Annealing is any heat treatment in which the microstructure, and therefore properties, of a material are altered. Annealing typically refers to heat treatment in which a cold-worked metal is softened by allowing it to recrystallize.
Annealing is used to reverse the changes made to a metal by cold working. This is achieved by subjecting that metal to a particular temperature for a particular amount of time. It is the most common of all heat-treatment processes. During the production process, every piece of metal is annealed at least once, and some parts many times, in the transition from raw material to finished part.
Annealing involves heating a material to above its critical temperature, maintaining a suitable temperature and then cooling. The annealing process improves bending consistency and reduces the chances of cracking. This process is used to:
- Improve ductility
- Relieve stress in the material
- Refine the material's crystal structure
- Improve bending properties
For example, in the semiconductor industry, silicon wafers are annealed so that dopant atoms, usually boron, phosphorus or arsenic, can diffuse into substitutional positions in the crystal lattice, resulting in drastic changes in the electrical properties of the semiconducting material.
There are three stages of annealing, with each stage producing different results:
- Recovery – This results in softening of the metal through removal of primarily linear defects called dislocations 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 – New strain-free grains nucleate and grow to replace those deformed by internal stresses.
- Grain growth – The microstructure starts to coarsen and may cause the metal to lose a substantial part of its original strength. All of the effects of cold working are eliminated at this point.
Grain growth can be detrimental to the properties of the material and can typically produce a rough surface appearance on components formed from sheet metal.
Temperature variations have a much stronger influence on the annealing of metals than time variations. The annealing temperature is dependent on the component's:
- Thickness
- Composition
- Geometry