Thermit welding is a fusion welding process in which no outside heat source is required for melting the components to be joined but still a very high temperature (3000 to 4000°C) is obtained from the exothermic chemical reaction (a reaction in which heat is released out) that takes place inside a thermit, which is employed for welding the joint. The thermit is a mixture of some metal oxide (mostly iron oxide or sometimes copper oxide) and a metal reducing agent, usually finely divided aluminium powder (although magnesium is also used sometimes).
Thermit welding is employed for welding steel and cast iron. Copper, nickel and manganese are also welded with this process. The main use of the process is for joining or repair of shafts, rails of railway tracks, machinery frames and gears and also for welding lugs to copper cables. Different types of thermits are employed for welding different metals.
The most commonly used thermit contains about one part by weight of aluminium and three parts of magnetic iron oxide (Fe3O4 ). For starting the chemical reaction, the thermit is required to be first heated to a temperature above 1206°C. A special ignition powder is employed to start the ignition of the thermit mixture by bringing it to the above mentioned temperature at which the chemical reaction starts. The ignition powder is, in turn, ignited by a burning magnesium ribbon, or with a match.
During thermit reaction, the aluminium oxide (Al₂O3) is formed and it floats as a slag over the molten iron that settles down in the bottom of the container (or mold prepared for the joint to be welded). The mold and crucible employed in a typical case of thermit welding are shown in Fig.
The chemical reaction is as below:
8A1+ 3Fe3O4 → Fe + 4A1₂03+ Heat
Another use of thermit welding technique is in the welding of lugs to copper cables. The joint to be made is housed in a graphite mold, and powdered copper oxide and powdered aluminium are placed in the joint and ignited by a spark. Process being exothermic, heat is given out which melts the ends of the cable leads and the lugs and a good joint is formed.
See More: Electroslag Welding(ESW)
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