抄録
Nickel (Ni)-base refractory alloys such as Alloy 625 are used for structural material of reducing roasting furnace in spent nuclear fuel reprocessing plant because they have good high temperature strength and corrosion resistance in nitric acid. The operating temperature of the furnace is around 1073 K. From past studies about thermal aging effect on microstructure of Ni-base alloys, it is reported that intermetallic compounds and carbides precipitate during thermal aging. For example, δ-phase (Ni_3Nb) precipitates after 30 hr at 1073K and δ-phase transforms to γ"-phase after 100〜300hr. It is considered that precipitation and phase transformation by thermal aging cause precipitation hardening and loss of creep ductility. Therefore, it is important to evaluate thermal aging effect on creep properties of Ni-base alloys in the furnace at the operating temperature to predict its lifetime but there is few report on the effect of thermal aging for creep properties. In this study, to evaluate the effect of thermal aging on creep properties of Alloy 625, we carried out creep tests on aged and solution-treated Alloy 625 at 1073 K. According to the creep test results, time-to-rupture decreased by thermal aging when test stress was more than 100 MPa, but did not change when test stress was less than 100 MPa for any specimens. In the solution-treated alloy, creep deformation behaviors changed over 100 MPa. These results show that time-to-rupture was constant because intermetallic compounds precipitated when the test stress was less than 100 MPa in solution-treated alloy. The observed relationship between creep strain rate and test time showed that the precipitation started after 100 hr for solution treated alloys. These results suggest that intermetallic compounds precipitate immediately after furnace operation. And it is appropriate to use creep data of thermal-aged Alloy 625 for the reducing roasting furnace lifetime prediction.
