Preheat Treatment is the application of heat to a joint immediately prior to welding and usually applied by a gas torch or Induction System, although other methods can be used.
Preheat is used when welding steels for a number of reasons and it helps to understand why,
⦁ Assist in removing Hydrogen from the weld
⦁ Slows down the cooling rate of the weld and HAZ, reducing the risk of hardened microstructures forming.
⦁ Removes moisture from the region of the weld preparation.
⦁ Improves overall fusion characteristics during welding.
⦁ Ensures more uniform expansion and contraction, lowering stresses between weld and parent metal.
Factors influencing the application of preheat is,
⦁ The Carbon Equivalent (CEV)
⦁ Thickness of the material
⦁ The Arc Energy or Heat Input of the welding process.