Preheating involves raising the temperature of the base metal or a section of the base metal above the ambient temperature before welding.  Preheat temperatures may vary from as low as room temperature (70° F) when welding outdoors in winter to as high as 600° and 1200° F when welding highly hardenable steels and ductile cast iron respectively. Preheating is a very effective means of reducing weld metal and base metal cracking. Preheating may improve weldability generally but has two major beneficial effects when it retards the cooling rates in the weld metal and heat-affected base metal, and reduces the magnitude of shrinkage stresses.  However, when welding quenched or age-hardened materials, the effects of preheating can be detrimental unless controlled within allowable limits.
