Notes on Thermit Welding

Thermit welding is the joining process in which the heat required for the melting and joining of components is obtained due to the exothermic chemical reaction. When ignition powder is initiated using the matchbox, the heat generated due to the burning of the igniting powder is used to trigger the chemical reaction in the thermit mixture, which is used for melting and joining plates, called thermit welding. The chemical reaction in thermit welding is

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

‘Fe₃O₄ + Al’ is called a thermit mixture, and due to the exothermic chemical reaction, three products are produced: 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 a much lower temperature, Such as Barium.

Thermit welding on Atlanta streetcar. Image source: Youtube video screenshot

Peroxide (Ba₂O₂), ' Ba₂O₂ + Al’, or powdered manganese are special igniting powders. A thin layer of this igniting powder, thereby initiating the reaction shown above, releases tremendous heat, producing a temperature of about 3000°C, changing the iron into a liquid state. The thermochemical reaction based on the ignition of the thermit mixture is

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

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