POSCO Lectures: The Bainite Reaction

Examination of a time–temperature–transformation (TTT) diagram for an eutectoid carbon steel (Fig. 1), bearing in mind the fact that the pearlite reaction is essentially a high temperature one occurring between 550C and 720C and that the formation of the martensite is a low-temperature reaction, reveals that there is a wide range of temperature ∼250–550C within which neither of these phases forms. This is the region in which fine aggregates of ferrite plates (or laths) and cementite particles are formed. The generic terms for these intermediate structures is bainite, after Edgar Bain who with Davenport first found these structures during their pioneering systematic studies of the isothermal decomposition of austenite. Bainite also occurs during athermal treatments at cooling rates too fast for pearlite to form, yet not rapid enough to produce martensite.