Austenite - Behavior in Plain-carbon Steel

Behavior in Plain-carbon Steel

As austenite cools, it often transforms into a mixture of ferrite and cementite as the carbon diffuses. Depending on alloy composition and rate of cooling, pearlite may form. If the rate of cooling is very fast, the alloy may experience a large lattice distortion known as martensitic transformation in which it transforms into a BCT-structure instead of into ferrite and cementite. In industry, this is a very important case, as the carbon is not allowed to diffuse due to the cooling speed, which results in the formation of hard martensite. The rate of cooling determines the relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness and tensile strength. Quenching (to induce martensitic transformation), followed by tempering will transform some of the brittle martensite into tempered martensite. If a low-hardenability steel is quenched, a significant amount of austenite will be retained in the microstructure.