EFFECT OF GRAIN-SIZE ON THE CREEP RESIDUAL LIFE EVALUATION IN THE FRAMEWORK OF THE API 579-1 STANDARD

Creep is a classic inelastic-strain accumulation mechanism of pressure components exposed to high temperature. Due to the long-term exposure, the residual life is usually checked accounting for the steady-state creep, namely the dominant regime in terms of contribution to the accumulated creep strain. The creep-strain accumulation directly depends on the stress field via a constitutive equation that reflects the underlying microstructural phenomenon, namely diffusion and dislocation glide+climb. Moreover, creep diffusion, dominant at low stress, is known to be grain-size dependent. Commonly, the grain-size effect is neglected for two reasons: I) the diffusive regime is completely ignored; II) the structure is considered to have a uniform texture and the same average grain-size. Unfortunately, every time a welding joint is made, the HAZ undergoes a microstructural change, often resulting in a local refinement of the grain-size. In two previous papers by two of the authors [1][2], the impact of a diffusive plus dislocational creep formulation on the API 579-1 [3] creep residual life was studied for a low-alloy ASME SA-335 P22 butt-weld. In this work the analysis was extended in order to include the impact of the microstructure in terms of grain-size, considering that it directly affects the diffusive creep rate. A FE inelastic model was built up for a butt-weld and the weld zones were characterized with respect to their grain-size. The creep constitutive equations were set up for diffusive and dis-locational secondary creep, including the grain-size contribution to the former. The custom user-subroutine employed in the FEA was able to determine the creep-strain accumulation and the API 579-1 creep damage based on the Larson-Miller theory within the different weld zones and the results indicated a damage concentration in the fine-grained intercritical HAZ, which is consistent with the Type-IV cracking of welds in the creep regime.