Annealing

Annealing is a heat treatment process which alters the microstructure of a material to change its mechanical or electrical properties. Typically, in steels, annealing is used to reduce hardness, increase ductility and help eliminate internal stresses.

Benefits of annealing

Annealing will restore ductility following cold working and hence allow additional processing without cracking. Annealing may also be used to release mechanical stresses induced by grinding, machining etc. hence preventing distortion during subsequent higher temperature heat treatment operations. In some cases, annealing is used to improve electrical properties.

Application & materials

One of the main applications of annealing is reversing the effects of work hardening. During cold forming, drawing, bending etc. the material can become hardened to the point where further working can be impossible or result in cracking. An annealing operation at this stage will make the material more ductile, permitting further forming. In a similar manner, annealing is utilized to remove the internal stresses which occur when welds solidify.

Besides steels, other metals may also benefit from annealing such as copper, aluminium, and brass.

Annealing process details

Annealing is a generic term and can be further classified according to temperature and atmosphere.

For steels, subcritical annealing takes place at 538°C – 649°C / 1000°F – 1200°F, so there is no crystal structure change. Intermediate annealing is carried out at 649°C – 760°C / 1200°F – 1400°F, so there is some transformation to austenite and full annealing involves completely austenitizing the work at 816°C – 927°C / 1500°F – 1700°F.

Parts can be annealed in a vacuum or reducing atmosphere where a bright surface finish is needed. Annealing in air is employed where surface finish is not an important factor and an endothermic/neutral atmosphere may be used during annealing to control decarburisation.