The influence of long-term service exposure at elevated temperatures on Gurson-Tvergaard-Needleman (GTN) model parameters for P91 steels

The P91 steel, which is widely used in critical components of power generation systems, undergoes significant microstructural deteriorations during long-term service under high-temperature and high-pressure conditions. These microstructural deteriorations highly coupled with parameters in meso-damage models like GTN, leading to the continuous decreasing mechanical performance, and ultimately posing serious challenges to the safety of the whole systems. In this case, this study aims to extensively investigate the influence of long-term service exposure on GTN parameters. Experiments, as well as the corresponding finite element calculations, are conducted on four P91 steels at different service stages (as-received, 50,000 h service exposure, 100,000 h service exposure and more than 200,000 h service exposure). It is found that extended service exposure can result in coarsening of secondary phase particles and interface degradation, increasing the number of initial damage and effective nucleation sites, leading to increasing f₀ and void nucleation parameters (f_N, s_N, ε_N). In addition, long-term service decreases dislocation densities, which reduces yield strength but enhances subsequent hardening behavior, resulting in decreasing trends for constitutive parameters (q₁ and q₂). Both failure parameters (f_C and f_F) depend on initial damage (f₀) and damage evolution. Short-term service exposure exhibits negligible effects, whereas long-term service causes higher void volume fraction at the same strain levels and enhanced matrix ductility, thus resulting in an increasing trend of failure parameters. Furthermore, the influence of parameter mismatching (referring GTN parameters from P91-New to serviced ones) on failure predictions of long-term serviced P91 pipelines is also discussed. This study provides technical guidance for failure predictions by meso-damage mechanics models.