Effects of mechanical and metallurgical variables on creep, fracture toughness and crack growth behavior of Alloy 617

Nickel base Alloy 617 has been identified to be a suitable structural material for heat exchanger applications in both hydrogen and electricity generation using nuclear heat. A maximum operating temperature of 950°C has been specified by department of energy (DOE) for both applications to achieve a maximum possible efficiency. Therefore, an extensive investigation has been pursued to evaluate time-dependent-deformation (Creep) of this alloy as functions of temperature and applied load. The results indicate that this alloy exhibited severe creep deformation, characterized by development of an instantaneous tertiary creep region at 850 and 950°C under applied stresses corresponding to its 35% yield strength (YS) values at these temperatures. However, this alloy satisfied the deformation acceptance criteria at 5, 10, 25 and 35 percent of its YS values when loaded at 750°C. The results of crack growth studies indicate that this alloy showed an enhanced cracking susceptibility when tested within a temperature range of 100 to 200°C at the lowest loading ratio of 0.1. The fracture toughness of this alloy in terms of JIC was not significantly influenced by variation in temperature. The results of stress-corrosion-cracking study suggest that the rate of crack growth was gradually reduced with longer testing duration due to a relaxation of load with time. Microscopic evaluations of tested specimens were performed using numerous conventional techniques.