What is Thermit Welding? – Complete Explanation


Thermit Welding is a welding process in which heat produced during an exothermic reaction is used to weld two metal pieces (conductors) together. No external heat source is required during this welding process, but only an external heat source is needed to initiate the process. thermite material ( mixture of a metal oxide, aluminium powder, and fuel) is used for the welding process. The chemical reaction that produces heat is an aluminothermic reaction. The reaction of aluminium powder with a metal oxide to produce heat is called an aluminothermic reaction.

  • This welding process is also known as exothermic welding, exothermic bonding, and thermit welding.
  • It was invented in the year 1898 by Hans Goldschmidt when he improved the aluminothermic reaction.

Working principle

In the thermit welding process, a thermite material which is a composition of a metal oxide, aluminium, and fuel is used to achieve an exothermic reaction. A reaction between the metal oxide (mostly iron (iii) oxide) and aluminium powder takes place. During the reaction, the metal oxide reduces to free elemental metal and aluminium get oxidized to aluminium oxide with production of a large amount of heat (about 3310 oC). The heat generated melts the elemental metal ( Fe) and then this molten metal is poured into the mold to join the two metal pieces together.

During the aluminothermic reaction, the reduction of iron oxide and oxidation of Al takes place. The large amount of heat generated during the process melts the iron and aluminium oxide and we get molten iron and refractory slag of aluminium oxide. The density of the aluminium oxide slag is much less than the liquid iron, so it floats above the molten iron.

So during the welding process, the molten iron is present at the bottom of the crucible, and slag floats above it.

If we talk about the composition of the metal oxide and aluminium powder, then 5 parts of iron oxide and 3 parts of aluminium powder by weight are mixed before initiating the welding process.

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Main Parts of Thermit Welding

Thermit Welding Process
  1. Crucible: It is that part which is capable to withstand in high temperature condition. In thermit welding process, the crucible contains thermite material. The exothermic process during the welding process takes place in the crucible. The molten iron present at the bottom of the crucible and slag of aluminium oxide floats over the molten metal. The crucible contains a taping device to discharge the molten metal to the mold for the welding.
  2. Thermite mixture: It is a pyrotechnic composition of metal oxide, aluminium powder, and fuel.
  3. Mold: A mold is created around the section to be welded. The mold consists of runner, riser, slag basin, heating gate. The molten metal is poured into the mold for the joining of the metal.
  4. Taping device: It is device which is used to discharged or pour molten metal form the crucible to the mold.

Working of Thermit Welding

  1. A mold is formed over and around the section to be welded. After the mold is formed thermite material is placed in the crucible.
  2. The thermite material is ignited by the use of flint sparker or magnesium ribbon. An exothermic reaction takes place in the crucible called an aluminothermic reaction.
  3. During the reaction metal oxide i.e. iron oxide reduces to iron and Al gets oxidized and forms aluminium oxide. A large amount of heat is generated. This heat generated melts the iron and aluminium oxide and form molten iron and slag of aluminium oxide.
  4. The molten metal so formed is poured into the mold through taping device. As the molten metal reaches the section, it fuses with the metal pieces to be joined. After solidification, it forms a very strong weld.

For a better explanation watch the video given below:

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  • In thermit welding, the weld formed has high mechanical strength and excellent corrosion resistance as compared with other forms of weld.
  • It is highly stable to repeated short circuit pulses and the electrical resistance remains unchanged over the lifetime of the installation.
  • It does not require any external power source.
  • It can be used at the place where power supply is not easily available.


  • It is a costlier welding process in comparison with other welding processes.
  • It requires the supply of replaceable molds.
  • Lack of repeatability.
  • It cannot be used in wet conditions or bad weather when working outdoors.


The thermit welding process is mainly used for rail welding in railways. It is also used or welding copper conductors and other metals that include cast iron, stainless steel, common steels, bronze, brass and monel.

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