Effects of High-Concentration Cu and Sn on the Nucleation and Growth Behavior of Graphite on Rare-Earth Compounds During the Solidification of Cast Iron

Ferrous scraps have been widely utilized as raw materials in the production of ductile irons. With increasing scrap use and repeated recycling, the concentrations of tramp elements such as copper (Cu) and tin (Sn) in the raw materials are expected to increase as they cannot be easily removed through conventional refining processes. To obtain fundamental insights into the effects of high-concentration Cu (up to 2.0 wt pct) and Sn (up to 1.0 wt pct) on the nucleation and growth of graphite on a foreign substrate, a novel approach through contact-melting of cast iron on lanthanum oxide (La₂O₃) substrate is utilized to study graphite nucleation and growth behavior at the alloy/substrate interface and in the bulk alloy. Enhanced graphitization at the alloy/substrate interface and in the bulk alloy are observed at varying Cu content. Copper promotes graphitization in cast iron by elevating the equilibrium liquidus temperature of graphite (T_G) and increasing the carbon supersaturation (ΔC). On the other hand, Sn demonstrates stronger graphitizing effects than Cu as only a lower concentration of Sn is required to achieve a similar increase of T_G and ΔC. More significantly, Sn enhances graphite nodule counts in the bulk alloy through the formation of fine La-Sn intermetallic compounds which act as effective sites for graphite nucleation.