Root Bead Welding of Duplex Stainless Steel Pipeline Girth Welds without Backing Gas

摘要

The pipeline industry has interest in implementing duplex stainless steel (DSS) instead of carbon steel for pipeline expansion projects due to the increase in corrosion resistance and strength. One disadvantage of using DSS, however, is the difficulty of producing acceptable root beads. As such, root bead trials were conducted using several welding processes and consumable combinations. The root bead trials pointed to one process/consumable combination as the most promising. This combination was submitted to BP Alaska and ConocoPhillips Alaska for procedure qualification using experienced pipeline welders. The qualification requirements included ASME Section IX, API 1104, and BP and ConocoPhillips DSS welding specification requirements for toughness, hardness and corrosion testing. The weldment passed all the requirements except the toughness requirement. The lack of toughness resulted in further development using different welding processes and consumables. The additional welds improved toughness but only one process combination, SMAW/GMAW, exceeded the toughness requirements. The results indicted SMAW as the best process for root bead deposition without backing gas and GMAW as the best process, after the root and hot pass, to fill the joint while improving toughness. Procedures have been developed that should pass industry standards, but have not yet been fully qualified.