Designing with Austempered Ductile Iron (ADI)

Transcription

Paper 10-129.pdf, Page 1 of 15
AFS Proceedings 2010 © American Foundry Society, Schaumburg, IL USA
Designing with Austempered Ductile Iron (ADI)
J. R. Keough and K. L. Hayrynen
Applied Process Inc. Technologies Division, Livonia, MI
G. L. Pioszak
University of Michigan
Copyright 2010 American Foundry Society
ABSTRACT
Austempered Ductile Iron (ADI) is a ferrous, cast
material with a high strength-to-weight ratio and good
dynamic properties. However, many designers are only
vaguely familiar with the savings related to near net shape
castings and totally unfamiliar with this material that can
compete favorably with steel and aluminum castings,
weldments and forgings. This paper will review the
design considerations for ADI to help the mechanical
designer in his/her material/process selection activity
early in the design process.
INTRODUCTION
The Austempering process is a high performance,
isothermal heat treating process that imparts superior
properties to ferrous materials. It was developed in the
1930's and, although in wide use, is familiar to only a
fraction of the design community. Ductile iron or
spheroidal graphite iron was developed in the 1940's.
Ductile iron, with its unique, spheroidal graphite
morphology, produces an iron that has tensile and impact
properties sufficient for products as varied as brake
calipers, pump impellers and steering knuckles .
The application of the Austempering process to ductile
iron produces a material called Austempered Ductile Iron
(ADI) that has a strength-to-weight ratio that exceeds that
of aluminum. ADI was commercialized beginning in the
1970's and has seen significant growth in the decades
following.
The selection of ADI as a material for design
consideration has been driven by the ductile iron
foundries and the Austempering suppliers and not by the
mechanical design community. That is the direct result of
the lack of shared information on the technology and a
near-absence of references to ADI in the most widely
used engineering textbooks and databases.
The design information necessary for the selection of ADI
as an option exists, but has largely been available in
fragments located in often obscure papers and texts. To
simplify the process for the selection of ADI, it is
important to have ADI design information readily
available in a format that mechanical designers can easily
interpret and use. This paper, and the references indicated
herein, are intended to aid the mechanical designer in the
consideration of ADI for a design solution.
WHERE TO BEGIN
A designer given a product or component to consider
must always start by narrowing down the entire world of
materials to those that might have appropriate properties,
have reasonable manufacturability and low cost. As
engineers, we would prefer that cost be no issue and be
able to deal only with making a perfect part. However,
we live in an imperfect world and cost is the ultimate
reality. All components will eventually fail. It is simply
a matter of how long we want them to live and how long
we can practically afford for them to live.
Narrowing down the material/process world for a specific
application includes such considerations as:
• Strength (tensile strength, yield strength, etc.)
• Dynamic Performance (toughness, fatigue
strength);
• Wear resistance (abrasion, rolling, sliding,
galling);
• Special features such as corrosion resistance,
noise damping, electrical resistivity, etc;
• Manufacturability (combining features,
machinability, near net shape, process reliability,
dimensional repeatability);
• Cost (cost of the material blank, cost of the
finished component, cost of inventory).
This paper is an attempt to guide the designer through the
consideration of Austempered Ductile Iron (ADI). The
authors’ goal is to provide the necessary comparative
information to allow one to filter through the first several
layers of decision making and get to the roots of an ADI
design….or not.
THE DUCTILE IRON PROCESS
Ductile iron is an iron-based alloy which contains a
carbon content that is high enough to exceed its solubility
in iron; resulting in the presence of pure carbon or
graphite dispersed within an iron matrix. In the case of
ductile iron, the shape of the graphite is spheroidal or
round and is described as having graphite nodules. (The
material is interchangeably referred to as ductile iron,
nodular iron and spheroidal graphite (SG) iron).
A complete listing of the minimum tensile properties to
meet the grades for ductile iron according to ASTM
International is given in Table 1.1 Other commonly used
standards for ductile iron include: SAE J434-04, ISO
1083:2004 and DIN EN 1563-2005.2-4
Table 1. Tensile Properties of Ductile iron per ASTM
A536-84(2009) Standard Specification for Ductile Iron
Grade
60-40-18
65-45-12
80-55-06
100-70-03
120-90-02*
UTS
min
psi / MPa
60 000 / 414
65 000 / 448
80 000 / 552
100 000 / 689
120 000 / 827
Yield Strength
min
psi / MPa
40 000 / 276
45 000 / 310
55 000 / 379
70 000 / 483
90 000 / 621
% Elongation
min
18
12
6
3
2
*120-90-02 grade is quenched & tempered
The properties of ductile iron are largely dependent on the
relative amounts of ferrite and pearlite present within the
matrix microstructure. Photomicrographs of two
commonly used grades of ductile iron, 65-45-12 and 8055-06, are shown in Figures 1(a) and (b), respectively. In
these photomicrographs, ferrite is the white phase
surrounding the round graphite nodules while pearlite is
the dark microconstituent. Ferrite is a soft, low strength
phase so the strength of the iron decreases as the volume
of ferrite increases.
(b)
Grade 80-55-06
Fig. 1. Photomicrographs of commonly used grades
of ductile iron taken at the same magnification.
Etched with 5% Nital.
The number and shape of the graphite nodules is
important when producing ductile iron. These
characteristics are described as the nodule count and
nodularity, respectively. Nodule count (number per mm2)
should be sufficiently high to minimize the presence of
porosity and carbides. Nodularity (% round) must be
sufficient to achieve the minimum ultimate tensile
strength (UTS) and elongation (%EL) levels, especially as
the yield strength of the material increases.
Ductile iron castings range in size from a few grams to
over 200 tonnes and can be produced using a number of
different molding methods. These methods include:
• Green sand mold;
• No bake sand mold;
• Permanent mold (mostly pipe);
• Lost foam;
• Investment cast (lost wax).
(a)
Grade 60-45-12
The mold method that is utilized will depend upon a
number of factors including:
• Size of casting;
• Complexity of casting shape;
• Production quantities;
• Surface finish;
• Linear dimensional tolerances;
• Cost.
Green sand molding is often used to produce engineered
castings because of its relatively low cost compared to
other methods and its versatility; allowing for the
production of both small and large castings. On the other
end of the spectrum is investment casting or the lost wax
process. Although this process is more expensive than
green sand molding, it is used for small castings that have
high tolerances and require better surface finish than can
be produced using a green sand process.
THE ADI PROCESS
In order to produce ADI, ductile iron must undergo a heat
treat process called Austempering. Austempering was
developed in the 1930’s and has subsequently been
applied to steel to produce a microstructure called Bainite.
While the steps for Austempering ductile iron are
essentially the same as those for steel, the resultant
microstructure is different. It is called Ausferrite and
consists of a mixture of high carbon Austenite and ferrite.
A schematic that illustrates the Austempering process is
shown in Figure 2. Austempering, in general, consists of
the following:
•
•
•
Heating to a temperature to produce Austenite;
Quenching rapidly to avoid the formation of
pearlite or other microconstituents to a
temperature above the Martensite start (Ms).
This quench temperature is referred to as the
Austempering temperature;
Holding at the selected Austempering
temperature for a time sufficient to transform the
Austenite to the desired end product; Bainite for
steel and Ausferrite for ductile iron.
A high nodule count is important to minimize segregation
of alloy elements which can promote the presence of
carbides as well as delay the rate of formation of the
Ausferrite microstructure. Additionally, a high nodule
count will prevent the formation of porosity or microshrinkage as well as promote the formation of small,
round graphite nodules.
Upon examining the grades of ductile iron in Table 1, one
can see that the chemistry of the iron is not part of the
specification. All that is required to certify ductile iron to
a particular grade is that the minimum tensile properties
are met. Conversely, most steels and aluminum alloys are
specified by chemical composition.
In order to be successful at Austempering ductile iron,
chemistry (or the hardenability) of the iron is important.
Hardenability refers to an ability to form Martensite or the
ability to cool from the austenitizing temperature to the
Austempering temperature without forming any
undesirable microconstituents like pearlite. Because
heavy sections cool more slowly, they require more
hardenability or more alloy additions.
A qualified heat treater can work with a designer to
choose the proper chemistry of ductile iron to be
Austempered. In general, most section sizes less than 20
mm can through harden without making alloy additions
provided the Austempering setup and apparatus is
adequate for the purpose. When alloy additions are
necessary, Cu, Ni or Mo are typically used.
Beyond meeting hardenability requirements, consistent
chemistry is necessary for lot-to-lot repeatability. The
chemistry of the iron will play an important role in
establishing the as-cast microstructure of the component.
The relative amounts of ferrite and pearlite that are
present in the as-cast material will affect the growth of the
component in response to Austempering. This is
especially important when machining is completed prior
to heat treatment as it will be desirable for the parts to
grow to the final dimensions for each heat treat lot.
Fig. 2. An isothermal transformation diagram that
illustrates the basic steps of the Austempering
process for a cast iron with >2% silicon.
HOW TO SELECT DUCTILE IRON FOR
AUSTEMPERING
Austempering is a heat treat process that is applied to
improve the properties of ductile iron. It will not be
successful if the base iron is not of high quality. For the
purpose of austempering, high quality can be defined as:
• Minimum nodule count of 100 per mm2;
• Minimum nodularity of 85%;
• Combined maximum of 1.5% of porosity;
carbides, inclusions and micro-shrinkage;
• Consistent chemistry.
If a high quality ductile iron component with the proper
alloy content is Austempered, its properties will depend
on the selection of the heat treatment temperatures and
times. ADI refers to a family of materials that encompass
a wide range of properties as indicated in Table 2. The
relevant SAE, ISO and DIN standards are listed in the
references section.5-8
It should be noted that the first grade of ADI listed in
Table 2, GR 750-500-11 (GR 110-70-11), is unique in
that the final microstructure contains some blocky
(proeutectoid) ferrite by design. As a result, the heat treat
rules and hardenability relationships for this grade are
slightly different compared to those previously described.
Once again, a knowledgeable heat treater can assist the
design engineer if this grade of ADI is utilized.
Table 2. The Six Standard Grades of ADI as
5
designated by ASTM A897-A897M-06.
Prior
Grading
System
Tensile
Strength
(MPa/ksi)
Yield
Strength
(MPa/ksi)
Elon
g.
(%)
Typical
Hardness
(HBW)
750 / 110
500 / 70
11
241-302
1
900 / 130
650 / 90
9
269-341
2
1050 / 150
750 / 110
7
302-375
3
1200 / 175
850 / 125
4
341-444
4
1400 / 200
1100 / 155
2
388-477
5
1600 / 230
1300 / 185
1
402-512
*Note: All properties are minimum requirements except
hardness which is typical.
Photomicrographs of two grades of ADI are provided in
Figure 3. These grades represent the range in
microstructure fineness that can be developed by varying
the Austempering temperature.
THE MECHANICAL DESIGN PROCESS- WHERE
TO START (The “Mouth” of the Funnel)
The mechanical designer has a tough job. He/she must be
able to satisfy the physical performance, aesthetics and
the cost of the component or system. The range of
material/process choices has broadened dramatically in
the past several decades. While steel properties have been
rather well defined for over 50 years, the properties of
materials like the various aluminum alloys, composite
materials, ceramic materials and polymers has been
evolving as the information “blanks” are being filled in
with experimental and experiential investigations.
In parallel with the materials developments have been
remarkable engineering and manufacturing process
developments in everything from 3-D, finite element
analysis (FEA) and stereolithographic prototypes, to new,
more efficient and accurate welding, casting, stamping,
cutting, forging and machining techniques.
Finally, the mechanical designer must decide for a
specific application if the material/process selections that
he/she makes are based on a product that is: life-anddeath and/or cosmetic and/or low/cost, etc.
The mechanical designer would, in fact, be happy to have
fewer choices because it would make his/her life easier to
choose from a smaller, rather than a larger range of
options. Today, we are not offered that simplicity and
must wade through a plethora of material/process
combinations, all with their own strengths and
weaknesses. Then, finally, we must choose.
(a)
Grade 900-650-09
In the previous sections, you have learned the basics of
the ductile iron process, the Austempering process and
ADI. Now how do we apply that knowledge to the real
material/process selection process? Let’s get started.
THE FIRST, MOST IMPORTANT DECISIONS
•
•
•
What is the function of the part under
consideration?
What is the mode of failure in precedent
parts/designs?
What are we trying to improve?
These are the broadest and most variable questions. For
example, if the design is a lever device for an agricultural
equipment application, one might be able to deduce the
following:
(b)
Grade 1600-1300-01
Fig. 3. Photomicrographs of ADI microstructures
(Ausferrite). Etched with 5% Nital.
•
•
•
•
The part will require a high strength-to-weight
ratio;
The part may be exposed to cold temperatures;
Nobody will die or be injured if this part fails;
The part will be loaded in low-cycle fatigue.
Narrowing the material/process combinations to those
options that adequately address the aforementioned
requirements/conditions constitutes what we will call,
getting into the mouth of the decision making funnel.
A high strength-to-weight ratio would eliminate
material/process combinations like zinc die castings, or
gray iron castings and most polymers or ceramic
materials.
Good properties at cold temperatures would further
narrow the range of steels and irons that would be
appropriate, but aluminum, having no ductile-to-brittle
transition temperature, would perform well in low
temperature conditions.
The fact that nobody will die or be injured if this part fails
allows the designer to be a bit more aggressive in his/her
mechanical safety factors which usually leads to reduced
cost. Cast, wrought and welded designs would all be
candidates. This decision would probably eliminate
exotic manufacturing processes (EDM, precision forging,
machining from bar stock) and materials (titanium alloys,
electro-slag remelted steel bar) as the design could be
accomplished with conventional processes that are lower
in cost.
The fact that the part will be loaded in low-cycle fatigue
may imply to the designer that we have a finite life issue
where the part will be highly loaded at a lower number of
fatigue cycles. We need only to design a component that
will have sufficient strength to survive this high loading
for just the number of cycles expected for the life of the
system. This will also reduce the cost of the chosen
material/process combination selected.
A key road sign for the designer is if a precedent part
failed. When there are failures, one can proceed
immediately to design a solution to the failure. Did it
wear out? If yes, we need to find a material/process
combination that gives us a part with sufficient strength
and dynamic properties that can survive the wear
conditions that the part is exposed to for the desired life
cycle.
A more difficult proposition is if there has never been a
failure in service on a like component. Why would one
change a part that has never failed in service? The nearly
universal answer to this is either cost, weight or
availability…..but usually cost.
If cost were not an issue, we would use cheap materials
and overdesign everything. If weight were the only issue,
we would use expensive, exotic light-weight materials
that would last forever. In any case, we need to acquire
the materials, and some materials and processes are just
scarce or being eliminated for environmental or
regulatory reasons. For example, lead is being eliminated
in metal solders and free-machining steels, chromium and
other heavy metals are an ongoing environmental concern
to water supplies and some polymers and composites
cannot be recycled at all.
In this “mouth of the funnel” decision making process,
ADI can be considered in the following, relative terms:
• It has a high strength-to-weight ratio;
• It has good dynamic properties;
• It has good wear resistance for a given hardness;
• It is a cast material and has the advantages of
near net shape processing and generally good
manufacturability;
• It is cost competitive with other common
engineering materials.
MONOTONIC PROPERTIES
The monotonic properties include such measures as
tensile (ultimate) strength, yield (proof) strength,
compressive strength, shear strength, elongation,
reduction in area, Young’s modulus (stiffness) and
Poisson’s ratio. All of these measure the deflection or
distortion of the material under a given, single-cycle,
stress up to, and including, failure.
Manufacturers over time have contented themselves with
supplying mechanical engineers with tensile strength,
yield strength and elongation because these three
properties are easily gathered in one test. This data is
familiar to us so we continue to gather it. After all, our
material standards are based on them. However, without
much fanfare, two of the three measures have become
largely meaningless.
Scores of interdependent property relationships related to
tensile strength continue to exist. For example, the
endurance ratio portends high cycle fatigue performance
for a given tensile strength. The problem is that for most
design applications, if the part has yielded (plastically
deformed or elongated), it is scrap. That is reflected in
the fact that FEA models do not consider either tensile
strength or elongation in their long list of coefficients and
exponents because those models are used to design parts
that are not plastically deformed. Those models also use
such values as Young’s Modulus (stiffness) and Poisson’s
Ratio (directional deflection) to accurately model the
dimensional response of a component to a given input.
Yield strength is a useful measure because it predicts the
onset of plastic deformation. Elongation is only useful to
the extent that it gives us a relative “feel” for the ductility
of a material. Unfortunately, the low speed at which the
load is applied in a tensile test is not often encountered in
the ductile failure of a component. As stated before, if a
component elongates in service, it is usually scrap and,
therefore, not a useful measure in design. Figure 4 shows
the relationship between elongation and yield strength for
several material/process combinations.
Fig. 4. Yield (Proof) Stength for ADI vs. various
common engineering materials.
Examination of Figure 4 shows that ADI is a rather
disruptive technology as it changes the order of things
that we knew. Historically, we knew that if we had a
design application with stress levels exceeding 500 MPa
(~73 ksi) that we had one material choice…..steel.
Neither the aluminum alloys nor the ductile iron alloys
could function for very long at those stress levels. Then
came ADI and our choices changed. Now the designer
had to choose a material instead of just defaulting to steel.
To make the traditional mechanical designer comfortable
with ADI, standards with minimum requirements defined
in tensile strength, yield strength, elongation and hardness
were developed. See Table 2.
As you can see from Table 2 and the comparative Figure
4, the ADI grades are viable alternatives to some of the
incumbent material/process combinations that designers
are more familiar with.
Figure 5 is a comparison of the elongation of ADI to steel,
aluminum, titanium, as-cast ductile iron and carbon fiber
composite materials. As can be seen in this figure, the
various grades of ADI (#20) are spread throughout the
distribution.
Fig. 5. Typical percent elongation for various
material/process combinations.
A note about when reduction in area is called for. In most
ductile materials, when the tensile bar is pulled, the bar
fails in a “necked” area. This is an area where the
deformation concentrates after the onset of plastic
deformation. Oddly, ADI does not neck. In a test bar,
this is manifested by the entire gage length getting smaller
in diameter with no specific smaller diameter section
surrounding the failure. As such, the percent reduction in
area for ADI is nearly identical to the percent elongation.
Yield strength is a useful value in both traditional infinite
life (stress controlled) mechanical design and finite life
(strain controlled) FEA designs. Practically, for most
designs, yield stress represents the load that you never
want your component to see…..the “stress ceiling”. As
such, many of the following comparisons use yield
strength as the constant comparative value and relate it to
various other properties to give the reader a relative
placement of ADI to other common (and not so common)
engineering materials.
Young’s Modulus (stiffness) is one property that seems to
require “re-invention” by each generation of engineers.
(The push to make systems lower in mass invariably leads
to systems that vibrate too much or make too much noise
or are felt to be “harsh” by the untrained end-user). To
make components lighter, the first place we look is the
low density materials, but the problem is that the low
density materials tend to have very low stiffness. Figure
6 shows the relationship between yield strength and
Young’s Modulus for several material families.
In this comparison, steel has the highest stiffness at about
205 GPa and aluminum has about one-third the stiffness
at 70 GPa. ADI is an excellent compromise (at about 165
GPa), having 2.3 times the stiffness of aluminum as well
as more than three times the strength. The limiting factor
with ADI in designing for stiffness is the minimum
section size achievable. In conventional sand molding,
the minimum ductile iron / ADI design thickness would
about 5mm generally and 3mm in specific areas. With
precision core sets, investment castings and other
processes, it is possible to achieve a general ductile iron
wall thickness of 3mm. A thin-walled ADI design can
replace a heavy-walled aluminum part at equal weight,
but ADI will not be able to replace an aluminum die
casting with a 2.5mm wall thickness.
Stiffness often has a dynamic inference. That is the case
with ADI when used in gear and rolling contact
applications. For example, in a gear tooth application,
ADI may have a lower allowable contact stress than
carburized and hardened steel. But because it has a lower
Young’s Modulus for a given input load, ADI will have a
larger “contact patch” and, thus, a lower contact stress for
a given input load. In this case, the lower Young’s
Modulus works to the advantage of the ADI as it
“elastically conforms” better to the mating part, assuming
that the increased backlash on the gear tooth is not a
functional issue.
the metal matrix generated by one of the aforementioned
processes increase the compressive stresses at the surface.
This manifests itself as a 5-20% increase in allowable
bending fatigue load. In addition to the creation of matrix
dislocations during surface working, the carbon-stabilized
Austenite in the Ausferrite structure undergoes a
metallurgical transformation to Martensite in a ferrite
“nest”. This results in a local volumetric expansion that
dramatically increases surface compressive stress and
allowable fatigue load, making ADI competitive with
carburized steel. Figure 8, compiled from AGMA 939A07 9 and AGMA 2001-D04 10 shows the comparative
allowable stresses of various material/process
combinations used in the manufacture of gears.
Fig. 6. Young’s Modulus (stiffness) for various
material /process combinations.
DYNAMIC PROPERTIES
Dynamic properties include such measures as fatigue
strength (rotating bending, rolling, gear tooth contact and
bending), wear resistance, galling resistance and
toughness.
Figure 7 shows the typical 10 million cycle allowable
rotating bending stress of ADI compared to several
material/process combinations. Examination of these
results shows that ADI is very competitive with neutral
hardened, medium carbon steel.
Fig. 8. A comparison of the allowable bending stress
for ADI (as machined and shot peened) vs. other,
conventional steel material/process combinations.
If one is designing with “strain controlled” FEA models,
the necessary coefficients and exponents are now
available. Sources for them are included in the references
with the most widely used one being the American
Foundry Society’s Research Report entitled “ Strain-Life
Fatigue Properties Database for Cast Iron” on CD.11 This
paper seeks only to familiarize you with the relative
fatigue strength of various material/process combinations
so that you can determine if ADI should even be
considered for a given fatigue application.
Fig. 7. Typical 10MM cycle allowable bending stress
(MPa) for various materials..
ADI has a few unique properties related to fatigue
strength. Figure 8 demonstrates them graphically. Unlike
all the other ferrous and non-ferrous materials, ADI’s
bending fatigue strength is at a maximum in the lower
strength grades. Furthermore, most materials exhibit an
increase in fatigue strength if they are shot peened, fillet
rolled or ground. This occurs because the dislocations in
The strain transformation of the ausferrite matrix as a
result of surface work also makes ADI wear better than its
bulk hardness would indicate. Figure 9 compares the pin
abrasion wear resistance of ADI with several other
materials, all at a bulk hardness of 40 HRC.
Table 3. Self Mated Galling Results for ADI, CarboTM
Austempered Steel, Carburized & Hardened Steel
and Bearing Bronze.
Material
Grade 900 ADI
Grade 1050 ADI
Grade 1600 ADI
C/A 8620 Surface
C/H 8620 Surface
SAE 660 Bronze
Fig. 9. Relative volume loss to abrasion of several
material/process combinations at 40 HRC.
This surface “strain transformation” effect also positively
affects the contact fatigue properties of ADI. Figure 10
draws on AGMA 9-10 for comparative data in contact
fatigue. ADI compares favorably with neutral (through)
hardened steel, nitrided steel and induction hardened
steel. ADI is perfectly adequate for contact stress levels
up to about 1600 MPa. Above 1600 MPa, carburized and
hardened steel is currently the only alternative.
Volume
Loss
(mm3)
10.9
10.7
9.4
10.6
10.6
70.1
Hardness
(HRC)
30
40
52
54
60
27(HRB)
Galling
Threshold
(MPa)
1527+
894
941
512
882
311+
+ Indicates no galling occurred during testing.
ADI is a moderately tough material for its strength. For
those familiar with designing with ductile iron, a general
rule of thumb for ADI would be that compared to as-cast
ductile iron, ADI will have twice the strength for a given
level of ductility.
The measures of toughness include impact strength
(notched and un-notched) and fracture toughness. Once
again, the existing standards have developed over time
with the tests that are easy to make. Charpy and Izod
impact tests are time honored measures. Unfortunately,
they do not offer one bit of data that is useful in FEA
design. In Charpy impact, ADI is better than as-cast
ductile iron and aluminum, but inferior to steel.
Fracture toughness (K1C) is a test that measures the energy
required to propagate an existing crack. In fracture
toughness, the performance of ADI is similar to that of
steel for a similar strength/hardness. Figure 11 shows the
relative value of fracture toughness for several
Fig. 10. Allowable contact stress for ADI (as
machined) compared to other, conventional steel
Galling resistance is often important for parts that twist
against each other in service. Table 3 shows the result of
galling tests on various grades of ADI, CarboAustempered™ steel, Carburized & Hardened steel and
bearing bronze. During testing, Grade 900 ADI did not
gall. This would imply that ADI 900 might be a very cost
effective alternative to expensive bronze in some galling
applications.
Fig. 11. Room temperature fracture toughness of ADI
compared to several material/process combinations.
Austenite is a face-centered-cubic (FCC) metallic matrix
structure. As such, it has no ductile to brittle transition
temperature. Aluminum is 100% FCC and that is why the
properties of an airplane’s skin and wings do not
deteriorate at -60°C (-76°F) during high altitude flight.
Similarly, ADI has FCC austenite as one of its principal
constituents and the lower strength grades of ADI (those
with the highest percentage of austenite in the
microstructure) have the most gentle ductile to brittle
profile. In fact, ADI maintains nearly 70% of its room
temperature fracture toughness at -40° as can be seen in
Figure 12.
Fig. 13. A comparison of typical specific gravities for
various material/process combinations..
In general, there is no free lunch for weight reduction.
The lowest density materials tend to have the lowest yield
strength and the lowest stiffness. More exotic materials
like titanium and some carbon composite materials can
escape that rule, but they tend to be very expensive, brittle
or have poor manufacturability. Note in the comparison
in Figure 14 that ADI has a relatively low specific weight.
Fig. 12. Fracture toughness of two types of ADI over a
range of temperatures. (Grade 1~302HBW and Grade
1.5~321HBW).
OTHER PROPERTIES
These “other” properties are as varied as the applications
being considered. They may include such measures as
density (specific gravity), corrosion resistance, coefficient
of thermal expansion, thermal conductivity, damping
coefficient and other measures as specific as magnetic
permeability and electrical resistivity.
Today, designers are often pressed for weight reductions
to either reduce energy requirements on moving systems
or to reduce shipping costs or to reduce the structural
needs of a system made up of many components. Figure
13 compares the densities of several material/process
combinations. The popularity of aluminum stems largely
from its low density and good manufacturability.
Low density, by itself, is insufficient to compare
materials. For instance, Styrofoam and balsa wood have
low densities, but their strengths are insufficient for most
component designs. Figure 14 compares the relative
weight per unit of yield strength of various materials.
Fig. 14. Relative weight per unit of yield strength for
several material/process combinations.
A material’s ability to damp noise is often important in
the perceived quality of a device or system. Gray iron
with large, coarse graphite flakes is referred to as
“damped iron” for its ability to damp noise. Conversely,
aluminum is a notoriously “noisy” material. Table 4
shows the relative damping capacities of various
materials. Note that Austempered ductile irons, with their
Ausferritic matrix, have better damping capacity than
regular ferritic/pearlitic ductile irons. The increase in
damping seems to be proportional to the size and
distribution of the ferrite plates in ADI’s Ausferrite
matrix. A higher strength grade of ADI (with a larger
volume of finer ferrite platelets) has a higher damping
coefficient than a lower strength grade of ADI (with
fewer, coarser ferrite platelets).
Table 4. Relative Damping Capacity for various
Material/Process
Coarse Flake (damped)
Gray Iron
Fine Flake Gray Iron
Austempered Ductile
Iron (ADI)
Ductile Iron
Carbon Steel
Carbidic (White) Iron
Aluminum (typical)
Relative Damping Capacity
100 - 500
20 - 100
9 - 30
5 - 20
4
2-4
0.4
Corrosion resistance is a material feature that must be
addressed in most designs; even finite life designs. Table
5 shows the galvanic series for selected metal alloys. The
light metal alloys (magnesium and aluminum) are subject
to rapid corrosion and must either be attached with (and
to) welds or fasteners of like galvanic behavior or
insulated from them. For example, magnesium alloy
(mag) wheels on cars must be coated and attached with
insulating (non-conductive) washers to prevent them from
coupling to ground through iron and/or steel components
and rapidly corroding in service.
The silicon-iron-graphite oxide that develops on cast iron
advances very slowly, once established. ADI is
incrementally more corrosion resistant than steels and
other cast irons due to the presence of graphite and
Austenite in the metal matrix. (Note the position of
graphite and the Austenitic materials in the Galvanic
Series). For example, a Grade 1050-7 ADI has 9%
graphite and approximately 30% Austenite in its structure
making the material more cathodic than ferritic/pearlitic
ductile irons or steels.
Ferrous alloys hardened to high tensile strengths can be
subject to environmentally assisted failure (EAF) under
the right conditions. Designers are familiar with the risk
of using quenched and tempered steels at elevated
hardnesses loaded at a constant elevated stress (near the
proof strength of the material). Liquids and other sources
of hydrogen ambient to the highly stressed region of the
component can induce brittle failures at bulk loads
calculated to be below the proof stress.
ADI is also subject to EAF.12 A failure of this type
requires the presence of three conditions: (1) A high and
constant stress near the proof stress and/or local plastic
deformation; (2) A slow strain rate and (3) a hydrogen or
liquid source of hydrogen ions. Therefore, in designing
with ADI, one should never use it in an application where
the parts are locally plastically deformed at a high (and
sustained) stress level.
Table 5. The (Relative) Galvanic Series for selected
metal alloys.
ANODIC / LEAST NOBLE / CORRODED
Magnesium Alloys
Zinc Alloys
Aluminum Alloys
Mild Steel and Wrought Iron
Alloyed Carbon Steels
Cast Iron (including Ductile Iron)
Austempered Ductile Iron (ADI)
Ferritic and Martensitic Stainless Steels
Ni-Resist (majority Austenitic Cast Iron)
Titanium
Lead
Tin
Inconel
Brass
Copper
Bronze
Austenitic Stainless Steel (fully Austenitic)
Silver
Graphite
Zirconium
Gold
Platinum
CATHODIC /MOST NOBLE / PROTECTED
Previously in this paper, we discussed the effects of the
FCC Austenite in ADI’s microstructure affecting its low
temperature toughness. The presence of Austenite in the
structure also produces other characteristics of note in
ADI.
We know that the Austenite in the Ausferrite structure is
thermally stable to very low (liquid helium) temperatures.
However, the Austenite can break down into ferrite and
carbide if exposed to elevated, long-term service
temperatures; resulting in a gradual degradation of tensile
strength and toughness. Earlier research13 has
demonstrated that the ADI microstructure is long-term
stable as long as operating temperatures did not exceed
about 60°C (108°F) less than the isothermal
transformation (Austempering) temperature. Table 6
shows estimated maximum continuous operating
temperatures for the various grades of ADI.
Table 6. Estimated maximum operating temperature
13
for the various grades of ADI.
Grade of ADI
750-500-11
900-650-09
1050-750-07
1200-850-04
1400-1100-02
1600-1300-01
Maximum Operating
Temperature
315°C (600°F)
315°C (600°F)
300°C (572°F)
290°C (554°F)
280°C (536°F)
260°C (500°F)
Austenite also has the affect of increasing the coefficient
of thermal expansion in ADI. Ferritic, pearlitic and
Martensitic irons and steels have a coefficient of thermal
expansion of about 11(mm/mm/°C)x10-6. ADI,
depending on the grade, has a coefficient of thermal
expansion ranging from about 13.5-14.5
(mm/mm/°C)x10-6 . By comparison, aluminum alloys
have a coefficient of thermal expansion of about 18
(mm/mm/°C)x10-6 . This property needs attention in
cases where tolerance fitting is required at temperature.
For example, a forged steel crankshaft rotating in an
aluminum block will require special design features to not
leak oil at operating temperatures. Conversely, an ADI
crankshaft operating in an iron block would require extra
cold clearance to allow for the greater crankshaft growth
at operating temperatures.
MANUFACTURABILITY AND COST
CONSIDERATIONS
Items considered in manufacturability include minimized
operational steps, near net shape, machinability, lowenergy production, recyclability, weldability, reduced
numbers of sub-components, availability, lot size, tooling
costs, and component size and shape.
Figure 15 shows a simple case of a forged steel end
connector as compared to an ADI casting. In this case,
the numbers are very clear. With ADI you buy less
material because ductile iron is 10% less dense and
because the holes have been cored into the casting. The
part is machined in the soft, as-cast condition.
Furthermore, ductile iron can be machined much more
quickly than forged steel with extended tool life. Unlike
the continuous, spring-like chips produced during the
machining of steel, the chips from ductile iron machining
are discontinuous, can be handled using standard
magnetic techniques and are 100% recyclable.
In the absence of an assignable cause failure, the designer
is most often asked to reduce the cost of the component to
make the producer’s product or system cost competitive
and more profitable. The task is to produce a component
or a system to the minimum engineering requirement for
the application at the lowest price.
Often, the cost of the material blank is eclipsed in this
consideration by the price of machining, plating,
transport, inventory, tooling, and so on. For instance, it is
common for an ADI blank to be 20-30% lower in cost
than a heat treated steel forging. However, the principal
savings may not be in the blank, but in the money saved
by machining the part in the soft, as-cast condition and
then Austempering. This can result in doubling
machining center throughput and greatly increased tool
life…aspects saving much more than the savings on the
heat treated blank.
Fig. 15. Cast ADI end connectors compared to forged
steel in a manufacturing sequence.
The lowest cost path is to cast the part, machine it
completely in the soft, as-cast condition and then
austemper. Because of the dimensional repeatability of
the ADI process, this is a viable option in about three
quarters of the applications. However, in applications
where the tolerances are on the order of 0.01mm, the part
will require machining after austempering.
Machining ADI can be, and is being, done every day;
even for ADI exceeding 400 HBW. The key is in
understanding how to correctly set up for it. For example,
if one uses their experiential knowledge with steel and
sets up to machine a 320 HBW ADI using the tools, setup and settings they use for a 320 HBW steel, the
machinist will mistakenly conclude that ADI cannot be
practically machined. However, if you know the critical
differences, ADI can be machined. Those differences are:
1. Difference: ADI has a 20% lower Young’s
modulus than steel with similar yield
strength for a given hardness, resulting in
excessive, high-frequency vibration and tool
wear.
Solution: The ADI part must be secured
with a very rigid chucking scheme and short
tool holder moments must be employed.
2. Difference: ADI undergoes a “strain
transformation” in front of the tool, similar
to some stainless steels,
Solution: A thicker chip (cut at an
appropriately lower speed) can move the
strain transformed area away from the
cutting edge of the tool and allow it to break
away cleanly. This requires, however,
greater power and, thus, more deflection of
3.
the tool setup during metal removal. A thin
chip taken at a high rate of speed can harden
over its entire cross section, generate more
heat and tool wear. High speed removal of
thin cuts may be acceptable with ceramic
composite tooling.
Difference: The high yield strength and the
in-situ strain transformation result in very
high tool-work interface temperatures.
Solution: A tool material capable of
withstanding high interface temperatures
that is tough enough for interrupted cuts is in
order. Aluminum Oxide tools with SiC
whiskers have been shown to provide good
results (even at higher surface speeds).
The casting process is the most direct, lowest energy
process from metal ore to finished component. All ductile
iron and ADI grades can be produced from up to 100%
recycled materials. Properly designed castings can
combine multiple part numbers into one, simplified
design, reduce weight and improve the appearance and
the functionality of the component.
Castings can put the metal right where you need it.
Casting processes allow us to cast holes and complex
passages into parts that cannot be forged in. They allow
for the ability to cast in threaded fittings, tubes, weld pads
and various fasteners needed for subsequent attachment or
function.
The advent of highly accurate finite element analysis tools
allows for freedom from preconceived engineering design
notions (like perfect circles, and right angle corners).
Today, the strain life fatigue coefficients and exponents
exist for the commercial ADI grades and engineers can
easily examine ADI in a proposed application before the
first bit of tooling or prototypes are built; thus, increasing
the accuracy of both the engineering and the cost models.
When FEA modeling was first becoming practical, a
North American automobile manufacturer had a problem.
A new model of a popular, high performance vehicle was
incorporating a new fuel tank design; displacing some of
the space previously allowed for the rear suspension. The
large, cast aluminum upper control arms slated for this
application would not fit in the package. The suspension
designer worked with a foundry and their casting designer
to develop a new, light-weight ADI design using FEA
optimization. The result was the configuration shown in
Figure 16 that fit handily in the available space and
provided the needed performance at a lower cost with
virtually no weight penalty.
Casting tooling is generally much lower in cost than
forging tooling. With the use of cores, one can design
holes or passageways in the as-cast component that could
not be achieved with forging, welding or by assembling
several pieces.
Fig. 16. These smaller, lighter-weight ADI upper
control arms replaced the larger aluminum design that
would not fit into the vehicle package.
Figure 17 shows a case study where a stamped, welded
and assembled steel suspension control arm was replaced
with an ADI design. The per-piece price savings for ADI
was 2%, but the tooling for the ductile iron castings was
54% lower in cost and the vehicle weight was reduced by
4 lbs (1.8kg).
Fig. 17. The stamped, welded and assembled steel
control arm on the left was replaced with the ADI
design on the right at a cost and weight savings.
Castings can be cost effective, even in very small lot
sizes. Manufacturers with in-house welding capabilities
often make the mistake of assuming that for a part that is
only 100 pieces per month, the welding together of three
parts is more cost effective than buying castings. When
one considers the production and inventorying of three
pre-weld part numbers and their drawings along with the
welding fixtures and gages that must be maintained to an
ISO standard, the price is often much higher for the
weldment. Since the costs are buried in the
manufacturer’s overhead and not easily defined, they are
typically miscalculated. With a casting, one’s ISO
controlled pattern is stored at the casting supplier. If the
customer needs 100 pieces, they need only to order them.
Many casting suppliers (for a small, additional per-part
fee) will even cast, machine and heat treat components in
bulk for cost savings and then ship in sub-groups as the
customer needs the parts; thus, eliminating inventory for
the purchaser.
Figure 18 shows a welded steel seed boot for a rangeland
seeder and its one-piece ADI replacement. The ADI
component is not only more visually appealing, it is 15%
lower in mass and 65% lower in cost. The lead time to
produce pieces went from six weeks with the steel
weldment to 3 weeks with the ADI casting (including heat
treatment).
Fig. 19. This one-piece ADI drive wheel replaced an
82-piece welded and fastened assembly.
For a given annual production volume, ADI is typically
20% lower in cost than a comparable steel component and
over 30% lower in cost than an aluminum component.
The design engineer is often buying strength. The lowest
strength grade of ADI is about three times stronger than
the highest strength aluminum and ADI’s density is only
2.4 times that of aluminum. This means that in certain
applications, ADI can replace aluminum at equal or lower
weight. With the cost per unit mass much lower for ADI
than steel or aluminum, ADI exhibits a lot of strength for
the money. Figure 21 shows the comparison of cost per
unit of yield strength for various engineering materials.
(a) Welded steel seeder boot
(b) ADI seeder boot
Fig. 18. The multiple-piece welded steel seeder boot
(a) was replaced with the one-piece ADI design (b)
with significant cost, mass and time savings.
Fig. 20. The ADI truck trailer hub (Left) is 2% lighter
and lower in cost than the aluminum hub (Right) that it
replaced.
Figure 19 shows a drive wheel for a rubber-tracked
crawler vehicle. The one-piece ADI conversion replaced
an 82 piece welded and bolted assembly at a 15% lower
mass and with a cost reduction of over 50%.
Figure 20 shows an ADI wheel hub for a Class 8 truck
trailer. The ADI hub was designed to take maximum
advantage of ADI’s high strength-to-weight ratio. It is
2% lighter than the aluminum hub that it replaced and
lower in cost.
Fig. 21. Relative cost per unit of yield strength for
various material/process combinations.
SUMMARY/CONCLUSIONS
ADI offers the designer an economical alternative to steel
and aluminum castings, forgings and weldments.
ADI’s high strength-to-weight ratio allows the designer to
even replace aluminum sand castings and forgings at
equal mass in applications with a minimum ADI wall
thickness of 3mm.
ADI’s bending and contact fatigue strength makes it
superior to aluminum and competitive with steel at a
similar hardness.
ADI offers the mechanical designer a practical material
choice at low cycle stress levels above 450 MPa.
It is impossible to capture the entire design process and to
address all the questions encountered in the design of
mechanical components in a single paper. The authors
have attempted to speak from a design perspective about a
material that is new to most designers. The comparative
relationships are insufficient for part design, but the
references referred to below would lead the designer to
the necessary documents and formulae to answer his/her
specific questions to allow for designing with ADI.
Manufacturers Association, Alexandria, VA,
www.agma.org.
11. American Foundry Society Research: Strain-Life
Fatigue Properties Database for Cast Iron, 2003,
AFS, www.afsinc.org.
12. Gagne, M and Hayrynen, K.L., “Environmental
Embrittlement of Ductile Iron”, Proceedings of the
8th International Symposium on Science and
Processing of Cast Iron, Beijing, China, 2006, pp.
452-457.
13. Hayrynen, K.L., PhD, Keough, J.R., P.E., Kovacs,
B.V., PhD, “Determination of Mechanical Properties
in Various Ductile Irons after Subjecting Them to
Long-Term Elevated Temperatures”; Research
Project No. 28, 1999, Ductile Iron Society, North
Olmsted, Ohio, USA; www.ductile.org .
FURTHER READING
•
PB89-190946, Austempered Ductile Iron (ADI)
Process Development Final Report, 1989, Gas
Research Institute, www.ntis.gov or 800-553-6847.
•
Project A4001, Austempered Ductile Iron Data Base,
1989, ASME Gear Research Institute, Naperville, IL.
•
1st International Conference on Austempered Ductile
Iron: Your Means to Improved Performance,
Productivity and Cost, Rosemont, IL, American
Foundry Society, individual papers from the
conference at www.afsinc.org.
•
2nd International Conference on Austempered Ductile
Iron: Your Means to Improve Performance,
Productivity and Cost, Ann Arbor, MI, American
Foundry Society, individual papers from the
conference at www.afsinc.org.
•
1991 World Conference on Austempered Ductile
Iron, Chicago, IL, American Foundry Society,
individual papers from the conference at
www.afsinc.org.
•
Proceedings of the 2002 World Conference on ADI,
Conference on Austempered Ductile Iron (ADI) for
Casting Producers, Suppliers and Design Engineers,
Louisville, KY, on CD-ROM, www.afsinc.org.
•
Ductile Iron Data for Design Engineers, revised
1998, Rio Tinto Iron & Titanium, Inc., Montreal,
Quebec, www.ductile.org/didata.
•
Iron Castings Engineering Handbook, 2003
American Foundry Society, www.afsinc.org.
•
Kovacs, B.V., PhD and Keough, J., PE, “Physical
Properties and Application of Austempered Gray
REFERENCES
1.
ASTM A536-84(2009), Standard Specification for
Ductile Iron Castings, ASTM International, West
Conshohocken, PA, www.astm.org.
2. SAE J434, Automotive Ductile (Nodular) Iron
Castings, SAE International, Warrendale, PA,
www.sae.org.
3. ISO 1083:2004, Spheroidal Graphite Cast Irons –
Classification, ISO, Switzerland, www.iso.org or
www.ansi.org.
4. DIN EN 1563-2005, Founding - Spheroidal Graphite
Cast Irons, Berlin, Germany, www.din.de .
5. ASTM A897/A 897M-06, Standard Specification for
Austempered Ductile Iron Castings, ASTM
International,
West
Conshohocken,
PA,
www.astm.org.
6. SAE J2477:2004, Automotive Austempered Ductile
(Nodular) Iron Castings (ADI), SAE International,
Warrendale, PA, www.sae.org.
7. ISO 17804:2005, Founding Ausferritic Spheroidal
Graphite Cast Irons – Classification, ISO,
Switzerland, www.iso.org or www.ansi.org.
8. DIN EN 1564:2006-03, Founding – Austempered
Ductile Cast Irons, Berlin, Germany, www.din.de .
9. AGMA 939-A07, Austempered Ductile Iron for
Gears, American Gear Manufacturers Association,
Alexandria, VA, www.agma.org.
10. ANSI/AGMA 2001-D04, Fundamental Rating
Factors and Calculation Methods for Involute Spur
and Helical Gear Teeth, American Gear
Iron”, AFS Transactions, 1993, Vol. 101, Paper #
93-141, pp. 283-291.
•
Metals Handbook Tenth Edition Volume 1,
Properties and Selection of Irons and Steels, 1990,
ASM International, www.asminternational.org.
•
Technical Library at www.appliedprocess.com.
ACKNOWLEDGMENTS
The authors would like to thank the employees and
customers of the AP Companies and our worldwide
network of licensees for their contributions to the
information and case studies referred to in this paper.
Special thanks to Terry Lusk, Justin Lefevre, Smith
Foundry, Dotson Company, Walther EMC, Benteler,
Toro, Citation Corporation and Chrysler.
DEFINITIONS/ABBREVIATIONS
In the comparative properties graphs the Key references
to various “CF” materials represent carbon fiber
materials.
There exists mixed convention regarding the
capitalization of the various forms of the words
“Austenite” and “Austemper”. The A is rightly
capitalized as the pre-fix “Aus” is a formal derivation
from the name of the metallic phase Austenite and its
principal discoverer, Sir William Chandler RobertsAusten (1843-1902), British metallurgist.
The same conundrum arises with the various conjugations
of Bainite, the metallurgical mixture of phases named
after its discoverer, Edgar Bain, and Martensite, a mixture
of phases named after the German investigator Adolph
Martens.
HBW is the convention for Brinell hardness taken from
an indentation made from the ISO required tungsten (W)
ball.

Designing with Austempered Ductile Iron (ADI)

Transcription

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