotating drums. The diffusion process occurs at temperatures
between 320ºC and 420ºC, during the vapor phase, and the zinc-iron alloy layers
then form in and on the surface.
•
Thermal Spraying of Zinc:
In this process, grit blasting is used to clean and prepare the surface for the hot
spray process. Also, the grits process creates a surface that has multiple 3
dimensional surfaces for zinc to react and adhere to. This eliminates the liquid
process used in the hot dip process. Finally, the zinc is melted and atomized and
sprayed onto the surface to be coated to a desired thickness.
•
Electroplating Galvanization:
Opposite of the hot-dip process, electroplating is applied in a cold electrolytic
bath rather than a molten zinc bath. Since the plating/coating is thinner than that
obtained from hot dipping, it is not suitable for extended outdoor exposure.
During this process, electric current is used to reduce cations of desired material
from a solution and coat a conductive object with a thin layer of metal. The steel
19
is immersed in an aqueous bath and electricity is used to move electrons from the
anode to the cathode, which induces the zinc anodes to be "oxidized" and dissolve
as zinc ions in the aqueous solution, be transported as ions through the solution,
and be "reduced" as metal onto the work.
1.8 Advance Steels
Advance Steels:
•
Dual Phase Steel: Dual phase steel (DPS) is a high-strength steel that is a combination of
ferrite and martensitic microstructure that enables high work hardening, elongation, and
energy absorption and is used mainly in a car. Applications for DP steel are rails, pillars,
exposed body panels, beams, cross members, fasteners and wheels. Dual phase steel
grade DP980 was used in the current testing. The samples were machined by Nucor Steel
and have 0.145 C, 2.27 Mn, 0.012 P, 0.002 S, 0.212 Si, 0.174 Cu, 0.219 Ni, 0.07 Cr,
0.156 Mo content.
•
Complex Phase (CP): These are steels with very high ultimate tensile strength of 800
MPa or greater. CP steel microstructure contains small amounts of martensite, retained
austenite and pearlite within the ferrite/bainite matrix. CP steels have high energy
absorption and high residual deformation capacity.
•
Armor Plate Steel: Applications of armor plates are used widely by the government.
Some applications include hummers, armored vehicles, ships, barges, tanks and body
armor. Also, these plates are used for domestic purposes, including armored truck service
vehicles and SWAT vehicles. The purpose of these plates is to defend from fired
projectiles and sniper rifles. Additionally, the surface should resist indentation,
20
scratching, abrasion, and cutting. High hardness of this material prevents penetration
from projectiles. However, high hardness with no tempering is likely to result in a brittle
material that ruptures upon impact. Increasing impact energy of the material will allow
the material to absorb more energy. A combination of both high hardness and impact
energy will cause the material to not be penetrated and absorb the energy in deformation.
The military produces detailed specifications that contain the procedures and
requirements that must be met in order for a material to be considered for its use. The
specification depends on the thickness of the test specimen, the level of hardness,
whether the metal is homogenous or not, and whether the metal is a wrought metal. Some
specification steels that the military use are MIL-A- 46100 D, MIL-A-32332, and MILA-12560.
1.
MIL-A- 46100 D: The specification for armor plate steel #46100D covers
quenched and high-hardness wrought armor plate steel for lightweight armor
applications for recommended thickness up to 2 inches.
Typical Brinell Readings: ranges from HB 477 to HB 534
Thickness: 1/8 of an inch to 2
2. MIL-A-12560: The specification for armor plate steel #12560 has been
approved by the MTL (Material Technology Lab) Department of the Army to
use in combat-vehicles and ammunition testing. Usually this grade of armor is
used to protect soldiers from land mines or explosive structures.
Typical Brinell Readings: ranges from HB 377 to HB 41522
21
Thickness: 3/16 of an inch to 3 inches [combat vehicles], ¼ on an inch to
12 inches [ammunition].
2.0 Background and Literature
2.1 Previous Research/ Experiments
One of the most important components of this project is to find effects of simulating the
cooling process of dual phase steel (DP980) and armor steel (GA0119). With these steels the
annealing process and cooling rate may have different effects on each material. Many
experiments were conducted before with different composition dual phase and armor steels other
than the Nucor grade steels that were provided; however, these still provided insight on the
different effects of the annealing and cooling process of dual phase and armor steels. The
research collected is from Science Direct
Experiment 1: The effect of intercritical heat treatment temperature on the tensile
properties and work hardening behavior of ferrite–martensite dual phase steel
sheets.
This research was conducted by P.Movahed, S. Kolahgar, S.P.H. Marashi, M.
Pouranvari, and N. Parvin to investigate the tensile properties and work hardening
behavior of dual phase (DP) steels. The steel used was a 2-mm in thickness SAE 1010
sheet steel with the chemical composition of 0.11 C, 0.53 Mn, 0.07 Si, 0.03 Ni, 0.03 Cr,
0.02 S, 0.02 P. The heat treatment temperatures were calculated to be 736 degrees and
22
852 degrees C, which all specimens were heated and held for 20 minutes in muffle
furnace and followed by water quenching. It was found that martensite volume fraction
increased by increasing the intercritical heat treatment temperature, which in turn
decreases the carbon content of this phase. This experiment is relevant to the current
experiment because it shows how dual phase steel reacts at different heat treatment
temperatures.
Experiment 2: Effect of heat treatment on mechanical and ballistic properties of
high strength armor steel.
This experiment was conducted by the Defense Metallurgical Research Laboratory in
Kanchanbagh, Hyderabad, India. UHS amour steel was used in this experiment where it
was austenatised at 910 degrees C followed by tempering at 200, 300, 400, 500 and 600
degrees C. Many results were concluded during this experiment; for example for the
microstructural observation a tempered martensitic structure was noted and for the
mechanical properties many results were accomplished at various temperatures.They
showed that at 200 degrees C tempering the best ballistic performance was observed.
23
3.0 Materials and Experimental
2.1 Experimental Procedures/ Materials
After the process of cleaning and cutting the steel (DP980 and GA0119), the SimpliMet
1000 Automatic Mounting Press was used to mount the structures for microstructure
viewing. After the samples were mounted the Buehler AutoMet 250 was used to grind
and polish the samples for clear microstructure pictures under the microscope. The PaxIt
computer program, which is connected to a microscope, was used to take pictures of the
samples at different magnifications.
(1)
(2)
(3)
(4)
(5)
(6)
Furnace
Cooling Simulator
SimpliMet 1000 Automatic Mounting Press.
Buehler AutoMet 250 Polisher and Grinder
Microscope/PaxIt Computer Program
CLC – 200R Hardness Test
Materials:
Two different types of steel (DP980 and GA0119) were used in the current study. The samples
were supplied from Nucor Steel, Decatur in plate form. The chemical compositions for these
steels are shown in the table below.
Table 1. Chemical composition of the steel grades used
Element
GA0119
DP980
C
0.30
0.145
Mn
0.60
2.27
P
0.007
0.012
S
0.0013
0.002
24
Si
0.28
0.212
Cu
0.10
0.174
Ni
0.48
0.291
Cr
0.94
0.07
Mo
0.36
0.156
Sn
0.005
0.004
V
0.004
0.0020
Nb
0.003
0.0030
B
0.0002
0.000
Al
0.035
0.019
Ti
0.002
0.004
Ca
0.002
0.000
N
0.008
0.01
Using SimpliMet 1000:
1. Turn on main water valve. (Twist 2 revolutions to the left)
2. Turn on water line slightly. (Behind the machine)
3. Turn on switch behind the machine.
4. Push white “on” button on the front panel.
5. Use Frekote to clean the inside of the machine after the top sampler holder comes out.
•
Clean the sampler holder and top fastener.
6. Let the machine go all the way down and swab with the Frekote.
7. Pour 1 ½ scoops of Bekalite into the machine on top of 1st sample facing one of the
longitude grains.
8. Put second sample holder on top and place the second sample in.
9. Pour 1 ½ more scoops of Bekalite into the machine.
•
Not exceeding the brim.
25
10. Position the fastener plates and lift the handle to hold it straight, tighten it.
11. Move the fastner over the hole and push down, twist and lock.
12. Push “cycle start” to start.
How to Clean SimpliMet 1000:
1. After removing samples, push “up” button till fully exposed.
2. Remove bottom plate and remove all black residues with damp paper towel.
3. Use Frekote to clean it. (Repeat steps 5 and 6) after putting the bottom sample holder
back in.
Using AutoMet 250:
1. Turn on main water valve. (Twist 2 revolutions to the left)
2. Turn on water line slightly. (Behind the machine)
3. Turn on switch behind the machine.
4. Push blue “on” button on the front panel.
5. Set time, base speed, head speed, and water release for either Sand Paper or Polish Pad.
•
Sand Paper: Time = 10 minutes, Head Speed = 30, Base Speed = 210, turn
water “on” on the front panel.
•
Polish Pad: Time = 10 minutes, Head Speed = 30, Base Speed = 130, turn water
“off” on the front panel.
6. Place samples in sample holder and secure tightly.
7. If using:
•
Grade Sand Paper: 120, 180, 240, 320, 400, 600.
•
Polish Pad: Ultra Pad, Trident, Micro Cloth.
Peel back the adhesive sheet and apply each sheet one at a time on the opposite side of a
magnetic plate.
8. Place magnetic side of plate on base.
26
9. Place sample holder in head.
10. Hold two green buttons on head at the same time until samples make contact with base.
11. After time duration remove samples from the head and retighten samples in holder.
12. Repeat steps 7 through 11.
•
For polish pads you must use suspension fluids that should sprayed on base every
2-3 minutes.
•
Ultra Pad (Red suspension fluid)
•
Trident (Green suspension fluid)
•
Micro Clothe (White suspension fluid)
Using CLC-200R Hardness Test:
1. Turn on switch behind machine.
2. Change hardness reading values to HBW.
3. Center sample on base.
4. Raise base from about ¾ inch away from tester.
5. Press green start button on the front of machine.
6. Reposition sample so that hardness value can be taking from the center and four corners
of sample.
Intercritical Annealing Procedures:
1. Samples were cut to specific size. GA119 has the dimensions of 1 x 5 inches with a
thickness of 2.51 inches (6.38 mm). DP980 has the dimensions of 1x5 inches with a
thickness of 1.59 inches (4.02 mm).
2. Attach thermocouple wire to sample using a tack welder. Once attached press record
on the data logger.
3. Samples were then places in the furnace at the set intercritical temperatures and held
for 5 minutes. (GA119: 1360 C/ 743 F ) (DP980:1270 C/ 682F )
4. Once placed in the furnace the timer would start.
5. Set cooling simulator to designated speed. (1-8)
27
6. After 5 minute hold the sample must then be moved to cooling simulator in a quickly
matter (5 sec), while simulator is on.
7. Once the sample has cooled to room temperature in the cooling simulator, remove
sample and analyze data.
8. Samples were then cut longitudinal and transverse directions for microstructural and
hardness evaluation.
9. Repeat steps for speeds 1-8 on GA119 and DP980.
28
4.0 Results
The Nucor grade GA119 falls in the characteristics as a medium carbon steel; therefore it can
be expected to pass the guidelines from MIL-A-46100D . The range for hardness values is
from HBW 477 to HBW 534. Due to the slow cooling, the GA119 was not able to meet
military specs for armor plate steels. The hardness was measured in Victor hardness and
convert to HBW (Table 2).
Table 2. GA119 Microhardness Values.
GA119
Phase 1
(Ferrite)
SPEED
Phase 2
(Bainite)
HBW value Ph. HBW value
1
Ph. 2
Average
HBW
1
256
320
244
304
274
5
276
357
262
338
300
8
306
492
290
464
377
Ph.1
Ph.2
Figure 4: Different test locations of microhardness.
29
Table 3. Microhardness Values for DP980
DP980
Phase 1
(Ferrite)
Speed
Phase 2
(Martensite)
HBW value Ph. 1
HBW value Ph. 2
1
251
365
239
346
5
271
392
259
371
8
349
479
331
452
The Nucor grade DP980 actually does achieve the full transformation in the microstructure after
intercritically annealing and cooling from speeds 1-8, with speed 8 completely transforming and
finer grains. The hardness values do not have an effect on the results for DP980 (Table 3).
Phase 1
Phase 2
Figure 5: Different test locations of microhardness.
30
Cooling Rates
The data collected from GA119 and DP980 were transferred from the data logger to the
computer and converted into cooling rates (C/s) and velocity (m/s). The DP980 cooled faster
than the GA119 thru all 8 speeds due to the different thickness in the two steels. The cooling
rate played a major role in the different microstructures and hardness of the steels. The higher the
cooling rates the better the results.
Table 4. Cooling Rates of DP980 and GA119.
Blower Speed
8
7
6
5
4
3
2
1
GA119
Velocity (m/s)
12.7
10.8
9.8
7.7
6.3
4.2
3.5
1.9
Cooling Rate
1.014
0.903
0.849
0.792
0.730
0.678
0.521
0.404
DP980
Velocity (m/s)
12.9
11.3
10.4
8.2
6.4
4.6
3.8
2.5
Cooling Rate
1.484
1.265
1.246
1.009
0.929
0.695
0.575
0.489
Fan Speed vs Velocity
This test was conducted before the cooling rates of the DP980 (figure 6) and GA119 (figure 7) to
calibrate the cooling chamber speeds. The speeds from test 1 and test 2 correlate with each other
in velocity. The max velocity was 14.9 m/s at speed 8 and the minimum 1.4 m/s at speed 1.
31
10
9
8
7
Fan Speed
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
Velocity (m/s)
10
11
12
13
14
15
Figure 6. Fan speed versus cooling rate for DP 980 steel.
10
9
8
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Velcoity (m/s)
Figure 7. Fan speed versus cooling rate for GA119 steel
32
14
15
Microstructure
DP980 steel parent (Figure 8 (a)) has a two phase microstructure (ferrite and pearlite).
When critically annealed at 1360 C/ 743 F and cooled at the different speeds the microstructures
of dual phase steel (ferrite and martensite) (Figures 8(b) (c) and (d)) were actually achieved. At
speed 1 the microstructure is achieved but with more bands; at speed 5 less bands were visible
and grains were finer; at speed 8 there were a few bands and much finer grains than for all the
speeds. Speeds 2 and 3 with correlate with speed 1; speeds 4 and 6 correlate with speed 5, and
speed 7 correlates with speed 8.
(a)
b) Cooling Rate: 0.489 C/s Velocity: 2.5
m/s
(c) Cooling Rate: 1.009 C/s Velocity: 8.2m/s
(d) Cooling Rate: 1.484 Velocity:
12.9m/s
Figure 8. Micrographs of the steel sample DP980 (a) parent (b) at speed 1 (c) at speed 5 (d) at
speed 8
33
GA119
The GA119Parent sample (a) consists of three different types of microstructures: ferrite,
and pearlite with small amounts of bainite. The intercritically annealed sample consists of
banded microstructure with phases as ferrite and bainite with martensite. The bainite is formed
in the bands. The banding is due to slow cooling from the intercritical range. From speeds 1-8
there are no changes in the microstructure. This is due to the slow cooling rate and the chemistry
of the GA119.
(a)
(b)Cooling Rate: .404 C/s Velocity: 1.9 m/s
(c) Cooling Rate: 7.7 Velocity: 0.792m/s
12.4m/s
(d) Cooling Rate: 1.014
Velocity:
Figure 9. Micrographs of GA119 armor steel sample (a) parent (b) interctitically cooled at speed
1, (c) speed (5) and speed (8)
34
5.0 Conclusions
In summary, based on the results of this experiment and others reported in the literature,
it is concluded that the higher the speed the higher the cooling rate. The results of this
experimentation showed that the necessary transformation needed for DP980 was achieved at
every speed. However, at higher speeds the grains were finer and there were fewer bands. For
GA119, due to the slow cooling rates, the necessary transformation was not possible; also the
resulting material did not pass the Military specifications for hardness of steel. The different
cooling speeds played a major role in the results.
35
References
What is steel?
http://www.acier.org/en/steel/what-is-the-steel.html
Types of Steel
http://en.wikipedia.org/wiki/Carbon_steel
http://www.jjsupplycompany.com/index_files/JJSupplyCarbonSteelInformation.htm
Elements in steel
http://www.chasealloys.co.uk/steel/alloying-elements-in-steel/
http://www.diehlsteel.com/technical-information/effects-of-common-alloying-elements-insteel.html
Steel process
http://www.azom.com/article.aspx?ArticleID=543
http://web.iitd.ac.in/~suniljha/MEL120/L4_Heat_Treatment_of_Metals.pdf
http://www.asminternational.org/content/ASM/StoreFiles/ACF180B.pdf
http://uhv.cheme.cmu.edu/procedures/machining/ch2.pdf
http://www.hnsa.org/doc/pdf/heat-treatment-steel.pdf
http://www.jjsupplycompany.com/index_files/JJSupplyCarbonSteelInformation.htm
http://navyaviation.tpub.com/14018/css/14018_659.htm
Mechanical Properties
http://www.technologystudent.com/joints/matprop1.htm
http://www.eng.morgan.edu/~desiree/IEGR363/part1steelpaper.html
http://science.howstuffworks.com/iron4.htm
http://www.leecosteel.com/low-high-carbon-steel-plate.html
http://metals.about.com/od/properties/a/Steel-Types-And-Properties.htm
http://en.wikipedia.org/wiki/Armour
36
http://en.wikipedia.org/wiki/Vehicle_armour
http://www.ehow.com/about_6638014_atomic-structure-steel.html
Different Phases
http://www.toolingu.com/definition-500230-18134-pearlite.html
http://www.wisegeek.com/what-is-pearlite.htm
http://www.toolingu.com/definition-500230-18106-bainite.html
http://www.toolingu.com/definition-500230-18105-austenite-phase.html
http://www.toolingu.com/definition-500230-18129-martensite.html
http://academic.uprm.edu/pcaceres/Courses/MatEng/MSE8-2.pdf
Galvanization Process:
http://www.galvanizeit.org/hot-dip-galvanizing/what-is-hot-dip-galvanizing-hdg/hdg-process/
http://www.galvanizeit.org/inspection-course/galvanizing-process/galvanizing/
http://www.metalplate.com/techdept/galvprocess.php
http://www.calphad.com/iron-carbon.html
http://www.sherart.nl/en/sherardizing/sherardizing
http://reneuxit-thermal-spray.com/Thermal_Spray_Galvanizing.html
Experiments:
http://www.sciencedirect.com/science/article/pii/S0921509309006248?np=y
37

Nucor Education and Research Center (NERC) 2013

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