Alloy Design and Material Properties of Ni-based Superalloy with Low Thermal Expansion for Steam Turbine

Abstract

Advanced 700°C class steam turbines require austenitic alloys to replace conventional ferritic 12Cr steels poor at creep strength and oxidation resistance above 650°C. The austenitic alloys, however, possess a higher thermal expansion coefficient than ferritic 12Cr steels. Therefore, Ni-based superalloys were tailored to reduce their coefficients to the level of 12Cr steels. A regression analysis of commercial superalloys proves that Ti, Mo and Al decrease the coefficient quantitatively in this order, while Cr increases it so significantly that Cr should be limited to 12 mass% to secure oxidation resistance. The newly designed Ni-18Mo-12Cr-1.1Ti-0.9Al alloy is strengthened by γ′ [Ni3(Al,Ti)] and also A2B [Ni 2(Mo, Cr)] phase precipitates. It bears a RT to 700°C mean expansion coefficient equivalent to that of 12Cr steels and far lower than that of low-alloyed heat resistant steels. It surpasses a current turbine alloy, Refractaloy 26, in tensile strength at RT to 700°C and SCC life in 330°C deaerated pure water. Its creep rupture life at 700°C is equivalent to that of Refractaloy 26.