Effect of Heating Rate and Silicon Content on Kinetics of Austenite Formation during Continuous Heating

The first step in a heat treating cycle is the austenitizing of the as-received material. Despite its importance, this step has received relatively little attention. In this work, the kinetics of austenite formation during continuous heating tests of steel samples with low and high silicon content was determined as a function of heating rate. The microstructural evolution was characterized through dilatometric analysis of cylindrical samples (7 mm × 20 mm), continuously heated in a protective atmosphere at constant heating rates ranging from 2 to 40 °C/min. The critical temperatures and the transformation kinetics were determined from the derivative of the relative length change as a function of temperature. As the heating rate increases the critical temperatures and the transformation temperature range increase; the addition of silicon produces a more marked effect. The transformation kinetics data were correlated using an Avrami-type equation. The kinetic parameter n is nearly independent of heating rate while the parameter k is a strong function of the heating rate; in both cases, slightly larger values were obtained for the high-silicon steel.