Influence of Bismuth on Graphite Nodule Count in Thin-section Spheroidal-graphite Cast Iron

Abstract

Cast irons containing 3.8% C and 2.2% Si were melted in a graphite crucible using a high-frequency induction furnace. Nodularisation treatment was carried out at 1530°C by the addition of 1.6% Fe-Si-Mg alloy and various amounts of bismuth as metallic bismuth, bismuth oxide and bismuth sulphide. When the magnesium reaction subsided, the melts were post-inoculated with Fe-Si and then poured into CO₂ Process sand moulds. The occurrence of chilled structure and graphite-nodule counts were determined using an optical microscope and an image analyser. X-ray microanalysis and emission spectrochemical analysis were carried out to detect Bi in the graphite nodules and also in nodules extracted with dilute hydrochloric acid. The nodule count was enhanced by the addition of extremely small amounts of metallic bismuth, bismuth oxide and/or bismuth sulphide. Some heterogeneous phases containing Bi, Mg, Ti, Fe and Si, with diameters of 1 μm or less were observed at the centres of graphite spheroids using back-scattered electron imaging and an energy dispersion X-ray microanalyser. Graphite crystallises on these phases during eutectic solidification. It is suggested that large numbers of fine liquid bismuth particles contribute to the formation of graphite spheroids by acting on the substrate.