During the winter of 2013-2014, Northern California experienced unusually cold temperatures (below 40°F). These low temperatures delayed Pacific Gas & Electric Company (PG&E) coating operations on new pipelines. Since liquid epoxy coatings capable of being applied at, or which fully cure at, temperatures below 50°F were not approved, PG&E chose to control the environment using small enclosures over the application area. This approach proved to be expensive and time consuming, so a novel method of pipeline surface temperature control was necessary to allow application of liquid epoxies at low ambient temperatures. 

Friction stir welding procedure development was initiated on steel grades S460 G2+M and S690QL1 in plate wall thicknesses of 10 and 15mm. In order to tackle the challenges of the high mechanical loads on the tool as well as its premature wear, a combination of preheating and optimized weld backing set-up was implemented. The inductive preheating allowed a 28% reduction in welding torque and a significant reduction of the tool wear, particularly during the critical initial plunge of the tool within the base material. A comparison of Mo-based and W-based tool was performed, allowing identifying the W-based tool as having a better combination of high temperature strength and wear resistance. Different backing arrangements and materials were investigated. Ceramic backing inlays were used in order to reduce the heat loss at the root area and maintain an acceptable stirring of this zone to achieve full penetration welds. The welds quality was assessed via metallurgical examination, bend tests, and confirmed the possibility to perform sound full-penetration, one-sided welds. The contribution of the preheating to the process as well as the quality and mechanical properties of the welds will be discussed here.