The addition of boron and nitrogen without the formation of any boron nitrides during normalizing heat treatment at high temperature improves long-term creep strength and rupture ductility of base metal and suppresses Type IV fracture in welded joints of 9Cr ferritic power plant steel at 650°C. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the stabilization of fine distribution of M23C6 carbides along boundaries in the vicinity of PAGBs, enhancing the boundary and subboundary hardening. 9Cr-3W-3Co-VNb steel (MARBN) with 120–150 ppm boron and 60–90 ppm nitrogen, in which no boron nitride formed during normalizing, exhibits not only much higher creep strength and rupture ductility of base metal than Gr.92 but also substantially no degradation in creep strength due to Type IV fracture in welded joints at 650°C. The formation of protective Cr2O3-rich oxide scale is achieved on the surface of MARBN by pre-oxidation treatment in argon gas, which significantly improves the oxidation resistance in steam. The present results suggest that MARBN is superior to Gr.91, Gr.92, and Gr.122 in terms of creep strength of base metal and welded joints and can be applied to boiler components in next-generation USC power plant.
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