What is PreHeating?
Preheating is the heating the base metal, either in its entirety or just the region surrounding the joint, to a specific desired temperature, called the preheat temperature, prior to welding.
Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source.
Why to PreHeat?
(1) it slows the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistance to cracking;
(2) the slower cooling rate provides an opportunity for hydrogen that may be present to diffuse out harmlessly, reducing the potential for cracking;
(3) it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints;
(4) it raises some steels above the temperature at which brittle fracture would occur in fabrication.
(5) it can be used to help ensure specific mechanical properties, such as weld metal notch toughness.