Development of Life Estimation Method for Nozzle Welds in Large Scale Piping of Modified 9Cr-1Mo Steel -Part II: Analytical Study

Creep strength-enhanced ferritic (CSEF) steels are widely used in fossil power plants operated in many countries and life management for the components made of these materials are of critical importance for plant operators. In particular, failures in welded joints in softened heat affected zone (HAZ) categorized as type IV failure have taken place widely in piping and boiler headers and prediction of such failure became a urgent requirement to be addressed. Following the simple longitudinal welds, weldments used for attaching various nozzles to main piping or boiler headers became the portions susceptible to type IV failure, stimulating the needs for prediction of their lives also. As a result, it turned out that a more sophisticated approach which can deal with complicated geometries involved in these weldments needs to be developed. With such a background, the authors carried out an internal pressure creep test at 650 °C using the pipe of P91 steel with the outer diameter of 457.2 mm. Outline of testing condition as well as the results including the periodic damage inspection by several methods is presented in the accompanying paper whereas related analytical efforts will be described in this paper. Detailed finite element analyses were performed on nozzles with different geometries, incorporating creep deformation properties of each continuant of welded joint. Moreover, failure criterion based on inelastic strain energy density has been developed for softened HAZ, taking into account of the dependency on temperature, creep strain rate as well as stress triaxiality factor. Results obtained using these properties were compared with the test results in terms of the location and extent of cracking due to creep damage accumulation. They showed reasonable agreement, demonstrating the soundness of the developed approach.