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

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.