Influence of iron carbide morphology on nucleation and grain grovvth of austenite

A medium plain-carbon steel has been heat treated to give three different starting structures: (a) normalized, (b) normalized and spheroidized, and (c) quenched and tempered. The grain-growth characteristics on heating through the A₁–A₃ range and then to 900°C have been studied using the photoemission electron microscope. It was found that a much larger grain size was obtained with the spheroidized than with the other two conditions of prior heat treatment. The reasons for these differences are discussed in relation to the heterogeneity developed in the austenite grain size (Z value) as the A₃ temperature is reached and exceeded. In the use of the spheroidized steel it has been observed that abnormal grain growth occurs which is a result of developing a large Z value at the A₃ temperature.