A process model for the microstructure evolution in ductile cast iron: Part I, The model

In the present investigation the multiple phase changes occurring during solidification and subsequent cooling of near-eutectic ductile cast iron have been modelled using the internal state variable approach. According to this formalism, the microstructure evolution is captured mathematically in terms of differential variation of the primary state variables with time for each of the relevant mechanisms. Separate response equations have then been developed to convert the current values of the state variables into equivalent volume fractions of constituent phases utilising the constraints provided by the phase diagram. The results may conveniently be represented in the form of C-curves and process diagrams to illuminate how changes in alloy composition, graphite nucleation potential and thermal programme affect the microstructure evolution at various stages of the process. The model can readily be implemented in a dedicated numerical code for the thermal field in real castings and used as a guiding tool in design of new treatment alloys for ductile cast irons. An illustration of this is given in an accompanying paper (Part II).