Using metal mold dressed with acetylene black, the change in chilling tendency of spheroidal graphite cast iron due to melting condition and addition of some elements was investigated and some factors affecting the spheroidization of graphites were also studied. It was found that melting conditions such as holding over a long period of time or at a high temperature before the spheroidizing treatment cause spheroidal graphite cast iron as well as flake graphite cast iron to chill. In order to clarify the effect of melting conditions on the chill, cast irons were melted in a high frequency vacuum furnace. Iron melts were thereafter held under either N2 (0.8 atm) or O2 (0.2 atm). It was found that in holding melts under vacuum, chill was markedly reduced, however in holding under atmosphere of N2 or O2, chill increased again remarkably. These results indicate that the effect of N and O on the chilling of cast irons are very important. Chilling tendency of the melt from which N and O were removed by addition of Al and Ca before spheroidizing treatment, was then examined. As a result, spheroidal graphite cast iron structures free from ledeburite cementite were obtained in samples of 20 mm diameter bar containing 3.4∼3.8%C, 3.5∼4.3%Si and 0.015∼0.02%Mg.
