Magnetic Particle Inspection on Welded Assembly
A magnetic particle inspection (MPI) is performed on a carbon steel weld overlay on a pressure vessel. The weld is 12 mm thick and the surface is prepared by grinding. The inspector uses a yoke with a lifting power of 4.5 kg (10 lb) on a 150 mm pole spacing. The yoke is placed on the weld and the area is dusted with dry magnetic particles. The particles are black iron oxide. The inspector observes a linear indication approximately 3 mm long, oriented transverse to the weld, and decides whether to accept or reject. The applicable code requires that any linear indication greater than 1.5 mm be evaluated. The inspector also notes that the yoke is due for calibration but the calibration sticker shows it was calibrated 11 months ago with a 12-month interval. The ambient lighting is 500 lux.
1. Evaluate the indication and determine the acceptance decision. Include considerations of indication type, size, and orientation.
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The indication is linear (length much greater than width) and 3 mm long, which exceeds the 1.5 mm threshold for linear indications. Since it is oriented transverse to the weld, it could be a crack or lack of fusion, which are typically rejectable. The code likely requires rejection of any linear indication exceeding the specified size, regardless of orientation, unless other criteria apply. Therefore, the indication is rejectable. The inspector should mark and document the location for repair.
2. Assess the equipment calibration status and its impact on the inspection validity. What actions should the inspector take?
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The yoke calibration is current because it was calibrated 11 months ago with a 12-month interval, so it is within the calibration period. However, the inspector should verify that the calibration certificate is on file and that the yoke's lifting power meets the code requirement (often 4.5 kg minimum at the specified pole spacing). The ambient lighting of 500 lux is adequate for proper evaluation of indications; typical requirements are 1000 lux for fluorescent and 500 lux for visible light, but for magnetic particle testing, 500 lux is often acceptable. The inspector should proceed with the inspection, but if the indication is critical, consider re-inspecting with a calibrated yoke after confirming its status. No immediate action is needed regarding calibration, but the inspector should ensure that all equipment is within calibration before starting any inspection.