Thermal-mechanical fatigue behaviour of 9–12%Cr power plant steels and pipes

This paper is concerned with the thermal-mechanical fatigue (TMF) behaviour of 9-12%Cr steels (P91 and P92) and of steam pipes made from them. A unified visco-plasticity model has been used in material behaviour characterization (cyclic hardening and creep relaxation), using the data obtained from iso-thermal and anisothermal experimental testing. A method for determining the material properties has been developed. The cyclic uniaxial test results have shown the capability of the model, in representing the thermalmechanical responses of the materials. The material model and the material properties obtained are then implemented into the FE package ABAQUS (and Z-mat code) to model a realistic main steam power plant pipe, using assumed plant thermal and mechanical loading histories. The detailed stresses and stress-strain loops within the pipe under the thermal-mechanical conditions are investigated. The results obtained will provide useful indication of possible crack initiation sites and could form a basis for the development of component lifing methodology under TMF conditions.