cementite. Martensite, the hardest constituent forms during severe quenches from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to about 0.7 wt%. If these unstable metastable products are subsequently heated to a moderately elevated temperature, they decompose to more stable distributions of ferrite and carbide. The reheating process is sometimes known as tempering or annealing. The transformation of an ambient temperature structure like ferrite-pearlite or tempered martensite to the elevated-temperature structure of austenite or austenite + carbide is also of importance in the heat treatment of steel. Hardenability ConceptsThe goal of heat treatment of steel is very often to attain a satisfactory hardness. The important microstructural phase is then normally martensite, which is the hardest constituent in low-alloy steels. The hardness of martensite is primarily dependent on its carbon content. If the microstructure is not fully martensitic, its hardness is lower. In practical heat treatment it is important to achieve full hardness to a certain minimum depth after cooling, that is, to obtain a fully martensitic microstructure to a certain minimum depth, which also represents a critical cooling rate. If a given steel does not permit a martensitic structure to be formed to this depth, one has to choose another steel with a higher hardenability. Principles of Tempering of SteelsMartensitic is a very hard phase in steel. It owes its high hardness to a strong supersaturation of carbon in the iron lattice and to a high density of crystal defects, especially dislocations, and high - and - low angle boundaries. However, except at low carbon contents, martensitic steels have insufficient toughness for many applications. Tempering of martensitic steels, by heating for a certain time at temperatures below the A1 , is therefore introduced to exchange some of t...
