It is known that the decomposition of austenite in spheroidal graphite cast iron (SGI) depends on numerous factors. Among them, the cooling rate and microsegregation of alloying elements (Si, Cu, Mn) are derived from the solidification step. The eutectoid transformation of the SGI has been investigated with a new dilatometer that was made for the investigation of cast iron alloys. The temperature dependence of the dilatations, similar to a function of time is shown by the dilatation curves. The investigation of the eutectoid transformation has been examined by the evaluation of dilatation curves, metallographic examinations and chemical compositions. It is possible to exactly follow the eutectoid transformation of austenite, which proceeds according to the stable reaction in ferrite and graphite (γ → α + graphite), the metastable reaction in perlite (γ → α + Fe 3 C), or dilatometric and differential dilatometric curves, respectively. The ratio between the perlite and ferrite phase in the matrix of the spheroidal graphite cast iron depends upon the extent of each reaction. The temperature difference between the dilatation extremes during the eutectoid reactions, as a function of the ferrite fraction, has been determined. A correlation between the amount of ferrite and the characteristic expansion owing to the graphite precipitation was found.

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