4 - Kinetics of phase transformations in steels

Abstract

This chapter deals with the kinetics of diffusional phase transformations in steels, in particular, the formation of allotriomorphic ferrite from an fully austenitic starting condition in low alloyed steels, and focuses on the macroscopically apparent transformation kinetics as described by the well-known Johnson–Mehl–Avrami (JMA) equation. While the JMA approach is very practical from an industrial perspective, fitting experimental transformation curves to JMA equations does not lead to insight into the underlying physics of the transformation process. The actual austenite to ferrite transformation proceeds via a nucleation and growth process. Recent insights into the physical nature of both the nucleation and the growth process are discussed and remaining challenges are identified. A survey of common and less usual methods to follow the transformation kinetics of the austenite decomposition is presented. The chapter ends with a short description of the industrial relevance of a better understanding of the transformation kinetics.