Revisiting Thermal Analysis of Hypereutectic Spheroidal Graphite Cast Irons

This study deals with early solidification of hypereutectic cast irons at varying carbon content and roughly constant alloying additions. Thermal analysis of such alloys shows that the start of the eutectic reaction occurs at a nearly constant temperature for mildly hypereutectic compositions. A similar trend is observed with more hypereutectic compositions but at a higher starting temperature. This jump in the start temperature of the eutectic reaction has not been previously evidenced and is here addressed by considering primary precipitation of graphite. Limiting the analysis to spheroidal graphite cast irons, it is demonstrated that simulation of primary graphite precipitation based on a 2D nucleation/lateral growth model allows substantiating the experimental distinction found between mildly and highly hypereutectic cast irons. This modeling explains that highly hypereutectic alloys start eutectic solidification in a limited temperature range that is nearly insensitive to the initial carbon equivalent of the alloy and to inoculation. This approach also suggests that the start of the eutectic solidification of mildly hypereutectic cast irons is shifted to lower temperature until growth of austenite enriches the liquid in carbon to such an extent that growth of graphite becomes possible.