Influence of microstructural aspects on the service 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. They show, compared to austenite grades, favourable physical properties - like good thermal conductivity and low coefficient of thermal expansion coupled with better service behaviour. In the frame work of the European research action COST 501 intensive investigations of these types of steels were performed. These alloys show during service conditions pronounced microstructural changes in the aged as well as in the stressed conditions. Therefore an extrapolation of short term creep tests to long times - i.e. 100 kh - using conventional methods is not successful because of changes in the acting creep mechanism governed by microstructural development. Therefore an intensive investigation and modelling of the microstructure of W- and Mo-alloyed advanced 9-12% Cr steels, mainly G-X12 CrMoVWNbN 10-1-1, was applied to understand the mechanisms. Hardness tests, optical microscopy, SEM, TEM methods and thermodynamic equilibrium phase diagram calculations were performed. Very promising is the use of the EFTEM (Energy filtering TEM) method to quantify the precipitation sequence of the different types of precipitate: M23C6, MX, Laves Phase and Z-Phase as a function of time, temperature and straining. A new approach ('Graz Model') was proposed for the improvement of the creep resistant alloys in terms of the α/γ-transformation and the Curie temperature respectively, which influence the microstructural stability. Both were found as important influencing factors. Experimentally it was found, that the alloys showing the highest creep resistance i.e. the Japanese grade NF616/P92 and the alloy B2, one of the COST 501 activity, show the highest degree of microstructural stability after creep exposure. Further investigations and modelling is necessary to develop an optimised alloy composition of advanced 9-12% Cr steels.