Graphite Degeneration in High Si, Mg-Treated Iron Castings: Sulfur and Oxygen Addition Effects

Abstract

S and O have a high capacity to react with nodularizing elements, while the resulting products could be useful as nucleation sites for graphite. Any consumption of these elements could change a nodular to compacted or just lamellar morphologies. The main objective is to evaluate the effects of inoculation and S- or O-addition after Mg-treatment and inoculation on the sensitivity of high Si ductile iron, solidified in mold media without S or O contribution, to graphite degeneration in the surface layer compared to the body of the casting. Mg-treated and ladle-inoculated cast iron is used, containing 0.031%Mgres, 3.37%C, 3.44%Si, 0.44%Mn, for 4.43% carbon equivalent. FeS2 or Fe2O3 powder is placed on the bottom of a standard ceramic cup, used in thermal analysis (7.3 mm cooling modulus). Structure characteristics are analyzed from the section of the ceramic cup samples. The skin thickness has different values, depending on the graphite nodularity or shape factors or matrix evaluation results. Inoculation has a beneficial effect on the graphite phase characteristics: with only 3% higher nodularity in the casting body, this treatment is very efficient to decrease the skin effect thickness. A stronger degenerative effect of S on the graphite morphology was found, compared to the stoichiometric equivalent O-addition after inoculation, not only in the casting but also as the thickness of the skin. At higher S- or O-addition, the casting skin thickness increased, but at different levels: from two to four times higher after S-addition, and up to 30% higher after O-addition.