Creep and creep-rupture of Alloy 617

The Ni-Cr-Mo-Co material Alloy 617 is the leading candidate for VHTR intermediate heat exchangers operating above 750 °C. Time-dependent properties are an important consideration in qualifying the alloy for construction of nuclear components. Creep behavior of several different heats of Alloy 617 has been evaluated in the temperature range of 800-1000 °C. Power law creep behavior was observed for the minimum creep rate, with a stress exponent of 5.6 and activation energy of approximately 400 kcal/Mol. The Monkman-Grant approach relating minimum creep rate to time to rupture gave a reasonable representation of the data for all of the testing with a slope of −0.84. Similarly, a modified Monkman-Grant fit the strain to failure data reasonably well. A Larson-Miller analysis was carried out to compare rupture behavior determined in the current experiments and historical data with well-known provenance over a wide range of conditions. It appears that the properties of modern heats of material are near the lower bound of rupture behavior when all of the data are considered in the same analysis.