Fatigue Crack Growth Properties of Austenitic Stainless Steels under the Influence of External/internal Hydrogen and Comparison with Those of Low Alloy Steels, Carbon Steels and Aluminum Alloys

In order to investigate the effects of external and internal hydrogen on fatigue crack growth (FCG) properties of austenitic stainless steels SUS304, SUS316 and SUS316L, two series of tests were conducted. As the external hydrogen series test, ΔK-increasing (ΔP-constant) and ΔK-constant (ΔP-decreasing) tests were conducted in 115 MPa hydrogen gas at room temperature and stress ratio, R, of 0.1 using non-charged specimens. As the internal hydrogen series test, ΔK-increasing (ΔP-constant), R-constant (ΔK-decreasing) and Pmax-constant (ΔK-decreasing) tests were conducted in air at room temperature and R = 0.1～0.9 using hydrogen-charged specimens exposed to 68 MPa hydrogen gas at 300℃. Comparing the results of the austenitic steels with those of low alloy steel SCM435, carbon steel SM490 and aluminum alloy 5083-O, the effects of material type and hydrogen on FCG rate, da/dN, and fatigue crack growth threshold, ΔKth, are discussed. FCG at R of 0.1 was accelerated by hydrogen in the intermediate ΔK regime. The relative FCG rates, (da/dN)H/(da/dN)air, in the external and internal hydrogenated conditions corresponded to each other. The upper limit of (da/dN)H/(da/dN)air was 10.6 for SUS304, 3.3 for SUS316 and 2.4 for SUS316L. On the other hand, the upper limit was 27.7 for SCM435 and 45.0 for SM490B. FCG was not accelerated by external hydrogen in 5083-O. ΔKth values of non-charged and hydrogen-charged specimens obtained under various R ranging from 0.1 to 0.91 were almost the same in SUS304, SUS316L and SCM435. That is, ΔKth was not affected by internal hydrogen. The fact was understood by considering that ΔKth is the critical stress intensity factor range below which dislocations are not emitted from fatigue crack tip.