Problems and Improvements on the Production of Large Casting with Hi–Si Ductile Iron

We have studied the problems and improvement on producing large casting with solid solution strengthened ferritic spheroidal graphite cast irons (hereafter Hi–Si). As the matrix of Hi–Si material becomes hardened with silico-ferrite, hardness, tensile strength and yield strength increase. But elongation and impact value decrease conversely. If silicon content increases or if the cooling rate decreases due to heavy section, as silicon segregation progresses, the elongation and impact value are remarkably reduced. When increasing the graphite nodule count with optimum inoculation and setting chiller, the segregation of silicon decreases and then the mechanical properties are recovered. In addition, when Hi–Si reaches around 400 °C, its elongation decreases largely, which is called 400 °C embrittlement. This high-temperature embrittlement can be prevented with setting the ratio of Mg/P less than 1.5 and reducing the free Mg, because the grain boundary fracture occurs due to the free Mg not fixed as Mg3P2 at the grain boundary.