Effects of hydrogen charge on microscopic fatigue behaviour of annealed carbon steels

The 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.