Surface morphology, internal structure and nucleation mechanism of graphite inclusions in ductile cast iron.

The surface morphology and internal structure of graphite inclusions in ductile cast iron are investigated. Based on this, the theory of graphite nucleation on solid inclusions in the melt is confirmed. It is shown that the morphology of the outer surface of spherical graphite inclusions can vary from almost smooth to consisting of successively overlapping graphite flakes. It is suggested that, depending on the thermophysical conditions of crystallization, the growth of inclusions along one of the crystallographic planes prevails: [1010] for slow cooling (scaly surface morphology of the “cabbage head” type) and [0001] for faster cooling (smooth surface). Metallographic studies of the internal structure of the spherical graphite inclusion has revealed its segmental‑layered structure with characteristic concentric wavy lines. Inside the inclusion, characteristic zones are distinguished: the center, sectors with characteristic junctions diverging from the center, and sometimes inclusions of siliceous ferrite are found. The polycrystalline structure of the inclusion with characteristic boundaries between graphite pyramids has been confirmed. The EPMA method revealed anomalies in the distribution of carbon, magnesium, silicon, sulfur, and oxygen concentrations in the graphite inclusion. In the center, content of the magnesium, sulfur, and oxygen is significantly increased. This confirms the theory of graphite nucleation on sulfides and oxides. In some cases, an increase in the oxygen content was observed on the outer part of the inclusion, which may be due to the displacement of oxides to the periphery during the growth of the graphite inclusion in the melt. The obtained results provide clarifi ations and supplementations to the theory of heterogeneous nucleation of spheroidal graphite particles in ductile cast iron, where the central part of the spheroid is a conglomeration of complex sulfides and oxides.