Mechanical properties and structure of austempered ductile iron

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ISSN (1897-3310)
of
Volume 7
Issue 1/2007
161 – 166
FOUNDRY ENGINEERING
35/1
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
Mechanical properties and structure of
austempered ductile iron -ADI
M. Kaczorowski a), A. Krzyńska b)
a)
Institute of Mechanics and Design
Institute of Materials Processing
Faculty of Production Engineering, Warsaw University of Technology,
ul. Narbutta 85, 02-524 Warszawa, POLAND
b)
Summary
The results of experimental study of austempered ductile iron are presented. The aim of the investigations was to look closer into the
structure – mechanical properties relationships of this very attractive cast material. The experiment was carried out with 500 7 grade
ductile iron, which was austempered using different parameters of heat treatment. The specimens were first solution treated 1 hour in
910oC and then isothermally quenched for different time in silicon oil bath of temperature 275, 325, 300 and 350oC. The mechanical
properties heat treated specimens were tested in tensile to evaluate yield stress R e, 0.2, tensile strength Rm and elongation A10. Additionally
hardness of heat treated samples was measured using Brinell-Rockwell hardness tester. Structure of the specimens was studied either with
conventional metallography, scanning (SEM) and transmission (TEM) electron microscopy. It followed from the study that conventional
grade ductile iron enabled to produce both low and high strength ADI, depend on heat treatment parameters. As expected the low
temperature isothermal quenching produced higher strength ADI compare to the same ductile iron but austempered at 350oC. It was
discovered however, that low yield strength ADI obtained for short time quenching at 275oC exhibited high strengthening effect while
strained in tensile. So it was concluded that this had to by cause by large amount of untransformed austenite, which FCC lattice is
characterized by high strengthening coefficient.
Key words: ADI; Heat Treatment Parameters; Structure; Properties.
1. Introduction
Austemperd ductile iron – ADI [1] looks to be probably the
most outstanding achievement in the metallurgy of cast iron. The
relative simple heat treatment, low cost and very interesting
combination strength and ductility make it very attractive material
for many applications. The ADI technology involve the solution
heat treatment at the temperature 850-950oC which is followed by
isothermal quenching in the temperature range 250 – 400oC.
Depends on the heat treatment parameters, high strength ADI with
Rm=1600MPa or mediate strength ADI with Rm= 800MPa but
10% elongation can be obtained [2]. These mechanical properties
results from specific metal matrix microstructure of ADI, which is
the mixture of carbon stabilized austenite and bainitic ferrite cold
ausferrite. The ductility of austempered ductile iron depends
mostly on relative amount of austenite with FCC lattice which
relative amount in case of high temperature austempering may
reach 40% or even more. On the other hand the high strength, low
ductile ADI is obtained in case of low temperature isothermal
quenching. In this case, except carbon stabilized austenite and
bainitic ferrite some amount of martensite forms. The relative
proportion between each component in ADI microstructure
depends not only on the temperature but also time of
austempering. This relationship was summarized in form of
graphs one of which was showed in fig.1 [3]. Attractive
mechanical properties, low cost and no special requirements
concerning chemical composition make ADI competitive not only
to conventional ductile iron but also with cast steel and in some
cases even aluminum alloys [4].
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161
stage is accompanied with carbide formation which decreases
strongly the plastic properties of ADI.
However there is no special demands concerning ADI,
except the good quality ductile iron, the decomposition of supercooled austenite proceeding at three stages described above
depend strongly on this chemical composition, and especially
segregation of alloying elements. It is known, that silicon
segregate mainly close to the graphite nodule while manganese at
eutectic cell boundaries [11]. This in turn will influence the offset
and speed of austenite decomposition into ausferrite and further
into bainitic ferrite and carbides.
The aim of this study is to find out how the time of
isothermal quenching influences the mechanical properties and
structure of ductile iron.
2. Experimental procedure
Fig. 1. The microstructure of ADI as function of austempering
time at temperature T = 300 - 350oC [3]
These are the reasons why ADI is the subject of interest
many researchers also in Poland [5-9]. Although, many
publications appeared since ADI was discovered, it will probably
take a long time before it would by understand as good as other
cast materials. On of the reason is that regular ductile iron which
is starting material before austempering, very often is nonhomogenous material from point of view its chemistry. For
example silicon segregates close to graphite nodules, while
manganese shows opposite segregation. The non uniform
chemical composition influences the process of decomposition of
super-cooled austenite, it mean start and course of austempering
process.
According to Taran and coworkers [10], the process of
decomposition of super-cooled austenite during isothermal
quenching can be described as follows:
a Stage 1: γ(Cγo) → α + γC(γsupersaturated)
b Stage 2: Stability of two phase (α + γC(γsupersaturated)) structure
c Stage 3: γC(γsupersaturated) → α + (ε or Fe3C) carbides
In the first stage of reaction the super-cooled austenite of
equilibrium carbon concentration - γ(Cγo) will decompose into
low carbon bainitc ferrite - α and enriched with carbon austenite –
γC (γsupersaturated). The high carbon concentration in austenite wills
it stabilize, so this carbon stabilized austenite together with
bainitic ferrite form microstructure – ausferrite, which is stable for
some time called “processing window”. This time region is
typically used for producing ADI because it enables to produce
ADI of the best combination strength and ductility. The third
162
The commercial EN-GJS-500-7 grade ductile iron was
selected for the study. The metal was melted in three ton
induction furnace. The spheroidizing treatment was carried out
with FeSiMg alloy using covered ladle method. Molten metal was
inoculated with FeSi75 alloy and then poured into green sand
moulds. The castings were 30mm diameter and 230mm length
rods. From these castings 4mm diameter mini-samples for tensile
were cut. The samples were solution heat treated 1h at 900oC and
then austempered at temperature 275, 300, 325 and 350oC. The
austempering time equaled: 15, 45 and 90 minutes for all
austempering temperatures and additionally 30 and 180 minutes
for austempering at the temperatures 275 and 300oC. The test
specimens were numbered with code use temperature and time
parameters, e.g. 275-45 – denote ADI specimen austempered 45
min. at T 275oC. After final heat treatment the specimens were
grinded to remove surface layer thickness of 0.2mm, which could
be decarburized while solid solution heat treated. The specimen
were tensile tested using Instron 1115 to evaluate yield strength –
Re,0.2, tensile strength - Rm and elongation - A10. For each heat
treatment parameter three specimens were used. Except tensile
test, Brinnel hardness measurements were carried out. In this case
2.5mm steel ball loaded 15s with force F = 187.5N was used.
For microstructure investigations light microscopy, scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM) were employed. Microstructure observations were carried
out on metallographic specimens prepared using conventional
grinding and polishing. For etching 4%HNO3 solution in C2H5OH
was used. The microstructure was observed in Olympus IX-70
light microscope using different magnifications and mode of
observations. The SEM investigations were performed in Hitachi
scanning electron microscope working at U = 20kV, using
secondary electrons (SE). TEM studies were carried out using thin
foil technique. The preparation of thin foils included cutting 3mm
diameter rods from the casting. From these rods 0.1mm thick
discs were sliced with load less wire saw IF-07A. These
preliminary slices were then very carefully grinded, dimpled and
finally ion milled using Gatan equipment. Thin foil were observed
in Philips EM 300 microscope operating at 100kV equipped with
goniometer for specimen tilting in the range ±45 deg.
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time increase. In case of short austempering time only part of
austenite has been enriched with carbon and the rest will
transform into brittle martensite. These results are not surprising,
because the lower austempering time the lower ductility which is
very well known as a property opposite to strength.
In fig.4 the results of hardness measurements in function time
and austempering temperature were depicted. As could be
expected the lower temperature of austempering the higher
hardness of ADI test specimens. So it is no surprising that
hardness measurements reflecting the strength properties of
material confirm the results presented in fig.2.
3. Results
3.1. Mechanical testing
The results of mechanical testing were given in form of
graphs (fig.2). As it could be expected the lower time of
austempering the higher strength properties of ADI test casting.
The increase of ductility while austempered in 350oC may
result from higher amount of stabilized with carbon austenite.
In next graph (fig.3) the changes of elongation with
austempering time and temperature are given. It follows from the
experimental date, that, except austempering at 350oC (dark
points), ductility of ADI test casting decrease with austempering
a)
b)
1600
1400
1400
1200
Re,0.2 & Rm [MPa]
Re,0.2 & Rm [MPa]
1200
1000
800
600
1000
800
600
400
400
Re,0.2
200
Rp,0.2
200
Rm
Rm
0
0
0
30
60
90
120
150
0
180
30
60
Time [min]
c)
d)
1200
120
150
180
1200
1000
1000
800
Re,0.2 & Rm [MPa]
Re,0.2 & Rm [MPa]
90
Time [min]
600
400
800
600
400
Re,0.2
200
Rm
Re,0.2
200
Rm
0
0
30
60
Time [min]
90
120
0
0
30
60
Time [min]
90
120
Fig.2. The values of yield – Re,0.2 and tensile strength – Rm as function of austempering time at the temperature: a – 275oC, b – 300oC, c –
325oC and d – 350oC
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163
4,0
275 deg
3,5
300 deg
325 deg
Elongation A 10 [&%]
3,0
350 deg
2,5
2,0
1,5
1,0
0,5
0,0
0
15
30
45
60
75
90
105 120 135 150 165 180 195
Austempering time [min]
Fig.3. The values of elongation in function of temperature and
time of austempering
Fig.5. The microstructure of austempered specimen (350-90)
450
Brinell Hardness
400
350
300
250
200
275 deg
300 deg
325 deg
150
350 deg
100
0
15
30
45
60
75
90
105
120
135
150
165
180
Austempering time [min]
Fig.4. The change of hardness in function of temperature and time
of austempering
3.2. Structure investigations
Metallography
Fig.4a illustrate an example of typical microstructure 500 7
grade ductile iron and fig.4b the microstructure this material after
90 minutes austempering at temperature 350oC. The needle like
matrix microstructure of austempered ductile iron consists of
bainitic ferrite and stabilized austenite. The high temperature
austempering prevent formation of martensite which usually is
one of phases formed in case of low temperature austempering.
SEM observations
In fig.6 the example of fracture surface of tensile tested high
temperature austempered ductile iron is presented. This mode of
fracture surface is typical for ductile materials. It is characterized
with dimples which depth reflects plasticity of matrix [12]. The
depth of these dimples reflect the ductility of the material in this
case ductility of ausferrite matrix.
164
Fig.6. The fracture surface of 350-90 ADI specimen
TEM observations
Next micrographs illustrate results of TEM observations. In
first of them (fig.7) the structure of low temperature austempered
ductile iron is visible. The needle-like structure is characteristic
for the matrix ADI in all cases, either for low- and high
temperature austempering. The most outstanding difference for
short time austempering at low temperature is martensite which
example was showed in fig.7b. In this picture it posses the shape
of needles with many microtwins [13] inside formed by shearing.
The next electron micrograph (fig.8) shows the example of
the same ADI but after 15 minutes austempering at T = 350oC.
First electron micrograph (fig.8a) shows austenite grain with
many staking faults inside. On the other hand ferrite grains are
distorted with high density dislocations tangles and very small
dislocation loops formed by vacancy coalescence (fig.8b).
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a.
clear increase of Re,0.2 is observed. This cause that for 15 minutes
austempering the difference ∆R = Rm – Re,0.2 ≈ 700MPa, while
after 180 minutes austempering this difference equals less than
170MPa (fig.2a). In case of short time austempering strong strain
hardening is observed. The phases which can exist in the structure
are: bainitic ferrite, carbon stabilized austenite and some
martensite. Knowing the crystal lattice and properties of these
phases it is easy to conclude that only austenite with FCC lattice
might be responsible for strain hardening. This conclusion is
partly confirmed by changes of elongation with time (fig.3). In
this picture it is clear visible that the highest elongation of ADI
specimen’s austempered at 275oC is observed for short time heat
treatment. As documented in fig.6b, except ausferrite, some
amount of martensite forms. The long needles of martensite are
highly distorted with enormous number very fine microtwins, the
thickness of 10nm.
b.
a.
↓ Martensite
Fig.7. The microstructure: a – needle-like structure 275-15
austempered ductile iron (x 18.000), b – martensite in the same
ADI observed at very high magnification (x 90.000)
b.
The next set of electron micrographs (fig.9) shows the
structure of ADI specimen austempered 180 minutes at 275oC. In
the upper right corner of first SAD pattern was located (fig.7a).
The careful analysis of SAD discovers very tiny diffraction
spots located close to the central 000 diffraction spot. These spots
correspond to the carbides which must be in shape of relative thin
plates. This conclusion based on the shape of the reflexes which
shows so called ‘streaks” [14]. As follows from the dark-field
[15] micrograph obtained with the use just one of those diffuse
tiny reflexes, the carbides are located at the ferrite grain
boundaries (see white rows of the carbides in fig.9b).
4. Discussion
Since there is no space enough for detailed discussion we start
with very short analysis the results of mechanical testing. First of
all is worth noting the specific behavior of tensile strength which
is different in value for different austempering temperature but
practically did not change a lot at the time region up to 90 min.
The similar behavior, although not so clear is observed for yield
stress, except that for austempering at 275oC. In this case very
Fig.8. The structure of 350-15 ADI: a – austenite with stacking
faults, b – austenite and bainitic ferrite (the last with many
dislocations and dislocation loops)
The 180 minutes austempering leads to decomposition of
carbon stabilized austenite from which thin plate-like carbides
precipitates. These are located at grain boundaries (fig.9b) which
are most preferred sites for nucleation and growth.
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165
a.
5. The decrease of elongation in high temperature austempering
with quenching time resulted partly from relative austenite
amount decrease and further from precipitation of plate-like
thin carbides at ferrite grain boundaries.
References
b.
Fig.9. The structure of 275-180 ADI: a – highly deformed
ausferrite – bright field and b – the dark-field micrograph from
the area shoved above (magnification x18.000)
5. Conclusions
On the basis of the experimental results including mechanical
testing and structure investigations using, either conventional
metallography, SEM and TEM observations and their analysis
presented above, following conclusion can be proposed:
1. The mechanical properties of ADI depend greatly on
austempering
parameters
which
produce
different
microstructure including in different proportion: ferrite,
austenite and sometimes martensite which forms in low
temperature austempering.
2. Short time isothermal quenching of ductile iron lead to
austempered ductile iron with mechanical properties
characterized by relative low ultimate yield strength compare to
its tensile strength.
3. The yield strength of low temperature austempered ductile iron
increase while tensile tested and it is caused by strain hardening
of austenite.
4. The decrease of elongation in low temperature austempered
ductile iron with time increase resulted from decrease of
relative austenite amount in matrix structure.
166
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