Preparation of White Heart Malleable Cast Iron in Na₂O-K₂O-SiO₂ Oxide Molten Salt

Abstract

The preparation process of white heart malleable cast iron by heat treatment of white cast iron in Na₂O-K₂O-SiO₂ oxide molten salts at lower temperature than usual production method was studied. The liquidus temperature of the K₂O added Na₂O-SiO₂ (64–36 mol%) system was measured by the hot-thermocouple method and the effect of the treatment temperature on the degree of decarburization of white cast iron was investigated. The result of measuring liquidus temperature indicated that the Na₂O-K₂O-SiO₂ (38.4–40–21.6 mol%) ternary oxide melted at about 893 K. After heat treatments of white cast iron (φ8mm×8mm) containing 3.34 mass% carbon in this oxide molten salt at 1123–1223 K for 72 h, the white heart malleable cast iron could be obtained. Although, the thickness of the surface decarburized layer decreased with the decrease of the treatment temperature, the white heart malleable cast iron having about 30 μm surface layer was obtained even at low temperature of 1123 K. The basicity of this molten salt was 2.53, which was too high to be suitable for the decarburization of white cast iron at 1323 K. It is found that white heart malleable cast iron can be prepared by the heat treatment in high basisity oxide molten salt at lower temperature than 1323 K.