Abstract
Structure formation of a ferrite-rimmed spheroidal graphite cast iron with a ferrite structure at the surface layer and a pearlite structure at the inner part has been investigated. The ferrite-rimmed structure hardly formed in small specimens. On the other hand, a large cylindrical specimen with a diameter of φ20 mm or more exhibited a distinct ferrite-rimmed structure. It was found from experiments using large cylindrical specimens with a free surface at the inner part that ferrite forms predominantly at the surfaces free from normal stress. Furthermore, the distribution of pearlite fraction on the cross section of the cylindrical specimen was shown to be related to the distribution of isotropic stress in untransformed austenite induced by ferrite formation from the surface. It is considered that elastic strain energy induced by the ferrite formation at the surface layer raises the eutectoid temperature and promotes pearlite formation at the inner part of the specimen.
