Abstract
Effects of carbon and nitrogen on creep strength and microstructure have been investigated on precipitation strengthened 15Cr ferritic steel. Creep rupture life is extended longer than of conventional ferritic creep resistant steel of ASME T92 with addition of 0.07mass% N and a combination of 0.05mass% C and 0.03mass% N. However, excess amount of carbon and nitrogen resulted in abrupt decrease in creep strength in the long-term. For the steel with 0.07mass% N, which showed excellent long-term creep strength, a lot of plate type fine precipitates and a few coarse block type one were observed and precipitate free zone was smaller than the other steels, and coarsening rate of the fine precipitates identified as Laves and χ phases was smaller than the other steels. It has been concluded that long-term creep strength of the present steel is improved by addition of 0.07mass% N through the precipitation strengthening effect of uniformly distributed a lot of stable fine particles.
