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We study the effect of grain size of austenitic and ferritic phases and volume fraction of δ-ferrite, which were obtained in different solution-treatment regimes (at 1050, 1100, 1150 and 1200 °C), on hydrogen embrittlement of high-nitrogen steel (HNS). The amount of dissolved hydrogen is similar for the specimens with different densities of interphase (γ-austenite/δ-ferrite) and intergranular (γ-austenite/γ-austenite, δ-ferrite/δ-ferrite) boundaries. Despite, the susceptibility of the specimens to hydrogen embrittlement, depth of the hydrogen-assisted surface layers, hydrogen transport during tensile tests and mechanisms of the hydrogen-induced brittle fracture all depend on grain size and ferrite content. The highest hydrogen embrittlement index IH = 32%, the widest hydrogen-affected layer and a pronounced solid-solution hardening by hydrogen atoms is typical of the specimens with the lowest fraction of the boundaries. Even though fast hydrogen transport via coarse ferritic grains provides longer diffusion paths during H-changing, the width of the H-affected surface layer in the dual-phase structure of the HNS specimens is mainly determined by the hydrogen diffusivity in austenite. In tension, hydrogen transport with dislocations increases with the decrease in density of boundaries due to the longer dislocation free path, but stress-assisted diffusion transport does not depend on grain size and ferrite fraction. The contribution from intergranular fracture increases with an increase in the density of intergranular and interphase boundaries.

Introduction. The development of hydrogen energy implies a decrease in the dependence of various human activities on fossil energy sources and a significant reduction in carbon dioxide emission into the atmosphere. Therefore, the requirements for the quality of structural materials, which have the prospect of being used for storage and transportation of hydrogen, as well as for the creation of infrastructure facilities for hydrogen energy, are increasing. Therefore, the scientific researches on the hydrogen-assisted microstructure and mechanical behavior of structural materials in various loading schemes are of great importance. The aim of this work is to establish the effect of chemical-deformation treatment, including rolling combined with hydrogen saturation, on the microstructure, phase composition, and mechanical properties of 316L-type austenitic stainless steel. Methods. Transmission electron microscopy and backscattered electron diffraction, X-ray diffraction, X-ray phase and magnetic phase analysis, microindentation and uniaxial static tension are utilized. Results and Discussion. It is shown experimentally that after rolling with 25 and 50 % upset, the morphology of the defect structure and the phase composition of 316L steel substantially depends on the deformation temperature (at room temperature or with the cooling of the samples in the liquid nitrogen) and on hydrogen saturation rate (for 5 hours at a current density of 200 mA/cm2). The main deformation mechanisms of the steel in rolling are slip, twinning, and microlocalization of plastic flow, which all provide the formation of ultrafine grain-subgrain structure in the samples. In addition, deformation-induced ε and α' martensitic phases are formed in the structure of the rolled samples. Regardless of the regime of chemical-deformation processing, grain-subgrain structures with a high density of deformation defects are formed in steel, but its morphologies are dependent on the processing regime. The experimental data indicate that both preliminary hydrogen saturation and a decrease in the deformation temperature contribute to the more active development of mechanical twinning and deformation-induced phase transformations during rolling. Despite the discovered effects on the influence of hydrogen saturation on the deformation mechanisms and the morphology of a defective microstructure formed during rolling, preliminary hydrogenation has little effect on the mechanical properties of steel at a fixed degree and temperature of deformation. These data indicate that irrespective of the morphology of the defective grain-subgrain structure, grain refinement, accumulation of deformation defects and an increase in internal stresses lead to an increase in the strength characteristics of the steel.

The features of crack formation near the fracture region and the fracture micromechanism are investigated during uniaxial tension of specimens of Ni–Cr stainless steels, Fe–19Cr–9Ni–0.1C and Fe–19Cr–9Ni–1.4Nb–0.1C, produced by the method of 3D wire-feed electron beam additive manufacturing (EBAM) as a function of the phase composition and phase distribution in the structure. It is found out that in the specimens with a twophase (austenite/δ-ferrite) microstructure the ferrite morphology and its volume content (up 25%) affect the plastic shear distribution in austenite and ferrite and exert a slight influence on the formation of strain localization micro- and macrobands in the pre-fracture stage, and the crack-formation mechanism is similar to that observed in cast stainless steels of similar compositions, which possess single-phase austenitic structure. In additively-manufactured steel containing niobium the formation of brittle niobium- and iron-based intermetallic phases favors the formation of pores and microcracks at the austenite/NbFeCrNi-phase or the δ- ferrite/NbFeCrNi-phase interfaces, while the processes of cracking in the strain localization bands (and their formation) turn out to be suppressed. Irrespective of the elemental and phase compositions of steel specimensformed by the method of 3D additive manufacturing, the principal micromechanism of their fracture, transgranular dimple fracture, is similar to that observed in the cast specimens of austenitic stainless steels.

This paper describes the results of an investigation into a microstructure formation on a wire-feed electron beam additive manufactured “steel–copper” bimetallic sample. The peculiarities of a gradient zone structure with a smooth change of components’ concentration are revealed. The heterogeneity of copper and steel distribution in the gradient zone is provided by copper solidification and precipitation mechanisms. Both solidification of coarse copper inclusions in the interdendrite areas or along the dendrite boundaries and precipitation of fine Cu-based particles at the cooling stage from the solid solution of Cu in γ-Fe are the main factors of structure formation during the double wire gradient zone deposition. The presence of such fine copper precipitates from the supersaturated solid solution was revealed by means of transmission electron microscopy. The shape of copper particles in the gradient zone varies from spherical to oblong and irregular. The shape of steel particles and/or grains is mainly determined by the peculiarities of the crystallization zone and is characterized by the primary crystallization of γ-iron dendrites from the liquid melt. A physical scheme describing a variation in phase composition and microstructure in gradient zone of the bimetallic specimen was proposed.

The authors studied the effect of duration of age hardening at the temperature of 700 °C on the microstructure, phase composition and microhardness of high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N (wt. %) steel. The study showed that age hardening at the temperature of 700 °C for half an hour causes the complex of phase transformations: the decomposition of δ-ferrite (with the formation of σ-phase and austenite) and the formation of cells of discontinuous decomposition on the austenitic grains boundaries (the formation of particles based on the chromium nitride Cr 2 N and the depletion of austenite by interstitials). After age hardening for more than 10 hours, besides the discontinuous decomposition of austenitic grains, a homogeneous (continuous) precipitation of chromium nitride occurs in those austenitic grains, which have not undergone discontinuous decomposition in the initial stages of aging. With an increase in the aging duration up to 50 hours, the authors observed the growth of decomposition cells in austenitic grains and the formation of mixed structure. Such structure consisted of austenite grains, which underwent discontinuous decomposition with the formation of lamellar precipitations of chromium nitride in austenite; austenitic grains with the dispersed particles formed by the mechanism of continuous decomposition; and the grains with σ-phase, chromium nitrides, and austenite formed as a result of the high-temperature ferrite decomposition during aging. The aging caused the increase in the microhardness, which value depends on the mechanism of precipitation hardening - continuous or discontinuous decomposition in austenite or the precipitation of intermetallic σ-phase and chromium nitrides plates in the grains of high-temperature ferrite.

In the present paper, the microstructure of heterogeneous material bimetal compound fabricated by wire-feed electron-beam additive technology from CrNiTi stainless steel and С11000 copper has been investigated. The bimetallic compound is characterized by the well-defined interface between the two materials and possesses two-phase transition areas on both sides of the interface. The heterogeneity of strength properties (microhardness) in the transition zones is associated with a solid solution hardening of the bimetal basis components and formation of composite structures in the transition zone of the bimetal: spherical inclusions of steel in the copper part and copper inclusions in the steel section. In the copper part of the bimetal sample, a heterogeneous grain structure is formed – areas with macroscale non-equiaxed grain structure and zones with spherical grains were observed. The heterogeneity of grain structure does not have significant influence on the yield strength, but affects the macroscopic deformation pattern of the bimetal copper part, as has been revealed by microstructural analysis of slip traces and grain structure calculated using the Hall–Petch relationship.

The paper studies the effect of electrolytic hydrogen charging on the plastic flow, strength properties, ductility, and fracture mechanisms in austenitic stainless steels Cr17Ni13Mo3C0.01, Cr18Ni10TiC0.12, and Cr18Ni9C0.17 with different stacking fault energies. The investigated steels are subjected to warm ABC-pressing and thermomechanical processing, including cold rolling and annealing, to produce the ultrafine-grained structure of different morphologies, such as ultrafine-grained (submicrocrystalline), misoriented grain-subgrain and mixed (grain and subgrain) structures of submicron scale. The strength properties of the steels after warm pressing and rolling with annealing exceed 3.5–6.0 times the properties of the quenched steels with coarse-grained structure. Electrolytic hydrogen charging of the studied steels with submicron-sized structure reduces the yield strength irrespective of the grain/subgrain size, structure, steel composition, and its stacking fault energy. The formation of a highly defective grain-subgrain structure with high dislocation density suppresses the effect of hydrogen embrittlement in Cr17Ni13Mo3C0.01 and Cr18Ni10TiC0.12 steels, in which no or a small volume fraction of strain-induced α′ martensite forms in tension. The tempering of the highly defective structure and the formation of a large fraction of high-angle misorientations in the stable Cr17Ni13Mo3C0.01 steel enhances the effect of hydrogen embrittlement in the specimens as compared to the specimens with a grain-subgrain structure with a high density of dislocations and low-angle boundaries. The hydrogen embrittlement effects are most pronounced in ultrafine-grained (submicrocrystalline) Cr18Ni10TiC0.12 and Cr18Ni9C0.17 steels with predominantly grain structure, which undergo induced γ→α′ phase transformation.

A study on elemental composition, phase composition, microstructure, nanohardness and fracture micromechanisms of the surface layers obtained by ion-plasma treatment of a stable CrNiMo austenitic stainless steel with different initial microstructures has been carried out. In cold-rolled specimens, containing a high dislocation density and deformation-induced low- and high-angle boundaries and possessing structural elements of 330 nm in size, interstitial concentration (carbon and nitrogen) is much higher than that for cold-rolled and annealed specimens with a similar size of structural elements. After ion-plasma treatment, a surface nanohardness of the pre-deformed specimens is higher by 8 GPa than one for specimens annealed after cold rolling. Deformation-assisted microstructure accumulates carbon more likely than nitrogen under ion-plasma treatment. These results provide experimental support for decisive role of high dislocation density and deformation-assisted well-developed microstructure in accumulation and bulk diffusion of interstitials under ion-plasma treatment of steel.

A comparative study of the mechanical properties, fracture mechanisms and hydrogen embrittlement peculiarities was carried out using the specimens of austenitic CrNi steel produced by two different methods: wire-feed electron beam additive manufacturing and conventional casting followed by solid-solution treatment. Hydrogen-induced reduction of ductility and the increase in the yield strength are observed in steel specimens produced by both methods. Despite hydrogen embrittlement index is comparable in them, the increase in the yield strength after hydrogen-charging is different: 25 MPa for cast steel and 175 MPa for additively manufactured steel. This difference is associated with the peculiarities of phase composition and phase distribution in steels produced by different methods.

A temperature dependence of the tensile mechanical properties, microstructure and fracture mechanism of high-nitrogen Fe-(19-23)Cr-(17-21)Mn-(0-3)V-(0.1-0.3)C-(0.5-0.9)N vanadium-free and vanadium-containing steels was investigated. For all steels, the 0.2% offset yield strength and strain-hardening drastically increase with a decrease in test temperature. This is associated with high interstitial solid solution strengthening of the steels and more pronounced twinning and stacking-fault formation during straining below room temperature. For the vanadium-free steel, a ductile-to-brittle transition was evaluated: at 77K specimens destroy by cleavage mechanism while at room temperature steels show ductile fracture. Vanadium-alloying provides a particle strengthening of the steels and, at the same time, reduce solid-solution strengthening. Increase of vanadium concentration fully or partially suppress brittle fracture of the steels at 77K. Particle strengthening changes interstitial solid-solution effect, dislocation arrangement and slip/twinning relation in vanadium-containing high-nitrogen steels compared to vanadium-free one.

We investigated the effect of electrolytic hydrogen-charging on regularities of plastic flow, strength and fracture mechanisms of AISI 316L and 321 austenitic stainless steels. In the steels, an ultrafine-grained structure of various morphologies was formed using methods of warm abc-pressing and thermomechanical treatment (cold rolling and annealing). Hydrogen-charging of ultrafine-grained steels reduces their yield strength and elongation. The high dislocation density and low-angle boundaries inhibit the effects of hydrogen embrittlement in 316L and 321 steels.

The structure and properties of the 17Cr13Ni3Mo0.01C stable austenitic steel subjected to high-temperature plastic deformation by the abc-pressing (multiaxial forging) are investigated in the temperature range from 800 to 600°С. The results of investigations demonstrate that after the abc-pressing the steel has a single-phase austenitic ultrafine-grained structure with the size of its elements (grains and subgrains) of (200 ± 140) nm. The formation of the ultrafine-grained state increases the strength properties (0.2 proof stress increases threefold) and decreases the elongation value of the steel at room temperature compared to the coarse-grained specimens. An analysis of the contributions from hardening during abc-pressing evidences in favor of the strength improvement being primarily due to the grain refinement; it is well described by the Hall–Petch relationship. In this case steel with ultrafine-grained austenitic structure exhibits an effect of structural superplasticity at the temperatures T > 750°С: the elongation value at Т = 800°С is found to be 180%.

Phase composition, microstructure, tensile properties and fracture mechanism of the high-nickel austenitic steel produced by a dual wire-feed electron beam additive manufacturing were studied. The joint using of NiCr-alloy and 321-type stainless steel wires in additive manufacturing process assists the formation a fully austenitic structure in as-built billet. Its microstructure is stable against deformation-induced γ → α’ martensitic transformation due to the 5–7 wt.% increase of nickel concentration relative to the initial steel wire composition. The high-nickel steel possesses high ductility (δ ≈ 54%) and plastic flow behavior similar to the conventional cast 321-type steel subjected to a solid-solution treatment.

The effects of electrochemical hydrogen charging on the stress relaxation and strain-aging of 〈111〉- and 〈001〉-oriented single crystals of Hadfield steel were studied under tension at room temperature. An orientation dependence of stress relaxation and strain-aging-assisted yield-drop phenomena was observed for hydrogen-free 〈111〉- and 〈001〉- oriented single crystals. Hydrogen charging up to five hours increased the stress relaxation rate for slip-associated deformation in 〈001〉-oriented single crystals and decreased it for twinning-assisted deformation in 〈111〉-oriented specimens. The present results demonstrate that different mechanisms dominate the interaction of hydrogen with dislocations, stacking faults and twins.

The phase composition, microstructure and microhardness of the vanadium-alloyed high-nitrogen steel produced by electron beam additive manufacturing have been studied and compared with conventionally cast and solution-treated counterpart. It has been shown that phase composition of the high-nitrogen steel is dependent on the method of fabrication. The homogeneous coarse-grained austenitic structure with high concentration of interstitial atoms (N) in solid solution and coarse vanadium-based carbonitrides have been formed in conventionally produced specimens, but additive manufacturing provides the formation of heterogeneous dendritic microstructure of high-nitrogen austenite with numerous precipitates. Additively manufactured specimen has columnar coarse austenitic grains elongated in building direction, and vanadium and chromium carbonitrides are located preferably along grain boundaries. The microhardness of the additively fabricated steel (5.5 GPa) is close to the value for cast steel (5.3 GPa).

In the present study, the effect of nitrogen alloying on hydrogen embrittlement in FeMnNiCoCr high-entropy alloys was investigated. In tension, hydrogen-free nitrogen-alloyed FeMnNiCoCrN alloy (0.37 wt.% N) demonstrated higher strength, strain hardening, and elongation-to-failure than the interstitial-free FeMnNiCoCr Cantor alloy. Despite the different tensile properties, both alloys fractured via a ductile dimple micromechanism. After hydrogen charging, the nitrogen-alloyed material demonstrated lower strain hardening and higher sensitivity to hydrogen-assisted embrittlement than the interstitial-free alloy. Both alloys featured a stable austenitic structure and similar grain size, yet, the nitrogen-alloyed FeMnNiCoCr alloy was more susceptible to hydrogen embrittlement. Although, the overall degradation effects appear similar, there are pronounced differences in mechanical behavior and hydrogen transport upon hydrogen charging when the high-entropy alloys are compared to conventional austenitic stainless steels, and the experiments reveales that nitrogen alloying enhances hydrogen diffusivity in the Cantor alloy.

The gradient material “stainless steel/NiCr alloy” has been designed using the additive manufacturing method – double wire-feed electron-beam melting. The effect of gradual addition of Ni-based alloy on phase transformation and microstructure in additively-fabricated “stainless steel/NiCr alloy” joint has been revealed. Despite the abrupt change in wire composition during the fabrication of the joint, the wide transition zone is formed between two parts of the additively-manufactured billet: γ-austenitic matrix and dendritic δ-ferrite in steel part gradually passing into Fe-based austenite, then into Ni-based austenite, and, finally, into γ-(Ni,Cr) austenite with cellular-shape dendrites in NiCr part. The steel-based transition zone of the as-built joint possesses pure austenitic structure stabilized by nickel in comparison with two-phase dendritic microstructure typical for additively-manufactured CrNi stainless steel. The increased microhardness (2.3–2.4 GPa) corresponds to stable austenitic steel structure of the joint.

Using different solid solution treatments, two structural states were produced in high-interstitial austenitic Fe-19Cr-22Mn-1.6V-0.4C-0.8N and Fe-22Cr-26Mn-1.3V-0.7C-1.2N (all in wt %) steels. Room temperature tensile properties of both steels are determined by a combination of several factors: solid-solution hardening of austenite with carbon and nitrogen, size and distribution of precipitates and grain size. Increase in solid-solution temperature from 1100 to 1200°C increases solid-solution hardening, decreases the particle strengthening effect in the steels and causes grain growth. The relation of these factors governs the value of a yield strength, ultimate tensile strength and elongation in the steel’s specimens.

The structure of EK-181 (RUSFER-EK-181, low-activation) and ChS-139 12% Cr ferritic–martensitic steels is investigated and their mechanical properties are tested after long-term (13500 h) aging at 450 and 620°C. The microstructure of the steels exhibits a high thermal stability, which provides the retention of their initial short-term mechanical properties at test temperatures.

The effect of solid solution temperature (1100 and 1200°C) on microstructure, phase composition and temperature dependence of 0.2% offset yield strength R0.2, in the temperature range of 77 to 673 K was studied for high-nitrogen austenitic Fe–19Cr–21Mn–1.5V–0.3C–0.9N (wt. %) steel. Increase in solid solution temperature provides a partial dissolution of (V, Cr)(N, C) particles and increases a concentration of carbon and nitrogen in austenite. A complex effect of solid solution hardening and particle strengthening provides strong temperature dependence of the yield strength in the steel independently on solid solution temperature. The mechanisms, responsible for the variation in the yield strength with test temperature and solution treatment, are described on the basis of the microstructural parameters of the steel.

The influence of hydrogen-charging on microstructure and microhardness of a high-nitrogen austenitic Fe– 19Cr–21Mn–1.5V–0.3C–0.8N (wt. %) steel processed by high-pressure torsion (HPT) was investigated. X-rays diffraction data indicate that hydrogenation of HPT-deformed specimens leads to an additional broadening of X-rays lines and their shifting, which testify to the formation of the stacking-faults and high residual stresses. Hydrogen-charging of HPT-processed steel promotes to γ–e martensitic transformation. Due to hydrogen-induced phase hardening, the microhardness of HPT-deformed specimens increases up to 6.8 GPa in hydrogen-charged condition (6.5 GPa in hydrogen-free one). After following exposure of hydrogen-charged specimens for 170 h at air condition, the release of hydrogen atoms occurs, and the X-rays lines shift back to positions corresponded to hydrogen-free HPT-processed specimens.

Effect of age hardening regime (at the temperatures of 700 and 800°C for 10 min to 10 h) on phase composition and microhardness of high-nitrogen steels was investigated. Before age hardening treatments, specimens of V-free and V-alloyed steels, Fe–23Cr–17Mn–0.1C–0.6N (0V-HNS) and Fe–19Cr–22Mn–1.5V–0.3C–0.9N (1.5V-HNS), were water-quenched after 1200°C (for 0.5 h) to produce supersaturated solid solution of interstitial atoms (nitrogen and carbon) in austenite (7% ferrite in 0V-HNS, no ferrite and

The effect of heat-treatment (HT) modes on the structural-phase state of heat-resistant ferritic-martensitic steel EK-191 is investigated. The use of intermediate (between quenching and tempering) annealing at reduced temperatures is shown to result in a higher dispersion of nanoparticles of vanadium carbonitride than that achieved by traditional heat treatment. The HT modes ensuring high density of nanoparticles and a simultaneous reduction of the martensite tempering intensity are found out.

The paper studies the influence of hydrogenation duration on structural and phase transformations, deformation mechanisms and mechanical properties of metastable austenitic steel Fe-18Cr-9Ni-0.5Ti-0.08C (in wt %) processed under cold rolling. Plastic deformation under rolling produces a two-phase (γ + α′) grain/subgrain structure in the steel. A yield stress and an ultimate tensile strength are reduced, but the elongation, on the contrary, is increased for hydrogenated and cold-rolled specimens in comparison with values for samples rolled without preliminary hydrogenation. Alloying with hydrogen prior to rolling increases the volume fraction of α’-phase and contributes to the appearance of e-martensite in steel structure. This effect is enhanced with the increase in hydrogen saturation duration.

The role of mechanical twinning on the fragmentation and the contribution to strengthening was studied using single crystals of Hadfield steel after high-pressure torsion and rolling at room temperature. Multiple twinning was found to be the basic deformation mechanism responsible for the fast generation of an ultrafine-grained microstructure with twin boundaries in Hadfield steel single crystals after severe cold plastic deformation. As a result, the hardness of the Hadfield steel increased noticeably.

The effects of hydrogen on the deformation mechanism of 〈0 0 1〉-oriented single crystals of Hadfield steel were studied under tension at temperatures of 77 and 300 K. An increase in intensity of twinning was observed for samples charged with hydrogen, and it is assumed that this is the reason for the increase in elongation to fracture. Despite the high elongation to failure, the fracture surfaces displayed a brittle component.

The role of aluminum alloying on strength properties and deformation mechanisms (slip, twinning) of 〈123〉 single crystals of Hadfield steel under tensile loading at T = 300 K is demonstrated. It is found out that aluminum alloying suppresses twinning deformation in the 〈123〉 single crystals and, during slip, results in a dislocation structure change from a uniform dislocation distribution to a planar dislocation structure.

The low stacking fault energy and high carbon content in Hadfield steel make twinning the basic deformation mechanism from the onset of plastic deformation in [1¯11] and [011] oriented single crystals in tension at T = 77 – 300 K. Alloying with aluminium (2.7 Al in wt.%) results in an increase of stacking fault energy from 0.03 J · m2 to 0.05 J · m−2 and moves twinning to higher degrees of deformation (∊pl > 15 %). In aluminium-free [1¯23] crystals twinning starts after 20 % strain. For [1¯23], [001] orientations, aluminium additions change the dislocation arrangement from a uniform distribution to a planar dislocation arrangment and also suppress twinning. Intersections of dislocation pile-ups were found to be the governing factor for hardening in the aluminium-alloyed [001] crystals.

The effect of hydrogenation on the strain-hardening behavior and the deformation mechanisms of 〈113〉-oriented single crystals of Hadfield steel was investigated under tension at room temperature. The stages of plastic flow and deformation mechanisms for hydrogen-charged specimens are similar to one in hydrogen-free state: slip → slip + single twinning → slip + multiple twinning. Hydrogen alloying favors to mechanical twinning, micro- and macrolocalization of plastic flow.

An influence of hydrogenation on a phase composition, a microstructure and mechanical properties of Cr Ni austenitic stainless steel under a plain rolling at room temperature was investigated. Hydrogen alloying provides an increase in volume fraction of a strain-induced α′-phase under rolling and, at the same time, allows to save an ultrafine-grained character of the steel structure.

The results of structural investigations and mechanical tests of low-activation 12%-chromium ferritic-martensitic steel EK-181 after long-term (13500 h) aging at 450°C and 620°C are presented. It is shown that the high thermal stability of steel microstructure ensures that its original short-term mechanical properties are maintained at T ≤ 620°C.