Examination of Nodular Graphite Formation and Austenite Solidification in Ductile Iron

Graphite growth at various stages in the ductile iron was captured by quenching experiments using spherical ceramic shell molds with inserted thermocouples. Statistical analyses on the graphite size relative to the quenching sequence were performed with a scanning electron microscopy equipped with Automated Feature Analysis software. Graphite growth was evaluated at different growth stages. Time and temperature dependent nucleation of graphite were observed. Examination of the etched microstructures revealed independent formation of spheroidal graphite and austenite phases at the early stage of solidification, which is followed by the austenite engulfment around the graphite particles. Shape of the graphite nodule varied with the degree of austenite encapsulation. The graphite feature size associated with completion of the encapsulating austenite shell was experimentally determined. A critical austenite growth velocity associated with the austenite engulfment at the graphite feature size was estimated by calculation. The experimental observations for the early solidification stages support a divorced eutectic model.