CREEP-BEHAVIOR OF SEAM-WELDED REHEAT STEAM PIPES IN THERMAL FOSSIL POWER-PLANT - FEEDBACK ANALYSIS AND LIFE ASSESSMENT METHODOLOGY

A methodology is developed to evaluate a residual life of the oldest s eam-welded reheat pipes. The first step consists of a design concept b ased 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 b efore crack initiation in the intercritical zone of a pipe tempered we ld. Creep tests on a simulated intercritical microstructure were subje cted to different creep life fractions. Creep damage was identified as the creep cavity density evaluated by image analysis. Tests results h ave shown that creep damage appears in the intercritical microstructur e in the Very end of life (last 16 +/- 2 % of creep life). These resul ts warrant a residual life of at least 24000 hours for a 150000 hours aged pipe under operating conditions, if no creep damage was found. Ex tra examinations have been performed on the oldest seam-welded (MMA pr ocess) reheat pipe of EDF thermal fossil power plants. Application of this metallographic approach have enabled one to estimate a residual l ife of at least 9000 hours before the creep crack initiation stage, in good agreement with creep tests performed on extracted welds. Applica bility of this new approach seems to be very interesting for in servic e seam-welded reheat pipes, but it requires the previously identificat ion of the most creep damaged section. In that way, NDT must be improv ed to be able to detect creep damage in the bulk.