Assessment of the influence of creep transition and nitridation in the creep-life prediction of Incoloy 800H

Accurate prediction of creep life of materials has been for years a matter of high research interest. The engineering design of such components is often performed following standardized analytical procedures aimed to empirically correlate state variables (mainly stress and temperature) with the chosen failure criteria (e.g., buckling, time-to-1% strain, time-to-rupture, etc). However, the accuracy of the chosen model ultimately depends on the microstructural properties of the material. As such, changes in thermomechanical treatments and environmental conditions can largely affect the creep behaviour of such components, thus making inadequate the use of simplified analytical models (R. W. Swindeman and D. L. Marriot, “Criteria for design with structural materials in combined-cycle applications above 815°F”, in Journal of Engineering for Gas Turbines and Power, vol. 116, pp. 352-359, 1993). Such is the case of Incoloy 800H, a solution-annealed austenitic Fe-Ni-Cr alloy of high industrial interest as it provides a good balance between production cost and high-temperature mechanical response. Under particularly low-stress and high-temperature loadings, this alloy is reported to exhibit a diffusion-to-dislocation transitional creep. Furthermore, the subsequent large dislocation-driven tertiary creep stage undergoes a nitridation-induced hardening while exposed to high-N environments (V. Guttmann and R. Bürgel, “Creep-structural relationship in steel alloy 800H at 900-1000°C”, in Metal Science, vol. 17, pp. 549-555, 1983).
In this work, the creep behaviour of the alloy is modelled using a Chaboche-type constitutive law (H. Morch et al., “Efficient temperature dependence of parameters for thermo-mechanical finite element modelling of alloy 230”, in European Journal of Mechanics / A Solids, vol.85, 2020) implemented in the MSM-team (ULiège) proprietary finite element software Lagamine. The results are later assessed with the aim of proposing a novel and efficient numerical creep micromechanics approach intended for the identification of Chaboche parameters while addressing the underlaying uncertainties that rule the creep behaviour of this alloy: diffusion-dislocation creep transition and nitridation.