Chapter 8 Strain Hardening, Hot Working and Annealing

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Chapter 8Strain Hardening, Hot Working and
Annealing
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Line Defects - Dislocations

• Line Imperfections in the lattice of a
  crystalline material.
• Movement of dislocations gt material deformation
• Interference with movement of dislocations gt
  material strengthened
• Types
• Edge
• Screw
• Mixed

Edge Dislocation
Screw Dislocation
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Blocking Slip Material Strengthening

• Slip Deformation of a material by movement of
  dislocations through the Crystal Lattice
• Movement of dislocations gt material deformation
• Interference with movement of dislocations gt
  material strengthened

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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

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Defects and Strengthening Mechanisms

• Strain Hardening
• Introduces Line Defects
• Varied Strengths
• Grain Size Refining
• Surface Defects
• Varied Strengths

Exceed Solubility Limit
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Strain Hardening

• Plastic deformation below recrystallization temp
• Simultaneously deforms and strengthens the metal

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Cold Working
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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

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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
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• 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

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Can we strain harden polymers?
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Deformation of Polymers
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Strain Hardening of Plastics

• Polymer chains become aligned
• Provides increase in tensile strength
• Anisotropic properties

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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

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Annealing

• Three stages
• Recovery
• Recrystallization
• Grain growth

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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 dislocationsgt strength reduced
  and ductility restored
• Grain growth
• grains grow larger gt lower strength and higher
  ductility
• Too large of grains generally undesirable

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Recrystallization and Grain Growth
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Annealing
3 s _at_ 580 C New grains start to appear
4 s _at_ 580 C Many new grains are present
Cold Worked Brass
Complete Recrystallization
Substantial grain growth
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Some Factors Affecting the Recrystallization
Temperature
Effect of Cold Work
Effect of Melting Temperature
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Heat Affected Zone in a Weld
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Hot Working
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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

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Cold Working Processes

• Cold Rolling
• Swaging
• Cold Forging
• Cold Extrusion
• Riveting
• Sizing
• Coining
• Hubbing
• Peening
• Burnishing
• Bending

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Hot Working Processes

• Rolling
• Forging
• Extrusion
• Hot Drawing
• Pipe Welding
• Piercing