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Thermite Welding is a chemical welding process using molten metal to join the conductors permanently. It uses heat to join or merge two metals with the help of an exothermic reaction to form one mass or whole. It is also known as Exothermic welding, exothermic bonding and thermit welding since it uses exothermic reactions for its task. The process requires no external source of heat or current since it employs an exothermic reaction of the powder consists of aluminium and the oxide of a metal such as iron which is called thermite composition which was due to the result of the chemical combination of the aluminium with the oxygen of the oxide.
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Key Takeaways: Thermite Welding, Exothermic Reactions, Chemical Reactions, Heat Transfer, Corrosion, Mechanical Strength, Aluminium
Principle of Thermite Welding
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Thermite welding is a chemical welding process in a liquid state, through which the joints are formed in molten state. It combines the process of welding and casting in which the molten iron is poured at the welding plates so that it can form a permanent strong joint. The molten state of iron is created without application of any external heat. In thermite welding, a mixture of aluminium and iron oxide is used in ratio 1:3 by weight therefore the common thermite is composed of iron oxide (78%) and aluminium powder (22%).
Thermite Welding Process
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| Transition Metals | Group 15 Elements | Physical Properties of Metals and Non-metals |
| Periodic Classification of Elements | Halogenation | Reactivity Series |
Properties of Thermite Welding
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An exothermic weld has higher mechanical strength and excellent corrosion resistance compared to other forms of weld. When it is subjected to repeated short-circuit pulses, it comes out to be highly stable and does not suffer from increased electrical resistance over the course of the installation. However, thermite welding is relatively costlier when compared to other welding processes.
Process of Thermite Welding
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It is an aluminothermic reaction which means that it is exothermic chemical reactions using aluminium as the reducing agent at high temperature and it is carried out between aluminium powder and a metal oxide is used to supply the essential heat energy. The chemical reaction involves the burning of thermit, which may be a mixture of fine aluminium powder and iron oxide within the ratio of about 1:3 by weight.
Despite reaching a temperature of 3000°C , the preheating of the thermite mixture up to about 1300°C is essential in order to start the reaction.
8 Al + 3 Fe3O4 \(\to \) 9 Fe + 4 Al2O3 + (3000oC, 35 kJ/kg of mixture)
Aluminothermic Reaction
Working of Thermite Welding
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Thermite welding is a foundry and casting process which involves patternmaking, moulding, melting, pouring, ejection, cleaning, fettling, and inspection. The steps involved in thermit welding are:
Working of Thermite Welding
- In the first place, we clean both the work pieces which are to be welded.
- Now a wax pattern is made round the weld cavity.
- A mould handle clamp is used to fix the moulding flask round the joint.
- Now the moulding sand is packed around the wax pattern to form a mould in which the molten metal will get poured and it involves the gating system consisting of sprue, runner, and gates.
- Now this mould is heated to get rid of the wax pattern.
- This will start the thermite reaction which liberates an enormous amount of heat and forms a molten state of iron oxide which flows from crucible to the sand mould.
- This molten metal obtained from the thermite reaction fills the weld cavity and fuses the parent metal to make a permanent joint and then it is allowed to cool down.
- After removing the flask, machining is done to get rid of the welding burr or other extra metal.
We use different Thermite mixtures for welding different metals like copper and chromium. In place of ferrous oxides, different metal oxides are being used . Below are some of the typical thermite mixture reactions with their temperature obtained are given below:
3 CuO + 2 Al → 3Cu + Al2O3 + (4860°C, 275 Kcal)
Cr2O3 + 2Al → 2Cr + Al2O3 + (3000°C, 540 Kcal)
Types of Welding
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There are 2 types of Welding:
Fusion Welding
It is a process that joins or fuses two or more materials by heating them to melting point using the thermite mixture as a filler metal but it may or may not be required.
Fusion Welding
Pressure Welding
In this type of welding, the components are joined together by applying heat and pressure which can be generated either via an electrical current (resistance welding) or by friction (friction welding). It includes many processes like gas pressure welding, friction welding, resistance welding, diffusion welding, ultrasonic welding, and explosion welding.
Pressure Welding
Advantages of Thermite Welding
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- It does not require any external energy input, so it can be used in areas where electrical power is not available.
- It is very simple to join two similar or dissimilar metals together quickly
- No power supply which is costly is required making the process cheap
- The joint formed between the steel rail and the copper conductor has an excellent current-carrying capacity and the ability to withstand corrosion, when Aluminium reduces copper oxide powder to form a metallurgical bond.
Disadvantages of Thermal Welding
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- It is essential to use thermite welding for parts of heavy sections of ferrous metals and not economical for welding cheap metals and light parts.
- It requires a supply of replaceable moulds and the rate of welding is also very slow and may contain gas and slag inclusion
- It suffers from a lack of repeatability, and can be impeded by damp or bad weather conditions if it is performed outdoors.
- When the temperature is high, it may cause a change in the grain structure and the weld metal can get distorted.
Uses of Thermite Welding
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Thermit Welding is used for fixing heavy castings and gears. It is used in welding of very huge cast and produced parts and large sections such as locomotive rails, ship hulls or joining railroad rails, steel wires and steel pipes. It is also used to weld a wide range of metals, which includes stainless steel, cast iron, common steel, brass, bronze, and Monel. The thermite welding process is also utilised to weld electrical conducting joints, particularly to supply electrical continuity for railroad signal systems. Aluminium reduces copper oxide powder to form a metallurgical bond between the steel rail and the copper conductor.
Things to Remember
- Thermite mixture comprises of a mixture of finely divided aluminium and metal oxide with the addition of any required alloying metals
- There is a mould which is created either by graphite or sand around parts that need to be welded and the molten metal is then poured onto it.
- Ignite powder can achieve temperature upto maximum 1300 degree centigrade which is essential to start thermite reaction.
- The hottest burning substance is thermite which is the man-made hottest burning substance that is available.
- Copper thermite is the best choice to weld railways rails for the electric joint using the copper oxide.
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Sample Questions
Ques: What equipment is needed for thermite welding? (2 Marks)
Ans: The equipment that are needed for thermal welding are: a thermite charge, which is made up of iron oxide and aluminium powder. To prevent oxidation, aluminium is encased in a protective paper or metal shell since aluminium tends to react with oxygen. Before beginning the process, the pieces of metal that are to be joined together are pre-heated. The thermite welding work requires heating of both the surfaces to be hot enough so that the reaction happens.
Ques: How does thermite welding work? (2 Marks)
Ans: The thermite reaction is the result of the oxidation of metal powders in presence of an exothermic reaction that produces so much heat which can be enough to melt the two metals and form a weld. In a thermite reaction, the first step involves mixing of two substances, metal oxide, and iron(III) oxide. The resultant mixture which is obtained from mixing will produce heat without any spark or flame needed. We mix the aluminium powder with another type of metallic fuel, like a magnesium strip or zinc dust and then we add a small amount of water to the mixture. After this, we ignite the mixture with a small flame. When heated, the aluminium reacts violently thus producing heat which is enough to melt any metals in its presence.The reaction will continue until all of the metallic fuel, like a magnesium strip or zinc dust has been consumed or the aluminium powder has been burned off.
Ques: Why do we use thermite welding? (2 Marks)
Ans: Thermite welding is simple, straight-forward and a quick friction-based joining technique that doesn’t take a lot of time when we compare it with other metal welding techniques. Thermite is a much simpler process that does not require the surfaces to be prepared in any way and can be carried out on unclean surfaces as well. It joins without the use of any flux thus making the job very easy to do. It does not require any air or oxygen because of the fact that the aluminium powder will combust with a small flame. This makes it an environmentally-friendly technique. Thus, Thermite welding is an excellent choice for many different reasons for joining metals!
Ques: Can Thermite Burn Under the water? (2 Marks)
Ans: A high temperature of a hot burning strip of magnesium is needed to ignite a mixture of iron oxide rust and aluminium powder (thermite). It is practically impossible to extinguish the fire, once the thermite mixture ignites. So much heat is released from the resulting reaction that it can burn through the pavement and engine blocks. The heat is so intensely high that it can burn even underwater.
Ques: Who shouldn’t bother with thermite welding? (2 Marks)
Ans: If anyone does not feel confident in working with thermite welding, then he should not attempt it. If you are using any type of unrefined metal like aluminium dross or recycled metal then you should also not try to use thermite welding. On hollow tube sections or on the materials that cannot be completely coated with molten metal during the process, the thermite welding is highly unsafe for use.
Ques: Is thermite welding safe? (2 Marks)
Thermite welding is considered to be a safe process when it is done with proper care. It should not be carried out by anyone who is a noob and does not have training and experience in using such a process. If you are using any type of unrefined metal like aluminium dross or recycled metal then you should also not try to use thermite welding.
Ques: What are the industrial uses of thermite welding? (2 Marks)
Ans: In Industry, the thermite welding is commonly used as an alternative to other types of welding methods, such as arc welding, gas tungsten arc welding (GTAW), and gas metal arc welding (MIG) when there’s a need for large volumes of solid and reliable welds. It can also be used on thin materials, allowing thermite welding to provide a more controllable process and is useful especially when joining parts of a different size together or needing very close tolerances for the finished product.
Ques: What is the maximum temperature in thermite welding? (2 Marks)
Ans: The maximum reachable temperature in thermite welding reaches upto 3090 °C (5600 °F)which is to be reduced quickly down to about 2480 °C (4500 °F) by adding non reacting pellets of ferroalloy, because of the fact that at 2500 °C (4530 °F) the aluminium vaporises. However , Because of the alumina slag (Al2O3), the Thermite-welding temperature cannot be much lower as it has to allow the liquid to float and separate from the metal, which would solidify at a temperature lesser than or equal to 2040 °C (3700 °F)
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