THERMITE

[from Thermit, a trade name], mixture of powdered or granular aluminum metal and powdered iron oxide. When ignited it gives off large amounts of heat. In wartime it has been used in incendiary bombs. A method for welding using thermite (invented by Dr. Hans Goldschmidt, a German chemist) is variously called the Goldschmidt process, the thermit process, or the aluminothermic process; it is used in welding large parts, e.g., castings, shafts, pipes, and steel rails. In the process the thermite, contained in a crucible, is ignited, e.g., by a strip of burning magnesium ribbon. The aluminum reduces the iron oxide to molten iron and forms a slag of aluminum oxide on its surface. The reaction is very exothermic; temperatures above 2,500 degrees Celsius (4,500 degrees Fahrenheit) are often reached. Typically, the molten iron is poured into the joint to be welded, providing both heat for fusion and filler metal. Excess metal may be removed when the weld cools. Because thermite reacts with explosive violence once ignited, it cannot be heated as a mass to its kindling temperature (about 1,550 degrees Celsius/2,800 degrees Fahrenheit); Goldschmidt was first to find a method for igniting thermite without explosion. He used a similar method to prepare various metals, e.g., chromium, manganese, and uranium, from their oxides.

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