Approaches to Consistently Achieve Full Penetration in Pipeline Steel Welds Using Friction Stir Welding

摘要

In on-going FSW studies, achieving consistent full penetration in pipeline steels has proven to be a difficult goal when using a portable (orbital) friction stir welding (FSW) system capable of operation in the field. Metallography, mechanical testing, and workmanship testing (root and face bend) demonstrate that full penetration can be achieved through much of the weld length, and when full penetration is achieved, mechanical properties are excellent. However, at times sections of the weld do not exhibit full penetration. As an example, metallography and root bend tests of the weld root have shown full penetration at many locations around the pipe circumference while from the same weld, locations are identified where remnant faying surfaces remain and full penetration was not achieved. Further, some welds exhibit full penetration and yet at some locations these welds are accompanied by a relatively continuous oxide path that remains at the weld root. Conversely, if the FSW tool penetrates into the support anvil, anvil material is drawn into the weld nugget. It is not known at what length a lack of penetration flaw becomes a defect, if the continuous oxide is a flaw or a defect, or under what conditions weld nugget contamination by the anvil is a defect. By definition, a flaw is an imperfection in the weld zone whereas a defect negatively impacts properties or performance. Indeed, the difference between being an innocuous flaw or a harmful defect that reduces service life is likely a function of the service environment and operating conditions. Lack of penetration, continuous oxide flaws, and anvil material contamination of the weld nugget are difficult flaws (defects) to locate using conventional nondestructive testing (NDT) methods. Accordingly, until proven otherwise, flaws of these types must be considered unacceptable in a pressurized gas pipeline structure. Weld properties achieved following FSW of X52 and X60 pipeline steels, examples of weld flaws that may occur, and briefly, the limits of NDT methods used to detect these flaws, will be illustrated.