Annealing is the heating of metal after it has been cold worked to soften it. Most of the energy expended in cold work is released as heat during the deformation. However, a small percent of the mechanical work is stored by dislocations and vacancies. The stored energy is the driving force for the changes during annealing. There are three stages of annealing. In order of increasing time and temperature, they are as follows:
Recovery – often a small drop in hardness and rearrangement of dislocations to form subgrains. Otherwise, overall grain shape and orientation remain unchanged. Residual stresses are relieved.
Recrystallization – replacement of cold-worked grains with new ones. There are new orientations, a new grain size, and a new grain shape, but not necessarily equiaxed. Recrystallization causes the major hardness decrease.
Grain growth – growth of recrystallized grains at the expense of other recrystallized grains.
The energy release during recovery is largely due to annealing out of point defects and rearrangement of dislocations. Most of the increase of electrical resistivity during cold work is attributable to vacancies. These anneal out during recovery, so that the electrical resistivity drops (Figure 6.1) before any major hardness changes occur. During recovery, residual stresses are relieved, and this decreases the energy stored as elastic strains. The changes during recovery cause no changes in microstructure that would be observable under a light microscope. Figure 6.2 shows the energy release and the changes of resisitivity and hardness with increasing annealing temperatures.