A great deal of research has been reported concerning the solidification of flake graphite cast irons. However, in spite of all this attention a number of questions have not been answered satisfactorily - among these is the role of proeutectic (or kish) graphite in the solidification of the eutectic. Part of this is due to the fact that research has been directed to the solidification of alloys of greater engineering significance,l-4 eutectic and hypoeutectic alloys. There is general agreement concerning the solidification of eutectic gray cast iron. Solidification proceeds by the co precipitation of austenite and graphite as, more or less, spherical cells. During solidification both the austenite and the graphite are in contact with the liqud. The graphite flake grows with the growing front projecting into the liquid. As growth progresses the flakes curl and branch and it is generally accepted that within a eutectic cell the flakes are interconnected.5•6 This branching of the flakes is attributed to dendritic branching and splitting of the flakes/·8 but most of these studies have been based on noncommercial Ni-C alloys. In hypoeutectic alloys solidification of the eutectic is preceded by dendritic austenite. As austenite forms, the composition of the remaining liquid shifts toward the eutectic where this liquid solidifies in a manner similar to that of eutectic gray cast iron. The solidification of hypereutectic gray cast iron begins with the crystallization of primary graphite (often referred to as kish). This primary graphite develops as straight graphite plates, with some branching, etc., growing while totally surrounded by liquid. The composition of the remaining liquid �hifts toward the eutectic where the liquid is believed to solidify in a manner similar to that of the eutectic cell, although the primary graphite may influence the size of the eutectic cell and the distribution of eutectic graphite.9 The literature2, 10 has summarized the solidification process through the use of sketches of the solidifying microstructure. The solidification of a hypereutectic gray cast iron is illustrated in Fig. 1.10 This sketch depicts the nucleation and growth of the eutectic cell occurring with no influence of the proeutectic kish graphite. It is this aspect of the solidification process which is to be discussed.
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