The 100 mm Cr-Mo Shell: Two Radiographers, Two Answers
A fabricator finishes longitudinal SAW seams in a 100 mm (4 in.) thick 2¼Cr-1Mo (SA-387 Gr.22 Cl.2, P-No. 5A) hydrocracker shell. The contract requires a 48 h hold before final NDE and full UT of all 'A' welds after PWHT. Under schedule pressure, the shop's Level II runs final UT 24 h after welding: 4 MHz single crystal, 60° shear wave, one side only, DAC from a 3 mm side-drilled hole in a carbon steel block. He records no indications. PWHT follows at 690 °C for 8 h. Post-PWHT MT is clean, the hydrotest passes, and the AI signs. A third party then re-scans one seam with 2 MHz dual-crystal and 45° probes and reports 6–10 mm long mid-wall planar reflectors in the coarse-grained HAZ at 40–60 mm depth.
1. The shop and the third party disagree. Which examination reflects the true condition of the seam, and why did the two results diverge? Address timing, mechanism, coverage and calibration.
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The third-party result must be treated as credible; the shop's 'clean' result is not evidence of a clean weld. Four independent reasons, and any one of them is fatal to the shop's conclusion:
- Timing vs. mechanism. Two mechanisms are live here. Hydrogen-assisted cold cracking nucleates and grows over tens of hours in a 100 mm section that retains hydrogen; a 24 h inspection precedes the contract's 48 h hold, so cracks that initiated later were not there to be found. Separately, reheat cracking occurs during PWHT in the coarse-grained HAZ of Cr-Mo steels. A pre-PWHT examination cannot, by definition, detect it. Any seam in this material must be judged post-PWHT.
- Coverage. A single 60° beam from one side does not insonify the full volume of a 100 mm weld. Reheat cracks and lack-of-fusion are planar and approximately normal to the weld axis; a 60° beam strikes them at a poor angle and returns weak specular energy. 45° is the better orientation, which is exactly why the third party found them.
- Calibration. A DAC built on a carbon steel block with a 3 mm SDH is not a validated reference for a 100 mm Cr-Mo path: velocity is similar but attenuation and beam spread over that path length are not, and no transfer correction is evidenced. Amplitude alone, without through-wall sizing, tells you the reflector is 'small' when it may be tall.
- The methods that 'passed' prove nothing about mid-wall. MT is surface-only. A hydrotest at 1.3× design can compress a tight planar flaw rather than open it; passing it is not a volumetric result.
Therefore: the reflectors are real until disproved by a properly qualified re-examination, and the question shifts from 'who was right' to 'what is the flaw population and does it meet acceptance'.
2. What must the Level III require before this shell is accepted or rejected? Lay out the decision sequence.
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- Characterise before you judge. Full 100% re-scan of the seam with a procedure demonstrated on a mock-up of the same thickness, P-No. and joint geometry containing known reflectors. Record for every reflector: position (surface distance and depth), length, through-wall height, orientation, and whether it is crack-like (branching, sharp, intergranular) or smooth planar (lack of fusion) or volumetric (slag). Acceptance cannot be applied to a length and a depth alone; the height drives the decision.
- Cross-check the process, not just the weld. Pull the PWHT chart: soak temperature uniformity, heating and cooling rates, and thermocouple placement. Reheat cracking in 2¼Cr-1Mo is a process signature - if the HAZ hardness and the thermal record are consistent with it, the finding is systemic across seams, not isolated. Extend the re-examination to the other 'A' seams.
- Apply the correct acceptance basis. UT substituted for RT must follow the UT-in-lieu-of-RT rules of the code of construction, with the AI's concurrence; the RT acceptance limits are not automatically transferable to an amplitude-based UT result. Where the contract invokes fitness-for-service, the assessment must be agreed in writing before it is used.
- Decide on evidence. If the flaws exceed acceptance, the disposition is excavation of the full flaw height plus a re-weld with the qualified WPS, re-PWHT, and re-examination after PWHT. Do not accept on the strength of the hydrotest or the AI's earlier signature - the earlier signature was based on an invalid examination.
- Document the nonconformance and notify the AI and the owner, including why the original examination was invalid. The integrity of the record matters as much as the weld.
3. Specify the technical changes you would mandate in the UT procedure for this seam, and justify each in one line.
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- Post-PWHT as the governing examination (plus a pre-PWHT check only if the 48 h hold is respected) - reheat cracks do not exist before PWHT.
- 1–2.5 MHz, dual-crystal or focused probes - lower frequency reduces grain-boundary scattering and attenuation over a 100 mm path and improves signal-to-noise in the coarse HAZ.
- 45°, 60° and 70° from both weld sides, full volume scan plan - no single angle covers near-vertical planar flaws and the root/volume regions simultaneously.
- Reference block of the same material and heat-treatment condition, or a documented, validated transfer correction - amplitude transfer from carbon steel over 100 mm is not defensible.
- TCG/DAC plus through-wall sizing (or PAUT/TOFD) and a written sizing-accuracy demonstration - acceptance depends on height, not amplitude.
- Documented signal-to-noise criterion and a mock-up demonstration with known flaws - this is the only evidence that the procedure can find what it claims to find.
- Defined scanning surface preparation and couplant control, and recording of every reflector above a stated threshold - otherwise 'no recordable indications' is an artefact of the threshold.