Fatigue fracture features of pressure vessel steel in simulated light water reactor environment

To evaluate fatigue crack growth of reactor pressure vessel steel at reactor operating conditions, tests were performed in air saturated hot water. The main test parameter was a loading frequency. Crack growth rate was increased with decreasing frequency up to a critical value. It was assumed through fractographic study and surface analysis that the enhancement of crack growth rate was environmentally assisted due to hydrogen embrittlement. In the low stress intensity factor range, cracks grew through crystallographic facets and tear ridges. Moreover, cleavage occurred at MnS inclusions. Larger cleavage occurred near MnS inclusions in the high stress intensity factor range. Brittle facets combined with voids and stair or step-like cracks with arrest marks appeared at sites some distance from the MnS inclusions. It was suggested that the environmentally assisted cracks were related to interactions of hydrogen with oxide film and also to dislocation motion at the crack tip.