Heat Treatment
 Heating
a metal or alloy to various definite
temperatures, holding these for various time durations
and cooling at various rates.
 Combination of controlled heating and cooling
determine not only the nature and distribution of
micro-constituents (which determine the properties of
a metal or alloy), but also the grain size.
Contd...
 Purpose of heat treatment:
1.To remove or relieve strains or stresses induced by cold
working or non-uniform cooling (for example welding):
Annealing
2. To increase strength or hardness of the material for
improved wear resistance: Hardening
3.To improve machinability: Annealing
4.To soften the material: Annealing
5. To decrease hardness and increase ductility and toughness.
(Tempering)
Contd...
6. To improve the cutting properties of tools.
7. To change or modify the physical properties of the
material such as electrical properties, magnetic
properties, corrosion resistance and heat resistance
etc.
Main Processes Include
 Annealing
 Full Annealing
 Isothermal Annealing
 Process Annealing
 Spheroidisation
 Normalizing
 Hardening and Quenching
 Tempering
Contd...
Annealing processes
 Annealing is a heat treatment process in which the
material is taken to a high temp. kept there for some
time and then cooled in furnace.
 Cooling is done slowly to avoid the distortion.
Contd...
 Benefits of annealing are:
• relieve stresses
• increase softness, ductility and toughness
• produce a specific microstructure
• modify electrical and magnetic properties
 Depending on the specific purpose, annealing is
classified into various types: process annealing, stress
relief, full annealing and normalizing.
Full annealing
 This consists of heating the steel to a temperature
above the transformation range, holding for one two
hours and then cooling at a predetermined rate to
obtain the desired microstructure. Grain refinement is
accomplished in this instance by the recrystallisation
of the steel in passing through the critical range in
both heating and in cooling.
Isothermal annealing
 It is a type of full annealing in which the steel first is
cooled to the temperature at which it is desired to have
transformation occur, at a rate sufficiently rapid to
prevent any structural change above the temperature.
The steel then is held at the selected temperature for
the time necessary to complete such transformation.
Process annealing
 This consists of heating the steel to a temperature first
under lower critical point and holding at this
temperature for the proper time(usually 2 to 4 hours)
followed by air cooling
Contd...
Spheroidisation
 This is accomplished by heating to a temperature just
above the critical point and cool very slowly through
the critical range.
 This treatment is used for practically all steels
containing over 0.6% carbon that are to be machined
or cold formed.
Contd...
Normalizing
Main objective
1. Refine grain, improve machinability, tensile strength and
structure of weld.
2. Remove cold worked stess.
3. Remove dislocations due to hot working.
Process
 Heat the steel approximately 4°C above its upper critical
temp, held about fifteen minutes and then allowed to cool
down in still air.
 Homogeneous structure provides a higher yield point,
ultimate tensile strength and impact strength with lower
ductility to steels.
Contd...
Hardening and Quenching
 Hardening is a process in which steel is heated to a
temperature above the critical point, held at this
temperature and Quenched (rapidly cooled) in water, oil
or molten baths.
 If a piece of steel is heated above its upper critical
temperature and plunged into water to cool it an
extrewmely hard, needle-shaped structure known as
martensite is formed. In other words, sudden quenching
of steel greatly increases its hardness.
Tempering
 Martensite structures formed by direct quenching of high
carbon steel are hard and strong, but unfortunately are also
brittle. They cannot be plastically deformed and have very
little toughness, and although strong are unable to resist
impact loads and are extremely sensitive to stress
concentrations. Some of the hardness and strength must be
sacrificed to obtain suitable ductility and toughness. This is
done by tempering the martensic steel.
Tempering
Objective of Tempering are:
 Increase toughness
 Decrease hardness
 Stabilise structure
 Relieve stresses
The process of Tempering consists of heating Quenched, hardened steel to some
pre determined temperature between room temperature and the critical
temperature of the steel for a certain length of time, followed by air cooling.
Fe-C equilibrium diagram
 The structural form of pure iron at room temperature
is called ferrite or -iron.
 Ferrite is soft and ductile.
 Since ferrite has a body-centred cubic structure, the
inter-atomic spaces are small and pronouncedly
oblate, and cannot readily accommodate even a small
carbon atom. Therefore, solubility of carbon in ferrite
is very low, of the order of 0.006% at room
temperature.
 The maximum carbon content in ferrite is 0.05% at 723
°C.
Contd…
 The face-centred modification of iron is called
austenite or -iron. It is the stable form of pure iron at
temperatures between 910°C and 1400°C. At its stable
temperature  austenite is soft and ductile and
consequently, is well suited for manufacturing
processes.
Contd…
 The maximum solubility is only 2% of carbon at 11
30°C.
 Above 1400°C, austenite is no longer the most stable
form of iron, and the crystal structure changes back to
a body-centred cubic phase called delta iron. This is
 -iron except for its
the same phase as the
temperature range.
 The solubility of carbon in  -ferrite is small, but it is
appreciably larger than In -ferrite, because of higher
temperature. The maximum solubility of carbon in
&iron is 0.1% at 1490°C.
Contd…
 In the reaction, the simultaneous formation of ferrite
and cementite from austenite results at the
temperature of 723°C and composition of 0.80%
carbon.
 Since the ferrite and cementite are formed
simultaneously,
they
are
intimately
mixed.
Characteristically, the mixture is lamellar, i.e., it is
composed of alternate layers of ferrite and cementite.
 This micro-structure is called pearlite which is very
important in iron and steel technology, because it can
be formed in almost all steels by means of suitable
heat treatments.
Contd…
 The alloy containing 0.80% of carbon is called the
eutectoid steel.
 Upon cooling the eutectoid steel below 723°C, all of
the austenite is transformed into pearlite.
 Alloys with less than 0.80% C are called hypo-
eutectoid steels and those with higher composition are
called hyper-eutectoid steels.
Contd…
 Fe-Fe3C phase diagram is characterized by five individual
phases,: α–ferrite (BCC) Fe-C solid solution, γ-austenite (FCC)
Fe-C solid solution, δ-ferrite (BCC) Fe-C solid solution, Fe3C
(iron carbide) or cementite - an inter-metallic compound and
liquid Fe-C solution and four invariant reactions:
 peritectic reaction at 1495 oC and 0.16%C, δ-ferrite + L↔ γ-
iron (austenite)
 eutectic reaction at 1147 oC and 4.3 %C, L ↔ γ-iron + Fe3C
(cementite) [ledeburite]
 eutectoid reaction at 723 oC and 0.8%C, γ-iron ↔ α– ferrite +
Fe3C (cementite) [pearlite]
Contd…
Fig. TTT diagram for eutectoid transformation in Fe-C
Fig. Transformations involving austenite for Fe-C system
Bainite Formation
 Bainite is an intimate mixture of ferrite and cementite,
as in pearlite. Pearlte has alternate layers of ferrite and
cementite. In bainite however, cementite apparently
exists as tiny spheroids uniformly distributed
throughout a ferrite matrix.
 In upper bainite(formed at temperatures just below
the nose of the TTT curve) there is evidence of some
patterns in the cementite arrangement since the
microstructure has a feathery appearance.
 In lower bainite(formed at temperatures approaching
Ms) the cementite becomes too fine for resolution and
an acicular(needle like) pattern is formed.
Contd…
 Bainite is normally harder, stronger and tougher than
fine pearlite of the same chemical composition(due to
differences in size,
cementite).
shape and
distribution of
Contd…
Martensite Formation
 The Martensite transformation occurs in a wide
temperature range. It begins at a temperature
corresponding to a point Ms(Martensite start). When
the cooling process passes through the point Ms,
austenite begins to transform into martensite.
 The lower the temperature, the more martensite will
be formed.
 At a definite temperature for each steel, further
transformation of austenite into martensite ceases.
 This temperature is usually denoted as Mf(Martensite
finish)
Contd…
 The positions of points Ms and Mf do not depend
upon cooling rate and are determined by the chemical
composition of austenite.
 More carbon in steel lowers points Ms and Mf.
 A characteristic feature of martensite formation is that
it is practically never completed. Thus, a hardened
steel contains retained autenite.
 The higher the carbon content the more austenite will
be retained.
 The presence of retained austenite is undesirable as it
has detrimental effect on its mechanical properties.
The End