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The oxidation behavior of GH2984 alloy in pure steam at 750℃ and 850℃ was investigated by using X-ray diffractometer (XRD) and scanning electron microscope equipped with energy dispersive spectra (SEM/EDS). Results show that GH2984 alloy is oxidized parabolically with time. The temperature rising greatly promotes the external and internal oxidation and also the evaporation of Cr. The composition and microstructure of the oxide films change obviously. At 750℃, a compact and single-layered (Cr,Mn)2O3 film is formed on alloy surface; the number of voids in oxide scale increases at 850℃, whereas a three-layered oxide scale consisting of thin Fe2TiO5 outer layer and much thicker (Cr,Mn)2O3 middle layer and thinner (Nb,Mo)2O5 inner layer is formed at the elevated temperature. Ti and Al are internally oxidized preferentially along grain boundaries to be TiO2 and Al2O3, respectively; both the number and the size of the two internal oxides increase with the temperature rising.

Unstressed exposure tests of Inconel 740H alloy tube were carried out at 750℃ for 500-3000h. The microstructure evolution and microhardness were studied by means of thermodynamic simulation, OM, FEG-SEM and microhardness testing. The results show that the tube is qualified if both chemical composition and tensile properties of the as-received alloy meet the corresponding requirements of ASME. After long term exposure, the main precipitates are γ' and M23C6, and no η and σ phase. With the prolonging of exposure time, the coarsening of γ' becomes faster and the law of relationship between the radius of γ' and time accords with LSW Ostwald ripening law; meanwhile, the change in size of M23C6 is not so obvious. During the whole process, microhardness increases firstly and then decreases, but the fluctuation is slight. The changes of microstructure and hardness indicate that, after long time exposure, the domestic Inconel 740H has good stability and can be used for further carrying out the investigation on the mechanical property of creep-rupture.