Abstract
The life time of dry steam pipes in fossil fuelled power plants in the Central European region, especially in Hungary, are expected to be about 30 years. Besides gauge changes followed by metric controlling methods, the microstructure of the steel materials is also deteriorating due to operation at about 540 °C at about 165 bar. High resolution X-ray line profile analysis reveals that large grain-to-grain, second-order residual internal stresses develop during the service period which finally, closer to the end of the expected life period of the materials decrease strongly. The evolution of grain-to-grain internal stresses is explained on the basis of elastic anisotropy of steel whereas, the decrease of these internal stresses is attributed to the deterioration of the grain boundary structure by precipitate coarsening and void formation during operation of the dry steam pipes at 540 °C and 165 bar.

Key words
life time of dry-steam pipe steel, intergranular stresses, second-order stresses, grain-to-grain internal stresses, high resolution X-ray diffraction,

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