The Influence of Mold/Metal Interactions on the Castings Microstructure—Bibliographical Research

Abstract

The phenomena on the mold/metal interface largely determine the microstructure of the casting surface layer, quality of the surface and casting performance characteristics (mechanical properties, wear resistance , corrosion resistance, etc.). Processes that occur here are of particular importance in the case of cast iron with vermicular or spheroidal graphite performed in sand molds (graphite degradation and graphite depletion ). It is assumed that the direct cause of graphite degradation , resulting in a layer of flake graphite, is the Mg depletion at the surface layer of the casting and the main reason (although not the only) for the decrease of the Mg concentration is the reaction of sulfur and oxygen. To explain this phenomenon, Fe–Mg, Mg–S, Mg–O and Fe–S systems have been discussed. Performed thermodynamic calculations have shown that the magnesium and sulfur and oxygen reactions in liquid iron occur in the gas phase, wherein the thermodynamic reaction of Mg vapors with oxygen gas is more privileged than with the sulfur vapors. To clarify the mechanism of the formation of the surface layer on the casting Stefanescu et al. developed a thermo-diffusion model 2-D. However, this model does not take the kinetic agent and the degree of undercooling during solidification of the casting into account, which affects the amount of primary austenite formed, which may also result in a reduction of the Mg concentration. To reduce the degree of transition of sulfur from molding sand to casting surface layer, the use of protective coatings for molds and cores, containing desulfurization ingredients such as CaO, MgO, talc, or Mg-FeSi, which binds sulfur and supplements magnesium at the same time is preferred.