What is Thermite Welding?
Thermite welding is a liquid state welding process in which the metal to metal joint created by melting of base material or applying filler material in liquid form. This welding is mostly used to welding railroads and electric connectors.
Principle
Thermite welding is a liquid state chemical welding process, in which joint formation takes place in a molten state. In this type of welding, a mixture of aluminum and iron oxide is used in ratio 1:3 by weight. This mixture chemically reacts as follow:
8 Al + 3 Fe3O4 –> 9 Fe + 4 Al2O3 + Heat
This reaction gives aluminum oxide and iron and also liberates a huge amount of heat which covers this mixture into a molten state. Because molten iron have a higher density compare to aluminum oxide so it remains bottom of the crucible. There is an opening at bottom of the crucible from where, this molten iron poured at the required plates where we want to create jointly.
Equipment
Refectory Crucible
The thermite mixture of iron oxide and aluminum reacts in a refectory crucible. This crucible is made by graphite which can handle temperature around 3000°C. There is a gate at bottom of the crucible for exit of molten metal.
Thermite Mixture
A mixture of aluminum and iron oxide in proper ratio known as thermite mixture. The ratio of aluminum to iron oxide is about 1:3 by weight.
Mold
In the thermite welding mold is created by either graphite or sand. Graphite molds are permanent mold which is used to make various similar joints. Sand mold are used where the joint design is different every time.
Wax Pattern
Wax pattern is used to make sand mold around the welding work pieces. The sand is rammed around the wax pattern to make sand mold. After proper ramming action, the mold is heated which removes the wax pattern by melting of it.
Molding Flask
The sand mold creates into the molding flask. The wax pattern, which is created around the weld cavity, is placed at the middle of the flask. The molding sand rammed into the flask to make sand mold.
Mold handle clamp
This is a clamp which is used to fix molding flask around the welding plates.
Ignite powder
It is a highly inflammable powder that can achieve a maximum temperature of 1300°C which is essential to start a thermite reaction.
Working
Its working can be summarized as follow-
- First both the work pieces which are needed to be weld, are cleaned.
- Now a wax pattern is created around the weld cavity.
- A molding flask is fixed around the joint with the help of mold handle clamp. This wax pattern is situated at the middle of the flask.
- Then the molding sand rammed around the wax pattern to create mold in which the molten metal will pour. This mold involves all necessary parts like runner, riser, pouring basin, gating system, opening for wax pattern etc. same as involves in casting.
- Now this mold is heated to remove wax pattern. The wax is melted and run off from the wax pattern outlet prepared at bottom of the sand mold.
- After that, the thermite mixture is taken into the refectory crucible. The ignite powder is placed over the mixture. This mixture is ignited by a magnesium ribbon.
- This will start the thermite reaction which liberates a huge amount of heat. This reaction form molten state of iron which flows from crucible to sand mould.
- The molten metal fills the weld cavity and fuses the parent metal to make a permanent joint. This will allow to cool down. After proper cooling, flask is removed from the joint.
- After removing the flask, machining is done to remove the welding burr or other extra metal.
