Heat treatment of steel is heating and cooling of steel to change its microstructure through phase transformation. It is based on Iron Carbon diagram. Following are the common heat treatment processes practiced in steel forging companies.
Normalizing: Heating steel to a temperature 30-50°C above A3 or Acm in austenite field range, soaking it at that temperature and then cooling it in still air is normalizing. This produces fine grains, ASTM grain size ranging 5-8. This improves both toughness and strength.Various forged components are normalised to improve machinability and increase toughness.
Quenching: In quenching, the soaking temperature is 30-50°C above A3 or A1. Here, the steel is cooled quickly in water, oil or salt bath. During fast cooling austenite cannot transform to ferrite and pearlite by atomic diffusion. Instead, it transforms to Martensite which is a supersaturated solid solution of carbon in α-iron with tetragonal body centered structure. Martensite has a needle-like structure and is very hard and brittle. This process is called hardening of steel.
Tempering: Tempering is carried out on hardened steels to remove the internal stresses and brittleness created during quenching. Without tempering, quenched forged components have high hardness and low machinability. The treatment requires heating the steel to a temperature below A1 depending upon the final properties desired. This heat energy diffuses carbon atoms out of the distorted lattice structure associated with martensite. This reduces the brittleness and increases the ductility, thus toughening the material making it capable of resisting certain degree of shock loading.