Research Progress on Friction Stir Welding of Pipeline Steels

Friction Stir Welding (FSW) has been widely commercialized to join aluminum alloys, but is yet to be broadly applied to structural steels. The primary difficulty in welding steels relates to severe loads and temperatures experienced at the interface between the FSW tool and the base material. These conditions are challenging even for the most advanced and expensive tool materials. However, within the last five years, tool advancements have begun to enable FSW of steels. Polycrystalline boron nitride (PCBN), tungsten-rhenium alloys, and mixtures thereof appear to be capable of producing sound welds in steel. This paper describes the results of a continuing study on the FSW of pipeline steels. Pipe grades from API X65 to X120 were subjected to FSW. Strength and toughness measurements using the crack tip opening displacement test were performed. The weld microstructure was evaluated using optical, scanning electron, and transmission electron microscopy. A computational fluid dynamics model was developed to better understand the effect of process parameters on thermal cycles, strain rates and strain experienced by material in the weld stir zone. The results indicate that the microstructure and properties of the welds have little dependence on the tool material, while significant variations in properties were observed between steels produced by different manufacturers. In general, obtaining high levels of toughness on par with gas metal arc mechanized girth welds appears difficult when using the FSW process. The results emphasize the need for a better understanding on the role of process parameters on microstructural evolution and weld quality during FSW of pipeline steels. As a full-scale demonstration of FSW on pipeline steels, several circumferential girth welds were produced in 762 mm (30 inch) diameter X80 pipe. The results of these efforts are discussed.