Influence of Cooling Rate on the Structural Characteristics of Ca and Rare Earth Inoculated, Low-S Grey Cast Iron

The structural characteristics of low S (0.025%), low Al (< 0.003%) and 4.0% carbon equivalent (CE) electric melted grey irons were studied at different solidification rates, for Ca and Ca, RE [Mischmetal] as the inoculating elements. Despite a relatively high CE, this base iron is sensitive to undercooling during solidification. A conventional Ca-FeSi inoculant is not recommended for these critical conditions, especially to avoid undercooled graphite morphologies. It is only adequate to control free carbides, for castings with more than 15mm section. Rare earth (RE) bearing CaFeSi alloy appears to be a more effective solution, at least for castings with more than 5mm section to control carbides, even the "end effect", and promote a homogeneous structure. Undercooled graphite presence was limited and avoided in more than 10mm section. Thin wall castings, less than 5mm section, requiring stronger control for carbides and undercooled graphite formation, should not be produced from this type of base iron, unless more efficient inoculation can be accomplished.