Welding After PWHT 1

[bobracha]

I'm having a bit of a debate within my group about whether or not ANY welding is permitted after PWHT without having to heat treat a second time. The situation is this:

We have a vessel with 1 1/4" thk shell sections. The rear head had a straight flange thickness of 1 3/8". The head was tapered, but 1 3/8" of weld was still applied, assume the vessel was not preheated to meet UCS-56 Note 2b.

My understanding of UW-40 tells me that the nominal thickness of this weld is the 1 3/8", thus requiring PWHT. However, the final line in UW-40(f) says "...[the nominal thickness] is the greatest weld thickness in any vessel or vessel part which has not previously been postweld heat treated."

My interpretation of that line says that I can make the girth seam weld connecting the shell and head, heat treat that weld, and then continue to weld to the vessel provided none of the welds made after PWHT exceed the 1 1/4". These are to be nozzle welds and other small attachments like insulation pins.

As long as we aren't into the heat affected zone, I don't see how the thicker material will be affected by these post heat treating welds? I couldn't find an interpretation that covered this topic, so if anybody can point me towards some official document that could support/rebuke my position I would appreciate it.

 

[metengr]

Yes, welding is permitted after PWHT provided the vessel is not for lethal service AND the nominal thickness of the weld(s) is below the thickness requirement for PWHT in accordance with ASME Section VIII, Div 1 requirements.

The sequence of PWHT is not addressed specifically in the Code because this is a process detail. If the Code requires PWHT based on nominal thickness, PWHT can be done after all welding is completed or during multiple cycles. However, for welds that are exempt from PWHT, these can be done at any time prior to hydrotesting.

 

[Duwe6]

metengr is right again.

Don't forget PWHT-for-Process, like wet H2S, amine or liquid NH3. The consensus on this board is that using Temper-Bead or elevated preheat in lieu of PWHT on processes that require heat-treat is a bad idea.

Also, you have to make VERY sure that your welders/fitters do not weld any temporary attachments, like lugs, to the thick PWHT'd areas. Only takes one idiot on 3rd-shift to force a re-PWHT.