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MM: Fachverband Metall- und Materialphysik

MM 35: Methods in Computational Materials Modelling IV: Steels

MM 35.4: Vortrag

Mittwoch, 18. März 2015, 12:30–12:45, H 0106

Z phase strengthened steels for ultra-supercritical power plants — •Daniel F. Urban, Christian Elsässer, and Hermann Riedel — Fraunhofer Institute for Mechanics of Materials IWM, Freiburg, Germany

To minimize fuel consumption and CO2 emission of fossil fired power plants, the thermal efficiency, and therefore the steam inlet temperatures, must be as high as possible. In the past 30 years sufficiently creep resistant 9% chromium steels were developed, allowing steam temperatures up to 615 °C. The increased creep resistance was obtained by controlled precipitation of fine (V,Nb)N particles. Further raise of the steam temperature calls for higher Cr contents for better corrosion and oxidation resistance. However, 11-12% Cr ferritic-martensitic steels strengthened by fine (V,Nb)N particles reveal that precipitation of the thermodynamically stable Z-phase, Cr(V,Nb,Ta)N, in long-term service is unavoidable and detrimental. Usually, coarse and brittle Z-phase particles grow at the expense of the desired fine nitride particles. We follow the idea to exploit the Z-phase as strengthening agent in martensitic creep resistant 12% Cr steels by controlling the precipitation of the Z-phase such that fine, thermodynamically stable Z particles are formed. We present atomistic DFT simulations which reveal the essential mechanisms underlying the Z-phase formation. Chromium atoms diffuse into nitride particles and subsequently cluster in a layered arrangement which finally yields the transformation of the nitride particles to Z-phase.