In regard to piping and tubing, preheating is the application of heat to a base metal (weld metal, HAZ, and adjacent base metal) prior to and throughout welding, as well as at the start of each pass in a multi-pass weld. The primary purpose of preheating is to drive off moisture prior to the start of welding, reducing the cooling rate, and increasing hydrogen diffusion within the weld to prevent cracking. Reducing the cooling rate, as a result of preheating, also redistributes solidification stresses and reduces the formation of hard or brittle microstructure components associated with a defective weld.
Temperature codes and specifications are based on material composition and thickness, however, preheat temperatures may also utilize fabrication codes to provide exemptions for Post Weld Heat Treatments (PWHT). For reasons explained above, most welding procedures specify minimum & maximum preheat/interpass temperatures that must be maintained whenever welding takes place.