Creep resistance of high-chromium steel and nickel-based alloy with oxidation-resistant coating for the components in advanced ultra supercritical steam generation

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The requirement of reducing the greenhouse CO2 emission has pushed the electrical power generation industry using the traditional fossil fuels to run steam turbines at temperatures above 700 degree C, which falls in the category of the so called advanced ultra supercritical (A-USC) steam turbines. To meet the high-temperature operating requirement of A-USC steam turbines (100,000 hours at 750oC), nickel-based alloy materials are to be used for parts that are directly in contact with the high temperature steam, including tubes, nozzle box (first stage nozzle), rotors and blades. Inconel 740 is one of the promising candidates. On the other hand, for economic reasons, parts that are not directly affected by the steam, for example, outer casings, are preferably made of traditional boiler materials, typically high chromium steel F91. For A-USC steam turbine components, the materials are all required to have high anti-creep strength and oxidation resistance at elevated temperatures. The existing materials, whether high Cr steels or Ni based superalloys, all exhibit deficiency in long-term creep performance because of the interference of oxidation. The available creep data for these materials were mostly obtained from short-term tests. The creep-oxidation interaction in these alloys has not been well understood as to predict their long-term behavior for application under the A USC condition. To solve these problems, the proposed research is aimed to further characterize the high-temperature creep resistance of Inconel 740 and F91; and in the meanwhile, to enhance the oxidation resistance of these substrates by applying a MCrAlY (where M represents Co, Ni or Co/Ni) coating on them; subsequently, to investigate the influence of the coating on the creep performance of these substrate materials. Since assurance by real material creep test may take a too long time for material design and characterization; a deformation mechanism based creep modeling approach will be utilized to depict the short-term creep behaviour and predict the long term creep resistance of these materials and their coating systems.