Session: MF-02-05 Materials for Hydrogen Service-High Alloy Materials 2

Paper Number: 122356

122356 - Hydrogen Effect on Fatigue-Life Properties of Cold-Rolled, Metastable Austenitic Stainless Steels With Artificial Defects 

The materials used for high-pressure hydrogen components such as hydrogen stations (HSs) and fuel cell vehicles (FCVs) are exposed to high-pressure hydrogen gas at around 100 MPa. Since the ductility of many materials is degraded by hydrogen, Japanese regulations stipulate that only materials that are negligibly embrittled by hydrogen are to be used in high-pressure hydrogen components to ensure safety. In this context, only stable austenitic stainless steels, Types 316 and 316L, with high nickel-equivalent values are authorized for use in high-pressure hydrogen gas environments. However, such nickel-equivalent materials are low in strength and high in cost. Therefore, to achieve widespread use of HSs and FCVs, it may be necessary to construct reasonable material selection and strength design methods in consideration of hydrogen embrittlement. Regarding the materials used for hydrogen components adopting an infinite life design, a new material selection method that enables the use of metastable austenitic stainless steels is being discussed on the basis that the hydrogen does not degrade the fatigue limit of the material, although it does degrade the slow strain-rate tensile (SSRT) properties. However, its steel has lower yield and tensile strengths compared to carbon and low-alloy steels; therefore, an improvement of the yield and tensile strengths of the metastable austenitic stainless steels is desired.

One of methods for improving tensile strength of metals is cold working; however, hydrogen effect on fatigue properties of cold-worked, metastable austenitic stainless steels is not clarified. In this viewpoint, uncharged and H-charged, plate specimens of cold-worked Types 301 and 304 steels with a small artificial defect (drilled hole or sharp notch) were prepared and then, its plane bending fatigue tests were conducted at test frequencies of 5 to 20 Hz and a stress ratio of –1 in air at room temperature. C/L-specimen with loading direction perpendicular/parallel to cold-rolling one was prepared and the hydrogen charging was conducted in hydrogen gas at 100 MPa and 543 K for 200 h. Although the fatigue lives of both steels were significantly degraded by hydrogen, these fatigue limits were equal to or slightly lower than those of the uncharged ones, irrespective of defect types. The fatigue limits of the drilled-hole, uncharged and H-charged specimens were determined by the critical stress for crack initiation and the H-charged specimens showed higher fatigue limits than the uncharged ones, because of hydrogen-induced increase in hardness. On the other hand, non-propagating cracks were detected from the sharp-notch specimens after fatigue tests and the fatigue limits were interpreted to be the threshold stress for crack propagation. Except for the H-charged, L-specimens of both steels, the experimental fatigue limits were successfully predicted by the √area parameter model where the fatigue limit can be predicted by the Vickers hardness and defect size and becomes higher with an increase in hardness and decrease in the defect size. On the other hand, the √area parameter model overestimated the experimental fatigue limit of the H-charged, L-specimens of both steels by around 20%. Although hydrogen contributed an increase in fatigue limit by elevated hardness (positive effect), there is a possibility that hydrogen reduced an emission stress of dislocations and caused an undeveloped crack closure by localized slip deformations enhanced and suppression of strain-induced martensitic transformation, leading to a decrease in fatigue limit (negative effect). It is deemed that the negative effect was enhanced in the H-charged, L-direction specimens of both steels and its fatigue limits were slightly degraded by hydrogen.

Presenting Author: Junichiro Yamabe Fukuoka University

Presenting Author Biography: He is a full Professor of Fukuoka University and continue to perform hydrogen-embrittlement study around 20 years. More than 100 peer-reviewed research papers have been published and many invited lectures have been made so far.

Authors:

Junichiro Yamabe Fukuoka University
Kento Hashiguchi Fukuoka University
Kentaro Wada National Institute for Materials Science (NIMS)