When austenite is rapidly cooled (i.e. quenched) to room temperature, an unexpected phase transformation can occur. Instead of forming ferrite and pearlite or cementite and pearlite, austenite can transform tomartensite,a highly stressed body-centered-tetragonal (BCT) phase. This reaction is not diffusion-controlled but rather takes place by a shear-type transformation and is only a function of temperature and not time. The result is a very hard but brittle microstructure. The hardness of the martensite depends on the carbon content of the austenite. The martensite and the austenite it came from have the same chemical composition.

As-quenched martensite is too hard and brittle for most practical uses, but it can be tempered to recover some toughness and ductility. Tempering consists of reheating the martensite to temperatures typically between 275-750°F (135–400°C) for several hours. During the temper heat treatment, carbides precipitate in the martensite matrix. This transformation increases the toughness of the steel by the carbide precipitates as well decreasing the carbon content of the martensite. In some steels, an increase in strength and toughness occurs.

For tempering, the important process parameters are temperature and time. The temperature must be selected to control the size and distribution of the carbides that precipitate during the tempering process and to convert any retained austenite to tempered martensite. The time must be long enough to heat the entire load to the desired temperature. In addition, time is needed for the nucleation and growth of the carbides to form the tempered martensite. The rate of cooling from tempering temperature, though not critical in many applications, must be rapid enough to avoid temper embrittlement (TE and TME) conditions.

Martensite: Rapidly cooled austenite. No carbon diffusion - trapped in martensite. Shear transformation involved. Supersaturated solution of carbon in alpha iron (ferrite). Trapped carbon induces built-in stresses. Martensite is very hard and brittle. Must be toughened by tempering. End result is still harder and stronger than typical slow-cooled pearlitic steel.

Upcoming topics
Part 5: Time-temperature-transformation (TTT) Diagrams