Heat treatment is an operation or combination of operations, involving heating and cooling of a metal or alloy in the solid state for the purpose of changing the properties of the metal. Heat treatment consists of three phases.
(a) Heating of the metal.
(b) Soaking of the heat into metal.
(c) Cooling of the metal.
Purpose of Heat Treatment
The purpose of heat treatment is as follows:
(a) To improve machinability.
(b) To improve mechanical properties, e.g., tensile strength, ductility, hardness and shock resistance.
(c) To relieve the stress induced during hot or cold working.
(d) To change or refine grain size.
(e) To improve magnetic and electrical properties.
(f) To improve wear resistance.
(g) To improve weldability.
Structure Of Iron And Steel
Various structures of iron and steel are as follows:
Ferrite. It is the name given to pure iron crystals which are soft and ductile. The slow cooling of low carbon steel below the critical temperature produces ferrite structure. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic. This is a Body Centered Cubic iron phase containing little or no carbon.
Cementite. This is formed when the carbon combines with iron in the form of iron carbide which is extremely hard in nature. Cementite increases generally with increase in carbon percentage. It is found in steel containing more than 0.87% carbon.
Pearlite. This is a mechanical mixture of about 87% ferrite and 13% cementite. It comprises of alternate layers of ferrite and cementite in steel. When seen with the help of a microscope, the surface has appearance like pearl, hence it is called pearlite. Hard steels are mixtures of pearlite and cementite while soft steels are mixtures of ferrite and pearlite.
Martensite. This is produced by entrapping carbon on decomposition of austenite when cooled rapidly. It is the main constituent of hardened steel. It is magnetic and appears as a needle like fibrous structure. It has carbon content up to 2%. It is extremely hard and brittle. The decomposition of austenite below 320°C starts the formation of martensite.
Austenite. It is a solid solution of ferrite and iron carbide in gamma iron. It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be hardened by usual heat treatment methods
Troostite. This is another constituent of steel obtained by tempering martensite. It is less hard and brittle than martensite. It is also produced by cooling the metal slowly until transformation begins and then cooling rapidly to prevent its completion. It has a dark appearance on etching. It is weaker than martensite.
Sorbite. This is also produced by the transformation of tempered martensite. It is produced when steel is cooled slowly from the temperature of the solid solution to normal room temperature. It has good strength and is practically pearlite. Its properties are intermediate between those of pearlite and troostite.