Friction stir welding of pipeline steels

In prior FSW studies, consistent full penetration in pipeline steel has proved to be a difficult goal when using a portable FSW system capable of operation in the field [1]. In a previous study, metallography, mechanical testing (tensile and Charpy impact), and workmanship testing (root bend) demonstrated 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 there remained sections of the weld that did 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 accompanied by a 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. Thus, different approaches need to be developed to assure consistent full penetration without anvil contact. In the study reported herein, two approaches to achieve consistent full penetration are being evaluated. These methods include 1) root arc welding followed by a partial penetration friction stir weld and 2) use of a sacrificial anvil. Arc welding of the root prior to FSW builds on a technique developed at ExxonMobil whereby an internal root arc weld can be used to provide support for the FSW process. After the internal root weld is made, the butt joint surfaces remain and a partial penetration friction stir weld penetrates into the arc weld root pass. In addition to providing structural support to resist FSW loads, with overlap of the two weld nuggets, there is no possibility of a root lack of penetration defect. This technique takes advantage of the efficiency afforded by internal root welding for onshore pipeline construction. Arc welding of the root is a commercialized widely used practice. For the arc weld / friction stir weld approach, mechanical properties and root bend test results are reported. 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 into the sacrificial anvil thus achieving consistent full penetration. Removal of the small sacrificial anvil may or may not be required.