Insights into the precipitation-dominated creep behavior of a 25Cr20Ni-Nb-N austenitic heat-resistant steel via interrupted creep

The creep behavior of austenitic heat-resistant steels (A-HRS) determines their application and safe operation in modern advanced ultra-supercritical power plants. To date, understating of the creep behavior and corresponding microstructural evolution has relied on creep rupture tests, therefore, the evolution of complex precipitates and their effects on properties remains debated. Here, a series of interrupted and ruptured creep tests were conducted on 25Cr20Ni-Nb-N (HR3C) steel at 700 °C under the stress of 180 MPa, 150 MPa and 120 MPa. It was found that the creep deformation was predominantly controlled by dislocation gliding that interacted with the secondary Z-phase dispersions in grain interior. While the associated fracture mechanism was the intergranular fracture dominated by wedge cracking that was accelerated by the σ-phase and coarse M23C6 at grain boundaries. It was further demonstrated that the creep strengthening was dominated by the shearing mechanism originated from the secondary Z-phase dispersions. Conversely, the contribution of Orowan bowing from M23C6 and primary Z-phase became negligible as their coarsened size. Furthermore, it was clarified that the dominant strengthening of the secondary Z-phase and the subgrains to the microhardness development, whereas the contribution of M23C6 and σ-phase is slight.