Alloy design of Ni-base superalloys aiming for over 750°C class A-USC steam power plant

To reduce CO₂ emissions, ongoing research and development on Advanced Ultra Super Critical (A-USC) power generation aims to enhance the efficiency of coal-fired power generation. In an A-USC plant, it is necessary to use higher strength Ni-base superalloys in addition to conventional heat-resistant steel. However, since the manufacturability of existing commercial Ni-base superalloys is not as good as that of heat-resistant steel, development of new materials with excellent manufacturability and high-temperature strength has been underway. 700°C class A-USC materials are the mainstream of current research, and trial production of 10 ton-class forged ingots has been reported. However, there have been no reports on the development of A-USC materials that can be used at higher temperatures. In this report, an alloy design for Ni-base superalloys that can be used at over 750°C is presented and the basic strength properties are reported. The alloy design was based on the Calculation phase diagram (CALPHAD) method to estimate the effect of each alloying element on phase stability. Eliminating Ti, Nb, and Ta, which are commonly alloyed to conventional Ni-base superalloys, is considered to contribute to a better balance between strength and hot-workability. A prototype alloy with a composition of Ni-23Co-18Cr-8W-4Al-0. 1C [mass %] exhibits twice the creep strength of Nimonic263, and the estimated 10⁵h creep resistant temperature for a steam turbine is 780°C.