Joining of Rail Lines by Thermite Welding

One of the oldest welding process that employs large amount of heat, produced by highly exothermic reactions of metal oxides. The weld made by thermite process are of extreme high quality and better mechanical properties and the process is still being used to weld large railway tracks. The process can be applied in two ways; either the heat produced from thermite process is used to melt the filler material to join the work parts or in the second case the parts to be welded are heated to forging temperature followed by pressing with force to join them.

How the Thermite Welding works?
Working Principle
Unlike an arc welding process, thermite (TW) don’t need the production of high temperature arc, instead uses the resultant heat of highly exothermic reaction, to make welds. Normally the reactants used in thermite welding are; metal and metal oxides. Highly active metal reduces other less active metal oxide to free metal. For example in joining of rail lines iron oxide and aluminum metal (powdered form) are employed where the aluminum metal reduces the iron oxide to free elemental form. This exothermic reaction results consideration amount of heat and the production of molten slag (aluminum oxide) and metal (iron). This is actually the molten iron which is used to fill the rail line joints while the slag floats on it due to its less density. Here the slag performs the same functions as required in any arc welding process.
                At first the ends of rail lines are prepared aligned with preset dimensions. Pre-heating of ends, is highly recommended in order to avoid any undesirable thermal gradient and frequent cooling of liquid metal. A (sand) mould is placed around the lines and pre-heating process is conducted with the use of heating source. A refractory crucible is placed on the mould assembly, where the thermite mixture is ignited for the exothermic reaction to take place. When the rail lines are heated to required temperature, the molten reaction products in the crucible are allowed to pour into the mould to fill the gap between the rail lines. The liquid metal enters into the joint gap while the molten slag floats on it due to its less density as mentioned earlier. After cooling the whole assembly is removed, disposing the slag off from the weld followed by finishing and grinding to make a clean and homogenized joint.  

Where the Thermite Welding can be applied?
However the welds produced, are of great mechanical strength having good corrosion properties due to high purity level of metals employed but the alloying materials can be included in thermite reactants to further enhance the quality of exothermic welds. The process is also applied to make non-ferrous (electrical conductive) welds; for example copper and copper alloys welding.

Moreover a wide range of materials and alloys can be fabricated by thermite process including; copper alloys, steels, cast iron etc. and even the best results were found in joining of dissimilar metals.