Investigation on the Effect of Mg on the 3D Graphite Morphology in Cast Iron Via X-ray Tomography

Despite the extensive development of cast iron, the mechanism behind the transition of 3D graphite morphology influenced by Mg has not yet been fully explained. In this study, X-ray tomography (XRT) was employed to systematically investigate the 3D morphology and size distribution of graphite in cast irons with varying Mg contents. The nucleation and growth behavior of graphite with different morphology were discussed. The results showed that Mg reacts with anti-spheroidal elements such as O and S to form Mg-O-S compounds, which act as a scavenger to a certain extent. The addition of Mg increases the undercooling of the melt, and promotes heterogeneous nucleation of graphite. The 3D analysis demonstrated that Mg causes the transition of graphite from lamellar (LG) to vermicular (VG), and further to spheroidal (SG). This transition is accompanied by an increase in both the sphericity and number of graphite particles, as well as an enhancement in the nodularity of the cast iron. LG precipitates at the austenite/liquid interface with high carbon concentration, while a small fraction nucleates heterogeneously on Mn-S compounds. The LG grows between austenite dendrites and interconnects to form large clusters. VG evolves from tadpole-like graphite, which originates from the distortion of SG. Lower Mg content favors the formation of complex coral-like morphology in cast iron. SG primarily nucleates heterogeneously on complex Mg-O-S compounds and grows via divorced growth. Once encapsulated by austenite, further growth of SG depends on the diffusion of carbon atoms.