Structure Characteristics of High-Si Ductile Cast Irons

The aim of the present study is to evaluate the actual status of silicon-alloyed ductile cast irons, as recent knowledge on the structure formation, and to review original data obtained by the authors from recent separate publications with additional unpublished data, specifically concerning the structure peculiarities. New data were added to deeply correlate the most important graphite parameters, as effects of inoculation and inoculating elements, casting section size and mold thermal properties. More than 4% Si-alloyed ductile irons are characterized by the presence of slightly irregular spheroidal graphite (Form V, ISO 945, 0.6–0.8 roundness shape factor), with the beneficial effect of inoculation. Ca, Ba-FeSi appears to be better than simple Ca-FeSi, while Ca, RE-FeSi led to higher graphite real perimeter and at lower shape factors. Inoculation also decreased the skin effect in high-Si ductile irons, including at the contribution of sulfur and oxygen from the mold coating. In the metal mold solidification of wedge casting (up to 20 mm thickness), 30% higher nodule count (> 75% at max. 15 µm size) is obtained and less depending on the casting section size, at higher values of the graphite shape factors, comparing to sand mold. Generally, higher is the graphite particles size class, lower is their compactness degree and higher is the effect of the casting wall thickness. Metal mold led to 8–10% higher graphite nodularity, for the entire wall thickness range. Higher is the minimum imposed graphite shape factor, lower is the graphite nodularity, for both metal and sand mold. As in high-Si ductile cast iron, the real graphite perimeter is strongly negatively affected; in the nodularity calculus, it is recommended to use the shape factor involving graphite real perimeter instead of its maximum size (such as stipulated in ISO 945-4-2019). Despite that higher silicon content generally suppresses the carbides formation, inoculation is still necessary to improve the quality of the graphite phase (to increase the Form VI rate) and to decrease the casting skin formation.