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Nickel-base alloys are required for many of the components in advanced ultra-supercritical steam and CO2 power systems operating at temperatures and pressures exceeding 1202°F (650°C) and 3.6 ksi (25 MPa). Age-hardened alloys offer a distinct advantage over traditional solid solution strengthened alloys by virtue of their significantly higher creep strength. This makes it possible to reduce wall thickness and thereby minimize total construction cost. INCONEL alloy 740H (UNS N07740) is an age-hardened alloy that was developed and extensively characterized for advanced ultra-supercritical steam boilers. Material testing by the A-USC Consortium and US Department of Energy led to ASME Code Case 2702 covering UNS N07740. Alloy 740H is the first age-hardened nickel-base alloy permitted for welded construction for use in the creep limited temperature regime. More recent development work on the alloy has focused on applications for supercritical CO2 systems. Various laboratories have reported on oxidation properties of the alloy under simulated operating conditions. This paper focuses on the manufacturing and properties of tubing and fittings that are being applied for the various advanced ultra-supercritical steam and supercritical CO2 projects now planned or underway. As many of the structures are constructed by welding, a review of welding practices is presented, including dissimilar welds and their properties.

Alloy 740H was developed specifically for boiler tube and steam transfer pipe for the Advanced-Ultrasupercritical steam boiler application. The composition was formulated to provide creep strength, weldability and oxidation and coal ash corrosion resistance. It is the first nickel-base age-hardened alloy to be approved under Section 1 of the ASME Boiler and Pressure Vessel Code. Although a significant amount of microstructure and creep data was submitted to support the code case, it was largely collected on tube and light plate, much of this from pilot-scale material. The data package did not cover other product forms, sizes, hot working methods and weld joint configurations that may be required to construct a power plant. These include large diameter pipe, bends, fittings, heavy section welds, dissimilar metal welds, etc. that have complex thermal cycles and composition gradients. An ongoing effort is underway to characterize the microstructure and properties of alloy 740H under a range of manufacturing processes and to project these results through the potential life cycle of a plant. This paper describes the current status of this work. Paper published with permission.

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