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. Based on the observations made during this initial pipeline FSW study, the most critical goal was to identify the most efficient means of assuring consistent full penetration in friction stir welded pipeline steel. Based on the hypotheses that 1) root notch lack of penetration is a flaw/defect that needs to be avoided, 2) achieving consistent full penetration in the field without anvil contact is impractical with current FSW methods, and 3) available NDT techniques are inadequate to locate small root notch lack of bonding flaws, new approaches for pipeline FSW are proposed. Two approaches to eliminate the potential for lack of penetration flaws at the weld root are being evaluated. These methods include 1) use of a sacrificial anvil and 2) root arc welding followed by a partial penetration friction stir weld. The sacrificial anvil approach uses a small insert in the structural anvil where metal of the same chemistry as the pipeline material is used as the insert material. In this approach, the FSW tool penetrates through the pipe wall thickness and into the sacrificial anvil achieving consistent full penetration in the pipeline steel. Removal of the small sacrificial anvil may or may not be required. Arc welding of the root prior to FSW builds on a technique developed at ExxonMobil whereby an internal root arc weld is used to provide support for the FSW process. This technique takes advantage of the efficiency afforded by internal root welding for onshore pipeline construction. After the internal root weld is made, the butt joint faying surfaces remain and a partial penetration friction stir weld penetrates into the arc weld root pass.
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