Steels can be heat treated to produce a great variety of microstructures and
properties. Generally, heat treatment uses phase transformation during
heating and cooling to change a microstructure in a solid state.
In heat treatment, the processing is most often entirely thermal and
modifies only structure. Thermomechanical treatments, which modify
component shape and structure, and thermochemical treatments which
modify surface chemistry and structure, are also important processing
approaches which fall into the domain of heat treatment.