Thermit Welding

Thermit welding is the joining process in which heat required for melting and joining of components is obtained due to the exothermic chemical reaction. When the initiation of ignition powder is given by using the matchbox, the heat generated due to the burning of igniting powder and it is used for initiating the chemical reaction taking place in the thermit mixture is used for melting and joining of plates called as thermit welding. The chemical reaction in thermit welding is

8 Al + 3 Fe3O4 –> 9 Fe + 4 Al2O3 + Heat

‘Fe3O4 + Al’ is called thermit mixture, and due to the exothermic chemical reaction, three products are produced, they are iron (used as filler material), aluminum oxide (as slag) and heat for melting.

Thermit mixture (8Al + 3Fe3O4) can be ignited only at about 1200°C but special igniting powders are used to initiate the reaction at much lower temperature Barium.

Thermit welding on rail track

Thermit welding on Atlanta street car. Image source: Youtube video screenshot

Peroxide (Ba2O2) or ‘Ba2O2 + Al’ or powdered manganese are used as special igniting powders. A thin layer of this igniting powder, thereby initiating the reaction shown above which releases a tremendous amount of heat producing a temperature of about 3000°C which will change the iron into a liquid state.

The thermochemical reaction based on the ignition of Thermit mixture is

Metal oxide + Al (powder)Al2O3 + metal + heat.

Thermit welding is developed for joining of railway track rails in the remote areas like hill areas and forest areas. Also, thermit welding is used for repair of heavy parts such as tracks, spokes of driving wheels, broken motor castings, connecting rods, especially in the welding of pipes.

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