Hitachi and Hitachi Metals have developed low thermal expansion Ni-base superalloy, Ni-20Cr-10Mo-1.2Al-1.6Ti alloy (USC141) for use as A-USC steam turbine material. The approximate 10 5 h creep rupture strength at 740° C is 100MPa, so USC141 can be expected to apply for blades and bolts. Now we have been studying to get better creep properties by microstructure controlling such as grain size or grain boundary morphology. In addition, the segregation test of USC141 shows good Freckle tendencies, it means that it would be easy to make a large ingot which could be used as rotors or pipes. From these calculation results, we have been tried to make an 850mmϕ ESR ingot of USC141.

To improve microstructure stability at temperature up to 700°C and avoid segregation of Nb during melting processes, we modified the chemical composition of conventional Ni-Fe base super alloy(Ni-36Fe-16Cr-3Nb-1.7Ti-0.3Al:Alloy706). It is known that Alloy706 is strengthened by γ'(Ni 3 Al) phase and γ”(Ni 3 Nb) phase. But these phases are unstable at high temperature and transform into Nb rich δ or η) phase after long-term exposure to elevated temperature. Therefore modified alloy contains lower Nb and higher Al than those of Alloy706, and it is mainly strengthened by γ’(Ni 3 Al) phase. In fact we could not find δ or η phase in the modified alloy after creep and aging at 700 °C. Tensile strengths of the modified alloy at temperature from room temperature to 700 °C were almost same as those of Alloy706. Yield strength of modified alloy at room temperature was slightly lower than that of Alloy706, but equivalent to that of Alloy706 at higher temperatures. Tensile and yield strengths of the modified alloy at temperature from room temperature to 700 °C were higher than those of Alloy706 after aging at 700 °C. In this paper, we discuss effects of Nb and Al on mechanical properties and microstructure at elevated temperature up to 700 °C, using mechanical testing, TEM observations and thermodynamics calculation results. And we show advantages of the microstructure stabilized Ni-Fe base superalloy(FENIX-700), which is a candidate material for 700 °C class USC steam turbine rotor.