Kinetics of the martensitic transformation in low-alloy steel studied by means of acoustic emission

Abstract

The kinetics of the martensitic transformation in three carbon steels (C60, C70 and C80) have been studied using the acoustic emission (AE) technique. It is demonstrated that the volume fraction of martensite f as a function of time t during cooling can be derived from the measured AE power $\text{℧}{}^2$, since $\text{℧}{}^2\text{∝}\text{d}\text{f}\text{/d}\text{t}$. An analysis of results shows that the fraction data as a function of temperature T can be described by the Koistinen and Marburger (KM) equation with high accuracy. This indicates that the nucleation of martensite takes place heterogeneously and that the average volume of martensite crystals is constant over the extent of the transformation. The change in kinetics with carbon content is attributed to the amount of dislocations created in the neighboring austenite, which influences the degree of autocatalysis. Also, the acoustic emission energy generated per unit volume of martensite has been found to scale with the change in dislocation density in the formed martensite as the carbon content is varied.