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Metal bridging furnace hazard needs fast remedial attention.

When cold charge material in the top portion of a furnace is not in contact with the molten metal in the furnace bottom as shown in Fig. 1, a condition known as "bridging" exists. The upper part of the charge is isolated from the molten metal and rapid increases in bath temperature can occur unexpectedly.

When bridging occurs, cold charge material no longer moderates the temperature of the now isolated bath. The air gap between the molten metal and the bridge acts as an insulator. Molten metal in the bottom of the furnace, under the impact of full melting power, will superheat quickly in an induction furnace, rapidly raising the temperature of the bath above the maximum rating of the refractory.

Excessive stirring of the melt due to the reduced metal mass, coupled with high-power density and a rising metal temperature, can cause rapid refractory damage by erosion and penetration or, possibly, complete refractory failure.

Without immediate attention to a "bridging" condition, a runout will occur. A runout through the bottom of the furnace can cause a fire under the furnace and in the pit area, precipitating the subsequent loss of hydraulics, control power and water cooling.

Keep the furnace pit area clean and dry, free of water or hydraulic fluid accumulation. The pit is designed solely to hold molten metal in case of an emergency.

If the molten metal melts through the furnace coil, it will come in contact with the water. This will instantaneously turn the water into steam with a 1600-to-l expansion ratio. Should water get under the molten metal, this rapid expansion rate will cause the molten metal to be forcibly ejected from the furnace. This explosion can cause extensive equipment damage and serious injury to personnel in the area.

Bridging can occur in any induction furnace, and operators must be able to recognize and solve a bridging problem and understand its inherent dangers. Bridging can be minimized by using proper charge materials and by making sure the different sizes of charge material are added correctly.

Corrective Actions

If a bridge occurs, power must be turned off until the bath temperature is known. If the bridge has completely sealed the top of the furnace, pressure may build between the molten metal and the bridge. In that event, the safest thing to do is to allow the molten metal to freeze.

Should the bridge fail to seal the top of the furnace and pressure has not risen, the furnace may be tilted to attempt to melt out the bridge.

Put a ladle in front of the furnace to catch any metal that may spill out. Then tilt the furnace carefully until the molten metal is in contact with the bridged material (approximately 45 |degrees~). The molten metal then will melt a hole in the bridge. All unnecessary personnel should be kept away from the furnace, and the area in front of the furnace should be cleared while melting out a bridge.

Under no circumstances should an oxygen lance or burning bar be used to cut through the bridge!

After a hole has been melted through the bridge, return the furnace to the upright position and check the temperature to make sure the molten metal is not overheated. Begin adding charge through the hole in the bridge to raise the bath level. This brings the molten metal into contact with the bridge, melting it into the bath. Adding charge also cools the molten metal and power may be reapplied, if necessary, to maintain the proper pouring temperature.

The furnace should be poured empty as soon as possible so the lining can be assessed for damage. If there is any doubt about the integrity of the lining, replace it!
COPYRIGHT 1993 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Title Annotation:Cast Facts
Author:Turner, Robert C.
Publication:Modern Casting
Date:Jul 1, 1993
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