An Alternative Fabrication Approach for Alloy 800H Helical Steam Generator Tubes: Autogenous Welding

Abstract Alloy 800H (N08810) has been historically used in the superheat regions of tube bundles in high temperature gas reactor (HTGR) steam generators, establishing its suitability as a material of construction in newer designs. In previous commercial scale HTGRs with helical coil steam generators, the full helical coil lengths were fabricated by circumferentially welding shorter tube segments together. To fit the requirements of a helical coil SG design without straight transition sections, one proposed solution calls for circumferential tube-to-tube welds to be bent into the final design geometry. Being subject to a HTGR environment, ASME Boiler and Pressure Vessel Code (BPVC), Section III, Division 5 design rules for Elevated Temperature Service could apply to these bent welds. Design rules currently exist for the use of filler metal welds in the creep regime, but their protruding root geometry on the inside of the tube is unfavorable to predictable flow development, inspection equipment accessibility, and may introduce crevices that become susceptible to water-side build-up and corrosion. Alternatively, an autogenous gas tungsten arc welding (GTAW) process or a hybrid method (i.e., an autogenous root pass with filler metal weld caps) for tube-to-tube circumferential welds is expected to produce a smoother weld root profile while also improving overall manufacturability; however, there is limited industry experience for cold bending autogenously welded tubes. Further, the relevant ASME design rules for high temperature reactors do not address tubes welded using autogenous GTAW processes. This paper presents initial examinations on circumferentially welded, cold bent, Alloy 800H seamless tubing segments using a GTAW autogenous root pass with filler metal caps welding process.