No-Bake S-Containing Mold/DI Metal Interactions: Consequences and Potential Applications
The ductile iron liquid metal is very reactive with environmental gases and solid molds after a nodulizing treatment. Mold-metal reactions could alter the spheroidal graphite shape to vermicular or flake. In this research, the metal-mold reactions were enhanced by various sulfur additions to the no-bake mold inserts to study these phenomena and to artificially produce layered dual-graphite structure in the casting. Thermodynamic and solidification modeling together with sets of different experimental techniques were applied to a test casting with internal core passages representing the cylinder head of a diesel engine. The effects of insert surface shape and location in the mold; content and type of sulfur-containing agent; coating; pouring temperature and additional chilling were experimentally studied and discussed. These results could be used to better understand the metal-mold interaction, prevent surface defects and design dual-graphite structure processes.
The Influence of Mold/Metal Interactions on the Castings Microstructure—Bibliographical Research
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.
Observation and analysis of the nucleus of spheroidal graphite in magnesium-treated ductile iron
The structure of spheroidal graphite in a magnesium treated ductile cast iron was studied by a combined use of high performance microscopic instruments: SEM with EDS, EPMA, AES and TEM. A spherical...