Cooing rate or heat transfer rate for carbon steel is of prime importance as it defines the required structure consequently essential properties. If the cooling rate is higher the critical cooling rate than quenching steel takes place. Quenching of carbon steel improves the hardness of materials due to the formation of martensite.
Quenching steel can be carried out in either gas medium or liquid medium. In the case of gas quenching, heat transfer is slow because of gases are poor conductors of heat resultantly low cooling rate.
But, in liquid quenching, heat transfer is very fast. It can take place in three stages as follows:
The vapor phase
Nucleate boiling
Convective stage