A Case Study on the Casting Skin Phenomena in Compacted Graphite Iron Cast Components

Abstract In sand casting, the surface microstructure often differs from the bulk microstructure due to mold–metal interactions. In nodular and compacted graphite iron, these interactions manifest mostly by a variation in graphite morphology, often attributed to magnesium depletion due to the release of sulfur and oxygen from the molding materials. Additionally, previous research suggests that solidification time, surface condition, and metallostatic height also affect the degree of surface microstructure deviations, though the mechanisms remain poorly understood. Therefore, the present case study targeted an in-depth analysis of the casting skin phenomena in two cylinder blocks sourced from two different foundries to ascertain its occurrence, severity, and main characteristics. Interfaces with molding materials were evaluated under varying solidification times and metallostatic heights, and measurements of the degenerated graphite layer (DGL) thickness and nodularity were performed; average surface roughness was estimated through image analysis. Correlations between these variables and the DGL thickness were assessed. Nital and Motz’s etchants were also employed to examine the DGL characteristics in greater detail, while scanning electron microscopy and energy-dispersive x-ray spectroscopy further explored the presence of particles at the mold–metal interfaces. Results revealed a wide range of DGL structures without a clear correlation to the studied process variables. In fact, some trends observed were opposite to those reported in the literature. A deeper understanding of the key mechanisms is still needed as the casting skin continues to challenge foundries in daily production.