Factors and mechanism of structure formation of ferrite-rimmed spheroidal graphite cast iron

摘要

Structure formation of a ferrite-rimmed spheroidalgraphite cast iron with a ferrite structure at the surface layer and apearlite structure at the inner part has been investigated. Theferriterimmed structure hardly formed in small specimens. On theother hand, a large cylindrical specimen with a diameter of Φ20mm or more exhibited a distinct ferrite-rimmed structure. It wasfound from experiments using large cylindrical specimens with afree surface at the inner part that ferrite forms predominantly at thesurfaces free from normal stress. Furthermore, the distribution ofpearlite fraction on the cross section of the cylindrical specimenwas shown to be related to the distribution of isotropic stress inuntransformed austenite induced by ferrite formation from thesurface. It is considered that elastic strain energy induced by theferrite formation at the surface layer raises the eutectoidtemperature and promotes pearlite formation at the inner part of the specimen.