Combined impact of creep aging and helium ion irradiation on P91 steel: Experiments and FE modelling

Understanding radiation damage resistance in Grade 91 steel (P91) is essential for the development of materials for future nuclear components. This study explores the combined effects of creep aging and helium ion irradiation on the microstructure and mechanical properties of P91 steel. Creep aging was conducted under a stress of 110 MPa at 625 °C for 475 h, followed by irradiation with 5 MeV helium ions to a fluence of 5.6 × 10 17 ions/cm 2 , creating a uniform radiation-affected zone with an average damage level of 0.6 dpa. Microstructural changes and mechanical responses were assessed through detailed microstructural observations and nanoindentation, supported by finite element modelling. The results show that creep aging led to a slight reduction in hardness from 2.66 GPa to 2.45 GPa, primarily due to carbide coarsening. Significant irradiation hardening was observed, with hardness increasing by 87 % in the as-received condition and by 99 % in the creep-aged condition. A three-dimensional finite element model was developed to reverse-engineer stress-strain relationship from nanoindentation load-displacement data. This study underscores the significant impact of combined creep aging and irradiation on P91 steel, with important implications for its use in nuclear applications. • Hardness of P91 steel creep-aged at 110 MPa and 625 °C for 475 h decreased to 2.45 GPa from 2.66 GPa of the steel in as-received tempered condition, due to the noticeable coarsening of M 23 C 6 carbides. • Irradiation hardness increased by 87 % in as-received P91 sample and 99 % in the creep-aged sample due to the helium ion irradiation with a damage of ∼0.6 dpa. • 3D FE modelling was developed to derive uniaxial stress-strain relationship of the irradiated and creep-aged P91 steels from nanoindentation measurements. Estimated yield stress increased by 550.8 MPa for the as-received sample and 579.1 MPa for the creep-aged sample due to helium irradiation.