Thermite Welding

Exothermic welding which is also known as thermite welding is a process that uses heat from an exothermic reaction to produce coalescence between two metals.  The name is derived from ‘thermite’ which is given to reactions happening between metal oxides and reducing agents. The thermite heat consists of metal oxides having a low heat of formation and metallic reducing agents which, when oxidised, have high heat of formation. The excess heat generated from the reaction products provides the energy source to form the weld between two metals.

The powder consists of aluminium and the oxide of other metals like iron. Once heated, it gives off an enormous amount of heat which is a result of the chemical combination of aluminium with the oxygen of the oxide. The reaction temperature can rise about 2400° C.

For joining the steel parts, thermite welding is mainly used. Thus the common components of thermite welding are the iron oxide that is present in about 78% and the aluminium powder that is present in about 22%. The proportion of 78% and 22% respectively is determined by the combustion reaction of the aluminium and the reaction is as follows:-

8Al + Fe₃O₄→ 9Fe + 4Al₂O₃

The product of the combustion reaction which is iron and aluminium oxide is heated up to 2500°C which is equal to  4500°F. Therefore the sand or the ceramic mould is filled up with liquid iron. The slag which is an aluminium oxide that then floats up to the surface is then removed from the weld surface.

To repair steel castings and forgings, for joining railroad rails, steel wires and steel pipes, for joining the large cast and forged parts, thermite welding is used. 

Advantages and Disadvantages of Thermite Welding

Advantages of thermite welding are as follows:

1. It is a very simple procedure or process to join two similar or dissimilar metals together quickly. 

2. As no costly power supply is required, this process is very economical and convenient. 

3. The thermite process is used in a place where there is no availability of electricity.

Disadvantages of thermite welding are as follows:

1. Thermite welding is essentially used for parts of every section of ferrous metals.

2. It is proven to be economical for the welding of light parts and cheap metals.

3. It has a very slow welding rate.

4. The presence of a very high temperature may cause distortion and a change in grain structure in the welded region.

5. The welded region also contains slag contamination and hydrogen gas.

What is Thermite Mixture? 

It is a mixture of fine aluminium powder and iron oxide in a ratio of about 1:3 by weight.

Thermite formula is given by Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat

In this topic we have discussed what thermite is, Let’s understand the process of Thermite Welding.

Process of Thermite Welding

It is the most effective, highly mobile method used for joining heavy section steel structures like rails. The high heat input and metallurgical properties of the thermit steel make the process ideal to weld high strength, high hardness steels which are used in modern rails.

Thermit Welding always requires skilled labour for the welding process and must not be undertaken by someone who has not been trained to use it. Thermite welding results in a thermite reaction that involves the burning of thermite which is a mixture of iron oxide and fine aluminium powder present in the ratio of 1:3 by weight.

As a result of the reaction that is preheating of the thermite mixture up to 1300° centigrade the temperature reaches up to 3000° centigrade but it is essential in order to start the reaction. The reaction is as follows along with the diagrammatic representation of the thermite welding process. 

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There is a greater affinity for aluminium to react with oxygen. Therefore the aluminium reacts with ferric oxide in order to generate pure iron and slag Aluminium oxide. Aluminium oxide then floats on the top of the metal pool in the form of slag and the pure iron that is formed settles below. It is because there is a large density difference between the two.

Steps to be Followed while Doing Welding

1. A proper gap must be prepared between the two rails, which must be accurately aligned by means of straight edges to ensure the finished joint is perfectly straight and flat.

2. After the first step, the second step involves pre-formed refractory moulds that are manufactured to accurately fit it around the specific rail profile, are clamped around the rail gap and then sealed in position. Equipment used in locating the preheating burner and the thermit container is then assembled afterwards.

3. The weld cavity which is formed inside the mould is preheated using an oxy-fuel gas burner with accurately set gas pressures for a given time. The quality of the finished weld highly depends on the precision of this preheating process.

4. On completion of the preheating process, the container is fitted at the top of the moulds, the portion is ignited and the subsequent exothermic reaction produces the molten Thermit Steel. The container consists of an automatic tapping system enabling the liquid steel – which is at a temperature in excess of 2,500°C – to discharge directly into the weld cavity.

5. The welded joint is allowed to cool down for a certain amount of time before the excess steel and the mould material are removed from the top of the rail with the help of a hydraulic trimming device.

6. Once cold the joint is cleaned of all debris, the rail running surfaces are inspected. The finished weld must be inspected carefully before it is passed as ready for service.

Types of Welding

There are two types of welding:

Fusion Welding: It will heat and fuse the metal parts, the thermite mixture will act as a filler metal also.

Pressure Welding: It will heat the metal part and raise them for forging temperature and forging force is applied in order to join them.

Conclusion

In this article, we have learnt about thermite welding, its advantages and disadvantages along with the process of thermite welding. We also learnt about what steps to be followed while doing welding, and what are the other types of welding.