Mechanical property assessment of austempered and conventionally hardened aisi

1. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
17 – 19, July 2014, Mysore, Karnataka, India
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 9, September (2014), pp. 08-14
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IJMET
© I A E M E
MECHANICAL PROPERTY ASSESSMENT OF AUSTEMPERED AND
CONVENTIONALLY HARDENED AISI 4340 STEEL
Dr. S.S Sharma1, Dr. Jagannath K2, Dr. P.R. Prabhu3, Gowri Shankar M.C4
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

2. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
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].
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Austempering is a method of hardening by heating to the austenitizing temperature i.e., 300C to 500C 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 350oC (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)

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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
17 – 19, July 2014, Mysore, Karnataka, India
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Fig. 3: Impact test specimen
(All dimensions are in mm)
series of emery papers before the test. The Rockwell hardness tester
non-heat treated,austempered and conventionally hardened AISI 4340 steel is
as-bought and conventionally hardened steel. The
steel. The conventionally hardened and
as-bought steel is higher than austempered and
ncrease pecimen. aviour as-bought
Figure 5: Load vs. elongation graph for
austempered specimen
ICCTEM -2014
ies one as compared to
fine
:

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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:

5. Figure 6: Tensile load vs. type of modification
Figure 9: Torque vs. Angular deflection graphs
foras bought specimen
17 – 19, July 2014, Mysore, Karnataka, India
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r and carbide phases.
Break deformation vs. type of modification
Figure7: Peak deformation vs. type of
modification
Figure 10: Torque vs. Angular deflection graphs
for austempered specimen
ICCTEM -2014

6. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management IC
Figure. 11: Torque vs. type of modification
3.3 Hardness test
Figure 12: Angular deflection vs. type of
modification
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:
17 – 19, July 2014, Mysore, Karnataka, India
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aviour specimens in all the three conditions at 500X magnification.
type well dispersed fine phases. This is the
Energy absorbed vs. type of modification
ICCTEM -2014
e

7. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India

8. Figure. 15: Microstructure of (a) as bought (b) Conventionally hardened (c) Austempered specimen at 500X
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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