Critical Issues for Development of High-Cr Ferritic Steels for USC Power Plant at 650°C

Abstract

Critical issues for the development of high-Cr ferritic steels for USC power plant at 650°C are described in terms of long-term creep strength and oxidation resistance. Creep tests were carried out for 9Cr-3W-3Co-VNb steel at 650°C for up to about 6 × 104 h. The addition of boron as high as 100ppm or more in combination with minimized nitrogen, which causes no formation of boron nitride (BN) during normalizing heat treatment, is essential for the improvement of long-term creep strength of base metal and welded joints. The addition of nitrogen causes the precipitation of fine MX carbonitrides, which significantly improves the creep strength. However, excess addition of nitrogen promotes not only the formation of BN during normalizing heat treatment and of Z phase during creep in base metal but also the formation of fine grained heat affected zone in welded joints. Therefore, the boron and nitrogen contents should be optimized for the improvement of long term creep strength of base metal and welded joints. On the other hand, the candidate steels for boiler at 650°C should exhibit oxidation resistance in steam at 650°C better than that of Gr.91 in steam at 600°C, because Gr.91 is being now used for long duration in power plants operating at 600°C. However, existing 9 to 12Cr steels, such as P92 and P122, cannot satisfy the criterion for oxidation resistance in steam at 650°C. The formation of Cr-rich oxide scale is achieved by pre-treatments such as pre-oxidation treatment and coating, which significantly improves oxidation resistance in steam at 650°C and satisfies the criterion for oxidation resistance. The assessment of resistance to exfoliation is a key issue for the Cr-rich oxide scale.