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Cold working and strain hardening

Short answer

Cold working plastically deforms metal below recrystallization temperature, increasing dislocation density. Common types include rolling, drawing, extrusion, bending. Strain hardening raises strength and hardness but reduces ductility. As % cold work increases, yield and tensile strengths rise, while ductility (elongation) decreases.

What is cold working? Name its common types. Explain strain hardening and the effect of increasing cold work on strength, hardness and ductility.

Cold working is plastic deformation of a metal below its recrystallisation temperature, so that the grains are deformed and strengthened without recrystallising.

Types of cold working

  • Cold rolling (sheets, strips)
  • Wire/bar drawing and tube drawing
  • Cold forging and extrusion
  • Bending, deep drawing and spinning of sheet
  • Shot peening (surface cold work)

Strain hardening

During cold work, the dislocation density rises from about 10610^{6}-10810^{8} cm−2^{-2} to 101010^{10}-101210^{12} cm−2^{-2}. Dislocations multiply (Frank-Read sources), tangle and block each other, so a higher stress is needed to move them. Hence the metal becomes stronger and harder as it is deformed. This is also called work hardening. The flow curve is σT=KεT n\sigma_T = K\varepsilon_T^{\,n}.

Effect of % cold work

% CW=A0−AdA0×100\%\,CW = \frac{A_0 - A_d}{A_0}\times 100 
 property
  |         tensile strength
  |       _..---
  |   _.-'
  |  /       yield strength lower than UTS
  |  \_
  |    `-._ ductility (% EL)
  +----------------------> % cold work
  • Yield strength, tensile strength and hardness increase (yield strength rises faster, approaching the UTS).
  • Ductility (% elongation, % RA) decreases.
  • Electrical conductivity falls slightly; corrosion resistance is reduced; grains elongate along the working direction and residual stress is introduced.

A heavily cold-worked metal can be restored by annealing.

✓ Answer checked by Claude Opus 5.5 (high effort)

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