Prediction of Creep Rupture Time Using Constitutive Laws and Damage Rules in 9Cr-1Mo-V-Nb Steel Welds

Long-term creep tests for the parent metal, a simulated heat-affected zone (HAZ) and a weld joint were conducted based on 9Cr-1Mo-V-Nb steel. The creep rupture time, t(D) for the weld joint was predicted by computationally simulating creep damage based on Norton's law combined with the time exhaustion rule (TER), where the equivalent stress (sigma(eq)), maximum principal stress (sigma(1)), or Huddleston stress (sigma(hud)) was used to evaluate the rupture time. Creep damage analysis was also conducted based on the Hayhurst-type damage mechanics rule (HDR), in which creep rupture time was evaluated in relation to the rupture stress, sigma(r). The computed creep rupture times and damage distributions were compared with the experimentally obtained rupture time and void distribution of the actual weld joint, respectively. Furthermore, the effect of the bevel angle of the HAZ was examined. The key findings from this study were as follows: (1) cautious predictions could be obtained for a loading stress of 80 MPa and higher; (2) the computed fracture initiation positions were on the HAZ boundaries, consistent with the type IV fracture observed at the stress of 80 MPa; (3) the magnitudes of the predicted rupture times were t(r) (sigma(1)) < t(r) (sigma(hud)) approximate to t(r) (sigma(r)) < t(r) (sigma(eq)); (4) the bevel angle dependence previously reported was reasonably reproduced with the TER models that used sigma(eq) and sigma(hud) and with the HDR model.