Grade 91 steel has been called out for use in advanced reactor intermediate heat exchangers and other components. The material has good high temperature creep resistance and thermal properties but has a complex microstructure that manifests as cyclic softening, work softening, and tension/compression asymmetry in its engineering mechanical response. We describe a unified viscoplastic model for the deformation of Grade 91 for an expected operating temperature range spanning from room temperature to approximately 650°C. The model transitions from a rate independent response at low temperatures and high strain rates to a rate dependent, unified viscoplastic response at high temperatures and low creep strain rates. The model captures work and cyclic softening in the material through combined isotropic-kinematic hardening and captures observed tension/compression asymmetry and related anomalous ratcheting effects through a non-J2 flow term. A particular focus of the model is on capturing the average response of Grade 91 as determined from a wide collection of experimental data at many different temperatures, rather than the response of a single set of experiments at a particular temperature. The final model is suitable for the engineering design of nuclear components via inelastic analysis using the ASME Section III, Division 5 procedures.

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