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Abstract
For several years there has been a debate on the subject of whether the graphite spheroids formed in SG iron are nucleated heterogeneously on foreign particles suspended in the iron melt. The authors have studied the specific case of spheroid nucleation in magnesium-treated chill-cast iron. Conclusive evidence of heterogeneous nucleation was obtained and information was acquired concerning (a) the shape, structure, and composition of the nuclei and (b) the role of inoculants in controlling the effectiveness of the nuclei. The authors' success may be directly attributed to the high-resolution analytical facilities provided by the combined electron microscope–microanalyser, EMMA–3. Salient features of the nuclei were (a) a duplex structure, with a sulphide core surrounded by an epitaxially orientated oxide shell, and (b) a platelet shape which presented two crystalline faces to the carbon-supersaturated melt upon which graphite could grow epitaxially. Evidence was obtained that surface-active sulphur could ‘poison’ the effectiveness of the nuclei.
