Demystifying the Role of Sulfur in Cast Irons

Sulfur is a key element in graphite nucleation in all of cast irons, so optimum sulfur content in the base iron is a prerequisite for each iron type. The effect of sulfur in cast irons varies greatly depending on the presence of Group IIA, IIIB, IVB and III A elements in the periodic table. This research summarizes much of the technical literature on the effects of sulfur in cast irons as well as new, experimental work on the inter-relationships between sulfur with other graphitizing elements. In grey cast irons, excessive sulfur levels can lead to slag inclusions, graphite flake degeneration and affect chill tendency. Sulfur levels less than 0.04% may solidify with high eutectic undercooling, promoting undercooled graphite and/or carbides. In ductile irons, low sulfur levels generally favor reduced nodulizer additions, reduced inclusion formation and result in higher magnesium recovery. However, excessively low sulfur levels may retard or minimize nodular graphite formation. In compacted graphite iron, controlling sulfur and magnesium to tighter ranges involves the aid of sophisticated thermal monitoring software. However, because of concerns regarding future availability of rare earth elements (REE), other production alternatives may be needed. An alternative involves making small and controlled S-addition after Mg-treatment (solo or in combinations with other elements) to promote less eutectic undercooling. Controlled resulfurization with briquetted FeS instead of pyrite powders is the most reliable method for precise sulfur control. Inoculation enhancing through additions of S, 0 and oxysulfides forming elements to commonly used conventional inoculants in a 1:3 ratio, had beneficial effects on solidification of all of standard irons (gray, ductile and compacted graphite irons). Inoculant consumption was reduced by 50% or more. Using this approach may eliminate the need for rare earth bearing treatment alloys in both nodularizers or/and inoculants.