This section is from the book "Welding Theory, Practice, Apparatus And Tests Electric, Thermit And Hot-Flame Processes", by Richard N. Hart. Also available from Amazon: Welding: Theory, Practice, Apparatus And Tests, Electric, Thermit And Hot-Flame Processes.
One of the most recent and successful methods of welding is called the Thermit Process. It was invented by Dr. Goldschmidt, of Essen, Germany, and is exploited by the company bearing his name. In this process a mixture of aluminum and oxid of iron is ignited. The aluminum reduces the iron from its oxid, and evolves an intense heat, about 2500 deg. Cent., or twice the temperature of molten steel. This molten steel, called thermit steel, is then poured around the metal to be welded and forms a melt-joint that is very strong when cold. Its present application is entirely in repairs of large metal pieces and in making continuous welded railroad track. It is used in repair shops for mending car axles, auto and electric motor cases, broken and defective castings, broken parts of reciprocating engines, broken rudder-posts, skegs, and sternposts of ships, and for repair work in general along this line. Special thermit mixtures are being advocated for toning up the melted steel in the ladle in foundry practice, for preventing "piping" of ingots; and the company is using the strong reducing property of aluminum in reducing a number of the less used metals, such as tungsten, chromium, and boron, to a pure metallic state.
Thermit is first of all a welding process. Its good and weak points may be summed up thus:
1. Simplicity of the apparatus.
2. No special skill needed to do the work.
3. Possibility of repairing breaks difficult of access and of repairing parts in situ that would otherwise have to be taken out.
4. Possiblity of intense local heating of large parts.
5. Time and money saved in most repair work.
6. Possibility of varying the chemical composition of thermit Steel so that its properties may be varied.
7. It is at present limited to rail welding and repair work.
8. Only iron and steel can be welded.
9. The cost, though much lower than the forge method of welding, is still often prohibitive.
The process is used by many of the leading railroads, shipyards, and machine shops of all of these countries, both for repair work and for special jointing, such as that of the third rail of the Paris subway. At present Dr. Goldschmidt is trying to produce chemically pure metals on a commercial scale. He has met with success in reducing metallic manganese, chromium, tungsten, vanadium, molybdinum, boron, etc., from their ores and oxids. This new field in metallurgy, now called aluminothermics, seems to promise as many new and interesting possibilities on its horizon as did the experiments of Moissan with his electric furnace.
The fundamental idea beneath thermit has been in the minds of metallurgists for at least a half-century. In the year 1869, a Mr. Budd1 describes a process for reducing the alloyed silicon in pig iron. His idea was to burn it out with hematite ore, the formula being:
3Si+2Fe203 = 4Fe+3SiO2.
He made a paste of hematite and smeared it over the bottoms of the pig molds. The molten iron, which appears to have been much too high in silicon, was run into the molds, and immediately the silicon began to burn out of the iron, first taking up the oxygen of the hematite mud on the bottom of the mold, and then uniting with some of the iron and coming to the top as a silicate-of-iron slag. Most of the iron reduced from the hematite added itself to the pig. Like the Goldschmidt method, this was the reduction of one metal by the transfer of its oxygen to another metal.
The fact that aluminum has the greatest affinity for oxygen has long suggested it as a final reducing agent. And its steady fall in price since its discovery by Woehler in 1857 finally brought it, about 1895, within range of the market. Woehler himself tried to smelt chromium from its chlorid by ignition with metallic aluminum. After an explosively violent reaction, he found he had an alloy of chromium with aluminum.
1 Transactions of the Iron and Steel Institute, 1869; "On a New Process for Removing Silicon from Pig Iron."
A number of later attempts were made to use aluminum as an agent for reducing the rare metals from their oxids. Yet, though it had an intense affinity for oxygen, the combustion was hard to start, and when started was hard to control. Experimenters mixed it as a powder with a metallic oxid and heated the mixture from the outside. Finely divided metallic aluminum will not burn at the temperature of molten cast iron. So that when the contents of the crucible began to react, the initial temperature was already so high that the reaction was an explosion. Dr. Goldschmidt overcame this by setting off the cold powder with a fuse of barium peroxid, BaO, which in turn was set off by a storm match. A charge of several pounds was found to burn in less than 30 seconds, and the temperature of the mass rose to an approximate 2500 deg. Cent. Larger quantities, though starting to burn from a cold and coarsely powdered sand, often boiled over. A premixture of cold steel turnings remedied this. The result of the burning was an intensely hot iron whose composition could be varied at will.
The commercial value of this invention is obvious. There are many processes and many emergencies where a very hot molten iron is invaluable, yet where it is difficult and expensive to get this heat by any known means. Take the case of a broken casting of some large machine that would in the ordinary course of repair have to be taken apart and shipped to the nearest forge to be welded. If, however, a definite quantity of iron, heated to twice its melting point, can be made on the spot, it can be poured around this break without dismantling the machine. It will then form a welded union, much as though one were to put the butts of two candles together and pour hot tallow over the joint. The tallow would melt into the candles before it itself cooled, and join the two with a homogeneous substance.
In order that the mechanical aspect of the thermit weld may be clear to the reader, a simple case of rail welding will be outlined. After which the appliances used in the process will be described in detail.
 
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