Besides its use in welding, thermit is being exploited for the repair of defective castings, which is not strictly a welding operation. Also for raising the temperature of the ladle before pouring for castings; for poling "burnt iron"; for the introduction of nickel, titanium, etc., into molten iron; for the formation of alloys; for the reduction of the less common metals from the refractory ores and earths. Though these last have nothing in common with welding, they will be treated of briefly, so that the treatise on thermit may be complete.

In the foundry, in the casting of large and expensive pieces, the loss by defective castings is sometimes equal to the price asked for the casting, due to cracks, bad flows, breaks, and inopportune blow holes. If the foundryman can save such pieces from the melting pot, he will greatly increase his profits.

For mending small surface defects, where the idea is to replace the surface without regard to the strength of the patch made, the following method is advised. First, chip out the defect to be sure that it is superficial. Heat the casting around the hole to a red heat. Then bank a basin of sand around the hole. Place a piece of abestos in the bottom of the basin, large enough to cover the hole. Pour thermit powder into the basin, using 18 ounces of thermit for every cubic inch estimated space of hole in the metal. If the casting is a large one, use a greater percentage of thermit, as more heat will be needed. Fire the thermit, and it will quickly melt the asbestos bottom, and the molten steel will be deposited in the hole in the metal. When cool, the protruding metal is machined off.

This reaction is too rapid for the complete separation of the slag, some of which may be lodged on the junction of the thermit metal and the casting. Also, it is likely that the local heating will cause weakening stresses in the patch when it cools; while it may also be full of blow holes if it cools rapidly, because of conduction. It is claimed1 that the shrinkage is so great that such a repair is unsafe and useless.

Thermit may be used for repairing fractures in castings before they leave the foundry. If the casting have a piece broken cleanly off, this may be joined at a less cost than the cost of recasting. Also cracks may be drilled out and thermit used.

Fig. 77. - Thermit can plunged into ladle.