Predicted life of thick-wall P91 steel for cyclic high temperature service

The ferritic-martensitic steel P91 (X10CrMoVNb9-1) is widely used in high temperature steam systems of power plants, and it is a candidate material for Gen-IV reactors. In comparison to austenitic steels, thick-wall P91 has relatively attractive mechanical and physical properties combined with resistance to stress corrosion cracking in water-steam environments. This study aimed to explore the combined cyclic, creep and relaxation behaviour of P91 material up to a component wall thickness of 60 mm. Uniaxial specimens were subjected to cyclic loadings with periodic forward creep or relaxation at peak stress. The results indicate that prior creep or intermediate relaxation periods up to 72 h will influence the subsequent cyclic softening of P91, but do not significantly reduce the cyclic life. In contrast, prior cycling has a detrimental effect on the subsequent creep life. A simplified creep-fatigue model is shown to predict life better than usual code-based approaches for cyclic service of P91 steel. Improved verification of all models would benefit from the availability of more extensive long-term data on P91 steel.