The document discusses various heat treatment processes for steels. It describes recrystallization annealing which involves heating below the eutectoid temperature to allow recrystallization. Spheroidizing annealing is used to change cementite morphology for improved machinability. Normalizing involves heating then air cooling to refine grains. Quenching and tempering improves hardness, strength and wear resistance by forming martensite then stress relieving through tempering. TTT and CCT diagrams are used to design heat treatments and predict microstructures. Martempering and austempering allow formation of bainite for high strength and toughness.

1. Heat Treatment of Steels :Recrystallization annealing
• The carbon and alloy steels were treated at a temperature of about
700°C, which is about 20 °C below the eutectoid temperature. The
holding time should be long enough for recrystallization to be
proceed.
• Recrystallization time depends on the degree of cold deformation.
The higher the degree of cold deformation, the lower
recrystallization time is.
Ref: Linde booklet:
Furnace Atmosphere No.2
3/21/2012Kitkamhorn, U.
29
Heat Treatment of Steels : Spheroidize annealing
• The purpose of this process is to improve machinability by changing
the morphology of cementite from continuous network or large plate
into globular. Therefore, machining become easier.
• Spheroidize annealing shall be applied only to medium and high
carbon steels.
3/21/2012Kitkamhorn, U.
Ref: http://www.astonmet.com/ss/images/win_ht1207_23.html
30
Heat Treatment of Steels : Spheroidize annealing
• The steel is heated to temperature just below or slightly above the
eutectoid temperature. Holding for 2-6 hr followed by slow
cooling
3/21/2012Kitkamhorn, U.
Modified from William D., and Jr. Callister, 2007 31
Heat Treatment of Steels : Normalizing
• The purpose of this process is to refine grain size or to produce
uniform grain size.
• It is also used for softening the heavily cold deformed materials. It is
quite similar to full annealing but the parts are cooled in air. Thus
the cooling rate is faster compare to annealing.
3/21/2012Kitkamhorn, U.
Modified from William D., and Jr. Callister, 2007 32

2. Heat Treatment of Steels : Normalizing
• Normalizing is considered to be out of equilibrium since the
air-cooling is considerably fast.
• Eutectoid temperature and composition become lower as
cooling rate increase.
• Ar3 and Ar1 merge and full pearlitic microstructure is obtained
in steels with >0.4%C
3/21/2012Kitkamhorn, U.
A1
Acm
A3
Modified from William D., and Jr. Callister, 2007
33
Heat Treatment of Steels : Quenching and Tempering
• The purpose of quenching and tempering is to improve
hardness, strength, and wear resistance of the parts.
• The desired microstructure is martensite or bainite or mixture
of both depending on the hardness, strength, and toughness
required in service.
3/21/2012Kitkamhorn, U.
[Ref: Heat Treater’s Guide, 1995]34
Heat Treatment of Steels : Quenching and Tempering
• The hardness of martensite is a
function of carbon contents only.
• The maximum hardness (fully
matensite) is then a function of
carbon contents in steels.
Ref: Linde booklet:
Furnace Atmosphere No.2
3/21/2012
This point is important for materials selection
Kitkamhorn, U.
35
Heat Treatment of Steels : Quenching and Tempering
• The steel parts are heated to a temperature above A3 for
hypoeutectoid steel and above A1 for hypereutectoid steel.
• Soaking time is about 30 minutes per 1 inch cross section.
• The parts are rapidly cooled (so called “quench) in either brine,
water, hot oil, cold oil, forced air, and still air.
temperature
time
austenitizing
quenching
tempering
3/21/2012Kitkamhorn, U.
Modified from William D., and Jr. Callister, 2007
36

3. Heat Treatment of Steels : TTT and CCT
Metallurgists use TTT and CCT diagrams to design the quenching
severity required to achieve the target hardness value.
TTT (time-temperature transformation diagram) indicates the
isothermal transformation versus holding time.
CCT (continuous cooling transformation diagram) indicates the
transformation by the different cooling rate.
3/21/2012Kitkamhorn, U.
37
Heat Treatment of Steels : TTT diagram
Ref: William D., and Jr. Callister, 2007
3/21/2012Kitkamhorn, U.
38
3/21/2012Kitkamhorn, U.
Heat Treatment of Steels : TTT diagram
Modified from William D., and Jr. Callister, 2007
39
Heat Treatment of Steels : TTT diagram
Curves on TTT
• Upper curves are of diffusion
transformation e.g.
• γ→ α +γ
• γ→ Fe3C +γ
• γ→ α +Fe3C
• Lower curve are of bainitic
transformation
• Horizontal lines are of
martensitic transformation
3/21/2012Kitkamhorn, U.
40

4. Heat Treatment of Steels : TTT diagram
3/21/2012Kitkamhorn, U.
41
Heat Treatment of Steels : TTT diagram
3/21/2012Kitkamhorn, U.
42
Heat Treatment of Steels : TTT diagram
TTT diagram of AISI 4340 steel
(JIS-SNCM 439, DIN 20CrMo5)
(0.4%C, 1.0%Mn, 0.8%Cr, 0.8%Mo, 1.85%Ni)
Ref: William D., and Jr. Callister
3/21/2012Kitkamhorn, U.
Additions of alloying elements
result in
• Separation of diffusion
transformation and bainitic
transformation curves.
• The position of the nose of
diffusion transformation curve
(upper curves).
• The position of the nose of
bainitic transformation curve
• The position of Ms.
43
Heat Treatment of Steels : TTT diagram
Important information of TTT
needed to be considered
• The position of the nose
which indicates the critical
cooling rate to suppress
diffusion transformation
• Ms temperature indicating that
austenite start to transform
into martensite at such
temperature.
• Fraction of martensite at a
temperature below Ms. This
indicates that after quenching
to room temperature, how
much retained austenite exist
in the steel. Modified from William D., and Jr. Callister, 2007
3/21/2012Kitkamhorn, U.
44

5. Heat Treatment of Steels : CCT diagram
Modified from William D., and Jr. Callister, 2007
3/21/2012Kitkamhorn, U.
45
Heat Treatment of Steels : CCT diagram
• CCT curves are slightly on
the lower right of TTT
curves
• Better to follow CCT since
most heat treatment
processes are based on
continuous cooling except
martempering and
austempering.
Ref: William D., and Jr. Callister, 2007
3/21/2012Kitkamhorn, U.
46
Heat Treatment of Steels : CCT diagram
TTT and CCT Diagram of AISI 4340 steel
(0.4%C, 1.0%Mn, 0.8%Cr, 0.8%Mo, 1.85%Ni)
Ref: William D., and Jr. Callister, 2007
3/21/2012Kitkamhorn, U.
47
Heat Treatment of Steels : How to read CCT
3/21/2012Kitkamhorn, U.
Hardness
%bainite
%proeutectoid ferrite
%pearlite
48

6. Heat Treatment of Steels : Quenching and Tempering
• Alloying elements except cobalt shift TTT to the right.
• This increases hardenability of the steels.
Ac1
Ac3
F+C
F+CA+F+C
A+F+C
A
Ms
½” 1” 2”
Temperature
Time
HRC
Distance from specimen core
Critical
cooling rate
Ac1
Ac3
F+C
F+CA+F+C
A+F+C
A
1”½” 2”
Ms
Temperature
Time
HRC
Distance from specimen core
Critical
cooling rate
3/21/2012Kitkamhorn, U.
49
Heat Treatment of Steels : Quenching and Tempering
• The hardenability of high alloy steels is
much better than that of carbon steels.
• Air-cooling is fast enough to quench the
high alloy steel.
• Alloying elements also decrease Ms and
Mf.
• Carbon strongly influences Ms
• In alloy steels with >0.4%C, some
retained austenite exists after quenching.
Ref: Linde booklet:
Furnace Atmosphere No.2
3/21/2012Kitkamhorn, U.
50
Heat Treatment of Steels : Quenching and Tempering
• As-quenched martensite is brittle because of strain
accompanied with phase transformation.
• There is a volume expansion when austenite transforms into
martensite.
• Tempering is a process used for stress-relieving of fresh
martensite by heating to a temperature at a range of 150-650°C
for 40-60 minutes or longer.
3/21/2012Kitkamhorn, U.
51
Heat Treatment of Steels : Quenching and Tempering
• At high temperature Some carbon atoms are rejected from martensite
and form carbide. The BCT- martensite become closer to BCC-ferrite
and the hardness decreases.
• Retained austenite can also decompose into ferrite and carbide.
• These phenomena are also accompanied with the volume change.
3/21/2012Kitkamhorn, U.
Ref:[Heat Treater’s Guide]
52

7. Heat Treatment of Steels : Quenching and Tempering
• Tool steels such as hot-work
tool steels, cold-work tool
steels, and high speed tool
steels, the tempering curves
exhibit secondary hardening at
tempering temperatures in a
range of 450-600°C.
• Such steels need to be tempered
twice or more.
Carbon steels
Low to medium alloy steels
high alloy steels
with high carbon
contents
high alloy steels
with medium carbon
contents
Ref: George E. Totten
3/21/2012Kitkamhorn, U.
Heat Treatment of Steels : Quenching and Tempering
3/21/2012Kitkamhorn, U.
54
Heat Treatment of Steels : Quenching and Tempering
• Sub-zero quenching is an option
treatment for high alloy steel
quenching.
• The purpose of this method is to
remove the retained austenite.
• After oil or air quenching to
room temperature, the parts are
dipped into cryogenic media such
as liquid nitrogen.
• The parts are then tempered
afterward.
temperature
time
austenitizing
quenching
tempering
Sub-zero quenching
3/21/2012Kitkamhorn, U.
55
Heat Treatment of Steels : Martempering
• The transformation of austenite
into pearlite or bainite is
delayed by addition of some
alloying elements.
• The high alloy steels can be
isothermal holding at a
temperature above Ms for long
time without undergoing phase
transformation
• Large or complex high alloy
steel parts can then be treated
by martempering to avoid
quench- cracking
temperature
time
austenitizing
tempering
3/21/2012Kitkamhorn, U.
56

8. Heat Treatment of Steels : Martempering
martemperingquenching
3/21/2012Kitkamhorn, U.
57
Heat Treatment of Steels : Austempering
temperature
time
austenitizing• Austempering produces bainite
microstructure by quenching to the
temperature above Ms and
isothermally holding until the
bainitic transformation is
completed.
• The austempered steel exhibit high
strength, high tougness, and high
ductility.
• Coil spring and disc spring are
examples of steel parts treated by
austempered
Ref: George E. Totten, 2006
3/21/2012Kitkamhorn, U.
58
Heat Treatment of Steels : Austempering
- Not all steels can be austempered practically.
- Their kinetics of bainitic transformation determined whether
austempering shall be applied to such steels.
Ref: George E. Totten, 2006
3/21/2012Kitkamhorn, U.
59
Heat Treatment of Steels : Summary of Metallurgical Factors during
heat treatment
3/21/2012Kitkamhorn, U.
Understand the concept of microstructural change due to thermal
treatment
Case 1
When crystallization of austenite and its transformation are the principles behind
then what needed to be considered
• Austenitizing temperature must be proper selected depending on the
chemical compositions of steels, the methods of cooling, and the final
microstructure required.
• When austenitizing treatment is in the single austenite region such as
hardening of medium carbon steels, treatment temperature and time should
not be too high since austenite grain coarsening may occur.
• Proper cooling rate (quenching method) is required.
• The final microstructure required
• There is a limitationof miximum hardness of steel, which can be reached
depending on the carbon conceration 60

9. 3/21/2012Kitkamhorn, U.
Case 2
When recrystallization of the same phase (α in α) is the principle behind
• Treatment temperature must be high enough to drive nucleation of
new strain-free grains.
• Proper time is required but too long time has no benefit or may cause
to large grain size.
• Only heavily cold-worked low carbon steels are treated with this
method since large cold deformation is required as a drivig force for
necleation.
61
Heat Treatment of Steels : Summary of Metallurgical Factors during
heat treatment
Heat Treatment of Steels : External Factors during heat treatment
3/21/2012Kitkamhorn, U.
• Oxidation
• Discoloration
• Carburization/decarburization
• Effectiveness of quenchant
• Stability of quechant temperature and its thermo-physical properties
• Transport of heat into/out of steel parts
• Parts configuration
• Jig Design
• etc
62
References:
• George E. Totten. Steel Heat Treatment Handbook : Metallurgy and Technologies, CRC Press, USA
2006
• George Krauss. S T E E L S :Processing, Structure, and Performance . ASM International, USA 2005
• Heat Treater’s Guide: Practice and Procedure for Iron and Steel. ASM International 2nd edition, USA
1995
• H. Eallentowitz et al. “Materials for future automotive body structure” Business
Briefing:GlobalAutomotive Manufacturing & Technology 2003
• Karen Connery and Len Switzer. “High Quality Heat Treatment: Atmosphere choice of critical” Heat
Treating Progress, September 2008
• Linde booklet: Furnace Atmosphere No.1 Gas carburizing and carbonitriding. Special Edition available
from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage
• Linde booklet: Furnace Atmosphere No.2 Neutral hardening and annealing. Special Edition available
from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage
• Linde booklet: Furnace Atmosphere No.3 Nitriding and nitocarburizing. Special Edition available from
http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage
• William D., and Jr. Callister MATERIALS SCIENCE AND ENGINEERING: AN INTRODUCTION
John Wiley & Sons, 7th 2007
• Dissemination of IT for the Promotion of Materials Science (DoITPoMS), Universiy of
Cambridge, http://www.doitpoms.ac.uk/about/dissemination.html
3/21/2012Kitkamhorn, U.
63