Radiography Too Soon After Thick Groove Welding
A 1.25 in thick A572 Grade 50 groove weld in a bridge girder is completed at 2:00 p.m. with SMAW E7018. The WPS specifies a 100°F minimum preheat and no postweld heat treatment. Ambient temperature is 45°F. At 5:00 p.m., the same day, the contractor requests radiographic testing so the girder can be painted the next morning. The RT technician produces acceptable film with no indications. The inspector is asked to sign off the weld as complete. The joint is a complete-joint-penetration groove weld with backing; surfaces are accessible for magnetic particle testing on the root and cap.
1. Is the 3-hour RT timing adequate for detecting hydrogen-induced cracking? Explain.
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No. Hydrogen-assisted cracking in thick, restrained groove welds is delayed; it can initiate many hours after welding, often after 24 to 48 hours, especially with a low preheat and cold ambient. Radiography performed 3 hours after completion may be clean simply because the cracks have not yet formed. In addition, RT is not the most sensitive method for tight planar cracks oriented unfavorably; it may miss cracks that are not aligned with the beam. A clean film at 3 hours is not evidence that the joint will remain sound.
2. What NDT methods and timing should be used for final acceptance?
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Hold final acceptance until a suitable delay, typically 48 hours for this thickness and hydrogen-risk condition, unless the governing code or WPS specifies otherwise. Perform MT or PT on accessible root and cap surfaces after the delay; these methods are sensitive to surface-breaking cracks. Use UT where internal planar discontinuities are a concern and access permits. RT can remain part of the required volumetric examination, but it should not be the sole basis for final acceptance at 3 hours. Document the inspection timing and require reinspection of any repaired area after the same delay.
3. What should the inspector do with the acceptable RT film?
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Accept it as an interim volumetric examination only. Do not sign off the weld as complete based on a 3-hour RT film. Record the time of welding, the time of RT, the preheat used, and the ambient conditions. Require delayed MT/PT and, where specified, UT after 48 hours. If delayed inspection reveals cracking, reject the weld, remove the cracks to sound metal, and repair under a qualified procedure with adequate preheat and electrode control. Final acceptance should be based on the delayed examination results, not the early film.