Formation mechanisms and characterisation of porosity in cast iron

Cast Iron, despite being used for thousands of years, is yet to be a perfected art, with a significant portion of losses caused by porosity in the cast components. Porosity leads to substantially increased costs, waste of embodied energy, and environmental emissions. Minimizing these defects demands a thorough understanding of their causes and effective corrective measures. Driven by the necessity to produce lighter and more efficient components, heavy-duty powertrains have increasingly adopted compacted graphite cast iron (CGI) as the current state-of-the-art in engine components. However, CGI’s recent adoption, facilitated by advanced process control methods, has also increased susceptibility to defects. The work presented in this thesis investigated the mechanisms surrounding porosity defect formation in CGI, particularly when compared to the extensively studied spheroidal graphite cast iron (SGI) and lamellar graphite cast iron (LGI). A case study was conducted in cooperation with several cast iron producers. Components containing porosity defects stemming from complex heavy castings were analysed in depth and compared to the literature information available regarding porosity defects. These analyses identified knowledge gaps, motivating the development of a novel experimental methodology capable of controlling key variables affecting pore formation, including gas composition within the pore, pore formation temperature, melt composition, and graphite morphology. Knowledge gaps highlighted in the case studies were targeted. Notably, although graphite film formation was previously largely attributed to the pore gas composition, its occurrence within inert argon-filled pores indicates the involvement of additional mechanisms. The present work also evaluated the partitioning behavior of nitrogen during solidification. A discrepancy in reported nitrogen partition coefficients was identified between existing literature and thermodynamic databases. Partitioning affects its segregation behavior and consequently the tendency for porosity formation. A hold and quench experiment has shown that nitrogen tends to partition towards the liquid phase during solidification, which suggests it can significantly contribute to pore nucleation and growth. This finding further stresses the importance of selecting databases that accurately represent the behavior of the system of interest, while simultaneously highlighting the need to improve the accuracy of existing ones. These findings offer critical insights for improved process control, ultimately supporting the production of more reliable and efficient CGI components.