3
Heat Treatment Processes
Introduction
In all the heat treatment processes, the
material is heated to certain specific
temperature range, held at this temperature
for the certain time period and then cooled
at a specific cooling rate.
This controlled heating and cooling results in
changes in microstructure as well as changes
in the distribution of microconstituents.
3.
4.
5.
6.
To improve strength and hardness
To improve machinability
To soften the material
To improve the ductility and reduce hardness
etc.
Labelling of Phase Boundaries
Fig. 3.1
Different types of heat treatment processes
are carried out:
Rack Your Brain
What causes the transformation of the
deformed martensite into austenite
phase?
1. To relieve the strain of hardening stresses
2. To relieve residual stresses
Fig. 3.2 Eutectoid Region of Iron-Iron Carbide Phase
Diagram
A1 (lower critical temperature line):
In the phase diagram, the lower critical
temperature is shown by drawing a horizontal
line corresponding to eutectoid temperature
(727° Celsius), and it is normally represented
by naming the horizontal line as A1.
It is worth mentioning that below the lower
critical temperature, austenite will convert
into two phases that is cementite and ferrite
phases.
Practice Questions
1.
What will be the microstructure formed
upon slow cooling of steel having
hypoeutectoid composition (less than
0.8% carbon by weight)
(A) Only ferrite
(B) Only pearlite
(C) Mostly ferrite and pearlite
Solution: (C)
Steel having a carbon percentage less than
0.8 is known as hypoeutectoid steel. When it
is cooled to a lower critical temperature line,
it contains ferrite and pearlite.
(D) Only cementite
Heat Treatment Processes
57
A3 (upper critical temperature line):
the upper critical temperature line (A3)
for hypo eutectoid steel so that alloy is
converted to austenite phase.
Case-2 Hypereutectoid steel: In the case
of a hypereutectoid Steel, the alloy is
heated to a temperature 50°C above the
A1 line (lower critical temperature line) to
form a cementite and austenite phase.
Application: Annealing is done to increase
the ductility and toughness of the material.
Generally, medium carbon steels and low
carbon steels are annealed.
Due to cooling in the furnace, the cooling
rate is slow hence coarse pearlite is
formed, which is comparatively soft and
ductile.
Results of full annealing:
1. Cold worked stresses are relieved.
2. Strength and hardness decrease.
3. Ductility and toughness improve.
4. Machinability improves.
The phase boundary is denoted as A3, and
it represents the upper critical temperature
line for hypoeutectoid steels.
Acm (upper critical temperature line):
The phase boundary is denoted as Acm, and
it represents the upper critical temperature
line for hypereutectoid steels.
Various Heat Treatment Process
1. Full annealing: The main characteristic of
this process is furnace cooling. After heating
to a particular temperature depending
upon whether it is hypo eutectoid or hyper
eutectoid steel, it is kept in the furnace
for several hours for cooling. Due to the
furnace cooling, the cooling rate is very
slow. Heating temperature of alloy steel is
decided on the basis of its composition.
Case-1 Hypoeutectoid steel: Steel alloy
is heated to a temperature of 50°C above
Practice Questions
2.
Full annealing of a component results in
______.
(A) Increase in toughness and yield point
(B) Reduction in ductility and resilience
(C) Removal of foreign impurities and
improved surface finish
(D) Increase ductility and machinability
Solution: (D)
Full annealing
machinability.
increases
ductility
and
2. Normalising: It differs to annealing in the
method of cooling. In normalising, cooling
is done in air as the cooling rate of air is
faster as compared to that of cooling rate
in the furnace hence fine grains are formed,
which leads to tough microstructure as
compared to the microstructure formed
by annealing.
Generally used for removing the effects of
previous heat treatment processes.
58
3.
A steel specimen is heated to 780°C
temperature, and after that, it is cooled
in the furnace (at the slowest possible
rate). Which property of the specimen
will increase by this process?
(A) Toughness
(B) Hardness
(C) Softness
(D) Tempering
Solution: (A)
When cooled at the slowest possible rate in
the furnace toughness of the specimen is
increased.
Rack Your Brain
In order to soften the hardened steel,
which of the heat treatment process is
preferably used?
Heat Treatment Processes
Case I: Hypoeutectoid composition:
In the case of an alloy having a carbon
percentage less than 0.8%C, normalising
is done by heating to at least 55°C above
the line A3, i.e. upper critical temperature
in case of hypo eutectoid steel.
Case II: Hyper-eutectoid composition:
In the case of an alloy having a carbon
percentage of more than 0.8%C,
normalising is done by heating to at least
55°C above the line A3, i.e. upper critical
temperature in case of hypo eutectoid
steel.
During normalising for both the cases,
steel is heated till it completely converts
into austenite phase – austenitising.
Then Final step in normalising is to cool
the heated element in the air.
Results of normalising:
1. Strain hardening stresses are released
2. Relatively higher yield point and
ultimate tensile strength.
3. Relatively lower ductility and toughness.
4. Relatively more hardness.
5. Relatively finer grains
Practice Questions
4.
Which one of the following statements is
NOT correct for normalising?
(A) During machining, surface finish
quality gets improved.
(C) In the case of medium carbon steel,
it leads to an increase in strength
(D) Coarse grains are formed
Solution: (D)
(B) In the case of casting, stress is
relieved by normalising
Normalising is used to refine the grains, not
to increase the grains size
3. Isothermal annealing: In case of several
grades of alloy steels, machinability is
very low, and it can’t be improved to the
required extent by full annealing, and in
such cases, isothermal annealing is done.
The specimen is heated above critical
temperature
[either
upper
critical
temperature or lower critical temperature]
and then rapidly cooled to a temperature
Just below A1 line [Lower critical
temperature] and is then held at this
temperature for sufficient time until the
full decomposition takes place; after that,
the specimen is finally cooled in still air.
Practice Questions
5.
In alloy steels, Isothermal annealing is
done to enhance ______.
(A) Machinability
(B) Toughness
(C) Ductility
(D) Weldability
4. Spheroidising: During spheroidising of
various steels (mainly medium, and high
carbon steels), spheroid are formed by
coalescence of cementite (Fe3C) particle,
which leads to a further reduction in
brittleness and hardness of medium and
high carbon steels.
Heat Treatment Processes
Solution: (A)
Isothermal annealing is done to improve
machinability. It is generally used for alloy
steels.
This increase in ductility due to the
formation of spheroids is helpful in
various applications involving machining
and plastic deformation of steel.
The main reason for the increase in
ductility and decrease in hardness is the
formation of spheroid structure.
59
Spheroidising heat treatment can take place by several methods:
If the spheroidising is done in the case of steel which has fine pearlite structure, then the
rate of spheroidising (how fast spheroids are formed) is higher as compared to that of if
spheroidising is done of a steel having coarse pearlite structure.
Practice Questions
6.
What will be the effect on steels after
spheroidising process?
(A) It improves the hardenability of low
carbon steel
(B) It improves the machinability of low
carbon steel
(C) It improves the hardenability of high
carbon steel
5. Process annealing: During cold working,
plastic deformation results in strain
hardening of the materials. If we want to
further deform it plastically it will resist
the deformation process due to strain
hardening effect.
Process annealing is a process to remove
this strain hardening effect, which occurs
due to prior to cold working.
Hence process annealing allows a large
amount of plastic deformation which is
required in applications like fabrication
processes.
60
(D) It improves the machinability of high
carbon steel
Solution: (D)
Spheroidising treatment heat is done to
improve the machinability of medium and
high carbon steel.
1
Recrystallization temperature ∝ degree of cold
working
Rack Your Brain
Which heat treatment process improves
machinability?
Heat Treatment Processes
Previous Years’ Question
Match the heat treatment process (Group A)
and their associated effects on properties
(Group B) of medium carbon steel.
Group A
P : Tempering Q : Quenching
R : Annealing
S : Normalising
Group B
I. Strengthening and grain refinement
II. Inducing toughness
III. Hardening IV. Softening
(A) P-III, Q-IV, R-II, S-I
(B) P-II, Q-III, R-IV, S-I
(C) P-III, Q-II, R-IV, S-I
(D) P-II, Q-III, R-I, S-IV
[GATE-2014, Set-4, 1 Mark]
Solution: (B)
Hint: Heat treatment processes and their
respective effects.
The effect of these factors are discussed
below:
1. Influence of alloy composition: To show
the influence of alloy composition, a term
is introduced named hardenability.
Hardenability: Hardenability is the ease of
martensite formation.
It tells about the rate at which the hardness
decreases as we move from the outer
surface to the inner surface. As in our earlier
discussion, we have studied that the cooling
rate varies while moving from outside
to inside because of which martensite
formation also varies. Don’t confuse
hardenability with hardness, as hardness
means resistance to scratch or indentation,
which is altogether a different property.
The high hardenability of a steel alloy
specimen means that it forms martensite
or hardens to a large extent towards the
inner surface of the specimen.
Hardenability of a specimen is known by
the use of the Jominy end-quench test.
Quenching
Traditionally
steels
having
martensite
structure are produced by first heating to the
temperature such that austenite formation
takes place and then rapid quenching of
the specimen in different quenching media
depending upon the required rate of cooling.
As quenching media comes in contact with
the outer surface, hence outer surface cools
at a faster rate as compared to the inner
surface, and because of this varying cooling
rate between the outer and inner surface
microstructure formed in the outer and inner
surface of materials also vary.
Major factors that influence the formation
of the martensitic microstructure during
heat treatment of steels are mentioned
below:
1. The alloy composition
2. The nature of the quenching medium
3. The dimension of the specimen
Heat Treatment Processes
Fig. 3.3 Jominy end-quench Specimen Mounted
During Quenching
61
Hardenability curves:
Fig. 3.4
Hardness is plotted as a function of
position from the quenched end known as
hardenability curve.
Previous Years’ Question
Hardenability of steel is a measure of
(GATE-2019, Set-2, 1 Mark)
(A)The maximum hardness that can be
obtained when it is austenitised and
then quenched.
(B)
The depth to which required
hardening is obtained when it is
austenitised and then quenched.
(C) The ability to harden when it is cold
worked
(D)The ability to retain its hardness when
it is heated to elevated temperatures
Solution: (B)
Hint: Hardenability is the ease of martensite
formation.
62
Fig. 3.5
The lower end, which is in direct contact with
the cooling medium, like in this case water
will have a maximum cooling rate, and hence
martensite will form to a maximum extent
(100% martensite) at this end. So, it will have
maximum hardness.
As we move away from the quenched end,
the cooling rate decreases so is the extent
to which martensite is formed, and so is the
hardness.
Thus if an alloy steel specimen is highly
hardenable, then that means martensite is
formed to a larger distance, and so it also
means hardness is high to a larger distance
from the quench end.
Jominy distance:
The distance at which there is 50% martensite
and 50% pearlite formation.
The hardenability curves also depend on
carbon content.
Heat Treatment Processes
a different rates of heat removal, so the
cooling rate varies with the use of different
quenching media.
The cooling rate is the property of quenching
media which is in contact with the surface.
Decreasing order of the severity of quench
media:
Water
oil
decreasing
order
↓
Air
Quenching mediums:
Relative
quenching rate
Brine
Fig. 3.6 This Effect is Demonstrated in Figure
for a Series of Alloy Steels in Which only the
Concentration of Carbon is Varied.
The hardness at any Jominy position
increases with the concentration of carbon.
2. Influence of nature of the quenching
media: As different quenching media has
Water + Alkali
(KOH/NaOH)
Water
Oil
Still air
1.2-1.3
1
<1
0.4 to 0.5
Decreasing cooling rate
Media
0.02
Practice Questions
7.
Which one of the following is used as a
quenching medium in Alloy steels?
(A) Water
(B) Ice
(C) Brine solution
(D) Oil
Solution: (D)
In the case of an alloy steels, oil is used as
quenching media.
Grey Matter Alert!
Rack Your Brain
Which quenching media has the highest
heat transfer equivalent?
For the same quenching medium, with an
increase in velocity of quenching medium,
cooling rate increases. Hence, agitation in
quenching medium causes removal of more
amount of heat which therefore increases
the effectiveness of quenching media.
Heat Treatment Processes
Oil quenching is suitable for alloy steels.
3. Influence of dimension (size and shape)
of specimen: Surface area influences the
rate of heat removal hence the cooling
rate. Larger the ratio of surface area to
mass, more will be the cooling rate and
hence deeper is the martensite formation
or hardening effect.
As a sphere has a minimum surface area
to mass ratio so hardening effect of these
63
shape material is very less as compared to
the irregular shapes with edges and corners.
Precipitation Hardening or Age Hardening
Enhancement in the strength and hardness
of some metal, alloys may be achieved by
the formation of extremely small uniformly
and homogeneously dispersed particles of a
second phase within the original phase matrix.
Rack Your Brain
Precipitation hardening is also known by
other names can you name it?
This must be accomplished by phase transformations that is induced by appropriate heat
treatments. This process is called precipitation
hardening because the small particles of the
new phase is termed ‘precipitates’.
2. A solubility limit that rapidly decreases in
the concentration of the major component
with temperature reduction.
In simple words, we can say that the second
phase must be soluble at an elevated
temperature
but
precipitates
upon
quenching and aging at lower temperature.
Both these conditions are satisfied by a
hypothetical phase diagram shown below:
The maximum solubility corresponds
to the composition at point M.
In addition, the solubility limit boundary
between the α and α + β phase
fields diminishes from this maximum
concentration to a very low B content in A
at point N. Furthermore, the composition
of a precipitation-hardenable alloy must
be less than the maximum solubility. These
conditions are necessary for precipitation
hardening to occur in an alloy system. An
additional requirement is discussed below.
Rack Your Brain
Can you name a few materials where agehardening is applicable?
As the alloys ages (time span passes),
strength develops with time, so the term
age-hardening is also used to describe
precipitation hardening process.
Examples:
Aluminium – Copper
Copper
– Beryllium
Copper
– Tin
Magnesium – Aluminium
Some ferrous alloys are also precipitation
hardenable.
Requisite features must be displayed by
the phase diagram of alloy system for
precipitation hardening:
1. An appreciable maximum solubility of the
component in the other, of the order of
several percent.
64
Fig. 3.7
Grey Matter Alert!
For most alloys, diffusion rates at T1 are
extremely slow, such that the single α
phase is retained at this temperature
for relatively long periods.
Precipitation hardening is accomplished by
two different heat treatments:
1. Solution heat treatment: In which all
solute atoms are dissolved to form a
single phase solid solution. Consider an
alloy of composition CO as shown in the
above figure.
Heat Treatment Processes
2. Precipitation heat treatment:
Representation of solution heat treatment
and precipitation heat treatment on the
temperature-versus-time plot:
increases, reaches a maximum value, and
then the yield stress (i.e. strength and
hardness) decreases. This decrement in yield
stress occurs after a very long period of time.
This phenomenon is known as overaging.
Fig. 3.8
Overaging:
From the below figure, we can easily
understand that as the aging time increases,
the yield stress (i.e. strength and hardness)
Heat Treatment Processes
Fig. 3.9
65
Previous Years’ Question
The process of reheating the martensitic
steel to reduce its brittleness without any
significant loss in its hardness is_________.
[GATE-2014, Set-1, 1 Mark]
(A) Normalising(B) Annealing
(C) Quenching(D) Tempering
Solution: (D)
Hint: Tempering is used for reducing
brittleness.
Natural aging:
For enriching the surface with this
extra carbon, a fully austenitic phase is
essential hence the required temperature
is between 950°C – 1000°C.
Upon cooling, the phase amount of
cementite increases at the surface, making
it hard while the core remains soft.
a) Pack carburising:
The components are surrounded by a
mixture of 50% charcoal, 25% BaCO3
and Rest Na2CO3 and CaCO3 inside a
ceramic which is packed from all around
by clay. The mixture is burnt.
When the precipitation process is carried
out at room temperature, it is called natural
aging.
Artificial aging:
When the precipitation process is carried out
above room temperature, it is called artificial
aging.
Maraging:
Fig. 3.11
Fig. 3.10
It is a precipitation hardening process, which
is generally used for a special group of high
strength iron-base alloys. In this process,
one or more intermetallic compounds are
precipitated in a low carbon martensite
matrix.
Typical uses of maraging steels are in dies
and tooling for casting, moulding, forging and
extrusion.
Case Hardening Process
In case of hardening processes, the surface
is hardened while the core is left to remain
soft. The hardened surface is called a case.
1. Carburising:
In case of carburising, the mild steel
components are enriched by C-atoms by
about 1%.
66
Charcoal burns and produces carbon
monoxide and a temperature up to
1000°C is produced, at this temperature
2CO + O2 
→ 2CO2
and then carbon dioxide breaks into
atomic carbon.
The carbon atoms diffuse at the surface
in which BaCO3 acts as an energiser, as
it increases the action of C-atoms at
the surface. Na2CO3 and CaCO3 acts as
a catalyst.
Thereafter the boxes are cooled down
to 450°C to 500°C, and now boxes are
opened, and components are quenched
to increase the phase amount of
cementite at the surface, this results
in a hardened case, while the core
remains soft.
Though the material cost is less, the
overall cost is very high due to large
production cycle and more labour cost.
Case thickness is not uniform.
Suitable for large components.
Heat Treatment Processes
The hardness of Fe3N is greater than Fe3C
[NaCN + NaCl + Na2CO3] — Cyanide bath.
This cyanide bath is sources of carbon and
nitrogen atoms.
Temperature: 820°C-860°C
b) Gas carburising:
Sources of carbon atoms: mixture of
[CO + CH4 (other alkanes)]
Temperature: 930°C to 950°C
Time required: [3-10 hours] – depends
on the thickness
Advantages of gas carburising over
pack carburising:
i. Cycle time is less.
ii. Relatively clean surfaces.
iii. Relatively uniform case thickness.
iv. Overall cost is relatively less.
c) Liquid carburising
Source of carbon atoms: Diluted NaCN
Bath [20% NaCN + 80% Water]
Temperature: 950°C
Both carbon and Nitrogen diffuse
into the surface but during quenching
most of the N-atoms came out being
smaller in size, and the C-atoms enrich
the surface, and upon quenching Fe3C
phase amount increases at the surface,
making it hard.
The process is most suitable for
medium and small size components.
Rack Your Brain
By what process maximum hardness is
obtained for a steel part?
2. Cyaniding:
Both carbon and nitrogen are diffused at
the surface, and the surface is enriched
by both Fe3C and Fe3N
Heat Treatment Processes
2NaCN + O2 
→ 2NaCNO
2NaCNO + O2 
→ Na2CO3 + CO + 2N
(
2CO 
→ CO2 + C Free carbon
)
Upon quenching, the surface is enriched
by both Fe3C and Fe3N, resulting more
hardness at the case as compared to
carburising.
3. Nitriding:
Only the Fe3N phase is increased at the
surface; this is done by heating the steel
in the atmosphere of ammonia.
Source of nitrogen – NH3 gas
Temperature – 500°C + 550°C
Time required – 24 hours to 40 hours
Cooling is done in the furnace itself with
liquid ammonia.
Mostly used in the automobile industry.
8.
Nitriding of a steel shaft improves
its________.
(A) Machinability
(B) Fatigue strength
(C) Torsional stiffness
(D) Surface finish
Solution: (B)
Nitriding is a case hardening process few
characteristics of the process areHard and highly wear-resistant surface, good
fatigue strength.
67
Outline for Various Heat Treatment process for surface hardening
68
Process
Metals
hardened
Element
added to
surface
Carburising
Low-carbon C
steel (0.2%
C), alloy
steels (0.080.2% C)
Carbonitriding
Low-carbon
steel
C and N
Cyaniding
Low-carbon C and N
steel (0.2%
C), alloy
steels (0.080.2% C)
Procedure
General
Typical
characteristics applications
heat steel
at 870950°C in an
atmosphere
of
carbonaceous
gases (gas
A hard, highcarbon surface
is produced,
hardness 55 to
65 HRC. Case
depth <0.5 to
carburising)
or carbon
containing
solids (pack
carburising)
Then quench.
1.5mm. Some
distortion
of part
during heat
treatment.
Heat steel
at 700800°C in an
atmosphere
of
carbonaceous
gas and
ammonia.
Then quench
in oil
Surface
Bolts, nuts,
hardness 55 to gears
62 HRC. Case
depth 0.07 to
0.5 mm. Less
distortion than
in carburising.
Heat steel at
760-845°C
in a molten
bath of
solutions of
cyanide (e.g.,
30% sodium
cyanide and
other salts)
Surface
hardness up
to 65 HRC.
Case depth
0.025 to 0.25
mm, some
distortion.
Gears, cams,
shafts,
bearings,
piston pins,
sprockets,
clutch
plates.
Bolts, nuts,
screws,
small gears
Heat Treatment Processes
Nitriding
Steel
N
(1% Al, 1.5%
Cr, 0.3% Mo),
alloy steels
(Cr. Mo),
stainless
steels, highspeed tool
steels
Heat steel at
500-600°C an
atmosphere
of ammonia
gas or
mixtures
of molten
cyanide salts.
No further
treatment.
Surface
hardness up
to 1100 HV.
Case depth 0:1
to 0.6 mm and
0.02 to 0.07
mm for high
speed steel.
Gears,
shafts,
sprockets,
valves,
cutters,
boring
bars, fuelinjection
pump parts
Boronising
Steels
B
Part is heated
using boroncontaining
gas or solid in
contact with
part.
Extremely
Tools and
hard and
die steels
wear resistant
surface. case
depth 0.025 to
0.075 mm.
Flame
hardening
Mediumcarbon
steels, cast
irons
None
Surface
is heated
with an
oxyacetylene
torch, then
quenched
with water
spray or other
quenching
methods.
Surface
hardness 50 to
60 HRC. Case
depth 0.7 to
6 mm. Little
distortion.
Gear and
sprocket
teeth, axles,
crankshafts,
piston rods,
lathe beds
and centers.
Induction
hardening
Same as
above
None
Metal part
is placed
in copper
induction
coils and is
heated by
high high
frequency
current, then
quenched.
Same as
above
Same as
above
Table. 3.1
Heat Treatment Processes
69