Graphite degeneration in the superficial layer of high Si-ductile iron casting as influence of inoculation and protective coating against sulphur diffusion into the iron melt

The objective of the present paper is to evaluate the occurrence of degenerate graphite in a surface layer on high Si ductile iron (0.032%Mgres, 3.37%C, 3.44%Si, 0.44%Mn, 4.43%CE), solidified in standard thermal analysis ceramic cup. S-bearing coating is applied on the inner surface of ceramic cup, with and without iron powder protective coating against S diffusion into the iron melt. Based carbonic material coating is also used as reference. Eutectic undercooling is normally 23% decreased by inoculation, but also depending on the nature of the applied coatings: less effect of S-bearing coating, while carbon and especially iron powder-bearing coatings reduced the beneficial effect of inoculation (higher addition, higher undercooling). Measured surface layer thickness in only Mg treated iron casting is higher by matrix evaluation (912 μm) compared to 716 μm obtained by graphite evaluation. Inoculation applied after Mg-treatment leads to decreased surface layer thickness to 195 μm and 108 μm, respectively. S-bearing coating increases the skin size of inoculated iron casting to 253 μm (matrix evaluation) and 176 μm (graphite evaluation). A layer iron powder addition on the S-bearing coating reduces the skin size to 132 μm and 66 μm, respectively, while supplementary iron powder addition avoids the surface layer formation, similarly to based carbonic material coating application.