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Exothermic welding, also known as exothermic bonding, thermite welding (TW), and thermit welding, is a welding process that employs molten metal to permanently join the conductors. The process employs an exothermic reaction of a thermite composition to heat the metal, and requires no external source of heat or current.
Thermit welding process: The process mainly used for joining steel pieces, here thermit comprises iron oxide and aluminum powder. The ratio is 78% iron oxide and 22% aluminum powder. The ratio is decided by the chemical reaction at the burning of aluminum:
8Al + Fe3O4 = 9Fe + 4 Al2O3 + heat (4500˚C, 35 kJ/kg of mixture)
The combustion of iron and aluminum oxide generates heat up to 4500 Degree Fahrenheit. As these two components have a different density, they become separated automatically. The liquid iron fills the ceramic mold built around the welded parts and aluminum oxide slag floats up and removed subsequently.
Steps of Thermit Welding: The thermit process developed by Hans Goldschmidt in the mid-1890s and now used in the repair of steel casting and forging. They design the thermite welding for joining railroad rails, steel pipes, steel wires, a larger cast, and forged components.
Essential Definitions:
1. Crucible - It is a vessel where thermite chemical reaction, aluminothermic happens is called a crucible.
2. Mould - The mold is created around the part that requires it to be welded. This mold is destined to receive the molten metal.
3. Mixture - The mixture is the term to describe granules of aluminum and metal oxide. and any alloying metals.
4. Reaction - The aluminothermic process is a chemical reaction between aluminum and metal oxide. The reaction results in super-heated molten material and aluminum oxide slag to be separated.
Thermit-welding-set-up:
1. It is mandatory to clean and prepare the edges properly the metal parts we will weld.
2. The wax pattern is the need for the joint to be fused. Pour the hot wax to obtain the one.
3. Place the molding box around the joint and the sand is loaded carefully all over the wax pattern. It provides the necessary pouring basin, riser, sprue, and gating system.
4. The bottom opening is used to drain the molten wax which helps to preheat the joint and make it ready for the welding.
5. It is time to mix the thermit in a crucible. This is made from refractory material to withstand the extreme heat and pressure which is produced during this aluminothermic reaction.
6. The igniter like barium oxide or magnesium is left on the head of the mixture and lighted with red hot metal rods.
7. The reaction completes in a very short time, a high heated molten iron flows into the mold cavity which we have created around the joint to be fused.
8. The heated molten material joins the parent metals & solidifies to a homogenous strong joint.
Thermit Welding Steps for Rail:
1. The rail edges to be welded preparation
2. Setting the weld gap
3. Apply clamp
4. Fix the mold
5. Leave the thermit mixture into the crucible
6. Preheating of the ends of the rail
7. Ignite and let steel flow
8. Time to remove the mold
9. Remove the excess head metal
10. Initiate the grinding
11. Final clean grinding
Pros of Thermit Welding:
1. It is a simple fast method of joining two similar or dissimilar conductive materials.
2. It is a very economical process where power supply is not required.
3. Thermit welding is possible in distant places where there is no electric supply.
Cons of Thermit Welding:
1. It is possible to use for ferrous metal parts of bigger sections.
2. Light metal and cheap metal fusion are not very economical.
3. Welding rate is slow.
4. High temperatures may cause a change in the grain structure and distortions of the weld metal.
5. Welding may contain gas and slag inclusion.
#thermitewelding #Exothermicwelding #railjointing