Graphite morphology in cast iron is analyzed in terms of the growth kinetics of graphite crystals in liquid iron. At small driving forces, i.e., low supersaturation or small kinetic undercooling, graphite growth is characterized by faceted growth, resulting in flake, compacted and spherulitic graphite morphologies. However, at large driving forces, there is a transition from facted to non-faceted growth, resulting in a dendritic growth morphology.
Flake morphology is rationalized 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 as 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 0, as a function of the residual concentration of the reactive elements, Mg, Ca, and Ce in a typical cast iron melt at 1500'C 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 section castings that freeze at faster cooling rates and large kinetic undercoolings, the basal spiral growth mechanism dominates over the impurity controlled prism growth mechanism, leading to deviations from predictions based simply on the graphite morphology control diagram. In the ase of compacted graphite, where growth on both the prism and basal faces is involved, the degree of nodularity increases with the cooling rate, giving rise to section sensitivity.
At large undercoolings, the prevention of the nucleation and growth of cementite is an essential feature of graphite morphology control. It is estimated that the mobility of the cementite interface exceeds that of the prism interface in flake graphite growth by an order of magnitude and that of the basal interface in spherulitic graphite growth by three orders of magnitude. In practice, the driving force for graphite growth is raised selectively through the addition of graphite stabilizing elements, such as silicon, which raise the temperature of the graphite eutectic and depress the temperature of the carbide eutectic. Kinetic growth undercooling can be decreased by increasing the number of heterogeneous nuclei for graphite growth through inoculation. The application of the above concepts for the control of graphite morphology in shaped automotive castings is discussed.