Mold Coatings to Reduce Graphite Degeneration in the Surface Layer of Ductile Iron Castings

The performance of coatings for furan resin sand molds using P-Toluol Sulfonic Acid (PTSA) hardener was evaluated for a surface layer of degenerated graphite in ductile iron test castings. It was reconfirmed that sulfur delivered by this mold type is an important contributor to graphite degeneration, especially in castings exhibiting lower graphite nodularity. A no-coating condition and especially a S-bearing coating led to a thicker surface layer and decreased graphite nodularity in the entire casting section, while MgO or (CaO + MgO + Talc) or Mg-FeSi blended coatings had beneficial effects, increasing in the order written, with (Mg + FeSi) coating the best performance. The applied coatings act mainly to control chemical interactions, such as S with Mg (or RE) rather than controlling heat transfer. Mg in a coating can offset Mg loss in the casting beyond the role of MgO or (CaO + MgO + Talc) coatings to inhibit S migration from the mold. This complex behavior could justify the application of coatings incorporating active Mg, from a fine sized Mg-FeSi alloy.