Annealing works by heating the metal above a critical temperature, holding it at a specific temperature and color and allowing it to cool to room temperature at a controlled rate. This process increases the rate of diffusion of the atoms towards its equilibrium state by providing the energy needed to break the atomic bonds. The movement of the atoms redistributes and destroys the dislocations in the structure of the metal.
Annealing occurs in three stages: recovery, recrystalization and grain growth. Recovery occurs in the lower temperatures of annealing and results in the softening of the metal through the removal of dislocations. Recrystalization results in the new strain-free grains that replace those deformed by dislocations. Grain growth results in the coarsening of the microstructure of the metal, which may cause a loss in strength if the metal is not hardened.
The thermodynamic process of stress relieving is spontaneous, but occurs slowly at room temperatures. The high temperatures of annealing accelerates this process. Annealing prepares the metal for further work, such as cutting, shaping or forming. Non-ferrous metals, such as copper, silver and brass, can be cooled quickly by quenching in water, while ferrous metals must be cooled slowly in still air. Aluminum can be melted if it is heated for too long, so it must be heated at a distance.