Improvement in creep strength of precipitation strengthened 15Cr ferritic steel by controlling carbon and nitrogen contents

The effects of carbon and nitrogen on the creep strength and microstructure of precipitation -strengthened 15Cr ferritic steel have been investigated. The creep rupture lives of 15Cr ferritic steel with the addition of 0.07mass%N and with a combination of 0.05 mass%C and 0.03 mass%N were extended longer than that of the conventional ferritic creep resistant steel of ASME T92. However, excess addition of carbon and nitrogen resulted in a decrease in long-term creep strength. For the steel with 0.07 mass%N addition, which showed excellent long-term creep strength, many plate-shaped fine precipitates and a few coarse block-type ones were observed. The volume fraction of precipitate free zone in this steel was smaller, and the coarsening rate of the fine precipitates identified as intermetallic compounds in this steel was lower than that of the other steels. The long-term creep strength of the present steel was improved by the addition of 0.07 mass%N through the precipitation strengthening effect of many uniformly distributed stable fine particles.