Galina Maier

The influence of temperature on microstructure and microhardness in high-pressure torsion of low-carbon steel

The ultrafine-grained structures produced by cold (20 ?C) and warm (450 ?C) high- pressure torsion in low-carbon steel were studied using transmission electron microscopy and X-ray analysis. After cold high-pressure torsion, the size of fragments is smaller (100 nm) and structure is more homogeneous in comparison with warm deformation (120 nm). As a result of high-pressure torsion, the microhardness of steel investigated has been increased up to 600 HV and 570 HV after cold and warm deformation respectively.

Influence of rolling temperature on structure, phase composition and mechanical properties of austenitic steel Fe–17Cr–13Ni–3Mo

We investigated the effect of temperature of plain rolling on structural peculiarities, phase composition and mechanical properties of austenitic steel (Fe–17Cr–13Ni–3Mo–0.01C, wt %, 316L-type). Plain rolling of steel Fe–17Cr–13Ni–3Mo provides a fragmentation of initial grain structure, formation of a high density of twin boundaries, slip dislocation and shear bands, and increases steel strength properties. Decrease in the deformation temperature increases the density of twin boundaries and causes an additional hardening effect. The plastic deformation at room temperature does not produce a substantial volume fraction of e-martensite and does not go with the γ–α’-martensitic transformation. Plain rolling of specimens with interpass cooling to 77 K is accompanied by γ–e, γ–α’-phase transformations, but their volume fraction is small (<5% each). The lower rolling temperature provides higher strength properties in steel.

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.

Thermal stability of nanostructured Hadfield steel produced by high-pressure torsion

The influence of annealing (400°C–800°C, 1 h) on the phase composition, microstructure and microhardness of Hadfield steel single crystals processed by high-pressure torsion at room temperature has been investigated. After the high-pressure torsion, the high microhardness values of Hadfield steel (690–750 HV) remain under annealing up to the temperature of 500°C. The mechanisms responsible for the high thermal stability are the formation of ultrafine α′-phase and retaining of a high-strength state of the austenite associated with stability of deformation-induced twin boundaries.

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 %).

Evolution of grain–subgrain structure and carbide subsystem upon annealing of a low-carbon low-alloy steel subjected to high-pressure torsion

The effect of annealing on the evolution of an ultrafine-grain structure and carbides in a 06MBF steel (Fe–0.1Mo–0.6Mn–0.8Cr–0.2Ni–0.3Si–0.2Cu–0.1V–0.03Ti–0.06Nb–0.09C, wt %) has been studied. The grain–subgrain structure (d = 102 ± 55 nm) formed by high-pressure torsion and stabilized by dispersed (MC, M3C, d = 3–4 nm) and relatively coarse carbides (M3C, d = 15–20 nm) is stable up to a temperature of 500°C (1 h) (d = 112 ± 64 nm). Annealing at a temperature of 500°C is accompanied by the formation in regions with a subgrain structure of recrystallized grains, the size of which is close to the size of subgrains formed by high-pressure torsion. The average size and distribution of dispersed particles change weakly. The precipitation hardening and the increase in the fraction of high-angle boundaries in the structure cause an increase in the values of the microhardness to 6.4 ± 0.2 GPa after annealing at 500°C as compared to the deformed state (6.0 ± 0.1 GPa). After 1-h annealing at 600 and 700°C, the microcrystal size (d = 390 ± 270 nm and 1.7 ± 0.7 μm, respectively) increases; the coarse M3C (≈ 50 nm) and dispersed carbides grow by 5 and 8 nm, respectively. The value of the activation energy for grain growth Q = 516 ± 31 kJ/mol upon annealing of the ultrafine-grained steel 06MBF produced by high-pressure torsion exceeds the values determined in the 06MBF steel with a submicrocrystalline structure formed by equal-channel angular pressing and in the nanocrystalline α iron.

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.