The effects of hydrogen on microstructural change and surface originated flaking in rolling contact fatigue were investigated using JIS-SUJ2 bearing steel specimens charged with hydrogen. Under clean lubrication conditions, subsurface originated flaking occurred and the rolling contact fatigue life was reduced and the amounts of the microstructural change called white structure that formed in the specimens increased as the hydrogen content increased. The localized microstructural changes were found in the hydrogen-charged specimens by electron microscope observations. It is supposed that the localization of plasticity was enhanced by hydrogen during the process of rolling contact fatigue. Under contaminated lubrication conditions, which included debris in the lubricating oil, surface originated flaking occurred and the rolling contact fatigue life of the hydrogen-charged specimens became shorter than the uncharged specimens, although white structure was not observed around the flaking. Enhancement of fatigue crack formations due to hydrogen was observed in specimens with artificial dents. It is presumed that hydrogen facilitated the formation of fatigue cracks on the raceway surface.

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A B S T R A C TThe effects of hydrogen charge on cyclic stress-strain properties, slip band morphology and crack behaviour of annealed medium carbon steels (JIS-S45C) were studied. The total strain range of the stress-strain hysteresis loop in the hydrogen-charged specimen was smaller than that in the uncharged specimen. Localized slip bands were observed in the hydrogen-charged specimen, while the slip bands were widely and uniformly distributed in the uncharged specimen. It is presumed that the decrease in the total strain range of the hysteresis loop is due to the slip localization caused by the hydrogen charge and cyclic loading. The sites of fatigue crack initiation were mostly at grain boundaries in the uncharged specimen. The sites of crack initiation in the hydrogen-charged specimen were not only at grain boundaries but also at slip bands inside ferrite grains. These results imply that hydrogen enhances dislocation mobility along slip bands and results in slip localization. These slip bands then attract hydrogen. This mechanism of hydrogen-slip band interaction may play an important role in the hydrogen-influenced metal fatigue.Keywords fatigue crack; hydrogen; hysteresis loop; localization; slip bands. N O M E N C L A T U R E2a = crack length C H = hydrogen content f = test frequency N = number of cycles N f = number of cycles to failure R = stress ratio t = time after hydrogen charge ε t = total strain range ε = strain σ = stress σ a = stress amplitude I N T R O D U C T I O NIt is known that fatigue failure can occur in the ultra long life regime of N > 10 7 in high strength steels and that the fracture origin is at a non-metallic inclusion contained in the material. Murakami et al. [1][2][3][4] pointed out that a typical morphology, named the optically dark area (ODA), exists around the non-metallic inclusion at the fracture origin and they have shown evidence that the formation of the Correspondence: H. Uyama.ODA is related to the hydrogen trapped by non-metallic inclusions. However, the exact mechanism of the formation of the ODA and the role of hydrogen has not been made clear.Recently, the demonstration and commercialization of fuel cell (FC) systems has led to the realization of their potential for solving the global warming problem. Many metallic components used in FC systems are directly exposed to hydrogen environments. The so-called 'hydrogen embrittlement' has been used to express the degradation of metals due to hydrogen. However, the

Effects of hydrogen charge on fatigue behaviour of two carbon steels, JIS-S10C (SAE1010) and JIS-S45C (SAE1045) were investigated. There was no hydrogen effect in the cyclic stress-strain hysteresis loops of S10C hydrogen-charged with 0.2 ppm. On the other hand, the strain amplitude was decreased in S45C hydrogen-charged with 0.8 ppm. The delayed yielding and the decrease in the saturated value of the strain amplitude were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C under the constant stress amplitude tests. It is supposed that the degree of influence of hydrogen on cyclic stress-strain properties depends or material structure and/or hydrogen content. The effect of hydrogen charge (H : 0.5 ppm) on the fatigue life, the fatigue limit and the crack growth curves of S45C were not remarkable, while there was a distinct difference in the morphology of the slip bands between the hydrogen-charged and uncharged specimens. The localized slip bands were observed in the hydrogen-charged specimen of S45C. Therefore, it is presumed that the decrease in the strain amplitude in hysteresis loop by hydrogen charge is caused by the localization of slip bands. More crack initiations from ferrite grains were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C. This phenomenon also corresponds to the localization and the formation of slip bands.

Effects of hydrogen charge and frequency on fatigue behaviour were studied in bearing steel (JIS-SUJ2) tempered at 823K. The crack growth rate of the hydrogen-charged specimen was faster than that of the uncharged specimen. The morphology of the crack observed on the surface of the hydrogen-charged specimen was thinner than that of the uncharged specimen. It is presumed that the slip deformation at the crack tip was restricted and localized by hydrogen. The definite dependency on test frequency in the crack growth rate was observed in the hydrogencharged specimens. The lower the test frequency, the faster the crack growth rate in the test of the hydrogencharged specimens at the frequency of 0.5~15Hz. However, there was no clear difference in the fracture surface morphology between the uncharged specimens and the hydrogen-charged specimens at the frequency of 0.5~15Hz. The transgranular fracture caused by fatigue was dominant and the intergranular fracture caused by so-called delayed fracture was little in all of the specimens. Therefore, it is presumed that the increase in the crack growth rate in the hydrogen-charged specimens at lower frequency was caused by enhancement of the fatigue mechanism related to hydrogen motion coupled with slip behaviour. Thus, it is presumed that the dependency on test frequency in the crack growth rate in the hydrogen-charged specimens was caused by the synergetic effect of the enhancement of hydrogen diffusion motion to the slip bands at the crack tip with time and easiness of slip motion by hydrogen.