WELDING OF PIPES The bevels for GTAW Root pass as per ASME B16.25 is Fig No-5 and Fig No-6 for thickness of pipes greater than 10 mm. The figures indicate a cylindrical surface of 5 mm in the inside and also a broken line indicating the transition from the welding groove to the body of the component. We have adopted a transition straight away in the inside instead of 5 mm straight machining. We have now come to know that the straight machining of 5 mm is preferable when compared to that of tapered transition for radiographic evaluation. But we have been adopting this transition in many jobs without experiencing any problem in evaluation of the welding. We request your views on this . In case you feel the transition is OK without straight machining, please provide your view points for justifying the same since our customer feels that straight machining of 5 mm is preferable.
06:00 Jun-06-2003 Doug Breeze Director, Consultant; R&D Inspection Software Limited, United Kingdom, Joined May 2000 20
Re: Radiographic Examination of Weld in Pipes
The surface you describe is (or used to be!) called a counterbore. It has lots of benefits and few drawbacks, except for the machining costs...
1. Machining an identical counterbore on both joining pipes should prevent root-misalignment. This is leads to a more easily welded root-run, with less likelhood of root defects.
2. The counterbore provides uniform geometry on each side of the root so that radiography (and ultrasonic) inspection can more readily differentiate real defects from false geometric indications.
3. The counterbore reduces the included-angle at each root edge, and also distributes stresses adjacent to the root over a larger area thus minimising the risk of cracking and/or stress corrosion in this critical area.