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PWHT time for P91
2

PWHT time for P91

PWHT time for P91

(OP)
I'm sure the question has been asked before, but what is the minimum hold time for PWHT on P91? The question is pertinent to thin sections (1/2" and lower say).

I believe ASME VIII (UCS-56) suggests it is possible to do as little as 15 mins on thin sections (<1/4"). However, I've seen some guidance, including:

http://preview.tinyurl.com/ok7ek38

and

http://www.sperkoengineering.com/html/Grade%2091%2...

Suggesting 2 hr min on the basis to ensure complete tempering.

Is anyone able to advise?

Regards

kclim

RE: PWHT time for P91

I have never heard of a 15 minute soak time. 2 hr is minimum for even a .216 thick alloy pipe wall.

RE: PWHT time for P91

You have to meet the requirements of the code . ASME sect I requires a min 2 hr soak.

NEM has published recommendations for PWHT time vs temperature in the magazine"Combined cycle quarterly" 3rd quarter 2005. There can be plotted a relationship of LMP vs weld hardness, where if the objective is a final weld hardness of Hv=270, then the correlation is

LMP=21= T*(20+Log(t))/1000 for hardness = 270Hv. For a lower hardness , set LMP=21.5 . For LMP=21 I get:
For T= 1425 F = 1047 K, t= 66 minutes = 1.1 hrs, but code requires 2 hrs min
T=1400F , t=2.1 hrs
T=1375 F, t= 4.0 hrs
T=1350F, t= 7.6 hrs

If you have in hand the material certs for the weld consumables, you can calculate the max permitted T AC1, and you would set the PWHT temp at least 50 F below that T Ac1. If you overshoot T Ac1, then you will need to recycle that section of pipe or submit it to a N+T.

This correlation is independent of wall thickness.

"Nobody expects the Spanish Inquisition! "

RE: PWHT time for P91

If you are going to use the 2011 Edition of ASME B&PV Code, for P-No 15E Group 1 material, the minimum PWHT time is 30 min at temperature, with 1 hour/inch of thickness up to 5" of nominal weld thickness per Section I, Table PW 39-5.

RE: PWHT time for P91

(OP)
Thanks to everyone for their feedback.

Ultimately we are going to be OK with 0.5 hr PWHT for thin wall tube:
- Qualification testing will show suitable properties are obtained
- Industry experience - 0.5 hr for thin wall tube is OK
- Metengr link to the EPRI document suggests oxide scaling can be an issue at long hold times, which is an issue with thin wall tube where scaling will consume a proportionally larger amount of material.

Cheers

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