Heat treatment Process Such as Hardening, Normalizing, Annealing and other related processes

1. Unit 3: Heat Treatment of Steels
Part 1
FACULTY: PROF. Y. M. KHAN
Subject: Materials & Metallurgy
Asst. Professor,
Dept. of Mechanical Engineering, ICEEM, Aurangabad

2. Heat Treatment
Heat treatment is a very broad term and includes any
heating and cooling operations or any sequence of two or
more such operations applied to any material in order to
modify its internal structure or alter its physical, mechanical
or chemical properties.

3. Objectives of Heat Treatment
1. To increase hardness, wear & abrasion resistance and cutting ability of steel.
2. To resoften hardened steel
3. To adjust its mechanical, physical & chemical properties such as T.S. ductility.
4. Eliminate internal residual stresses
5. To induce controlled residual stresses e.g compressive stresses on the surface sharply
increase the fatigue life components
6. To stabilize steel dimensions & size
7. To produce special microstructure to increase machinability & corrosion resistance
8. To eliminate entrapped gases
9. To change composition of the surface

4. Types of Heat Treatments
A. Softening
1. Annealing
2. Normalizing
B. Hardening
1. Conventional Hardening
2. Austempering
3. Martempering
4. Maraging
5. Ausforming
6. Surface and case Hardening

5. 1. Full Annealing
The important characteristics of full annealing is the slow cooling of
steel during transformation.
Slow cooling results in the formation of spheroidal carbides or coarse
lamellar pearlite which are soft & ductile and results in softening of
steel.

6. i) Process
a) Heating
It involves heating steels up to austenitic region
For Hypo eutectoid steel temperature range is A3 + 50oc
For Hyper eutectoid steel temperature range is A1 + 50oc
Temp oC
% C by weight
0.025 0.8 A1
A3
727
768
Full Annealing Temperature Range
For annealing do not heat steel above Acm
Dislocations
Pearlite
Fe3C

7. b) Holding/ Soaking
After heating steel above the austenitic temperature it is held at this temperature for some
time period. Generally 1 hr for each 25 mm thickness of diameter. During this uniform
structure is obtained.
c) Slow Cooling
The steel is then cooled very slowly ( slow cooling rate). This is done by cooling steel in furnace
itself by turning ‘off’ the furnace and keeping steel in it.
b) Holding
Temp 0c
time
a) Heating
c) Slow Cooling in Furnace
727
Coarse Pearlite
Stages of Full Annealing

8. 727
Temp 0C
Log time
γ
Mf
Ms
IT Curve
CCT Curve
Coarse
Pearlite
Full Annealing in TTT Diagram

9. ii) Purpose
1. To relieve internal stresses
2. To increase ductility & % elongation
3. To refine grain size
4. To make homogeneous chemical composition
5. Increase machinability
6. Make steel suitable for subsequent heat treatments

10. iii) Microstructure Changes
Austenite is converted into coarse pearlite by slow cooling
Austenite

11. iv) Disadvantages
1. Time Consuming
2. Simultaneous(parallel) annealing is not possible

12. v) Applications
1. Modify properties of steel castings
2. Relieve internal stresses of steel sheets and strips
3. Increase machinability of forgings

13. 3. Cyclic Annealing
This heat treatment is given to high carbon & air hardened steels to
soften & increase machinability.
This heat treatment is also called as Spherodising or spherodise
annealing.

14. i) Process
a) Heating
Steels are heated up to 7270C when the temperature is reduced just below
7270C. Due to eutectoid reaction austenite tries to transform in pearlite which
is a mixture of α ferrite and cementite. These Fe3C cementite particles are hard
& difficult to machine.

15. b) Thermal Cycling
After heating steel above the austenitic temperature they are cooled just below
727 where pearlite formation starts as soon as it is formed, they are reheated up
to austenitic temperature. This process of heating cooling and reheating is done
several times.
b) Thermal Cycling
Temp 0c
time
a) Heating
c) Cooling in air
727
Pearlite
Stages of cyclic Annealing
γ
γ γ
γ
P
γ
P P P

16. Spherodization of pearlitic cementite
Pearlite
cementite
α ferrite
Pearlite
Cementite layer breaks
Pearlite
cementite
α ferrite
Pearlite
Spheroids of
Cementite
α ferrite matrix

17. c) Slow Cooling
The steel is then cooled in air. There is no change in
microstructure during cooling.

18. ii) Applications
To increase machinability of
1. High Carbon Steels
2. Air hardened steels
3. Safety razor blades
4. Needles

19. 4. Isothermal Annealing
In this heat treatment the components are slightly fast cooled from
usual austenitic temperature of conventional annealing to a
temperature just below A1.
Then component is held at this temperature for sufficient time
period for the completion of the transformation and then cooled to
room temperature in air.

20. 727
Temp 0C
Log time
γ
Mf
Ms
Isothermal Annealing
Full annealing
Heat Treatment Cycles of Full Annealing & Isothermal annealing
A3
A1

21. Advantages
1. Reduce annealing time
2. Because of equalization of temperature, the transformation occurs
at same time throughout the cross section. This leads to more
homogeneity in structure.
3. It shows improved machinability, improved surface finish after
machining and less warping.

22. Application
1. Used for medium & high carbon steel
2. Used for some alloy steel

23. 5. Diffusion Annealing
This process is applied to alloy steel ingots to reduce dendritic &
inter crystal segregation which increases susceptibility of steel
subject to plastic working to be failure.
It is also know as ‘homogenization’.
Dendritic segregation reduces ductility and toughness of alloy steel.
Hence large castings are homogenized in many cases

24. Process
a) Heating
Heating up to 1100 to 1200 0C so as to make composition uniform or homogenous.
b) Holding
Holding time ranges from 8 to 20 Hrs depending on composition and mass of charge.
c) Cooling
Cooled in furnace
b) Holding( 8 to 20 Hrs)
Temp 0c
time
a) Heating
c) Cooled in furnace
1100 to 1200
Total Process Time: 50 to 100 Hrs
After homogenization foundry casting
undergo full annealing or normalizing
to refine grain structure & improve
their properties.

25. Normalizing
The purpose of the process is same as that of annealing.
For hyper eutectoid steels the process may also be used to
eliminate the cementite network that may have formed due
to slow cooling in the temperature range from Acm to A1.

26. i) Process
a) Heating
It involves heating steels up to austenitic region uniformly
For Hypo eutectoid steel temperature range is A3 + 50oc
For Hyper eutectoid steel temperature range is Acm + 50oc
Temp oC
% C by weight
0.025 0.8 A1
A3
727
768
Normalizing Temperature Range

27. b) Holding/ Soaking
After heating steel above the austenitic temperature it is held at this temperature for some
time period. Generally 1 hr for each 25 mm thickness or diameter. During this uniform
structure is obtained.
c) Cooling
The steel is then air cooled (medium cooling rate ). This is faster than annealing. This cooling is
non uniform cooling and hence austenite transforms to medium pearlite.
b) Holding
Temp 0c
time
a) Heating
c) Air Cooling
727
Medium Pearlite
Stages of Normalizing

28. 727
Temp 0C
Log time
γ
Mf
Ms
IT Curve
CCT Curve
Medium
Pearlite
Normalizing in TTT Diagram
Air Cooled

29. ii) Microstructure Changes
Austenite is converted into medium pearlite by air cooling
Austenite

30. Difference between annealed and
normalized pearlite

31. iii) Purpose
1. To Increase machinability
2. To relieve internal stresses
3. To modify & refine cast dendritic structures.
4. To modify & refine grain size
5. Make steel suitable for subsequent heat treatments

32. iv) Advantages
1. Faster Process
2. Requires less furnace time as compared to annealing due to air
cooling outside the furnace

33. v) Applications
1. Modify properties of cast metals
2. Make uniform grains in forged, rolled and extruded parts.
3. Increase machinability of low and high carbon steel.

34. Difference between normalizing and
annealing
Sr.
No
Annealing Normalizing
1 Less hardness, tensile strength &
toughness
Slightly more hardness, tensile strength &
toughness
2 Cooling Medium – Furnace Cooling Medium – air
3 Austenite – Coarse Pearlite Austenite – Medium Pearlite
4 Grain size distribution is more uniform Grain size distribution is less uniform
5 Total Time required is more Total Time required is less

35. Normalizing
The purpose of the process is same as that of annealing.
For hyper eutectoid steels the process may also be used to
eliminate the cementite network that may have formed due
to slow cooling in the temperature range from Acm to A1.

36. i) Process
a) Heating
It involves heating steels up to austenitic region uniformly
For Hypo eutectoid steel temperature range is A3 + 50oc
For Hyper eutectoid steel temperature range is Acm + 50oc
Temp oC
% C by weight
0.025 0.8 A1
A3
727
768
Normalizing Temperature Range

37. b) Holding/ Soaking
After heating steel above the austenitic temperature it is held at this temperature for some
time period. Generally 1 hr for each 25 mm thickness or diameter. During this uniform
structure is obtained.
c) Cooling
The steel is then air cooled (medium cooling rate ). This is faster than annealing. This cooling is
non uniform cooling and hence austenite transforms to medium pearlite.
b) Holding
Temp 0c
time
a) Heating
c) Air Cooling
727
Medium Pearlite
Stages of Normalizing

38. 727
Temp 0C
Log time
γ
Mf
Ms
IT Curve
CCT Curve
Medium
Pearlite
Normalizing in TTT Diagram
Air Cooled

39. ii) Microstructure Changes
Austenite is converted into medium pearlite by air cooling
Austenite

40. Difference between annealed and
normalized pearlite

41. iii) Purpose
1. To Increase machinability
2. To relieve internal stresses
3. To modify & refine cast dendritic structures.
4. To modify & refine grain size
5. Make steel suitable for subsequent heat treatments

42. iv) Advantages
1. Faster Process
2. Requires less furnace time as compared to annealing due to air
cooling outside the furnace

43. v) Applications
1. Modify properties of cast metals
2. Make uniform grains in forged, rolled and extruded parts.
3. Increase machinability of low and high carbon steel.

44. Difference between normalizing and
annealing
Sr.
No
Annealing Normalizing
1 Less hardness, tensile strength &
toughness
Slightly more hardness, tensile strength &
toughness
2 Cooling Medium – Furnace Cooling Medium – air
3 Austenite – Coarse Pearlite Austenite – Medium Pearlite
4 Grain size distribution is more uniform Grain size distribution is less uniform
5 Total Time required is more Total Time required is less

45. Hardening
The purpose of the process to make steels hard as per the
need of applications.
Hardening maybe defined as rapid cooling of steel from
austenite phase.
The rapid cooling is obtained by immersion of heated steel
in a liquid bath such as water or oil.
The fast cooling of the steels from austenite phase results in
the formation of the metastable ‘martensite’

46. i) Process
a) Heating
It involves completely heating steels up to austenitic region
For Hypo eutectoid steel temperature range is A3 + 50oc
For Hyper eutectoid steel temperature range is A1 + 50oc
Temp oC
% C by weight
0.025 0.8 A1
A3
727
768
Hardening Temperature Range
If temperature of heating Hypo eutectoid is less than A3 temp., then
‘pro eutectoid ferrite’ appears & remains after quenching and gives
‘soft spot’ lowering the hardness.
In plain carbon steels heating is not done abovee Acm temp. because
of
1. Acm is steep line, high temp is required & due to which austenite
grains becomes coarse & gives coarse martensite, a brittle form.
2. Quenching from high temp results in more distortions, more
chances of cracking, more oxidation , decarburization and more
retained austenite

47. b) Holding/ Soaking
After heating steel above the austenitic temperature it is held at this temperature for some
time period. Generally 1 hr for each 25 mm thickness or diameter. During this uniform
structure is obtained.
c) Cooling (Quenching)
The steel is then quenched (faster cooling rate ). Steel components are rapidly cooled with
cooling rate just exceeding the ‘critical cooling rate’.
b) Holding
Temp 0c
time
a) Heating
c) Water or Oil Cooling
727
martensite
Stages of Hardening

48. 727
Temp 0C
Log time
γ
Mf
Ms
Surface
Core
Martensite
Hardening in TTT Diagram
Tempering Process
Brittle
Martensite
CCR

49. ii) Microstructure Changes
Austenite is converted martensite by quenching
Austenite Needles of Martensite

50. Martensite under Optical

51. iii) Purpose
1. To Increase hardness of steel
2. To Increase wear resistance of steel
3. To Increase service life of steel

52. iv) Disadvantages
1. Steels formed after hardening are brittle and hence required
further heat treatment like tempering process.

53. Hardening Defects
Feature Causes of defect by
heating quenching tempering
Steel to soft hardening temperature too low,
too little heated, work piece too
much cooled prior to quenching
quenching bath too hot,
quenching bath too
small, wrong quenching
medium, quenching
speed too slow,
quenching time too
short
tempering temperature too
high, wrong temper colour,
when tempering from inside,
too slowly cooled down
Steel irregularly
hard
irregular heating, sulphur taking
in from fuel gas, scaled
workpiece, sticking melting bath
when using melting baths
too big tongs' bit,
unclean quenching bath,
pieces to be hardened
lie too crowded,
unsuitable covering,
wrong move in the bath
(vapour bulbs),
annealing skin and scale
irregularly heated

54. Feature Causes of defect by
heating quenching tempering
Steel too hard too high hardening temperature quenching medium too
coarse
tempering temperature too
low
Distorted work-
pieces
due to great cross-section
differences heated wrong,
unfavorable position in
annealing furnace, heated too
quickly and unevenly, work piece
partly superheated, covered
inadequately or even not, too
long kept onto hardening
temperature
cooled down too
coarsely, emerged wrong
-
Work pieces with
cracks
irregularly and too much heated,
sharp screwing not covered, not
preheated
irregularly quenched
- quenched too coarsely
- suspended wrong
Hardening Defects

55. Retained Austenite
Austenite that does not transform to martensite upon quenching is
called retained austenite.
This retained austenite occurs when the steel is not quenched to a
temperature low enough to form 100% martensite.
This retained austenite can cause loss of strength and
increased brittleness.

57. Remember the martensite transformation never reaches 100%, since
there is always some austenite is left untransformed into martensite.
This is due to the slower cooling rates at core and faster cooling rate
at surface.

58. 727
Temp 0C
Log time
γ
Mf
Ms
Surface
Core
Martensite
Hardening in TTT Diagram
Tempering Process
Brittle
Martensite
CCR

59. Sub Zero Treatment
The steel components are cooled to a temperature below Mf by using a suitable cooling
medium.
727
Temp 0C
Log time
γ
Mf 90%
Ms
Martensite
Ms 50 %
Sub Zero Treatment
M
RT
-80
Due to this the retained austenite gets
transformed to martensite.

60. If quenched steel is held at room temperature for some time before
subzero treatment, less austenite is transformed to martensite. i.e.
retained austenite gets stabilized, therefore the subzero treatment
should be done immediately after hardening.
Sr No Cooling Medium Min Temp obtained in0C
1 Ice + salt (NaCl) -23
2 Ice + salt (CaCl) -55
3 Acetone + CO2 -76
4 Liquid air -183
5 Liquid Nitrogen -196
6 Liquid Hydrogen -253
7 Liquid Helium -269
Various Cooling Mediums used in Subzero treatment

61. Application
This treatment can be used to eliminate retained austenite from
hardened components like tool steel & die steel.

62. Tempering
Tempering is a heat treatment process applied to hardened materials
to relive the internal stresses, to reduce hardness and to increase
ductility toughness of a material and eliminate retailed austenite

63. Process
b) Holding 1- 2hrs
Temp 0c
time
a) Heating(100- 700)
c) Air Cooling
727
Stages of Tempering

64. Types of tempering
-Reduction in hardness up to 60RC
Martensite Low carbon Martensite ε carbide
Tempered martensite Transition carbide
1. Low temperature tempering
-Temperature range= 1000c -200 0c
-Micro structural changes
Important Structural Changes
Due to separation of ε carbide the structure etches rapidly by Nital & appears dar under
microscope so its called as black martensite
No change in % of RA
Internal stress decreases
Decrease in brittles of martensite

65. Types of tempering
-Reduction in hardness up to 40RC
Low carbon Martensite ε carbide
Tempered martensite Transition carbide
2. Medium temperature tempering
-Temperature range= 2000c -500 0c
-Micro structural changes
Important Structural Changes
Retained Austenite –> Bainite/Martensite
Increase in ductility and toughness
Fine Cementite Ferrite
Troostite

66. Types of tempering
-Reduction in hardness up to 20RC
Fine Cementite Ferrite
3. High temperature tempering
-Temperature range= 5000c -700 0c
-Micro structural changes
Increase machinability
Coarse Cementite Ferrite
Troostite Spherodite

67. Temper Brittleness (Embrittlement)
Alloy Steels containing Ni, Mn, & Cr, when cooled slowly from
temperature about 200- 4000C becomes brittle in impact.
This is probably because of precipitation of films on particles of
carbides at grain boundaries.
Due to temper brittleness tensile strength & ductility is not
seriously affected but there is reduction in impact value & inter
crystalline fracture occurs.
If components are quenched in water it eliminates this defect.
Addition of 0.5% Mo to steel also eliminates this defect.

68. Tempering Colors
In tempering there is need to control accurate temperature. As the
temperature varies it gives wide range of change in properties.
As the steel is heated the oxide film begins to form on the surface of
the component.
Initially it shows & gradually thicken with
increase in temp. until it is
pale yellow color
dark blue

69. Tempering Colors for Plain Carbon Steel Tools
Temp 0C Color Type of Component
220 Pale Yellow Scrapers, Hacksaws, Turning & Parting Tools
230 Straw Screwing Dies, Hammer Faces, Slots
240 Dark Straw Shear Blades, Milling Cutter, Drills, Boring Tools, Reamers
250 Light Brown Penknife Blades, Taps, Dies
260 Purplish Brown Plane Blades, Stone Cutting Tools, punches, reamers, twist drills
270 Purple Axes, gimlets, surgical tools, press tools
280 Deeper Purple Cold Chisels, chisels for wood, plane cutters for wood
290 Bright Blue Cold Chisels, screw drivers
300 Dark Blue Wood saws, Springs

70. Austempering
Mass effect of heat treatment

71. Surface and Case Hardening
Most of the load bearing structure such as cams, gears &
shafts requires hard and wear resistant surface and at the
same time tough and shock absorbing cores due to the
following reasons

72. 1. In service metal piece is never stressed uniformly throughout its
section
2. Rotating shafts have maximum stress at the surface and minimum
stress at the center
3. Wear & fatigue starts at surface
4. On the surface stress concentration appears from scratches, tool
marks, poor fillets etc.

73. Case Hardening
It may be defined as a process for hardening a ferrous material in
such a way that the surface layer known as ‘case’ is substantially
harder than the remaining material known as ‘core’.
This can be achieved in two ways:
1. By changing chemical composition at surface called as case
hardening
2. Without changing chemical composition at surface called as surface
hardening

74. Case Hardening Types
1. Carburizing
2. Nitriding
3. Carbonitriding
4. Cyaniding

75. Carburizing
1. Pack Carburizing
2. Gas Carburizing
3. Liquid Carburizing

76. Pack Carburizing
1. Solid Carburizing Medium
Charcoal- 55 %
Coke – 35 %
BaCO3 Or CaCO3 OR NaCO3 – 10 %
2 . Carburizing reaction
Surface of steel absorbs atomic carbon produced by
the decomposition of carbon monoxide as follows:
2 Charcoal + O2(air) = 2CO
2 CO(dissociation)= CO2 + C (atomic)
CO2 + C ( charcoal) = 2 CO
BaCO3 = BaO + CO2
b) Holding 6-15hrs
Temp 0c
time
a) Heating
c) Air Cooling
950
Carbon medium
Steel box Component
Carbon layer

77. Advantages
1. More Case Depth 1-2.5 mm
2. More Grinding Allowance
3. Less Dimensional changes
4. Process is cheap

78. Disadvantages
Batch type production
Dirty Process
Case layer is non uniform
Surface finish is poor
More space required

79. Gas Carburizing
PROCESS
It is carried our in muffle furnace . Carbonaceous gas is created
Component is heated to 900degrees in the presence of carburizing gas and hold for 1-2 hrs for
obtaining uniform layer
1. Gas Carburizing Medium
Natural Gas, Butane Ethane, Methane, Coke Oven Gas, Propane
These gases are partially burnt in furnace & are diluted with a carrier gas in order to produce required
atmosphere.
Carrier Gas: Mixture of Nitrogen, hydrogen, and CO.
2. Gas Carburizing reaction
CH4 2H2 + C (at work surface)
CO + H2 H2O + C (at work surface)
b) Holding 1-2 hrs
Temp 0c
time
a) Heating
c) Air Cooling
900

80. Advantages & Disadvantages
Less time required
Good Surface Finish
Automation is possible
Clean
Uniform Layer
Accurate Case depth
Less labor cost
Highly Skilled Labor to maintain require
case depth

81. Liquid Carburizing
1. Liquid Carburizing Medium
Sodium Cyanide – 20 to 50 %
Sodium Carbonate – upto 40 %
Sodium Chloride– Balance
b) Holding 5 min to 1 hrs
Temp 0c
time
a) Heating
c) Air Cooling
950
2. Gas Carburizing reaction
2NaCN + O2 2NaCNO
3NaCNO NaCN +Na2CO3 + C +2N
BaCO3 + 2NaCN Ba(CN)2 + NaCO3
Ba(CN)2 BaCN2 + C

82. Advantages & Disadvantages
Rapid Heat treatment
Low Distortion
Uniform Depth
Less time required
Depth up to 0.1 to 0.5 mm
Poisonous
Explosion can happen
Disposal is difficult

83. Hardenability of Steel
It is the ease with which a steel piece can be hardened by
martensitic transformation.
It is most commonly measured by Jominy End Quench Test

84. Jominy End Quench Test
Specimen
Cylindrical Shape- 25 mm dia
Length- 100 mm
Machined Shoulder

85. Process
Heating specimen above austenitic temperature
for a fixed time and then transferred to a fixture
(quenching jig). Where its end is quenched by
water sprayed by a pipe.

86. Setup

87. Hardness Measurement

88. Variation of Hardness