Inconel Alloy 617 is a high temperature creep and corrosion resistant alloy and is a leading candidate for use in Intermediate Heat Exchangers (IHX) of the Next Generation Nuclear Plants (NGNP). The IHX of the NGNP is expected to experience operating temperatures in the range of 800°–950°C, which is in the creep regime of Alloy 617. A broad set of uniaxial, low-cycle fatigue, fatigue-creep, ratcheting, and ratcheting-creep experiments are conducted in order to study the fatigue and ratcheting responses, and their interactions with the creep response at high temperatures. A unified constitutive model developed at North Carolina State University is used to simulate these experimental responses. The model is developed based on the Chaboche viscoplastic model framework. It includes cyclic hardening/softening, strain rate dependence, strain range dependence, static and dynamic recovery modeling features. For simulation of the alloy 617 responses, new techniques of model parameter determination are developed for optimized simulations. This paper compares the experimental responses and model simulations for demonstrating the strengths and shortcomings of the model.

Premature creep cracking in fabricated large bore branch connections in Grade 91 steel (9Cr-1Mo-VNbN) piping continues to be a commonly observed failure mechanism in high energy applications. Failures have been observed in components fabricated to the requirements of both ASME Section I and B31.1 codes. This paper presents the application of a physically-based creep continuum damage constitutive model developed for Grade 91 steel to the assessment of a large bore fabricated branch connection. For a specific component geometry and operating conditions, model predictions for the expected location and timing of crack initiation as well as for the crack growth behavior have been made. In addition, as validation, trends in the simulated behavior are compared to information from case studies of large bore branch cracking and failure in service. The physically-based continuum damage model is shown to accurately predict both the location and timing of local crack initiation as well as the observed crack growth behavior.

One of the most challenging tasks in the design code development for Next Generation Nuclear Plant (NGNP) is the design of intermediate heat exchanger (IHX), whose operation temperature may range from 800°C–950°C (1472°F–1742°F). The ASME design code does not have any design provisions for any material at this temperature range. Hence, different candidate materials are under consideration for IHX and one of the leading candidate materials is Ni based Alloy 617. The operating temperature of IHX will be in the creep regime of Alloy 617 and low-cycle creep-fatigue and creep-ratcheting failure mechanisms of Alloy 617 need to be understood. This study is developing a unified constitutive model (UCM) for Alloy 617 based on a broad set of uniaxial and multiaxial creep-fatigue and creep-ratcheting experiments at high temperatures. The experiments were conducted at different temperatures, loading rates and strain ranges prescribing different loading histories. The unified constitutive model was developed based on the Chaboche viscoplastic model framework. Model improvement was performed by including cyclic hardening/softening, dynamic and static recovery, strain rate and strain range dependence, nonproportional loading parameter and multiaxial ratcheting features. The simulated responses of the modified UCM were compared against the broad range of experimental data to demonstrate the robustness of the improved model. The strengths and shortcomings of the model are discussed. Paper published with permission.