2015 年 81 巻 824 号 p. 14-00433
Gigacycle fatigue tests were conducted on two heats of actual materials of storage cylinders used in hydrogen stations for verification tests. The storage cylinders were made of SCM435 steel and the test specimens were extracted in both longitudinal and circumferential directions. Ultrasonic and servo-hydraulic fatigue tests were carried out on hydrogen-charged and uncharged specimens. The two heats of the materials showed difference in microstructures, resulting in different mechanical properties. The hydrogen-charging introduced about 1.2 ppm of diffusible hydrogen, which decreased reduction of a cross-sectional area in tensile tests. In the fatigue tests on the uncharged specimens, only surface fracture occurred, showing negligible difference of fatigue strength between the heats and between the directions. The hydrogen-charged specimens, on the other hand, revealed different fatigue properties between the heats. Internal fractures originating from inclusions occurred in one heat and the gigacycle fatigue strength was decreased. The other heat showed no internal fracture and the degradation of the fatigue strength was negligible. The origins of the internal fractures were different between the directions, while the sizes were close. Accordingly, the difference of the fatigue strength was small between the directions. In summary, fatigue properties were not so match affected by microstructure unlike mechanical properties, while careful attentions were necessary for inclusions. The inclusion-induced internal fractures were strongly affected by hydrogen and this research demonstrated that this type of internal fractures could occur in actual materials.
