Preheating involves 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. The interpass temperature, defined as the base metal temperature between the first and last welding passes, cannot fall below the preheat temperature. Interpass temperature will not be discussed further here. Preheating can produce many beneficial effects; however, without a working knowledge of the fundamentals involved, one risks wasting money, or even worse, degrading the integrity of the weldment.