Annealing

Annealing

Annealing is a process involving the heating of a metal above the critical temperature and subsequent slow cooling. The purpose of such heating may be to remove stresses; induce softness; alter ductility, toughness, electrical, magnetic, or other physical properties; refine crystalline structure; remove gases; or produce a definite microstructure.

b. Specific heat treatments which fall under the term annealing are:

(1) Full annealing. This is the heating of iron base alloys above the critical temperature range, holding them above that range for a proper period of time, followed by cooling in a medium which will prolong the time of cooling.

(2) Process annealing. This is the heating of iron base alloys to a temperature below or close to the lower limit of the critical temperature range, followed by cooling as desired.

(3) Normalizing. This is the heating of iron base alloys to approximately 100°F (38°C) above the critical temperature range, followed by cooling to below that range in still air at ordinary temperature.

(4) Patenting. This is the heating of iron base alloys above the critical temperature range, followed by cooling below that range in air, molten lead, a molten mixture of nitrates, or nitrates maintained at a temperature usually between 800 to 1050°F (427 to 566°C), depending on the carbon content of the steel and the properties required of the finished product.

(5) Spheroidizing. This is any process of heating and cooling steel that produces a rounded or globular form of carbide. Methods of spheroidizing generally used are:

(a) Prolonged heating at a temperature just below the lower critical temperature, usually followed by relatively slow cooling.

(b) In the case of small objects of high carbon steels, the spheroidizing result is achieved more rapidly by prolonged heating to temperatures alternately within and slightly below the critical temperature range.

(6) Tempering (also called drawing). This is reheating hardened steel to some temperature below the lower critical temperature, followed by any desired rate of cooling.

(7) Malleablizing. This is an annealing operation performed on white cast iron to partially or wholly transform the combined carbon to temper carbon, and in some cases, to wholly remove the carbon from the iron by decarburization.

(8) Graphitizing. This is a type of annealing of gray cast iron in which some or all of the combined carbon is transferred to free graphite carbon.

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