Empirical modelling of the isothermal transformation of pearlite in hypereutectoid steel

The rate controlling mechanism for pearlite growth in a hypereutectoid steel composition was examined through an analysis of the microstructural characteristics of the pearlite structure, namely interlamellar spacing, for different isothermal transformation and austenitising conditions. From a metallographic analysis of the pearlite structure as a function of the austenitisation and undercooling conditions applied to the hypereutectoid steel, the interlamellar spacing was observed to increase with increasing austenitising temperature and increasing isothermal transformation temperature. Through the application and experimental validation of a theoretical model (Zener and Hillert) for the calculation of the pearlite interlamellar spacing as a function of the undercooling, the growth rate of pearlite in the hypereutectoid steel was determined to be controlled by the volume diffusion of carbon in austenite during isothermal transformation in the temperature range of 550-620°C.