Quenching is an accelerated method of bringing a metal back to room temperature, preventing the lower temperatures through which the material is cooled from having a chance to cause significant alterations in the microstructure through diffusion. Quenching can be performed with forced air convection, oil, fresh water, salt water and special purpose polymers.
The slower the quench rate, the longer thermodynamic forces have a chance to alter the microstructure, which is in some cases desirable, hence the use of different media. When quenching in a liquid medium, it is important to stir the liquid around the piece to clear away steam from the surface; steam pockets locally defeat the quench by air cooling until they are cleared away.
Most commonly performed to harden steels, water quenching from a temperature above the austenitic temperature will cause carbon to be trapped inside the austenitic lath, resulting in the hard and brittle martensitic phase. Typically, the steel will be subsequently tempered to restore some of the ductility and toughness lost by conversion to martensite.