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Thermit Welding (TW) is a chemical reaction welding process. The weld joint is
produced by pouring superheated molten oxide metal around the joint to be welded.
Thermit welding basically involves a mixture of finely divided metal oxide and a
metal reducing agent as aluminum. The necessary heat for joining metal of thermit
welding is obtained from the chemical reaction of metal oxide and metal reducing
agent. Usually, iron oxide is used as a metal oxide and aluminum or magnesium is
used as a metal reducing agent. The strong chemical attraction of aluminum for oxygen
is the basis for the thermit process. First, the thermit mixture is ignited by a burning
magnesium ribbon. The ignited temperature of thermit is about 1200 °C. When
ignited in one spot of mixture, the heat reaction spreads through the mass. The
aluminum reacts with the oxygen of metal oxide, setting free the iron, which is
deposited on joint portion into the mold as a highly superheated liquid metal. The
theoretical temperature of thermit is about 3000 °C, and due to the chilling effect of
crucible, the temperature will be reduced to about 2500 °C. So it is sufficient for
welding temperature. Thermit material is a mechanical mixture of metallic aluminum
and processed iron oxide. Molten steel is produced by the reaction in a refractory
crucible. At the bottom of the crucible, there is a hole through which molten metal
is poured into the space that is going to be joined (Figure 1). The crucible is charged
by placing the correct and calculated quantity of portion. In preparing the joint for
welding, the parts to be welded must be cleaned, aliened and held firmly in place. If
necessary, excess metal is removed from the joint by oxy LP gas to permit a free flow
of the metal into the joint. A wax pattern is then made around the joint in the size
and shape of the intended weld. A mold made of refractory sand is built around the
wax pattern and joint to hold the molten metal after it is poured.
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