To reduce the risk of cracking in weld metal and heat-affected zone (HAZ), heating before actual welding is carried out. The level and need for preheat is determined by the applicable welding code and is dependent on the following:
- Heat input during welding
- Carbon equivalent derived from the chemical composition of steel
- Thicknesses of the materials being welded
- Diffusible hydrogen
Preheating results in the following:
- Prevents formation of brittle weld metal/HAZ and allows more time for hydrogen to escape the molten weld by lowering the cooling rate of the weld.
- Reduces shrinkage by lowering the temperature differential between weld metal/HAZ and the surrounding metal.
- Removes hydrogen source from the part by removing moisture.