Solidification Patterns of Ductile and Grey Cast Irons Produced from the Same Base Iron Melt: Thermal (Cooling-Curve) Analysis

The cast iron foundry industry has undergone important changes in melting operations, with induction furnaces becoming a major source of molten iron. In many foundries, grey (GI) and ductile (DI) irons are produced simultaneously, including from the same base iron prepared in the same melting furnace. The response of a common base iron to the specific metallurgical treatments required for GI and DI production may differ and can be evaluated by thermal (cooling curve) analysis (TCCA). Experiments were conducted to evaluate, by TCCA, the effects of inoculation on near-eutectic DI (4.10–4.15% CE) and medium-strength GI (3.7–3.8% CE) produced from the same base iron melts, with a limited Mn content and less than 0.02% S. Based on the average values from four melts, inoculation decreased the austenite liquidus temperature by 7.2 °C in DI and 1.8 °C in GI, increased the eutectic undercooling temperature by 16 °C in DI and 2.2 °C in GI, and produced a substantially larger increase in the end-of-solidification temperature in DI. Eutectic recalescence increased by approximately 72% in both irons, while the first-derivative minimum at the end of solidification became more negative after inoculation. The results indicate that the common base iron is suitable for DI production under the specified compositional constraints for ferritic–pearlitic and pearlitic matrices. In contrast, the high degree of undercooling during solidification of the resulting GI increased susceptibility to free-carbide formation and undercooled graphite. Under the present conditions, inoculation had only a limited ability to reduce undercooling during the eutectic reaction and at the end of solidification and to promote sufficient graphitization in GI. Based on previous research on low-sulphur GI, possible improvements are recommended, including stronger inoculation, preconditioning with oxide-forming elements, a more effective inoculation technique, and/or improved graphite nucleation through an increase in the (%Mn) × (%S) factor.