Abstract
Chilled samples of base iron and magnesium-treated iron were surveyed using optical microscopy, SEM and computer assisted microanalysis. Numerous voids, 0.5–1 m m in size, were observed in magnesiumcontaining chilled samples. Graphite spheroids at an early stage of growth, 3–5 m m in size, and inclusions were also observed. Voids were not found in the base iron samples but inclusions were observed. Magnesium enrichment was detected at the voids, graphite and inclusions of the magnesium treated samples. It is proposed that the voids represent the traces of magnesium vapour bubbles formed in the melt. Some graphite spheroids consist of a thin graphite layer at their surface and a hollow core, a morphology that supports nucleation on the void walls followed by growth towards the centre. Although inclusions were occasionally observed in the centres of graphite spheres, they do not appear to be essential for spheroidisation to occur. It is concluded that gas bubbles formed by free magnesium contribute to the formation of spheroidal graphite by supplying nucleation and growth sites.
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