Austenitic steels for complex and variable stress temperature, pressure and environmental conditions of next generation ultra supercritical power plants (AUSPLUS) : final report

The project aims at supplying practical guidelines, sound metallurgical evidences and production routes to develop new hot corrosion/oxidation, creep/ creep-fatigue-resistant austenitic steels, for the extreme service conditions of next generation advanced Ultra Supercritical (A-USC) pulverized coal-fired power plants. New alloys have been designed, produced, aged (max 10000h) and characterised, starting from commercial state of the art austenitic stainless steels for the project target application, i.e. Sanicro25 and TP310. Good hot corrosion and oxidation resistance as well as good tensile properties were obtained in the experimental materials, by an increase of chromium and nitrogen, while reducing nickel, with respect to the reference commercial steels. The microstructural stability during service was a critical issue: the precipitation of sigma phase was favoured by the high amount of chromium, and only partly limited by the austenite stabilising action of nitrogen. However, during the project, the effectiveness of nitrogen addition was proved to be increased by strict control of the chemical composition, aiming at suppressing the formation of undesired high temperature precipitates (such as Ti nitrides). The excessive sigma phase precipitation impaired the ductility of the experimental materials, which were all characterised by a brittle behaviour during creep. However, considering the creep rate in the pseudo-stationary creep stage, the experimental materials are placed among the commercial reference ones. Modelling supported the whole project in: alloy design, mechanical tests design, interpretation of the test results and analysis of the aged microstructures. Thermodynamic databases review and data on aged microstructures are delivered for further validations of thermodynamic and kinetic models.