Planar indications found by PAUT after radiography has passed
A 38 mm thick SAW girth weld in an SA-516 Gr 70 pressure vessel has been 100% radiographed and accepted to ISO 17636 class B, quality level 2. The client now requires phased array UT before hydrotest. The encoded PAUT scan reveals a planar indication 10 mm long at 14 mm depth with amplitude above the reference level, and a second indication 6 mm long at 30 mm depth near the root. The radiographs show nothing at either location apart from faint slag lines. The vessel is due for hydrotest in five days. A repair would require re-PWHT at 620 °C for one hour, and the nozzle set has already been stress-relieved once. The welding coordinator argues that RT is the contractually agreed method, so the vessel should pass.
1. Why can PAUT detect what radiography missed, and how would you confirm and characterise the indications?
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RT and PAUT respond to different defect geometries. Radiography is a through-thickness transmission method: it detects volumetric defects such as porosity and slag well, but it is poor at tight planar defects that are not aligned with the beam, especially lack of sidewall fusion and cracks in thick SAW welds. A 10 mm planar indication at 14 mm depth in 38 mm plate can be invisible on a class B radiograph if its plane is near-parallel to the beam. PAUT is a reflection method: it sees planar reflectors and can size them by tip diffraction or by the 6 dB drop, and a root indication at 30 mm is exactly the kind of lack of penetration or root slag that RT often under-calls. The right response is not to dismiss the PAUT but to validate it: check the calibration block, the encoder and the scan coverage, scan from a second direction, and repeat with TOFD or a different probe angle to characterise the reflector as planar or volumetric and to measure its height. The faint slag lines on the film at the same locations support the PAUT rather than contradicting it. Have a second operator repeat the scan before any decision is made.
2. What disposition would you recommend, and how should the inspection plan change?
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Acceptance is governed by the specification, not by which method was contractual. If the indications are confirmed as planar and exceed the acceptance level - for example ISO 11666 quality level 2 for 38 mm, or the equivalent ASME acceptance criteria - the weld must be repaired even though the radiograph passed it. A 10 mm planar lack of fusion at 14 mm depth is a real defect and would not be accepted on its merits. Before cutting, do two things: verify the PAUT with a second operator and a second technique so you are not repairing on a false call, and review whether a local repair is feasible at that depth. The re-PWHT decision depends on the applicable code and on whether the vessel was originally stress-relieved; do not let the cost of re-PWHT drive the acceptance decision. Do not hydrotest first: hydrotest can grow a planar defect and turn a repairable weld into a scrapped vessel. For the future, the inspection plan should require PAUT or TOFD in addition to RT for thick-section girth welds, applied before PWHT and hydrotest, with the acceptance criteria stated in the plan and agreed with the client at the start of the contract.