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 s olidification and subsequent cooling of near-eutectic ductile cast iron hav e been modeled using the internal state variable approach. According to thi s formalism, the microstructure evolution is captured mathematically in ter ms of differential variation of the primary state variables with time for e ach of the relevant mechanisms. Separate response equations have then been developed to convert the current values of the state variables into equival ent volume fractions of constituent phases utilizing the constraints provid ed 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 c omposition, graphite nucleation potential, and thermal program affect the m icrostructure evolution at various stages of the process. The model can rea dily 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 article (Part II).