Writing a Procedure for a Thick-Section Weld with Limited Access
A Level III must write a UT procedure for a 60 mm thick low-alloy steel weld in a pressure vessel. Access is limited to one side, and the weld cap is ground flush on that side only. The procedure must cover detection and sizing of lack of fusion and transverse cracks. Available probes are 2.5 MHz and 5 MHz, 45, 60, and 70 degree shear-wave wedges, plus a 0-degree longitudinal probe. The vessel code requires demonstration of coverage on a reference block with the same thickness and material. The customer asks for a written procedure that a Level II can follow without further engineering judgement.
1. What beam angles and probe frequencies would you specify, and why?
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For 60 mm thick low-alloy steel:
- 45 degrees is the primary angle for through-wall coverage of the fusion faces; it gives the shortest leg and the best chance of hitting lack of fusion oriented near-vertical.
- 60 and 70 degrees supplement for near-surface and for fusion-face orientation that the 45 misses. At 60 mm, a 70-degree beam has a long leg and is prone to mode conversion and attenuation; it should be used selectively.
- 2.5 MHz is preferred for the thicker section because attenuation and grain scattering increase with path length; 5 MHz may be used for near-surface resolution but must be justified against penetration.
- A 0-degree longitudinal probe is needed to check for laminations or delaminations that would affect shear-wave propagation.
The procedure should state the angle for each weld region, not a single angle for the whole weld.
2. How should the procedure address coverage demonstration and transverse crack detection?
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Coverage demonstration: the reference block must have the same thickness and material, with reflectors at the fusion faces and at the root. The procedure should require a plotted beam path showing that each fusion face is interrogated by at least two angles, and that the root and cap regions are covered. The demonstration should record the amplitude and position of each reference reflector.
Transverse cracks: these require a beam normal to the weld axis, which means a probe oriented across the weld rather than along it. The procedure must specify a separate scan pattern for transverse cracks, with the probe rotated 90 degrees, and a reference reflector oriented to simulate a transverse flaw. Without this, the procedure does not cover transverse cracking.
3. What must the procedure contain so a Level II can apply it without engineering judgement?
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The procedure must be prescriptive:
- Scan patterns and index positions with numeric values, not 'as required'.
- Calibration block, hole sizes, and DAC or TCG setup steps with tolerances.
- Scanning speed, overlap, and couplant requirements.
- Recording thresholds and the exact acceptance criteria clause from the code.
- Sizing method (e.g., 6 dB drop or tip diffraction) with the steps and the allowed error.
- Disposition rules for signals that cannot be classified, including the requirement to escalate to a Level III.
If any step requires judgement, the procedure is incomplete for the stated purpose.