Ir-192 Weld Radiograph with Two Problems
A 25 mm thick double-V butt weld in carbon steel is shot with Ir-192 at 500 mm source-to-object distance, film in contact on the far side, 3 mm effective source size. Density measures 2.8 in the parent metal but 4.2 under the weld cap; your procedure requires 1.8 to 4.0 in the area of interest. A wire IQI on the source side shows the 0.50 mm wire, which satisfies the 2% sensitivity requirement for the section. One film, exposed with the beam at 90 degrees to the weld, shows a 1.5 mm round, sharply defined dark spot at mid-thickness. A second film of the same area, exposed with the beam skewed 15 degrees, does not show it. The film carries no visible scratches and the cassette was reloaded between exposures.
1. Calculate the geometric unsharpness and state whether the technique's geometry and sensitivity are acceptable.
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Using Ug = F x T / D, where F is the effective source size, T is the distance from the source side of the object to the film, and D is the source-to-source-side distance:
Ug = 3 mm x 25 mm / 500 mm = 0.15 mm
Against the usual limit of 0.51 mm for material in the 25 to 50 mm range, this is comfortably acceptable - the source is small and the source-to-object distance is generous for this thickness.
Sensitivity: the required 2% of 25 mm is 0.50 mm, and the 0.50 mm wire is visible, so the required sensitivity is met.
Two conditions to verify before signing this off:
- T is taken to the film, so any gap between the part and the cassette adds directly to T. A 25 mm gap would raise Ug to 3 x 50 / 500 = 0.30 mm, still acceptable but no longer negligible.
- The IQI density and location must be in the area of interest and within the density range; an IQI read at 4.2 is not a valid sensitivity demonstration.
The geometry is fine. The problem with this radiograph is density, not unsharpness.
2. The density under the cap is 4.2. What does that mean for the film and what are your options?
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The area of interest includes the weld, and 4.2 is outside the 1.8 to 4.0 range. The film is therefore not acceptable for interpretation of the weld, regardless of how good the sensitivity looks in the parent metal.
The cause is straightforward: the cap adds thickness, so the beam is attenuated less in the parent metal than under the cap. The exposure was set for the parent metal and the cap ran hot.
Options, in order of preference:
- Re-shoot with higher energy (or a source better matched to 27–30 mm of steel) so the relative difference in attenuation between parent and cap is smaller and both fall inside 1.8 to 4.0.
- Thickness compensation: place a shim of approximately the cap thickness over the parent metal so the two regions record similar density, then confirm both are in range.
- Multi-film technique with separate exposures/interpretation for the cap region, if the procedure allows it.
Do not simply interpret the 4.2 area - the density limit exists because film contrast and the readability of small indications change outside the qualified range, so a defect could be missed or misread.
3. How do you resolve the 1.5 mm spot that appears on one exposure only?
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First establish whether the second exposure actually covered the same volume. A 15 degree skew moves the projected image of an internal feature, so 'not seen on the second film' may mean 'not in the same place on the second film' rather than 'not in the object'.
Then work through the possibilities:
- Artifact: dirt or a screen defect, a static mark, a processor roller mark, or a handling mark in the emulsion. Examine the original radiograph under magnification with a bright light; a feature that sits only in the emulsion, or that has a lighter halo, is not a defect in the steel.
- Real internal discontinuity: a gas pore or a small slag pocket at mid-thickness. At 1.5 mm it is well above the demonstrated 0.5 mm sensitivity and would be relevant if real.
- Geometry/cap effect: unlikely at mid-thickness with this beam orientation, but worth checking against the cap profile.
Action: re-shoot the region, ideally with the same technique plus a second skewed view that is positively located over the same weld length, and inspect the film in the darkroom before processing is complete where possible. Only after the indication reproduces on a properly located, in-density-range film do you classify it and evaluate against the acceptance criteria.