Microstructural factors of the complex creep rate change in 18Cr–9Ni–3Cu–Nb–N steel

The unique properties of 18Cr–9Ni–3Cu–Nb–N steel have prompted its wide application in various fields. However, the complex creep behavior of this material needs to be investigated for its further development. In this study, the microstructural factors of the two-step minima in the creep rate vs time curve of 18Cr–9Ni–3Cu–Nb–N steel at 873 K under an applied stress of 280 MPa were investigated through the microstructural characterization of a virgin sample, and five creep-interrupted and creep-ruptured specimens. The constituent of the first local minimum creep rate was identified as the precipitation strengthening by the coherently precipitated NbX phase and softening by the dynamic recovery driven by the initial plastic strain. The transition to the second deceleration is ascribed to the precipitation of the M23C6, NbX, and Cu phases on the grain boundaries, and precipitation of the Cu phase in the grain interior. In other words, the effectiveness of grain boundary precipitation strengthening and Cu phase precipitation strengthening for 18Cr–9Ni–3Cu–Nb–N steel was directly clarified. Moreover, the results noted that these strengthening mechanisms did not function from the beginning of the creep test due to the incubation time of the precipitates at 873 K. The retarded precipitation during creep at 873 K was suggested as the cause of the different trend of the creep data at 873 K compared with that at 898–1073 K.