Effect of graphite morphology on shrinkage cavity formation of spheroidal graphite iron cast in metal mold.

Abstract

This paper describes the effect of graphite crystallization morphology on shrinkage cavity formation of hypereutectic spheroidal graphite iron cast in metal mold. Metal mold used can be considered as designed for riserless casting.In the melts containing 3.5–3.6%C, large spheroidal graphite nodules were observed with increasing silicon content beyond 3.0% (carbon equivalent beyond 4.5), and their counts increased with increasing silicon content (carbon equivalent). When the number of these large graphite nodules was more than 10 counts/mm2, shrinkage cavities were seen to be generated, whose area then increased with increasing large graphite nodule counts.In the melts with high carbon content of 3.8–3.9%, large graphite nodules of 25–30 counts/mm2 were observed in low silicon content range from 2.6 to 3.0%. In this range, however, the shrinkage cavity could not be found through detailed observation.In the melts with low carbon content about 3.3%, on the other hand, shrinkage cavities were produced in spite of no presence of large graphite nodules even in 3.7% Si content.Large graphite nodules were considered to be generated in spheroidizing treatment and to have been in existence before pouring. This caused the ratio of those graphite area which was crystallized in metal mold after pouring to decrease. It was followed by the decrease in expansion of cast iron, subsequently by the shrinkage cavity formation.It was quite clear, from the study on graphite morphology with image analysis, that sound cast irons were produced when graphite amounts corresponding to the carbon content more than 3.3% was crystallized during the solidification in metal mold.