9 % Cr martensitic steels are perspective candidates for elements of boilers, steam pipes and tubes for novel fossil fuel power plants which are able to work at ultra-supercritical parameters of steam (T = 600 - 620 °C, P = 25 - 30 MPa). Crept structure and creep properties of 0.1С-9Сr-3Co-3W-VNbBN steel after creep tests at 650 °C under a stress of 100 – 220 MPa with a step of 20 MPa were studied. During first 5.000 h, Laves phase particles are essentially stable their sizes remain unchanged, while after 5.000 h the extensive coarsening of these precipitates starts to occur. The coarsening of the Laves phase particles was also calculated using the Prisma software. The results of the calculations are corroborated by experimental observations. There are three features of microstructural evolution of the Laves phase particles during creep in the steel studied. First, volume fraction of Laves phase increases with time. Second, there is well-defined peak in the size distribution of Laves phase. Third, interfacial energy of Laves phase changes with transition from short-term creep to long-term creep.Keywords: Creep-resistant steel, tempered lath martensite, Ostwald ripening, Laves phase particles
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