Structure Characterization of Ca/Ba, Ca-FeSi Inoculated, Low Sulphur, Electrically Melted, thin Wall Grey Iron Castings

Abstract

Much of the base iron in grey iron foundries is electrically melted in an acid lined induction furnace. The performance of the induction furnace allows superheating above 1500 oC, which is appropriate for thin wall casting production. With higher levels of superheat, the base iron characteristics are totally different from cupola melted iron, resulting in changes to the final casting microstructure. Previous experiments illustrated that eutectic undercooling of this type of base iron is excessively high, demonstrating an increased need for inoculation. The high dissolution rate of residual graphite in superheated iron and difficulties in forming complex (Mn,X)S compounds as active nucleation sites of graphite can be due to very low residuals of Al (< 0.003%) and Zr (< 0.0003%), especially at less than 0.03%S content. This results in increased tendencies for chill and undercooled graphite morphologies, even in inoculated irons. The structural characteristics of low-S (0.025%), low-Al (<0.003%) and 4.0 wt.% carbon equivalent for electrically melted grey irons were studied at different solidification cooling rates in wedge castings up to 20mm wall thickness, using Ca and (Ba + Ca) inoculating elements in FeSi based alloys with the same Si and Al contents. Under these conditions, Ca inoculation had minimal effect at less than 8mm wall thickness, while a Ca-Ba combination improved most of the structural parameters, including those in thin wall castings: less than 10% carbides for 2.5 mm and no carbides at more than 5mm section size, which also showed the highest graphite amount with a uniform distribution over the casting section.