Experimental investigation of chunky graphite formation in small-section ductile iron castings

The formation of chunky graphite has generally been related to the thermal centre of heavy – section iron castings. Previous works carried out on large blocks with cooling rate in the liquid state of about 6. 3 – 6. 5 K/min allowed highlighting some of the factors related to the extent of chunky graphite. In the present study, attempts to obtain chunky graphite in small-section castings were made by enhancing the factors found to promote its appearance. Degenerate graphite was observed in standard keel-block castings with cooling rates above 100 K/min in the liquid period. Well-formed graphite spheroids were always observed in the bottom parts of the keel-blocks where chunky graphite was not detected. On the other hand, all the upper parts of castings were affected by chunky graphite. The effect of post inoculation, of Si content in the melt, of Ce or Mg-Ce treatment and oxygen activity will be discussed. The total oxygen content has been compared with some other values obtained from ductile, compact and grey iron samples.