Thermit 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 oxidized, 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.
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 is thermite, Let’s understand the process of Thermit Welding.
Process of Thermite Welding
It is a 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.
Steps to be Followed While Doing Welding
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.
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.
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.
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.
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.
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.