Creep-fatigue behavior of Alloy 617

Alloy 617 is the leading candidate material for the intermediate heat exchanger for the very high temperature reactor (VHTR). It is thought that creepfatigue resistance is likely to be the life-limiting material property for this application. As part of evaluating the behavior of this material in the expected service conditions, creep-fatigue testing of Alloy 617 was performed at 850 and 950°C for several strain ranges and hold times. Creep-fatigue degradation was observed, characterized by reduced cycles to failure when a hol d time was added at peak tensile strain, compared to fatigue values. At 850°C, increasing the tensile hold duration continued to degrade the creep-fatigue resistance, at least to the investigated strain-controlled hold time of up to 60 minutes at the 0.3% strain range and 240 minutes at the 1.0% strain range. At 950°C, the creep-fatigue cycles to failure are not further reduced with increasing hold duration, indicating saturation occurs at relatively short hold times. The creep and fatigue damage fractions have been calculated using the ASME methodology and plotted on a creep-fatigue interaction D-diagram. The impact of selecting different parameters on calculation of the creep damage fraction has been evaluated and will be discussed in this paper. Additional damage fractions from creep-fatigue tests at 800 and 1000°C on a different heat of Alloy 617 h ave been determined and are plotted on t he D-diagram along with results of the current testing. The combined data represent a c omprehensive evaluation of Alloy 617 be havior under the ASME methodology and result in a consistent creep-fatigue damage envelope.