It has been known that a thin surface layer (casting skin) is formed on the surface structure of grey cast iron castings. This surface layer is said to consist of a reaction zone where the metal has reacted chemically with the mold materials and the gasses in the mold and a transition zone which formed because of the cooling condition of the mold. There is no precipitated graphite in this layer so that is offers greater resistance to machining. The formation of the layer is influenced by various factors such as the composition of cast iron, casting temperature, cooling rate, molding materials, mold dressing, innoculation and so on. Though it is difficult to describe sufficiently the effect of these factors on the formation of the layer, the main purpose of this study is to examine how it is formed.
  In this investigation, five kinds of raw iron materials were used; ductile iron (A) and (B), charcoal iron, low phosphorus iron and electrolytic iron. Under constant molding and casting conditions, the iron of a standard composition (3.5%C-2.0%Si) was cast into a unified green sand mold. The effect of heredity of the raw iron materials, solidification characteristics, innoculation and alloying elements (Mn, P, S) on the formation of the layer were examined. The results obtained were that the formation of the surface layer of the grey iron castings depended not only on the decarburization of the metal-mold chemical reaction which was already reported, but also on the solidification characteristics of the molten metals.