In order to clarify the mechanism of the formation of spheroidal and compacted/vermicular graphite in cast iron, two kinds of experiment were performed using molten iron treated with rare earth metal (REM) which has a less fading effect than Mg. At first, the interfacial energies between REM-treated molten iron and specific planes of graphite crystal were measured by the sessile drop technique. Next, Ni–C alloy sticks with spheroidal or flaky graphite were immersed into the REM-treated molten iron, and then the morphologies of graphite grown from the graphite of the stick into the molten iron were observed.
From these experiments, the following conclusion is drawn. The interfacial energy between the molten iron and the basal plane of graphite was almost independent of the residual Ce content, while the energy between the molten iron and the prism plane of graphite increased with increasing Ce content in the same way as in the case of Mg-treatment.
When the Ce content is low, the interfacial free energy ratio of single crystal is lower than that of polycrystal, and consequently the single crystal (flaky graphite) becomes more stable. When the Ce content is high, the ratio of polycrystal is lower than that of single crystal, and the polycrystal (spheroidal grahite) becomes more stable. When the Ce content is moderate, the ratios of both crystals are almost equal, and CV graphite is formed.
Therefore, the morphology of graphite grown into the molten iron is dependent on the interfacial energy between the molten iron and the specific plane of graphite, whether the graphite substrate is spheroidal or flaky.