annealing

just keep some basic in mind, its give u enough information about this topic.

1. Dr. H. K. Khaira
Head
Deptt. of Mat. Sci. and Met. Engg.
ANNEALING

2. Heat Treatment
Heat treatment is defined as heating a metal to a specified
temperature, keeping it at that temperature for some time
followed by cooling at a specified rate.
It is a tool to get required microstructure and properties in
the metal.

3. Handouts 23
Heat treatment
Heat treatment - controlled heating and cooling basically
The basic steps of heat treatment are:

Heating → Soaking → Cooling

4. Handouts 24
Heat treatment
Heating -> Soaking -> Cooling
Temperature Time of soaking Rate of cooling

Medium of cooling
- Different combinations of the above parameters
- Different compositions of materials and initial phases of materials
Give rise to different heat treatments

5. Heat Treatments
There are different types of heat treatments.
Annealing is one of the heat treatments given to metals.
Main aim of annealing is to increase the ductility of the
metal.

6. Annealing
Annealing is a heat treatment in which the metal is heated to a
temperature above its recrystallisation temperature, kept at that
temperature some time for homogenization of temperature
followed by very slow cooling to develop equilibrium structure in
the metal or alloy.
The steel is heated 30 to 50
o
C above Ae
3
temperature in case of
hypo-eutectoid steels and 30 to 50
o
C above A
cm
temperature in
case of hyper-eutectoid temperature
The cooling is done in the furnace itself.

7. Aims of Annealing
-Increase ductility
-Reduce hardness and brittleness
-Alter microstructure to soften the metal prior to shaping by improving
formability
-Recrystallize cold worked (strain hardened) metals
-Remove internal stresses
-Increase toughness
-Increase machinability
-Decrease electrical resistance
-Improve magnetic properties

8. Types of Annealing
1.Full annealing
2.Stress relief annealing
3.Process annealing
4.Spheroidizing annealing

9. Full annealing
It is heating the steel 30 to 50
o
C above Ae
3
temperature in case
of hypo-eutectoid steels and 30 to 50
o
C above A
cm
temperature
in case of hyper-eutectoid temperature, keeping it at that
temperature for some time for homogenization of
temperature followed by cooling at a very slow rate.
The cooling rate may be about 10
o
C per hour.
It is to get all the changes in the properties of the metals like producing
equilibrium microstructure, increase in ductility, reduction in hardness,
strength, brittleness and removal of internal stresses.
The microstructure contains coarse ferrite and pearlite.

10. Annealing on TTT Diagram
The cooling rate during
annealing is very slow, about
10
0
C per hour.

11. Stress Relief Annealing
In stress relief annealing, the metal is heated to a lower
temperature and is kept at that temperature for some time to
remove the internal stresses followed by slow cooling.
The aim of the stress relief annealing is to remove the internal
stresses produced in the metal due to
Plastic deformation
Non-uniform cooling
Phase transformation
No phase transformation takes place during stress relief annealing.

12. Spheroidizing Annealing
In spheroidizing annealing, the steel is heated to a
temperature below A
1
temperature, kept at that temperature
for some time followed by slow cooling.
The aim of spheroidizing annealing is to improve the
machinability of steel.
In this process the cementite is converted into spheroidal
form.
The holding time varies from 15 – 25 hours.

13. Process Annealing
In process annealing, the cold worked metal is heated above
its recrystallisation temperature, kept for some time
followed by slow cooling.
The aim of process annealing is to restore ductility of the
cold worked metal.
During process annealing, recovery and recrystallization
takes place.

15. Heat Treatment Temperature
The temperature
ranges to which the
steel has to be
heated for different
heat treatments
←Acm
A
3
→

16. Fe-Fe
3
C phase diagram in the vicinity of eutectoid, indicating
heat treating temperature ranges for plain carbon steel

17. Recrystallization and Melting Temperatures
Recrystallization proceeds more rapid in pure metals that in alloys.
For pure metals, the recrystallization temperature is about 0.3Tm (Tm is absolute melting
temp.) For some alloys, the recrystallization temperature can be as high as 0.7Tm.

18. Process
Heat the metal to a temperature
Hold at that temperature
Slowly cool
Purpose
Reduce hardness and brittleness
Alter the microstructure for a special property
Soften the metal for better machinability
Recrystallize cold worked (strain hardened) metals
Relieve induced residual stresses

19. Annealing
In case of annealing of steels, the steel is heated to different
temperatures depending upon the aim of annealing followed
by furnace cooling.

20. Annealing
Annealing is a heat treatment designed to
eliminate the effects of cold working. The
properties of a metal may revert back to the
precold-worked states by Annealing, through
recovery, recrystallization and grain growth.

21. Spheroidizing: held at A1 or 700 °C for 15-25 hrs.

23. Stages of Annealing
There are three stages of annealing
1.Recovery
2.Recrystallization
3.Grain Growth

24. Recovery
the relief of some of the internal strain energy of a
previously cold-worked material.

25. Recovery
Let us now examine the changes that occur when a sample is
heated from room temperature. At first, recovery occurs in which
there is a change in the stored energy without any obvious change
in the optical microstructure. Excess vacancies and interstitials
anneal out giving a drop in the electrical resistivity but little
change in hardness .
Dislocations become mobile at a higher temperature, eliminate
and rearrange to give polygonisation.

26. Changes in Mechanical Properties
during Annealing
Annealing temperature
and Mechanical Properties
for a Brass Alloy

27. Polygonisation
The misorientation between grains can be described in terms of
θ
dislocations
Inserting an edge dislocation of Burgers vector b is like
forcing a wedge into the lattice, so that each dislocation is
associated with a small change in the orientation of the lattice
on either side of the extra half plane.
If the spacing of dislocations is d, then
θ = b/d

28. Recrystallization
the formation of a new set of strain-free grains
within a previously cold-worked material.

29. Recrystallization
The dislocation density decreases only a little during
recovery and the deformed grain structure is largely
unaffected by recovery. It takes the growth of new grains to
initiate a much larger change, i.e. recrystallisation.

30. Recrystallization
The nucleation of new grains happens in regions of high
dislocation density.
Nucleation begins in a jumble of dislocations. The recrystallised
grain will essentially be free from dislocations.
A greater nucleation rate leads to a finer ultimate grain size.
There is a critical level of deformation below which there will be
no recrystallisation at all. A critical strain anneal can lead to a
single crystal on recrystallisation.

31. Changes in Mechanical Properties
during Annealing
Annealing temperature
and Mechanical Properties
for a Brass Alloy

32. Grain Growth
the increase in average grain size of a
polycrystalline material.
D - D
o
= kt
1/2

Where D is the grain diameter after time t and
And D
o
is initial grain diameter

33. Grain Growth
A grain of radius r has a volume 4/3(πr
3
) and surface area 4πr
2
. The
grain boundary energy associated with this grain is 2πr
2
where is
γ γ
the boundary energy per unit area. and we have taken into
account that the grain boundary is shared between two grains. If
follows that:
energy per unit volume = 3γ/2r 3γ
≡
/D
where D is the grain diameter
It is this which drives the growth of grains with an equivalent
pressure of about 0.1 MPa for typical values of = 0.3Jm
γ
−2
and D
= 10 m.
μ
This is not very large so the grains can readily be pinned
by particles (Zener drag).

36. Changes in Mechanical Properties
during Annealing
Annealing temperature
and Mechanical Properties
for a Brass Alloy

37. Changes in Microstructure during
different stages of Annealing

38. Annealing
Annealing is a heat treatment designed to eliminate the effects of cold working.
The properties of a metal may revert back to the precold-worked states by
Annealing, through recovery, recrystallization and grain growth.
Recovery: the relief of some of the internal strain energy of a previously cold-worked material.
Recrystallization: the formation of a new set of strain-free grains within a previously cold-worked
material.
Grain Growth: the increase in average grain size of a polycrystalline material. An elevated
temperature heat treatment (annealing) is needed for these 3-processes.

39. Recrystallization and grain growth of
brass
(a) Cold-worked
(b)
Initial stage of
recrystallization
c
a b
(3 s at 580°C)
(c) Partial replacement
of cold-worked
grains by
recrystallized ones
(4 s at 580°C)
(d) Complete
recrystallization
(8 s at 580°C)
(e) Grain growth after
15 min at 580°C.
(f) Grain growth after
10 min at 700°C
f
d e

40. Changes in Mechanical Properties
during Annealing
Annealing temperature
and Mechanical Properties
for a Brass Alloy

41. Grain Growth
Grain Growth
dn - d0n = Kt

43. Annealing
Cold work : mechanical deformation of a metal at relatively low
temperatures. Thus, cold work of a metal increases significantly dislocation
density from 10
8
(annealed state) to 10
12
cm/cm
3
, which causes the metal to be
hardened.
ex) rolling, forging, and drawing etc.
•% cold work = (A
0
- A
f
)/A
0
x 100%
, where A
0
is the original cross-
sectional area and A
f
is the final
cross-sectional area after cold
working.
•With increasing % cold work, the
hardness and strength of alloys
are increased whereas the
ductility of the alloys are
decreased.
Cold-rolling
Cold-drawing

44. Annealed crystal (grain) deformed or strained crystal
When a metal is cold worked, most of energy goes into plastic deformation to
change the shaped and heat generation. However, a small portion of the energy, up
to ~5 %, remains stored in the material. The stored energy is mainly in the form of
elastic energy in the strain fields surrounding dislocations and point defects
generated during the cold work.
Annealing : a cold worked grains are quite unstable due to the strain energy. By
heating the cold worked material to high temperatures where sufficient atomic
mobility is available, the material can be softened and a new microstructure can
emerge. This heat treatment is called annealing where recovery and recrystallization
take place.
Cold work(high energy state)

45. Recovery and recrystallization
Recovery :
A low temperature annealing.
The concentration of point defects is decreased and dislocation is allowed to move to
lower energy positions without gross microstructural change.
Modest effects on mechanical behavior while electrical conductivity increases
significantly.
Recrystallization : occurs at 1/3 to 1/2 T
m
.
during recrystallization process, new equiaxed, strain-free grains nucleate at high-stress
regions in the cold-worked microstructure, and hence hardness and strength decrease
whereas ductility increases. Recrystallization temp. is that at which recrystallization just
reaches completion in 1 hour.
recrystallization annealing
deformed crystal undeformed crystal

46. The processed of recovery and recrystallization of a cold worked represent a
structural transformation, not true phase transformations. The driving force for
recovery and recrystallization is associated with the strain energy stored in the
crystal as a result of cold work.

↑
the amount of cold work

↑
grain size before cold work number of strain-free nuclei
↑

↑
annealing temp.

47. Influence of annealing temperature on the
tensile strength and ductility of a brass alloy
Variation of recrystallization
temperature with percent
cold work for iron

48. Grain growth
Grain growth :
A large concentration of grain boundaries (fine grain structure) is reduced by
grain growth that occurs by high temp. annealing. The driving force for the
grain growth is the reduction in the grain boundary surface energy.
Stages of the
recrystallization
and grain growth
of brass