Creep Damage Assessment Procedure for Weldment Parts of P91 Boiler Piping in USC Plants: Part I—Development of Stress Analysis Code for Boiler Piping Systems

High-chromium steels have excellent high temperature strength properties and are therefore used for piping systems of ultra-super-critical (USC) power plants. Recently, type-IV damages occurred in heat affected zone (HAZ) and had often caused unexpected shutdown. The type-IV damages are due to dense concentration of creep voids on grain boundaries, and then quantitative prediction of void growth is an important subject to prevent a critical accident. To that end, it is necessary to obtain stress conditions under plant operation with high accuracy. The hoop or circumferential stress caused by steam or internal pressure can easily be estimated by a conventional FEM code. However estimation of system load caused by thermal expansion of pipe materials is difficult because of the complex structures of piping system. In this study, we developed a three-dimensional FEM code which can estimate both thermal expansion and creep deformation of complex piping system. Consequently, accurate stress analyses of weldments under plant operation become now enabled.