Cold Working and Annealing. Cold working is deformation carried out under conditions where recovery processes are not effective. Structural changes during cold working of polycrystalline metals and alloys. Effect of cold work on properties. Annealing. Recovery

1. Mr. MANICKAVASAHAM G, B.E., M.E., (Ph.D.)
Assistant Professor,
Department of Mechanical Engineering,
Mookambigai College of Engineering,
Pudukkottai-622502, Tamil Nadu, India.
Email:mv8128351@gmail.com
Dr. R.Narayanasamy, B.E., M.Tech., M.Engg., Ph.D.,
(D.Sc.)
Retired Professor (HAG),
Department of Production Engineering,
National Institute of Technology,
Tiruchirappalli-620015, Tamil Nadu, India.
Email: narayan19355@gmail.com

2. Cold Working
Cold working is deformation carried out under conditions where recovery processes are
not effective.
Hot working is deformation under conditions of temperature and strain rate such that
recovery processes take place simultaneously with the deformation.
Hot Working

3. Structural changes during cold working of polycrystalline
metals and alloys
(1) Changes in shape and size of grains: The equi axed
grains on deformation are elongated in the direction of acting
force i.e. stretched in the direction of main tensile deformation
stress–say, in the direction of rolling or wire drawing.
Cold Working

5. (2) Changes in orientation of grains: Preferred orientation or
texture of is the state of severely cold worked metal in which
certain crystallographic planes of the grains orient themselves
in a preferred manner with respect to the direction of the stress
(or maximum strain).
Cont.

6. (3) Changes in internal structure of grains:
 During cold working around 15% of the work of the deformation gets
absorbed in the material (rest is lost as heat).
 This stored energy is the form of energy of crystal defects. Plastic
deformation increases the concentration of point defects.
 With increase of cold working, the number of stacking-faults
increases, thus density of extended dislocations increases.
Cont.

7. Cont.
 The number of kinks, jogs, dipoles, prismatic loops increase.
 The most important internal change of structure is increase in density
of dislocation from 106 – 108 cm-2 in annealed state to 1010 – 1012 by
moderate cold working.

8.  Cold working or strain hardening is the increase in the stress
required to cause further slip because of previous plastic deformation.
 This is an important industrial process that is used to harden metals or
alloys that do not respond to heat treatment.
 It changes various mechanical, physical and chemical properties of
metals and alloys.
Effect of cold work on properties

9.  With increase in amount of cold work, Ultimate Tensile Strength,
Yield Strength, Hardness increases but ductility (elongation and
reduction in area) or Formability decreases.
 Cold worked texture and mechanical fibering leads to Anisotropy in in
properties of materials.
 The ductility and impact toughness is much lower in transverse
section rather than in longitudinal section.
Cont.

11.  As the internal energy of cold worked state is high, the chemical
reactivity of the material increases i.e. the corrosion resistance
decreases, and may cause stress corrosion cracking in certain alloys.
 The rate of strain hardening (slope of flow curve) is generally lower in
HCP metals than cubic metals.
 High temperatures of deformation also lower the rate of strain-
hardening.
Cont.

12. Annealing of Cold worked materials
 In certain applications materials are used in the cold-worked state to
derive benefits of increased hardness and strength.
 The cold worked dislocation cell structure is mechanically stable, but
not thermodynamically stable.
 It is necessary to restore the ductility to allow further cold
deformation or to restore the optimum physical properties such as
electrical conductivity essential for applications.
Cont.

13.  The treatment to restore the ductility or electrical conductivity with a
simultaneous decrease in hardness and strength is Annealing (or
Recrystallization annealing).
 It is heating cold worked metal to a temperature above
recrystallization temperature, holding there for some time and then
slow cooling.
Cont.

14. The process of Annealing can be divided into three fairly distinct stages
(1) Recovery
(2) Recrystallization
(3) Grain growth
 There is no change in composition or crystal structure during
annealing.
 The driving force for recovery and recrystallization is the stored cold-
worked energy, whereas for grain growth is the energy stored in grain
boundaries.
Annealing

16. Annealing

17.  Recovery It is restoration of the physical properties of the cold
worked metal without of any observable change in microstructure.
 It is the Annihilation and rearrangement of point imperfections and
dislocations without the migration of high angle grain boundaries.
 Recovery is initially very rapid, and more when the annealing
temperature is high.
 Electrical conductivity increases rapidly toward the annealed value
and lattice strain measured using XRD is appreciably reduced.
Recovery

18.  Properties those are sensitive to point defects are affected, and
strength properties are not affected.
 With increasing time at constant temperature the recovery becomes
slower.
 The greater the initial cold work, the more rapid is the initial rate of
recovery.
 The rate of recovery of fine grains is higher than that of coarse grains.

19. Polygonization one of the recovery processes which leads to
rearrangement of the dislocations, with a resultant lowering of the
lattice strain energy.
It is a process of arranging excess edge dislocations in the form of tilt
boundaries, and the excess screw dislocations in the form of twist
boundaries, with the resultant lowering of the elastic strain energy.
Climb and slip of dislocations are essential for polygonization.
The presence of solute atoms in a metal reduces the rate of
polygonization.
Polygonization

20. Recrystallization: It is nucleation and growth of new strain-free crystals from the cold
worked metal. Kinetics of recrystallization resembles a phase transformation.
Two distinct nucleation mechanisms have been identified.
(1) Strain-induced boundary migration, where a strain-free nucleus is formed when one
of the existing grain boundaries into its neighbor, leaving a strain-free recrystallized
region.
Recrystallization

21. 2) New grains are formed in the regions of sharp lattice curvature
through sub grain growth. This seems to predominate at high strains,
with nuclei appearing at grain boundaries or at inclusions or second
phase particles. Mechanical properties change drastically over a very
small temperature range to become typical of the annealed material.
Electrical resistivity decrease sharply.

24. Comparison of Mechanical Properties During Recovery, Recrystallization and
Grain Growth.

25. Factors influence recrystallization behavior are
(1) Amount of deformation
(2) Temperature
(3) Time
(4) Initial grain size
(5) Composition
(6) Amount of recovery or polygonisation
(7) Method of deformation.

26.  Hence recrystallization temperature is not a fixed
temperature in the sense of a melting temperature.
 It can be defined as the temperature at which a given alloy
in a highly cold-worked state completely recrystallizes in
1h.
Recrystallization Temperature and its Variables

27. The laws of recrystallization are:
(1) A minimum amount of deformation is needed to cause
recrystallization.
(2) Smaller the degree of deformation, higher the temperature required to
cause recrystallization.
(3) Recrystallization rate increases exponentially with temperature.
Doubling the annealing time is approximately equivalent to increasing
the annealing temperature 10°C.
(4) Greater degree of deformation and lower annealing temperature, the
smaller the recrystallized grains.

28. (5) Larger the original grain size, the greater the amount of cold-work
required to produce equivalent recrystallization temperature.
(6) The recrystallization temperature decreases with increasing impurity
of motel. Alloying always raise recrystallization temperature.
(7) The amount of deformation required to produce equivalent
recrystallization behavior increases with increased temperature of
working.

29. Solute and Pinning effects:
 The impurity in metal segregate at grain boundary and retard the
migrating boundaries during recrystallization.
 This is known as the solution drag effect.
 When fine second phase particle (carbides) lies on the migrating
boundary, the grain boundary area is reduced by an amount equal to
cross sectional area of particle.
 When the boundary moves further, it has to pull away from the
particle and thereby create new boundary are equal to cross sectional
area of particle.
Solute and Pinning Effects

30.  This increases energy and manifests itself as a pinning acting on the
boundary.
 Consequently the rate of recrystallization decreases.

31.  It is uniform increase in the average grain size following
recrystallization.
The grain size distribution does not change during normal grain
growth.
During abnormal grain growth called secondary recrystallization
because the phenomenon shows kinetics similar to recrystallization,
the grain size distribution may radically change i.e.
some very large grains present along with the fine grains.
Grain Growth

32. The driving force for abnormal growth is decrease in surface energy.
Solute drag and pinning action of second phase particles retard
movement of a migrating boundary during grain growth as well.

33. Thank You
References:
Authors of Technical articles and Scopus Journals are
Acknowledged.