DISCUSSION


Pouring, solidification and high casting temperatures enhance the breakdown rate of both H and N, increasing gas solubility in the liquid metal. High pouring temperatures also have a significant effect on liquid metal surface tension, which affects porosity formation.

Chemical analyses showed that considerable pickup of both H and N occurred in the immediate subsurface layers, when conditions favoring porosity were employed. At depths 0.25 in. (0.6 cm) below the cored surface, H and N levels were low and representative of the base metal. Just before solidification, momentary supersaturation of both H and N may exist just under the casting surface.

Further, if a considerable amount of nascent N is dissolved in a casting from unbalanced binder ratios favoring excessive polyisocyanate components, even a small amount of H will lower the overall solubility of N. H may exert a catalytic effect on N to enhance porosity formation. Alloying elements have the same effect on gas solubility.

To further aggravate conditions, if the melt initially has a high gas content resulting from poor charge metallics or carbon additives, then the tolerance for additional solution of nascent mold or core gases is reduced considerably, and porosity formation is extremely likely.

Typical microstructures in sound and porosity-containing castings were taken at the mold-metal interface. In all cases, no differences in matrix structure or graphite morphology were found. Both microstructures contained the same ferritic-type matrix with Type A graphite. Although H and N are carbide stabilizers and favor formation of pearlite and other graphite structures, insufficient time was available during solidification and subsequent cooling through the transformation temperatures for such phases to form.

Although most gas holes exhibited a bright or shiny interior of a graphitic nature, no such films were observed. Further examination of these areas by SEM showed distinct layers of a crystalline graphitic coating in the gas holes.

Although gas holes were located just underneath the surface and most extended no more than 0.25 in. (0.6 cm), a few castings contained gas fissures almost 0.5 in. (1.27 cm) long. Because of the sub-surface nature of the defects, large amounts of alloying elements that form stable N compounds may not be needed, since only these sub-surface layers are affected.

Incorporating N-stabilizing elements or "scavengers," which include both Ti- and Zr-based ferroalloys, may offer additional possibilities for treating binder-induced porosity defects. Likewise, in-the-mold inoculating tablets incorporating Zr for N control and small amounts of Se for H control, also show promise for defect elimination.

It is not well understood how small amounts of red iron oxide were so effective in eliminating subsurface porosity in the castings. The red iron oxide may be exerting some type of "catalytic effect" on binder decomposition products that minimize or alter the generation of N and H gases.

When exposed to the sudden high temperatures of iron casting, red iron oxide readily releases oxygen. This oxygen immediately reacts with N from the binder to form stable NOX compounds. Since hematite has a much higher concentration of oxygen compared to magnetite and, based on its performance, this is feasible.

Table of contents
[Paper Header]
[Porosity Study]
[Binder Ratio] [Binder Level] [Casting Temperature] [Section Size]
[Sand Effect] [Iron Oxide Additions] [ Binder Dispersion/Mixing]
[Metal Composition] [Core Age]
[Eliminating Porosity]
[Ti and Zr Additions] [In-the-Mold Additions] [ Core Washes]
[Core Post-Baking]
[Discussion]