Casting Skin of Compacted Graphite Iron Part I: Evaluation and Mechanism of Formation

The purpose of this Part I of a two-part paper is to study the morphology of the casting skin for compacted graphite (CG) iron and to quantify its value as a function of some casting variables, including molding sand, casting thickness (cooling rate), metallostatic height and shot blasting. Two CG iron heats were produced through the ladle treatment process. The iron was cast in sodium silicate and phenolic-urethane bonded sand molds. A total of 78 test castings were produced. The thickness of the cast test plate was 7.6 mm (0.3 in.), 10.2 mm (0.4 in.) and 15.2 mm (0.6in.). The characterization performed on the test castings included graphite shape factors (nodularity), graphite area, percent of graphite and amount of pearlite, as well as the roughness average. It was found that after very low levels in the proximity of the surface the nodularity increases and then gradually decreases toward the center of the plate castings. In addition, the percentage pearlite increases with distance from the surface. These quantities were used to define the casting skin. Casting skins of 0.05 to 0.39 mm were observed in these experiments. The sodium silicate bonded molding aggregate produced higher roughness and thicker pearlitic rim than the phenolic-urethane bonded sand molds. Faster cooling rate tends to encourage graphite degradation. Finally, removal of the casting skin through shot blasting was observed when shot blasting time for 5 min. Selected samples were further examined using a color metallography technique. The special etching outlined the interdendritic regions through the segregation of silicon and was found to be a useful tool in the study of the casting skin.