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Work Hardening and Annealing
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Principal of Strengthening
The ability of a metal to plastically deform
depends on the ability of dislocation to move.
Restricting or hindering dislocation motion
renders a material harder and stronger
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Metal Working Methods
• Change in cross-sectional area
• %CW = (A0 – A)/A0 x 100
Rolling
Forging
A
Ao
Ao
A
Extrusion
Drawing
A
Ao
Ao
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A
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Strain (Work) Hardening
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Tensile Test – Work Hardening
*
n is the strain-hardening exponent
K is the strength coefficient
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Tensile Test – Work Hardening
log-log curve of  vs. 
- Slope of the curve is n
Various hardening components
- Lower n means poor respond to work
hardening
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Values for n and K for metals at room temperature
Metal
Condition
n
K, psi
0,05% C steel
Annealed
0,26
77000
SAE 4340 steel
Annealed
0,15
93000
0,60% C steel
Quenched and tempered 1000oF
0,10
228000
0,60% C steel
Quenched and tempered 1300oF
0,19
178000
Copper
Annealed
0,54
46400
70/30 brass
Annealed
0,49
130000
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Strain (Work) Hardening
Cold work will lead to:
•Increase of Yielding Strength
•Increase of Tensile Strength
•Reduction of Elongation
Material becomes stronger
but more brittle
Effect of cold work on mechanical
properties of Cu-1.5Ti alloy
S. NAGARJUNA, K. BALASUBRAMANIAN,
1997
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Strain (Work) Hardening
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Strain (Work) Hardening
Reason: Increasing dislocation density and the
interaction between dislocations, which
reduces dislocation mobility. As a result,
larger stresses must be applied in order that
additional deformation may taken place.
Dislocation Multiplication:
- Before deformation a metal contains about 106 cm of
dislocation line per cubic centimeter of metal. The
number of dislocations may increase to 1012 cm of
dislocation line per cubic centimeter of metal.
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Strain (Work) Hardening
– Dislocation Multiplication
Frank-Read Source
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Strain (Work) Hardening
Dislocation Interactions make it difficult to move
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Strain (Work) Hardening
The intersection of two dislocations creates jogs which in
magnitude and direction are equal to the other dislocation’s
Burgers vector.
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Microstructure Change after Cold Work
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Annealing
• Heating a cold-worked metal above a recrystallization temperature
(0.3–0.5 TM) eliminates most of the defects (dislocations, etc)
– In 1 hr, substantial amount of recrystallization occurs
• Heating process referred to as annealing
– Annealing consists of heating to a high enough temperature
followed by cooling at a suitable rate
• During annealing, metals undergo recovery and recrystallization
– Highly-strained grains are replaced by new strain-free grains
• Amount of recrystallization is dependent on both time and temperature
• Annealing leads to
– Reduction in yield strength and hardness and increase in ductility
as the dislocations are removed
– Increase ductility, softness
– Development of desired microstructure and properties
• Cold-working and annealing are often cycled to assist in production
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Recovery
When the cold-worked metal is annealed:
 Recovery occurs first (at 0.1TM)
– Thermal energy allows some dislocation motion
– Dislocation density goes down slightly due to
annihilation and rearrangement
– Hardness and ductility are almost unchanged
 Major changes come from recrystallization that occurs at
higher temperatures (at 0.3–0.5TM)
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Recrystallization
• Recrystallization:
– New grains nucleate and grow at the expense the highly-strained
grains until the whole of the metal consists of strain-free grains
– Nucleation usually occurs in the most deformed portion of the grain
– boundary or slip plane
» Driving force is the strain energy of the deformed grains
– Dislocation density returns to original value (before cold working)
– Hardness and ductility return to original value
• Recrystallization also used to control grain size
– High temperatures and long crystallization times can lead to grain
growth of the strain-free grains (driven by reduction in grain
boundary area) tend to produce a large grain size
– Grain growth due to surface tension - big grains eat little grains
– High initial deformation tends to produce small recrystallized grains
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Cold Working and Annealing
Starting material with low
dislocation density
Cold-worked material
has greatly increased
dislocation density
Further annealing leads to
nucleation and growth of new grains
Annealing leads to
recovery
Fully recrystallized metal with new
(smaller) strain-free grains
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Recrystallization & Growth
0 sec
3 sec
4 sec
8 sec
Cold worked brass at recrystalization temp 580oC
Mechanical & Aerospace Engineering
Callister, Mat Sci & Eng an Intro,
5th
ed.
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Example: Design a process to produce 0.20-in diameter copper
wire.
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