Why PWHT holding time has no range?

[srijey]

Please refer to ASME Section 1 Table PW-39-1 to 13 or ASME Section VIII Table UCS-56. There is only minimum holding time for PWHT requirement. I don't see a re-qualification requirement when the holding time exceeds in a disproportionate manner....actually it happened - the contractor cooked the metal for almost 13 hours! Of course material properties deteriorated particularly notch toughness. My fundamental question: What is the rationale behind not specifying a range or limit on holding time for the PWHT? Is this again a case of common sense, engineering judgement, or due diligence? We need to draw a line somewhere.....

 

[metengr]

I can give you first hand rationale being a Code volunteer. The simple reason for not specifying a maximum holding time is you have PWHT time at temperature based on thickness per hour requirements, which provides maximum flexibility for most users. Yes, common engineering sense and basic math skills must be used to arrive at an aggregate time for PWHT.

You have handled the nonconformance correctly. The Code cannot prevent mishaps; this is why you are required to have descriptive nonconformance processes and method of disposition in your Quality Manual.

 

[jtseng123]

Are you talking about cooking the vessel for 13 hours or the coupons ? Or perphase I do not understand the question. We have coupon cooked almost 20 hours, which is not unusual for 2 1/4Cr-1/2Mo material. The code does not draw a line, however, it is the engineer to draw the line, and the contractor to follow.

 

[gtaw]

I believe the contractor is required to qualify the WPS per Sction IX when working to ASME B&PV code. The type of PWHT is an essential variable as is the time at temperature. The PWHT must represent 80% of the time at temperature used in production.

Did I miss something?

Best regards - Al

 

[stanweld]

gtaw,
You are correct if impact toughness is mandated. QW-407.2 is a Supplementary Essential variable. Otherwise it does not apply. QW-407.9 limits on time at temperature applies for corrosion resistant weld overlays.

 

[srijey]

Thank you folks for responding. The actual material (carbon steel) was over-cooked not the test coupons. PWHT is towards the wall thickness (>60 mm) not for corrosion sake. Our remedy was: Normalize the material again wherever possible as this was a skirt portion of the huge conical bottom atmospheric pressure hopper (doesn't necessarily follow a code); re-qualify the WPS. Though the minimum time depends on the thickness, still there seems to be infinite flexibility on the upper limit. Number of PWHT cycles also does not cover this excessive one time hold time. There was an Australian paper for different metal which seems to pin down the heat input rate as the major factor in reduction of notch toughness under such high hold time PWHT. Anyway that is not relevant here. I thought may be a note on these PWHT tables would caution the user. My lack of knowledge is what will go wrong if a limit is specified such as do not exceed the computed hold time, by say, 1 hr. I can hesitantly accept the rationale behind this omission of hold time range given by Metengr. I always struggle how many pages on the code can be eliminated based on 'common sense, engineering judgement, or due diligence'........Please remember, I've highest regards for those volunteers in code committees; my ranting is my interpretation issue. In this case of non-conforming(?) PWHT, we managed to work around by conducting combination of retesting of the samples, reinforcing, re-qualifying, EHTing and living with.

 

[stanweld]

Whether you need to do what you are proposing depends on your design (stress), the carbon steel grade, the weld filler metal composition and the PWHT temperature. Your proposed normalizing PWHT may well reduce the weld metal strength far more than the 13 hrs at PWHT.