Use of PE-GTAW to Control Microstructure in Duplex Stainless Steels

Nathan Ames Dr.Christopher Frye Knut Larsen EWI ExxonMobil Development Sandvik

Columbus, OH, USA Brisbane, Australia Sandviken, Sweden

ABSTRACT A combination of fundamental research and applied engineering led to an improvement in super duplex weldability. Work completed jointly by EWI, Sandvik and Swagelok Co. (Ames, Johnson, and Holmquist, 2000) concluded that through proper use of the penetration enhanced gas tungsten arc welding (PE-GTAW) super duplex microstructure could be controlled through a wide range of heat inputs. With the help of ExxonMobil, the PE-GTAW process was implemented in a production umbilical fabrication facility and deployed on a test umbilical qualification string. Results demonstrated superior welds produced at lower cost. This paper provides a comprehensive look at the entire program, including the history of the PE-GTAW process, the academic research looking into the microstructure effects of the process and final the applied engineering required to deploy the process. KEY WORDS: PE-GTAW, Super Duplex, A-TIG, Flux SAF 2507, Welding INTRODUCTION Oil and gas exploration continues to push existing technology as it moves into deeper waters and higher pressure systems to access today’s reserves. One of the key enabling alloy systems for this exploration is duplex stainless steels. These alloys have excellent corrosion resistance and mechanical properties, both of which are required to meet the industry's needs. As with most alloy systems, the advantages of super duplex stainless steel’s enhanced corrosion and mechanical properties comes with the negative side affects of difficult weldability. Historically, welds made on duplex and super duplex stainless steels required tight control of heat input, filler materials and shielding gas. Several deepwater umbilical applications in recent years have found that traditional welding techniques are marginally acceptable in terms of their ability to meet the required performance. Additional microstructural control is needed in order to attain acceptable and repeatable welds in real world fabrication. A combination of academic research and applied engineering led to an improvement in super duplex weldability. Work completed during the last few years (Ames, Johnson, and Holmquist, 2000;Ames, Johnson,

and Lippold, 2002;Ames, Ramberg, Johnson, and Johns, 2002;Nystrom, Ramberg, Marshall, Williams, Larsen, Wuertz, and Ames, 2001) has illustrated that through proper use of PE-GTAW, super duplex microstructure can be controlled through a wide range of heat inputs. ExxonMobil was one of the first organizations to investigate the PE-GTAW process for microstructural for a production application. A team comprised of ExxonMobil, Sandvik Materials Technology and EWI implemented this technique into a production umbilical fabrication facility and deployed it on a test umbilical qualification string. The information contained in this paper explains the basic research conducted to understand the underlying physics behind the PE-GTAW process and more importantly their impact on the performance of a welded production umbilical. The combination of improved weld properties and the mechanical properties of super duplex stainless steel have generated positive results. The work completed to date has found that PE-GTA welding yields improved corrosion and mechanical properties during manufacturing – all with less weld rejects and thus reduced fabrication costs. BACKGROUND A vast majority of the early research pertaining to the activated tungsten inert gas process or A-TIG (as it was originally known) originated in the Ukraine from the Paton Welding Institute. Publications dating back to the mid-1960’s (Goryachev and Zelenenin, 1964;Gurevich, Zamkov, and Kushnirenko, 1965) discuss the use of activation fluxes used in combination with the GTAW process. These publications are typically introductory papers and offer minimal information regarding the mechanisms controlling the process. Similarly, there was little (in terms of today’s technology) information present regarding the net effect on resultant microstructures. The important commonality of the early publications was that simply welding through or over these activation fluxes enabled the GTAW process to perform more like a high energy process than is typically expected. The activation fluxes when used in combination with a conventional GTAW system yielded penetration increases of 100% to 300% on a wide variety of materials. As the cold war ended and technology exchange increased between eastern and western cultures, there was a notable increase in