Microstructural changes and creep-strength degradation in 18Cr-9Ni-3Cu-Nb-N steel

Highlights: • Creep strength degradation was observed in the long-term in 18Cr-9Ni-3Cu-Nb-N steel. • The dislocation density in grain interior was kept high even after creep rupture in the long-term. • Precipitation free zone (PFZ) was formed around σ phases on grain boundaries after long-term creep. • The hardness was lower in PFZ than in grain interior. The mechanism of long-term creep-strength degradation was investigated for 18Cr-9Ni-3Cu-Nb-N (KA-SUS304J1HTB, ASME Code Case 2328) steel, with a focus on changes in fracture mode and microstructure. At 650 to 800 °C, the long-term creep strength deviated from the trend of short-term data plots. The stress dependence of the minimum creep rate in the low-stress regime was different from that in the high-stress regime at 650 to 750 °C. The creep ductility initially increased with increasing Larson–Miller parameter and subsequently decreased at high values of the Larson–Miller parameter. M₂₃C₆, NbX, and Cu phase particles were confirmed to exist after short-term creep exposure, whereas a modified Z-phase and σ-phase were observed after long-term creep exposure. The fracture mode changed from creep void formation on grain boundaries to cracking at the interface between the σ-phase and the matrix in the long-term. The dislocations were pinned by precipitates in the grain interior, and the dislocation density in the grain interior remained high, even in the long term, at 600 to 700 °C. A precipitate-free zone formed around the σ-phase on grain boundaries after long-term creep exposure. The dislocation density was also low in the precipitate-free zone. The hardness of the precipitate-free zone was lower than that of the grain interior. We believe that the change in the fracture mode and the formation of a precipitate-free zone are responsible for the observed long-term degradation in the creep strength.