Hydrogen-related Fatigue Fracture under Various Test Frequencies in Low-carbon Martensitic Steel

The present study investigated the hydrogen-related fatigue fracture under various test frequencies in low-carbon martensitic steel. In the hydrogen-charged specimen, although the number of cycles to failure decreased with decreasing test frequency, the time to failure was almost the same regardless of the test frequency. Observation of fracture surface revealed that the transgranular surface was a main component in the uncharged specimen, while the intergranular surface was often observed especially at the lower test frequency in the hydrogen-charged specimen. In addition, for the transgranular fracture, cracks often propagated across the laths regardless of test conditions. The high-strained region was observed over a relatively wide area in the uncharged specimen. On the other hand, the hydrogen-related fatigue-crack propagation was accompanied by intense localized plastic deformation, which could accelerate crack growth. The intergranular cracking and high localization of plastic deformation could be the possible reasons for decreasing the fatigue life by the presence of hydrogen.