Thermit Welding – Diagram ,Process , Advantages and Disadvantages
Introduction to Thermit Welding:
It is a process in which a mixture of aluminium powder and a metal oxide, called Thermit, is ignited to produce the required quantity of molten metal by an exothermic non-violent reaction. The superheated metal so produced is poured at the desired place which on solidification results in a weld joint. It is thus a casting cum welding process.
This process has been used since 1895 for joining rails at site and for the repair and salvage of heavy castings. The process is, however, slowly going out of use due to its high cost and because of the development of more efficient, highly mechanised and less costly processes like flash butt welding and electroslag welding.
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:
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.
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.
(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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