Thermit Welding Process – Main Parts, Working Principle with Application

 Railroad thermite welding is a common method in railway construction. As we know, there is an interface at the rail joint between two steel rails. There is an issue in the railway industry. When the train passes away, the enormous pressure will cause shake even crack. These problems will threaten the service life of the steel rails and the safety of trains. So how solving the interface is an important project for rail technology.

 

With the development of the manufacturing industry, there are two main methods to weld the railroad tracks. One is electric soldering, and another way is thermite welding. What is thermite welding? Railroad thermite welding is regarded as the most high-efficiency method to weld railroad tracks. Because of the high efficiency, reliable quality and high adaptability, railroad thermite welding becomes the most popular way of railroad construction. The application of thermite welding is widespread. In railroad construction, it is used to weld railroad tracks. The seamless rail can be made by thermite welding technology.

 

The working principle is easy to understand. After setting the thermite welding machine, we preheat the steel rails to 600 degrees. Then put the welding portion into the crucible and light the welding portion. The welding portion will produce molten steel and slag. The high-temperature molten steel will fuse the interface between two steel rails.

 

Main Parts of Thermit Welding

Crucible: It is that part that is capable to withstand high-temperature conditions. In the thermite 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 aluminum oxide floats over the molten metal. The crucible contains a taping device to discharge the molten metal to the mold for welding.

Thermite mixture: It is a pyrotechnic composition of metal oxide, aluminum powder, and fuel.

Mold: A mold is created around the section to be welded. The mold consists of a runner, riser, slag basin, and heating gate. The molten metal is poured into the mold for the joining of the metal.

Taping device: It is a device that is used to discharge or pour molten metal from the crucible into the mold.

 

Working principle

In the thermit welding process, a thermite material which is a composition of metal oxide, aluminum and fuel is used to achieve an exothermic reaction. A reaction between the metal oxide (mostly iron (iii) oxide) and aluminum powder takes place. During the reaction, the metal oxide reduces to free elemental metal and aluminum gets oxidized to aluminum oxide with the 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 aluminum oxide and we get molten iron and refractory slag of aluminum oxide. The density of the aluminum 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 aluminum powder, then 5 parts of iron oxide and 3 parts of aluminum powder by weight are mixed before initiating the welding process.

 

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Operation of Thermit Welding:

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 molding box is kept around the joint and refractory sand is packed carefully around the wax pattern, providing the necessary pouring basin, sprue, 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 extremely 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.

 

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.

 

The 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.

 

Application

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.

Rail welding expert

CRS is a leading manufacturer of thermit welding materials in China, we have more than 30 years experience of designing thermit welding to the rails. Focusing on rail welding used for railroad, metro, tram, crane rails, turnel construction and maintenance. If you are interested in our products, please contact us now!