I don't know about you guys, but this thread is getting long and really screwed up.
The terms PWHT and stress relieve are used interchangablily in the Code to mean the same heat treatment; which is to heat treat a welded joint below the lower transformation temperature. "Purpose" and "result" of this heat treatment is:
1. to release locked in stress caused by welding or deformation at a joint. Thus, call "stress relieve" by mechanical engineers. Although stress relieving can be achieved by mechanical means (like peening, ultrasonic vibration, etc.,)as well but is generally not permitted as an instrustry practice.
2. to restore some ductility in the weld metal and lower the hardness in the HAZ. That's why metallurgist calls this a "heat treatment".
Hence, the two terms used in the PV industries and in Codes/Standards (see Sect VIII, API-650, etc) are one and the same with respect to actually doing it. You get both benefits in one shot.
Because tensile and other mechanical properties degrade when subjected to high temperature, you only want to do a PWHT (or call it stress relieve if you feel like it) once in the shop only, not counting a repair. Don't do it twice. Now, when we get into 1.25Cr and 2.25Cr steel then there more intermediate heat treatment terminologies, but let not get into that. If I were your Client, I will want you to explain to me why you doing 2 PWHT in the shop.
Hope this clears it up.