Stress-Rupture Behavior of Alloys 230 and 617 for High Temperature Applications

Austenitic Alloys 230 and 617 have been identified to be the two most suitable structural materials for heat exchanger application within the purview of the next generation nuclear plant (NGNP) program. The NGNP program is aimed at developing electricity and hydrogen using heat from very-high-temperature-reactor (VHTR). A maximum operating temperature of 950 °C has been recommended to achieve the highest possible efficiency in both electricity and hydrogen generation. The identification of Alloys 617 and 230 as heat exchanger materials was based on their excellent resistance to high-temperature degradations including creep, stress-rupture, fatigue and tensile deformation. Extensive efforts have been made to evaluate the creep and stress-rupture behavior of both alloys at temperatures relevant to the NGNP application. This paper presents the results of stress-rupture testing involving these alloys as functions of applied stress and temperature. The time to rupture was gradually reduced with simultaneous increase in stress and temperature, leading to a gradual reduction in the Larson-Miller-Parameter (LMP) indicating enhanced rupture tendency.