Beneficial role of the grain boundary precipitates for intergranular creep fracture of 25Cr-20Ni-Nb-N steel

Creep deformation and fracture behavior of pre-aged 25Cr-20Ni-Nb-N steel were examined to determine the effect of the M23C6 phase precipitated on the grain boundary in the initial microstructure. Aging at 873 K, 923 K, and 973 K for 500 h resulted in an area fraction of the precipitates on the grain boundary of 51%, 79%, and 89%, respectively, and the time to rupture at 873 K under 280 MPa increased with the area fraction. Additionally, the creep elongation was significantly improved by aging at 973 K, whereas that at 873 K and 923 K did not. Because the coherent matrix/M23C6 interface exhibited high crack propagation arrestability, the crack propagation path in the microstructure was the incoherent matrix/M23C6 interface and uncovered grain boundaries. The different creep deformation behaviors between the aging at 923 K or below and 973 K are discussed using the percolation theory of the crack propagation path. It was suggested that the enhanced creep properties due to the aging at 973 K were attributed to a sufficiently high area fraction of the precipitates on the grain boundary, which fragmented the crack propagation path and retarded the intergranular creep fracture.