Valentina Moskvina

Structure, phase composition and mechanical properties of austenitic steel Fe–18Cr–9Ni–0.5Ti–0.08C subjected to chemical-deformation processing

The effect of rolling combined with hydrogen charging on the structural and phase transformations and mechanical properties of metastable austenitic stainless steel Fe–18Cr–9Ni–0.5Ti–0.08C (in wt %) was investigated. Deformation of steel is accompanied by the refinement of the structure due to the accumulation of deformation defects and strain-induced γ–α′ transformation. Hydrogenation promotes the formation of e-martensite and increases the volume fraction of α′-phase in steel structure under rolling, as compared to the state after rolling without hydrogenation. Mechanical properties of austenitic steel increase under rolling as compared to the initial state, but preliminary hydrogen charging has no significant effect on their magnitudes. Hydrogen alloying before rolling increases specimen elongation compared to rolling without hydrogenation.

Effect of Hydrogen Charging on Mechanical Twinning, Strain Hardening, and Fracture of ‹111› and ‹144› Hadfield Steel Single Crystals

This paper studies the effect of electrolytic hydrogen charging on the plastic deformation and fracture of Hadfield steel single crystals oriented for tension along the ‹111› and ‹144› directions, which the major deformation mechanism is mechanical twinning. Electrolytic hydrogen charging for five hours at a current density of 100 A/m2 slightly affects the stages of plastic flow, deformation mechanism, and the value of uniform elongation of ‹111› and ‹144› single clreystals. Hydrogen saturation causes shear microlocalization and a decrease of the strain hardening coefficient in twinning in one system, but slightly affects the strain hardening characteristics in multiple twinning. Hydrogen charging increases the fraction of the brittle component on fracture surfaces and leads to microand macrocracking near the fracture zone on the lateral surface of deformed specimens. It has been found experimentally that the stress relaxation rate in loaded ‹111› single clreystals after hydrogen saturation decreases. Mechanisms of describing this phenomenon have been proposed.

Influence of hydrogenation regime on structure, phase composition and mechanical properties of Fe18Cr9Ni0.5Ti0.08C steel in cold rolling

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.

Effect of rolling on phase composition and microhardness of austenitic steels with different stacking-fault energies

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 α′-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 α′-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.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 α′-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 α′-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.

Influence of thermomechanical treatments on mechanical properties and fracture mechanism of high-nitrogen austenitic steel

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 cold-rolled specimens possessing high strength properties, but extremely low elongation (σYS = 1200 MPa, σUTS = 1650 MPa, EL=1%, PSE=1.7 GPa %).

The effect of hydrogen on strain hardening and fracture mechanism of high-nitrogen austenitic steel

High-nitrogen austenitic steels are perspective materials for an electron-beam welding and for producing of wear-resistant coatings, which can be used for application in aggressive atmospheres. The tensile behavior and fracture mechanism of high-nitrogen austenitic steel Fe-20Cr-22Mn-1.5V-0.2C-0.6N (in wt.%) after electrochemical hydrogen charging for 2, 10 and 40 hours have been investigated. Hydrogenation of steel provides a loss of yield strength, uniform elongation and tensile strength. The degradation of tensile properties becomes stronger with increase in charging duration - it occurs more intensive in specimens hydrogenated for 40 hours as compared to ones charged for 2-10 hours. Fracture analysis reveals a hydrogen-induced formation of brittle surface layers up to 6 μm thick after 40 hours of saturation. Hydrogenation changes fracture mode of steel from mixed intergranular-transgranular to mainly transgranular one.

The effect of severe plastic deformation by high-pressure torsion on structure and phase composition of high-nitrogen austenitic steel

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.

The effect of hydrogenation on strain hardening and deformation mechanisms in 〈113〉 single crystals of Hadfield steel

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.