Annealing is a heat treatment process which alters the microstructure of a material to change its mechanical 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.
One of the main applications of annealing is reversing the effects of work hardening, at this stage it 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. Beside steels, other metals may also benefit from annealing such as copper, aluminium, and brass.
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Annealing is a generic term and can be further classified according to temperature and atmosphere. The temperature range for process annealing ranges from 260 °C to 960 °C, depending on the alloy in question. The material is heated up to a temperature just below the lower critical temperature of steel. Cold-worked steel normally tends to possess increased hardness and decreased ductility, making it difficult to work. Process annealing tends to improve these characteristics. This is mainly carried out on cold-rolled steel like wire-drawn steel, centrifugally cast ductile iron pipe etc.
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.