Search

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.

The paper studies the structure and microhardness of the heat-affected zone, welding area in low carbon and low alloy steel 09G2S (AISI A516 grade 55), and the coating created by two-pass manual metal arc welding with the addition of titanium carbonitride nanoparticles. It is shown that the microhardness dependence on the distance to the coating surface is divided into four zones. The microstructure of each zone is studied by using the optical and scanning tunneling microscopies. It is found that the formation of highly dispersed acicular ferrite and globular bainite structures in the deposition superheat zone of surfacing is favorable for the improvement of the coating–base metal composition impact toughness.

The possibility of the structural turbulence of plastic flow and ductile fracture in low-carbon and low-alloy steel 09Mn2Si is shown by fracture toughness measurements and uniaxial tension of chevron-notched specimens at 20°C. The conditions for the structural turbulence of plastic deformation of solids are determined which provide high fracture toughness. The decisive functional role is played by the lattice curvature, the appearance of mesoscopic structural states in the lattice interstices at the nanoscale, the activation of self-consistent rotational motion in the hierarchy of mesoscopic structural levels, the presence of free volume, and the possibility of structural transformations at the nanoscale. A nonlinear theory of describing structural turbulence in a deformable solid is discussed.

The paper analyzes the nucleation and growth of bainite in low-carbon low-alloy 09Mn2Si steel doped with titanium carbonitride nanoparticles in impact toughness testing. The analysis shows that such particles segregate at low-angle boundaries, retarding the formation of high-angle ones, and when impacted into the steel, they curve the lattice and generate a new bainite phase at the curvature interstices. The mechanism of bainite nucleation and growth is sympathetic, obeys the angular momentum conservation law, and provides the formation of multilayered packets of bainite plates capable for unlimited thinning to sub-sub-subunits during deformation. Such bainite plates can respond to their stress-strain state by one or another rotation, showing a high relaxation capacity and providing a high impact toughness of the steel at low temperatures.

The paper is devoted to an experimental investigation of a high-temperature deformation in V-alloyed high-nitrogen austenitic Fe-19Cr-22Mn-1.5V-0.3C-0.6N steel processed via different thermo-mechanical treatments. Simple thermo-mechanical processing regimes (cold rolling or rolling with single post-deformation anneal) do not allow to realize a substantial elongation in high-nitrogen steel during high-temperature tensile tests. For fine-grained austenitic structure with an average grain size of 3 µm, the maximal value of elongation to failure of 150% was realized at temperature 950 °C. Using a multi-stage thermo-mechanical treatment included cold rolling and intermediate anneals, a heterophase grain/subgrain structure with high density of deformation-induced defects and precipitates was produced. When heated to a deformation temperature, this deformation-assisted microstructure recrystallizes into a stable fine-grained structure and demonstrates the attributes of superplastic flow (values of elongation to failure higher than 400%) in the temperature range of 850-1000 °C. The maximum elongation of 900% is achieved at temperature of 950 °C and an initial strain rate of 10-4 s-1.

The effect of age-hardening regime on peculiarities of hydrogen-assisted fracture and tensile properties in two high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N and Fe-19Cr-22Mn-1.5V-0.3C-0.9N steels was studied. A large number of intergranular (austenite/austenite) and interphase boundaries (austenite/coarse particle) provides high fraction of trapping sites for hydrogen atoms in V-alloyed steel. This leads to a change in fracture regime from transgranular brittle mode in coarse-grained V-free steel to intergranular brittle fracture of hydrogen-assisted surface layers in fine-grained V-alloyed steel with coarse (V,Cr)(N,C) particles. The formation of cells (Cr 2 (N,C) particles and austenite) along the grain boundaries due to discontinuous precipitate-hardening reaction facilitates predominantly interphase hydrogen-assisted fracture for both steels. The complex reaction of the particle-strengthening mechanisms including discontinuous precipitation with formation of austenite/Cr 2 (N,C)-plates interfaces or homogeneous nucleation of coherent (V,Cr)(N,C) particles in austenite (age-hardening regime 700 °C, 10 h) promotes mainly transgranular cleavage-like fracture mode under hydrogen-charging. The structural scheme is proposed to describe a change in hydrogen-assisted fracture micromechanisms and tensile properties of the steels with different density and distribution of interphase and intergranular boundaries.

Tensile properties, work-hardening, microstructure and fracture mechanisms were studied in two vanadium-containing high-nitrogen austenitic steels, Fe-19Cr-21Mn-1.3V-0.22C-0.81N and Fe-17Cr-10Mn-7Ni-1.0V-0.09C-0.65N, in low-temperature deformation regime. Vanadium-alloying provides several concurrent effects – decreases a solid solution hardening in the steels (softening effect) due to formation of nitrides(carbonitrides), causes precipitate hardening by vanadium-based particles (hardening effect) and suppresses grain growth during solid solution treatment (hardening effect). The yield stresses at room temperature deformation and 77 K-deformation are analyzed taking into account the contributions from these softening/hardening mechanisms. As a result of complex hardening and softening effects, both steels possess high tensile properties, show striking temperature dependences of the yield strength, an ultimate tensile strength and an elongation (the ultimate tensile strength and elongation at 77 K reaches 2600 MPa and 14% for Fe-19Cr-21Mn-1.3V-0.22C-0.81N steel) in low-temperature deformation regime. Striking temperature dependence of the yield strength for particle-containing steels is in accordance with deformation behavior of high-nitrogen particle-free steels with high solid-solution strengthening. In spite of similarities in temperature dependence of strength properties and deformation mechanisms with precipitate-free high-nitrogen steels, V-containing steels do not undergo ductile-to-brittle transition during deformation at cryogenic temperatures and they fracture in ductile manner even during tensile deformation at 77 K. Deformation microstructures at different strain levels and test temperatures are studied in order to reveal the dominating structural parameters responsible for fracture micromechanisms. Despite the activation of mechanical twinning and planar dislocation slip as dominating deformation mechanisms during tension at 77 K, precipitate-hardened high-nitrogen steels show rather high elongation values of 14–15% and ductile fracture with dimple micromechanism. Vanadium-based precipitates of ≈ 300 nm in diameter do not cause strong precipitate hardening, but strongly influence dislocation arrangement and suppress low-temperature brittle fracture in high-nitrogen austenitic steels.

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.

The Institute of Strength Physics and Materials Science (Siberian Department, Russian Academy of Sciences) has developed a process for electron-beam deposition of powdered composite materials on metallurgically-produced parts. A unique electron-beam deposition device, the LUNa-10, for reinforcement of blower nozzles used in blast-furnace casting and for reinforcement of crystallizers in continuous steel casting machines was developed and placed into service at the West Siberian Metallurgical Combine (Novokuznetsk). This paper describes some characteristics of the electron-beam deposition process for composite coatings, and the properties of such coatings as a function of composition and purpose.

Electron-beam deposition (EBD) is an advanced technique intended for the formation of protective coatings that exhibit high strength and wear, and fatigue resistance. The investigation of the structure of coated materials and the development of mesoscale plastic deformation shows that these properties depend heavily on EBD modes. It is proposed that the latter can be optimized by: (i) choosing the electron-beam parameters that ensure a uniform structure of the coating, appropriate width and proper structure of the transient zone, (ii) the trial-and-error method of selecting such deposited powder composition that ensure high strength properties, and (iii) mechanical test aimed at distinguishing the coatings whose performance has been advantageously increased by EBD.