Simple Solutions That Work! Issue 11

slag additives and atmospheric humidity. Nitrogen behaves in a similar manner and can result from the processing of some ferroalloys as well as decomposition products from chemical mold and core binders. Appearance: Pinhole defects are a form of gas porosity. They are usually small, elongated holes (about 1/16 in. in diameter) at or immediately below the casting surface with the long axis of the pinhole perpendicular to the casting surface. The walls of pinholes in steel are often smooth and bright. When the pinholes extend to surface of the casting, the interior may appear oxidized and show a faint dendritic structure. Pinholes may occur at or below any casting surface but will tend to occur more readily in thin and intermediate transition thicknesses, where re- entrant angles can serve as hot spots. Figure 3 illustrates typical subsurface pinholes in a high alloy steel casting. Sources of Gas from Mold-Metal Interface Reactions: With few exceptions, core and mold binders used in the steel foundry industry are organic based systems. Being organic, they are based on the elements carbon, hydrogen, and oxygen and in some cases nitrogen. The approximate chemical make-up of some of the common binder systems is shown in Table 1 (a typical Western Bentonite bonded green sand is shown for comparison). Newer binder formulations have been developed in recent years to minimize hazardous air pollutants (HAPs), and these modifications have had a direct effect on the chemical makeup of the binder systems. More recently, TEOS solvent systems (a silicate-based solvent combining both organic and inorganic characteristics) have replaced traditional aromatic and aliphatic solvents. The primary performance advantages over aromatic hydrocarbon or biodiesel solvents is reduced smoke and odor. These newer binders can reduce HAP's at pouring, cooling and shakeout. At ferrous casting temperatures, the presence of these elements (carbon, hydrogen, oxygen and nitrogen) and their subsequent decomposition products can produce a variety of casting defects. The following gaseous reactions are thermodynamically possible and under the right conditions may occur at the mold-metal interface 56 . Hydrogen is absorbed into 5 MAKING YOUR INSTALLATION ASUCCESS Continued on next page Figure 3: Steel axle bracket from a mining car illustrating sub- surface porosity Table 1. Approximate chemical compostion of current foundry binders systems