The Bainite Reaction

Examination of the TTT diagram for a eutectoid carbon steel, Fig. 1, bearing in mind the fact that the pearlite reaction is essentially a high temperature one occurring between 550°C and 720°C and that the formation of martensite is a low temperature reaction, reveals that there is a wide range temperature, usually 250-550°C, when neither of these phases forms.

Fig.1: Time-Temperature-Transformation (TTT) diagram for a 0.89 carbon steel
(US Steel Co., Atlas of Isothermal Diagrams)

This is the region in which lath-shaped fine aggregates of ferrite and cementite are formed, which possess some of the properties of the high temperature reactions involving ferrite and pearlite as well as some of the characteristics of the martensite reaction.

The generic term for these intermediate structures is bainite after Edgar Bain who with Davenport first found them during their pioneer systematic studies of the isothermal decomposition of austenite. Bainite also occurs during thermal treatments at cooling rates too fast for pearlite to form, yet not rapid enough to produce martensite. The nature of bainite changes as the transformation temperature is lowered. Two main forms can be identified, upper and lower.