High-Cycle Fatigue Behavior of Incoloy Alloy 800H in a Simulated HTGR Helium Environment Containing High Moisture Levels

A study has been carried out to evaluate the high-cycle fatigue strength of Incoloy alloy 800H in a high-temperature gas-cooled reactor helium environment containing significant quantities of moisture. As-heat-treated and thermally aged materials were tested to determine the effects of long-term oxidation in the helium test gas. Results from these tests were compared to those for a standard air environment. It was found that the mechanisms of fatigue failure were complex and involved recovery recrystallization of the surface-ground layer on the specimens, sensitization, work hardening, oxide scale integrity, and oxidation at the tips of propagating cracks. For certain situations, a corrosion-fatigue process seems to be important. However, for the helium environment studied, the fatigue strength was nearly always higher than that for air.