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Effect of bismuth on refinement of graphite in thin wall spheroidal graphite cast iron

Abstract Although it is known that the addition of bismuth refines the graphite nodule in spheroidal graphite cast iron, the refinement mechanism has not yet been clarified. In this research, the effect of bismuth on the refinement has been investigated by examining the microstructure of the spheroidal graphite cast iron containing a small amount of bismuth. Bismuth was added at 0˙01 mass% to the spheroidal graphite cast iron melt containing 3˙5–3˙7 mass% carbon and 2˙0– 2˙8 mass% silicon, then the melt was poured into the mould to obtain the stepped test bar with 2, 3, 5 and 10 mm thicknesses. The graphite nodule increased as the bismuth content increased. The diameter of the graphite nodule decreased as the thickness decreased, namely, as the cooling rate increased. The graphite nodule was further refined by the addition of bismuth. The increase in silicon content increased the graphite nodule count and the ferrite in the matrix. It postulated that bismuth exists as simple substance or a compound in the vicinity of the nucleus of the graphite.