Effect of Hydrogen on High Cycle Fatigue Failure of High Strength Steel, SCM435

High cycle fatigue properties of high strength steel were investigated. In the Fuel Cell(FC) system, various metals are used in hydrogen environment under cyclic loading. In this study, hydrogen was artificially charged into specimen of a Cr-Mo high strength steel, and the fatigue properties were compared with those of the specimens without hydrogen charge. Fatigue strength and fatigue life decreased with increasing hydrogen content. The fracture origin of hydrogen charged specimens showed smaller ODAs than as-heat-treated specimens. It implies that the fatigue threshold of the microstructure which contains high hydrogen content is much lower than that of the as-heat-treated microstructure, and the upper bound of the critical hydrogen content level resulting ODA lies between 1.5ppm and 2.3ppm. Hydrogen desorption properties near the fracture origin were measured with Secondary Ion Mass Spectrometer(SIMS) and Thermal Desorption Spectrometer(TDS). Measurements with TDS and SIMS revealed that hydrogen trapped in microstructure is diffusive one and the hydrogen trapped by inclusion is non-diffusive one.