Hastelloy X is widely used in the pressure vessel and piping (PVP) industries, specifically in nuclear and chemical reactors, pipes and valves applications. Hastelloy X is favored for its resistance to extreme environments, although it exhibits a rate-dependent mechanical behavior. Numerous unified viscoplastic models proposed in literature claim to have the ability to describe the inelastic behavior of superalloys subjected to a variety of boundary conditions; typically limited experimental data is used to validate their performance. In this paper, two unified viscoplastic models (Miller and Walker) were experimentally validated for Hastelloy X creep behavior. Both constitutive models are coded into ANSYS Mechanical as user programmable features (UPF). Creep behavior is simulated at a broad range of stress levels. The results are compared to an exhaustive database of experimental data to fully validate the capabilities and performance of these models. Material constants are calculated using the recently developed Material Constant Heuristic Optimizer (MACHO) software. This software uses the simulated annealing algorithm to determine the optimal material constants by using an extensive database of experimental data. A qualitative and quantitative discussion is presented to determine the most suitable model for Hastelloy X PVP components.

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