Per UCS 56 you must PWHT P1 materials when weld thickness exceeds 1.25" unless a preheat of 200F is applied and maintained throughout welding. Joints in P1 materials greater than 1.5" thick must be PWHT'd. Some applications require PWHT of thinner joints for services such as those considered lethal. See UW 2
Keep in mind the differences between nominal thickness as used in table UCS 56 and weld thickness. Both are not the same in some applications. For the definition of nominal thickness review UCS-56.
Good point! UCS 56 refers you to UW 40 where there are very detailed instructions as to how to handle various situations, such as dissimilar thicknesses, fillet welds, welds in combination, etc.
