Effects of test frequency on fatigue behaviour in a tempered martensitic steel with hydrogen charge
Hideyuki UYAMA, Yoji MINE and Yukitaka MURAKAMI
Abstract:Effects of hydrogen charge and frequency on fatigue behaviour were studied in bearing steel (JIS-SUJ2) tempered at 823K. The crack growth rate of the hydrogen-charged specimen was faster than that of the uncharged specimen. The morphology of the crack observed on the surface of the hydrogen-charged specimen was thinner than that of the uncharged specimen. It is presumed that the slip deformation at the crack tip was restricted and localized by hydrogen. The definite dependency on test frequency in the crack growth rate was observed in the hydrogen-charged specimens. The lower the test frequency, the faster the crack growth rate in the test of the hydrogen-charged specimens at the frequency of 0.5`15 Hz. However, there was no clear difference in the fracture surface morphology between the uncharged specimens and the hydrogen-charged specimens at the frequency of 0.5`15 Hz. The transgranular fracture caused by fatigue was dominant and the intergranular fracture caused by so-called delayed fracture was little in all of the specimens. Therefore, it is presumed that the increase in the crack growth rate in the hydrogen-charged specimens at lower frequency was caused by enhancement of the fatigue mechanism related to hydrogen motion coupled with slip behaviour. Thus, it is presumed that the dependency on test frequency in the crack growth rate in the hydrogen-charged specimens was caused by the synergetic effect of the enhancement of hydrogen diffusion motion to the slip bands at the crack tip with time and easiness of slip motion by hydrogen. Key Words:Hydrogen,Fatigue crack, Frequency, Tempered martensitic steel, Bearing steel, Fuel cell