Abstract
The evaluation of factors influencing the formation of spheroidal graphite in untreated cast irons was examined, which were synthetized from electrolytic iron, electrode graphite powder and metallic silicon and were desulfurized to 10 ppm of sulfur in magnesia or lime crucible under lime-fluospar flux. (1) Spheroidal graphite can be produced without a deliberate addition of any spheroidizing elements by controlling the carbon equivalent, impurities and cooling rate. In the range of composition corresponding to the maximum carbon solubility limit of austenite, the whole of the graphite can separate in spheroidal form. However, in iron with higher carbon content, the attempt to produce fully spheroidal graphite structure by desulfurization alone was not successful. (2) Rapid cooling has a favorite effect on spheroidal graphite formation especially in high carbon iron. Unidirectional freezing can bring the formation of a periodic structure consisting of alternate layers of spheroidal and flaky graphite in eutectic and hypereutectic iron of extremely low sulfur content. (3) Vacuum melting of the desulfurized iron shifted the range of composition producing fully spheroidal graphite structure to a lower level of carbon content and moreover showed no clear tendency to promote the formation of spheroidal graphite in high carbon iron.
