Annealing – It is a process of heating steel slightly above the eutectic temperature of steel (723ºC) and letting it cool very slowly in the furnace itself.
Following are few types of annealing processes:
Full Annealing – In this, the steel is heated 30 to 50 degrees Centigrade above the critical temperature of steel and soaked at that temperature for a specified period of time, then allowing the material to slowly cool down inside the furnace itself with no other means of cooling. This process is usually applied to medium and high carbon steel.
Process Annealing – This process involves heating of steel to a temperature just below the lower critical temperature (723ºC) of steel. Usually, cold worked steel has high hardness and low ductility making it difficult to work. These characteristics can be improved by process annealing. This process is usually applied to low carbon steel.
Isothermal Annealing – In this process, steel is heated above the upper critical temperature (910ºC) causing the structure of the steel to be converted rapidly into austenite structure. The steel is then cooled by forced means to a temperature below the lower critical temperature about 600ºC-700ºC. The steel is soaked at this temperature for a specified period of time for homogeneous structure. This is applied to low carbon and alloy steels to improve their machinability.