Thermite Welding

Exothermic welding, also known as exothermic bondingthermite 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. The chemical reaction that produces the heat is an aluminothermic reaction between aluminiumpowder and a metal oxide.

Overview

In exothermic welding, aluminium dust reduces the oxide of another metal, most commonly iron oxide, because aluminium is highly reactive. Iron(III) oxide is commonly

\mathrm{Fe_2O_3 + 2 \ Al \longrightarrow 2 \ Fe + Al_2O_3}

The products are aluminium oxide, free elemental iron,and a large amount of heat. The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation.

Commonly the reacting composition is five parts iron oxide red (rust) powder and three parts aluminium powder by weight, ignited at high temperatures. A strongly exothermic(heat-generating) reaction occurs that via reduction and oxidation produces a white hot mass of molten iron and a slag of refractoryaluminium oxide. The molten iron is the actual welding material; the aluminium oxide is much less dense than the liquid iron and so floats to the top of the reaction, so the set-up for welding must take into account that the actual molten metal is at the bottom of the crucible and covered by floating slag.

Other metal oxides can be used, such as chromium oxide, to generate the given metal in its elemental form. Copper thermite, using copper oxide, is used for creating electric

{\displaystyle \mathrm {3\ Cu_{2}O+2Al\longrightarrow 6\ Cu+Al_{2}O_{3}} }

Thermite welding is widely used to weld railway rails. One of the first railroads to evaluate the use of thermite welding was the Delaware and Hudson Railroad in the United States in 1935 The weld quality of chemically pure thermite is low due to the low heat penetration into the joining metals and the very low carbon and alloy content in the nearly pure molten iron. To obtain sound railroad welds, the ends of the rails being thermite welded are preheated with a torch to an orange heat, to ensure the molten steel is not chilled during the pour. Because the thermite reaction yields relatively pure iron, not the much stronger steel, some small pellets or rods of high-carbon alloying metal are included in the thermite mix; these alloying materials melt from the heat of the thermite reaction and mix into the weld metal. The alloying beads composition will vary, according to the rail alloy being welded.

The reaction reaches very high temperatures, depending on the metal oxide used. The reactants are usually supplied in the form of powders, with the reaction triggered using a spark from a flint lighter. The activation energy for this reaction is very high however, and initiation requires either the use of a “booster” material such as powdered magnesium metal or a very hot flame source. The aluminium oxide slag that it produces is discarded.

When welding copper conductors, the process employs a semi-permanent graphite cruciblemould, in which the molten copper, produced by the reaction, flows through the mould and over and around the conductors to be welded, forming an electrically conductive weld between them.When the copper cools, the mould is either broken off or left in place.Alternatively, hand-held graphite crucibles can be used. The advantages of these crucibles include portability, lower cost (because they can be reused), and flexibility, especially in field applications.

Properties

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. However, the process is costly relative to other welding processes, requires a supply of replaceable moulds, suffers from a lack of repeatability, and can be impeded by wet conditions or bad weather (when performed outdoors).

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.The process is marketed under a variety of names such as APLIWELD (in tablet form), American Rail Weld, Harger ULTRASHOT, ERICO CADWELD, Quikweld, Tectoweld, Ultraweld, Techweld, TerraWeld, Thermoweld, Ardo Weld, AmiableWeld, AIWeld, FurseWeld and Kumwell.

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 labelled, but some engineering specifications require that completed exothermic welds be examined using X-ray equipment.

Remote welding

Remote exothermic welding is a type of exothermic welding process for joining two electrical conductors from a distance. The process reduces the inherent risks associated with exothermic welding and is used in installations that require a welding operator to permanently join conductors from a safe distance from the superheated copper alloy.

The process incorporates either an igniter for use with standard graphite molds or a consumable sealed drop-in weld metal cartridge, semi-permanent graphite crucible mold, and an ignition source that tethers to the cartridge with a cable that provides the safe remote ignition.

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