THERMIT WELDING AND ITS PROCESS

Thermit Welding is primarily a fusion-welding process in which the weld is effected by pouring superheated liquid Thermit steel around the parts to be united. In the case of thermit pressure welding, only the heat of the "thermit" reaction is utilized to bring the surface of the metal to be welded in a plastic state and mechanical pressure is then applied to complete the weld.

The thermit process for welding metal is based on the chemical reaction between finely divided aluminum and iron oxide.

During the reaction, the oxygen leaves the iron oxide and combines with aluminum, producing aluminum oxide , or slag, and superheated thermit steel.

The thermit is a mixture of finely divided aluminum and iron oxide, the ratio by weight being approximately three parts of iron oxide to one part of aluminum. The mixture, placed in a refractory-lined crucible, is ignited with the aid of highly inflammable powder composed largely of barium peroxide. The temperature produced by the thermit reaction is approximately 3000*C or about twice the temperature of the melting point of the steel.

In making pressure welds by the thermit process, a pattern of wax is shaped around the parts to welded. A sheet-iron box is placed around the wax pattern and the space between the pattern and box is filled and rammed with sand. Pouring and heating gates, and risers, are cut in the melts and drains out but the heating is continued to raise the temperature of poured into the mould in order to prevent chilling of the steel. Then the burner or torch is removed, and the preheating gate is plugged with sand. The superheated metal produced by the thermit reaction in a crucible is poured into the mould surrounding the surfaces to be welded. After the welding temperature is reached mechanical pressure is applied to complete the weld.

The thermit pressure welding is used to great extent in the welding of pipes, cables , conductors, rails, shafts and broken machinery frames and rebuilding of large gears etc.

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