Chapter 8
Strain Hardening, Hot Working
and Annealing
Line Defects - Dislocations
• Line Imperfections in the lattice of a crystalline
material.
• Movement of dislocations => material deformation
• Interference with movement of dislocations =>
material strengthened
• Types:
– Edge
– Screw
– Mixed
Edge Dislocation
Screw Dislocation
Blocking Slip =
Material Strengthening
• Slip: Deformation of a
material by movement of
dislocations through the Crystal
Lattice
• Movement of dislocations =>
material deformation
• Interference with movement of
dislocations => material
strengthened
Some Ways to Strengthen a Metal By
Disrupting the Crystal Lattice
• Alloy:
– Solid Solution Strengthening
– Dispersion Strengthening
– Precipitation Hardening
• Heat Treatment:
– Controlled heating and cooling of metals for purpose of altering
properties
• Strain Harden or Cold Work
Strain Hardening
–Introduces Line Defects
–Varied Strengths
Grain Size Refining
–Surface Defects
–Varied Strengths
Defects and Strengthening
Mechanisms
Solid Solution Strengthening
–Single Phase
–Point Defects
–Low Strengthening Effect
Exceed Solubility Limit
Precipitation Hardening
–Two Phase, Coherent
–Point and Surface Defects +
–High Strengthening Effect
Dispersion Strengthening
–Two Phase, Non-Coherent
–Point and Surface Defect
–Medium Strengthening Effect
Strain Hardening
• Plastic deformation below recrystallization temp
• Simultaneously deforms and strengthens the metal
Cold Working
Simple Cold Rolling
% Cold Work = ((Ao - Af)/Ao) x 100%
Where:
Ao = original x-sectional area
Af = final x-sectional area
Other Cold Working Techniques:
•Forging
•Rolling
•Drawing
•Extrusion
•Stretch Forming
•Misc
Microstructural Mechanism Responsible for
Strain Hardening in Metals
• Increases number of dislocations (line imperfections)
• Dislocations get in the way of other dislocations
trying to move or slip
• Provides higher strength, hardness but lower
toughness.
Frank - Read Source
Advantages of Cold Working
Metals
– Provides excellent surface finish
– Simultaneously strengthen metal
and produce desired shape
– Inexpensive for producing large
number of small parts
– Properly controlled residual
stresses and anisotropic behavior
may be beneficial
– May be the only method available
for accomplishing certain tasks
– FCC metals respond well to
coldworking
– HCP metals do not respond well to
coldworking
Disadvantages of Cold
Working Metals
– Not practical for all types of metal
– Ductility, corrosion resistance,
electrical conductivity impaired by
cold working
Can we strain harden polymers?
Deformation of Polymers
Strain Hardening
of Plastics
• Polymer chains
become aligned
• Provides increase in
tensile strength
• Anisotropic
properties
Stages of Annealing – Team
Work
– Recovery
• Rearranges dislocations, but does not eliminate them
• So-----how might this affect the properties?
– Recrystallization
• Small grains nucleate at boundaries of dislocations, eliminate
most of the dislocations
• So---how might this affect the properties
– Grain growth
• grains grow larger
• So---how might this affect the properties
Annealing
Three stages:
– Recovery
– Recrystallization
– Grain growth
Annealing
• Heat treatment used to eliminate all or part of the effects of cold working
• Three stages
– Recovery
• Reduces or eliminates residual stresses
• Does not change mechanical properties
• Restores electrical conductivity and improves corrosion resistance
• Relatively low temp heat treat
• Produces a polygonized subgrain structure
– Recrystallization
• Material heated above recrystallization temp.
• Small grains nucleate at boundaries of polygonized structure
• Eliminates most dislocations=> strength reduced and ductility
restored
– Grain growth
• grains grow larger => lower strength and higher ductility
• Too large of grains generally undesirable
Recrystallization and Grain
Growth
Cold Worked Brass
3 s @ 580 C
New grains start to appear
4 s @ 580 C
Many new grains are
present
Annealing
Complete Recrystallization
Substantial grain growth
Some Factors Affecting the
Recrystallization Temperature
Effect of Cold Work
Effect of Melting
Temperature
Heat Affected Zone in a Weld
Hot Working
Hot Working
• Plastically deforming a material above its recrystallization
temperature
• No strengthening occurs (simultaneously annealing the
material!)
• Essentially no limit on the amount of deformation
• Suitable for a wider range of crystal structures since all
materials become more ductile and weaker at elevated
temperatures
• Surface finish is not as great (like rolling warm cookie dough as
opposed to hot cookie dough)
• Surface may oxidize
• Properties typically not anisotropic
• Dimensional accuracy not as great since thermal expansion
occurs
Cold Working Processes
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Cold Rolling
Swaging
Cold Forging
Cold Extrusion
Riveting
Sizing
Coining
Hubbing
Peening
Burnishing
Bending
Hot Working Processes
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Rolling
Forging
Extrusion
Hot Drawing
Pipe Welding
Piercing