Metastable austenitic steels react to plastic deformation with a thermally and/or mechanically induced martensitic phase transformation. The martensitic transformation to α’-martensite can take place directly or indirectly via the intermediate stage of ε-martensite from the single-phase austenite. This effect is influenced by the stacking fault energy (SFE) of austenitic steels. An SFE < 20 mJ/m² is known to promote indirect conversion, while an SFE > 20 mJ/m² promotes the direct conversion of austenite into α’-martensite. This relationship has thus far not been considered in relation to the hydrogen environment embrittlement (HEE) of metastable austenitic CrNi steels. To gain new insights into HEE under consideration of the SFE and martensite formation of metastable CrNi steels, tensile tests were carried out in this study at room temperature in an air environment and in a hydrogen gas atmosphere with a pressure of p = 10 MPa. These tests were conducted on a conventionally produced alloy AISI 304L and a laboratory-scale modification of this alloy. In terms of metal physics, the steels under consideration differed in the value of the experimentally determined SFE. The SFE of the AISI 304L was 22.7 ± 0.8 mJ/m² and the SFE of the 304 mod alloy was 18.7 ± 0.4 mJ/m². The tensile specimens tested in air revealed a direct γ → α’ conversion for AISI 304L and an indirect γ → ε → α’ conversion for 304 mod. From the results it could be deduced that the indirect phase transformation is responsible for a significant increase in the content of deformation-induced α’-martensite due to a reduction of the SFE value below 20 mJ/m² in hydrogen gas atmosphere.

The scope of this research project covers the examination of the microstructure of different austenitic steels with regard to H embrittlement. It is known from the literature that the alloy X2CrNi18-9 is susceptible to H embrittlement, whereas steel X2CrNiMo18-14-3 is highly resistant to the embrittlement process. Austenitic steels with a low γ stability are susceptible to form α‘ martensite during plastic deformation. The γ stability can be estimated using empirical formulas, while one drawback of this approach is that only a limited amount of alloying elements is considered by those equations. A thermodynamic approach has proven effective for the estimation of the γ stability of multicomponent alloys. Hence, results from the microsegregation analysis for alloy X2CrNi18–9 using energy dispersive X-ray spectroscopy are presented and their influence on the local γ stability is discussed based on property distribution images.