Annealing of cold-worked metals

Annealing is heating a cold-worked metal to a suitable temperature, holding, and cooling slowly to remove strain hardening, relieve residual stress and restore ductility.

Stages

1. Recovery (low temperature, below about $0.3\,T_m$)

• Point defects annihilate and dislocations rearrange into low-energy arrangements (polygonisation).
• Internal stresses are relieved; electrical conductivity partly recovers.
• Hardness, strength and ductility change very little; grain structure stays deformed.

2. Recrystallisation (about $0.3$-$0.5\,T_m$)

• New strain-free equiaxed grains nucleate at high-energy sites (grain boundaries, slip bands) and grow, consuming the deformed grains.
• Dislocation density drops sharply; strength and hardness fall, ductility rises to near the annealed values.

3. Grain growth (higher temperature or longer time)

• Large grains grow at the expense of small ones to reduce grain-boundary energy.
• Strength and hardness fall further and the surface may look rough (orange peel).

 property
  |\ strength
  | \_______
  |  |       \___
  |  |   |       `--- 
  |  |   | ductility ___--
  |__|___|_____..--'
  +--|---|----------------> temperature
   Rec. Recryst. Grain growth

Recrystallisation temperature

It is the temperature at which a cold-worked metal completely recrystallises in about one hour. For pure metals it is about $0.3$-$0.5\,T_m$ (absolute), e.g., about 200 °C for copper, 450 °C for iron, near room temperature for lead.

Factors affecting it: (i) larger prior cold work gives a lower temperature; (ii) smaller initial grain size lowers it; (iii) higher purity lowers it, since alloying atoms and impurities impede boundary movement; (iv) longer holding time lowers it.

Importance

Annealing permits further cold work without cracking, since ductility is restored. Working above the recrystallisation temperature is called hot working; below it, cold working.