Undercooling and nodule count in thin walled ductile iron castings

Casting experiments have been performed with eutectic and hypereutectic castings with plate thick¬nesses from 2 to 8 mm involving both temperature measurements during solidification and micro¬structural examination afterwards. The nodule count was the same for the eutectic and hypereutectic castings in the thin plates ( 4.3 mm) while in the 8 mm plate the nodule count was higher in the hypereutectic than in the eutectic castings. The minimum temperature prior to the eutectic recalescence (Tmin) was 15 to 20C lower for the eutectic than the hypereutectic castings. This is due to nucleation of graphite nodules which begins at a lover temperature in the eutectic than in the hypereutectic castings The recalescence (Trec) was however also larger for the eutectic casting and in the thin plates the maximum temperature after recalescence (Tmax) was the same in the eutectic and hypereutectic plates. This is due to that higher undercooling gives higher driving force for the solidification process and the equal nodule counts therefore give the same Tmax. The higher undercooling before recalescence increases however the risk for formation of carbides during the solidification. In the 8 mm plates the lower nodule count in the eutectic plates gave also a lower Tmax than in the hypereutectic castings.