Proceedings of the 2 ICCTEM-2014 - Manipaleprints.manipal.edu/140204/1/ICCTEM-2014 SSS-1.pdf · Proceedings of the 2 nd International Conference on Current Trends in Engineering and

Proceedings of the 2nd

International Conference on Current Trends in Engineering and Management ICCTEM-2014

17 – 19, July 2014, Mysore, Karnataka, India

Copyright © 2014 by VVCE, Mysore

Mechanical Property Assessment of Austempered and Conventionally Hardened AISI 4340 Steel

S. S. Sharma1, P. R. Prabhu

2, Gowrishankar M. C

3, A. V. B. Sudhakar

4

1Professor, Department of Mechanical & Manufacturing Engineering, MIT Manipal

2Asst. Professor Sr. Scale, Department of Mechanical & Manufacturing Engineering, MIT Manipal

3Asst. Professor Sr. Scale, Department of Mechanical & Manufacturing Engineering, MIT Manipal

4Student, Department of Mechanical & Manufacturing Engineering, MIT Manipal

1E-mail ID – [email protected]



Abstract: The chemical composition and mechanical properties of steel decide its applicability for manufacturing various components in different areas of engineering interests. Heat treatment processes are commonly used to enhance the required properties of steel with or without change in chemical composition. The present work aims to perform conventional hardening and Austempering treatment with experimental investigation of the effect of austempering and conventional hardening (quenching) on AISI 4340 steel. Different tests like tensile, torsion, hardness, impact and microstructure analysis are carried out in as bought and heat treated conditions. It was found that Austempering improves tensile, torsional and impact strength whereas a marginal decrease in hardness is found as compared to conventional hardening (direct quenching).Lower bainitic and martensitic structures are observed in austempered and conventionally hardened specimens.

Keywords: Heat treatment, austempering, martensite, bainite, hardening, tensile. 1. INTRODUCTION

In today’s world, structural materials require various

properties such as high strength, excellent toughness and

wear resistant due to the demands for high performance

and severe service environments of machine components.

In order to meet these demands, many studies have been

performed on steels especially alloy steels. However,

little attention has been paid to the tensile and torsional

behaviour, toughness and hardness of specimens which

have been given a heat treatment.

Steel over the years has proved to be the most important,

multi-functional and most adaptable material in

automotive, aircraft and general engineering applications.

Nickel, Chromium, Molybdenum, silicon steels are best

suited for applications requiring high tensile strength and

toughness. In recent years, extensive studies on the

improvement of mechanical properties of these materials

have been carried out. Austempering as a heat treatment

process on engineering materials increases the yield

strength, wear resistance, hardness and toughness

properties. Engineered systems are often set by intended

or unintended stresses due to heavy machining, rapid

solidification, bombardment of foreign materials, heat

treatment conditions adopted and thermal cycling on

components. The conventional hardening process may

increase the hardness and ultimate tensile strength but

results in the reduction of toughness of the material.

Hence, a criterion to enhance the properties of materials

such that the maximum load that a component can sustain

is paramount importance.

Steel is one of the important alloy where a variety of

properties are possible by altering heating and cooling

cycle i.e., heat treatment. The tailor made properties are

possible in steels by selecting suitable heat treatment

process according to the application. A wide variety of

thermal hardening techniques are available in the heat

treatment engineer tool kit like direct quenching, stepped

quenching, timed quenching, spray quenching (hardening

with self-tempering), martempering, austempering etc.

Out of these treatment methods austempering method has

the unique advantage of moderate hardness combined

with good toughness and tensional strength. At the same

time generally there is no retained austenite and residual

stresses if the process is designed accordingly. The micro

structure consists of needle like ferrite and well dispersed

carbides as saturated phases. In appearance it resembles

like single phase martensite because of the degree of

fineness of the micro constituents [1–4]. Austempered or

interrupted quenching steels possess optimum hardness

balanced with tensile properties, known as toughness.

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Commonly austempered steels include AISI 1090, 4140,

4340, 6050, EN 31and SAE 52100 [5–8].

Austempering is a method of hardening by heating to the

austenitizing temperature i.e., 300C to 50

0C above upper

critical temperature in the case of hypo eutectoid steel

followed by isothermal quench in a medium maintained

above temperature, but below the nose of isothermal

transformation diagram and holding the steel in this

medium until austenite completely transforms into

bainite. Lower the temperature range better is the

dispersion of two saturated phases, which enhances

toughness of steel. The quenching severity must be faster

enough so that continuous cooling curve do not cut the

transformation beginning curve of isothermal

transformation diagram i.e., cooling rate is equal to or

greater than critical cooling rate (CCR) and temperature

and duration of isothermal holding in later stage is

designed in such a way that decomposition of austenite

into a well dispersed tiny two phase mixture as ferrite and

carbide is fully completed. In the case of conventionally

hardenable steels like HSLA, Cr-Mo, Ni-Mo where

martensite forms on air cooling, bainite formation also

takes place by continuous slow cooling [9–13]. In such

cases bainitic formation results with retained austenite

and martensite so that bainitic transformation is

incomplete. This type of transformation results in

marginal residual stresses compared to isothermal

transformation. Higher the temperature range of bainite,

lower is the hardness and strength with increased ductility

[14–19].

In this view different tests like hardness, impact, wear

and microstructure analysis, are carried out before and

after heat treatment process. It is found that as bought

steel has less hardness and more wear prone, while

martempered steel is hardest and least vulnerable to wear.

Austempered steel has got highest impact strength and it

is depend upon isothermal holding duration. Least

toughness is observed in conventionally hardened. On the

other hand, qualitative and quantitative studies are

performed to ascertain the influence of austempering heat

treatment process on the properties.

2. EXPERIMENTAL METHOD

The chemical composition of the

investigated steel is determined by optical emission

spectrometer and shown in Table 1.

Table1: Composition of steel used

Component C Si Mn Ni Cr Mo

Wt % 0.4 0.25 0.7 1.85 0.8 0.25

The specimens are prepared by machining from as-

bought steel according to ASTM standard in three sets for

tensile, torsion and impact. Each set consists of three

specimens each for tensile, torsion and impact tests. The

lower bainitic temperature range for AISI 4340 steel is

between 280oC and 350

oC (From Isothermal

Transformation diagram).

One set of as-bought (without heat treatment) specimens

are subjected to austempering on heating to 850oC for 2

hours and quenching in oil bath maintained at 300oC for

about 200-220 minutes isothermally. Second set is

conventionally hardened by heating to 850oC for 2 hours

and quenching in oil bath maintained at room temperature

(30oC). The third set is tested without heat treatment to

compare the properties between austempering,

conventional hardening and without hardening.

2.1 Mechanical testing

Tensile test: All the tensile specimens are subjected to

tensile test on Electronic Tensometer. The load versus

elongation graphs are recorded and analysed.

Fig. 1: Tensile test specimen (All dimensions are in

mm)

Torsion Test: All the torsion testing specimens are

subjected to torsion test on torsion testing machine. The

torque versus angular deflection graphs are plotted and

analysed.

Fig. 2: Torsion test specimen (All dimensions are in

mm)

Fig. 3: Impact test specimen (All dimensions are in

mm)

Hardness test: The specimens are polished with 200

series of emery papers before the test. The Rockwell

hardness tester is employed for the hardness

measurement.

135





Impact test: The charpy test is conducted for all the

samples. The energy absorbed before failure of the

specimen is noted in each case.

Microstructure examination: samples are prepared by

polishing with different grades of emery papers and

etched with Nital solution. Micro structure of the non-

heat treated, austempered and conventionally hardened

AISI 4340 steel is recorded using metallurgical

microscope.

3. RESULTS AND DISCUSSION

3.1 Tensile test

Figures 4 and 5 show the Load versus deformation

graphs for as-bought and conventionally hardened steel.

The as-bought specimen shows clear cut yield point, the

typical ductile behaviour of steel. The conventionally

hardened and austempered specimens do not show clear

yield points. The ductility of as-bought steel is higher

than austempered and conventionally hardened steel. The

area under the load versus deformation is larger for

austempered one as compared to other two. This is the

measure of toughness. The marginal loss in strength is

Fig. 4: Load vs. elongation graph for as-bought

specimen

observed in austempered specimen over conventionally

hardened with the benefit of higher toughness. The

increased deformation with higher strength shows the

increase in stiffness of the material. This is the typical

behaviour of lower bainitic structure. The better

dispersion of fine ferrite and carbides is responsible for

this behaviour. Tensile results especially ductility is poor

in conventionally hardened specimen. A little permanent

elongation is recorded in conventionally hardened

specimen. The fractured surface shows almost brittle

failure without necking. It is the typical behaviour of

unaged martensitic structure. Figures 6, 7 and 8 show the

tensile behaviour of the specimen in with and without

treatment condition.

Fig. 5: Load vs. elongation graph for

austempered specimen

7900

16200 15800

0

5000

10000

15000

20000

Ult

ima

teT

en

sile

Lo

ad

(N

)

Tensile test results

Fig. 6: Tensile load vs. type of modification

3.8

1.3

2.8

0

1

2

3

4

Pe

ak

de

form

ati

on

(mm

)


Fig. 7: Peak deformation vs. type of modification

5.6

1.4

3.9

0

1

2

3

4

5

6

Bre

ak

de

form

ati

on

(mm

)


Fig. 8: Break deformation vs. type of modification

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 1 2 3 4 5 6 7

Lo

ad

in

N

Deformation in mm

Load Vs Deformation

136





3.2 Torsion Test

Figures 9, 10, 11 and 12 show the torsional

behaviour of the specimen in the given condition. Higher

torque is observed in austempered one as compared to as

bought specimen. Conventionally hardened specimen also

shows lesser torque with lesser yield angular

displacement. Austempered shows higher yield angular

deflection and is at par with as bought specimen. It also

indicates the increase in shear strength of the material

during Austempering. This behaviour is due to the

uniform dispersion of fine ferrite and carbide phases.

Fig. 9: Torque vs. Angular deflection graphs for as

bought specimen

Fig. 10: Torque vs. Angular deflection graphs for

austempered specimen

10400 890012850

0

10000

20000

To

rqu

e (

Kg

-cm

)

Torsion test results

Fig. 11: Torque vs. type of modification

Fig. 12: Angular deflection vs. type of modification

3.3 Hardness test

Figure 13 shows the bulk hardness of the specimen

with respect to the treatment given. Excellent hardness

value is observed in conventionally hardened specimen

compare to as bought. A marginal decrease in hardness is

due to the behaviour of super saturated solid solution

martensite structure.

Fig. 13: Rockwell hardness number vs. type of

modification

3.4 Impact test

Figure 14 shows the ability of the specimen to resist

impact load. The energy absorbed before failure under

impact load is extremely higher in austempered specimen

compare to the other two conditions. It also suggests that

further tempering may not be required after the treatment.

Fig. 14: Energy absorbed vs. type of modification

137





3.5 Microstructure examination

Figure 15 shows the microstructure of different

specimens in all the three conditions at 500X

magnification. Clear distinguished carbide and ferritic

phases are seen in as bought specimen. Conventionally

hardened specimen Shows typical band like single

martensitic phase. Austempered one shows needle type

well dispersed fine phases. This is the typical pattern of

bainitic structure.

(a)

(b)

(c)

Fig. 15: Microstructure of (a) as bought

(b) Conventionally hardened (c) Austempered

specimen at 500X

4. CONCLUSIONS

The UTS of conventionally hardened and austempered

specimens are comparable but peak and break elongation

of conventionally hardened is far less than that of

austempered specimen. This indicates the increase in

elastic limit of the material during austempering compare

to conventionally hardened one. However, the following

conclusions are made during metallography, tensile,

torsion, impact and hardness tests.

· Tensile graph shows clear and sharp yield strength in

as bought specimen.

· Ductility of as bought specimen is higher than

austempered and least in conventionally hardened.

· Yield torque of austempered one in torsion test is

higher but angular deflection is comparable with as

bought specimen. Torsional strength of

conventionally hardened is far less compare to heat

treated one.

· Hardness of austempered and conventionally

hardened are almost similar but far higher than that

of as bought specimen.

· The toughness (energy absorbed before failure) of

the austempered specimen is far ahead compare to as

bought and conventionally hardened. It indicates the

ability of the specimen to undergo self-tempering

during austempering. It also reduces the processing

cost of the specimen to induce toughness compared

to conventionally hardened one.

· Microstructure reveals the clear martensitic structure

in conventionally hardened, needle type bainitic

structure in austempered and ferritic and carbide

structure in as bought specimen.

· There is overall improvement in mechanical

properties of austempered one compared to

conventionally hardened one.

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Proceedings of the 2 ICCTEM-2014 - Manipaleprints.manipal.edu/140204/1/ICCTEM-2014 SSS-1.pdf · Proceedings of the 2 nd International Conference on Current Trends in Engineering and