B31.1 Lower Critical Temperature

By cb7cwi Date 04-10-2010 09:54

Need some help here. A subcontractor that was performing PWHT for me on P-1 to P-5a material 2" thick just exceeded the lower critical temperature for the P-1 material per table 129.3.2. It only went over about 100 degrees for about 30 minutes but by the time he figured out what was going on, he killed the power to that circuit and the temperature dropped faster than it went up. We just performed hardness reading on the pipe and I think I'm ok as far as that is concerned but what other damage could have been caused other than hardness and should I be concerned about any thing else. Note: P-1 hardness readings averaged 150-160 2" from the weld, 160-170 HAZ, and 200 plus in the weld with no variance in reading to the area that was over exposed.

By js55

Date 04-10-2010 15:53

Since Table 129.5 lists the lower transformation at 1340 this must mean you were cooking at 1440.
This is too high for P5A as well. Its LTT is listed at 1480. Since Table 129.5 is only an approximation, and thermocouples are anything but accurate for the entire weldment, and conduction is unpredictable, you may have hit that one too.
What the hell was he thinking???
Please tell us this was NOT a PWHT subcontractor.
You have most likely violated your WPS and therefore the code as well. You need to place the piece on hold, write an NCR, and qualify a procedure with PWHT above transformation. Just re-cooking may be a metallurgical solution but it does not make the code violation go away.
And because he panicked you have also most likely exceeded the cooling rate in B31.1 (para 132.5) so you will need to NCR and disposition for that as well. I would think that your customer would be justifiably concerned about through thickness stress and cracking on the P5A side so you may need to do UT to mitigate their concerns.
Its interesting, the violation is the carbon steel but the biggest concern is the P5A. Most likely the carbon steel side is fine but you have to deal with the code issues. The P5A side would be the concern.

By cb7cwi Date 04-13-2010 15:59

Of course this was a PWHT Subcontractor, would you expect anything less. I really can't blame the sub, equipment failures do happen with these machines but his reaction time bothered me more than anything. This was a 2 hour and 15 minute soak at 1300 F plus 25 F minus 0 and it happened right at the 2 hour point in the soak. Well as it turns out, I waiting on the clients NCR response which I think they are going to accept as is. I like the idea of performing UT in this area, makes sense, thanks.

By MBSims

Date 04-11-2010 00:43

Metallurgically, I would say there appears to be no damage. The hardness values look good and are in the expected range for both materials. P22 max hardness is 235 HB for A335-P22base material. Since you are welding P-5A to P1, and the maximum design temp in B31.1 for P-1 is 750-800 F, the creep properties of the P-5A would not be a concern. There may be some grain growth, which could lower yield point slightly, but the hardness values suggest no major changes have occurred. Re-performing the heat treatment would do nothing for possible grain growth and the hardnesses are acceptable, so I see no benefit to doing the PWHT again.

On the Code side, a heat treatment above the lower critical temp was probably not covered by the PQR. Possible resolutions are 1) do a PQR with similar PWHT temps, 2) cut out the section of pipe/fittings and replace about 2 ft. on each side of weld, or 3) accept as-is. I only see option #3 being viable if this is non-boiler external pipe and the AE agrees.

By cb7cwi Date 04-13-2010 15:52

I agree, option #3 is the direction we are heading. I had discussions with the client about re-performing the heat treatment if the hardness reading showed different.

By saitsa Date 04-10-2010 14:02

You can check Welding Research Council Bulletin 452 which discusses recommendations for local heat treatment. When you killed the power did it cool to room temperature or did you just cool to the correct PWHT temperature and continue with the remainder of the PWHT?

You are correct to do the hardness checks since the heating and cooling through the lower tansformation temperature may cause some increase in hardness. Fortunately even without power the cooling rate is not very quick (it is not like a quench).

Bulletin 452 states if you exceeded the lower critical temperature you should decrease the temperature to the correct PWHT temperature and hold at this temperature for the specified time and then cool to room temperature as you would normally do. Their idea is this will temper any harder structures. Then they suggest hardness tests to prove there are no hard structures.
Hopefuly that helps.