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A comparative analysis of the effect of hydrogen-charging on the mechanical properties and fracture mechanisms of the Nb-free and Nb-alloyed austenitic steels produced by an additive manufacturing was carried out. Asbuilt steels possess different phase compositions: austenite and -ferrite (Nb-free steel) and austenite, □-ferrite and Nb-enriched phases (Nb-alloyed steel). After hydrogen-charging, the yield strength increases, and the plasticity of both steels is reduced. The Nb-alloying increases the resistance of the steel against hydrogen embrittlement. The hydrogen embrittlement index and thickness of the brittle hydrogen-affected layer in Nb-alloyed steel (IH = 10%, DH = 18 ± 5 µm) are lower than those in Nb-free steel (IH = 17%, DH = 35 ± 7 µm). The Nb-assisted changes in the microstructure and phase composition in the additively manufactured austenitic steel leads to a change in the fracture mechanisms of the hydrogen-affected layer. [ABSTRACT FROM AUTHOR]

The effect of high-pressure torsion on the microstructure was studied in high-nitrogen austenitic steels with different vanadium content: Fe-23Cr-19Mn-0.2C-0.5N, Fe-19Cr-21Mn-1.3V-0.3C-0.8N and Fe-18Cr-23Mn-2.6V-0.3C-0.8N, wt %. Independently of the chemical composition of steels, high-pressure torsion causes the refinement of the microstructure of high-nitrogen steels due to the formation of a high density of twin boundaries, dislocations and shear bands. Vanadium-alloying promotes the change of dominating deformation mechanism from planar slip and twinning (vanadium-free steel) to slip with pronounced shear band formation (vanadium-alloyed steels) under high-pressure torsion. Increase in vanadium concentration in steels provides more misoriented microstructure in comparison with V-free steel. [ABSTRACT FROM AUTHOR]

The influence of test temperature in the range from –60 to 60°C and strain rate from 1×10–4 to 1×10−2 s−1 on tensile mechanical properties and peculiarities of a plastic flow has been experimentally investigated using the vanadium-alloyed ultrahigh-interstitial austenitic Fe-22Cr-26Mn-1.3V-0.2Ni-0.7C-1.2N (mass %) steel specimens subjected to a 1-hour solid-solution treatment at temperature of 1200°C. It has been established that the decrease in deformation temperature and the increase in strain rate does not change the dominating deformation mechanism of the steel— dislocation slip—but increase a planarity of a dislocation arrangement and promote mechanical twinning as the additional deformation mechanism. The ultrahigh-interstitial steel demonstrates striking temperature and strain rate dependencies of a yield strength in considered loading conditions. [ABSTRACT FROM AUTHOR]

Austenitic Fe-23Cr-19Mn-0.53N-0.2C steel was processed by high-pressure torsion (HPT) to reveal the influence of high strain values on phase composition, structural parameters and microhardness of a high-nitrogen austenite. Evaluation of X-ray diffraction data indicates that HPT-processing leads to broadening and shifting of X-ray lines for austenitic phase as a result of refinement of the structure, stacking-faults formation and high residual stresses build-up. Increase in HPT strain promotes an increase in value of stacking-faults probability and weakly changes a lattice parameter of austenitic phase. As a result of HPT-processing, the microhardness of the steel increases from 300 HV in initial pre-HPT state to ≈600 HV in HPT-processed one. Experimental results reveal that the homogeneity of distribution of microhardness along HPT-processed specimens (disks) increases with the strain. [ABSTRACT FROM AUTHOR]

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–ε 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. [ABSTRACT FROM AUTHOR]

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 <5% VN precipitates in 1.5V-HNS were formed). V-alloying changes a kinetics of age-hardening in high-nitrogen steels. According to XRD data, aging at 700 and 800°C results in decomposition of austenite and ferrite in 0V-HNS and formation of duplex microstructure, which consist of high-nitrogen austenite, ferrite, Cr2(N, C) precipitates and intermetallic σ-phase. Such duplex microstructure consists of two types of grains: (1) austenitic grains with precipitates mainly along grain boundaries possess lower microhardness characteristics (about 3.5 GPa); and (2) decomposed multiphase grains have high microhardness (4.5–5.5 GPa depending on aging duration). After aging of vanadium-containing steel at 700 and 800°C, rather homogeneous composite structure forms, which consists of high-nitrogen austenite, ferrite and (V, Cr) (N, C) particles randomly distributed both on grain boundaries and in grain bodies. This stricture possesses high microhardness values about 4.0–4.5 GPa. Independently on age hardening regime and steel composition, the most pronounced effects of hardening occur during short-time age hardening regimes (0.5–1 h). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, the mechanical properties and fracture mechanisms of the high-nitrogen austenitic steel Fe- 17Cr-10Mn-7Ni-0.95V-0.8N-0.1C (in wt %) processed by different thermomechanical treatments are investigated. Cold rolling and short-time solid solution hardening contribute to the formation of a rather homogeneous fine-grained structures in the steel, which possess high strength, sufficient plasticity and exhibit excellent product of strength and elongation (σYS = 540-570 MPa, σUTS = 900-950 MPa, EL = 36-37%, PSE = 33-35 GPa %) in comparison with coldrolled specimens possessing high strength properties, but extremely low elongation (σYS = 1200 MPa, σUTS = 1650 MPa, EL = 1%, PSE = 1.7 GPa %). [ABSTRACT FROM AUTHOR]

The influence of multi-pass cold rolling on the phase composition and microhardness of austenitic Fe-18Cr- 9Ni-0.21C, Fe-18Cr-9Ni-0.5Ti-0.08C, Fe-17Cr-13Ni-3Mo-0.01C (in wt %) steels with different stacking fault energies was studied. The metastable Fe-18Cr-9Ni-0.5Ti-0.08C steel undergoes γ → α' phase transformations during rolling, the volume fraction of strain-induced a'-martensite in steel structure is increased with increasing strain. Metastable austenite Fe-18Cr-9Ni-0.21C steel does not undergo the formation of an appreciable amount of strain-induced a'-martensite under rolling, but the magnetophase analysis reveals a small amount of ferrite phase in the structure of steel after rolling. The structure of stable Fe-17Cr-13Ni-3Mo-0.01C steel remains austenitic independently under strain. Investigations of microhardness of the steels show that their values are increased with strain and are dependent on propensity of steels to strain-induced martensitic transformation. [ABSTRACT FROM AUTHOR]

The authors investigate microhardness homogeneity and microstructure of the high-nitrogen austenitic steel Fe-23Cr-19Mn-0.2C-0.5N (wt %) subjected to high-pressure torsion. High-pressure torsion causes a significant increase in the microhardness of steel from 300 HV in the initial state up to 600 HV after torsion for one full revolution. Experimental results reveal a homogeneous distribution of microhardness values across the discs after deformation for N = 1/4 revolution. The control of the top and bottom surfaces of steel specimens shows that microhardness radial distributions are independent of the side of the specimens (position of a moving plunger during plastic deformation). Under high-pressure torsion, the spacings between twin boundaries decrease, while the dislocation density increases. The formation of a twin net in high-nitrogen steel in high-pressure torsion for N = 1/4-1 revolutions provides a homogeneous distribution of microhardness across the discs under torsion. [ABSTRACT FROM AUTHOR]

The effect of grain orientation relative to tensile load on the strain hardening behavior and fracture mechanism of directionally solidified high-nitrogen steel Fe-20Cr-22Mn-1.5V-0.2C-0.6N (in wt %) was studied. The tensile samples oriented along the longitudinal direction of columnar grains demonstrated the improved mechanical properties compared to specimens with the transversal directions of columnar grains: the values of tensile strength and strain-to-fracture were as high as 1080 MPa and 22%, respectively, for tension along the columnar grains and 870 MPa and 11%, respectively, for the tension transversal to the columnar grains. The change in the grain orientation relative to the tensile load varies a fracture mode of the steel. The fraction of the transgranular fracture was higher in the samples with longitudinal directions of the columnar grains compared to the transversal ones. [ABSTRACT FROM AUTHOR]

We study the effect of high-pressure torsion (6 GPa) for 0 (upset), 1/4, 1/2, and 1 revolutions at room temperature on the microstructure and microhardness of high-nitrogen austenitic steel Fe-18Cr-23Mn-2.7V-0.2C-0.7N (wt %). Slip, twinning, formation of localized microbands, and precipitation hardening are the main deformation mechanisms of steel under HPT. The level of solid solution hardening of steel after deformation remains as high as after quenching. As the result of severe plastic deformation, steel microhardness increases by 1.5 times. Mechanical twinning facilitates strain hardening due to high density of high-angle twin boundaries, prevents the formation of misoriented grain/subgrain structure with common type boundaries and contributes to the homogeneity of the structure and microhardness across the specimens. [ABSTRACT FROM AUTHOR]