Physico-chemical causes of the formation of chunky graphite in austenitic ductile iron

Chunky graphite (CHG) is a well-known phenomenon which occurs in heavy section spheroidal graphite iron (SGI) castings or SGI castings with higher alloying content. Chunky graphite results in a decrease of mechanical properties, in particular a decrease in elongation. Despite numerous studies, the root cause of the CHG formation remains unidentified. In publications analyzing the different stages of CHG formation based on quenched samples, it is assumed that carbon diffusion through the austenite halo is inhibited due to the size of the eutectic grain. Furthermore, due to the carbon supersaturation CHG nucleates near the liquid-austenite interface of the eutectic grain. Additionally, other studies show that partly incomplete graphite spheres can be interconnected with CHG where no nucleation is required. Using thermodynamic-kinetic material simulation two morphologies, the decoupled SGI growth and the coupled CHG growth, were modeled to simulate their impact on the solidification kinetics. In the present study, it was shown for the first time that with increasing Ni content conditions arise in the austenite halos which strongly reduce the diffusive flux of carbon towards the graphite spheroid and thereby decelerate the kinetics of SGI growth. With a content above 5 wt% Ni, the competing kinetics of CHG growth exceeds the SGI growth. Therefore, the preference in kinetics and the present thermodynamic constraints are critical to the morphology change from SGI to CHG. - Graphical abstract: Comparison of the transformation kinetics between the SGI and the CHG growth model.