PAUT sizing disagreement on a thick-section butt weld
During fabrication of a 40 mm thick S355J2 pressure-retaining butt weld, access prevents RT. The fabricator uses encoded PAUT with a 2.5 MHz, 64-element probe, 45° shear wave, and a 1 mm side-drilled hole reference. Acceptance is ISO 5817 level B, with UT according to ISO 17640 and acceptance according to ISO 11666. An indication is found at 28 mm depth, 22 mm long, with amplitude 4 dB above reference and a sharp, straight tip signal. Operator A sizes it at 12 mm; Operator B sizes it at 26 mm using a different focal law. No macro section is available. The client wants to know whether the weld is acceptable and whether repair is required.
1. Interpret the indication and explain why the two operators obtained such different sizes. What defect type is most likely and why is it significant?
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The indication is planar, sharp-tipped and located at 28 mm depth in a 40 mm butt weld, which is consistent with lack of fusion or a crack rather than porosity or slag. A 22 mm length is substantial, and the straight tip signal suggests a planar reflector with a defined edge. Amplitude alone is not decisive; length, height and orientation determine acceptance.
The sizing difference of 12 mm versus 26 mm is too large for a reliable acceptance decision. Possible causes are different focal laws and beam widths, different scanning directions relative to the defect, the use of tip diffraction versus echo amplitude, incorrect sound-path or index calibration, and lack of a qualified sizing procedure. A planar lack-of-fusion may be oriented so that one probe angle sees it specularly while another sees only a weak tip. The 45° shear wave may not be optimal; additional angles and a focused beam are needed.
2. Determine the acceptance decision and the additional checks required before a repair is ordered.
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The weld should not be accepted on the present data. A 22 mm long planar indication at 28 mm depth in a 40 mm butt weld would normally exceed the ISO 11666 acceptance level corresponding to ISO 5817 level B, but the exact acceptance depends on the length and height and must be checked against the applicable table. The disagreement in sizing means the acceptance case is not established.
Re-scan the area with a qualified PAUT procedure using at least two probe angles, a focused beam and encoded recording. Measure the indication height by tip diffraction or a focused probe, and compare with the acceptance criteria. If access permits, use RT as a cross-check; otherwise, remove the indication by grinding and confirm with magnetic particle or penetrant testing. Only then decide whether repair is required. Do not accept on amplitude 4 dB above reference alone.
3. Describe the repair and re-inspection sequence, including the root-cause checks you would make.
Read the worked answer
Excavate the indication by grinding or gouging until the defect is removed, and confirm removal by magnetic particle or penetrant testing. Maintain the required preheat for S355J2 if the section and procedure require it. Repair with a qualified WPS, controlling heat input and interpass temperature, and blend the repair flush. Re-inspect the repaired area by PAUT or RT, and repeat the examination after a suitable delay if hydrogen cracking is a concern.
Check the root cause: groove angle and root gap, fit-up, welding technique, and whether the original weld was made with the qualified parameters. Review the PAUT procedure and operator qualification if sizing remains uncertain. Document the defect type, size, repair and final acceptance. The client should receive the revised inspection report and the repair record before the vessel is accepted.