THE AUSTENITE TRANSFORMATION IN FERRITIC DUCTILE CAST-IRON

The isothermal austenitization transformation in an unalloyed ferritic ductile cast iron was studied over the temperature range from 880 to 953-degrees-C. During austenitization, austenite nucleates preferentially at cell boundaries and graphite-ferrite interfaces and grows by thickening or lengthening the existing nuclei with the rate at the cell boundaries higher than that at the graphite-ferrite interfaces. The growth of austenite is accomplished by an up-hill boundary diffusion of carbon to cell boundaries and a volume diffusion of carbon from graphite to ferrite; the former mechanism was found to be dominant. Austenitization kinetics can be described by an Avrami equation with exponent n ranging from 1.31 to 1.51. The activation energy of austenitization is 267.2 kJ mol-1 and is about half of that required by an interface-controlled process. The combined evidence of microstructural observations, transformation kinetics and activation energy suggest that austenitization is a carbon diffusion-controlled process.