The primary purpose of an annealing treatment is to reduce the hardness of a material and facilitate the progress of subsequent manufacturing operations. Annealing is commonly used after casting, forging or rolling to soften materials and minimise residual stresses, improve machinability, and increase ductility by carefully controlling the microstructure. Many steels in strip form are annealed, as are most tool steels and stainless steels. Non-ferrous alloys are also annealed.
There are several process variations that qualify as annealing treatments:
- Full annealing is performed on steels by heating to a high temperature (typically 830-950°C), then cooling slowly to ambient temperature. Non-ferrous materials are softened and refined by fuII annealing at temperatures appropriate for each alloy.
- Isothermal/cyclic annealing is performed by heating steels to the full annealing temperature, cooling to an intermediate temperature (typically 550 - 700°C) and soaking for a long period to allow transformation to proceed slowly, followed by cooling to ambient temperature.
- Intercritical annealing is applied by heating steels to below the full annealing temperature (typically 723- 910°C) according to composition. A prolonged soak is followed by cooling to ambient temperature.
- Subcritical annealing takes place at a temperature for steels of typically 650 - 720°C, allowing a prolonged soak before cooling to ambient temperature.
- Homogenisation annealing can be applied to both ferrous and non-ferrous materials and is a prolonged high-temperature soak intended to break down segregation in the material's structure.
- Solution annealing is applied commonly to austenitic stainless steels, typically at 1010-1150°C. With unstabilised grades, the treatment must be followed by fast cooling or quenching. It is applied as a softening process during manufacture or to optimise corrosion resistance (e.g. after welding).