M. V. Kostina

Special Features of Steels Alloyed with Nitrogen

The role of nitrogen as an alloying element that stabilizes austenite in steels and makes it possible to replace nickel, manganese, and other austenization promoters without deterioration of mechanical and special properties of the metal is considered. Put into practice this could reduce the volume of mining of the mentioned elements. Methods for introducing nitrogen into iron alloys are described. The mechanical properties of nitrogen-bearing steels are considered. It is shown that such steels with a structure of nitrogen martensite possess a specific strength under static and cyclic loads no worse than the strength of light structural alloys and have better fracture toughness and technological properties than the latter. The replacement of light alloys by such steels should reduce the consumption of energy in the production.

Influence of rare-earth metals on the high-temperature strength of Ni3Al-based alloys

The influence of the content of reaction- and surface-active alloying elements (rare-earth metals (REMs)) and the method of their introduction into cast high-temperature γ′-Ni3Al-based intermetallic alloys, which are thermally stable natural eutectic composites, on their structure-phase state and the mechanical properties is studied. The life of low-alloy heterophase γ′ + γ cast high-temperature light Ni3Al-based alloys is shown can be increased at temperatures exceeding 0.8Tm (Tm is the melting temperature of Ni3Al) due to additional stabilization of the single-crystal structure of these alloys with submicron and nanometer-sized particles of the phases formed by refractory and active REMs. It is also shown that stage-by-stage fractional introduction of all components into alloys during vacuum induction melting with allowance for their reaction activities (most refractory metals are introduced in the form of low-melting-point master alloys at the first stage of vacuum induction melting, and lanthanum is introduced with a master alloy in the optimal contents of 0.1–2 wt % into the charge of VKNA-1V and VKNA-25 alloys at the final stage) leads to the formation of a modified structure stabilized by nanoprecipitates of nickel and aluminum lanthanides and the phases formed by refractory metals. This method increases the life of VKNV-1V-type alloys (0.5 wt % Re) at 1000–1200°C by a factor of ∼1.7 and that of VKNA-25-type alloys (1.2 wt % Re and Co) by a factor of ∼3.

Effect of hot-rolling and heat-treatment conditions on the structure and mechanical and technological properties of nitrogen-bearing austenitic steel 05Kh22AG15N8M2F-Sh

The effect of hot-rolling conditions on the structure, strength, ductility, fracture toughness, and technological properties of the nonmagnetic steel 05Kh22AG15N8M2F-Sh containing 0.55% N has been studied. A homogeneous and fine-grained austenitic structure forms in the steel upon rolling at 1000–1050°C and a reduction of more than 60–70%. This structure provides the following properties: σ0.2 = 1044 N/mm2, σu = 1172 N/mm2, δ = 32%, ψ = 64%, and KCV = 1.06 MJ/m2 at −70°C. The possibility of recrystallization of the hot-rolled steel (deformed at 10–90% reductions) is checked upon its subsequent heating to 850–1200°C followed by water quenching. The steel is shown to have high strength, ductility, and fracture toughness and to retain an austenitic structure without cracks or exfoliation upon hot plastic deformation by rolling up to a 90% reduction.

Effect of heat treatment on the structure of high-nitrogen austenitic corrosion-resistant 04Kh22AG17N8M2F and 07Kh20AG9N8MF steels

The structures of high-strength austenitic 04Kh22AG17N8M2F and 07Kh20AG9N8MF steels are studied after various heat-treatment conditions, and the relation between these structures and the mechanical and chemical properties of these steels is analyzed. The phase compositions of the steels, the morphology of phases, the fine structure of austenite, and the mechanism of its decomposition upon heating are investigated by X-ray diffraction and transmission electron microscopy. The homogeneous decomposition of the supersaturated γ solid solution in the 04Kh22AG17N8M2F steel at 500 °C is shown to be accompanied by the formation of the CrN nitride, which is isomorphic to the matrix, and to increase the strength and elastic stresses in the austenite, decreasing the stress corrosion cracking (SCC) resistance of the steel. Heating at 800 °C facilitates stress relaxation and increases the SCC resistance of the steel.

Corrosion properties of austenitic Cr-Mn-Ni-N steels with various manganese concentrations

The structure and corrosion properties of two high-nitrogen 05Kh20AN8MF steels additionally alloyed with 9 and 17% Mn have been studied. Metallographic, X-ray diffraction, and fractographic studies show that both steels have an austenitic structure and high plasticity properties after quenching from 1100 and 1100°C and subsequent aging at 500°C for 2 h. The steel alloyed with 9% Mn and 0.58% V exhibit a higher strength. Both steels have a higher corrosion resistance in a 3.5% NaCl aqueous solution than 12Kh18N9T steel. After aging at 400–600°C, the corrosion rate and the sensitivity to stress corrosion cracking increase.

Phase transformations in a corrosion-resistant high-chromium nitrogen-bearing steel

The structure, phase composition, and mechanical properties of an austenitic corrosion-resistant high-chromium nitrogen-bearing (∼0.5% N) steel are studied in the as-cast state and after homogenizing heat treatment (HT) followed by quenching. The main structural constituents of the as-cast steel are austenite and the σ phase (12%), which forms as an interdendritic metal during solidification, and δ ferrite and M23C6-type chromium carbides are absent. Homogenizing HT at 1100–1200°C leads to the σ → γ transformation through the stage of the formation of intermediate δ ferrite via the restructuring of the tetragonal into the bcc lattice. Upon long-term homogenizing HT, the chromium concentration in ferrite decreases due to diffusion chromium redistribution and the δ → γ transformation takes place. The austenite in both the as-cast steel and the steel subjected to homogenizing HT followed by water quenching contains numerous (Cr, V)N nanoparticles. The twofold yield strength of this steel (∼400 MPa) as compared to nitrogen-free 18Cr-10Ni-type steels can be explained by not only the solid-solution hardening of austenite by nitrogen but also by precipitation hardening.

Effect of heat treatment on the structure and properties of a high-nitrogen austenitic corrosion-resistant 03Kh20AG11N7M2 steel

The structure and mechanical and corrosion properties of a high-strength austenitic 03Kh20AG11N7M2 steel after quenching and aging at 500 and 800°C are analyzed. The phase composition of the steel and the mechanism of the decomposition of austenite during heat treatment are studied by electron-probe microanalysis and transmission electron microscopy. This steel is thermally stable upon heating to 800°C for 1 h and is stable to the γ → α and γ → ɛ martensitic transformation during deformation up to tensile strains leading to fracture. The homogeneous decomposition of a supersaturated γ solid solution at 500°C leads to the formation of disperse CrN nitrides, which increase the strength of the steel and insignificantly decrease its plasticity. In this case, the stress corrosion cracking resistance slightly decreases and the passivation of the steel increases in an corrosive medium without loading.

Machinability of the high-strength corrosion-resistant high-ductility austenitic steel 06Kh22AG15N8M2F

The machinability of the high-nitrogen corrosion-resistant austenitic steel 06Kh22AG15N8M2F during turning is studied. The specific features of the structure of the surface layers in steel workpieces after turning are revealed. The cutting conditions that provide the lowest wear of VK8 alloy cutting tools upon turning are found: the cutting speed is 21–74 m/min, the feed is 0.15–0.60 mm/rev, and the cutting depth is 0.15–0.75 mm. The presence of a large amount of Cr2N-type chromium nitrides in the structure of the steel annealed at 800°C for 2 h and a high nitrogen content in the austenite of the steel quenched from 1100°C increase the wear of the cutting tools. As to turning of the forged steel, the wear resistance of the cutting tools upon turning of the 06Kh22AG15N8M2F steel is higher than that upon turning of 08Kh18N10T steel, in which deformation martensite forms (in surface layers) during turning.

Pitting corrosion resistance of chromium-nitrogen steels with an overequilibrium nitrogen content

The effect of alloying elements, such as chromium and nitrogen (at their overequilibrium contents), and heat-treatment conditions (quenching from 1200°C, tempering at 400 and 600°C for 2 h) on the pitting corrosion resistance of chromium-nitrogen steels containing 15–21% Cr and 0.4–1.22% N in chloride solutions is studied. A relation is established between the structural and phase states of the chromium-nitrogen steels and the character of anode polarization curves and the potentials of pitting formation and repassivation. It is found that the pitting corrosion resistance of the chromium-nitrogen steels tempered at a temperature of 600°C decreases significantly because of the formation of chromium nitrides.

Effect of the heat treatment and plastic deformation of high-strength corrosion-resistant nitrogen-bearing steels on their wear resistance during sliding friction

Wear-resistance sliding-friction tests of high-strength corrosion-resistant nitrogen-bearing 08Kh14AN4MDB and 05Kh22AG15N8M2F steels have been performed for different structural states under sliding-friction conditions. The wear resistance of these steels depends on the intensity of the strain hardening of austenite. The martensitic-austenitic 08Kh14AN4MDB steel has a higher wear resistance if it has a high nitrogen-austenite concentration. The dependence of the weight loss of the 05Kh22AG15N8M2F and 12Kh18N10T steels on the test time is found to be linear.