EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Ductile cast iron is of increasing importance for transportation and renewable energy sectors. A critical factor for the mechanical characteristics of ductile iron castings is the morphology of the embedded graphite nodules which, under certain solidification conditions, degenerate and form irregular features, resulting in inferior mechanical properties. In situ time-resolved high-speed synchrotron X-ray tomography was used to capture the evolution of graphite nodules during solidification of cast iron, revealing the development of degenerate features and the carbon concentration field in the matrix during repeated re-melting. The results reveal that the dendritic austenite phase and the distribution of the carbon concentration in the surrounding liquid phase control the graphite morphology. Changes in the sphericity of the nodule population under varying conditions are examined, showing an increase in degeneration with the number of re-melting cycles. A numerical model was developed to predict the formation of graphite protrusions during degeneration, correlated with the carbon concentration in vicinity. Changes in diffusion and field inhomogeneity are taken into account by varying the austenite shell thickness. The model shows that the protrusions form towards higher carbon concentration regions in the surrounding liquid, confirming our hypothesis of openings forming in the surrounding austenite. These results provide a strong fundament for the development of more advanced predictive numerical models and provide critical microstructural insights for the design of advanced cast iron alloys.