Thermit welding is a chemically reaction welding process. The weld joint is produced
by pouring of superheated molten metal around the joint to be welded, applying with
or without of pressure. Thermit welding basically called a mixture of finely divided
metal oxide and a metal reducing agent as aluminium.
Thermit welding principle:
The necessary heat for joining metal of thermit welding is obtained from chemical
reaction of metal oxide and metal reducing agent. Usually iron oxide is used as a
metal oxide and aliminium or magnesium is used as metal reducing agent. The strong
chemical attraction of aluminium for oxygen is the basis for thermit process. First
the thermit mixture is ignited by a burning magnesium ribbon. The ignited temperature
of thermit is about 12000C. When ignited in one spot of mixture, the heat reaction
spreads through the mass. The aluminium merging with the oxygen of metal oxide and
setting free the iron, which is deposited on joint portion into the mold as a highly
superheated liquid metal. If theoretical temperature is about 30000C of thermit,
due to chilling effect of crucible the temperature is reduced about 25000C. So it
is sufficient for welding temperature.
The thermit reaction is required about thirty second to ignite and it is non-explosive
Thermit welding diagram
Some chemical reaction:
3Fe3O4 + 8Al → 9Fe + 4Al2O3(719.3 Kcal)
Fe2O3 + 2Al → 2Fe + Al2O3(181.5 Kcal)
3MnO + 2Al → 3Mn + Al2O3(403 Kcal)
3CuO + 2Al → 3Cu + Al2O3(275.3 Kcal)
Thermit welding application :
It has large application in railway track repairing on the spot.
It is used in large frame like locomotive frame welding.