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In this paper, the influence of irreversible hydrogen on the fatigue crack growth in high-strength steel is discussed. The fatigue crack growth rate was accelerated by irreversible hydrogen, and the acceleration increased with a decrease in stress cycle frequency. However, the stress ratio did not influence the fatigue crack growth rate evaluated in terms of Δ K eff . The behavior of irreversible hydrogen under cyclic loading was visualized by a hydrogen microprint technique. This proved that irreversible hydrogen desorbed out from the trap site under cyclic loading. The mechanical condition for irreversible hydrogen desorption was investigated by stress analysis and EPMA analysis.

The fatigue tests were conducted for cold-drawn eutectoid steels having different activation energies and microstructures to investigate the mechanism of fatigue strength degradation by irreversible hydrogen. The fatigue strength of a sample with low activation energy was decreased by irreversible hydrogen, whereas the fatigue strength of a sample with high activation energy was not. When the activation energy for irreversible hydrogen becomes small, the desorption of irreversible hydrogen from its trap site is easily induced by cyclic loading, which results in a decrease in fatigue strength. When a sample having high activation energy was annealed at 473 K, the fatigue strength was decreased by irreversible hydrogen although the activation energy barely changed. This is because the precipitation of fine carbides due to annealing causes higher sensitivity to hydrogen embrittlement. High sensitivity to hydrogen embrittlement was concluded to induce the fatigue strength degradation by irreversible hydrogen. Therefore, the degradation of fatigue strength by irreversible hydrogen depends on the activation energy and sensitivity to hydrogen embrittlement.

Fatigue tests were conducted for cold-drawn eutectoid steels having different activation energies for irreversible hydrogen trap sites and sensitivities to hydrogen embrittlement; the purpose was to investigate the mechanism of fatigue strength degradation by irreversible hydrogen. The fatigue strength of a sample with low activation energy was decreased by irreversible hydrogen in the material, whereas the fatigue strength of a sample with high activation energy was not. When the activation energies for irreversible hydrogen were almost equal, a higher sensitivity to hydrogen embrittlement induced fatigue strength degradation by irreversible hydrogen. Therefore, fatigue strength degradation by irreversible hydrogen depends on the activation energy for irreversible hydrogen trap sites and sensitivity to hydrogen embrittlement.

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, ΔKinc, calculated from stress and inclusion size. ΔKinc giving the same Nf 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.

This paper discusses the influence of irreversible hydrogen on the fatigue crack initiation life in cold drawn eutectoid steel. Fatigue tests were conducted for the sample having an artificial small notch fabricated by focused ion beam (FIB) system. Samples were cathodically hydrogen charged, and internal hydrogen states were changed as follows : (a) virgin sample and (b) the one that contained only irreversible hydrogen. In the notched sample, the fatigue strength was lower than that of smooth sample. The slope of the S-N curves became steeper and the influence of irreversible hydrogen was not observed. This is because the fatigue life was almost dominated by fatigue crack propagation life, resulted from a crack being easily initiated at a notch root owing to high stress concentration. The surface fatigue crack propagation rate, obtained by using fatigue life data for notched sample, was not affected by irreversible hydrogen. Using the obtained fatigue crack propagation rate and the rate in high vacuum corresponding to the propagation in a fish-eye region, the crack initiation life from an internal inclusion for smooth sample was estimated, and it shows that the irreversible hydrogen reduced the fatigue crack initiation life, compared with those of a virgin sample. This is a reason why the fatigue strength of a smooth sample was decreased by irreversible hydrogen.

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.

The influence of irreversible hydrogen on the fatigue crack growth in cold-drawn high-strength steel was investigated to clarify the mechanism of fatigue strength degradation by irreversible hydrogen. The fatigue crack growth rate was accelerated by irreversible hydrogen in low ΔKeff region, and the acceleration increased with a decrease in ΔKeff. Moreover, the fatigue crack growth rate under irreversible hydrogen charged condition increased as the stress cycle frequency decreased. The behavior of irreversible hydrogen under cyclic loading was visualized using a hydrogen microprint technique. It was proved that irreversible hydrogen desorbed out from its trap site under cyclic loading.

The effect of microshot peening on the surface characteristics and fatigue life of spring steel SUP9 was investigated. In the experiment, the projective method of the microshot was of the compressed air type. The microshot of 0.1 mm diameter was high-carbon cast steel, and the workpiece used was the commercially spring steel JIS-SUP9. The effect of the intermittent shot peening on the surface hardness was also studied. The surface layer of the workpiece was sufficiently deformed by microshot peening. The microshot peening process was very efficient in improving the fatigue life of spring steel. It was found that the use of the microshot was found to cause a significantly enhanced peening effect for spring steel.