In this article, we shall discuss on an example having a PQR qualified with a PWHT (post weld heat treatment). What must be kept in mind while applying PWHT as an essential variable as well as supplementary essential variable? A question asked in an Interpretation has been used to explain the point.
Consider the following question asked in Interpretation IX-07-06.
“Background: A weld test coupon for a PQR was welded to itself using P-No. 1, Group 2 plate material and postweld heat treated at 1110°F to 1130° F (600°C to 610°C) for 10 hr. The PQR documents all supplementary essential variable notch toughness requirements. Notch toughness is required and all qualification ranges are supported by the PQR for production welds.
Question: Will this PQR support a WPS that specifies a PWHT temperature range below the lower transformation temperature, provided the time at temperature does not exceed 12.5 hr?”
It has been specified that notch toughness is required for production welds. So, all supplementary essential variables become additional essential variables.
For above question, QW 407.1 which is an essential variable, and QW 407.2 which is a supplementary essential variable, are pertinent for discussion.
As per QW 407.1, “a separate procedure qualification is required” for “PWHT below lower transformation temperature.”
This means that if the production job is to undergo a PWHT at any temperature below lower transformation temperature (this temperature is generally quoted at 723°C for carbon steels and carbon-molybdenum steels), the procedure qualification must have been qualified with a PWHT at a temperature below lower transformation temperature.
The PQR cited in the question meets this requirement.
As per QW 407.2, “The procedure qualification test shall be subjected to PWHT essentially equivalent to that encountered in the fabrication of production welds, including at least 80% of the aggregate times at temperature(s)”.
The production weld described in the question is to undergo a PWHT of 12.5 hours. The PWHT time adopted in procedure qualification is 10 hours, which is 80% of that required in production weld. Hence, the ‘80%’ requirement of QW 407.2 is satisfied.
Now, let us see about the ‘essentially equivalent’ part. Some inspectors think that this term conveys that the soaking temperature of test coupon must be exactly equal (or with a very minor tolerance) to that of production weld.
The reply to the above question has been given as ‘yes’ by ASME. In other words, the Committee agrees with not using the exact same soaking temperature (while doing PWHT on production weld) as the test coupon. It is fine if it is any other temperature below lower transformation temperature is used.
The PQR in question thus meets the requirement of QW-407.2 too.
Hence, the PQR in question is good enough to support the WPS described therein. The answer given in the Interpretation is also, therefore, “yes”.
This was all on this. Please comment below if you any clarification or comment.
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