The low cycle fatigue tests of SA508 Gr.1a low alloy steel were carried out to investigate the fatigue crack growth mechanisms in high temperature water. The fatigue life in 310oC deoxygenated water was shorter than that in air. Furthermore, the reduction in the fatigue life in 310oC deoxygenated water was enhanced with a decreasing strain rate, from 0.4 to 0.008 %/s. The ductile striations with the streamed down features, which may indicate the occurrence of the metal dissolution, were mainly observed at the strain rate of 0.008 %/s. And the flat facets and the brittle cracks, which may be evidences for the HIC, were primarily observed in the strain rate range from 0.04 to 0.4 %/s. From the analysis of microstructure, it is thought that the HIC contribute dominantly to the reduction in the fatigue life in the strain rate range from 0.04 to 0.4 %/s and the metal dissolution is mainly responsible for the reduction in the fatigue life at the strain rate of 0.008 %/s.

Tensile properties of SA508 Cl.3 reactor pressure vessel (RPV) steel were investigated at room temperature and at 288 °C before and after hydrogen charging by electrolysis. At room temperature, the charged hydrogen induced distinct hardening and ductility loss, where quasi-cleavage features were observed around inclusions. These results may be due to interactions between the dissolved hydrogen and dislocations and an increase of hydrogen concentration near the inclusions. On the other hand, at 288 °C, the charged hydrogen induced some softening, which was explained in terms of the hydrogen shielding effect, and of strain localization by dynamic strain aging (DSA). Further, at 288 °C, the fracture surfaces of the hydrogen-charged specimens showed brittle regions, where the hydrogen might have been trapped in microvoids, leading to internal pressurization.

The low cycle fatigue tests of SA508 Gr.1a low alloy steel in 310oC deoxygenated water were conducted to investigate the effect of cyclic strain rate on the environmentally assisted cracking (EAC) mechanisms. The flattened striations and the blunt crack tip, which indicate the occurrence of the slip dissolution/oxidation, were mainly observed for the specimen tested at 0.008 %/s. On the other hand, the brittle cracks and the blunt main crack with microcracks, which are the evidences of the hydrogen-induced cracking (HIC), were observed for the specimens tested at 0.04 and 0.4 %/s. Through this study, it is thought that the slip dissolution/oxidation dominantly contributes to the reduction in the fatigue life at a strain rate of 0.008 %/s and the HIC is mainly responsible for the reduction in the fatigue life at strain rates of 0.04 and 0.4 %/s.