Thermit Welding: Process, Operation and Uses (With
Diagram)
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Process of Thermit Welding:
Thermit welding is a chemical welding process in which an exothermic chemical reaction
is used to supply the essential heat energy. That reaction involves the burning of Thermit,
which is a mixture of fine aluminum powder and iron oxide in the ratio of about 1:3 by
weight.
Although a temperature of 3000°C may be attained as a result of the reaction, preheating
of the Thermit mixture up to about 1300°C is essential in order to start the reaction.
The mixture reacts according to the chemical reaction:
8 Al + 3 Fe3O4 → 9 Fe + 4 Al2O3 + heat (3000˚C, 35 kJ/kg of mixture)
Aluminum has greater affinity to react with oxygen; it reacts with ferric oxide to liberate
pure iron and slag of aluminum oxide. Aluminum oxide floats on top of molten metal pool
in the form of slag and pure iron (steel) settled below, because of large difference in
densities.
Operation of Thermit Welding:
Thermit welding process is essentially a casting and foundry process, where the metal
obtained by the Thermit reaction is poured into the refractory cavity made around the
joint.
The various steps involved in Thermit welding are:
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1. The two pieces of metal to be joined are properly cleaned and the edge is prepared.
2. Then the wax is poured into the joint so that a wax pattern is formed where the weld is
to be obtained.
3. A moulding box is kept around the joint and refractory sand is packed carefully around
the wax pattern as shown in Fig. 7.40, providing the necessary pouring basin, sprue, and
riser and gating system.
4. A bottom opening is provided to run off the molten wax. The wax is melted through this
opening which is also used to preheat the joint. This makes it ready for welding.
5. The Thermit is mixed in a crucible which is made of refractory material that can
withstand the extreme high heat and pressure, produced during the chemical reaction.
6. The igniter (normally barium peroxide or magnesium) is placed on top of the mixture
and is lighted with a red hot metal rod or magnesium ribbon.
7. The reaction takes about 30 seconds and highly super-heated molten iron is allowed to
flow into the prepared mould cavity around the part to be welded.
8. The super-heated molten metal fuses the parent metal and solidifies into a strong
homogeneous weld.
9. The weld joint is allowed to cool slowly.
There are different Thermit mixtures available for welding different metals,
such as copper and chromium. They use different metal oxides in place of
ferrous oxide. Some typical Thermit mixture reactions with their
temperature obtained are given below:
3 CuO + 2 Al → 3Cu + Al2O3 + Heat (4860°C, 275 Kcal)
Cr2O3 + 2Al → 2Cr + Al2O3 + Heat (3000°C, 540 Kcal)
Application and Uses of Thermit Welding:
Thermit welding is a very old process and now-a-days, in most cases, it is replaced by
electro-slag welding. However, this process is still in use.
Some applications are:
(i) Thermit welding is traditionally used for the welding of very thick and heavy plates.
(ii) Thermit welding is used in joining rail roads, pipes and thick steel sections.
(iii) Thermit welding is also used in repairing heavy castings and gears.
(iv) Thermit welding is suitable to weld large sections such as locomotive rails, ship hulls
etc.
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(v) Thermit welding is used for welding cables made of copper.
Advantages of Thermit Welding:
1. Thermit welding is a simple and fast process of joining similar or dissimilar metals.
2. This process is cheap, as no costly power supply is required.
3. This process can be used at the places where power supply is not available.
Disadvantages of Thermit Welding:
1. Thermit welding is essentially used for ferrous metal parts of heavy sections.
2. It is uneconomical for welding cheap metals and light parts.
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