1. The document discusses various heat treatment processes for steels including annealing, normalizing, and hardening. 2. Annealing involves heating and slow cooling to soften steel by refining grain structure. Types include stress relief, spheroidizing, and full annealing. 3. Normalizing refines grain size by heating above the critical temperature and slow cooling in air. 4. Hardening increases hardness and wear resistance by heating and quenching in water or oil to form martensite.

1. Prepared by
Prof. Naman M. Dave
Assistant Professor,
Mechanical Engg. Dept.
Gandhinagar Institute of Technology
MATERIAL SCIENCE &
METALLURGY
2131904
Chapter 8
Heat Treatment Processes

2. Please do not blindly follow
the presentation files only, refer
it just as reference material.
More concentration should
on class room work and text
book-reference books.

3. Steels Can be heat treated to alter properties by either ...
B) Heating and rapid cooling (Quenching)
1. Hardening
1. Full hardening / Through Hardening
2. Surface Hardening / Case
Hardening / Selective Hardening
(i) Nitriding
(ii) Induction Hardening
(iii) Carburising – (a) Liquid
Carburising (b) Gas Carburising (c) Pack
Carburising
(iv) Cyaniding
(v) Electron Beam Hardening
(vi) Flame Hardening
(vii) Laser Beam Hardening
2. Tempering
1. Low Temperature Tempering
2. Medium Temperature Tempering
3. High Temperature Tempering
3. Aus-tempering
4. Mar-tempering
A) Heating and slow cooling
1. Annealing
1. Stress-relief annealing
2. Process annealing
3. Spheroidize annealing
4. Full annealing
5. Bright and Black
Annealing
6. Iso-thermal annealing
2. Normalising
Prof. Naman M. Dave

4. Prof. Naman M. Dave

5. Objectives / Purposes
 Softening the steel
 Refining grain structure
 Improve mechanical, physical, electrical and
magnetic properties
 Relieving Internal Stresses
 Improve machinability
Heat Treatment Processes for
Steels
Annealing
Prof. Naman M. Dave

6. Heating Temperature >
Normally heating temperature for
annealing is as under
• Hypo-eutectoid Steels
Upper Critical Temp. :
(723-910OC) + 30-50OC
• Eutectoid Steel
Critical Temp.
(723OC) + 30-50OC
• Hyper-eutectoid Steels :
Lower Critical Temp.
(723OC) + 30-50OC
However, some special annealing
cycles (like spheroidise annealing
and process annealing) are carried
out below lower critical
temperature also.
Annealing
Prof. Naman M. Dave

7. Holding Time
1 minute per mm of maximum thickness cross
section OR
1/2 hr. Per 1 inch of maximum thickness cross
section
Cooling Rate : Very slow e.g. 25-30OC per
hour. Refer CR1 shown in the T.T.T.
Diagram here. >
Cooling Medium : Cooling in furnace or heap
of ashes or in an insulated chamber
Final Micro-Structure
Hypo-eutectoid Steel : Coarse Pearlite + Ferrite
Eutectoid Steel : Coarse Pearlite
Hyper-eutectoid Steel : Coarse Pearlite+Cementite
Annealing
Prof. Naman M. Dave

8. Types of Annealing
There are different types of Annealing
Processes. Some of the most widely used
processes are…
1.Stress Relief Annealing
2.Process Annealing
3.Spheroidize Annealing
4.Full Annealing
5.Bright Annealing and Black Annealing
6.Isothermal Annealing
Prof. Naman M. Dave

9. Stress Relief Annealing
• Stress relief annealing process consists of three steps.
• The first step is heating the cold worked steel to a temperature between 5000C
and 5500C i.e. below its recrystallization temperature.
• The second step involves holding the steel component at this temperature for
1-2 hours.
• The final step is to cool the steel component to room temperature in air.
• The stress relief annealing partly relieves the internal stress in cold worked
steels without loss of strength and hardness i.e. without change in the
microstructure.
• It reduces the risk of distortion while machining, and increases corrosion
resistance. Since only low carbon steels can be cold worked, the process is
applicable to hypo eutectoid steels containing less than 0.4% carbon. This
annealing process is also used on components to relieve internal stresses
developed from rapid cooling and phase changes.
Annealing
Prof. Naman M. Dave

10. Spheroidise Annealing
• Spheroidise annealing is one of the variant of the annealing process that
produces typical microstructure consisting of the globules (spheroid) of
cementite or carbides in the matrix of ferrite. The following methods are used
for spheroidise annealing.
• Holding at just below A1:Holding the steel component at just below the lower
critical temperature (A1) transforms the pearlite to globular cementite
particles. But this process is very slow and requires more time for obtaining
spheroidised structure.

11. Spheroidise Annealing
• Thermal cycling around A1: In this method, the thermal cycling in the narrow
temperature range around A1 transforms cementite lamellae from pearlite to
spheroidal. Figure depicts a typical heat treatment cycle to produce
spheroidised structure. During heating above A1, cementite or carbides try to
dissolve and during cooling they try to re-form. This repeated action
spheroidises the carbide particles. Spheroidised structures are softer than the
fully annealed structures and have excellent machinability. This heat treatment
is utilized to high carbon and air hardened alloy steels to soften them and to
increase machinability, and to reduce the decarburization while hardening of
thin sections such as safety razor blades and needles.

12. Recrystallization or Process Annealing
• Recrystallization annealing process consists of heating a steel
component below A1 temperature i.e. at temperature between 6250C
and 6750C (recrystallization temperature range of steel), holding at
this temperature and subsequent cooling.
• This type of annealing is applied either before cold working or as an
intermediate operation to remove strain hardening between multi-
step cold working operations. In certain case, recrystallization
annealing may also be applied as final heat treatment.
• The cold worked ferrite recrystallizes and cementite tries to
spheroids during this annealing process. Recrystallization annealing
relieves the internal stresses in the cold worked steels and
weldments, and improves the ductility and softness of the steel.
Refinement in grain size is also possible by the control of degree of
cold work prior to annealing or by control of annealing temperature
and time.
Annealing
Prof. Naman M. Dave

13. Full Annealing
• Full annealing process consists of three steps. First step
is heating the steel component to above A3 (upper
critical temperature for ferrite) temperature for
hypoeutectoid steels and above A1 (lower critical
temperature) temperature for hypereutectoid steels by
30-500C (Figures 1 and 2). In Figure 2, the terms α, γ
and Fe3C refer to ferrite, austenite and cementite
phases.
• The second step is holding the steel component at this temperature for a definite
holding (soaking) period of at least 20 minutes per cm of the thick section to assure
equalization of temperature throughout the cross-section of the component and
complete austenization. Final step is to cool the hot steel component to room
temperature slowly in the furnace, which is also called as furnace cooling. The full
annealing is used to relieve the internal stresses induced due to cold working,
welding, etc, to reduce hardness and increase ductility, to refine the grain structure,
to make the material homogenous in respect of chemical composition, to increase
uniformity of phase distribution, and to increase machinability.
Annealing

14. Isothermal Annealing
• Isothermal annealing consists of four steps. The
first step is heating the steel components similar
as in the case of full annealing. The second step
is slightly fast cooling from usual austenitizing
temperature to a constant temperature just
below A1. Third step is to hold at this reduced
temperature for sufficient soaking period for the
completion of transformation and the final step
involves cooling the steel component to room
temperature in air. Fig. depicts heat treatment
cycles of full annealing and isothermal
annealing. The terms α, γ, P, PS and PF.
• Reduced annealing time, especially for alloy steels which need very slow cooling to
obtain the required reduction in hardness by the full annealing.
• More homogeneity in structure is obtained as the transformation occurs at the same time
throughout the cross section.
• Improved machinability and surface finish is obtained after machining as compared to that
of the full annealed components.
• Isothermal annealing is primarily used for medium carbon, high carbon and some of the
alloy steels to improve their machinability.
Annealing

15. 1. Stress Relief Annealing
• Heating up to a temperature of 500-550 OC
• Iso-thermal holding
• Cooling in still air up to room temperature
2. Process Annealing
• Heating up to a temperature of 600-650 OC
• Iso-thermal holding
• Cooling in still air up to room temperature
3. Spheroidize Annealing
• This Annealing process is used to convert the carbides in a steel
into globular form
• Heating & cooling alternately in the temperature range of 650-
700 OC
• Iso-thermal holding
• Very Slow Cooling (furnace cooling) up to room temperature
Annealing

16. 4. Full Annealing
• Heating up to UCT (723-910 OC) + 30-50 OC for hypo-eutectoid
steels (less than 0.8% C) &
• LCT (723 OC) + 30-50 OC for eutectoid and hyper-eutectoid
steels (more than 0.8% C)
• Iso-thermal holding
• Very Slow Cooling (furnace cooling or insulated chamber
cooling) up to room temperature
5. Bright Annealing and Black Annealing
• Annealing process carried out in protective atmosphere
prevents dis-colouration of the steel. This process is called
as Bright Annealing.
• When components to be annealed are surrounded by
reducing agents like charcoal and annealed in a box then
oxidation of these components is reduced. This process is
Box or Black annealing.
Prof. Naman M. Dave
Annealing

17. 6. Isothermal Annealing
• Medium or High Carbon Steels have their T-T-T curves
positioned substantially away from temperature axis. This
demands pro-longed annealing cycles.
• The steels are fast cooled up to the subcritical i.e. below A1 and
then held isothermally there until the transformation is
completed. Subsequently these steels are cooled to room
temperatures. This saves cycle time substantially.
Prof. Naman M. Dave
Annealing

18. Temperature(OC)
Time
550
500 (1) Stress Relief Annealing Range
(2) Process Annealing Range
(3) Spheroidize Annealing Range
25
960
LCT
600
UCT-1910 (4) Full Annealing Range
763
723
700
650
UCT-21130
Prof. Naman M. Dave
Annealing

19. Objectives / Purposes
 Softening the steel
 Eliminate carbide network at grain boundaries
 Relieving Internal Stresses
 Refining grain structure
 Improve mechanical, physical, electrical and
magnetic properties
Heat Treatment Processes for
Steels
Normalizing
Prof. Naman M. Dave

20. Heating Temperature >
• Hypo-eutectoid Steels:
Upper Critical Temp. (723-910OC) + 50-70OC
• Eutectoid Steel:
Critical Temp. (723OC) + 50-70OC
• Hyper-eutectoid Steels:
Upper Critical Temp. (723-1130OC) + 50-70OC
Normalizing
Prof. Naman M. Dave

21. Holding Time
About 15 minutes
Cooling Rate : Slow e.g.
50-100OC per hour.
Refer CR3 shown in
the T.T.T. Diagram
here. >
Cooling Medium :
Cooling in still
ambient air
Final Micro-Structure:
• Hypo-eutectoid Steel:
Fine Pearlite + Ferrite
• Eutectoid Steel:
Fine Pearlite
• Hyper-eutectoid Steel:
Fine Pearlite +
Cementite
Prof. Naman M. Dave
Normalizing

22. 910
Temperature(OC)
Time
763
25
LCT
UCT-1
1130
Normalising Range
UCT-2
723
1180
Prof. Naman M. Dave
Normalizing

23. Objectives / Purposes
1. To increase the hardness
2. To increase the wear resistance
Heating Temperature >
• Hypo-eutectoid Steels:
Upper Critical Temp.
(723-910OC) + 10-30OC
• Eutectoid Steel:
Critical Temp.
(723OC) + 10-30OC
• Hyper-eutectoid Steels:
Lower Critical Temp.
(723OC) + 10-30OC
Heat Treatment Processes for
Steels
Hardening
Hardening

24. Holding Time
• 1 minute per mm of maximum
thickness cross section OR
• 1/2 hr. Per 1 inch of maximum
thickness cross section
Cooling Rate >
• Faster than Critical Cooling
Rate. For e.g. If CCR is 250OC
per hour then we
• have to cool at say 250 + 30 to
50 i.e. 280-300OC. In other
words the
• temperature drop per hour
should be more than 250 OC.
Prof. Naman M. Dave
Hardening

25. Cooling Medium :
Quenching medium are many. For e.g.
1) 5-10% Caustic Soda (Very Drastic Quench)
2) 5-20% Brine (NaCl)
3) Cold Water
4) Warm Water
5) Mineral oil
6) Animal oil
7) Vegetable oil
8) Air
Normally water is recommended for quenching of plain carbon
steels and oil is
recommended for quenching alloy steels
Prof. Naman M. Dave

26. Final Micro-Structure >
 Steels with C% below 0.3% do
not harden.
 For Steels with C% higher than
0.3%, the final microstructure
will be Martensite + Retained
Austenite + Carbides.
 Relative amount of these
phases will depend on
composition of Steel,
hardening temperature, soaking
time and temperature of
quenching medium.
 For e.g. If the cooling rate
becomes slower than critical
cooling rate then it can result
in formation of pearlite and
bainite also. Needle type micro-structure indicating Martensite,
white coloured structures are a mixture of
carbides and retained austenite
Prof. Naman M. Dave

27. 27
910
Temperature(OC)
Time
763
25
LCT
UCT-1
1130
Hardening Range
UCT-2
723
960
Prof. Naman M. Dave
Hardening

28. Hardenability of Steels
 Hardness is a measure of resistance to wear.
 Hardening is done to improve the hardness of steel i.e. To improve the
wear resistance of steel.
 Hardenability of steel means the depth and distribution of hardness
attained by quenching. It is defined in following ways :
1. It is ease with which the steel can be hardened or ability of steel to get
hardened.
(Qualitative definition)
2. It is the length of the test bar up to which 50% martensite is achieved
OR
It is the length of the test bar up to which 50 RC hardness is achieved.
(Quantitative definition).
 It is noted by a notation like this : J50 = 5. It means 50 RC hardness can
be obtained in the test bar upto 5/16 inch length from the quenched end.
 For testing the hardenability of a steel, there is a test called Jominy
Quench Test. This will be explained in lab.
 All steels are not hardenable. For e.g. low C steels are not hardenable.
This is due to their low Carbon content.
Hardenability
Prof. Naman M. Dave

29. 29
Note :
Only the alloys highlighted in blue colour can
be full hardened
Hardenability
Prof. Naman M. Dave

30. Factors affecting hardenability are as under...
 Alloying content : Higher alloying means good
hardenability
 Homogeneity of Austenite : Higher homogeneity
means good hardenability
 Grain size of Austenite : Higher the size better is the
hardenability
 Presence of un-dissolved carbides : Lesser carbides
better hardenability
 Section of steel : Smaller the section better is the
hardenability
 Quenching medium : Selecting the quenching
medium as per the size of the casting will give better
results for full hardening.
Hardenability

31. Objectives / Purposes
1. To reduce hardness
2. To eliminate retained austenite
3. To relieve internal stresses induced by
quenching
4. To improve toughness and ductility
5. To improve other mechanical properties
Heating Temperature >
Low temperature tempering : 150-250OC
Med. temperature tempering : 350-450OC
High temperature tempering : 500-650OC
Heat Treatment Processes for
Steels
Tempering
Tempering
Prof. Naman M. Dave

32. Holding Time :
• Low temperature tempering cycle :
Heating results in formation of low C martensite from Martensite. Hence holding time
depends on Martensite content in the micro-structure (after hardening). Holding should be
done untill C% in Martensite decreases and reaches the value of 0.3%.
Tempering
Cooling Rate : ->
Slower than hardening See the curve shown on T.T.T. Dig. Here
• Medium temperature tempering cycle :
Heating results in formation of Bainite
from retained austenite. Hence holding
time depends on Retained Austenite (RA)
content in the micro-structure (after
hardening). The holding should be done
untill all the RA converts to Bainite.
• High temperature tempering cycle :
Heating results in formation of ferrite
from Martensite. Hence holding time
depends on Martensite content in the
micro-structure (after hardening). The
holding should be done untill all
Martensite converts to ferrite by losing
carbon.

33. Tempered Martensite (Black colour)
+
Carbides (White colour)
+
Retained Austenite (White colour).
Cooling Medium : Cooling in air
Final Micro-structure : ->
Tempered Martensite (having lower C% than
Martensite formed after hardening) +
‘ε’ or Epsilon Carbide (Fe2.4C) (having higher C%
than Cementite i.e. Fe3C +
Retained Austenite. The quantity of retained austenite
after tempering is lesser than before.
Tempering
<-Tempering Colors and
Temperatures

34. Surface Hardening Concept
Surface
- Pearlite +
Cementite
- High Hardness
- Highly Wear
Resistant
Core
- Ferrite
- Low hardness
- Ductile
- Capable of
withstanding
stress
Cross section of a case
hardened gear teeth
 The Carbon content in the steel determines whether it can be directly hardened or
not. If the Carbon content is low (for example less than 0.3%) then an alternate
means exists to increase the Carbon content of the surface. The part then can be
heat-treated by either quenching in liquid or cooling in still air depending on the
properties desired.
 Note that this method will only allow hardening
on the surface, but not in the core, because the
high carbon content is only on the surface. This
is sometimes very desirable because it allows
for a hard surface with good wear properties (as
on gear teeth or knife), but has a tough core that
will perform well under impact loading.
 It is also possible to add additional carbon or
chromium or boron to the outer surface of a
component low in carbon which will make the
surface sufficiently hard.
 This process is known as case hardening or
surface hardening or selective hardening.
Prof. Naman M. Dave

35. I. Thermal Treatments
1) Induction Hardening
2) Flame Hardening
3) Laser Beam
Hardening*
4) Electron Beam
Hardening*
II. Thermo-Chemical Treatments
1) Nitriding
a) Gas Nitriding
b) Liquid Nitriding
2) Carburizing*
a) Solid / Pack Carburizing*
b) Gas Carburizing*
c) Liquid Carburizing
(Cyaniding)
3) Carbo-nitriding*
4) Chroming*
5) Boronizing*
Surface Hardening Concept
Prof. Naman M. Dave

36. 36
Thermal Treatment
Induction Hardening
 In Induction hardening, the steel part is
placed inside a electrical coil which has
alternating current through it. This induces
Eddy Current on the outer surface of the
component and heats it up.
 Depending on the amperage of current,
frequency of current and heating time
depth of hardening can be controlled .
 Temperature is around 750-800OC.
 The heated region is then quenched by
water jets to achieve the desired hardness.
 Tempering can be done to eliminate
brittleness.
 Maximum 6 mm of depth can be achieved
in about 6 seconds time.
 Example of application : Gear teeth
hardening
Coil
Work
Piece
Work Piece
(Gear)
Coil
Prof. Naman M. Dave

37. Flame Hardening
 A high intensity oxy-acetylene flame is applied to the selective region. The
temperature is raised high enough to be in the region of Austenite transformation.
 The "right" temperature is determined by the operator based on experience by
watching the color of the steel.
Flame hardening of a flat cross section work piece Flame hardening of a circular cross section work
piece using lathe
(MOVING)
(STATIONARY)
(Stationary)
Work piece
(Rotating)
Thermal Treatment
Prof. Naman M. Dave

38.  The overall heat transfer is limited by the torch and thus the interior never
reaches the high temperature.
 The heated region is quenched to achieve the desired hardness.
 Tempering can be done to eliminate brittleness.
 The depth of hardening can be increased by increasing the heating time.
 Maximum 6 mm of depth can be achieved.
A gear teeth being surface hardened
using flame hardening method
Thermal Treatment
Flame Hardening
Prof. Naman M. Dave

39. Flame Hardening
 Large parts, which will not normally fit in a furnace, can be heat-treated using
this method.
 Example of application : Lathe Bed Hardening, Large Gears’ Teeth hardening,
Large Sprockets’ teeth hardening, etc.
Thermal Treatment
Prof. Naman M. Dave

40. Thermo-chemical Treatment
Nitriding
 Introduction of nitrogen into the outer surface of steel parts in
order to give an extremely hard, wear resisting surface is called
as nitriding or nitrogen hardening.
 Nitride compounds precipitate out during one of the following
processes
a) Gas nitriding - heat in ammonia
b) Liquid nitriding - dip in molten cyanide bath
 A schematic layout of gas nitriding plant is shown in the next
slide.
Prof. Naman M. Dave

41. 41
Gas Nitriding
 The part to be nitrided is placed in a box made of
heat resistant steel. This box also has provision
for inlet and outlet of ammonia gas (2 holes
drilled on the walls – one for inlet of ammonia
gas and another for outlet of ammonia gas)
 The box (along with the component) is then
placed in the furnace and heated to around 450-
550OC.
 Then a steady flow of Ammonia gas is injected
in the box.
 On coming in contact with the steel, the
ammonia vapour gets dissociated and nascent
nitrogen is released. This nitrogen combines with
other elements in outer surface of steel like C, Si,
Mn, Ni, Cr, Mo, etc and forms nitrides which are
extremely hard. After that, quenching is done.
 Case thicknesses are between 0.5 to 0.8 mm with
hardness up to 70 RC.
Thermo-chemical Treatment
Prof. Naman M. Dave

42. Solid Carburizing or Pack carburizing
 The component is placed in a Cast Iron box
along with a carbon rich material (e.g.
charcoal). The box is placed in a furnace and
heated to a temperature around 900-950 oC and
maintained at that temperature for some period
(1 hr for 0.1 mm depth).
 The carbon is absorbed into the austenite on the
surface of the component.
 The depth of penetration of carbon into the
surface depends on the temperature and the
time spent in the furnace.
 After enriching the surface with carbon, oil
quenching is done.
Carburizing
 Low-carbon steel is heated in a carbon-rich environment and then
quenched.
 Depth achieved by carburizing is 0.025 to 4 mm with hardness around 60
RC.
Thermo-chemical Treatment

43. 43
Gas carburizing
1. A mild carburizing gas is generated from LPG
(Liquified Petroleum Gas). This gas is fed into the
furnace containing the heated steel component.
2. Another method of gas carburizing is called liquid
feed or drip feed method. In this method, an organic
liquid such as a mixture of iso-propyl or methyl
alcohol with benzene is dripped directly into the
furnace for generating a specific furnace atmosphere.
• In any case, the gas mixture breaks to form an
atmosphere containing N2, Ha, CH4, CO2, and H2
(due to heat). The component to be carburized is
heated externally and made to travel through this
atmosphere of the furnace and thus their surface
becomes rich in Carbon content. It is then quenched
to form a hard external surface whereas the core
remains soft and ductile. The depth depends upon
time and temperature of exposure to carbon rich
atmosphere.
HIGH FREQUENCY ELECTRIC
CURRENT GAS-CARBURISING
UNIT
Gas Carburizing is conceptually the same as pack carburizing, except that gas is
supplied to a heated component through the furnace.
Thermo-chemical Treatment

44. 44
Liquid Carburizing (Cyaniding)
 The steel parts are immersed in a molten carbon
rich bath maintained at 850-950OC for periods
ranging from 15 minutes to 3 hours.
 Sometimes the heating / holding can be intermittent
i.e. there can be two baths – one at 850-950OC and
another at 760OC. This done so as to minimize the
distortion of shape of product during heat
treatment. After that quenching is done.
 This process produces a thin, hard shell that is
harder than the one produced by other carburizing
methods, and can be completed in 3 hours
compared to several hours as in other methods so
the parts have less opportunity to become distorted.
 Sketch on the left illustrates an electric salt bath
furnace used for cyaniding.
 It is typically used on small parts such as bolts,
nuts, screws and small gears.
 The major drawback of cyaniding is that cyanide
salts are poisonous. Therefore, safety concerns
have led to non-toxic baths that achieve the same
result. ELECTRIC SALT BATH FURNACE
Thermo-chemical Treatment

45. Carbo-nitriding
 Use both carbon and nitrogen – hardness around 70 RC –
maximum depth of hardened surface 0.07 – 0.5 mm
Chromizing
 Pack or dip in chromium-rich material - adds heat and
wear resistance
Boronizing
 Pack or dip in boron-rich material - Improves abrasion
resistance, coefficient of friction
Thermo-chemical Treatment
Prof. Naman M. Dave

46. Sr.
No.
Thermo-
chemical
Treatment
Hardness of
Surface
Depth of Hardened
Layer from Surface
1. Nitriding 70 RC 0.5 – 0.8 mm
2. Carburizing 60 RC 0.025 - 4 mm
3. Carbo-
Nitriding
70 RC 0.07 – 0.5 mm
4. Chromizing 70 RC -
5. Boronizing 70 RC -
Comparison of different thermo-chemical Treatment
Thermo-chemical Treatment
Prof. Naman M. Dave

47. Thank
You