Accurate very-high temperature creep-life prediction of Incoloy 800H addressing effects of creep mechanism transition and nitridation

Incoloy 800H is a solution-annealed Fe-Ni-Cr alloy, interesting for the industry as it exhibits a good balance between cost efficiency, creep-life and corrosion resistance. For this alloy, a standard mechanical creep response is observed at temperatures below 760°C, where the three main creep stages are clearly identified. At very-high temperature and low-stress loadings, the creep behaviour of Incoloy 800H is characterized by three unusual micromechanical stages: (1) An initial diffusion-driven creep stage, (2) a subsequent diffusion-to-dislocation creep mechanism transition and (3) a final large dislocation-driven creep stage. Furthermore, a nitridation-induced creep hardening effect is observed under long-term high-temperature exposure in high-N atmospheres.
In this article, accurate creep-strain prediction of alloy 800H is achieved by implementing a modified Graham-Walles (mGW) viscoplastic model within a Chaboche-type constitutive law in Lagamine (in-house developed finite element software, MSM team, ULiège). Phenomenological modelling of the nitridation-induced hardening effect is achieved by including a new nitridation-dedicated function within the mGW model. Coupled damage evolution is predicted using a GW-type creep-fatigue approach developed in Darmstadt institute. The macromechanical model is validated using literature-available information, in conjunction with new experimental data addressing pristine and nitrided samples of 800H alloy. Results show that the chosen numerical model exhibits remarkable capabilities and accuracy for prediction of creep response and creep-life of Incoloy 800H. It addresses the creep mechanism transitional phenomenon and alloy nitridation. Furthermore, an extended Wilshire-type set of equations is introduced as a reliable approach for interpolation of viscoplastic law parameters.