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Exothermic welding is a joining technique used to create a permanent connection between two metallic components. It involves a chemical reaction that generates heat, known as an exothermic reaction. This process is particularly noted for the durability of the bond produced and for preserving good electrical conductivity between the joined components.
Creating a bond by exothermic welding typically involves heat created by a chemical reaction between some type of heavy metal oxide and a reducing agent. For example, iron oxide is a commonly used metal oxide and aluminum is a common reducing agent. These reactants produce heat extremely rapidly when ignited, thereby achieving the high temperatures needed for welding.
Such heating is generally initiated once the parts to be joined are fitted together in a mold which contains the materials and the reaction as it takes place. Filler metal in liquid form is produced by this reaction and mixes with melted metal from the parts being joined to form a bond shaped by the mold. Molds used in exothermic welding may be made of graphite, ceramic, or other appropriate materials.
Welds formed by this method exhibit two particularly useful characteristics. First, they form permanent molecular bonds between the materials. The resulting joint, therefore, does not loosen over time, nor does it corrode at a different rate than the joined components. Pressure-based connections, on the other hand, are highly susceptible to loosening and corrosion. Welds between railway track sections or segments of reinforcing steel bar in concrete may be constructed by the exothermic technique to take advantage of this durability.
Joints formed in this way also maintain good electrical performance. They typically exhibit uniform electrical conductivity and do not increase in electrical resistance over time. Exothermic welding is, therefore, commonly prescribed by professional industry standards for use in joining electrically conductive components used in grounding systems.
Another advantage of this welding method is its portability. No external power source or heat source is required for exothermic welding, so it can be done almost anywhere. It can be used with a variety of materials, ranging from common metals such as copper or steel to rare metals like niobium. These features make exothermic welding suitable for a wide variety of applications.
While the exothermic reaction that takes place in this welding method is not explosive, it is very rapid and occurs at a high temperature. Safe operation begins with the proper outfitting of the operator, including a full face mask, protective clothing, and gloves — all designed to sustain contact from hot metal. All materials and molds involved in the operation must be completely dry in order to prevent steam buildup, which has the potential of ejecting hot metal out onto the user. The added precaution of operating at a distance is also prudent whenever practical.