THERMIT. Thermit is a mixture of aluminium powder and iron oxide. On ignition the reaction, gives a temperature estimated to be between and 2,700° Centigrade. The reaction, stated in weights, means that 217 parts of aluminium plus 732 parts magnetite (iron oxide) equals 54o parts steel plus 409 parts slag, or approximately, three parts of aluminium plus ten parts of magnetite will produce, on combus tion, seven parts of steel. The steel thus produced represents about one-half of the original thermit by weight and about one third by volume.
Thermit was discovered by Dr. Hans Goldschmidt, of Essen, Germany, in 1895, while trying to reduce chromium and man ganese. Dr. Goldschmidt's principal discovery related to a simple and safe method of ignition, as the action of aluminium when mixed with various oxides, sulphides and chlorides was well known. Fine aluminium will not burn below the temperature of molten cast-iron, and previous experimenters had resorted to heating their mixtures in a crucible. This made the initial temperature so high at the moment of ignition that there was an explosion. Dr. Gold schmidt obtained ignition of a cold mixture by means of a barium peroxide fuse, which was set off by a storm match. Later, magne sium powder or ribbon was used, being set off in the same way. A red-hot iron rod may also be used to set off the magnesium, which in turn ignites the thermit. Dr. Goldschmidt's original American patent No. 615700 was granted March 16, 1897, and re lated principally to the use of aluminium as a reducing agent for the production of carbon-free metals such as cobalt, chromium, magnesium, tungsten, etc., by what is known as the aluminother mic process. Thermit is used considerably in the foundry for puri fying iron and steel in the ladle. For this purpose the thermit is placed in a can on the end of a rod and plunged to the bottom of the molten metal. The intense heat generated tends to liberate many impurities which are carried away in the slag. The princi pal and better-known use for thermit, however, is in the various forms of welding.
Thermit Welding.—Two methods are used, known as the plastic and the fusion. The first is used for welding pipe and the latter for solid or large sections. In the plastic method, in which the thermit is used merely for heating purposes, the ends of two pieces of pipe are machined square and clamped in a cast-iron mould with the ends butted together. This mould is in two parts, so arranged that the pipe ends may be forced together when heated. The thermit is placed in an open top crucible lined with magnesia tar, and ignited. After the reaction takes place, the slag, which is of course lighter in weight, rises to the top of the molten metal, and is first poured into the mould. This slag forms a protective coating on the pipe and on the inside of the mould, and keeps the thermit from melting or burning through. The thermit flows into the mould and forces out the bulk of the slag, but leaves a coat ing as mentioned. When the pipe ends become plastic they are forced together, completing the weld. After cooling, the mould is easily knocked off, since the slag coating prevents adhesion. To weld a pipe takes from 4 to 1 minutes. (For a detailed descrip tion of thermit fusion welding, see the article WELDING.) Some of the principal uses to which thermit is put are in repairing broken rudder frames, propeller shafts, locomotive frames, steel rolling mill pinions, and other heavy sections, but it cannot be economi cally used for welding thin sheet metal sections. Welds have been made where from 3,00o to 4,000 lb. of thermit were used. For commercial purposes there are now produced three varieties of thermit, known as plain thermit, railroad thermit and cast-iron thermit. The plain thermit is simply a mixture of aluminium and iron oxide, as already given. Railroad thermit is plain thermit with the addition of i% nickel, 1% manganese and 15% mild steel punchings. Cast-iron thermit is plain thermit with the addi tion of 3% ferrosilicon and 20% mild steel punchings. The various names of these mixtures indicate very closely their princi pal uses. (E. V.)