Graphite morphology control in cast iron

Graphite morphology in cast iron is analysed in terms of nucleation and growth kinetics of graphite crystals in liquid iron. The high interfacial energy between the graphite/melt interface does not allow the homogeneous nucleation of graphite in the melt. The efficacy of the heterogeneous substrates influences the growth undercooling.At small driving forces, i.e., low supersaturation or small kinetic undercooling, graphite growth is characterised by faceted growth resulting in flake, compacted and spherulitic graphite morphologies. However, at large driving forces, there is a transition from faceted to non-faceted growth, resulting in a dendritic growth morphology.Flake morphology is rationalised in terms of impurity dependent crystal growth mechanisms, whereas a spherulitic morphology is attributed to a defect controlled spiral growth mechanism. Compacted graphite morphology is considered a transition between flake and spherulitic morphology.A thermodynamic approach is used to inter-relate the residual concentrations of impurities of technological interest, i.e., S and O as a function of the residual concentration of the reactive elements, Mg, Ca, and Ce in a typical cast iron melt at 1500C and atmospheric pressure. Such a diagram that quantitatively relates graphite morphology in thick cast iron sections to soluble concentrations of impurities is referred to as a graphite morphology control diagram.In thin sections with characteristic large undercoolings, the defect controlled spiral growth mechanism dominates over impurity dependent growth mechanisms, leading to deviations from predictions based on graphite morphology control diagram. Estimates of the growth kinetic curves for cementite, austenite and graphite (flake and spherulitic) are presented. The nucleation and growth of cementite can be prevented in thin sections through the provision of an adequate number of heterogeneous nuclei for graphite, and a selective driving force for graphite growth through the addition of graphite stabilising elements. There is an optimum dispersion of graphite nuclei for a given section size to promote the required degree of interconnected growth of graphite in compacted morphology.