In conclusion, differences in the cooling rate appear to
provide dramatic effects on the microhardness of steels,
depending on the carbon content of steels. The microhardness
increases with the increasing cooling rate and
carbon content due to solid solution hardening and
formation of the martensite phase. Thus heat treatment
(heating and cooling) is used to obtain desired properties
of steels such as improving the toughness, ductility or
removing the residual stresses