The Influence of Increased Cr Content and Austenite Grain Size on the Kinetics of Phase Transformations when Cooling Hypo-Eutectoid Steel

Abstract

The paper presents the influence of the grain size and a little higher Cr content on the kinetics of austenite phase transformations during continuous cooling of hypo-eutectoid steel. The kinetics of austenite phase transformations during continuous cooling were determined by means of analysis of the dilatometric curves and structure investigations. The influence of the austenite grain size and the higher Cr content was analysed in two hypoeutectoid steels containing about 0.4% C. One of them had, Cr content higher, by about 1%. In both steels, the austenite grain size was changing insignificantly up to the austenitising temperature of about 950fl. Above that temperature, the austenite grain size in carbon steel grew much quicker than that in the steel with Cr addition. The austenite grain in the Cr enriched steel was smaller than that in carbon steel and, in spite of that, the duration of cooled austenite transformations were several times longer. This means that the phase transformations are much more strongly influenced by the addition of chromium slowing down carbon diffusion in austenite, than by the austenite grain size. For each phase transformation in the examined steels, the activation energy of the transformation has been determined. The activation energy of all the phase transformations varied slightly with the increase of austenitising temperature. On the basis of the obtained results, curves of true isothermal transformations have been developed for the beginning of the phase transformations in both steels, related to infinitely quick cooling down to the transformation temperature.