Influence of hydrogen saturation on the structure and mechanical properties of Fe-17Cr-13Ni-3Mo-0.01С austenitic steel during rolling at different temperatures

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Introduction The development of methods and technologies gives reason to believe that hydrogen fuel cells will play an important role in energy production. Advances in hydrogen energy will help one to reduce the depen- dence on fossil fuels and significantly decrease carbon dioxide emission. Safe storage (ultrahigh-pressure tanks and containers) and transport of hydrogen (piping, valves, sleeves, springs, and gauges to regulate pressure) are key issues in the widespread using of hydrogen energy. In this regard, the requirements grow for the quality and service properties of structural materials that are exposed to hydrogen [1–3]. One of the important areas of research in this field is the evaluation of the mechanical behavior of structural materials subjected to hydrogen exposure under various loading conditions. Austenitic stainless steels (SS) have good corrosion resistance and are less susceptible to hydrogen embrittlement than other steels [4, 5]. Therefore, such steels are candidate materials for various compo- nents of hydrogen transportation and storage systems. Cold plastic deformation of austenitic SS causes the formation of various types of deformation defects in the structure, and in some cases is accompanied by γ→ε and γ→αʹ phase transformations [6–13]. This leads to deformation and fragmentation of the grain structure and, consequently, to a change in the mechanical properties of steel under cold deformation (an increase in microhardness, a yield strength and an ultimate tensile strength, and a decrease in plasticity). The choice of materials for hydrogen energy should consider the effect of hydrogen as an alloying element on deformation-induced processes in austenitic stainless steels. Numerous works indicate that SS with high stability of austenite to phase transformations (for example, Fe-17Cr-14Ni-3Mo or Fe-18Cr-20Ni-2Si) are less susceptible to hydrogen embrittlement than steel grades with low stability (for example, Fe-18Cr-8Ni, Fe-18Cr-10Ni, Fe-18Cr-10Ni-Ti) [3, 5, 14–18]. The stability of austenitic SS to phase transformations depends on stacking fault energy (SFE), which is determined by the steel chemical composition [19–22]. At the same time, it is shown in [4] that not only deformation phase transformations, including those hydrogen-induced, cause hydrogen degradation of the mechanical properties of austenitic steels, but also the type of deformation-assisted dislocation arrangment. Stable steels with high SFE, in which a planar dislocation structure develops, are more susceptible to hydrogen embrittlement than those with wavy slip [4]. All above-mentioned studies indicate that hydrogen effectively affects both the type of microstructure and phase transformations in austenitic steels. The steel of composition Fe-17Cr-13Ni-3Mo-0.01С (analog of AISI 316L) is a type of austenitic chromium-nickel stainless steel. It possesses high strain hardening rate, low tendency to deformation phase transformations at room temperature due to its high SFE [7, 10, 11, 19]. Despite the fact that the processes of hydrogen embrittlement for chromium-nickel steels with different SFE is studied in detail in uniaxial tension [3–5, 14–18, 23], there are almost no data on the effect of hydrogen on the structure refinement and hardening of these steels under other types of loading, for example, during rolling. In this work, we investigated the effect of chemical-deformation processing regimes, including multipass rolling with preliminary saturation with hydrogen of the specimens, on phase composition, microstructure, deformation mechanisms and mechanical properties of Fe-17Cr-13Ni-3Mo-0.01С austenitic steel. Methods The industrially melted stable austenitic stainless steel Fe-17Cr-13Ni-3Mo-0.01С is chosen as the material for investigation. The steel billets with the shape of the rectangular plates were cut using an electric spark machine. After cleaning in the aqua regia, the plates were kept at a temperature of 1100 °C for an hour and water-quenched at room temperature. Heat treatment was carried out in an inert gas (helium) environment. After heat treatment, the plates were mechanically ground and electrolytically polished in a solution of 25 g CrO 3 + 210 ml H 3 PO 4 . Before hydrogen saturation, all plates had the same size of 10×20×1 mm 3 . The first portion of specimens was electrolytically hydrogen saturated for 5 hours at room temperature. Hydrogen saturation was carried out in 1N aqueous solution of sulfuric acid (H 2 SO 4 ) with add of