To enhance long-term creep strength at 650°C, stabilization of the lath martensitic microstructure near prior austenite grain boundaries has been investigated for a 9Cr-3W-3Co-0.2V-0.05Nb steel. This was achieved by adding boron to stabilize M23C6 carbides and dispersing fine MX nitrides. Creep tests were conducted at 650°C for up to approximately 3 × 104 hours. Adding a large amount of boron exceeding 0.01%, combined with minimized nitrogen, effectively stabilized the martensitic microstructure and improved long-term creep strength. The amount of available boron, free from boron nitrides and tungsten borides, is crucial for enhancing long-term creep strength. Reducing the carbon concentration below 0.02% led to a dispersion of nano-sized MX nitride particles along boundaries and in the matrix, resulting in excellent creep strength at 650°C. A critical issue for the 9Cr steel strengthened by MX nitrides is the formation of Z-phase, which degrades long-term creep strength. Excess nitrogen additions of 0.07 and 0.1% promoted Z-phase formation during creep. The formation of a protective Cr-rich oxide scale was achieved through a combination of Si addition and pre-oxidation treatment in argon, significantly improving the oxidation resistance in steam at 650°C.

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