PQR Coverage for a Thicker Nozzle
A fabrication shop qualified a GTAW root plus SMAW fill/cap procedure on 12 mm (0.47 in) carbon steel plate to ASME Section IX. The PQR records a single tensile test set (two specimens) passing at 470 MPa, one 180-degree bend pair passing, and Charpy impact tests at minus 20 C with 42 J average. A new order requires the same joint design on 25 mm (0.98 in) plate with impact testing at minus 20 C. The welding engineer proposes using the existing PQR without requalification, arguing that thicker base metal is only a supplementary essential variable for impact-tested procedures and that the shop's standard practice already covers 25 mm. Production wants to start Monday.
1. Determine whether the existing PQR supports the 25 mm production joint. Identify the governing variable and state what requalification, if any, is required.
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Base metal thickness is an essential variable for procedure qualification and, when impact toughness is specified, also a supplementary essential variable. A PQR qualified on 12 mm plate does not automatically cover 25 mm.
Key points to establish:
- Essential variable check. For the SMAW and GTAW processes involved, base metal thickness ranges are limited by the qualified thickness. A single PQR on 12 mm typically qualifies a range that does not extend to 25 mm without additional testing.
- Supplementary essential variable (impact). Because impact testing is required at minus 20 C, the thickness range is further restricted. The 42 J average on the 12 mm PQR cannot be extrapolated to 25 mm; thicker sections have different heat input, cooling rate, and restraint.
- Tensile and bend coverage. The existing tensile specimens demonstrate strength at 12 mm only. A 25 mm joint needs a tensile test on a full-thickness or reduced-section specimen representing the production thickness.
Conclusion: The existing PQR does not support 25 mm. A new PQR (or a supplementary PQR) is required with base metal at or near 25 mm, impact specimens at minus 20 C, and tensile/bend tests. The engineer's argument that thickness is only supplementary is incorrect - thickness is essential, and impact makes it supplementary as well. Starting Monday without requalification is not acceptable.
2. The shop proposes welding the 25 mm test coupon with the same parameters as the 12 mm coupon, then testing only impact specimens. Evaluate this shortcut.
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Testing only impact specimens is insufficient. The PQR must demonstrate all required mechanical properties for the production thickness:
- Tension test: Confirms the joint achieves the required tensile strength. A thicker coupon with the same parameters may have different dilution and bead sequence, so a tensile test is mandatory.
- Guided bend test: Confirms ductility and soundness of the root and cap. Bend specimens from a 25 mm coupon are required unless the code permits alternative specimens; the shop cannot rely on the 12 mm bends.
- Impact test: Required at minus 20 C, but must be performed on specimens taken from the 25 mm coupon, not the 12 mm coupon.
Additionally, welding the 25 mm coupon with identical parameters is not necessarily appropriate - heat input, preheat, interpass temperature, and number of passes will differ. The procedure must specify parameters suitable for 25 mm and the test coupon must be welded using those parameters.
Recommendation: Weld a full 25 mm test coupon per the proposed WPS, perform tensile, bend, and impact tests, and document all results on a new PQR before production.