Creep behavior of seam-welded reheat steam pipes in thermal fossil power plant: feedback analysis and life assessment methodology

Abstract

A methodology is developed to evaluate a residual life of the oldest seam-welded reheat pipes. The first step consists of a design concept based using a weld parameter depending on welding process, in order to classify pipes by their creep life fraction. This part is in progress. The second step consists of a metallurgical approach to optimize life assessment of the most damaged pipes, by estimating a residual life before crack initiation in the intercritical zone of a pipe tempered weld. Creep tests on a simulated intercritical microstructure were subjected to different creep life fractions. Creep damage was identified as the creep cavity density evaluated by image analysis. Tests results have shown that creep damage appears in the intercritical microstructure in the very end of life (last 16 ± 2 % of creep life). These results warrant a residual life of at least 24000 hours for a 150000 hours aged pipe under operating conditions, if no creep damage was found. Extra examinations have been performed on the oldest seam-welded (MMA process) reheat pipe of EDF thermal fossil power plants. Application of this metallographic approach have enabled one to estimate a residual life of at least 9000 hours before the creep crack initiation stage, in good agreement with creep tests performed on extracted welds. Applicability of this new approach seems to be very interesting for in service seam-welded reheat pipes, but it requires the previously identification of the most creep damaged section. In that way, NDT must be improved to be able to detect creep damage in the bulk.