Heat Treatment Heating a metal or alloy to various definite temperatures, holding these for various...
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Heat Treatment of Metals
Heat Treatment Heating a metal or alloy to various definite temperatures, holding these for various time durations and cooling at various rates. Combination
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...
Slide 3
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...
Slide 4
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.
Slide 5
Main Processes Include Annealing Full Annealing Isothermal
Annealing Process Annealing Spheroidisation Normalizing Hardening
and Quenching Tempering Contd...
Slide 6
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...
Slide 7
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.
Slide 8
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.
Slide 9
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.
Slide 10
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...
Slide 11
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...
Slide 12
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 4C 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...
Slide 13
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. Hardening and Quenching
Slide 14
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
Slide 15
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. Tempering
Slide 16
Slide 17
Slide 18
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
Slide 19
The face-centred modification of iron is called austenite or
-iron. It is the stable form of pure iron at temperatures between
910C and 1400C. At its stable temperature austenite is soft and
ductile and consequently, is well suited for manufacturing
processes. Contd
Slide 20
The maximum solubility is only 2% of carbon at 11 30C. Above
1400C, 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 the same phase as the -iron except for its
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 1490C. Contd
Slide 21
In the reaction, the simultaneous formation of ferrite and
cementite from austenite results at the temperature of 723C 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
Slide 22
The alloy containing 0.80% of carbon is called the eutectoid
steel. Upon cooling the eutectoid steel below 723C, 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
Slide 23
Fe-Fe 3 C phase diagram is characterized by five individual
phases,: ferrite (BCC) Fe-C solid solution, -austenite (FCC) Fe-C
solid solution, -ferrite (BCC) Fe-C solid solution, Fe 3 C (iron
carbide) or cementite - an inter-metallic compound and liquid Fe-C
solution and four invariant reactions: peritectic reaction at 1495
o C and 0.16%C, -ferrite + L - iron (austenite) eutectic reaction
at 1147 o C and 4.3 %C, L -iron + Fe 3 C (cementite) [ledeburite]
eutectoid reaction at 723 o C and 0.8%C, -iron ferrite + Fe3C
(cementite) [pearlite] Contd
Slide 24
Slide 25
Fig. TTT diagram for eutectoid transformation in Fe-C
Slide 26
Fig. Transformations involving austenite for Fe-C system
Slide 27
Slide 28
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
Slide 29
Bainite is normally harder, stronger and tougher than fine
pearlite of the same chemical composition(due to differences in
size, shape and distribution of cementite). Contd
Slide 30
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 Martensite Formation
Slide 31
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.