Chills: solidification trickery. (Casting Facts).

As a clever metalcaster once remarked, "Chills are the foundryman's way of doing thermal trickery." It isn't magic, but a molder can pull a metal chill out of his hat to effectively "trick" the metal into solidifying the way he wants it to.

Metalcasting chills work much in the same way as icing down cans of beer on a hot summer day. When ice comes in contact with the can, it doesn't cool the beer--it removes the heat from it.

In the same way, chills placed in a sand mold absorb the heat caused by the molten metal filling the mold. And by absorbing and removing this heat, it hastens solidification-and therefore reduces shrinkage problems from "hot spots"--for a sound casting.

Through the use of external and internal chills, directional solidification can be accomplished for ideal casting conditions. By using these simple shapes, known as chills, in molding, the metal at the portion of the casting that is farthest from the gating area can be chilled quickly for even solidification.

External Chills

External chills are shapes usually made of steel, iron, graphite, chromite or copper. Placed where hot spots or slow freezing rates may occur, these chills are normally rammed up with the pattern and become part of the mold wall. They not only promote directional solidification, but can also, in some cases, engender temperature gradients that reduce the possibility of microporosity.

External chills are used effectively at junctions or other portions of the casting that are difficult to feed with risers. When possible, external chills should be placed in the drag surface of the mold to ensure contact with the casting surface. They should not be placed near gates or in the feed paths of risers, or these areas may solidify too soon.

Chill size is determined by the cooling requirement. Generally, a chill's thickness shouldn't be less than that of the metal section it is chilling. They are frequently covered with a protective wash, silica flour or other refractory material.

Internal Chills

Internal chills are placed internally at locations in the mold cavity that can't be reached with external chills. During pouring, they become surrounded by molten metal and actually become part of the casting. They are often placed in areas of the casting that will be machined.

Internal chill use is more problematic than external chills. In external chills, the makeup isn't as critical because they are outside the cavity; in internal chills the metal used must be compatible with the metal being poured. For instance, because the internal chill becomes part of the casting, aluminum alloy chills would normally be used for aluminum castings.

In addition, the chill must have a melting temperature nearly equal to that of the metal being poured. In addition, internal chills do not always fuse completely with the casting, thus establishing points of weakness, such as lack of pressure tightness and radiographic unsoundness. Depending on their location, chills may also alter the mechanical properties.

Because internal chills will be completely surrounded by metal, it is critical that they be clean. Gas created from unclean internal chills can't readily escape.

Benefits of Chilling

By using chills to speed up solidification, foundrymen can:

* promote directional solidification;

* help risers feed shrinkage;

* promote steep thermal gradients;

* refine the microstructure;

* reduced dendrite arm spacing;

* help reduce macro gas cavities.

Chills must be clean and free from rust or other surface matter, and must be thoroughly dried before they are used. If the chill is preheated equal to or slightly higher than the temperatures of the mold and core materials before being placed, conditions improve dramatically. This helps prevent sweating and gas-related defects.

If you don't preheat, you should pour the mold shortly after it's made. If chills stand too long in the mold, condensation will form and cause defects.

Other Options

Besides risers and chills, there are other ways to combat poor solidification situations. The same effects can be accomplished by using a variety of molding materials that can change the cooling characteristics of the mold.

Molding aggregates such as zircon or chromite sand are effective because of their ability to quickly diffuse heat. By properly placing these materials in the mold, foundrymen can achieve improved solidification without having to handle and care for more costly metal chills.

For More Information

"Resurrecting Chill Wedges for Ductile iron Production," D. Reimer, MODERN CASTING, December 2002, p.34-35.