Annealing

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

1. Dr. H. K. Khaira
Professor
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. Types of Heat Treatments
1. Annealing
2. Normalizing
3. Hardening
4. Tempering
5. Precipitation Hardening

7. Annealing
Annealing is a heat treatment in which the metal is heated to a
temperature above its recrystallisation temperature, kept at that
temperature for 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
1
temperature in case
of hyper-eutectoid temperature
The cooling is done in the furnace itself.

8. Fe-Fe
3
C phase diagram indicating heat treating
temperature ranges for plain carbon steel

9. Aims of Annealing
-1. Increase ductility
-2. Reduce hardness
-3. Improving formability
-4. Recrystallize cold worked (strain hardened) metals
-5. Remove internal stresses
-6. Increase toughness
-7. Decrease brittleness
-8. Increase machinability
-9. Decrease electrical resistance
-10. Improve magnetic properties

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

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

12. 1. 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
1
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 (furnace
cooling).
The cooling rate may be about 10
o
C per hour.

13. 1. Full annealing
It is to get all the changes in the properties of the metals like
1. Producing equilibrium microstructure,
2. Increase in ductility,
3. Reduction in hardness, strength, brittleness and
4. Removal of internal stresses.
The microstructure after annealing contains coarse ferrite and pearlite.

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

15. 2. 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.

16. 3. 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.

17. Figure 12.6 The microstructure of
spheroidite, with Fe
3
C particles
dispersed in a ferrite matrix (´ 850).
(From ASM Handbook, Vol. 7, (1972),
ASM International, Materials Park, OH
44073.)

18. 4. Process Annealing
In process annealing, the cold worked metal is heated above
its recrystallisation temperature, kept for some time
followed by slow cooling.

19. 4. Process Annealing
The aim of process annealing is to restore ductility of the cold
worked metal.
During process annealing, recovery and recrystallization takes
place.
During process annealing, 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 annealing
deformed crystal undeformed crystal

20. Process Annealing
Cold work : mechanical deformation of a metal at relatively low
temperatures. Thus, cold working of a metal increases significantly dislocation
density from 10
8
(annealed state) to 10
12
cm/cm
3
, which causes hardness and the
strength of the metal.
Example --- 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.
•For further deformation, the ductility
has to be restored by process
annealing.
Cold-rolling
Cold-drawing

21. Process Annealing
Annealed crystal (grain) deformed or strained crystal
When a metal is cold worked, most of energy goes into plastic deformation to
change the shape 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.
Process Annealing : 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 process annealing where recovery and
recrystallization take place.
Cold work (high energy state)

22. Process Annealing
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

23. Variation of recrystallization temperature with
percent cold work for iron

24. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
™
is a trademark used herein under license.
Figure 12.4 Schematic summary of the simple heat treatments for (a)
hypoeutectoid steels and (b) hypereutectoid steels.

25. Example 12.2 SOLUTION
From Figure 12.2, we find the critical A1, A
3
, or A
cm
, temperatures
for each steel. We can then specify the heat treatment based on
these temperatures.

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

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

28. Recovery
Relieves the stresses from cold working
Recovery involves annihilation of point defects.
Driving force for recovery is decrease in stored energy from cold work.
During recovery, physical properties of the cold worked material are
restored without any observable change in microstructure.
Recovery is first stage of annealing which takes place at low temperatures of
annealing.
There is some reduction, though not substantial, in dislocation density as
well apart from formation of dislocation configurations with low strain
energies.

29. Recovery
The concentration of point defects is decreased and dislocation
is allowed to move to lower energy positions without gross
microstructural change.

30. 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.
Dislocations become mobile at a higher temperature, eliminate
and rearrange to give polygonisation.
Modest effects on mechanical behavior

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

32. Polygonisation
Polygonisation occurs during recovery.
Dislocations become mobile at a higher temperature, eliminate
and rearrange to give 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

33. Polygonisation
(a)Random
arrangement of
excess parallel
edge dislocations
and
(b)alignment into
dislocation walls

34. Changes in Microstructure during
different stages of Annealing

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

36. Recrystallization
This follows recovery during annealing of cold worked material. Driving
force is stored energy during cold work.
It involves replacement of cold-worked structure by a new set of strain-free,
approximately equi-axed grains to replace all the deformed crystals.
This process ocurs above recrystallisation temperature which is defined as
the temperature at which 50% of material recrystallises in one hour time.
The recrystallization temperature is strongly dependent on the purity of a
material.
Pure materials may recrystallize around 0.3Tm, while impure materials may
recrystallise around 0.4Tm, where Tm is absolute melting temperature of the material.

37. Effect of alloying elements on
Recrystallisation Temperature
Increase in the recrystallisation temperature of pure copper
by the addition of 0.01 atomic percent of the indicated
element
Added elementIncrease in recrystallisation Temp. (C)
Ni 0
Co 15
Fe 15
Ag 80
Sn 180
Te 240

38. Recrystallization laws
A minimum amount of deformation is needed to cause recrystallisation (Rx).
Smaller the degree of deformation, higher will be the Rx temperature.
The finer is the initial grain size; lower will be the Rx temperature.
The larger the initial grain size, the greater degree of deformation is
required to produce an equivalent Rx temperature.
Greater the degree of deformation and lower the annealing temperature, the
smaller will be the recrystallized grain size.
The higher is the temperature of cold working, the less is the strain energy
stored and thus Rx temperature is correspondingly higher.
The recrystallisation rate increases exponentially with temperature.

39. Rate of Recrystallisation
Rate of Recrystallisation = Ae
-Q/RT

Taking log of both sides Log (Rate) = -Q/RT

Or Log(Rate of recrystallisation) is

proportional to 1/T

40. Recrystallization
Recrystallisation : occurs at 1/3 to 1/2 T
m
.
during recrystallisation 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 temperature is that temperature at which
recrystallization just reaches completion in 1 hour.
recrystallization annealing
deformed crystal undeformed crystal

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

42. 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.

43. Recrystallization
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.

45. Changes in Microstructure during
different stages of Annealing

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

50. 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

51. Grain growth
Grain growth follows complete crystallization if the material is left at
elevated temperatures.
Grain growth does not need to be preceded by recovery and
recrystallization; it may occur in all polycrystalline materials.
In contrary to recovery and recrystallization, driving force for this process is
reduction in grain boundary energy.
Tendency for larger grains to grow at the expense of smaller grains is based
on physics.
In practical applications, grain growth is not desirable.
Incorporation of impurity atoms and insoluble second phase particles are
effective in retarding grain growth.
Grain growth is very strongly dependent on temperature

52. 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.

53. Grain Growth
Grain Growth
dn - d0n = Kt
1/2

54. Changes in Microstructure during
different stages of Annealing

55. 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

56. 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

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Figure 12.5 The effect of
carbon and heat treatment on
the properties of plain-carbon
steels.

58. Effects of microstructure
Hardness Ductility