Thermit Welding (TW)

Thermit Welding is a welding process utilizing heat generated by exothermic chemical reaction between the components of the thermit (a mixture of a metal oxide and aluminum powder). The molten metal, produced by the reaction, acts as a filler material joining the work pieces after Solidification.

Thermit Welding is mainly used for joining steel parts, therefore common thermit is composed from iron oxide (78%) and aluminum powder (22%).
The proportion 78-22 is determined by the chemical reaction of combustion of aluminum:

8Al + Fe₃O₄ = 9Fe + 4Al₂O₃
The combustion reaction products (iron and aluminum oxide) heat up to 4500°F (2500°C). Liquid iron fills the sand (or ceramic) mold built around the welded parts, the slag (aluminum oxide), floating up , is then removed from the weld surface.

Thermit Welding is used for repair of steel casings and forgings,for joining railroad rails, steel wires and steel pipes, for joining large cast and forged parts.

Advantages of Thermit Welding:

• No external power source is required (heat of chemical reaction is utilized);
• Very large heavy section parts may be joined.

Disadvantages of Resistance Welding:

• Only ferrous (steel, chromium, nickel) parts may be welded;
• Slow welding rate;
• High temperature process may cause distortions and changes in Grain structure in the weld region.
• Weld may contain gas (Hydrogen) and slag contaminations.