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Thermite welding or alumino-thermic welding is a fusion welding process in which the work pieces are joined by the heat obtained from a chemical reaction of the hermit mixture. Pressure may or may not be applied during the process.
The hermit mixture is a mixture of iron oxide and aluminum powder, and when this mixture is brought to its ignition temperature of about 1200 degree centigrade, reaction starts, producing molten iron and slag (Al2O3) releasing enormous amount of heat.
3FeO+8Alà9 Fe+4Al2O3+ heat
The molten iron (Fe) obtained is poured into the cavity (Gap between the two work pieces) and upon solidification, complete fusion takes place. The figure shows the welding of rail joint (I-section) using hermit welding process.
Operation of Thermite Welding:
The edges of the work piece are cut flat and cleaned to remove dirt, grease and other impurities to obtain a sound weld. A gap of about 1.5-6mm is left between the edges of the two work pieces.
A wax heated to its plastic state is poured in the gap between the work pieces to be joined and allowed to solidify. Excess wax solidified around the joint is removed.
A mould box is placed around the joint and packed with sand providing necessary gates and risers. A hole or heating gate is made in the mould connecting to the joint.
The wax material is melted out by means of flame directed into the heating gate, so that it leaves a cavity at the joint which will later be occupied by the molten metal. The heating gate is then closed with a sand core or iron plug.
Exothermic reaction occurs to form molten iron and slag which floats at the top. The temperature resulting from this reaction is approximately 25000C.
The plug at the bottom of the crucible is opened and the molten metal is poured into the cavity. The molten metal acts as a filler metal, melts the edges of the joint and fuses to form a weld.
After the weld joint cools and solidifies, the mould is broken, risers are cut and the joint is finished by machining and grinding.
Advantages of Thermite Welding:
Heat required for welding is obtained from the chemical reaction of the hermit mixture. Hence, no costly power supply is required.
The process is best suitable, particularly in remote locations where sophisticated welding equipments and power supply cannot be arranged.
Disadvantages of Thermite Welding:
Process is applicable only to ferrous metal parts.
Process consumes more time.
Applications of Thermite Welding:
Thermit welding is mainly used in repair and welding of large forgings and castings, pipes, mill housings and heavy rail sections.