Film Density and IQI Sensitivity Failure on a Dissimilar Thickness Weld

The density split (1.6 thin, 2.8 thick) is inverted from what simple thickness alone would predict only if the exposure was set for the thin section and the thick section is under-penetrated - but here the thick section is denser, which points to a different cause: the beam is likely angled or the part is not flat, so the effective path length differs; more importantly, at 220 kV with 0.025 mm Pb front screen on stainless, the front screen is too thin to provide meaningful electron emission and the back screen (0.1 mm Pb) is doing most of the work. The likely dominant cause is that the exposure was set by time alone without a density step-wedge check, so the film is under-exposed in the thin section (density 1.6 is below the 1.8 minimum) and over-exposed in the thick section (2.8 is within 1.8-4.0 but high).

The IQI hole not visible: the 2-2T IQI on the source side is correct placement, but at density 1.6 the contrast is too low to resolve the 2T hole. Sensitivity is density-dependent; below ~1.8 the film is on the toe of the curve and contrast is poor. Also confirm the IQI is the correct size for the thick section (2-2T means 2% of thickness for the thick section), and that the IQI is not in a region of weld reinforcement that adds scatter.

Order of operations:

1. Verify the actual exposure chart and the density step wedge (or pre-exposure test strip) for this material and kV. Do not just add time - establish the correct mAs from a density vs. log E curve.
2. Confirm screens: for 220 kV on stainless, 0.025 mm Pb front is acceptable but consider 0.05-0.1 mm Pb front and 0.1-0.15 mm Pb back to improve contrast and reduce scatter; ensure screens are in intimate contact and not wrinkled.
3. Check geometry: beam perpendicular to the weld, no angulation that changes path length across the transition; use a mask or collimator to reduce scatter from the thicker section.
4. Re-place the IQI: source side, on the thick section, in a region of uniform thickness, with the 2T hole oriented for the viewing direction.
5. Reshoot a test strip at the thick section to hit density ~2.3-2.5, then adjust for the thin section; if a single exposure cannot satisfy both, use a dual-film or graduated technique, or shoot the transition in two exposures with overlap.
6. Re-read density with a calibrated densitometer and confirm IQI sensitivity meets code before releasing.

Do not accept on that basis alone. Density 2.8 is within the stated 1.8-4.0 window, so it is not a density violation per se. But the acceptance criterion that matters is IQI sensitivity, and the 2T hole was not visible. Density alone does not guarantee sensitivity; the film must resolve the required IQI hole at the specified density. If the hole is not visible at 2.8, the film is not acceptable regardless of density.

Also, a density of 2.8 in the thick section paired with 1.6 in the thin section means the two sections are on very different parts of the characteristic curve, so contrast and sensitivity are inconsistent across the weld. The code intent is uniform, demonstrable sensitivity over the required coverage. The correct response is to fix the technique so both sections meet density and the IQI hole is visible, not to defend the number.