One of the oldest welding process that employs a large amount of heat, produced by highly exothermic reactions of metal oxides. Welds made by thermite process are of extremely 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 a press force to join them.
Things to know in thermite process
Crucible – high-temperature resistant container in which exothermic chemical reaction takes place.
Mold – hollow part placed around the joint, in which molten metal is poured and solidified. A mold should have smooth contours to allow homogenized and appropriate distribution of liquid metal.
Mixture (Charge) – The reactants that take part in chemical reaction usually in fine powdered form.
Wax Pattern – placed inside the mold, giving the required shape and size to the joint.
Thermite Temperature – The maximum temperature required to ignite the thermite mixture to start reaction in crucible. After ignition, high temperature of over 4000 degree Fahrenheit is attained that helps in producing molten metal.
What is thermite?
A chemical mixture of finely ground aluminum and a metallic oxide (usually in powdered form), which on ignition makes a high temperature exothermic reaction, producing a large amount heat and molten metal.
Video of Joining Rail Road tracks
How does the Thermite Welding work?
Unlike an arc welding process, thermite (TW) don’t need the production of the 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 oxides 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 the 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 of the slag from the weld followed by finishing and grinding to make a clean and homogenized joint.
The joint to be welded should be prepared as per the recommended requirements. A fragile-wax pattern is placed between the joint and mold which melts and dries out, on heating to produce the cavity of required shape. Crucible is mounted on the mold assembly. The molten metal thus produced by high-temperature chemical reaction of thermite charge (mixture) in a crucible, is then poured into the mold. A mold should have proper ventilation to facilitate the escape of gases. The molten metal fills inside the cavity produced by wax pattern and solidifies into a firm weld joint. After solidification, the crucible is removed from the mold assembly and the mold assembly is dismantled for final grinding and finishing of weld surface.
As stated earlier the parts to be joined should have a clean surface finish free from all unwanted foreign particles like; grease, oil, metal fins, etc. The rail lines must be aligned as per the recommended tolerances and requirements as a slight negligence may cause severe defects in the final weld.
A mold assembly is set in place by making a sand mold around the joint. A pattern along with the sand mold is clamped around the lines and a suitable gas torch is used to pre-heat the sand mold and joint parts.
Thermite Ignition in Crucible
The crucible with thermite charge is ignited when the parts gain an adequate temperature. A liquid metal is prepared as a result of high temperature reaction between aluminum, metallic oxides and other alloying elements, which is then tapped into the mold at crucible bottom.
The liquid metal flows into the sand mold between the gap and making a firm weld after solidification. A mold must have the following components to allow appropriate distribution of metal inside it; pouring gate, basin, riser, and ventilation holes. A slag produced as a by-product, being lighter floats over the liquid metal where it can be easily separated.
After cooling for some time, the whole assembly is removed away from joint. Weld is then cleaned by chipping or grinding procedures to give a smooth finished shape.
Where the Thermite Welding can be applied?
The welds produced, are of great mechanical strength having good corrosion properties due to high purity level of metals employed. However, some alloying materials can also 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 welding of copper and copper alloys.
Moreover, a wide range of materials and alloys can be fabricated by thermite process including; copper alloys, steels, cast iron, etc and even best results have been found in joining of dissimilar metals.