PWHT temperature issue of P91 weld

[qualitypro]

On a furnace PWHT load of P91 piping spool welds that our pipe sub ran yesterday, one of the temperature controller dropped out and one of three thermocouples dropped below 1375 by about 10 deg for about 30 to 45 minutes. They have a WPS requirement of holding the PWHT temperature from 1375F min -1425F max. Is there any tolerance allowed per B31.1 Code and are the joints acceptable as such?

 

[metengr]

qualitypro;
First off, the tolerance is the specified range of 1375-1425 deg F. You now have a nonconformance because you fell below the 1375 deg F minimum.

What do you need to do? Well, you need to disposition this nonconformance by hardness testing and surface replication to compare with accepted pipe pieces. Grade 91 material is nothing to fool around with because it is a creep strength enhanced ferritic steel. Anything short of what I mentioned, I would reject as the customer – meaning you need to reheat treat.

 

[stanweld]

B31.1 specified Holding Temperature Range is 1350 F to 1425 F and permits increasing the range dependent on %Ni+%Mn. As metengr states, the fabricator has a nonconformance to its own procedure, which may reflect the Purchaser's or Owner's specifications. The fabricator must address the nonconformance, which may require acceptance by the Purchaser and the AI, if applicable.

 

[qualitypro]

Metengr & Stanweld,
Thanks for the responses.
I asked the pipe sub to share his recorder downloads so I can review the exact details. Based on my review I observe that the total furnace soak time was 2 hours and 21 minutes. The WPS has a requirement of holding the PWHT temperature from 1375F min -1425F max for 2 hours minimum. There were 3 thermocouples used T1, T2 & T3. While T2 was always in the specified PWHT range during soak period T1 dropped to 1365 F for 15 minutes and T3 dropped to 1364F for 20 minutes. If we take the aggregate I see that the requirement of having a soak temperature of 1375 F minimum for the required holding time of 2 hours minimum has been achieved. Does B31.1 state that the PWHT temperature at soak time has to be continuously held? Is there any code case that you are aware of that clarifies this aspect?

 

[metengr]

As long as the PWHT temperature is held within the specified band for the specified time duration based on governing thickness, this is acceptable. For example, your PWHT varies from 1380 to 1375, this is acceptable. You should be working off of the specified PWHT procedure for heating/cooling rates and hold time within the specified temperature range.

 

[stanweld]

If the T/Cs were furnace T/Cs only, you may have some locations that did not receive adequate heat treatment. I have seen weld hardness in excess of 310 BHN at weld locations when only 3 furnace T/Cs were in use. We mandate multiple work located T/Cs to assure more uniform PWHT on all pipe spools.

 

[Duwe6]

You only 'count' the time that the temp was at [or above] the PWHT 'window'. Thus T1 cannot count 15 minutes of saok time, and T3 can't count 20 minutes of soak. You cannot "add 'em up and average them". Tech T/C represents a discrete area to the weld [ASSuming that an adequate number and location of T/C's were used.

So you have a valid PWHT. Aren't you glad that a little extra 21 minutes of soak time was used? Lesson: If 1-hr is needed, give it 70 or 75 minutes, and scale up from there.