1. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 1
Q. Define heat treatment.
“Heat treatment is defined as an operation involving heating and
cooling of metals or alloys in its solid state with the purpose of changing the
properties of the material.” The physical and mechanical properties of the
materials depend upon the size, shape and form of the micro-constituents
present. The micro-constituents generally present in steel are ferrite,
troostite, sorbite, austenite and cementite.
Steel possesses many properties like strength, cheapness and
workability in addition to toughness, stiffness creep resistance, fatigue
resistance, impact strength, etc. Proper heat treatment of steel plays an
important part in engineering. Heat treatment of all components, whether
cast forged or rolled, is necessary before actual use.
Q. Which factors are to be considered in Heat Treatment Processes?
1. Chemical composition of the material.
2. Mode of manufacture of the material, i.e. cast, ingot, rolled or forged,
etc.
3. Whether any previous heat treatment operation has been carried out
on the material and what is its structure.
4. Heat treatment operations to be performed and properties and
structure required.
Q. Which are the advantages or objects of heat treatment process?
Or
Why heat treatment is done?
The following are the main objects of the heat treatment of steel.
1. To soften the steel that has been hardened by the previous heat
treatment or mechanical working.
2. To harden the steel and increase its strength.
3. To adjust its other mechanical and physical properties like ductility,
malleability, permeability corrosion resistance, etc.
4. To stabilize the dimensions of the steel instruments so that they do not
expand or contract with time.
5. To refine the grain size of the steel and to reduce internal stresses.

2. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 2
Q. Draw cooling curve of pure iron.
Fig 2.1: Cooling curve of pure iron
Q. Draw Fe-C Phase transformation diagram. (Iron-Iron carbide
equilibrium diagram)
The various phases existing in the diagram are as below:
(i) α (Ferrite): Ferrite is a solid solution of carbon in low temperature B.C.C. α
iron. It is almost pure iron and the name ferrite comes from the Latin word
ferrum which means iron. It is a relatively soft and ductile phase
(ii) γ (Austenite): Austenite is a solid solution of carbon in F.C.C. γ - iron. It
can dissolve upto 2.0% carbon at 1147°C. The phase is stable only above
727°C. It is a soft, ductile, malleable and non-magnetic (paramagnetic) phase
(iii) δ (δ - ferrite): It is a solid solution of carbon in high temperature B.C.C. δ-
iron. It is similar to α-ferrite except its occurrence at high temperature.
(iv) Fe3C (Cementite): It is an intermetallic compound of iron and carbon with
a fixed carbon content of 6.67% by weight. It is extremely hard and brittle
phase. It is also called Iron Carbide.

3. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 3
Fig 2.2: Iron-Iron carbide equilibrium diagram
The above diagram contains three different transformations which are
described below:
(i) Peritectic transformation
The peritectic region is the upper left hand corner of Fig. 2.2. In Fe-C
system, this transformation occurs at point P and is as below:
δ of 0.1% C combines with liquid of 0.55% C at 1492°C and forms γ of
0.18%C.
(ii) Eutectic transformation
In Fe-C system, this reaction occurs at 1147°C and 4.3% C and is as
below:

4. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 4
Liquid of 4.3% carbon transforms at constant temperature of 1147 °C
and gives a eutectic mixture of austenite (of 2% carbon) and cementite. This
eutectic mixture of austenite and cementite is called ledeburite.
(iii) Eutectoid transformation
The eutectoid region is in the lower left hand side of Fig 2.2. Eutectoid
transformation in Fig. 2.2 occurs at point E and is as below:
Q. Explain with sketch TTT Curve.
Time Temperature Transformation diagrams or Isothermal diagrams
are also called S curve or C curve due to their shape. For each steel
composition, different IT diagram is obtained. Fig 2.3 shows TTT diagram of
eutectoid steel (i.e. steel containing 0.8% C).
Austenite is stable above eutectoid temperature 727 °C. When steel is cooled
to temperature below this eutectoid temperature, austenite is transformed
into its transformation product. TTT diagram relates transformation of
austenite to time and temperature conditions. Thus, TTT diagram indicates
transformation product according to temperature and also time required for
complete transformation.
Curve 1 is transformation begin curve while curve 2 is transformation end
curve. The region to the left of curve 1 corresponds to austenite (A‟). The
region to the right of curve 2 represents complete transformation of austenite
(F+C). The interval between these two curves indicates partial decomposition
of austenite into ferrite and Cementite (A‟+F+C).

5. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 5
Fig 2.3: TTT diagram of eutectoid steel
At temperatures just below eutectoid temperature, austenite
decomposes into pearlite; at lower temperatures (600 °C) sorbite is formed
and at 500 – 550 °C troostites is formed. If temperature is lowered from
500 °C to 220 °C acicular troostite or bainite is formed. In eutectoid steels,
the martensite transformation begins at MS (240 °C) and ends at MF (50
°C). The change in the hardness of the structures is shown in Rockwell units
(RC) at the right hand side of the diagram.

6. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 6
Q. List common heat treatment processes.
Common heat treatment processes can be classified as follows:
Q. Why annealing is done? Explain their types briefly.
The annealing operation is carried out mainly to obtain the following
properties.
1. To soften the steels.
2. To improve machinability.
3. To relieve internal stress induced by some previous treatment (rolling,
forging, extrusion, uneven cooling).
4. To remove coarseness of grains.
5. To produce a completely stable structure.
Annealing treatment is applied to castings, forgings, cold worked sheets and
wires. The operation consists of (i) heating the steel to- a certain
predetermined temperature (ii) soaking at a constant temperature for a
sufficient time to allow the necessary changes to occur and (iii) cooling at a
predetermined very slow rate.
1. Annealing
(i) Full annealing
(ii) Process annealing
(iii)Isothermal annealing
(iv)Spheroidize annealing
(v) Homogenizing
2. Normalizing
3. Hardening
4. Tempering
5. Surface hardening

7. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 7
1. Full Annealing:
Purpose:
(i) To reduce internal stresses produced due to cold working, welding etc.
(ii) To reduce hardness and increase ductility.
(iii) To refine the grain size.
(iv) To increase machinability.
(v) To make the steel suitable for further heat treatment.
Process:
Hypoeutectoid steel (steel containing less than 0.8 % C) is heated to
30-50 °C above the upper critical temperature and hypereutectoid steel (steel
containing more than 0.8 % C) is heated to 50°C above the lower critical
temperature. The steel is soaked at the annealing temperature (soaking time
depend upon the thickness of steel parts). Then these steel parts are slowly
cooled at the rate of 20 to 40°C per hour. The cooling is carried out in the
furnace.
2. Process Annealing:
It is also known as sub- critical annealing or recrystalization.
Purpose:
(i) To soften the component to restore the ductility.
(ii) To remove the internal stresses produced in the casting by welding or
by previous heat treatment.
Process:
Steel is heated to a temperature from 600 to 650 °C, holding at that
temperature, and then cooling in air or in furnace. By this process, high
degree of softening takes place due to removal of stress from pearlite. No
phase change takes place and the ferrite & pearlite simply rearrange
themselves to induce softening in materials.
3. Isothermal Annealing:
This process is suitable for small rolled and forged components and not for
large components. It is faster than full annealing and saves much time.
Purpose:
(i) To obtain stable structure

8. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 8
(ii) To save the time required for heat treatment
Process:
The process is similar to ordinary annealing but it is first cooled rapidly in air
or by blast in furnace to temperature 600-700 °C. The steel is held
isothermally at this temperature for certain duration then it is rapidly cooled in
air.
4. Spheroidize Annealing:
The process of producing a structure of globular pearlite is known as
Spheroidizing or spheroidizes annealing.
Purpose:
(i) To improve machinability of the steel
(ii) To reduce hardness
(iii) To prevent chances of cracking during cold working.
Process:
This operation is generally applied to the hypereutectoid steels. Steel is
heated just above the lower critical temperature (740 to 770 0
C), held for the
required time and cooled very slowly upto 600 0
C in furnace. Further cooling
is conducted in still air. The cooling rate varies from 20 to 25 0
C per hour. It
should be noted that heating much above Acm will produce lamellar pearlite
instead of granular cementite.
5. Homogenizing:
It is also known as diffusion annealing.
Purpose:
(i) To remove non uniformity of castings this is caused by coring. Coring
means variation in the composition from centre to surface of a grain.
(ii) To improve the structure of steel.
Process:
The steel is heated as rapidly as possible up to 1150 °C and is held at this
temperature for sufficient time so that diffusion takes place. It is then cooled
in 6 to 8 hours to a temperature of 800 to 850 °C and then further cooled in
air. After homogenizing, the full annealing is done to refine the grain
structure.

9. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 9
Q. Explain the normalizing process.
Purpose:
1. To eliminate coarse-grained structure.
2. To remove internal stresses that may have been caused by working.
3. To improve the mechanical properties of the steel.
4. To increase the strength of medium carbon steels to a certain extent
(in comparison with annealed steels)
5. To improve the machinability of low carbon steels
Normalizing is frequently applied as a final heat treatment for items which are
to operate at relatively high stresses.
Process:
1. Heating the metal to temperatures within the normalizing range usually
40°C to 50°C above Ac3 (for Hypoeutectoid steels) and Acm (for
hypereutectoid steels)
2. Holding at this temperature for a short time (about 15 minutes).
3. Cooling in air.
Normalized steels have a higher yield points, tensile strength and
impact strength than if they were annealed, but ductility and machinability
obtained by normalizing will be somewhat lower.
Q. Give Difference between annealing and normalizing
Annealing Normalizing
 Less hardness, toughness.
 For plain carbon steel the
microstructure shows pearlite.
 Pearlite is coarse and usually
gets resolved by the optical
microscope.
 Grain size distribution is more
uniform.
 Internal stresses are least.
 Slightly more hardness,
toughness.
 Microstructure shows more
pearlite.
 Pearlite is fine and appears
unresolved with optical
microscope.
 Grain size distribution is slightly
less uniform.
 Internal stresses are slightly
more

10. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 10
Q. What are hardening and quenching processes?
Hardening:
Objectives:
(i) To improve mechanical properties, like elasticity, strength, ductility,
toughness, etc.
(ii) To enable the metal to cut other metals,
(iii) To develop desired hardness.
Process:
The process consists of heating the metal to a temperature above
critical point. The metal is held at this temperature for a considerable time
and then it is rapidly cooled. The cooling media used varies between water,
oil or molten salt.
Hardening is applied to tools and machine parts to perform the
operations more efficiently.
Quenching:
The rapid cooling of a metal in a bath of liquid during heat
treatment is known as quenching, e.g. Steel is heated above its critical
temperature and plunged into water to cool it, an extremely hard, needle
shaped structure known as „martensite‟ is formed. The rapidity with which
heat is absorbed by the quenching bath has different effects on the hardness
of the metal. Cold clean water is used as quenching media, while addition of
salt increases the hardness considerably. Oil gives the best balance between
hardness, toughness and distortion. Special soluble oils are used as quenching
media.
The parts which are subjected to hardening have good tensile
strength, but poor ductility, toughness and impact strength.
Q. What is tempering and why it is done after hardening?
Objectives:
(i) To reduce internal stresses developed during previous heating,
(ii) To reduce the hardness developed during hardening,
(iii) To give the metal a right structural condition (To stabilize the
structure).

11. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 11
After hardening, when a metal is removed from the quenching media,
it is very hard and brittle and there are several other inequalities in the
structure of the metal. Tempering is done to restore ductility and reduce
hardness. The process involves re-heating of the metal below critical point,
then holding it for a considerable time and then slowly cooling it. Tempering
should be done immediately after quenching in order to relieve hardening
strains. The temperature at which tempering is done varies with the carbon
content of the metal and mechanical properties desired in the finished article.
Lathe tools, chisels in which only the cutting ends need hardening
may be hardened and tempered in one operation only. The whole tool
is heated to the hardening temperature and the cutting end is quenched.
When the cold end is rubbed bright and the heat from unquenched portion
causes tempering, when the colour is satisfactory, the whole tool is quenched.
Three types of tempering processes are classified as:
(i) Low temperature tempering: This type of tempering is done in the range
of 200 - 250° C. At this range, hardness changes to a very small extent.
Tensile strength is increased. Internal stresses are reduced comparatively.
(ii) Medium temperature tempering: Tempering done in this case at a range
of 350° to 450° C. In this case, the properties of the structure are improved,
mostly employed for coil and laminated springs. Highest elastic limit and
toughness are achieved.
(iii) High temperature tempering: This tempering is performed in the range of
550° C to 600° C. Eliminates the internal stresses completely. Comparatively
high strength and toughness are achieved.
Q. What is case hardening? List some of them.
A large number of industrial components like cams, change-over switch
shafts, drive worms brake drums, gears, etc. require a hard wear resistant
surface (also called case) and a soft core, so that it is tough and shock
resistant too. No plain carbon steel and even alloy steels possess both the
requirements, i.e. hard surface and tough core to resist shock. It is noticed
that steel containing 0.1% carbon is tough whereas the steel containing 0.8
%C is very hard and brittle. Both these properties are obtained by the case

12. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 12
hardening process. The heat treatment process of producing a hard wear-
resistant carbon rich case (surface layers) on a tough and soft core of steel
part is known as case hardening. Low carbon steel is used for the case
hardening processes except in induction, hardening, where medium carbon
steel or high carbon steel is used. The processes generally employed for case
hardening are as follows.
1. Carburising
2. Cyaniding
3. Nitriding
4. Carbonitriding
5. Flame hardening
6. Induction hardening.
Q. Explain the process of case carburizing
Process: Curburising is a method of depositing carbon on the surface
layer of low carbon steel in order to produce a hard case. Carburising is also
known as cementation. Roughly, the machined parts of the low carbon steel
are packed with carburising mixture in a steel box as shown in Fig. The
carburising mixture contains 70% charcoal, 10% barium carbonate, 10%
calcium carbonate and 10% sodium carbonate. A layer of the carburising
mixture of nearly 25 mm thickness is placed at the bottom. Then the
components are so placed that no component touches one another or even
the sides of the box. The box is covered and the lid tightly sealed with fireclay
to avoid the entry or escape of gases.
Fig: Packing components for solid carburising

13. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 13
The portion which is not to be case hardened is protected by electroplating on
the surface which does not absorb carbon. The boxes are placed in a furnace
and heated to a temperature of 900 to 980°C for 6 to 8 hours. Temperature
and time of heating depends upon the depth of the case required. After
heating, the box is allowed to cool along with components inside the furnace.
Carbon percentage increases on the surface as the austenite has a tendency
to absorb carbon at high temperatures.
Depth of the case obtained in this case varies from 1 mm to 1.5 mm with the
carbon content on the outer surface at 1.1 to 1.2%.
Q. Explain Cyaniding process for case hardening.
The process of providing a hard wear resistant case with a tough core
to the low carbon steels by liquid cyanide bath is called cyaniding.
Process: The cyanide mixture (20 to 50 % Sodium cyanide and 40% Sodium
carbonate) is heated to a temperature of 870 to 930°C, and the work pieces
contained in a wire basket are immersed in the molten bath of cyanide. The
soaking period varies from component to component depending on the depth
of the case, but generally, it varies from-10 minutes to 3 hours.
Nitrogen produced in atomic form also dissolves on the surface and increase
in hardness takes place due to the formation of nitrides. In nitriding, a portion
of the surface to the parts to be kept soft is coated with such materials which
are not affected by the bath. Careful handling of cyanides is needed as these
salts are very poisonous.
Q. Explain the Nitriding process.
The heat treatment process which produces a hard-wear resistant layer
of nitrides on a tough core of low carbon steel is known as nitriding.
Process: The process is suitable for the steels containing 1%
aluminium, 1.5% chromium and 0.2 per cent molybdenum. The percentage of
carbon in these steels varies from 0.2 to 0.5.
The process consists of heating machined and heat treated components to a
temperature of 500°C for 40 to 90 hours in a gas tight box through which
ammonia gas is circulated. The essential requirement of the operation is close

14. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 14
adherence to a temperature of 500°C. The component is allowed to cool in
the furnace after switching of the supply of ammonia.
When ammonia vapours come in contact with the steel, they get dissociated
NH3 = 3H +N and nascent nitrogen so produced diffuses into the surface of
the workpiece forming hard nitrides.
Q. Give difference between Carburizing and Nitriding.
Carburizing Nitriding
1) Carburizing is a method of heat
treatment by which carbon content at
the surface of a ferrous material is
increased.
1) Nitriding is a case hardening
process by which nitrogen content at
the surface of steel is increased.
2) High temperature (930°C).
Quenching is done.
2) Temperature employed ≤=600°C.
Quenching is not required.
3) Hardening and tempering is
needed.
3) No need of hardening and
tempering.
4) This process is very simple and
inexpensive.
4) This process is complex and
expensive.
5) Grain refinement is not necessary. 5) Before nitriding, grain refinement
is necessary.
6) Inferior surface finish as compare
to nitriding.
6) Surface finish is very good.
Q. Write short note on Carbonitriding.
The process of producing a hard case by the addition of carbon and nitrogen
on the surface of the steel.
Process: Hydrocarbons, carbon monoxide and ammonia gases are used for
Carbonitriding. Carbonitriding is carried out at a temperature of 800 to 875°C
for 6 to 10 hours and the case depth obtained is 0.5 mm. Carbonitriding is
applied to the low carbon steels (steels used for carburising). Nitrogen in the
surface layer of the steel increases its hardenability and permits hardening in
oil quenching. Thus, chances of distortion and cracking are eliminated. The
portion of components which is not to be carbonitrided is protected by copper
plating.

15. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 15
Q. Explain any one surface hardening.
Surface hardening involves the following two methods.
1. Flame hardening:
The process of heating the metal with a flame of an oxyacetylene torch and is
then almost immediately quenched is called as flame hardening.
Fig: Principle of flame hardening
Process: The surface to be case hardened is heated by means of an
oxyacetylene torch for sufficient time and Quenching is achieved by sprays of
water which are integrally connected with the heating device. The heating is
generally accomplished for sufficient time so as to raise the temperature of
the surface of the specimen above the critical temperature. As the
temperature desired is achieved immediately, spraying of water is started. In
mass production work, progressive surface hardening is carried out where it is
arranged to have the flame in progress along with quenching.
Advantages:
 Selective surface can be hardened even on very large components.
 There is less distortion than in ordinary methods.
Disadvantages:
 Temperature can not be precisely controlled.
 Hardening is restricted to parts which are affected by wear.
2. Induction hardening:
The process of the surface hardening by inductive heating is
known as induction hardening.
Process: A high frequency current is passed through the inductor
blocks which surround the surface to be hardened without actually touching

16. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 16
it. The inductor block current induces current in the surface of the metal
which the block surrounds. The induced eddy current and hysterisis losses in
surface material effect the heat required. When the surface, to be hardened,
is heated upto a proper length of time, the circuit is opened and water is
sprayed immediately on the surface for quenching. It is extensively used for
hardening of crank shaft, cam shafts, axles and gears.
Advantages:
(i) Time required for this process is less
(ii) Deformation is reduced.
(iii) Hardening can be controlled by controlling the current
(iv) Depth of hardening can be controlled.
Disadvantages:
(i) High equipment cost
(ii) High maintenance cost
(iii) Method is suitable only for large scale production.

17. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 17
Q. Give difference between Flame and Induction hardening.
Flame Hardening Induction Hardening
 Material is heated with
oxyacetylene flame at a required
temperature, and then it is
followed by water spraying.
 Holding time is required.
 Oxidation and decarburization
are minimum.
 Irregular shape parts can be
flame hardened.
 Flame hardening requires more
care in control of temperature.
 Material is heated by using high
frequency induced current and
then it is followed by water
spraying.
 Due to very fast heating, no
holding time is required.
 No scaling and decarburization.
 Irregular shape parts are not
suitable for induction hardening.
 Easy control of temperature by
control of frequency of supply
voltage.
Q. Give applications of heat treatment processes.
(a) Heat treatment of steel castings
(b) Heat treatment of forgings
(c) Heat treatment of gears
(a) Heat treatment of Steel Castings:
Cause: After solidification, steel castings have a coarse grained structure.
Coarse grained structures have poor mechanical properties, poor
machinability, high hardness and high internal stresses.
Application: Steel castings are subjected to full annealing. Steel castings are
charged in the furnace at a temperature of 300 to 400 °C and heated at slow
rate to the annealing temperature. Steel castings are held at this temperature
for sufficient time and then cooled to low temperature.
Cooling media: Air for low carbon steels while medium and high carbon
steels are cooled in furnace.
(b) Heat treatment of Forgings:
Cause: After forging operation, component loses its mechanical properties
(which were improved by adding different alloying elements).
Application: Forging components are placed in the furnace and heated at a
slow rate to the annealing temperature. Then they are normalized. After

18. -Heat Treatment-
© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 18
normalization, the forged components are subjected to hardening and
tempering to get desired properties.
Cooling media: Air- for low carbon steel forgings
In furnace- for high carbon steel forgings
(c) Heat treatment of Gears:
Cause: Gear teeth are subjected to severe stresses when in use. Thus they
must possess high strength to withstand large torques combined with very
high wear resistance to protect them from wearing away in service.
Application: Plain carbon steel gears (containing 0.4 to 0.5% C) are
hardened and quenched in water from 820 to 850 °C, followed by tempering
from 500 to 550 °C to obtain a Brinell hardness of 220 to 260.