Thermite Welding of Rails (8 Step Process with Images)

Thermite Welding of Rails

What is Thermite Welding?
Definition: Thermite welding (also known as exothermic welding or exothermic bonding) is a fusion welding process in which the joining of metals takes place with the help of superheated molten metal and slag resulting from chemical reaction between aluminum and metal oxide.

During the thermite welding process, the pressure may or may not be applied depending upon the applications.

The heat produced in thermite welding process is just because of the exothermic reaction between aluminum and the metal oxide. No external heat or current is applied during the process.

Thermite Welding process is generally used for welding of rails. Hence the process is also referred as Thermite Rail Welding.

Principle of thermite Welding

The thermite welding principle is based on the Thermite reaction.

Chemical reaction (Thermite reaction) takes place between the metal oxide (usually iron oxide is used) and aluminum and a large amount of heat is obtained by this thermite reaction.

This heat ranges from 3000 °C to 4800 °C, which is enough to melt the metal.

This extreme heat melts the metal oxide as well as parent metal and finally the welded joint is formed on solidification.

Thermite Welding of Rails (8 Steps)

Rail thermite welding process can be done in 8 steps as mentioned below.

Step 1: Clean the joints

Step 1 Clean the joints

First of all, the metal surfaces (or the rail surfaces) which are to be welded are cleaned to get the perfect strong weld.

The surfaces can be cleaned by heating them using an oxy acetylene torch.

The dust particles, oil, grease, oxides, scales, etc must be removed during the cleaning process.

Step 2: Setup the rail gap

Step 2 Setup the rail gap

The rail tracks are aligned properly such that their axis coincides.

The gaps are necessary between the two rails so that the proper homogeneous welding takes place between the two rails.

Generally, 25 mm gap is laid between the two rails. For more information about the gaps between the rails, visit the table from this article.

Step 3: Align the rail and setup the clamps and mold

Step 3 Align the rail and setup the clamps and mold

While aligning the rails, it is necessary to look after the four important alignments.

  • Vertical alignment
  • Horizontal alignment
  • Rotational alignment
  • Distance between the rails

After this step, the clamps (or mold shoes) are applied to the rails joints.

The slag bowl is attached to the mold shoes which collects the overflowing slag and molten metal during the railroad thermite welding process.

Step 4: Prepare a thermite mixture in the crucible

Step 4 Prepare a thermite mixture in the crucible

The thermite mixture is filled in the crucible furnace or a reaction vessel.

This crucible or reaction vessel is conical in shape and it contains magnesia tar lining.

Outer surface of the crucible furnace is made up of sheet steel.

Step 5: Preheat the mold

Step 5 Preheat the mold

In thermite welding, preheating of mold is done for the following reasons.

  • To bring the rail part at a desired temperature to prevent the chilling of molten metal at the surface of the rails.
  • To dry the mold completely, otherwise the final welded joint may contain some impurities or porosity that weakens the weld.
  • To melt away and remove the impurities present in the mold.

There are various methods used for preheating the mold. A torch is specifically designed which burns propane, natural gas or gasoline.

Step 6: Ignite the thermite mixture

Step 6 Ignite the thermite mixture

To ignite the thermite mixture, it is contacted with a hot rod.

This ignition starts the reaction and liberates a high amount of heat, which will melt the thermite mixture.

This molten metal is passed between the rail gaps. 

The excess molten metal comes out and it is collected in the slag bowl.

Ignite the thermite mixture

Because of the extreme heat of the molten metal, the edges of the rails melts and the fusion welding takes place.

Step 7: Open the mold

Step 7 Open the mold

After the strong homogeneous fusion is made due to thermite welding, the mold is opened and some extra metal portion is removed with a hammer or any other tool.

Step 8: Finish the weld

Step 8 Finish the weld

The rail welds are finally given a finishing surface by grinding or machining.

Cutting torch is also used to remove the excess metal or to cut the gates or risers.

Thermite welding equations

The thermite welding chemical equations are mentioned below.

3Fe3O4 + 8Al  —->  9Fe + 4Al2O3 + 2O3 (3088 °C) 719.3 kcal

Fe2O3 + 2Al  —->  2Fe + Al2O3 (2960 °C) 181.5 kcal

3CuO + 2Al  —->  3Cu + Al2O3 (4865 °C) 275 kcal


External links:
Image credits: User:欅, CC BY-SA 3.0, via Wikimedia Commons, Trio3D via Wikimedia commons, Cjp24, CC BY-SA 4.0, via Wikimedia Commons, PetrS., CC BY-SA 3.0, via Wikimedia Commons

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