Abstract
In order to obtain a more detailed knowledge of the role of silicon in promoting graphite formation, the distribution behavior of silicon during the solidification of iron-carbon-silicon alloy with or without graphite-spheroidizing treatment was investigated. Carbon and silicon contents of alloys were varied from about 0% to 4% and 0.2% to 3.5%, respectively. The cooling rate employed was about 2°C/sec in the molten state. The distribution of silicon was detected by means of a potentiostatic etching technique. The following became clear after this study : (1) The distribution behavior of silicon is largely influenced by the carbon content of alloy, irrespective of the shape of the graphite phase deposited, indicating that the solidification process of spheroidal graphite cast iron is not much different from that of flake graphite iron. The segregation intensity of silicon, however, seems to be stronger in spheroidal graphite iron than in flake graphite cast iron. (2) The normal segregation of silicon is observed around the primary austenite, but is inverted at eutectic. So the primary austenite can be distinguished from the eutectic one by silicon content. Most of the graphite are found in the region rich in silicon and it is thought that the build-up of silicon as well as carbon in front of the growing austenite dendrite plays an important role in the formation of graphite in hypo-eutectic cast iron. (3) Also in hyper-eutectic iron, dendritic austenites can be observed, but they are enriched with silicon in contrast to the primary austenite occurring in hypo-eutectic iron and can be regarded as divorced eutectic austenite. (4) Thus, it is thought that the solidification process of both flake and spheroidal graphite cast iron can be explained according to the iron-carbon-silicon phase diagram.
