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Abstract. Hypereutectic ductile iron was cast in green sand moulds with four plates with thickness of 1.5, 2, 3 and 4 mm in each mould. Temperatures were measured in the 3 and 4 mm plate. The temperature curves showed that eutectic solidification was divided into two stages: primary and secondary eutectic solidification. The first stage, which was relatively short, had none or very little recalescence. Further under cooling, followed by reheating during recalescence, was necessary to initiate the second part of the eutectic solidification. Both the secondary under cooling and recalescence was larger in the 3 mm plates than in the 4 mm plates. All 1.5 mm plates contained carbides but the other plates solidified without carbides. Metallographic examination showed two populations of graphite nodules. A small group of nodules was larger than rest of the nodules. Colour etching revealing the segregation of Si showed a higher Si content in the ferrite around the larger nodules compared to the ferrite around the rest of the nodules. This indicates that solidification took place along the following path: The solidification starts with nucleation and growth of primary graphite nodules. This probably starts during the filling of the mould. The primary nodules act as nuclei for austenite. As austenite easily nucleates on graphite the temperature will be the same for the 3 and 4 mm plate for the first part of the eutectic solidification. This first part of the solidification ends when concentration of carbon around austenite dendrites is too large and new nodules have to nucleate and grow. The larger under cooling for the 3 mm plates compared to the 4 mm indicates that the nucleation of new nodules is governed by kinetics even in very well inoculated melts.

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 (T min ) was 15 to 20uC 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 DT rec was however also larger for the eutectic casting and in the thin plates the maximum temperature after recalescence (T max ) 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 T max . 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 T max than in the hypereutectic castings.