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Thermit Welding (TW)


Process description

A charge of iron oxide and aluminum powder is ignited in a crucible. The alumino-thermic reaction produces molten steel and alumina slag. On reaching the required temperature, a magnesite thimble melts and allows the molten steel to be tapped off to the mold surrounding the pre-heated joint area. On cooling, a cast joint is created.


Carbon and low alloy steels, and cast iron only.

Thermite Welding

Process variations

  • Molds can be refractory sand or carbon.
  • Can be used to repair broken areas of structural sections using special molds.

Economic considerations

  • Production rates very low. Cycle times typically 1 h.
  • Lead time a few days.
  • 20 per cent of welding metal lost in runners and risers.
  • Scrap material cannot be recycled directly.
  • Economical for low production runs. Can be used for one-offs.
  • Manual operation only.
  • Tooling costs low to moderate.
  • Equipment costs low to moderate.
  • Direct labor costs moderate to high. Some labor involved.
  • Finishing costs moderate. Excess metal around joint not always removed, but gates and risers must be ground off.

Typical applications

  • Site welding of rails to form continuous lengths
  • Joining heavy structural sections and low-loaded structural joints
  • Machine frame fabrication
  • Shipbuilding
  • Joining thick cables
  • Concrete reinforcement steel bars
  • Repair work

Design aspects

  • The cross section of the parts to be joined can be complex, otherwise limited design freedom.
  • Joint gaps typically 20–80mm.
  • Butt joint design possible only (see Appendix B – Weld Joint Configurations).
  • Minimum sheet thickness =10 mm.
  • Maximum thickness =1000 mm.

Quality issues

  • Weld quality fair.
  • The cast joint has inferior properties than that of the base material.
  • Pre-heating times ranging 1–7 min depending on section thickness. Small section thicknesses may not require pre-heating.
  • Joint area must be cleaned thoroughly.
  • Joint edges must be aligned with a suitable gap dependent on section size.
  • Alloying elements can be added to the charge to match physical properties of materials to be joined.
  • Exothermic chemical reaction has safety concerns and proper precautions and ventilation necessary.
  • Surface finish poor to fair.
  • Fabrication tolerances a function of the accuracy of the component parts (hot-rolled sections usually which have poor dimensional accuracy) and the clamping/jigging method used, but typically ±1.5 mm.