In
General the term Annealing refers to heating
& cooling operations, that is usually applied to induce softening.
Annealing is Classified into Four types
1.
Full
Annealing,
2.
Process
Annealing,
3.
Spherodise
Annealing
4.
Iso-Thermal
Annealing
1) Full Annealing
=> The term annealing, when
used in connection with heat treatment of the steel which means Full annealing.
=> Full
annealing may be defined as the Process of heating
steels to austinite phase & then cooling slowly within a closed furnace
by putting off the supplied heat.
=> The
Purpose of Full Annealing are
1.
To
soften the steel,
2.
To
relieve the internal stresses,
3.
To
improve machinability,
4.
To
refine grains
5.
To
reduce Hardness
6.
To
make the material hardness
Process of Full Annealing
=> The process consist of heating the steels above A3
temperature for hypo eutectoid steels & above A1 temperature for hyper
eutectoid steels by varying 30°C to 50°C.
=> Hold the material for a sufficient time to allow necessary
changes occurs.
=> After some time slowly cool the steel below A1 temperature
(or) below room temperature.
=> Too high temperature or Too long time of heating may produce coarsening(unpleasant)
of grains produces. which results in the loss of toughness,Hardness &
Strength.
=> The temperature to which steel is heated is depend upon the
Carbon content.
The Suitable temperature for various steels are given below
|
Sl.no
|
Type
of Steel
|
Carbon
Content %
|
Annealing
temp.
|
|
1
|
Dead
Mild Steel
|
Less
than 0.12%
|
875°C
- 925°C
|
|
2
|
Mittal
Steel
|
0.12
- 0.25%
|
840°C
- 875°C
|
|
3
|
Low
carbon Steel
|
0.25
- 0.5%
|
815°C
- 840°C
|
|
4
|
Medium
Carbon Steel
|
0.5
- 0.9%
|
780°C
- 815°C
|
|
5
|
High
Carbon Steel
|
0.9
- 1.3%
|
760°C
- 780°C
|
=> Annealing is commonly employed for Casting & Forging to reduce the core
grain structure prior to machining.
Slow cooling results the equilibrium structure which are
predicted from the equilibrium diagram.
The micro structure of annealed steels are as follows.
|
Sl.no
|
Type
of Steel
|
Microstructure
of Annealing
|
|
1
|
Hypo
Eutectoid Steel
|
Ferrite
+ Pearlite
|
|
2
|
Eutectic
Steel
|
Pearlite
|
|
3
|
Hyper
Eutectoid Steel
|
Pearlite
+ Cementite
|
2) Process Annealing
The purpose of Process Annealing are
1.
To reduce the distortion of the crystal lattice & to
provide by cold working.
2.
To remove the Strain Hardening(brittleness) i.e., to improve
the ductility.
3.
To relieve the internal stresses.
4.
To obtain the Homogenius structure.
5.
To refine the grains
Process of Process Annealing
=> Process annealing consist of heating a steel to a temperature
of 600-700°C. i.e., below critical point A1,holding at this
temperature for a pro-long period & then allowed to cool down slowly in the
furnace.
=> It should be noted that the process annealing is a critical
operation. i.e., operation should be carried below lower critical
temperature.
=> For this reason, there is no phase change occurs &
constituents of ferrite & cementite remains in the structure throughout the
process.
=> Process annealing is used extensively in heat treatment of steel
& Wires.
=> It is also applied in cold working steel in order to restore
ductility & Softness.
3)
Spherodise Annealing
=> Spherodise Annealing transforms laminal
type into Globical(round) type. i.e., lamina of pearlite is transformed into matrix of
ferrite with cementite & some % of carbon in the form of spherodise shape.
This process is called spherodise Annealing.=>There are various techniques
which are adopted to obtain spherodise structure.