Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea Local strains in 1.4301 austenitic stainless steel with internal hydrogen Thorsten Michler a, ⁎ , Enrico Bruder b a Opel Automobile GmbH, Ruesselsheim, Germany b Physical Metallurgy Division, Materials Science Department, Technische Universität Darmstadt, Germany ARTICLE INFO Keywords: Hydrogen embrittlement Austenitic stainless steel Digital image correlation Microstructure Martensite ABSTRACT 304 austenitic stainless steel was precharged with hydrogen (∼ 0.3 at%) and subsequently slow strain rate tensile tested at room temperature. The motivation of this investigation was to further clarify the nature of crack initiation, and especially the role of martensitic transformation, in metastable austenitic stainless steel under the influence of hydrogen. However, it must be distinguished between martensite formed by pre-straining, i.e. γ-α′- transformation prior to the exposure to hydrogen and γ-α′-transformation during plastic deformation under the influence of hydrogen. In the first scenario martensite contents formed by pre-straining could not be correlated with HE effects in tensile tests. In the second scenario crack initiation could always be attributed to locations with local strain incompatibilities like twin-grain boundary triple junctions, γ-α′ phase boundaries, α′ marten- site, δ-ferrite stinger and strain incompatibility sites within γ grains. The results clearly show that γ-α′ trans- formation only plays a role in two of the five failure modes indicating that γ-α′ transformation is not the sole root cause for the high susceptibility of 304 stainless steel to hydrogen embrittlement. 1. Introduction Hydrogen embrittlement is a well-known damage phenomenon af- fecting most metallic alloys [1]. Among the most important materials used for gaseous hydrogen storage and transport applications are Cr-Ni austenitic stainless steels (SS) with Ni contents higher than 12 wt% (type DIN 1.4435, SUS 316L) because of low susceptibility to hydrogen embrittlement (HE) at relevant operating conditions [2]. Experimental results clearly show that the resistance to HE of SS increases with in- creasing Ni content in the steels [2]. Since Ni is a strong austenite stabilizer, it appears self-evident that the resistance to HE increases with increasing austenite stability. However, this correlation has never been proven resulting in an ongoing debate about the role of marten- sitic transformation on HE of SS. It is important to emphasize that it must be distinguished between martensite formed by pre-straining, i.e. γ-α′-transformation prior to the exposure to hydrogen and γ-α′-trans- formation during plastic deformation under the influence of hydrogen. In the case of γ-α′-transformation prior to the exposure to hydrogen it has been clearly shown that α′-martensite contents formed by pre- straining cannot be correlated with a reduction in tensile ductility [15,38]. In the case of γ-α′-transformation during plastic deformation under the influence of hydrogen, the high susceptibility of low Ni SS is often attributed to the low austenite stability. This statement is justified by the detection of high amounts of α′-martensite along crack paths and fracture surfaces [3–12]. In addition to the inherent low austenite sta- bility (due to the low concentration of austenite stabilizing alloying elements), there is growing evidence that hydrogen enhances the lo- calization of deformation [27,34,36], i.e. decreasing the local stacking fault energy and thus facilitating γ-α′-transformation. It is then argued that upon γ-α′-transformation, the α′-martensite is supersaturated by hydrogen due to the significantly lower solubility of hydrogen in the α′- martensite leading to premature failure. First of all, one weak point of this argumentation is that the huge set of available tensile test results could not be correlated with the well known austenite stability or stacking fault energy figures in a satisfying manner [39,40] implying that there is no simple correlation between HE effects and austenite stability. Secondly, due to the large plastic strains at a crack tip, α′- martensite is detected on fracture surfaces independent of hydrogen, i.e. the detection of α′-martensite at fracture surfaces is not a direct proof of the detrimental effect of γ-α′-transformation but just the effect of high local strains at the crack tip [13]. Thirdly, hydrogen diffusion modelling results predict critical hydrogen concentrations in the aus- tenite next to the α′-martensite and not in the α′-martensite ahead of the crack tip [41] explaining the experimentally observed crack in- itiation in the austenite and not in the α′-martensite [37]. Finally, there are numerous examples of stable austenitic steels showing severe de- gradation of tensile properties under the influence of hydrogen [14] which suggests that γ-α′-transformation is neither necessary nor suffi- cient to explain hydrogen effects in SS. To sum up this brief review: https://doi.org/10.1016/j.msea.2018.04.011 Received 8 January 2018; Accepted 4 April 2018 ⁎ Corresponding author. E-mail addresses: Thorsten.dr.michler@opel.com (T. Michler), e.bruder@phm.tu-darmstadt.de (E. Bruder). Materials Science & Engineering A 725 (2018) 447–455 Available online 05 April 2018 0921-5093/ © 2018 Elsevier B.V. All rights reserved. T