mechanical property assessment of austempered and conventionally hardened aisi

Proceedings of the 2nd

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

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

8

MECHANICAL PROPERTY ASSESSMENT OF AUSTEMPERED AND

CONVENTIONALLY HARDENED AISI 4340 STEEL

Dr. S.S Sharma1, Dr. Jagannath K

2, Dr. P.R. Prabhu

3, Gowri Shankar M.C

4

1, 2, 3, 4

(Mechanical& Manufacturing Engineering Dept, Manipal Institute of Technology, Manipal, Karnataka, India)

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

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

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ISSN 0976 – 6359 (Online)

Volume 5, Issue 9, September (2014), pp. 08-14

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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. 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 PROCEDURE

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)


International Conference on Current Trends in Engineering and Management IC

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)

Hardness test: The specimens are polished with 200 ser

is employed for the hardness measurement.

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

recorded using metallurgical microscope.

3. RESULTS AND DISCUSSION

3.1 Tensile test Figures 4 and 5 show the Load versus deformation graphs for as

as-bought specimen shows clear cut yield point, the typical ductile behaviour of

austempered specimens do not show clear yield points. The ductility of as

conventionally hardened steel. The area under the load versus deformation is larger for austempered

other two. This is the measure of toughness. The marginal loss in strength is 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 fin

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.

Figure 4: Load vs. elongation graph for as

specimen austempered specimen



10

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 Fig. 3: Impact test specimen


The specimens are polished with 200 series of emery papers before the test. The Rockwell hardness tester

The charpy test is conducted for all the samples. The energy absorbed before failure of the specimen is

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

Figures 4 and 5 show the Load versus deformation graphs for as-bought and conventionally hardened steel. The

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



ncrease in stiffness of the material. This is the typical behaviour of lower bainitic structure. The better dispersion of fin


pecimen. A little permanent elongation is recorded in conventionally hardened specimen. The fractured


aviour of the specimen in with and without treatment condition.

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

austempered specimen

Figure 5: Load vs. elongation graph for



19, July 2014, Mysore, Karnataka, India

testing specimens are subjected to torsion test on torsion testing machine. The torque

Fig. 3: Impact test specimen


ies of emery papers before the test. The Rockwell hardness tester

The charpy test is conducted for all the samples. The energy absorbed before failure of the specimen is

samples are prepared by polishing with different grades of emery papers and etched with

heat treated,austempered and conventionally hardened AISI 4340 steel is

bought and conventionally hardened steel. The

steel. The conventionally hardened and

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



ncrease in stiffness of the material. This is the typical behaviour of lower bainitic structure. The better dispersion of fine


pecimen. A little permanent elongation is recorded in conventionally hardened specimen. The fractured


: Load vs. elongation graph for




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 a

Figure 8:

7900

16200

0

5000

10000

15000

20000

Ult

ima

teT

en

sile

Lo

ad

(N

)

Tensile test results

Figure 6: Tensile load vs. type of modification

Figure 9: Torque vs. Angular deflection graphs

foras bought specimen



11

Figures 9, 10, 11 and 12 show the torsional behaviour of the specimen in the given condition. Higher torque is


r displacement. Austempered shows higher yield angular deflection and is at par with as


due to the uniform dispersion of fine ferrite and carbide phases.

Figure 8: Break deformation vs. type of modification

15800

Tensile test results

Figure7: Peak deformation vs. type of

modification

Tensile load vs. type of modification

Figure 10: Torque vs. Angular deflection graphs

for austempered specimen

Torque vs. Angular deflection graphs



Figures 9, 10, 11 and 12 show the torsional behaviour of the specimen in the given condition. Higher torque is


r displacement. Austempered shows higher yield angular deflection and is at par with as


Peak deformation vs. type of

Torque vs. Angular deflection graphs

for austempered specimen



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.

Figure 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.

3.5 Microstructure examination

Figure 15 shows the microstructure of different

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 typ

typical pattern of bainitic structure.

Figure 14:

Figure. 11: Torque vs. type of modification



12



aviour of super saturated solid solution martensite structure.

Rockwell hardness number vs. type of modification

Figure 14 shows the ability of the specimen to resist impact load. The energy absorbed before failure under


further tempering may not be required after the treatment.

Figure 15 shows the microstructure of different specimens in all the three conditions at 500X magnification.


typical band like single martensitic phase. Austempered one shows needle type well dispersed fine phases. This is the

Figure 14: Energy absorbed vs. type of modification

Torque vs. type of modification Figure 12: Angular deflection vs. type of

modification





Figure 14 shows the ability of the specimen to resist impact load. The energy absorbed before failure under


specimens in all the three conditions at 500X magnification.


e well dispersed fine phases. This is the

Angular deflection vs. type of




13

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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