Thermit Welding Method

Thermit welding method (TW) is a welding method with a process that uses heat from an exothermic reaction to produce coalescence between metals. The name is derived from ‘thermite’ the generic name given to reactions between metal oxides and reducing agents. The thermite mixture consists of metal oxides with low heats of formation and metallic reducing agents which, when oxidized, have high heats of formation. The excess heats of formation of the reaction products provide the energy source to form the weld. The reaction of aluminium powder with a metal oxide to produce heat is called an aluminothermic reaction.
Let show you an example; When applied to the reduction of Iron oxides, the exothermic reaction generates sufficient energy to raise the
reaction product temperature to in excess of 3,000°C at which both the metal and aluminium oxide are both liquids:

Iron Oxide + Aluminium -> Aluminium Oxide + Iron + Heat

In this post, let’s see:  What are the thermit welding method, the processes and the principle.

History of Thermit Welding Method

In 1837 Theodore Goldschmidt founded the Goldschmidt Company in Berlin to supply chemicals to the textile industry, and established a research and development facility investigating processes for refining metals and their oxides. In March 1895, Dr Hans Goldschmidt was granted German Patent #96317 for a “process to manufacture metals and alloys” based upon the reduction of heavy metal oxides by a more reactive metal such as aluminium.

Thermit Welding Processes

The process was first used to provide the thermal energy for a method for forge welding rails in 1899, when a number of welded joints were installed in the Essen Tramway. The process was first used in the UK to weld tram rails installed in Leeds in 1904.

Thermit Welding Processes

Further development leading to the addition of alloying elements to the basic Aluminothermic reaction produced a steel with a compatible metallurgy to the parent rails, thus enabling a full fusion welding process to be developed. While early welds were produced by casting the Thermit steel into hand produced moulds encompassing the two rails to be joined together, subsequent development lead to the introduction of pre formed refractory moulds designed to fit specific rail profiles.

While the basic Aluminothermic process still forms the heart of the Thermit welding processes, continuous development coupled with modern production technology, statistical process control and quality assurance, has resulted in processes which more than match the service demands of modern high speed, high axle load railway systems.

  • Conventional “Flat bottom” or “Vignole” Rails
  • Special Section Railway rails
  • Grooved Tram Rails
  • Heavy Section Crane Rails
  • Electrical Conductor Rail

In addition, products are available to suit special types of track support, confines spaces, environmental restrictions, and for joining rails of differing types or with differing degrees of wear.
In each case, should products not be available from our own manufactured stock, we will either design and manufacture the products to suit, or call on the extensive range available from within the Thermit Group.

The Principle of Thermit Welding

Thermit welding is an effective, highly mobile, method of joining heavy section steel structures such as rails. Essentially a casting process, the high heat input and metallurgical properties of the Thermit steel make the process ideal for welding high strength, high hardness steels such as those used for modern rails.

Thermit Welding is a skilled welding process and must not be undertaken by anyone who has not been trained and certificated to use it.

Detailed operating instructions  are provided for each of our processes, but the welding methods all comprise of 6 main elements:

  • A carefully prepared gap must be produced between the two rails, which must then be accurately aligned by means of straightedges to ensure the finished joint is perfectly straight and flat.
  • Pre-formed refractory moulds which are manufactured to accurately fit around the specific rail profile are clamped around the rail gap, and then sealed in position. Equipment for locating the preheating burner, and the Thermit container is then assembled.
  • The weld cavity formed inside the mould is preheated using an oxy fuel gas burner with accurately set gas pressures for a prescribed time. The quality of the finished weld will depend upon the precision of this preheating process.
  • The Thermit Portion is manufactured to produce a steel with a metallurgy compatible with the specific type of rail to be welded. On completion of the preheating, the container is fitted to the top of the moulds, the portion is ignited and the subsequent exothermic reaction produces the molten Thermit The container incorporates 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.
  • The welded joint is allowed to cool for a predetermined time before the excess steel and the mould material is removed from around the top of the rail with the aid of a hydraulic trimming device.
  • When cold the joint is cleaned of all debris, and the rail running surfaces are precision ground the profile. The finished weld must then be inspected before it is passed as ready for service

Once correctly installed, the finished weld is expected to last the life of the rail, with no further maintenance.

It is estimated that over 100,000 Aluminothermic welds are manufactured in track each year, with a total population in excess  of 1.8 million. The processes are used throughout the year on site with no heavy  equipment other than that which may be carried to site by the two man welding team. All the equipment, ancillary tools such as lighting, gases and consumables can be transported to site by small commercial vehicles.

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.

Refer the video below to know more details: (Youtube source: Wolfgang Lendner Chanel)

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