Effect of Nonmetallic Inclusions on Solidification of Inoculated Spheroidal Graphite Iron

Inoculation treatment of spheroidal graphite cast iron (SGI) controls graphite nodule heterogeneous nucleation and is used for elimination of solidification microporosity and improvement in casting performance. In this study, thermodynamic simulations were performed to predict precipitates formed in the inoculated melt above a liquidus temperature (primary precipitates) and during solidification (secondary precipitates). The experimental inoculation treatments were designed targeting formation of primary precipitates (Ti and Zr additions) and secondary precipitates (S and N additions to inoculant). An automated SEM/EDX analysis was applied to analyze the graphite nodule distribution statistics and a family of nonmetallic inclusions in the experimental castings. In inoculated SGI, the observed bimodal distributions of graphite nodules were related to continuous nucleation with the second nucleation wave that occurred toward the solidification end. The measured microporosity in the castings was linked to graphite nucleation. The origin of the continuous graphite nodule nucleation and the possibility of engineering nonmetallic inclusions to control casting soundness are discussed.