Duplex stainless steel for pipeline applications

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Duplex stainless steel for pipeline applications

Singh, A.K. / Bagdasarian, A.J.

The duplex stainless steel, alloy 2205 (UNS S31803), is an Fe-Cr-Ni alloy with a microstructure consisting of ferrite and austenite in approximately equal proportions. Recently Santa Fe Braun specified this material for transporting mixed produced fluids and gases on a major Alaskan North Slope project. The gas phase of this line contains a large amount of carbon dioxide, and the associated brine contains up to 300 ppm H2S. The initial problem with the use of this material in the given service was that the effects of welding on corrosion properties were not known. It was also considered necessary to optimize the selected welding method (GTAW) from the standpoint of welding technique, corrosion resistance and adequate low temperature toughness in the weld and heat-affected zone. A welding procedure was developed for field joining of large diameter duplex stainless steel pipes. The variables studied were filler metals, joint geometry, type of purge gas and heat input. Corrosion properties were studied of welds made with varying parameters. Tests were conducted for intergranular corrosion, pitting corrosion and sulfide stress corrosion cracking. The test results showed that welds made with a proper welding technique do not significantly alter the corrosion properties of duplex stainless steel. These tests also established the need for proper ferrite control in order to obtain optimum weld metal properties. The weldment produced using optimized parameters had mechanical properties and a ferrite/austenite ratio similar to those of the pipe material.