Thermite welding is the process of igniting a mix of high energy materials, (also called thermite), that produce a molten metal that is poured between the working pieces of metal to form a welded joint.
It was developed by Hans Goldschmidt around 1895. For non-ferrous welding, or other uses of the thermite-type reactions, see the main thermite article.
Commonly the reacting composition is 5 parts iron oxide red (rust) powder and 3 parts aluminium powder by weight, ignited at high temperatures. A strongly exothermic (heat-generating) reaction occurs that produces through reduction and oxidation a white hot mass of molten iron and a slag of refractory aluminium oxide. The molten iron is the actual welding material; the aluminium oxide is much less dense than the liquid iron and so floats to the top of the reaction, so the set-up for welding must take into account that the actual welding material is on the bottom and covered by floating slag.
Thermite welding is widely used to weld railroad rails. The weld quality of chemically pure thermite is low due to the low heat penetration into the joining metals and the very low carbon and alloy content in the nearly pure molten iron. To obtain high-quality railroad welds, the ends of the rail being thermite welded is usually preheated with a torch to induce a good fusion with the working pieces of metal. Because the thermite reaction yields relatively pure iron, not the much stronger steel, some small pellets or rods of high-carbon alloying metal are included in the thermite mix; these alloying materials melt from the heat of the thermite reaction and mix into the weld metal.
The method was patented by John H. Deppeler Jr. in 1928 while working for the Metal and Thermit Corporation.
See also PEI/RP800.