Low Cycle Fatigue of Alloy 690 in the PWR Environment

International audience ; Environmentally Assisted Fatigue (EAF) is one of the concerns for the extended operation of nuclear power plants. Over time, certain components experience thermal cycling while being exposed to the chemically active environment of Pressurized Water Reactors (PWRs). While many components in the primary loop are constructed from stainless steel, some critical parts are made from Alloy 690, a nickel-based alloy. Compared to the extensive research on EAF in stainless steels, only a limited focus has been placed so far on Alloy 690. This study aims to assess the impact of the PWR environment on the fatigue life of Alloy 690. Low cycle fatigue tests were conducted in two different environments, namely air and PWR environments, at different strain amplitudes and strain rates. To investigate the mechanisms of EAF in Alloy 690, fracture surfaces were analyzed. In particular, fatigue crack growth rates as a function of crack depth were derived from striation spacing measurements. Additionally, microstructural changes, as influenced by Dynamic Strain Aging, in the air were examined using Transmission Electron Microscopy (TEM) on the specimens from interrupted tests at different stages of the fatigue life. A twofold ratio was expected according to the design rules, but the results indicate that there is minimal or almost no influence of the environment on the fatigue life of Alloy 690 in PWR conditions when compared to its behavior in an air environment. This observation is consistent with the crack growth rates, which demonstrate a constant growth in both the environments.