Heat Treatment
Heat treatment is done to obtain some desirable properties. It is a process in which materials are heated and cooled in solid state. The mechanical properties are improved due to change in size and structure of the grains of which it is composed.
The main purpose of heat treatment is to
– To improve the mechanical properties like ductility, hardness, tensile strength etc.
– To refine the grain size
– To increase the hardness of surface
– To improve the machinability
– To increase resistance to wear, tear and corrosion
The various heat treatment processes are:
Normalising – It is the process in which the iron based alloys are heated 40-50°C above the upper critical temperature and remain there for a specific time and followed by cooling in stir air at room temperature.
Normalising is carried out for large castings and forgings of steel to improve its machinability. It eliminates the coarse grain structure.
Annealing – Annealing is softening process. It is the process in which iron base alloys are heated above the transformation range, kept there for a specific time and cooled down below the transformation range within the furnace.
It is performed to
– increase the ductility
– improve machinabilty
– soften the metal
– to refine the grain size
Hardening – In this process steel is heated 20°C above the transformation temperature, held there for specific period to penetrate the temperature inside the part, and then cooled at room temperature by quenching in suitable quenching media (brine solution, water or oil).
The hardened parts are brittle and good in tensile strength.
Tempering – It is a process in which the heating of hardened steel is carried out at a temperature below the lower critical temperature to increase the ductility and toughness. The hardness and strength decreases. The steel is reheated from 150°C to 650°C depending upon the usefulness.
The purpose of tempering is to
– Increase ductility
– Improve toughness
– Reduce hardness
Case hardening:
Some of the components like gears, bearings and cams require hard surface (case) with soft inner core. This heat treatment process is called case hardening. Coatings are applied to the surface in order to improve resistance to wear, tear and corrosion. The processes used for case hardening are as follows
Carburizing – It is the process of introducing the carbon into iron base alloys to produce a hardened surface. Low carbon steel is hardened by this method. The carbon containing substances (liquid, solid or gas) are used to introduce carbon into low carbon steel. Low carbon steel is heated in contact with these substances. A hardness of 65 HRC can be obtained by this method.
Nitriding – It is the method of introducing the nitrogen into the surfaces of steel containing Aluminium and Chromium. The steel is heated and held there in contact with partially dissociated ammonia for specified time.
Cyaniding – The steel is heated and held there in contact with molten cyanide bath for a specified time. It is used for the case hardening of Screws, nuts and bolts.
Flame hardening – It consists of heating the steel by means of a flame at temperature within transformation range immediately followed by quenching.
Induction hardening – The surface is heated electrically to avoid over heating or burning. The surface is heated by means of an alternating magnetic field immediately followed by quenching.