Research Progress on Friction Stir Welding of Pipeline Steels

Abstract

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.

Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

“…However, there are some outliers with CTOD values of 0.13 mm and 0.07 mm respectively, which were associated with material inhomogeneity due to the FSW deformation and thermal cycle. Samples with notches located at the HZ showed a steady behavior, mostly brittle with little plastic deformation before failure, and values below the acceptance limits, 0.1 to 0.2 mm for pipeline design or defect acceptation[41].…”

Modeling of thermal cycles and microstructural analysis of pipeline steels processed by friction stir processing

“…The heat in the process is mainly generated from viscous dissipation and frictional sliding in the contact region between the tool and the workpiece and is controlled by a spatial stickingsliding parameter based on the tool radius. The model is useful to predict the welding-affected zones and to provide thermal cycle evolution during FSW [19,20]. Thus, by merging computational modeling with post-processed microstructural analysis, a better understanding of FSP can be reached.…”

A Review of Orbital Friction Stir Welding

“…Regarding linear FSW, there is vast research and published work on successfully joining diverse aluminum, magnesium [61], copper, titanium [62][63][64], and steel [65][66][67][68][69][70] alloys, as well as dissimilar materials. However, as expected, published work related to OFSW is much more limited, although there are reports of successful welds on tubular specimens, mainly composed of aluminum [22,37,42,49,[71][72][73], steel [17,45,74], titanium [63], copper, magnesium, or zinc alloys, and also dissimilar materials (AL3003 and pure copper) [40]. FSW/OFSW performed on aluminum alloys that are considered nonweldable, such as the AA6XXX series, tend to be the main focus material of published work, which is understandable given the superiority of FSW/OFSW over the fusion methods for these alloys, filling an industrial "void" after so many years.…”