Influence of Internal Hydrogen State on the Fatigue Behavior in Cold-Drawn High-Strength Steel

Abstract

Slow strain rate tests (SSRT) and fatigue tests were conducted to investigate the influence of internal hydrogen state on the quasi static SSRT strength and fatigue strength for cold drawn eutectoid steel specimens which were cathodically hydrogen charged. Internal hydrogen states were changed as follows : (a) virgin sample or non-charged one, (b) the sample that contained both diffusive and non-diffusive hydrogen, and (c) the one that contained only non-diffusive hydrogen. The SSRT strength and the fracture displacement of only non-diffusive hydrogen charged samples were the same as that of non-charged sample, whereas those of the sample having both diffusive and non-diffusive hydrogen were smaller than the others. This indicates that only the diffusive hydrogen caused hydrogen embrittlement under the quasi-static loading condition. The fatigue strength of the specimen having both diffusive and non-diffusive hydrogen decreased compared with non-charged specimens. However, S-N curves had a large scatter and the influence of non-diffusive hydrogen on the fatigue strength was unclear ; because the fatigue crack was initiated at internal inclusion. Therefore, the fatigue strength was discussed based upon the stress intensity factor, ΔKinc, calculated from stress and inclusion size. ΔKinc giving the same Nf decreased in the order of the non-charged specimen, only non-diffusive hydrogen charged specimen and the specimen having diffusive and non-diffusive hydrogen, indicating non-diffusive hydrogen caused a decrease in fatigue strength.