There are four basic processes for the heat treatment of iron and steel: annealing, normalizing, quenching and tempering. Quenching (hardening) is a heat treatment process in which steel is heated to above critical temperature for a certain period of time and then cooled at a cooling rate greater than the critical cooling rate to obtain unbalanced structure dominated by martensite (also bainite or retaining single-phase austenite as required). Quenching is the most widely used process in the steel heat-treatment process.