Cold Working is Actually Strain Hardening

1
Cold Working is Actually Strain Hardening

• Basic equation relating flow stress (strain
  hardening) to structure is
• so si aGbr1/2

• so is the yield stress
• si is the friction stress overall resistance
  of lattice to dislocation motion
• a is numerical constant 0.3 0.6
• G shear modulus
• b is the burgers vector
• r is the dislocation density

Yield stress increases as r increases
2
Effects of Cold Work
As cold work is increased

• Yield strength (sy) increases
• Tensile strength (TS) increases
• Ductility (EL or AR) decreases

3
Other Cold Work Effects

• Usually a small decrease in density (few 10ths of
  a percent)
• An appreciable decrease in electrical
  conductivity (increased number of scattering
  centers)
• Small increase in the thermal coefficient of
  expansion
• Because of increased internal energy chemical
  reactivity is increased (decreased resistance to
  corrosion)

4
s- e Behavior vs. Temperature
Results for polycrystalline iron
sy and TS decrease with increasing test
temperature. EL increases with increasing
test temperature. Why? Vacancies help
dislocations move past obstacles. Climb
of Edge Dislocations Never Screw
Positive Climb
5
Strain Energy Related to Cold Work

• Mentioned that 10 of the energy imparted during
  cold working is stored as strain energy
• Amount of strain energy is increased by
  increasing the severity of deformation, lowering
  the deformation temperature, and by impurity
  additions
• The strain energy increase is stored in the
  highly deformed microstructure dislocation
  tangles
• Metastable microstructure!

Figure Stored energy of cold work and fraction
of the total work of deformation remaining as
stored energy for high purity copper
Source Reed-Hill Abbaschian, Physical
Metallurgy Principles, 3rd Edition, PWS
Publishing Company, 1994.
6
Annealing

• Can we release the stored strain energy? YES!
• The material is in an unstable state but there
  is an activation energy barrier to releasing that
  energy
• By heating the material and adding energy to the
  system we can increase the probability of moving
  past the activation barrier
• Heat treating cold worked material is called
  Annealing

7
Release of Stored Energy

• What happens as we heat up cold worked material?
• Curve to the left is an anisothermal anneal curve
• Two samples one cold worked and the other not
• Samples are heated continuously from low
  temperature to a higher temperature
• Energy release is determined as a function of
  temperature
• Difference in power to heat the specimens at same
  rate

Figure Anisothermal anneal curve for
electrolytic copper
Source Reed-Hill Abbaschian, Physical
Metallurgy Principles, 3rd Edition, PWS
Publishing Company, 1994.
8
Annealing Stages

• The cold worked state is thermodynamically
  unstable.
• With increasing temperature it becomes more and
  more unstable
• Eventually the metal softens and returns to a
  strain-free condition
• Complete process is known as Annealing
• Annealing is easily divided into 3 distinct
  processes

1. Recovery
2. Recrystallization
3. Grain Growth

9
Recovery

• Defined as Restoration of physical properties of
  a cold worked metal without any observable change
  in microstructure
• Electrical conductivity increases and lattice
  strain is reduced
• Strength properties are not affected
• Involves
• Dislocation Annihilation
• Polygonization
• Removal of grain curvature created during
  deformation
• Regrouping of edge dislocations into low angle
  boundaries within grains
• Reduces the energy of system by creating reduced
  energy subgrains

Source 1 G. Dieter, Mechanical Metallurgy, 3rd
Edition, McGraw-Hill, 1986.
Source 2 Reed-Hill Abbaschian, Physical
Metallurgy Principles, 3rd Edition, PWS
Publishing Company, 1994.
10
Recrystallization

• Recrystallization is
• The replacement of the cold worked structure by
  the nucleation and growth of a new set of strain
  free grains
• Density of dislocations is reduced
• Strain hardening is eliminated
• The hardness and strength is reduced and the
  ductility is increased
• Driving force for recrystallization is the
  release of stored strain energy
• Note this is also the driving force for recovery
  and therefore they are sometimes competing
  processes

Source 1 G. Dieter, Mechanical Metallurgy, 3rd
Edition, McGraw-Hill, 1986.
11
How does it work?

• Nucleation of strain free grains occurs at points
  of high lattice curvature
• Slip line intersections
• Deformation twin intersections
• Areas close to grain boundaries
• Several models (unproven) that propose mechanisms
  for nucleation
• Grain boundary bulging due to a local variance in
  strain energy
• Sub-boundary rotation and coalescence

Source 2 Reed-Hill Abbaschian, Physical
Metallurgy Principles, 3rd Edition, PWS
Publishing Company, 1994.
12
Recrystallization
New grains are formed that -- have a
small dislocation density -- are small --
consume cold-worked grains.
13
Further Recrystallization
All cold-worked grains are consumed.
14
º
TR recrystallization temperature
º
15
Variables for Recrystallization

• Six main variables influence recrystallization
  behavior

1. The amount of prior deformation
2. Temperature
3. Time
4. Initial grain size
5. Composition
6. Amount of recovery or polygonization prior to the
   start of recrystallization

Because the temperature at which
recrystallization occurs depends
? Recrystallization temperature is not a fixed
temperature like melting point The practical
definition for recrystallization temperature
is The temperature at which a given alloy in a
highly cold worked state completely recrystallizes
in 1 hour.
Source G. Dieter, Mechanical Metallurgy, 3rd
Edition, McGraw-Hill, 1986.
16
Affect of Variables on Recrystallization

1. Minimum amount of deformation is required
2. The smaller the deformation, the higher the
   temperature required for recrystallization
3. Increasing annealing time decreases required
   recrystallization temperature. Temperature is
   more important than time. Doubling annealing
   time is approximately equivalent to increasing
   annealing temperature 10oC
4. Final grain size depends most on the degree of
   deformation and to lesser extent on the annealing
   temperature. The greater the deformation the
   lower the annealing temp., the smaller the
   recrystallized grain size.
5. The larger the original grain size, the greater
   the amount of cold work required to produce same
   recrystallization temp.

Source G. Dieter, Mechanical Metallurgy, 3rd
Edition, McGraw-Hill, 1986.
17
Affect of Variables on Recrystallization

1. The recrystallization temperature decreases with
   increasing purity of the metal. Solid solution
   alloying additions ALWAYS raise the
   recrystallization temperature.
2. The amount of deformation required to produce
   equivalent recrystallization behavior increases
   with increased working temperature
3. For a given reduction in cross-section
   different metal working processes produce
   different effective deformations. Therefore,
   identical recrystallization behavior may not be
   obtained.

Source G. Dieter, Mechanical Metallurgy, 3rd
Edition, McGraw-Hill, 1986.