Effect of prior austenite grain size on its diffusionless decomposition into martensite

Abstract

Diffusionless transformation involves the movement of atoms in a coordinated manner. Such movement gets interrupted due to the presence of strong defects such as precipitates, grain boundaries, etc., leading to stabilization of austenite. Austenite stabilization, a phenomenon that takes place in many steels typically means the increase in the resistance of the decomposition from austenite to martensite. Stabilization occurs mainly due to the influence of three major factors, i.e., mechanical, chemical, and thermal. One of the most elementary method to analyze the effect of a particular factor on the austenite stability is by examining its impact on the MS temperature (Martensitic start temperature). The present work outlines the effect of PAGS on the decomposition behavior of austenite to martensite in carbide-free bainitic steels. It is basically observed that refinement in grain size leads to decrement in the MS (Martensite start temperature) temperature and higher decomposition rates during the initial stages. Later on, it is observed that on formation of about (0.3-0.4) fraction of martensite, the rate of decomposition decreases at a much rapid rate for fine PAGS whereas it continues to maintain faster rate in case of microstructures with coarser grains. Dependence of decomposition kinetics of austenite on PAGS is ignored frequently, but it does exist, and adequate experimental evidence is presented in the present study for the same. Dilatation data were analyzed in order to determine the fraction of martensite formed as a function of supercooling along with decomposition kinetics of austenite. The kinetics of austenite decomposition with respect to PAGS primarily depends on the strength of the austenite phase and the density of nucleation sites.