Mechanism of the silicon influence on chilling tendency index and chill of ductile iron in thin wall castings

Abstract

In the present work an analytical expression that combines the susceptibility of liquid cast iron to solidify according to the Fe-C-X metastable system (also known as the chilling tendency of cast iron), is proposed. A relationship between the chilling tendency index of cast iron and several factors has been presented. The results can be also used as a guide for a better understanding of the effect of technological variables such as the melt chemistry, the holding time and temperature, the spheroidizing and inoculation practice, the resulting nodule count and the type of mold material and pouring temperature, on the resultant chill of the ductile iron. Theory was experimentally verified using silicon as an example. In particular, it has been shown that as a result of increasing silicon content the critical nodule count increases, and the temperature range between the graphite eutectic equilibrium temperature and formation temperature for cementite eutectic. Such variations lead to decreasing the chilling tendency index and in consequence reducing chills in cast iron. The chilling tendency index has been related to the critical wall thickness, below which the chill is formed. Theoretical calculations of the critical wall thickness were made and then compared with experimental outcome for ductile iron melts.