What Is Exothermic Welding?
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.
Exothermic welding is a process used for joining similar metals, such as copper to steel or copper to copper. It employs an exothermic reaction to form a molecular bond between the two pieces.
The connection is produced in a purpose-designed graphite mold. The weld employs an exothermic reaction of thermite (powdered metal) to heat the metal. Put simply, the exothermic process involves a chemical reaction that releases heat and requires no external source of heat. The chemical reaction that produces the heat is an aluminothermic reaction between aluminum powder and metal oxide.
Exothermic Welding Process
The process of Exothermic Welding is a method of making electrical connections of copper to copper or copper to steel in which no outside source of heat or power is required. In this process, a special weld material is placed into a high-temperature resistant mold and ignited.
The process of igniting the particles creates extremely high heat and molten metal (exothermic reaction) up to 1400 degrees Celsius and produces a molten metal slag. This liquid copper metal flows into the weld cavity, filling any available space and completing the weld.
The weld is then allowed to cool and solidify before it is removed from the mold. The mold is ready for the next weld after a brief cleaning with a brush. This entire process takes only seconds to complete.
Exothermic Welded connections produce a permanent connection, superior in performance to any known mechanical or pressure-type surface-to-surface contact connector. Because the connection produced is a molecular bond, an exothermically welded connection will not loosen or increase in resistance over the lifetime of the installation.
FEATURES
- It has a superior electrical conductivity than the conductors themselves.
- It does not corrode oxide or degrade with time and is resistant to galvanic coupling.
- It is able to withstand repeated electrical discharges.
- It never increases its resistance.
- It has higher mechanical and squeezing resistance than the conductors themselves.
- It offers a permanent welding and a low resistance connection, essential for achieving long-wearing and trustworthy results in earthing
It guarantees the most common connections not only between copper cables but also for welding tapes and metallic pieces made of brass, stainless steel, and copper-coated steel earth rods.
When Exothermic Welding is used?
Exothermic welding is most commonly used for permanently joining copper conductors. It is the only acceptable means of bonding copper to galvanized cable. It can also be used on stainless steel, cast iron, steel, brass, bronze, and more. If joining two dissimilar metals, exothermic welding is a good option.
Process for Exothermic Welding
Step 1: Always wear protective safety glasses and gloves while working with exothermic welding products.
Step 2: Gather all the proper materials and equipment/accessories for the type of connection you are making. The typical Weld system requires a graphite mold, handle clamp, welding material, natural bristle brush for mold cleaning, wire brush for cleaning/preparing conductors, flint igniter, and propane torch. Check to ensure the graphite mold is not worn or broken, which could cause leakage of molten weld metal.
Step 3: Slide the handle clamp into the pre-drilled holes with the proper orientation for the thumbscrews. Tighten the clamp thumbscrews onto the mold and close the grips to tightly lock the mold. Make adjustments to tighten/loosen the handle clamp.
Step 4: The material to be welded (cable, rod, tape) must be clean and dry using the brush included in the set of accessories. Thus, the oxide layer and superficial impurity is eliminated. Given that the graphite mold also absorbs moisture, this should be removed by preheating with a gas welding torch to avoid porous welding. After the first welding is done, it is not necessary to re-heat the mold if the next welding is done within 15 minutes as it conserves the previously generated heat.
Step 5: Place the conductors in the mold and close the handle clamps to avoid material leakages during the reaction.
Please Note – If there is even a minor gap between the two Conductors, apply a Sealing Compound at the places where the conductors are passing out at the time of Welding, the Powder will spread out as a flame resulting in improper joint
Step 6: Obstruct the tap hole with the metallic disk. Empty the contents of the welding mixture package.
Step – 7: Empty 50% of the starting powder above Exothermic Weld Powder (Don’t Mix, just scatter) and then Close the Mold Mouth then the rest 50% Starting Powder, sprinkle it on the Mold Mouth near the small hole given on the top of the Mold Mouth.
Step 8: Ignite the starting powder extended on the top/side of the mold using the flint igniters. Once started, the reaction will take 3-4 seconds during which it is recommended to stand clear of the mold.
Step 9: After at least 2 minutes of bonding, open the mold by undoing the handle grip. Remove the mold from the joint and clean the joint for any slag. Once open, clean the slag sticking to the mold with the relevant tool and clean the cavities. The mold will be ready now to use again without having to reheat it as it is already warm.
MAINTENANCE & STORAGE
- Mold is usually good for 50 – 60 connections in field conditions.
- The equipment is fragile and should be handled carefully while in use.
- Cleaning of molds should be done using an appropriate brush/tool after the mold is reasonably cool after a welding process. Avoid hot mold cleaning.
- Cavity cleaning should be carefully done to avoid damage/chipping.
- On completion of a task, mold should be well cleaned from inside and from outside using a soft cloth. It should be properly wrapped in Bubble Plastic Packing while storing it.
- The molds and the weld powder should always be stored in cool & dry places.
- All tools and accessories must be cleaned before storing for safe reuse.
Applications
Exothermic welding is usually used for welding copper conductors but is suitable for welding a wide range of metals, including stainless steel, cast iron, common steel, brass, bronze, and Monel. It is especially useful for joining dissimilar metals.
Because of the good electrical conductivity and high stability in the face of short-circuit pulses, exothermic welds are one of the options specified by §250.7 of the United States National Electrical Code for grounding conductors and bonding jumpers.
It is the preferred method of bonding, and indeed it is the only acceptable means of bonding copper to galvanized cable.
The NEC does not require such exothermically welded connections to be listed or labeled, but some engineering specifications require that completed exothermic welds be examined using X-ray equipment.
An exothermic weld has higher mechanical strength than other forms of weld, and excellent corrosion resistance It is also highly stable when subject to repeated short-circuit pulses, and does not suffer from increased electrical resistance over the lifetime of the installation.