Annealing

Annealing, is a heat treatment that alters a material to increase its ductility and to make it more workable. It involves, heating material to above its critical temperature, maintaining a suitable temperature, and then cooling. Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

Ferrous materials are cooled slowly to anneal, copper, silver and brass can be cooled slowly in air, or quickly by quenching in water.

Thermodynamics:

Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat increases the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and in ceramics. This alteration in dislocations allows metals to deform more easily, so increases their ductility.

Stages:

The three stages of the annealing process that proceed as the temperature of the material is increased are:

•  Recovery: The first stage is recovery, and it results in softening of the metal through removal of primarily linear defects called dislocation and the internal stresses they cause. Recovery occurs at the lower temperature stage of all annealing processes and before the appearance of new strain-free grains. The grain size and shape do not change.

•  Recrystallization: The second stage is recrystallization, where new strain-free grains nucleate and grow to replace those deformed by internal stresses.

•  Grain growth:  If annealing is allowed to continue when recrystallization has completed, then grain growth the third stage occurs. In grain growth, the micro structure starts to coarsen and  cause the metal to lose a substantial part of its original strength.