Abstract
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.
Highlights
Metastable austenitic steels can potentially undergo a thermally-induced phase transformation due to undercooling
Two metastable CrNi steels of the type AISI 304L were investigated with respect to the direct γ → α’ and indirect γ → ε → α’ phase transformation dependent on the stacking fault energy as a result of plastic deformation at room temperature (RT)
As a result of plastic deformation, deformation-induced formation of the intermediate stage of ε-martensite was detected in the alloy with an stacking fault energy (SFE) of 18.7 ± 0.4 mJ/m2, whereas the alloy with an SFE of 22.7 ± 0.8 mJ/m2 directly transformed via the γ → α’ sequence
Summary
Metastable austenitic steels can potentially undergo a thermally-induced phase transformation due to undercooling. At or near room temperature (RT), austenitic steels generally exhibit a face-centered cubic (fcc) lattice structure, which has implications regarding the thermal phase stability. These steels are able to partially transform into α’-martensite through deformation. The common feature of the aforementioned austenitic steel grades is the relation between the stacking fault energy (SFE) and the deformation mechanism, which is characterized by a direct or indirect martensitic transformation. In this regard, metastable austenitic stainless CrNi- and
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