Understanding metal penetration in green sand: cast iron.

Penetration defects cost American iron and steel foundries about $65 million per year. Despite all of the research performed on the subject, the problem still plagues iron casters.

Listed below are conclusions from more than five years of studying penetration in cast iron. Some, you will find, are rather surprising.

1. In cast iron, penetration is caused by mechanical pressure of metal forcing itself into the sand mold. The pressure of the liquid metal is what forces it into the mold. No chemical reactions occur that might cause the metal to react with the sand. The only reaction that could occur would be if the molten iron reacted with oxygen (in the air in the mold, or in the moisture in the sand) to form iron oxide; this oxide could then react with the sand.

However, there is so much carbon dissolved in the iron that it will always react with the oxygen in the mold to form carbon monoxide gas. The burning of the carbon monoxide gas is what causes the flames that shoot out from the molds when they are poured.

To resist the pressure of the molten metal, use a fine grade of sand at the casting surface, and be sure that the mold is rammed hard and uniformly. Mold and core coatings prevent penetration because their small particle size fills up the spaces between the sand grains.

2. Most penetration defects are caused by poor casting design, not poor molds. This was a surprise, but it makes sense. As shown in "A" Fig. 1, most penetration problems in cast iron are found in corners where heat is concentrated (a hot spot).

[Figure 1 ILLUSTRATION OMITTED]

This area of the casting is the last to solidify. Remember that graphite formation during solidification causes the metal to expand. If the casting has frozen solid on both sides of the hot spot but not in the hot spot ("B"), when the graphite in this last metal expands on solidification, where will the liquid metal in its vicinity be pushed? It cannot penetrate the solid metal on either side, so it will go out into the sand, causing penetration.

The solution is redesigning the casting to avoid the hot spot, or taper the sides of the section back to a riser ("C"). This is the principle of directional solidification, but in reverse. Steel and nonferrous founders use directional solidification and taper walls to deliver feed metal to the last place to solidify so that the casting will be sound. But in cast iron, it may be necessary to taper sections so that liquid metal can be forced into the riser so that penetration doesn't form. The rules for casting design are the same: a path of liquid metal must be supplied to where the last metal in the casting solidifies. In steel, it is to feed the shrink, in cast iron it is to prevent penetration.

This also explains why the addition of phosphorous to cast iron reduces penetration defects: the phosphorous reduces the expansion of the graphite, so metal no longer is pushed into the sand. This often causes shrink in castings, but it eliminates the penetration. It also is probably the same phenomenon that causes mold wall movement in well-rammed molds.

3. The tendency for a given iron composition to penetrate a mold can be predicted. Because penetration is caused by the pressure of the molten iron squeezing liquid metal in between sand grains, a simple pressure balance can be written that will predict whether the pressure of the metal will force it into the sand. As a result of this project, an equation has been written, evaluated and validated in commercial foundries. It is in the form of a spreadsheet that runs on a laptop computer. Called the "Penetration Prognosticator," it is available from AFS.

The probability of penetration occurring increases with:

* higher pouring temperatures;

* higher silicon and carbon levels;

* higher casting heights;

* faster pouring speeds.

The software program combines these variables into a spreadsheet, which shows whether penetration is likely for a given casting poured into a given mold with a given metal composition. In addition, it also provides information on how to reduce the likelihood of penetration.

4. Seacoal acts by expanding to fill the spaces between the sand grains, not by producing a reducing atmosphere in the mold. Most foundrymen were taught that seacoal acts by burning and producing a reducing atmosphere, which prevents oxidation of the cast iron, and consequently prevents the formation of fayalite and ultimately Penetration. That thought is partially true: the seacoal does burn, and in burning does provide a reducing atmosphere.

But, as noted earlier, because there is so much dissolved carbon in cast iron, the atmosphere is reducing in the mold whether or not seacoal is added. When the composition of the mold atmosphere was measured at the interface in molds with seacoal additions and in those without seacoal or other carbonaceous additions, it was discovered that there was little difference between the two--the atmospheres in both molds were reducing.

Seacoal is effective because it swells and fills the space between the sand grains when the hot metal hits it. In doing this, it behaves the same way that coal does when it's turned into coke. Examine some coke: notice that it appears "swollen" and has obviously deformed during the coking process. This is what happens to the powdered coal in seacoal in your mold. This swelling action closes off the spaces between the sand grains, and resists penetration, giving the casting a better peel.

This observation is important to firms that are trying to find a substitute for seacoal. Unless the alternative material expands during metal solidification to fill the space between the sand at the sand/metal interface, it won't prevent penetration in cast iron--even if it produces a reducing atmosphere during solidification.

These remarks are valid for cast iron. Further research is underway on steel castings and, while the two metal groups share much of the rules, there are some important differences, especially regarding the conditions under which chemical penetration can occur. In steel, for instance, a carbonaceous addition to the sand is necessary to prevent iron oxide formation and chemical penetration.

Prevention of penetration in cast iron is a combination of mold practice and casting design. Nearly all cast iron penetration problems will vanish if both are done correctly.

The complete report of this research can be found in AFS Transaction papers 96-206, 96-207 and 96-208. The program was sponsored by AFS, a consortium of foundries and suppliers and the Dept. of Energy.