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YUKITAKA MURAKAMI, TOSHIHIKO KANEZAKI, YOJI MINE, and SABURO MATSUOKAThe basic mechanism of the hydrogen embrittlement (HE) of stainless steels under fatigue loading is revealed as microscopic ductile fracture, resulting from hydrogen concentration at crack tips leading to hydrogen-enhanced slip. Fatigue crack growth rates in the presence of hydrogen are strongly frequency dependent. Nondiffusible hydrogen, at a level of 2 to approximately 3 wppm, is contained in ordinarily heat-treated austenitic stainless steels, but, over the last 40 years, it has been ignored as the cause of HE. However, it has been made clear in this study that, with decreasing loading frequency down to the level of 0.0015 Hz, the nondiffusible hydrogen definitely increases fatigue crack growth rates. If the nondiffusible hydrogen at O-sites of the lattice is reduced to the level of 0.4 wppm by a special heat treatment, then the damaging influence of the loading frequency disappears and fatigue crack growth rates are significantly decreased.