Kinetics approach to understand “martensitic” transformations in steel.

2022-11-17   Lecture by dr.  Matteo Villa.

Abstract:
In the classical description as taught in undergraduate, and many graduate, materials science courses, martensite in steel begins to form during cooling austenite below a critical temperature. In absence of partitioning, the transformation proceeds on continued cooling, irrespective of the applied cooling rate. In this classical picture, interrupted cooling implies discontinuation of martensite formation and isothermal martensite is considered an annoying anomaly. Furthermore, the classical approach considers martensite to have structural and morphological characteristics that are fully determined by the chemical composition of the parent austenite.

A holistic collection of experimental evidence reveals the existence of various types of “martensitic” transformations that can occur simultaneously. Each of these transformations has its own mechanism with associated kinetics and structural characteristics. In this holistic picture, the chemical composition of austenite is only one aspect.

In the present contribution, experimental work at the Technical University of Denmark is coupled with key observations in the literature to showcase the limitations of the current mainstream theory and advance our current understanding of “martensitic” transformations in steels. We distinguish between suppressible “martensite” and insuppressible martensite, where insuppressible martensite forms instantaneously upon reaching a critical driving force for martensite formation, while suppressible “martensite” can only form if sufficient thermal energy is available. The suppressibility is linked to the structural and morphological characteristics of the developing martensite, hence to the internal structures in martensite and adjacent austenite.