Analyzing the Mold-Metal Interface.

In hopes of better understanding the mold-metal interface and how mold atmosphere relates to the surface quality of iron and steel castings, W.D. Scott revisited his research (in conjunction with CE. Bates) of 25 years ago in "The Atmosphere at the Mold Metal Interface" (082).

The research was conducted to determine which gases evolve at the mold-metal interface with the thermal decomposition of organic binders during the pouring and cooling of ferrous castings. The 11 sand binder systems investigated included versions of: sodium silicate (ester and carbon dioxide), low-nitrogen furan, medium-nitrogen furan, phenolic, phenolic hotbox, furan hotbox, phenolic urethane and shell. The experimental approach consisted of pouring either iron or steel into sand molds made with these binders and sampling the gases that evolved in the mold metal interface as a function of time. The sand-binder mixes were consistent with the foundry practices and formulations of 25 years ago.

According to Scott, the principal gases that evolved were hydrogen (I-I), carbon monoxide (CO), carbon dioxide, methane and nitrogen, with lesser amounts of oxygen (0), ethane,. ethylene and acetylene. For the most part, the amounts of these gases were measured as consistent with the formulations of the binders, however, the concentrations of H at the mold-metal interface (greaterthan 40%) were unexpectedly high, Scott said.

After the castings cooled, they were examined as-cast, after wire brushing and after shotblasting to characterize surface condition such assand penetration and gas porosity. Selected castings were sectioned and examined for pinholes, subsurface porosity, decarburization and ferrite skin. Through this analysis, correlations between the gas concentrations and surface finish were made.

According to Scott, all of the castings produced, except those molded with ester-catalyzed sodium silicate, exhibited a clean surface finish. This behavior was deemed consistent with the reducing atmosphere rich in H and CO. Scott said that the study also substantiated that several reactions can take place at the mold-metal interface, including: surface oxidation with consequent burn-on or penetration; carburization/decarburization, including removal of both graphite and carbon from austentite that otherwise would produce pearlite; and sulfurization and phosphorization of the casting surface when these elements were present in the binder.

Each reaction is controlled by the C, O, sulfur or phosphorous potentials existing between the metal and gas at the mold metal interface, said Scott. There was no evidence, however, that common foundry variables like mold age, binder percentage or, to a lesser extent, type of organic resin has a major role in the composition of the mold atmosphere.