Austempering Heat Treatment of Carbon Alloy Steels

1. AUSTEMPERING HEAT TREATMENT
OF CARBON ALLOY-STEELS
ADVANCED DESIGN PROJECT:- 4th YEAR 1st SEMESTER,2017-2018
REPORT SUBMITTED BY:- ANIRBAN SADHU
B.E. 4th YEAR, MECHANICAL ENGINEERING
JADAVPUR UNIVERSITY.
ROLL NUMBER – 001411201042.
UNDER THE GUIDANCE OF:- PROF. AVIJIT MUKHERJEE
DATE OF SUBMISSION:- 29th NOVEMBER,2017.

2. ACKNOWLEDGEMENT
I sincerely convey my gratitude to my Respected Guide, Prof. Avijit Mukherjee of
Mechanical Engineering Department, Jadavpur University, whose tiring efforts and
continual guidance has helped me throughout the process of writing this
dissertation. He has helped in pointing out the need of the project and his valuable
suggestions and encouragement has inspired me during the course of the project-
work.
I also thank The Almighty for giving me the opportunity of working on this project
and I am grateful to my parents and all Respected Faculty Members of Mechanical
Engineering Department, Jadavpur University for constantly supporting me.

3. 1.INTRODUCTION
Mechanical parts are manufactured from steels that are in their softest possible
condition. This makes cutting, bending, stamping etc. as easy as possible. However, in
the final application, the part may need to be hard, wear resistant, tough or strong or
some combination of these. Hardening, tempering and austempering [1]
processes are
used to transform the part from the initial form to the final desired form.
At room temperature, soft steel is a mixture of iron and iron carbide. When it is heated
above 720ºC, the iron carbide begins to dissolve in the iron to form a solid solution
called austenite. If this were slow cooled again, it would go back to iron and iron
carbide. However, if it is quenched, there is insufficient time for this to happen. The
carbon becomes trapped in the austenite as it tries to get to the low temperature
structure, forming what is known as martensite. Because of the trapped carbon, the
martensite is hard, the more carbon in the steel, the harder and stronger the martensite,
but the more brittle it becomes. After quenching, the steel is termed fully hardened. This
condition is unsuitable for most applications because it is too brittle; so it is tempered.
In tempering, the steel is heated to a temperature usually in the range of 150 to 450ºC.
This provides some energy to allow some of the trapped carbon atoms to pop out,
thereby relieving the stress. The higher the temperature, the more the stress is relieved
and the softer but tougher the steel becomes. Austempering is a sort of short-cut. Instead
of quenching down to room temperature, the parts are quenched into a molten salt bath

4. at a temperature between about 230 and 330ºC. The steel transforms to a material called
Bainite which is very much like Tempered Martensite, but even tougher.
In the present project-work, our target is to go through the literature reviews to gather
hands-on knowledge, to be acquainted with the recent trends of austempering processes
of different grades of carbon-alloy steels.
Special emphasis is then given to study the austenitic transformation of bearing alooy-
steel, AISI 52100 through laboratory experimentation.

5. 2.LITERATURE REVIEW
Austempering [2]
is a method of hardening steel by quenching from the austenitising
temperature into a heat extracting medium (usually molten salt), which is maintained at
specified temperature level between 200°C and 400°C and holding the steel in this
medium until austenite is transformed to Bainite. This method is used to increase
strength, toughness and to reduce distortion.
The two sub-processes involved in Austempering are:-
(1) Heating a medium-to-high carbon ferrous metal to an austenitic condition
(2) Then cooling the object rapidly enough to a temperature above Martensite Start
(Ms) temperature(to avoid the formation of pearlite) and isothermally holding the
part for a time sufficient to produce the desired microstructure.
But these two processes are generally limited to small components for large-scale
industrial purposes. Due to their high applicability, these processes are explored by
many researchers.
The amount of retained austenite in Cr-Mo steels (used in mill-liners) was studied
by Shaeri et al. The effects of heat treatments including direct quenching, martempering,
and austempering on the retained austenite existing in the microstructure of these steels
were investigated. Specimens were austenitised at 950°C followed by direct quenching
using compressed and still air. The specimens were also isothermally quenched in salt
bath at 200°C and 300°C for 2, 8, 30, and 120 min. The existence of the retained
austenite[10],[12]
in the microstructure of this steel led to some drawbacks. Wear
resistance of the material was reduced as a result of the presence of phase with low

6. hardness and strength. Unfavourable dimensional variations appeared in the specimens
resulting from the transformation of austenite to martensite during tempering or upon
severe impacts applied to the liners during milling process. Transformation of austenite
to martensite during tempering gave rise to a volume change in austenite, resulting in
the formation of a severe compressive stress at the austenite- boundary. Such a defect
formed a suitable place for crack nucleation and propagation, therefore reducing
durability of the specimen. The results showed that the lowest amount of retained
austenite in the microstructure was obtained in the specimens quenched isothermally at
300°C for 120 minutes.
The effect of austempering treatment on microstructure and mechanical properties
of high-Silicon steel was studied by Mandal et al.
MacIejewski and Regulski studied the fracture assessment of martempered and
quenched-and-tempered AISI 4140 low alloy steel. The reported advantages of
Martempering include less distortion, elimination of quench cracking, improved fatigue
resistance, and improved absorbed impact energy. Data regarding improved impact
energy are sparse and appear to be most widely reported for the high-carbon steels. The
results of impact-energy and tensile strength that are compared between quenched and
tempered to that of modified martempered had no much difference, and the analyst must
check for the martempering process.

7. Wear resistance properties of austempered ductile iron (ADI) were studied by
Lerner and Kingsbury. A detailed review of wear resistance properties of ADI was
undertaken to examine the potential applications of this material for wear parts, as an
alternative to steels, alloyed and white irons, bronzes, and other competitive materials.
Two modes of wear were studied: adhesive (frictional) dry sliding and abrasive wear.
In the rotating dry sliding tests, wear behaviour of the base material (a stationary block)
was considered in relationship to counter surface (steel shaft) wear.
In this wear mode, the wear rate of ADI was only one-fourth that of pearlitic ductile
iron (DI). Only quenched DI with a fully martensitic matrix slightly out performed ADI.
No significant difference was observed in the wear of steel shafts running against ADI
and quenched DI. The excellent wear performance of ADI and its counter surface,
combined with their relatively low-friction coefficient, indicate potential for dry sliding
wear applications. In the abrasive wear mode, the wear rate of ADI was comparable to
that of alloyed hardened AISI 4340 steel, and approximately one-half that of hardened
medium carbon AISI 1050 steel and of white and alloyed cast irons. The wear resistance
of ADI may be attributed to the strain-affected transformation of high-carbon austenite
to martensite that takes place in the surface layer during the wear tests.
Jetley reported improvement in wear properties of aircraft brake steel rotors by
martempering. Martempering process using oil- and water-based quenchants at lower
temperature is adopted in this work. The test samples were evaluated for hardness,
distortion, and wear under accelerated simulated tests. The results show that although
both hardness and wear resistances were lower compared to the austempering, they met

8. the design intent. Also the wear rate of martempered samples was more consistent
which may provide advantages for maintenance purposes.
Wear Of Hard-turned AISI 52100 steel was studied by Bartha et al. High precision
machining such as hard turning changes the surface and the material properties of steel
alloys. A sliding block-on-cylinder wear tester was used for the purpose of testing the
wear performance of AISI 52100-bearing steel. The effect of microstructure on the wear
performance of hard-turned steel showed that the white layer and overtempered
martensite (OTM) had a higher wear resistance than martensite. The wear mechanism
dependence on the surface hardness was attributed to this increase in wear performance.
The near-surface residual stress of the material was shown to become more compressive
as the material wore down. The applied normal loads affected the surface roughness,
residual stresses, and, in turn, the wear performance of the material.
Prof. Andrew Clark of University of Windsor examined the relevance of
Austempering as a replacement for the current quench-and-tempering processes and as
a method for heat-treating carburised low-carbon alloy-steel automotive components
(of grades SAE 8620, SAE 4320 and SAE 8822). He collected the data with 12
austempering parameters (on 12 runs). His studies included:-
(i) Case and core microstructures
(ii) Distortion (characterised by Navy C-ring samples)
(iii) X-ray diffraction (for measuring residual stress and retained austenite
percentage)

9. (iv) Charpy Impact test (V-notch testing for toughness).
(v) Rockwell Hardness test (on Scale-C).
His observations were:-
(1) Austempering [3]
produced improved distortion characteristics[11]
(planar
distortion was almost negligible).
(2) Residual stresses for Austempered specimens were far less.
(3) Austempering helped to eliminate the costly reworking step associated with
traditional quench-and-tempering.

10. 3.Theoretical Framework
Quenching has the disadvantage that stretching effect generated during martensite
formation results in excessive stresses developed in the material.
Basically, two main types of Martensite are usually observed for steels:-
(1) Lath Martensite (steels with <0.6% C)
(2) Plate Martensite (steels with >1% C).
3.1 Martensitic Transformation
Martensitic Transformation involves change of crystal structure from FCC to BCT
(body-centred tetragonal), which stretches the vertical axis of the unit cell in a shear
process. The temperature, while cooling, at which the martensitic transformation begins
is known as the Martensite Start temperature (denoted as MS).
The following formulae have been experimentally established by scientists for
estimating the martensite start temperatures of various alloy-steels:-
[The symbols of each element indicate the percentage composition of the element by
volume in the microstructure.]
K.W. Andrews, 1965
MS (0
C) = 539 – 423 C – 30.4 Mn – 12.1 Cr – 17.7 Ni – 7.5 Mo
Liu et al (upto 0.02% by weight of C)
MS (0
C) = 525 – 350 (C – 0.005) – 45 Mn – 35 V (Nb + Zr + Ti) – 30 Cr – 20 Ni
– 16 Mo – 8 W – 5 Si + 6 Co + 15 Al

11. Nehrenberg
MS (0
C) = 500 – 300 C – 33 Mn – 22 Cr – 17 Ni – 11 Si – 11 Mo
Grange and Stewart
MS (0
C) = 538 – 361 C – 39 Mn – 19 Ni – 39 Cr
Steven and Hans
MS (0
C) = 561.1 – 473.9 C – 21.1 Mn – 16.7 Ni – 16.7 Cr
In case of austempering, our target to avoid the formation of martensite, rather to
produce Bainite.
3.2 Bainite
Bainite [4]
is a non-lamellar mix of ferrite and cementite formed at an intermediate range
of cooling rates. Based on temperature ranges, two types of bainite may be formed:-
3.2.1 Lower bainite ― It has a more needle-like appearance and forms with a shearing
mechanism similar to martensite, although there are some diffusion effects as well. It is
formed upon cooling to a temperature just above MS and holding. It has a higher tensile
strength and hardness.
3.2.2 Upper bainite — It has a more feathery appearance, and is produced
predominantly by diffusion mechanism. It is formed by cooling past the pearlite shelf
of the TT curve and holding.

12. Although there is no exact Bainite-start point, Zhao-et-al’s Equation may be used to
estimate to a rough extent, as stated below:-
BS (0
C) = 630 – 45 Mn – 40 V – 35 Si – 30 Cr – 25 Mo – 20 Ni – 15 W.
It is assumed that Bainite Start Temperature marks the “knee” or “nose” of the cooling-
curve that has to be passed before the bainite can be formed. But the equation above
does not take into account the necessary cooling rate to avoid pearlitic formation.
Using the above equation as a guide, it is possible to approximate TT curve for a given
steel, using empirical testing to find the time required for transformation. Upon crossing
the Bainite Start line on the TT curve, bainite will begin to form.
Now, if we interrupt the transformation by a quench, the austenite will be partially
transformed to bainite and rest of the austenite will either turn into martensite upon
quenching or remain trapped in the bainite matrix as “retained austenite’
(untransformed austenite) at room temperature.
3.3 Alloying Effect
It is not possible, with a plain carbon steel, to design a heat-treatment schedule to form
bainite and martensite from austenite. The pro-eutectoid and pearlite-start lines on the
TTT curve do not allow enough time to quench to an austempering temperature or MS
before pro-eutectoid and eutectoid reactions begin. Hence, any microstructure will
contain a finite amount of pearlite and a pro-eutectoid phase.
Hardenability (ease of martensite formation) can be enhanced by alloying. These
alloying elements shift the pearlite start line to longer times, allowing sufficient time
for bainite and martensite formation.

13. Fig. 1. Schematic Representations of the Austempering Process in steels. [5]

14. 3.4 Salt-bath used
The modern molten salt bath[6]
is the ideal medium for low-distortion, interrupted
quenching processes such as martempering and austempering. Molten salts have been
used for quenching for more than 50 years. Their wide operating temperature range
makes them ideal for many quenching processes aimed at minimizing distortion of iron
and steel parts. Their unique characteristics coupled with recent advances in salt quality,
pollution abatement, and material handling make salt bath quenching more efficient and
economical than ever before.
When the quenching medium is water, brine, a polymer solution, or fast oil, it is
generally referred to as conventional quenching. Steels with low hardenability can be
quenched this way. However, this method can cause distortion and in some cases, non-
uniform hardness or even cracking. The tendency to distort increases with increasing
steel hardenability. Causes of distortion usually can be traced to uneven or non-uniform
quenching, thermal stresses, and transformational stresses. These factors often can be
mitigated by adopting an interrupted quenching technique.
For most steel and alloys, the temperature at which the quench is interrupted is usually
in the 175 to 370°C (350 to 700°F) range. Water, brine, polymer solutions and most
quench oils cannot be used at these temperatures. Attempts at using molten lead and
fluidized beds do not appear to have met with much success. Some oils can be used at
temperatures up to 230°C (450°F), but for higher temperatures, molten salt is the
natural, practical choice.
The most distinct advantages of salt over oil is its wide operating temperature range-
150 to 595°C (300 to 1100°F) for a typical composition. Thus, salt can be used for any

15. interrupted quenching process. Oil, however, cannot be used above 230°C (450°F),
which restricts its use to low temperature processes. The quenching mechanism also is
considerably different. Most of the heat extracted during salt quenching is by convection
(the third stage of liquid cooling), and is therefore at a uniform rate. In oil quenching,
heat is extracted during all three stages with varying rates. As a result, salt quenching
causes less distortion and produces more uniform and consistent hardness. Other
important advantages of salt over oil include:
i. Quench severity can be controlled to a greater degree by varying temperature,
agitation, and water content of the salt bath.
ii. Productivity is higher because parts attain temperature equalization faster.
iii. The excellent thermal and chemical stability of salt means that the only
replenishment required is due to dragout losses.
iv. A salt bath provides satisfactory quenching performance for many years. In contrast,
oil deteriorates with use, requiring closer control and sometimes partial or complete
replacement.
v. Non-flammable salt poses no fire hazard, whereas oil at a comparable temperature
poses a serious hazard.
vi. Salt can be easily washed off with water and recovered for reuse, if desired.
Choosing to recover salt not only eliminates disposal but also reduces operating costs.
In contrast, washing of oil requires special cleaners and equipment; and its recovery is
not simple.
There are relatively few limitations to salt as a quenching medium. It has to be used
above its melting point of about 150°C (300°F). And, because it is a strong oxidizer,

16. combustible or incompatible materials should definitely be kept out of the salt bath to
avoid the possibility of violent reactions. Salt may appear to present safety and
environmental problems, but the technology for dealing with them is well developed
and they are no longer viewed as a deterrent to its use.
Quench severity refers to the ability of a quenchant or quenching system to extract heat
from a test specimen, part or workload. It can be determined by measuring either its
hardening power or cooling power. Methods based on hardening power such as the
Grossmann techniques and Jominy end-quench test are time-consuming. More
preferable are methods based on cooling power such as the GM nickel ball, hot wire
and cooling curve tests. Among them, cooling curve (and cooling rate curve) analysis
has emerged as the most useful tool for measuring quench severity, as well as for
understanding the quenching mechanism and studying the effects of quench variables.
In a salt bath, the cooling rate is nearly constant throughout the temperature range of
interest confirming that quenching in molten salt occurs at a uniform rate.
Temperature, agitation, water content, and residence time are the main variables in salt
bath quenching.
In general, the lower the bath temperature, the faster the cooling rate for any medium.
In salt bath quenching, the effect of this variable is generally marginal. However, in
some cases, salt bath temperature can be manipulated to achieve remarkable results. For
example, low-hardenability steels can be austempered using two salt baths, each at a
different temperature. Similarly, parts having thick sections can be martempered by
quenching them in water or brine for a short time and then transferring them to a salt
bath.

17. The effect of agitation is greater than that of temperature. Increasing agitation
results in a considerable increase in quench severity. Like temperature, agitation can be
controlled to advantage. Agitation can be provided by propeller-type agitators or by a
pump with a draft tube. Depending on the size of the bath, one, two or four propellers
with single or dual impellers may be required. If a pump is used, salt flow from the
bottom to the top of the bath is preferred. Agitation is a must when water additions are
to be made.
A small addition of water to a salt bath increases its quench severity significantly.
Water content of the bath typically varies from 0.5 to 2%, usually in the operating
temperature range of 150 to 290°C (300 to 550°F). Note that the combination of
agitation and a water addition increases quench severity three-fold, compared with an
unagitated, “dry” salt bath. Safety considerations dictate that water is always added to
a well-agitated bath and never to an unagitated bath. Instead of fresh water, salt solution
from the washing operation can be used. Some heat treaters also use low pressure steam
instead of water. A fringe benefit of a water addition is that it lowers the salt’s melting
point, increasing its working range. For example, 1% water lowers the melting point by
11°C (20°F) and 2% lowers it by 19°C (35°F).
The time that parts remain in the bath (i.e, the residence time) generally depends on
steel composition, section thickness, quench severity and the process being performed.
In martempering, a few minutes are enough for temperature equalization. A longer
residence time may produce a microstructure other than the desired martensite. In
austempering, on the other hand, a few hours may be needed to complete the
transformation to bainite. A longer time is not harmful, but it will increase processing

18. costs. Successful salt bath quenching also requires that steel composition, austenitising
temperature, the section thickness and configuration of parts be considered. Note,
however, that these are not strictly quenching variables. When parts are quenched from
an atmosphere furnace into a salt bath, splashing of salt into the furnace is prevented by
use of a separating curtain (salt curtain) or an intermediate chamber or vestibule.
Quenching salt is a eutectic mixture of nitrates and nitrites of sodium and potassium.
There are nearly a dozen compositions available, ranging in melting point from 135 to
260°C (275 to 500°F). Selection of a quenching salt primarily depends on the lowest
temperatures at which it is going to be operated and its melting point. The difference
between the two should preferably by 55°C (100°F) or greater. Once molten, these salts
all exhibit nearly the same physical properties (as displayed in Table-1 below).
Table-1 : Salt Properties
Quenching salts used to be available only in granular or crystalline form, which caused
dusting when added to a quenching bath, and frothing and scum for a few hours or a
day. This adversely affected quenching severity until the bath stabilized. These
problems can now be avoided to a great extent by using improved quality salt in
briquette or flake form.

19. 3.5 Modifications of Austempering
3.5.1 Carboaustempering:- Austempering of low-carbon-content steels after
carburizing produces a high-carbon bainitic case and either a martensitic or bainitic
core, depending on steel composition and quench severity. What makes this process
unique is that the core becomes hard first, and then hardening processes to the surface,
further minimizing distortion. In addition, while the surface is being austempered, any
martensite in the core is being tempered simultaneously. Carboaustempered[7]
parts
have excellent fatigue strength and wear resistance, and are dimensionally and
functionally superior to carburized and conventionally quenched parts.
3.5.2 Modified austempering:- [8]
In this modification of austempering, quench
severity is intentionally decreased to force the cooling curve to intersect the pearlite
nose of the TTT diagram. Because a mixed microstructure of bainite and pearlite results,
hardness is relatively low – usually in the 30 to 42 HRC range – but ductility is
extremely high and strength moderately high. Wire patenting at 510 to 450°C (950 to
1000°F) is a fine example of modified austempering. The method also should be
considered for carbon steels and heavier sections where ductility greater than that
possible by standard austempering is required. Some trial and error will be necessary to
develop the optimum cycle for the particular steel, section thickness, and property
requirements.
3.5.3 Other variations:- Additional austempering variations involve the use of two
quenching baths and manipulation of agitation. Low-hardenability steel can be

20. austempered by using two quenching baths instead of one. The temperature of the first
bath is maintained at just above MS to avoid the pearlite nose of the TTT diagram, while
the other bath is at normal austempering temperature. For example, parts made of AISI
1080 steel with a section thickness of about 12 mm (0.5 in.) are first quenched for 30
seconds at 260°C (500°F) and then immediately transferred to the second, 315C°
(600°F) bath, where they’re held for the normal duration of austempering.
Austempering of thin-walled parts such as cylinder liners can be enhanced by
manipulating agitation. For example, to minimize thermal shock, agitation is not used
during the first 15 to 20 seconds of the quench. It is then turned on automatically to
complete the standard austempering operation. This procedure helps to minimize
distortion without any sacrifice in required mechanical properties.

21. 4.PROPOSED EXPERIMENTAL PLAN
We have procured specimens of AISI 52100 directly from the market as available. The
approximate percentage composition of different alloying constituents in the specimen
are indicated in Table 2 below. [9]
Table - 2 : Composition of specimen
Alloying Element Percentage Composition
Carbon (C) 0.92
Manganese (Mn) 0.62
Silicon (Si) 0.25
Phosphorus (P) 0.035
Sulphur (S) 0.03
Nickel (Ni) 0.04
Chromium (Cr) 1.25
Molybdenum (Mo) 0.02
Hence, the Martensite Start Temperature for the above-stated specimen is estimated by
Nehrenberg’s formula as:-
MS = 500 – 300 C – 33 Mn – 22 Cr – 17 Ni – 11 Si – 11 Mo
= 500 – (300 ×0.92) – (33×0.62) – (22×1.25) – (17×0.04) – (11×0.25) – (11×0.02)
= 172.39 O
C .

22. It was planned that the specimens would then be heated in a Muffle Furnace (whose
specifications are stated in Table-3 on the next page) at three different temperatures
9250
C, 9400
C, 9550
C. The dwelling time inside the furnace is fixed at 1 hour.
Table – 3 : Specifications of the Muffle Furnace used
Type Resistance heating
Brand/Maker N.R. Scientific
Maximum Temperature 1100O
C
Working Temperature 1000O
C
Heating-space Size 10”×5”×5”
Fig. 2. The Muffle-furnace in Closed
Condition, Open Condition respectively,
and its Temperature-Control/Monitoring
Panel.

23. After that, the austenitised specimen would be immediately quenched in a molten salt
bath (constituting 50% NaNO3 + 50% KNO3 — such a mixture forms an eutectic
composition which melts at a much lower temperature than the individual melting
temperatures of the two salts) at about 172O
C (as estimated earlier numerically).
Fig. 3. The salt-bath and its Temperature-Control Panel
The specimens would be allowed to dwell in the furnace for about 30 to 45 minutes to
allow uniform austempering to take place.
The specimen is then taken out of the salt-bath and mounted with Bakelite. Then, it is
first it is ground manually with 5 grades of Emery-paper (coarse to fine), then polished
on a rotating nap-cloth with aluminium-oxide solution and finally etched with Nital
solution (3% HNO3 + 97% ethanol). Etching makes the grain-boundaries more
prominent.
The specimen is then observed under the metallographic optical microscope
(VERSAMET) at a resolution of 500X (50X × 10X). [Maker ― Olympus].

24. 5.CONCLUSION
AISI 52100 steel was subjected to austempering heat treatment for enhancing the
material properties. From the present study, the following conclusions are drawn.
(i) Austempered samples have very high impact strength. The impact strength increased
with soaking time in austempered samples up to a certain level.
(ii) Austempered steel can efficiently and reliably be used for manufacturing
automotive components like bearings, differentials, shafts etc.
(iii) In the near future, austempering can replace conventional quench-and-temper
process to a great extent, with the research and development efforts going on all over
the world on more energy-efficient furnaces, better quenching medium and salt-
cleaning and recovery methods.
(iv) Austempering leaves minimal hazardous footprints on the environment, compared
to conventional heat-treatment processes because petroleum-based oils are replaced by
molten-salt bath in case of austempering.
(v) Austempering can enhance other desirable properties in medium and high carbon
steels, such as wear resistance, resistance to corrosion etc.

25. 6.REFERENCES
[1] Austempering. (2013, December 8). Retrieved from
http://en.wikipedia.org/wiki/austempering.
[2] Austempering, Process Benefits. (n.d.). Retrieved from
http://www.atmosphereheattreat.com/benefits.asp?pid=42
[3] Austempering, Process Overview. (n.d.). Retrieved from
http://www.atmosphereheattreat.com/process.asp?pid=39
[4] Bainite. (2013, May 21).
Retrieved from http://en.wikipedia.org/wiki/bainite
[5] Claus, Laurence. (2012). Heat Treating Basics. Available from NNI Training
and Consulting Inc.
[6] Godding, A.D., Keough, John R., Laird, W. James. (1991)
ASM Handbook Vol. 4- Heat Treating, Austempering of Steel (pp. 152-163).
Materials Park, OH: ASM International.
[7] International Nickel Inc., (n.d.), Isothermal Transformation
Diagrams of Nickel Alloy Steels, INCO, Park 80 West- Plaza Two, Saddle
Brook, NJ, (p. 12).
[8] Roy A. Lindberg, Processes And Materials of Manufacturing (2006) 4th
edition, Prentice Hall India (pp. 280-340).
[9] P. Vamsi Krishna, R. R. Srikant, Mustafa Iqbal, and N. Sriram, Research
Article on Effect of Austempering and Martempering on the Properties of
AISI 52100 Steel, ISRN Tribology, Volume 2013 (2013), Article ID -
515484, 6 pages. Retrieved from http://dx.doi.org/10.5402/2013/515484.
[10] Arnell, R. D., K. A. Ridal, and J. Durnin. "Determination of Retained
Austenite in Steel by X- Ray Diffraction." J IRON STEEL INST 206, no. 10
(1968): 1035-1036.
[11] Ruud, C. Measurement of Residual Stresses, Handbook of Residual Stress
and Deformation of Steel, ASM International, 2002, p 100-117.
[12] Jatczak, C. F. "Retained Austenite and its Measurement by X-Ray
Diffraction." Society of Automotive Engineers, Pp.20, 1980 (1980)
[13] Suffredini, R. L. "Factors Influencing Austempering." Heat Treating.
Handbook-12, serial no. 1 (1980) (pp. 14-16, 18-19)