Thermite Welding or Exothermic welding is a welding process for joining materials that employs molten metal to permanently join the conductors.
Exothermic welding process employs an exothermic reaction of a thermic composition of heat the metal and requires no external source of heat or current.
An aluminothermic reaction between aluminium oxide powder and metal oxide is the chemical reaction that produces the heat.
Exothermic welding was developed by Hans Goldschmidt around 1895. The first non-ferrous application for exothermic welding was developed in 1938 by Dr. Charles Cadwell, a professor at the Case School of Applied Science, later Case Western Reserve University, in Cleveland, Ohio. The original use of the process was to weld signal bonds to railroad tracks.
The method was patented by John H. Deppeler in 1928 while working for the Metal and Thermit Corporation. It is United States patent number 1671412.
In the United States, an investigation into exothermic rail welding was conducted by the Committee On Welded Rail Joints,with the goal of improving and standardizing rail welding. Composed of members from the American Bureau of Welding and the American Electric Railway Engineering Association, this committee conducted extensive physical testing of welded rail joints, as well as testing of various parameters of the welding process.
An exothermic weld has higher mechanical strength than other forms of weld, and excellent corrosion resistance It is also highly stable when subject to repeated short-circuit pulses, and does not suffer from increased electrical resistance over the lifetime of the installation. However, the process is costly relative to other welding processes, requires a supply of replaceable moulds, suffers from a lack of repeatability, and can be impeded by wet conditions or bad weather (when performed outdoors)
Thermite welding is widely used to weld railway rails. One of the first railroads to evaluate the use of thermite welding was the Delaware Hudson in 1935 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 sound railroad welds, the ends of the rails being thermite welded are preheated with a torch to an orange heat, to ensure the molten steel is not chilled during the pour. 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 alloying beads composition will vary, according to the rail alloy being welded.