Heat treatment is used to avoid or alleviate the detrimental effects of high temperatures, which are unavoidable during the welding process. The heat treatment required and the recommended temperatures and holding times vary among metals. They are specified in ASME B31.3, Table 331.1.1, Requirements for Heat Treatment, except were specified differently in the same document. Materials are grouped under P numbers, which originate from BPV Code, Section IX, QW/QB-422. When using this table, generally, the thicker of the two components is considered to be the governing thickness when specifying heat treatment.
The method of heating the work piece should be applied uniformly under a controlled condition, which includes an enclosed furnace, local flame, electric induction, electric resistance, or any other approved method. The cooling-down method also must be in a controlled environment, which allows uniform temperature reduction.