strain hardening, work hardening , pile up theory, recovery, recrystallization, grain growth

1. Module-1
1.4-Strain Hardening and
Recrystallization
By Prof. F.J.KHAN
DEPT.OF MECHANICAL ENGG.
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2. � DEFINITION :
Strain hardening or work hardening is a phenomenon which results in an
increase in hardness and strength of metal when subjected to plastic
deformation at temperature lower than the recrystalization range .
� RECRYSTALIZATION TEMPERATURE :
Recrystallization temperature is a particular temperature point below the
melting point of a metal (or material).
� WHY IS STRAIN HARDENING DONE?
With increasing stress on a material i.e. by applying load , there are
possibilities that a material may fail before reaching the desired stress
value
✔To improve the hardness of a substance so that it is able to sustain
more load in the elastic region strain hardening is done
✔Here ductility is compromised to get hardness and strength.
Introduction
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3. Strain(Work) Hardending
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4. � All the theories of work hardening believe that work
hardening is due to the increased dislocations, when the
sample is subjected to cold working.
� Some dislocations become struck inside the crystal and act as
sources of internal stress which opposes the motion of other
dislocation motions i.e. work hardening occurs due to
restriction to motion of the dislocations.
Theory of Work Hardening
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5. 5

6. • This stage is followed immediately after the yield point of
the material. Dislocations are able to move over relatively
large distance without accounting barriers.
• Length of this region depends upon the characteristics such
as orientation, purity and size of crystals .
• The end of stage I is not a particularly reproducible
phenomenon and is sensitive to material purity and stress
raisers on the specimen surface.
Stage - I [Easy Glide Region]
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7. • The region shows a rapid increase in work hardening rate,
the slope of which is approximately independent of applied
stress, temperature or alloy content.
• In this region, slip occurs on both the primary and secondary
slip systems.
• As a result, several new lattice irregularities may be formed
such as forest dislocations.
• Lomer-Cottrell barriers and Jog produced either by moving
dislocations cutting through forest dislocations.
• The theories linear hardening regiobs are the Pile-Up Theory
, Forest Theory and the Jog Theory.
Stage - II [Linear Hardening Region]
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8. � According to this theory, some dislocations given out by
FRANK-REED source are stopped at barriers e.g. Lomer
Cottrell barriers in lattice and other dislocations pile up
behind.
� As deformation proceed, the number of barriers increase until
each source becomes completely surrounded by barriers.
� As per this theory, the hardening is principally due to long
range internal stresses from piled up groups interacting with
guide dislocations.
Pile-up Theory (Stage Ii)
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9. 9

10. � It is a region of decreasing rate of strain hardening. At
sufficiently high stress value or temperature in region III, the
dislocations held up in stage-II are able to move by a process
that had been suppressed at lower stresses and temperatures.
� The screw dislocations which are held up in stage-II, cross slip
and possibly return to the primary slip plane by double cross-
slip.
� By this mechanism, dislocations can bypass the obstacles in
the glide plane and do not have to intersect strongly with them.
� For this region, stage-III exhibits a low rate of work
hardening.
� Stage III corresponds to a steady decrease of work hardening
rate and is sensitive to temperature and strain rate.
Stage – III [Parabolic Hardening
Region]
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11. }Strain hardening (also called work-hardening or cold-working) is
the process of making a metal harder and stronger through
plastic deformation.
}When a metal is plastically deformed, dislocations move and
additional dislocations are generated.
}The more dislocations within a material, the more they will
interact and become pinned or tangled.
}This will result in a decrease in the mobility of the dislocations
and a strengthening of the material.
}This type of strengthening is commonly called cold-working.
}It is called cold-working because the plastic deformation must
occurs at a temperature low enough that atoms cannot
rearrange themselves. 11
Strain Hardening

12. }When strain hardened materials are exposed to elevated
temperatures, the strengthening that resulted from the
plastic deformation can be lost.
}Heat treatment can be used to remove the effects of strain
hardening.
}Three things can occur during heat treatment:
}Recovery
}Recrystallization
}Grain growth
}
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Effects of Elevated Temperature on
Strain Hardened Materials

13. }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. 13
Recovery

14. }Polygonisation occurs during recovery.
}Dislocations become mobile at a higher temperature,
eliminate and rearrange to give polygonisation.
}The misorientation θ between grains can be described in
terms of 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
}
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Polygonisation

15. 15
Polygonisation

16. 16
Properties changes during cold working
and annealing process

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18. 18

19. }Recrystallization is a process by which deformed grains
are replaced by a new set of defect-free grains that
nucleate and grow until the original grains have been
entirely consumed.
}Recrystallization is usually accompanied by a reduction in
the strength and hardness of a material and a simultaneous
increase in the ductility.
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Recrystallization

20. }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 20
Recrystallization

21. }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. 21
Grain growth

22. 1.Melting Point
2.Purity of Metal
3.Degree of cold work
4.Heating Time
5.Grain Size
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Factors Affecting Recrystallization

23. Thank You..
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