Introduction of Annealing

Annealing is a process in which a metal is heated to a particular high temperature, held there for a period of between several hours and several days, and allowed to cool. In order for the annealing process to occur correctly, this cooling process must be slow in steels and other ferrous metals. Metals are generally annealed at a temperature slightly above the point at which recrystallization occurs. Special furnaces are used in the annealing process. Conditions in these furnaces are tightly controlled to ensure that the expected changes are taking place.

Annealing has several uses. Annealing can increase ductility and alleviate internal stresses that contribute to brittleness. Annealing can also increase toughness and homogeneity of metals. Furthermore, metals may be annealed to prepare for further heat treatment, in order restore ductility to the metal prior to further working.

Annealing of steel tube

Physically, the annealing process involves diffusion of particles within a material. The material must be heated sufficiently to allow free movement of particles; in steels this critical temperature is approximately 723 degrees Centigrade. Steels must be cooled slowly to allow for adequate diffusion of molecules. In fact, the speed of cooling is a critical difference between annealing and tempering.

Finally, the environment in which a material is annealed is critical. Steels can be annealed in a carbon-rich environment to facilitate carburization, which has certain desirable advantages. However, if carburization, oxidation, or other defects are to be avoided, annealing must be carried out in an environment free of such elements.

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