Elena Astafurova

The influence of orientation and aluminium content on the deformation mechanisms of Hadfield steel single crystals

Abstract 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.

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 77u2005K 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.

Grain refinement and mechanical properties of low-carbon steel by means of equal channel angular pressing and annealing

Abstract Equal channel angular pressing of low-carbon steel produces an ultrafine-grained microstructure with an average (sub-)grain size of 325 nm and both coarse and fine carbides, a microhardness of 330 HV and 0.2 % offset yield strength of σ0.2 = 815 MPa. The average (sub-)grain size and the mechanical properties of the steel after equal channel angular pressing were found to be stable up to 500 °C. The composition of the carbides, their sizes and their distribution in the microstructure were analyzed in order to understand their contribution to the mechanical properties and the thermal stability of the steel. Both the high strength and the thermal stability of the steel after equal channel angular pressing are associated with the grain refinement, the substructure strengthening, and precipitation hardening.

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 Grain Refinement on the Elemental Composition and Nanohardness of the Surface Layers in AISI 316L Austenitic Steel Subjected to Ion-Plasma Hardening

We study the effect of the grain refinement on the elemental composition and nanohardness of the surface layers in AISI 316L austenitic steel processed by ion-plasma hardening. Ion-plasma hardening of the samples with (1) grain-subgrain (with high dislocation density) and (2) coarse-grained structures causes a surface hardening and formation of the composite layers with a thickness of about 20 μm. The nanohardness and depth profiles of elemental concentration of nitrogen, carbon and oxygen in the ion-plasma hardened layers depends on pretreatment regime of the steel specimens. Cold rolling causes an increase in the grain and subgrain boundaries fraction and dislocation density in steel specimens, provides more intensive accumulation of interstitial atoms in thin surface 5 μm-layer, leads to additional surface hardening and suppress carbon diffusion into depth of the specimens as compared with coarse-grained structure.

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 = 540–570u2005MPa, σUTS = 900–950u2005MPa, EL = 36–37%, PSE=33–35u2005GPa %) in comparison with cold-rolled specimens possessing high strength properties, but extremely low elongation (σYS = 1200u2005MPa, σUTS = 1650u2005MPa, EL=1%, PSE=1.7u2005GPa %).

Influence of ion nitriding regime on mechanical properties and fracture mechanism of austenitic steel subjected to different thermomechanical treatments

The effect of thermomechanical treatments and low-temperature ion nitriding on mechanical properties and a fracture mechanism of stable austenitic stainless steel Fe–17Cr–13Ni–1.7Mn–2.7Mo–0.5Si–0.01C (in wt %, 316L-type) was investigated. Irrespective of initial heat treatments of steel and the regime of nitrogen saturation, traditional ion nitriding and nitriding with hollow cathode effect do not influence the stages of plastic flow and strain hardening; instead, they contribute to surface hardening of steel samples and reduce their plastic properties due to formation of a brittle surface layer. Ion nitriding leads to formation of a hardened surface layer with the microhardness of 12u2005GPa. Formation of a high-defective grain/subgrain structure with high dislocation density contributes to strengthening of steel samples under ion nitriding and formation of a thicker strengthened layer in comparison with fine-crystalline and coarse-crystalline samples.

Microstructural features and microhardness of Fe-Mo-Nb-V-C low-carbon steel processed by high-pressure torsion: The significance of the initial structural state

The effect of the initial heat treatment (quenching or tempering) of low-carbon steel (Fe-Mo-Nb-V-C) on special features of the ultrafine-grained structure and microhardness produced by high-pressure torsion was investigated. High-pressure torsion promotes the more apparent refinement of structural elements of the steel (dpr = 55u2005nm for the quenched state and 74u2005nm for the tempered state) and an increase in structural homogeneity of microhardness of quenched specimens in comparison with tempered ones. Experimental results reveal a high significance of the initial structural state for the final deformation-processed microstructure and microhardness (radial distribution) of steel specimens.