Age hardenable nickel-based alloy developments and research for new high temperature power cycles

Abstract

Advanced Ultrasupercritical (A-USC) steam Rankine cycles and Supercritical Carbon Dioxide (sCO2) Brayton cycles are under intensive development to enable low carbon generation of electricity. These high-efficiency power cycles, aimed at fossil and in some cases renewable energy, require higher tem­peratures and pressures compared to traditional steam cycles for pressuring retaining components such as tubing, piping, heat exchangers, and turbine casings. Extensive research and development to produce and characterize age-hardenable nickel-based alloys containing Al, Ti, and Nb in judicious amounts have allowed designers to now consider supercritical fluid temperatures up to ~760 °C which is much greater than today’s supercritical steam technology based on steel metallurgy up to ~610 °C. This paper will focus on the alloys developed around the world to enable these advanced power cycles, and a discussion on their key properties: long-term creep strength (100,000 h+), fabricability, and weldability/weld perfor­mance. Most of these alloys contain less than 25% gamma prime, such as alloy 740H, 263, and 282, due to the need for heavy section weldability, unique to these applications. While welding processes have now been developed for many of these alloys using a variety of filler metals and processes, key research questions remain on the applicability of processes to field power plant erection, the potential for cracking to occur during service, and the long-term weld creep and creep-fatigue performance.