Thermit welding is the only welding process employing an exothermal chemical reaction for the purpose of developing a high temperature. It is a fusion process in which weld is effected by pouring super-heated liquid thermit steel around the parts to be united. The heat of “thermit” reaction is utilised to bring the surfaces of metal to be welded into plastic state and mechanical pressure is then applied to complete the weld.
It is based upon the chemical reaction between finally divided aluminium and iron oxide. The proportions are approximately three parts of iron oxide or magnetic iron scale or one part of aluminium.
The reaction that takes place is represented by the following chemical equations:
8Al + 3Fe3O4 —> 9Fe + 4Al2O3
During the reaction, oxygen from the iron oxide separates from it and combines with aluminium (which has a strong affinity for it) to form aluminium oxide, or slag or superheated thermit steel. During the reaction a very high temperature of the order of 2700°C is developed.
This temperature is nearly twice the temperature of the melting point of steel. The reaction is non-explosive and requires 30 seconds to complete. Handling and storing of mixture is dangerous, since an initial temperature of 1100°C is needed for ignition. The thermit mixture is kept dry, because if it becomes wet or damp, it cannot be restored to original state.
In making the pressure welds by thermit process, a wax pattern is made around the parts to be welded. Around this pattern a sheet box is placed as shown in Fig. 9.55 and the space between the pattern and box is filled with sand which is rammed. Pouring, heating and rising gates are cut in the sand. A gas flame is used which preheats the parts to be welded and melts the wax pattern.
This preheating is done before the liquid metal is poured into the mould, in order to avoid the chilling effects of the steel. Then the flame is removed and the heating gate is plugged. The superheated steel from the refractory lined crucible is poured into the mould surrounding the surfaces to be welded. When the welding temperature is reached mechanical pressure is applied to complete the weld.
Thermit welding is applicable in the repair of heavy parts such as tracks, spokes of driving wheels, broken motor castings, connecting rods and especially in the welding of pipes.
The advantage of using this process in welding large sections is that broken parts can be welded in their place but its application is limited as it requires the use of moulds etc.
The thermit welded joints have an average tensile strength of 5000 kg/cm2 with the yield strength of about 2500 kg/cm2 and elongation from 35 per cent to zero, thus approaching the properties of forged steel.