Solidification dynamics, microstructure and toughness of a next-generation high-CE (4.9 %) ductile iron

Understanding and optimizing high-CE DCI is key to meeting future demands for large, high-performance cast components. The microstructures obtained in ductile cast iron (DCI) processing are closely linked to the resulting mechanical properties. Both the graphite distribution and morphology, as well as the structure prevalent of the Fe-rich matrix, impact toughness and ductility. Although some studies in the literature address these aspects, few advances have been made in modeling cooling rates in high-Carbon Equivalent (CE) DCI solidified in green sand-based molds. Furthermore, establishing correlations between microstructures, morphologies, cooling rates, and carbon transport in the melt during the solidification process is essential for optimizing the as-cast properties. This study explores various methods for measuring and calculating solidification cooling rates, defining two distinct cooling rates based on the Flemings’ model —one at the center (70 mm from the surface) and another at 20 mm from the surface of the DCI casting after testing this model against experiments. By using a thermocouple at 20 mm from the casting surface to support models and methods for determining the cooling rate, the cooling rates have been determined as well as correlate them with the graphite nodules morphometric parameters. It becomes evident that in a high-CE DCI (Equivalent carbon of 4.86), segregation occurs towards the central region, resulting in a manifest bimodal distribution containing both primary and eutectic graphite, with an increased quantity of finely refined eutectic graphite particles. The microstructure optimized for higher toughness and ductility (according to Inglis' stress concentration criterion) is that corresponding to samples taken 20 mm from the surface with smaller graphite particle density (205 nodules/mm 2 ), more uniform graphite size, and a smaller overall aspect ratio of 1.45 (i.e., less elongated ellipses), particularly for eutectic graphite nodules. In sum, both regions show similar tensile strengths (∼500 MPa), but the 20 mm sample exhibits higher elongation and toughness, reflecting its more uniform microstructure.