Effect of Hydrogen on the Gigacycle Fatigue Properties of Low Alloy Steel

Gigacycle fatigue tests were conducted for hydrogen charged SCM440 low-alloy steel in order to investigate the effects of hydrogen on fatigue properties of high-strength steel. 200 and 550°C tempered samples were prepared and an ultrasonic fatigue testing machine was used for the fatigue tests. The base steel without hydrogen charging showed only fish-eye fractures in case of the 200°C tempered version, while the 550°C tempered version showed surface and fish-eye fractures in short and long life regions, respectively. On the other hand, the hydrogen charged steel of both versions showed only fish-eye fracture and the fatigue strength largely dropped. These results meant that the hydrogen effects were larger on the fish-eye fracture than on the surface fracture. However, no extension of ODAs was observed in the hydrogen charged steels, so the degradation of fatigue strength due to hydrogen had nothing to do with ODAs. Moreover, in this research, it was confirmed that the ultrasonic fatigue testing machine was effective in evaluating gigacycle fatigue properties of the hydrogen charged steel. The ultrasonic fatigue testing machine enabled us to finish the gigacycle fatigue test before the specimen lost a large amount of hydrogen due to emission, and no frequency effects were observed in case of fish-eye fracture even in the hydrogen charged steel.