| Summary: | Changes in size- and spatial distribution of precipitates and their effect on stationary creep rate of two grades of creep-resistant 9-12% Cr steels, X20 and P91, as a function of ageing at two different conditions were investigated. Ageing of both steels was performed at 650°C lasting up to 2 years and at 750°C lasting up to 6 months. Prior to the ageing, simulation of two weld heat-affected zone HAZ regions, i.e., coarsegrained microstructure (γ), and grain-refined microstructure (α+γ) was performed. Accelerated creep tests at 580°C and 170 MPa, lasting up to 100 hours were performed on the above microstructures prior to- and after the ageing. In addition, optical and SEM imaging were performed on metallographic specimens, followed by image analysis. Deterioration of the creep performance show a proportional behaviour with the precipitate coarsening, which is more pronounced after the ageing at 750°C. Comparatively, the increase of the stationary creep rate after the ageing at 650°C was more pronounced in the steel X20 than in the P91. In both steels and both tempering temperatures, the most troublesome region with respect to the creep behaviour was observed to be the simulated intercritical (α+γ) HAZ microstructure.
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