2019 Volume 68 Issue 9 Pages 673-679
The long term creep degradation mechanism and the feasibility of creep damage analysis by means of hardness method have been evaluated for γ’-Ni3(Al,Ti) precipitation strengthened wrought superalloy, TOS1X-2, developed for 700°C class Advanced USC steam turbine rotor material. EBSD quantitative analysis of crept specimens suggested that creep deformation occurred at both grain boundary and grain interior. Very fine particles of γ’ phase at grain interior showed Ostwald type ripening in accordance with LSW theory, and PFZ appeared at grain boundary in crept portion. The linear relationship between hardness and γ’ precipitation density suggested that the softening found at simple aged condition is the result of the change in γ’ precipitation strengthening, which follows Orowan’s dislocation by-pass model. Also the relationship between hardness and local creep strain suggested that the hardening occurred at creep aged condition is due to work hardening. The procedure of creep damage assessment for precipitation strengthened superalloy by hardness was established, by means of estimating the amount of local creep strain in stress concentrated portion.