Heat tratment bykotkar ss

Chapter no-03
Heat Treatment
Heat Treatment:- (two marks for explanation)
The heat treatment is very broad term and includes any
heating and cooling operation or any sequence of two or more
such operation- applied to any material in order to modify its
nternal structure or to a ter its physical, mechanical or
chemical properties.
Usually it consists of heating the material to some specific
temperature, holding at this temperature for a definite period
and cooling to room temperature or below room temperature
with a definite rate.

Application :- ( two marks for any
four application)
1. It is used for the hardening of a
component like, shaft, crankshaft, etc.
2. It is used to increase the wear and
abrasion resistance of cutting tools.
3. It is used for removing internal stresses
formed during cold working operations.
4. It is used to increase corrosion
resistance of steel

Two Objectives of Heat Treatment.- ( any Four x ½
mark each )
1. To refine grain structure.
2. To improve machinability.
3. To relieve internal stresses.
4. To increase strength and wear resistance.
5. To increase corrosion resistance.
6. To increase Hardness and toughness of
metal surface.
7. Formation of stable phase.

Needs of Heat Treatment-
1. To refine grain structure.
2. To improve machanibility.
3. To relive internal stresses.
4. To increase strength and wear
resistance.
5.To increase corrosion
resistance.
6. To increase hardness and
toughness of metal surfaces

Figure 1. Heat-treatment temperature Ranges of Classes of Carbon Steels in Relation to
the Equilibrium Diagram

ANNEALING -
Explain any two methods of annealing.
1)Full annealing or Conventional Annealing :
-Full annealing implies annealing a ferrous Alloy by
austenitizing and then cooling slowly in the furnace
itself through the transformation range. The
austenitizing temperature range for hypoeutectoid
steels is usually between 723ºC to 910 ºC and for
hypereutectoid steels, temperature is 723ºC to 1130 ºC
full annealing thus involves.
-Heating steel to proper annealing temperature in
the austenitic zone

Holding the steel object at that temperature for a
definite period of time depending upon its
thickness or diameter so that it becomes
completely austenitic and then.
-Cooling very slowly the steel object through the
transformation range preferably in the furnace
upto room temperature. The purpose of full
annealing is to reduce hardness, to refine grain
size, to make material homogeneous

Isothermal Annealing :-
-In this process transformation occurs at constant
temperature. Steel is heated up to austenitic range then
fast cooled to a constant temperature below AC1, and
held at this temperature for sufficient period for the
completion of transformation and then cooled to room
temperature in air.It reduces the annealing time as
compared to full annealing. Because of equalization of
temperature, transformation occurs at the same time
throughout the cross-section. This leads to more
homogeneity in structure

3)SpheroidiseAnnealing :-(2M)
This heat treatment is given to high carbon and air
hardening alloy steels to soften them and to increase
machinability. Following methods produce spheroidised
structures.
i)Hardening and high temperature tempering: -Due
to tempering of hardened steelsat 650ºC
-700 ºC for a long time, cementite globules are
formed in the matrix of ferrite.
ii)Holding at just below AC1 :-Due to holding for a
long time at just below the lower critical temperature,
cementite from pearlite globularises. The process is
very slow

b) Describe spheroidise annealing in brief? (
04 marks for appropriate answer)
This heat treatment is given to high carbon and air
hardening alloy steels to soften them and to increase mach
inability. The microstructure, typical of this heat treatment
shows globules of cementite or carbides in the matrix of
ferrite. Any heat treatment that produces a structure of the
above is called spheroidises annealing.
Following methods produces spheroidised structure:
i) Hardening and high temperature tempering
Due to tempering of hardened steels at 650-700°c for a
long time, cementite globules are formed in the matrix of
ferrite from martensite.
Martensite → cementite (in globular form) + ferrite
.

ii)Thermal Cycling Around Ac1 :
-Due to thermal cycling in a narrow temperature
interval around AC 1, cementite lamellae from
pearlite becomes spheroidal. During heating above
A1, Cementite or carbides try to dissolve and during
cooling they try to form. This repeated action
spherodises the carbide particles.

4)Process Annealing :-
Process annealing is usually subcritical
annealing and is applied to remove the effects
of cold work, to soften and permit further cold
work as in sheet and wire industries.
Ferrous alloys are heated to a temperature
close to but below the lower limit of the
transformation range (550ºC to 650 ºC) are held
at that temperature and then cooled usually in
air in order to soften the alloy for further
cold working as in wire drawing.

5)Bright Annealing :-
Annealing of steel components is carried out using some
protective medium to prevent oxidation and surface
discoloration. Such type of annealing keeps the surface bright
and
Hence it is called bright annealing. The surface protection
is obtained by the use of an inert gas such as argon or
nitrogen or by using reducing atmospheres.
6)Box annealing: -
Here annealing is carried out in a sealed container under
conditions that minimize oxidation. The components are
packed with cast Iron chips, charcoal or clean sand and
annealed in a way similar to full annealing(Any two methods
from above.)
Figures are not essential but if draw it would be considered

Normalizing
Explain the procedure in normalizing. (4m)
Normalizing is similar to annealing.
The process consist of heating to above the upper critical
temperature AC3 for hypoeutectoid steels and above Acm for
hypereutectoid steel by 30 to 50ºC, holding long enough at this
temperature for homogeneous austenization and cooling to room
temperature in still air.
Due to air cooling which is slightly fast as compared to furnace
cooling employed in full annealing, normalized components show
slightly different structure and properties than annealed components.
Hypereutectoid steels are usually normalized from above Acm
temperature. Normalising produces microstructures consisting of ferrite
and pearlite for hypoeutectoid steels. For eutectoid steels, the
microstructure is only pearlite and it is pearlite and cementite for
hypereutectoid steels

Purpose Of Normalising
(Any Four) ½ Mark each)
1. To increase strength and hardness.
2. To obtain more refined grains than the
annealing.
3. To remove the internal stresses induced by
heat treating, welding, casting etc.
4. To improve machinability of low carbon steel

• Distinguish between annealing &Normalising.
( At least 4 points 1 Mark for each point – 4 Marks)
ANNEALING NORMALISING
Main purpose of annealing is to
relieve internal stresses
Main purpose of normalizing
is to improve mechanical
properties of steel.
Less hardness, more T.S. &
toughness
Slightly more hardness, less
T.S. and
toughness
Pearlite is coarse and usually
gets
Pearlite is fine and usually
appears
unresolved with optical
microscope
Grain size distribution is more
Grain size distribution is

Internal stresses are least. Internal stresses are slightly
more.
Furnace cooling is employed Air cooling is employed.
Cooling rate is slow Cooling rate is fast.
Temp range :
Hypoeutectiod Steel – AC1 + 50o C
Eutectiod steel – AC1 + 50o C
Hypereutectoid Steel – AC1 + 50o
C
Temp range :
Hypoeutectiod Steel – AC1 +
50o C
Eutectiod steel – AC1 + 50o C
Hypereutectoid Steel – ACm+
50o C
Costly and inconvenient Economical and more
convinient.

Flame hardening :- (four marks for explanation )
Flame hardening is process of heating the surface layer of a hardenable steel ( or cast
Iron ) to above its upper critical temperature by means of oxyacetylene flame followed
by water spray quenching or immersion quenching to transform austenite to martensite.
Flame hardening can be done in different ways such as by spot or local area of the
component is heated by one or more flames followed by quenching in water

Flame hardening is process of heating the
surface layer of a hardenable steel ( or cast Iron ) to
above its upper critical temperature by means of
oxyacetylene flame followed by water spray
quenching or immersion quenching to transform
austenite to martensite. Flame hardening can be done
in different ways such as by spot or local area of the
component is heated by one or more flames followed
by quenching in water.

. In progressive method, heating and quenching devices are moved
over the component surface at a controlled rate. Spinning method is used
for parts having a rotational summetry in which the flames are held
against a rotating workpiece and when heating is complete the parts is
quenched by water spray or by complete immersion in water. In
combination method the work is rotated and the flames are transferred
for heating followed by quenching in water or by water spray.
The depth of hardened layer depends on the following parameters :
1. Distance between the gas flames and the component surface.
2. Gas pressure and ratio.
3. Rate of travel of flame head or component.
4. Type, volume and application of quench.
Flame hardening causes less distortion than conventional hardening
and due to high heating rate, oxidation and decarburization are minimum.

Advantages of Flame Hardening.
(Any Four) ½ Mark each
1. It is a fastest process.
2. There is less distortion of surface.
3. It is economical and useful method.
4. Large part can be surface hardened economically
5. The hardened zone is generally much deeper than
that obtained by carburizing. its range from 3 to 6 mm
depth.
6.Thinner case ( 1. mm ) can be obtained by increasing
the speed of heating and quenching

These processes are similar in principle to flame-hardening,
except that the component is held stationary- whilst the whole
of its surface is heated simultaneously by electro-magnetic
induction, as shown in figure , The component is surrounded
by an inductor block through which a high-frequency current
in the region of 2ooo Hz, passes. This raises the temperature of
the surface layer to above its upper critical in a few seconds.
The surface is then quenched by pressure jets of water which
pass through holes in the inductor block.Thus, as in flame-hardening,
the induction processes make use of the existing
carbon content (which must be above 0-4%), whilst in both
case-hardening and nitriding an alteration in the composition
of the surges layer of the steel takes place.

Write the advantages and disadvantages of Induction hardening.
Advantages :-
1) Fast heating and no holding time leads to increase in production
rates.
2) It can be applied to both external and internal surfaces.
3) No scaling and decarburization.
4) Less distortion because of heating of only surface.
5) Through proper design of the heating coils, the shape of the
hardened portion can be controlled very closely.
6) Depth of hardening can be controlled by selecting current of
appropriate frequency
7) This process is automatic so it can be carried out with unskilled labour
Disadvantages :-
1) Irregular shaped parts are not suitable for Induction hardening.
2) Cost of equipment is high.
3) Steels having less than 0.4 % carbon cannot be induction hardened.
4) It is beneficial in mass production only.
5) Associates high maintenance cost

CASE HARDENING -
What do you mean by case hardening / state
its advantages.
Various machine components like cams, gears,shafts,
etc require a hard wear resistant surface and a
relatively soft,tough and resistant inside ,called a core.
Both these requirements may be achieved by
employing a low carbon steel having soft , tough core
and then adding carbon, nitrogen or both to the surface
of the component to provide a hard case. This
treatment is known as case hardening.case hardening
is a technique whereby both surface hardness and

fatigue life are enhanced for steel alloys. This is accomplished by a
carburizing or nitriding process whereby a component is exposed to a
carbonaceous or nitrogenous atmosphere at a elevated temperature.
( 02 Marks)
Advantages : ( any four advantages, ½ mark each)
1.Hard wear resistant case and soft,tough core is obtained .
2.Most of the methods suitable for mass production .
3.Good corrosion & wear resistance.
4.Good fatigue resistance.
5.Uniform case depth can be obtained.
6.Negligible change in dimension after case hardening

Describe Case Hardening.
Numerous industrial applications such as cams, gears, etc. require a
hard wear resistant surface called the case and a relatively soft, tough and
shock resistant inside called the core. No plain carbon steel can possess
both these requirements at the same time, because a low carbon steel,
containing about 0.1 % carbon will be tough, while a high carbon steel of
0.9 % or more carbon will possess adequate hardness when suitable heat
treated.
However both these requirements may be met by employing a low
carbon steel with suitable core properties and then adding ( or
penetrating ) carbon, Nitrogen or both to the surface of the steel part in
order to provide a hardened case ( or layer ) of a definite depth. These
treatmentsare known as case hardening.
The processes used to createhardened cases are

i)Carburizing :-Increasing the carbon on the surface of a
low carbon ( o.1 –0.2 % C) and subsequently heat treating the
component in a specific manner to produce hard and wear
resistant surface and tough center.
ii)Nitriding:-Introducing nitrogen in the surface ofa tough
steel so as to produce hard nitrided
case with no subsequent heat treatment.
iii)Carbonitriding :-Introducing carbon and nitrogenin the
surface of a tough steel and produce hard and wear resistant
case.
iv)Flame Hardening.

Tempering :-
Tempering process consists of heating the hardened
components to a temp.
between 100 0C & 700 0C. ( Below A1) holding at this temp.
for specific period ( 1 to 2 hrs.) & cooling to room temp.
usually in air.
Tempering is classified as
1) Low temp. tempering (100 0C & 200 0C)
2) Medium temp. tempering (200 0C & 500 0C)
3) High temp. tempering (500 0C & 700 0C)

Tempering is done because of following purposes :-
1) To relieve the internal stresses developed due to
rapid cooling of steels during
hardening process & volume changes occurring in
above transformation
2) To reduce brittleness of material .
3) The high internal stresses produced due to
hardening are likely to cause cracking of
components , if tempering operation is delayed.
4) To reduce hardness & to increase ductility &
toughness.

EXPLAINTEMPERING ROCESS
(4M)
A quench-hardened plain carbon steel is hard, brittle
and hardening stresses are present. In such a condition it is
of little practical use and it has to be reheated, or tempered,
to relieve the stresses and reduce the brittleness.
This temperature will remove internal stress setup
during quenching, remove some, or all, of the hardness,
and increase the toughness of the material.
Tempering causes the transformation of martensite into
less brittle structures. Unfortunately, any increase in
toughness is accompanied by some decrease in hardness.
Tempering always tends to transform the unstable
martensite back into the stable pearlite of the equilibrium
transformations.

Tempering temperatures below 200 °C only relieve the hardening
stresses, but above 220 C the hard, brittle martensite starts to
transform into a fine pearlitic structure called secondary troostite (or
just'troostite'). Troostite is much tougher although somewhat less
hard than martensite and is the structure to be found in most
carbon-steel cutting tools.
Tempering above 400 C causes any cementite particles present to
"ball-up" giving a structure called sorbite.
This is tougher and more ductile than troostite and is the
structure used in components subjected to shock loads and where a
lower order of hardness can be tolerated, for example springs. It is
normal to quench the steel once the tempering
For most steels, cooling form the tempering temperature may be
either cooling in air, or quenching in oil or water. Some alloy steels,
however, may be become embrittled if slowly cooled temperature has
been reached. from the tempering temperature, and these steels have
to be quenched

Martemparing-: (Four marks for explanation)

In this process the austenitized steel is cooled
rapidly avoiding the nose of the TTT. diagram to a
temperature between the nose and MS soaked at this
temperature for a sufficient time for the equalization of
temperature but not long enough to permit the
formation of bainite and then cooled to room
temperature in air or oil. Since the component has to
be held for some time for equilization of temperature,
the process will be applicable to steels of slightly high
hardenability such as high carbon steels and low alloy
steels. The process produces martensitic structures
with the following advantages.

i) It results in less distortion and warping, since the
matensite formation occurs at the same time
throughout the cross section of the component.
ii) There is less possibility of quenching cracks
appearing in the component.
This is a hardening process and therefore the name
martempering is a misonomer for the
treatment

e)Differentiate between Martempering And Austempering.
Martempering Austempering.
• Martempering is a hardening
treatment
• Austempering is not a
hardening treatment
• Martempering gives
martensite product.
• Austempering gives bainite
product.
• More distortion and
quenching cracks.
• Less distortion and quenching
cracks.

• Tempering is needed after
martempering
• Tempering is not needed after
austempering.
• It requires less time. • It requires more time.
• Low ductility and toughness
obtained
• Greater ductility and toughness
obtained

CARBURIZING:-
Define and explain the carburizing. State its
applications.
Defn-The method of increasing the carbon on
the surface of a steel is called carburizing
OR
Carburising is a method of introducing, carbon
into solid iron basealloys
such as low carbon steels in order to produce a
hard case (surface).

CARBURIZING:-
Define and explain the carburizing. State its
applications.
Defn-The method of increasing the carbon on the
surface of a steel is called carburizing
OR
Carburising is a method of introducing, carbon into
solid iron basealloys
such as low carbon steels in order to produce a
hard case (surface).
..

Explanation :-
Carburising increases the carbon content
of the steel surface by a process of
absorption and diffusion. It consists of
heating the steel in the austenitic region in
contact with a carburizing medium, holding
at this temperature for a sufficient period
and cooling to room temperature

In the austenitic region, the solubility of carbon is
more and hence the carbon from medium diffuses into
the steel i.e.in the austenite. High carbon content on
surface does not mean high hardness of the surface,
unless the carbon is present in the martensitic form.
Hence after carburizing hardening treatment is
necessary to bring the carbon is the martensitic form.
Depending on the medium used for carburizing it is
classified as
• solid carburizing
• Gas carburizing
• liquid carburizing

Application :
-Gears, Camshaft, bearings, crank shaft sin these
components hard and wear surface is required and tough core
to withstand impact loads
or
The method of increasing the carbon on the surface of a
steel is called carburizing.
It consists of heating the steel in the austenitic region in
contact with a carburizing medium , holding at this temp for a
sufficient period and cooling to room temperature.
In the austenitic region the solubility of carbon is more and
hence the carbon from medium diffuses in to the steel in the
austenite.

High carbon content on the surface does not
mean high hardness of the surface ,unless
carbon is present in the martensitic form. hence
after carburizing ,hardening treatment is
necessary to bring the carbon in the martensitic
form.
Therefore the hardening heat treatment that
follows the carburizing operation is as important
as the carburizing itself. It is also known as case
hardening .depending on medium used for
carburizing it is classified as solid carburizing,
gas carburizing, liquid carburizing.

Advantages of Carburising (two marks for any two
advantages )
1. With this process carbon is deposited on the
surface of lower or medium carbon steel material and
with proper heat treatment it can be hardened.
2. Certain depth of hardening can be provided on
the surface of metals. Hence it is called as case
hardening.
3. distortion and cracking during hardening is less.
4. It can be done in the solid, gas and liquid
carburizing medium.
5 Depth of hardness can be controlled easily

Demerits :- ( Any two ½ M each ) –1 Mark
i) Some times uniform case depth is difficult
to obtain.
ii) Skilled personnel are required.
iii) In case of liquid carburizing necessary
care is to be taken.
iv) In case of solid carburizing more time
required for process

Nitriding process-
Describe Nitriding process.
( Suitable description.04 Marks)
Nitriding accompanies the introduction of nitrogen
into the surface of certain types of steel ( e.g.
containing A1 and Cr) by heating it and holding it at a
suitable temperature in contact ith partially
dissociated ammonia or other suitable medium. This
process produces a hard case without quenching or
any further heat treatment.

Nitriding is accomplished by heating the steel in
contact with a source of atomic nitrogen at a
temperature of about 550 ºC. The atomic nitrogen
diffuses into the steel and combines with iron and
certain alloying elements present in the steel and
forms respective nitrides. These nitrides increase the
hardness and wear resistance of steels. The atomic
nitrogen source can be a molten salt bath containing
NacN.

In gas nitriding, the components are placed in a
heat resistant metal container which is then filled
with ammonia. When it is completely purged, it is
sealed, placed in a furnace and raised to a
temperature of approximately 500 ºC .At this
temperature the ammonia dissociates.
NH3gives3H + N and N is absorbed in the surface
layer of steel. Parts are maintained at 500 ºC for
between 40 to 100 Hours depending upon the depth
of case required, afterwhich parts are allowed to cool
in the container

Heat tratment bykotkar ss

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