Application of computational thermodynamics and kinetics to control of high Mn ductile iron austempering

Abstract

To provide sufficient hardenability for austempering of ductile iron, alloying elements like Mn, Ni, Cu or Mo are frequently added. But the segregation of these alloying elements, occurring during solidification, changes the kinetics of austenitisation and austempering reaction. In this work, using the calculated alloying elements segregation profile, the related thermodynamics equations were used to calculate the Ae3 and A1 phase boundaries in the different regions of matrix. The results show the variation of austenitisation kinetics as a function of alloying elements segregation. Needles to say that as a consequence of alloying elements segregation and variation of austenitisation kinetics, kinetics of austempering should be a function of alloying elements local concentration. To verify the austempering kinetics, three high carbon Si-Mn steel compositions corresponding to three different parts of ductile iron matrix (i.e. near graphite nodules, intercellular regions and the area between these two regions) were prepared. The iron and steel samples were all subjected to different austempering cycles in dilatometry equipment. In addition, to predict time-temperature-transformation diagrams the classical nucleation theory and related equations were used to calculate the time of bainitic start transformation.