Undercooling and nodule count in thin walled ductile iron castings

Casting experiments have been performed with eutectic and hypereutectic castings with plate thicknesses from 2 to 8 mm involving both temperature measurements during solidification and microstructural 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 before the eutectic recalescence (Tmin) was 15 to 20ºC lower for the eutectic than for the hypereutectic castings. This is due to nucleation of graphite nodules which begins at a lower 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 because higher undercooling gives a larger driving force for the solidification process and the equal nodule counts therefore give the same Tmax. However, the higher undercooling before recalescence increases the risk for formation of carbides during the solidification. In the 8 mm plates, the lower nodule count in the eutectic plates also gave a lower Tmax than in the hypereutectic castings.