Visualization of Hydrogen Diffusion in a Hydrogen-Enhanced Fatigue Crack Growth in Type 304 Stainless Steel
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IN fuel cell systems, many engineering components, such as hydrogen tanks, pipes, valves, and springs, are exposed to high-pressure hydrogen. Recently, to ensure the safer use of fuel cell systems in the near future, great efforts have been made to obtain reliable data about the strength properties of various materials when exposed to hydrogen. Austenitic stainless steels are obvious candidate materials for hydrogen pipes, valves, and springs. It is well known that hydrogen can enhance crack growth under both static and cyclic loadings in these steels. In the static cases, e.g., slow crack growth (SCG) problem,[1–5] it is frequently postulated that, in order for a crack to propagate or accelerate, hydrogen has to be transported to and accumulated in a region at or near the crack tip;[4] that is, the degree of hydrogen effect can be controlled by the rate of the hydrogen supply. Perng and Altstetter[5] point out that deformation-induced a¢ martensite enhances the hydrogen diffusivity and, thereby, accelerates the hydrogen transportation to the crack-tip region in metastable austenitic stainless steels. This is the HISAO MATSUNAGA, Associate Professor, is with the Department of Mechanical Engineering, Fukuoka University, Fukuoka 814-0180, Japan. Contact e-mail: [email protected] HIROSHI NODA is with the Needle Roller Bearing Technology Department, Automotive Bearing Technology Center, NSK Ltd., Gunma 370-3344, Japan. Manuscript submitted May 4, 2010. Article published online March 29, 2011 2696—VOLUME 42A, SEPTEMBER 2011
reason for metastable austenitic stainless steels, such as type 304, generally being more susceptible to hydrogen embrittlement than stable steels such as type 310.[5] Also, in the case of cyclic loading, some studies have reported that type 304 steel exhibits a poorer crack growth resistance against hydrogen than type 316L and 310S steels having a greater austenitic stability.[6–9] This propensity is also true in the combination of type 301 and 302 steels.[10,11] Murakami et al.[6] point out a possible process, similar to Altstetter et al.’s model, for the fatigue crack growth acceleration, in which a deformation-induced martensite increases the hydrogen diffusivity locally and, therefore, enhances the hydrogen concentration in a region near the fatigue crack tip. These models suggest that deformation-induced martensite plays a key role in the hydrogen-enhanced crack growths in austenitic stainless steels. However, the microscopic behavior of diffusion and segregation in the processes has not been directly confirmed. In this study, the influence of the resultant diffusion of hydrogen on the fatigue properties of an AISI type 304 stainless steel was investigated using specimens charged with hydrogen. The hydrogen emission site in the hydrogen-charged specimen was visualized using the hydrogen microprint technique (HMT). In addition, the martensitic transformation due to cyclic deformation was examined using an X-ray diffraction (XRD) technique and electron backscatter diffraction (EBSD) analysis. The
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