Chapter 25 - Bainite in Steels

Abstract

This chapter considers a general classification of the products of austenite decomposition and an understanding of the mechanisms that are involved in the conclusion of the brief account of bainite. Bainite is the name given to the microstructural constituent formed in steels by the decomposition of austenite at temperatures above those of martensite formation but below those at which fine pearlite is formed. This transformation is not easy to understand and has given rise to much controversy, of which only an outline account has been given in the chapter. Bainite consists of ferrite plates or laths (subunits) and a dispersion of carbide precipitates. The ferrite plates are in aggregates called sheaves and are separated from each other by regions of retained austenite or, after cooling, by transformed austenite—that is, martensite. The carbide (also called as cementite) is precipitated among the ferrite plates. In lower bainite, the carbide is precipitated inside the ferrite, it takes the form of plate-shaped particles orientated at about 60 to the axis of the ferrite plates. Two micrographs in the chapter are depicted to present the upper and lower bainite, respectively.