In order to improve long-term creep strength at 650°C, stabilization of lath martensitic rnicrostrueture in the vicinity of prior austenite grain boundaries has been investigated for a 9Cr-3W-3Co-0.2V-0.05Nb steel by the addition of boron to stabilize MasCe carbides and by a dispersion of only fine MX nitrides. Creep tests were carried out at 650°C for up to about 3×10{sup}4 h. The addition of large amount of boron exceeding 0.01%in combination with minimized nitrogen is very effective to stabilize martensitic microstructure and to improve long-term creep strength. The amount of available boron free from boron nitrides and tungsten borides is essential to improve long-term creep strength. A dispersion of nano-size MX nitride particles along boundaries as well as in the matrix is achieved by reducing carbon concentration below 0.02%. This gives rise to excellent creep strength at 650°C. Critical issue for the 9Cr steel strengthened by MX nitrides is the formation of Z-phase, which degrades long-term creep strength. Excess addition of nitrogen, 0.07 and 0.1 %, promotes the formation of Z phase during creep. The formation of protective Cr-rich oxide scale is achieved by a combination of Si addition and pre-oxidation treatment in argon, which significantly improves the oxidation resistance in steam at 650°C.
展开▼
