The austenite transformation in ferritic ductile cast iron

Abstract

Abstract The isothermal austenitization transformation in an unalloyed ferritic ductile cast iron was studied over the temperature range from 880 to 953°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 the 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.