effects of section thickness on the microstructure and...
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Journal of Materials Sciences and Applications
2018; 4(2): 17-30
http://www.aascit.org/journal/jmsa
ISSN: 2381-0998 (Print); ISSN: 2381-1005 (Online)
Effects of Section Thickness on the Microstructure and Mechanical Properties of Austempered Ductile Iron
John Oluyemi Olawale*, Simeon Ademola Ibitoye, Kunle Michael Oluwasegun
Department of Material Science and Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria
Email address
*Corresponding author
Citation John Oluyemi Olawale, Simeon Ademola Ibitoye, Kunle Michael Oluwasegun. Effects of Section Thickness on the Microstructure and
Mechanical Properties of Austempered Ductile Iron. Journal of Materials Sciences and Applications. Vol. 4, No. 2, 2018, pp. 17-30.
Received: February 8, 2018; Accepted: March 1, 2018; Published: March 23, 2018
Abstract: This study aimed at evaluating the effects of castings dimensions on the microstructure and mechanical properties
of austempered ductile iron (ADI). Ductile iron that conforms to ASTM A536 65-45-12 grade was produced, cast into Y-block,
machined into section thicknesses ranging from 5 to 25 mm, and isothermally heat treated at 300°C and 375°C austempering
temperatures to produce ADI. Thereafter, the microstructure and mechanical properties were characterized. The
microstructures were characterized using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) method. Their
strength and hardness were also evaluated in accordance with ASTM standard procedures. The microstructure revealed
significant coarsening of ausferrite as the section thickness increases with 375°C austempering temperature coarser. The
mechanical test results indicated that strengths and hardness value decreases with increase in section thickness while the
percentage elongation and impact strength increases with it. The study concluded that the structure and mechanical properties
of ADI strongly depends on the castings dimensions.
Keywords: Ductile Iron, Austempered Ductile Iron, Ausferrite, Austempering Temperature, Section Thickness
1. Introduction
Austempered ductile iron (ADI) is an engineering material
with exceptional combination of mechanical properties and
marked potential for numerous applications [1-3]. Nearly
twice as strong as pearlitic ductile iron, ADI still retains high
elongation and toughness [4-6]. This combination provides a
material with superior wear resistance and fatigue strength,
thus enabling designers to reduce component weight and
costs for equivalent or improved performance [7-10]. These
properties are achieved by heat treatment of ductile iron
using an austempering process. The mechanical properties of
ADI are primarily determined by the metal matrix.
Austempered ductile iron has a unique matrix called
ausferrite [11-13]. The ausferrite microstructure consists of
acicular ferrite in carbon-enriched austenite.
Austempered ductile iron offers the design engineer the best
combination of low cost, design flexibility, good machinability,
high strength-to-weight ratio and good toughness, wear
resistance and fatigue strength [14]. It offers this superior
combination of properties because it can be cast like any other
member of the ductile iron family, thus offering all the
production advantages of a conventional ductile iron casting.
Subsequently it is subjected to the austempering process to
produce mechanical properties that are superior to
conventional ductile iron, cast and forged steels.
The production of a high quality casting is essential but, by
itself, not a sufficient condition to ensure optimum properties
in ADI. The casting must be heat treated properly taking into
account the interaction between casting section thickness,
composition, microstructure and the desired properties in the
austempered casting. Therefore, to ensure that the desired
mechanical properties are obtained precise control of the
austempering transformation is necessary [15]. This is
achieved through proper control of iron chemistry and quality,
and strict control of austempering temperature and time [16].
An understanding of the relationship between the
austempering transformation and the resultant microstructures
18 John Oluyemi Olawale et al.: Effects of Section Thickness on the Microstructure and Mechanical
Properties of Austempered Ductile Iron
developed is the key to effective process control.
The role of alloying elements [9, 17-23],
effects of
austenitising temperature [7, 9, 24-26], effects of
austenitising time [27], effects of austempering temperature
[9, 27-33], and effects of austempering time [31, 32, 34, 35]
on the microstructure and mechanical properties of ADI have
been investigated. However, the effects of casting section
thickness have not been reported, hence this study.
2. Methodology
2.1. Production of ADI
The ductile iron used for this study was produced to
conform to ASTM A536 65-45-12 grade of ductile iron [36].
The charges which consist of pig iron, spheroidal graphite (SG)
returns, steel scraps, 75% foundry grade ferrosilicon, 70%
grade ferromanganese and petroleum coke were melted in 500
kg capacity of coreless induction. The molten iron was treated
in the tundish ladle at 1450°C with Fe-Si-Mg master alloy
producing Mg content in the range of 0.04 to 0.05 wt% in the
melt. Proper post inoculation was also carried out during
tapping and pouring of metal to ensure the high level of nodule
count. The final chemistry of the treated iron was given in
Table 1. The treated iron was poured into a green sand mold to
cast Y block as per ASTM 897/897M-16 [37].
Table 1. Chemical composition of produced ductile iron.
Elements C Si Mn P S Cr Ni Mo Cu Mg
Composition 3.61 2.54 0.40 0.017 0.023 0.048 0.015 0.003 0.016 0.05
From the leg part of this block flat samples of 5, 10, 15, 20
and 25 mm thickness were machined. Thereafter, ADI was
produced by an isothermal heat treatment known as
austempering. The austempering heat treatment involved two
steps. The sample was initially heated to an austenitising
temperature of 820°C, held it at this temperature for one hour
sufficient to get the entire part to the temperature and to
saturate the austenite with carbon such that the matrix was
fully austenitic (γ). It was then cooled rapidly to avoid
formation of pearlite to an intermediate temperature of 300°C
and held at this temperature for two hours for complete
transformation of austenite to ausferrite. Finally, it was air
cooled to room temperature. For another set of samples, the
procedure was repeated but austempered at intermediate
temperature of 375°C and soaked at this temperature also for
two hours.
2.2. Microstructural Characterizations
Microstructural characterizations of ADI’s under
investigation were carried-out using Scanning Electron
Microscopy (SEM) and X-Ray Diffraction (XRD) method.
The samples were prepared for micro-examination in
accordance with ASTM E3 – 01 [38] and were etched in
accordance with ASTM E407 – 07 [39]. Samples from each
of the various section thicknesses were taken through
metallography process: sample selection, mounting, grinding,
polishing and etching. Thereafter, morphology of acicular
ferrite and carbon-enriched stabilized austenite in series of
austempered ductile iron produced were characterized by
SEM after etching with 2% nital. XRD was performed at 40
KV and 100 mA using a Cu- Kα target diffractometer.
Scanning was done in angular range 2θ from 42° to 46° and
72° to 92° at a scanning speed of 0.25°/min and 1°/min
respectively. The profile was analyzed on computer by using
X‟ Pert High Score Software to obtain the peak position and
integrated intensities of the austenite and ferrite.
The mean free path of dislocation (�) in ferrite (�) was
determined (which represents the mean particle size of ferrite)
from the X-ray diffraction peaks of ferrite using the Scherrer
equation [40]:
�� = �.�
�� � (1)
where � is the wave length, � is the breadth of (211) peak of
ferrite at half height in radians and � is the Bragg angle.
2.3. Mechanical Testing
Mechanical testing was conducted on ADI specimens. The
material properties namely: yield strength, tensile strength,
percentage elongation and hardness stated in ASTM
A897/897M-16 ADI specification were determined. The
impact strength of the specimens was also determined.
2.3.1. Tensile Testing
Tensile testing of all the specimens was conducted as per
ASTM Standard E-8 [41]. Five test specimens for each
section thickness were tested at room temperature with a
crosshead speed of 1 mm min−1
using a computerised Instron
Electromechanical Testing Machine (Model 3369). Load –
displacement plots were obtained on an X – Y recorder and
ultimate tensile strength, yield strength and percentage
elongation values were calculated from this load –
displacement diagrams.
2.3.2. Hardness Testing
The specimens for each section thickness were subjected
to the Brinell hardness test according to ASTM E10 – 15a
[42] using Monsato Tensometer (Model W) in compression
mode. A 10 mm indenter made of a hardened steel ball was
mounted in a suitable holder and forced with a load of 3000
kgf into prepared surface of the specimens polished to 600
microns using a dwell time of 15 seconds. The diameter of
the impression left by the ball was measured using the Brinell
calibrated hand lens and the corresponding Brinell hardness
number was determined. The hardness of each test pieces
was taken at five different points and the average was
determined as the hardness value. The Brinell hardness
Journal of Materials Sciences and Applications 2018; 4(2): 17-30 19
number (BHN) was evaluated according to Equation 2:
��� = ��
��[� � �(�� � ��)] (2)
Where
! = "#$%&'� (%)� (*+,)
- = -.)#'/'0 %, /ℎ' &$ℎ'0.2)( .3�'3/'0 (##)
� = -.)#'/'0 %, /ℎ' 0'&4(/.3+ .3�'3/)/.%3 (##)
2.3.3. Impact Testing
Impact testing of all the test specimens was conducted as
per ASTM Standard E23 – 07 [43]. Five test specimens were
tested for each section thickness. The tests were carried out
using Izod impact test method on Houndsfield Balance
Impact Testing Machine. The amount of impact energy
absorbed by the specimen before yielding was read off on the
calibrated scale attached to the machine as a measure of
impact strength in Joules.
3. Results and Discussion
Figures 1 and 2 are SEM image of specimens’
austempered at 300 and 375°C respectively. The structures
showing a matrix of acicular ferrite in carbon-enriched
austenite (called ausferrite). The structure of the specimens
austempered at 300°C have fine needles of acicular ferrite
with carbon stabilized austenite regions present as silver
between them (Figure 1) while the structure of specimens
austempered at 375°C have broad acicular ferrite needles
within blocky carbon enriched stabilized austenite (Figure 2).
This increase in length of acicular ferrite needles can be
attributed to the greater coarsening of carbon stabilized
austenite grains as the section thickness increases. Hence,
significant coarsening of ausferrite was observed as the
section thickness increases in the considered austempering
temperatures (Figures 1 and 2).
The structure resulted from austempering at such relatively
higher temperature of 375°C (Figure 2) showed a
homogeneous structure of coarse ausferrite associated with
relative higher diffusion and growth at such temperature.
Lowering the austempering temperature to 300°C, both
diffusion and growth rates are decreased and the structure
consists of fine needles of ausferrite. Since nucleation
depends on the supercooling, at lower austempering
temperature the degree of supercooling of austenite is large,
more ferrite is nucleated, and at the same time, because of
lower diffusion rate of carbon at this temperature, the growth
rate of ferrite is low, so the ferrite becomes finer in nature.
Thus, at austempering temperature of 300°C, the acicular
ferrite volume fraction is higher, i.e. the carbon stabilized
austenite volume fraction is lower, and both acicular ferrite
and carbon stabilized austenite are finer in nature. However,
at a higher austempering temperature, because of the lower
supercooling, the nucleation of ferrite is less, and at the same
time, the higher diffusion rate of carbon causes the ferrite to
become coarse in nature. Thus, at austempering temperature
of 375°C we obtain a higher volume fraction of austenite, but
both acicular ferrite and carbon stabilized austenite are
coarser in nature. Also, as the section thickness increases the
length of the ferrite needles is generally found to be
increasing. The increase in the length of the ferrite needles
favors the coarsening of carbon stabilized austenite grains
and hence results in coarse structure.
The XRD pattern of produced ADI is as presented in
Figures 3 to 7. From the pattern, the peak values were
predominantly austenite (γ) and ferrite (α) phases. These
diffraction peaks were identified as 5(111), �(110), 5(002),
�(200), 5(220), �(211) and 5(311)when Bragg conditions
is satisfied at 50.2°, 52.3°, 58.4°, 77.1°, 88.5°, 99.7° and
109.5° respectively. The mean free path of dislocation (d) in
ferrite (α) as presented in Table 2 was determined from the
XRD profile from the breadth of (211) diffraction peaks of
ferrite using Equation 1. The plot of ferritic cell size (��)
against the section thickness is as presented in Figure 8.
From this figure the mean particle size or mean free path of
dislocation motion were found to increase with section
thickness and austempering temperature.
The observed trend in the mean particle size with section
thickness and austempering temperature is due to the breadth
of diffraction curve of �(211). The breadth (�) is broading
of diffraction line measured at intensity equal to half
maximum intensity. This according to Cullity (1978) is used
to determine the particle size effect of any crystal. The more
the value of angular width the finer is the particle size. As
shown Figures 3 – 7 and as presented in Table 2 the width of
diffraction curve (�) decreases as the section thickness and
austempering temperature increases because the angular
range decreases as these parameters increases. The increase
in length of the ferrite needles as observed in Figures 1 and 2
with section thickness and austempering temperature can be
attributed to the increase in the mean particle size. This is
also confirming while the ausferrite becomes coarse with
section thickness and austempering temperature.
20 John Oluyemi Olawale et al.: Effects of Section Thickness on the Microstructure and Mechanical
Properties of Austempered Ductile Iron
Figure 1. SEM image of ADI austempered at 300°C showing graphite nodules in ausferrite matrix for casting section thickness of (a): 5 mm, (b): 10 mm, (c):
15 mm, (d): 20 mm, (e): 25 mm.
Journal of Materials Sciences and Applications 2018; 4(2): 17-30 21
Figure 2. SEM image of ADI austempered at 375°C showing graphite nodules in ausferrite matrix for casting section thickness of (a): 5 mm, (b): 10 mm, (c):
15 mm, (d): 20 mm, (e): 25 mm.
22 John Oluyemi Olawale et al.: Effects of Section Thickness on the Microstructure and Mechanical
Properties of Austempered Ductile Iron
Figure 3. X-ray diffraction pattern of ADI austempered at (a) 300°C and (b) 375°C with 5 mm section thickness.
Journal of Materials Sciences and Applications 2018; 4(2): 17-30 23
Figure 4. X-ray diffraction pattern of ADI austempered at (a) 300°C and (b) 375°C with 10 mm section thickness.
Figure 5. X-ray diffraction pattern of ADI austempered at (a) 300°C and (b) 375°C with 15 mm section thickness.
24 John Oluyemi Olawale et al.: Effects of Section Thickness on the Microstructure and Mechanical
Properties of Austempered Ductile Iron
Figure 6. X-ray diffraction pattern of ADI austempered at (a) 300°C and (b) 375°C with 20 mm section thickness.
Journal of Materials Sciences and Applications 2018; 4(2): 17-30 25
Figure 7. X-ray diffraction pattern of ADI austempered at (a) 300°C and (b) 375°C with 25 mm section thickness.
Table 2. Mean particle size of ferrite.
Section Thickness
(mm)
Austempering
Temperature (°C)
Bragg Angle (2:) of
(211) Peak of Ferrite : =
;:
;
Breath of (211) Peak of
Ferrite (<)
Particle Size of Ferrite
(nm) => � ?.@ A
<BCD:
5 300 99.662 49.831 0.94 0.2655
375 99.721 49.861 0.48 0.5203
10 300 99.797 49.899 0.77 0.3246
375 99.610 49.805 0.30 0.8316
15 300 99.773 49.887 0.62 0.4031
375 99.742 49.871 0.21 1.1896
20 300 99.710 49.855 0.50 0.4995
375 99.643 49.822 0.16 1.5598
25 300 99.786 49.893 0.41 0.6096
375 99.747 49.874 0.12 2.0819
The results of mechanical properties are presented in Table
3. The ultimate tensile strength, 0.2% offset yield strength
and percentage elongations were obtained from the stress
versus strain plot. The correlation between tensile strength
and section thickness is shown in Figure 9. The figure
indicated that tensile properties of specimens’ austempered at
300°C are higher than those austempered at 375°C. In
addition, a decrease in tensile properties was noticed as
section thickness increases. For samples austempered at
temperature EF of 300°C, the ultimate tensile strength
decreases from 1,226 to 903 MPa as section thickness
increases from 5 to 25 mm, whereas at EF of 375°C it
decreases from 1,144 to 852 MPa. However, percentage
elongation increases linearly with increase in the section
thickness (Figure 9). Figure 9 also reveals that percentage
elongation increases with the austempering temperature.
Table 3. Results of mechanical testing.
Section Thickness
(mm)
Austempering
Temperature (°C)
Yield Strength
(MPa)
Tensile Strength
(MPa) Elongation (%) Hardness (BHN)
Impact Strength
(J)
DI - 365 536 11 185 25
5 300 883 1226 6 372 -
375 794 1144 8 340 -
10 300 814 1164 8 365 43
375 788 1138 9 338 48
15 300 768 1118 9 347 51
375 728 1078 11 313 59
20 300 652 986 12 319 63
375 606 925 13 289 72
25 300 590 903 14 297 75
375 551 852 16 270 86
26 John Oluyemi Olawale et al.: Effects of Section Thickness on the Microstructure and Mechanical
Properties of Austempered Ductile Iron
Figure 8. Influence of section thickness and austempering temperature on the mean particles size of acicular ferrite (��).
Figure 9. Effect of section thickness and austempering temperature on strength and percentage elongation.
Figure 10 presents the average hardness values measured
at different section thickness (See Tables 4 and 5). It is
observed that specimens austempered at temperature of
300°C show higher hardness values compared to those
austempered at 375°C. At 300°C, the average hardness value
decreases from 372 to 297 BHN whereas at EF 375°C it
decreases from 340 to 270 BHN with increasing section
thickness from 5 to 25 mm. The effect of section thickness on
impact energy of specimens’ austempered at 300 and 375°C
is also illustrated in Figure 10 (See Tables 6 and 7). The
results show that the impact energy increases gradually with
increasing section thickness and austempering temperature
for which higher impact energy is observed at 375°C (48 –
86 J) than at 300°C (43 – 75 J).
Journal of Materials Sciences and Applications 2018; 4(2): 17-30 27
Figure 10. Effect of section thickness and austempering temperature on hardness and impact energy.
Table 4. Hardness test results of samples austempered at 300°C.
Section Thickness
(mm)
Diameter of Indentation (mm) Brinell Hardness
Number (BHN) Samples
Mean 1 2 3 4 5
DI 9.80 9.54 9.27 9.80 9.27 9.54 189
5 3.16 3.15 3.17 3.16 3.16 3.16 372
10 3.19 3.19 3.20 3.18 3.19 3.19 365
15 3.30 3.30 3.27 3.24 3.24 3.27 347
20 3.39 3.39 3.43 3.41 3.43 3.41 319
25 3.50 3.55 3.55 3.49 3.54 3.53 297
Table 5. Hardness test results of samples austempered at 375°C.
Section Thickness
(mm)
Diameter of Indentation (mm) Brinell Hardness
Number (BHN) Samples
Mean 1 2 3 4 5
DI 9.80 9.54 9.27 9.80 9.27 9.54 189
5 3.32 3.28 3.28 3.30 3.32 3.30 340
10 3.33 3.33 3.30 3.31 3.30 3.31 338
15 3.41 3.42 3.45 3.48 3.44 3.44 313
20 3.57 3.55 3.56 3.57 3.60 3.57 289
25 3.73 3.69 3.65 3.68 3.70 3.69 270
Table 6. Impact test results of samples austempered at 300°C.
Section Thickness
(mm)
Impact Energy (J)
Standard Deviation Samples Mean
1 2 3 4 5
10 4.29 43.4 43.0 42.4 43.4 43 0.37
15 51.4 50.0 54.4 49.0 52.0 51 1.54
20 65.5 60.0 62.1 64.4 65.6 63 1.93
25 76.5 74.5 73.9 78.1 72.0 75 2.11
28 John Oluyemi Olawale et al.: Effects of Section Thickness on the Microstructure and Mechanical
Properties of Austempered Ductile Iron
Table 7. Impact test results of samples austempered at 375°C.
Section Thickness
(mm)
Impact Energy (J)
Standard Deviation Samples Mean
1 2 3 4 5
10 48.4 47.2 51.0 45.4 48.0 48 1.82
15 58.2 61.5 61.5 50.7 63.2 59 4.46
20 70.0 72.8 73.6 75.2 68.3 72 2.50
25 83.5 87.0 82.0 85.5 92.0 86 3.45
The improvement in yield strength and ultimate tensile
strength are attributed to the grain size and can be explained
from the microstructures perspectives. The finer these grains
are the more the boundaries. During plastic deformation, slip
or dislocation movement must take place across these grain
boundaries. Since polycrystalline grains are of different
crystallographic orientations at the grain boundaries, a
dislocation passing from one grain to another will have to
change its direction of motion. Such changes of direction
cause impediment to dislocation movement, and increases
both the yield strength and ultimate tensile strength. Since 5
mm section thickness samples have the highest number of
grain boundaries, dislocation movement becomes more and
more difficult during plastic deformation. This is responsible
for highest yield strength and ultimate tensile strength
observed as the section thickness of the samples decreases.
Also, at high austempering temperature (375°C) the grain
structures become coarse and hence have lower yield and
ultimate tensile strength when compared with samples
austempered at lower temperature (300°C).
Percentage elongation increases as the section thickness
increases from 5 to 25 mm and with the austempering
temperature. This is partly due to increase in grain coarsening
which leads to an increase in the grain boundary area. This
increases the amount of energy required for the movement of
dislocations needed to cause fracture [44-46]. Thus, the
material is able to withstand a higher plastic deformation
before the final fracture. The impact strength followed the
same trends as percentage elongation with thicker sections
and higher austempering temperature having the highest
values. This is because impact strength is also a measure of
material’s ductility, and ductility is inversely related to
strength.
4. Conclusion
From the outcome of this study it can be concluded that
the structure and mechanical properties of ADI strongly
depends on the castings section thickness. The microstructure
revealed significant coarsening of ausferrite as the section
thickness increases with higher austempering temperature
coarser. The ausferrite becomes coarse, mean particle size of
acicular ferrite increases, and length of ferrite needles of
ausferrite increases as the section thickness increases. The
mechanical test results revealed that strengths and hardness
value decreases while the ductility and impact strength
increases with the section thickness.
Acknowledgements
This work was technically supported by Nigerian
Foundries Limited, Sango-Ota, Ogun State, Nigeria. The
authors are sincerely grateful to their contribution.
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