Topic Strain Hardening Hot Working and Annealing

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Topic Strain Hardening Hot Working and Annealing Powered By Docstoc
					          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                  Defects and Strengthening
    –Introduces Line Defects
    –Varied Strengths                   Mechanisms
Grain Size Refining
    –Surface Defects
    –Varied Strengths

 Solid Solution Strengthening
     –Single Phase
     –Point Defects
     –Low Strengthening Effect

                     Exceed Solubility Limit

Precipitation Hardening              Dispersion Strengthening
   –Two Phase, Coherent                 –Two Phase, Non-Coherent
   –Point and Surface Defects +         –Point and Surface Defect
   –High Strengthening Effect           –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               Disadvantages of Cold
         Metals                              Working Metals
  – Provides excellent surface finish   – Not practical for all types of metal
  – Simultaneously strengthen metal     – Ductility, corrosion resistance,
    and produce desired shape             electrical conductivity impaired by
  – Inexpensive for producing large       cold working
    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
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
– Recovery
              Work
   • 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                 4 s @ 580 C
                             New grains start to appear    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

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

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