Microstructural aspects on the creep behaviour of advanced power plant steels

There is a world-wide substantial demand to increase the application temperature and design stresses of advanced creep resistant ferritic-martensitic 9-12% Cr steels to increase the efficiency of thermal power plants. These alloys show during service conditions pronounced microstructural changes in the aged as well as in the stressed conditions. Therefore an intensive investigation and modelling of the microstructure of the cast steel material G-X12CrMoVWNbN10-1-1, was applied to understand the mechanisms. Hardness tests, optical microscopy, TEM methods and thermodynamic equilibrium phase diagram calculations were performed. By means of the EFTEM (Energy filtering TEM) method a quantification of the precipitation sequence of M23C6, MX, Laves Phase and Z - Phase could be established at 600 degrees C as a function of exposure time. A new approach ("Graz Model") was proposed for the improvement of the creep resistant alloys in terms of the alpha/gamma - transformation temperature and diffusion behaviour respectively, which influence the microstructural stability. Both were found as important influencing factors.