Effect of hydrogen on fatigue fracture of type SUS316L stainless steel rod

Abstract

The rotate bending fatigue tests for type 316L austenitic stainless steel rods were carried out in air and in an electrolyte with electrochemical cathodic hydrogen charging. The electrolyte used was composed of 0. 05kmol•m^-3 sulfuric acid with 1. 4kg•m^-3 thiourea, and the employed current density was 49Am^-2. Fatigue strength was noticeably decreased by hydrogen charging and the fatigue limit disappeared at the number of cycles up to 10⁷. A considerable amount of α' martensite phase was detected on the fracture surface by X-ray diffraction measurement. The electron probe micro analysis made clear that nickel segregation existed in the central part of the rods, from which the fatigue test pieces were taken. In some area, nickel content was as low as 11mass%, which was almost 1mass% lower than the lower limit of the specification of the average nickel content for 316L. It is considered that the area with low nickel content transformed to martensite phase at the fatigue crack tip having high strain when fatigue cracks propagated. As the martensite phase was believed to be vulnerable against hydrogen, it is considered that the nickel segregation was the major cause of the degradation of fatigue properties in the hydrogen circumstance.