Abstract
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.
Highlights
Studies on creep resistance of modified 9–12% Cr steels have been largely focused on the rupture strength of the parent metal
Long-term experience of creep exposed welded structures has shown that the heat affected zone (HAZ), a narrow zone of parent metal adjacent to the weld fusion line altered by the weld thermal cycle, in respect to the creep strength, is often regarded as the weakest link in welded constructions [1-3]
The aim of the present work was to investigate the changes in the number density (n), size (d), surface share (f), and mutual spacing (λ) of particles in the parent metal (α) and two regions of the HAZ, α+γ and γ of two 9–12% Cr steel grades, the X20 and the P91, as a function of ageing, and the effect of these changes on the stationary creep rate (έ)
Summary
Studies on creep resistance of modified 9–12% Cr steels have been largely focused on the rupture strength of the parent metal. Particles with different size, spacing, and distribution are present in the HAZ after a multilayer welding. Studies [4-6] have shown that there is a correlation between the size, spacing, and distribution of precipitates on the one hand and creep properties on the other. The aim of the present work was to investigate the changes in the number density (n), size (d), surface share (f), and mutual spacing (λ) of particles in the parent metal (α) and two regions of the HAZ, α+γ and γ of two 9–12% Cr steel grades, the X20 and the P91, as a function of ageing, and the effect of these changes on the stationary creep rate (έ)
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