A treatment process in which a material in the solid state is heated, then cooled at a slow rate. See HEAT TREATMENT.
The annealing temperature and the rate of cooling depend on the material and the purpose of the treatment. Annealing is used to effect any of the following changes in metal:
(a) Relieve stresses
(b) Induce softness
(c) Alter ductility, toughness, electrical, magnetic,
(d) Refine the crystalline structure
(e) Remove gases
(f) Produce a definite microstructure or other physical properties
Annealing is a comprehensive term. Some specific annealing heat treatments are:
Full Annealing-
Heating iron-base alloys above the critical temperature range, holding above that range for a specified period of time, followed with slow cooling to below that range.
The annealing temperature is generally about 55°C (100°F) above the upper limit of the critical temperature range, and the holding time is usually not less than one hour for each inch of section of the workpiece being treated. The workpiece is then allowed to cool slowly in the furnace. In some applications, the workpiece is removed from the furnace and placed in a medium which will cool the material at a slower rate than unrestricted cooling at room temperature.
Heating iron-base alloys to a temperature below or close to the lower limit of the critical temperature, followed with cooling as specified.
Any process of heating and cooling steel that induces a rounded or globular form of carbide. The spheroidizing methods generally used are:
(1) Prolonged heating at a temperature just below the lower critical temperature, usually followed by relatively slow cooling.
(2) For small parts made 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.
(3) Tool steel is generally spheroidized by heating to a temperature in the range of 750 to 800°C (1380 to 1480°F) for carbon steels (higher for many alloy tool steels), holding at heat from 1 to 4 hours, and cooling slowly in the furnace.
An annealing operation performed on white cast iron partially or wholly to transform the combined carbon to temper carbon, and in some cases wholly to remove the carbon from the iron by decarbonization. Temper carbon is free graphite carbon in the form of rounded nodules composed of an aggregate of minute crystals.
Graphitizing-
Graphitizing is a type of annealing for gray cast iron in which some or all of the combined carbon is transferred to free graphitic carbon.