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Ni-based superalloys are commonly used in hot gas path parts such as turbine blades because of their higher strength under high temperature. Their high strength results from an intermetallic compound such as a cubical γ' phase precipated in a γ phase. However, the γ' phase tends to show coarsening and lafting. The re-heat process has been adopted to repair the creep damage in the Ni-based superalloys. The re-heat process can restore the γ' phase by carrying out thermal cycles with an appropriate combination of temperature and duration. In this report, we focused on creep damage under the bending load. First, the creep constitutive law was developed to calculate creep strain more accurately. Then, we investigated the relationships between the effect of the re-heat process and creep crack initiation under the bending load through a mock-up creep test.The time-dependent hardening law was selected as a creep constitutive law. The calculated creep deformation showed relatively good correspondence with the results of uni-axial and mock-up creep tests. Also, creep crack initiation seemed to be controlled by the re-heat process at an appropriate stage. Re-heated specimens showed critical creep deformation, corresponding to creep crack initiation, about half that of virgin specimens. This seemed to be because of transgranular precipitation, not all of which can be removed using the re-heat process. In uni-axial creep tests, we found some critical value for cumulative creep strain, which causes harmful precipitation during the re-heat process. Therefore, we conclude that creep damage in Ni-based superalloys can be repaired effectively using the re-heat process before cumulative creep strain reaches a critical value that is restrained by the initiation of harmful precipitation or creep cracks.

In order to confirm the validity of the model for creep damage mechanism constructed from a series of experiments on reheat pipe elbow of mod. 9Cr-1Mo steel with a practical size, microstructural observation and uniaxial creep rupture tests were carried out using the welded joint of the hot reheat pipe elbows (No. 5 and No. 25) taken from a practical plant after long-term service. The obtained results are as follows : (1) Nucleation and growth of creep voids were observed in the fine-grained HAZ region of the inside-elbow region, where the triaxiality factor of stress was high compared to the other regions in the HAZ. The phenomenon of creep damage in the long-term used elbows was consistent with that in the practical sized experimental elbow used for the model construction. (2) The degree of creep damage observed for No. 5 and No. 25 elbows corresponded to the early stage of creep damage. But the creep damage of No. 5 elbow was more serious than that of No. 25 elbow due to the difference in degree of circularity between the two elbows. (3) Considering that the long-term used elbows underwent creep damage to some extent, the creep rupture strength except for the repair-welded specimen exhibited high enough to exceed the demand strength estimated from the life assessment curve for the welded joint of high chromium ferritic steels presented by Nuclear and Industrial Safety Agency.

An experimental investigation is reported of the zero pressure gradient boundary layer along a 90 degree streamwise corner. Turbulent and non-turbulent conditional sampling measurements in the outer intermittent region of the boundary layer as well as a comprehensive set of conventional measurements were made in a cross section 1.7m downstream from the leading edge. The results of a turbulent zone-averaged secondary velocity field indicate a pair of closed streamwise vortex patterns near the corner, as in conventional measurements. A large difference was found in the outer part of the boundary layer, where the turbulent zone-averaged secondary flow moves from the wall to the free stream direction. On the contrary, the non-turbulent zone-averaged counterpart moves from the free stream to the wall and it includes almost no streamwise vorticity, indicating that the secondary flow of the second kind is generated only when the flow is turbulent.

An experimental investigation of the zero pressure gradient turbulent boundary layer along a 90 degree streamwise corner is reported. Turbulent non-turbulent conditional sampling measurements in the outer intermittent region of the boundary layer as well as a comprehensive set of conventional measurements were made in a cross section at 1.7 m down stream from the leading edge. The results of a turbulent zone averaged secondary velocity field indicate a set of closed streamwise vortex patterns near the corner, as in the conventional measurements. A large difference was found in the outer part of the boundary layer, where the turbulent zone averaged secondary flow moves from the wall to the free stream direction. On the contrary, the non-turbulent zone averaged counterpart moves from the free stream to the wall and which includes almost no streamwise vorticity, indicating that the secondary flow of the second kind is generated only when the flow is turbulent.