Class A Design Data for Alloy 709 for Use With the ASME Boiler and Pressure Vessel Code Section III, Division 5 Rules

Abstract Alloy 709 is an advanced austenitic stainless steel with improved high temperature performance compared to other reference materials for high temperature reactor construction, like 316H and 304H stainless steels. While Alloy 709 is more expensive per unit weight than either of those two reference materials, its improved high temperature performance means that many reactor designs would be able to reduce costs by switching from 304H or 316H to Alloy 709. Making Alloy 709 available to designers planning future high temperature reactors requires qualifying Alloy 709 for use with Section III, Division 5 of the ASME Boiler & Pressure Vessel Code. This paper summarizes a set of preliminary design data for Alloy 709, informed by an extensive testing campaign, targeting the Class A design rules in Section III, Division 5, Subsection HB, Subpart B. The design data described in this paper includes the allowable stresses required for use with the Class A design rules, along with the technical basis for extrapolating these allowable stresses from tensile and creep test data. The paper considers several strategies for time extrapolation of the creep data and makes a recommendation based on accuracy and robustness, i.e. how much data can be removed from the database while still retaining accuracy in long-term extrapolation. Techniques considered include both region split and standard Larson-Miller approaches. The paper describes a method for evaluating competing approaches by selecting the models that best extrapolate the short-term creep data. The paper also develops hot tensile and isochronous stress-strain curves for Alloy 709, again based on tension and creep test data. These curves are provided in terms of a calibrated mathematical model for the elastic, plastic, and creep strain accumulated in Alloy 709 as a function of temperature and time. We recommend a Voce model for plastic strain accumulation, a Kocks-Mecking model for minimum creep rate, and an exponential model for transitioning through primary creep to the minimum creep rate. The parameters for each component of the model are calibrated and validated against the available test data. Finally, the paper describes the process of calculating time-dependent creep buckling exemption charts based on the isochronous stress-strain curves. This paper will serve as part of the technical basis for an ongoing Code qualification effort for Alloy 709. We anticipate the final design data recommended to the cognizant Code Committees will be similar to the values presented here, updated to account for the results of ongoing long-term tests.