Abstract
The dislocation structure in dry-steam pipe steel has been determined by X-ray line profile analysis as the function of long-term service from the initial state up to 220.384 h. The specimens are taken from a Hungarian fossil fuelled power plant where the dry-steam pipe has been operating at about 540 °C at 165 bar. X-ray diffraction patterns were taken in a conventional θ-2θ powder diffractometer and in a special high resolution double crystal diffractometer dedicated for line profile analysis. The θ-2θ powder diffractograms reveal narrower peaks in the low angle range than the peaks of the NIST SRM-660a LaB₆ standard specimen. In the high angle range the peaks of the steel specimens are broader than the standard peaks indicating the presence of microstrain. Dislocation densities were evaluated in individual grains by using the convolutional multiple whole profile fitting procedure and were obtained to vary between 10¹³ and 10¹⁴ m^–2, decreasing towards the end of the service life of the tube. Data from metallographic micrographs are in good correlation with X-ray diffraction results.

Key words
dislocation density, intergranular stresses, life time of dry-steam pipe steel, high resolution X-ray diffraction

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