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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