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Slow strain rate tensile (SSRT) tests and fatigue tests were conducted to investigate the influence of irreversible hydrogen on the quasi static SSRT strength and fatigue strength for cold drawn eutectoid steels which were cathodically hydrogen charged. Internal hydrogen states were changed as follows: (a) virgin sample, (b) the sample that contained both diffusible and irreversible hydrogen, and (c) the one that contained only irreversible hydrogen. The SSRT strength properties of only irreversible hydrogen contained sample were the same as that of virgin sample, whereas those of the sample having both diffusible and irreversible hydrogen were smaller than the others. This indicates that only the diffusible hydrogen caused hydrogen embrittlement under the quasi-static loading. Contrary to this, the fatigue strength of the sample having both diffusible and irreversible hydrogen was lower than that of virgin sample. However, the influence of irreversible hydrogen on the fatigue strength was unclear since S-N curves had a large scatter. This is due to the fact that the fatigue crack was initiated at internal inclusion. Therefore, the fatigue strength was discussed based on the stress intensity factor, ∆K inc , calculated from stress and inclusion size. ∆K inc giving the same N f decreased in the order of virgin sample, irreversible hydrogen charged sample and the sample having diffusible and irreversible hydrogen, indicating irreversible hydrogen caused the fatigue strength degradation.

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