Effects of Low Temperature on Hydrogen-Assisted Crack Growth in Forged 304L Austenitic Stainless Steel
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INTRODUCTION
AUSTENITIC stainless steels such as 304L are attractive for high-pressure hydrogen fueling applications, since they retain significant ductility in the presence of hydrogen, in comparison to other types of steels. Given the low ambient temperatures that hydrogen fueling systems may experience (as in hydrogen fueling stations and fuel cell electric vehicles), and the numerous studies indicating synergistic effects of hydrogen and low temperature on tensile ductility loss in austenitic stainless steels,[1–14] characterization of the fracture resistance of these alloys in low-temperature hydrogen environments is needed. However, few studies at any temperature have characterized effects of hydrogen on fracture resistance of forged austenitic steels
HEATHER JACKSON, formerly Postdoctoral Appointee with Sandia National Laboratories, Livermore, CA, is now Consultant with Structural Integrity Associates, San Jose, CA. Contact e-mail: [email protected] CHRIS SAN MARCHI, BRIAN SOMERDAY, and DORIAN BALCH, Distinguished Members of Technical Staff, are with Sandia National Laboratories. JOSEPH MICHAEL, Distinguished Member of Technical Staff, is with Sandia National Laboratories, Albuquerque, NM. Manuscript submitted December 14, 2015. Article published online June 6, 2016 4334—VOLUME 47A, AUGUST 2016
using fracture mechanics (or crack growth methods) and well-characterized hydrogen concentrations.[6,15–18] Previous tensile studies[1–13] also showed that low temperature alters fracture micromechanisms in hydrogen-exposed austenitic steels, with effects more pronounced for metastable 300-series alloys with low Ni content. These environmental (hydrogen, low temperature) and metallurgical (low Ni) factors are associated with increased propensity for localized deformation[19–22] (although it should be noted that other interpretations for effects of low temperature and low Ni have been proposed, such as the promotion of a¢-martensite formation at low temperature emphasized in References 10 through 12). Hydrogen-induced localized deformation has been associated with degraded fracture resistance,[18,20,23,24] and it has been proposed that temperature-sensitive localized deformation in low-Ni (or otherwise predisposed) alloys is exacerbated by hydrogen exposure.[25] The objective of the present study is to characterize effects of low temperature on hydrogen-assisted crack propagation in forged 304L stainless steel. Fracture mechanics specimens were thermally precharged in hydrogen gas, and fracture initiation toughness and crack growth resistance curves were measured at room temperature and 223 K (50 C). The role of microstructural features in damage initiation and METALLURGICAL AND MATERIALS TRANSACTIONS A
microcrack formation was characterized via electron microscopy of fracture surfaces and fracture profiles.
II.
EXPERIMENTAL
A. Forged 304L The starting materials were two heats of 304L bar stock having similar compositions (Table I). Forgings were prepared by two processes, referred to here as 3-stage (‘‘higher-stre
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