Refurbishment of Creep Damage Using Re-Heat Process for Ni-Based Superalloy under Bending Load

Abstract

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.