Degradation due to ageing in extraction steam piping of a nuclear power plant

Abstract

"Ageing assessment of extraction steam piping, which was limited to wall loss and corrosion, has been extended to microstructural degradation in this paper. Metallography, radiography, ultrasonic thickness and hardness measurements have been carried out for examining the virgin and twelve years old Seamless Carbon Steel SA 106B pipe in this study. In addition to commonly observed degradation mechanisms i.e. pitting and wall thinning, microstructural degradation has also been observed. The transformation of bands of pearlite and ferrite phases, present in virgin microstructure, into uniformly distributed pearlite and irregularly distribute graphite nodules have been observed due to ageing. The depletion of pearlite phase from 40% to 25% and consequent reduction in hardness up to 20.4% maximum has been observed. The more reduction in hardness of internal surface has been observed as it comes in direct contact with steam. Due to microstructural degradation with simultaneous pitting, wall thinning and reduction in mechanical properties, frequent inspection has been suggested. (C) 2016 The Authors. Published by IASE.