Materials & Welding: [MW:23633] PWHT as per latest 31.3 (2014)

Thursday, October 1, 2015

[MW:23633] PWHT as per latest 31.3 (2014)

All,

If we refer to ASME B 31.3 latest issued (2014), the Table 331.1.3 (Exemption to Mandatory Postweld Heat Treatment), the tabulation for P 1 (all groups), it indicated that "All thicknesses" for "All of type of welds" there is no need to perform PWHT. Except that >25mm they need to perform preheat at 95degC.

So based on above, it now looks all thicknesses are exempted from PWHT, and there is no need to even perform preheat for weld thicknesses <=25mm; and if we just perform preheat the PWHT is now exempted for any thicknesses. Table 330.1.1 -2014 indicated Preheat for <=25mm is 10degC (again it is not clear on this 10degC requirement).

Whereas, B 31.4, B31.8, ASME Sec VIII à all these 3 codes indicate that PWHT is mandatory for thickness >1.5", and preheat mandatory for weld thickness >1.25" for all P1 materials.

So, we have to be clear as many of our Operation / Maintenance / PMT are in need of a clear understanding of the B31.3-2014 requirements on PWHT.

B 31.3 – 2014 – my understanding (based on para 331.1.1(a), (b), (c) is PWHT can be exempted, provided the WPS/PQR is qualified (without preheat for less <=25mm, and with preheat for thickness >25mm). The PQR qualification tests shall be as per ASME Sec IX as minimum (and any extra ex: Impact testing if low temperatures, hardness test if NACE etc).

Assuming that the PQR tests pass (without the coupon undergoing PWHT for any thickness in P1), now the code B 31.3 is giving exemption to PWHT.

And if any of the required tests during PQR fail, then the fabricator to re-perform this PQR with PWHT, and re-test to ensure that all test results pass.

(otherwise I do not see any reason, how Code B 31.3-2014 has given this blank concession for all thicknesses, as preheat >25mm or no preheat for <=25mm does not guarantee that PQR test results will pass).