What phase transformation does a steel undergo to become martensite?

The transformation of steel into martensite occurs through a process of rapid cooling, often referred to as quenching. During this phase transformation, steel is typically heated to a high temperature, where its crystal structure becomes austenitic (face-centered cubic). Once it reaches this austenitic phase, rapid cooling is applied, usually by immersion in water or oil. This rapid change in temperature does not allow time for carbon atoms to diffuse out of the austenite structure, leading to an unfavorable arrangement of atoms which creates the martensite phase.

Martensite is characterized by its hard and brittle properties, which are a result of the retained carbon in the structure and the transformation that takes place during the rapid cooling. Thus, this process is essential for enhancing the hardness and strength of steel used in many applications.

Other cooling methods, such as slow cooling and isothermal transformations, would lead to different microstructures like pearlite or bainite rather than martensite, while annealing involves controlled heating and cooling to relieve stresses but does not yield martensite.