New Ferritic Heat Resistant Steels for 650°C USC Boilers

摘要

In order to develop advanced ferritic steels for thick section boiler components of ultra-supercritical (USC) power plant at 650°C, the improvement of creep rupture strength and oxidation resistance has been investigated for high-Cr ferritic steels. For the improvement of long term creep strength, special attention has been paid to stabilize the microstructure in the vicinity of grain boundaries. At first, a tempered martensitic 9Cr-3W base steel containing fine MX is alloy-designed taking into account of maximized W concentration but no δ-ferrite in 9Cr steel. The fine precipitates of Fe-Pd L1_0 type ordered phase and the boron addition combined with minimized nitrogen are very effective to stabilize the martensitic microstructure and significantly improve the creep rupture strength of the 9Cr-3WVNb steel. The improvement of creep strength is also achieved for innovative steels, such as carbon-free Fe-Ni-Co martensitic alloy and full-annealed ferrite matrix 15Cr steel. The weight gain of the 9Cr steels during oxidation in steam at 650°C decreases by the addition of Si or Pd. For the improvement of oxidation resistance in steam, the stabilization of protective Cr-oxide scale is a key issue.