Structure and mechanical properties of a nonmagnetic high-nitrogen corrosion-resistant 05Kh22AG15N8M2F cast steel produced by high-gradient directional solidification

The structure and properties of an austenitic high-nitrogen corrosion-resistant 05Kh22AG15N8M2F cast steel produced by high-gradient directional solidification (HGDS) and equiaxed-grain solidification (ES) have been studied and compared. In contrast to ES, HGDS allows one to substantially decrease the degree of dendritic segregation of alloying elements, to eliminate porosity, and to increase the strength and plasticity of the steel.     

Electron-microscopic study of the structure of the surface layer in high-nitrogen 05Kh22AG15N8M2F steel after face turning

The structure of the surface layer in high-nitrogen 05Kh22AG15N8M2F steel workpieces subjected to face turning is studied by electron microscopy. It is found that improved machinability by VK8 alloy cutting tools is achieved at a cutting depth of 0.25 mm and that the cutting-tool life decreases sharply when the cutting depth increases to 1 mm. A nanocrystalline structure with nanocrystal sizes from several to several tens of nanometers forms in the surface layer upon face turning in the as-cast, hot-rolled, and thermally deformed states. The structure of the surface layer is characterized by a high dislocation density and large austenite fragments with broad subgrains and deformation twins.     

Cyclic strength of corrosion-resistant high-nitrogen austenitic 05Kh22AG15N8MF steel during repeated tension

The static and fatigue cyclic strength of high-strength corrosion-resistant nitrogen-bearing austenitic 05Kh22AG15N8MF steel are studied in various structural states after hot rolling at 1100°C (water quenching from 1150°C, subsequent annealing at 800°C for 1 and 10 h, and cooling in air). The maximum life and a higher fatigue limit (400 MPa) are shown to be characteristic of hot-rolled samples with a finegrained austenitic structure and numerous twins. The mechanisms of fatigue crack propagation are studied.     

Structure and mechanical properties of corrosion-resistant high-nitrogen 04Kh22AG15N8M2F and 05Kh19AG10N7MFB steels after hot deformation

The structure and mechanical properties of corrosion-resistant high-nitrogen austenitic 04Kh22AG15N8M2F and 05Kh19AG10N7MFB steels are studied after hot rolling at 950 and 1100°C. The following specific features of the structure of hot-rolled 04Kh22AG15N8M2F steel are revealed: the presence of coarse grain-boundary precipitates of the molybdenum-rich ?? phase and its nonuniform distribution over the volume of austenite grains. The 05Kh19AG10N7MFB steel hot rolled at 950°C contains ultrafine carbonitrides particles and has the best combination of a high strength and a sufficient elasticity and impact toughness. The structures of the hot-rolled steels have no ferrite, martensite, and traces of recrystallized austenite grains.     

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.     

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.     

Weldability of corrosion-resistant high-nitrogen austenitic Kh22AG16N8M-type steels

The influence of thermal treatment on the structures and mechanical properties of welds of corrosion-resistant high-nitrogen austenitic 05Kh22AG16N8M-type steels is studied. In these steels, austenite is found to be highly resistant to discontinuous precipitation and the formation of σ phase and δ ferrite upon cooling regardless of the temperature of heating for quenching (from 900 to 1250°C) and the cooling conditions (water, air, furnace). Welding of these steels can produce high-strength welds with an enhanced impact toughness.     

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: σ_0.2 = 1044 N/mm², σ_u = 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.     

Structural and phase transformations in high-nitrogen austenitic steel 05Kh20AG10N3MF during thermal action

The structural and phase transformations in high-nitrogen austenitic steel 05Kh20AG10N3MF under various thermal actions are studied. The transformations are shown to occur only in the Fe-Cr diffusion couple, and they resemble the phase transformations in an Fe-20% Cr binary alloy. At 1200°C, hightemperature phase separation is detected; at 550°C or below, low-temperature separation is detected; and at 700 and 800°C, ordering with the formation of a Laves phase is observed. The ordering-low-temperature separation phase transition in the steel differs substantially from that occurring in Fe-Cr binary alloys.     

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.     

Fracture of high-nitrogen 05Kh20G10N3AMF steel during impact loading

Specimens with V- and U-shaped notches made of austenitic high-nitrogen corrosion-resistant 05Kh20G10N3AMF steel are subjected to impact tests in the temperature range from +20 to ?196°C, and stress-strain diagrams are recorded. The test data are used to estimate impact toughness KCV and KCU, dynamic fracture toughness J _id at the stage of crack nucleation, and crack propagation energy A _p. The microrelief of the fracture surfaces is studied. As compared to forging, quenching from 1100°C is found to increase the impact toughness and the dynamic cracking resistance of the steel during impact loading and to decrease the ductile-brittle transition temperature. The steel is shown to exhibit the cold brittleness behavior characteristic of bcc materials. A model is proposed for the formation of cleavage facets in austenitic steels. It is based on easy slip along lattice planes under the low shear stress at a notch tip and the development of fracture at a stress lower than the yield strength of the material.     

Development of a Commercial Technology for Plasma Arc Remelting of a Nitrogen-Bearing 05Kh21AG15N8MF Steel

Steel 05Kh21AG15N8MF forgings 250 mm in diameter with a nitrogen content higher than 0.60%, which is necessary for a high-strength austenitic structure, are fabricated by plasma arc remelting. An experimental batch and strips are made of this steel. The chemical composition, the macrostructure, the mechanical properties, the intergranular corrosion resistance, and the hardness of the forgings are studied.     

Structure and properties of high-temperature austenitic low-carbon 01Kh15N22AG2V4TYu and 02Kh18N12AG11MFB steels

Principles of multicomponent alloying of high-temperature steels are formulated on the basis of reported and obtained experimental data. The short- and long-term strength, the structure, and the phase composition of high-temperature austenitic low-carbon steels 01Kh15N22AG2V4TYu and 02Kh18N12AG11MFB are studied in the structural states that form upon forging and aging and ensure the maximum hardening. These steels are found to have a high short-term strength and high-temperature strength. The tests of these steels performed for 8 × 10³ h without failure of specimens at a temperature of 680°C and stresses of 100–120 MPa show that they are promising materials that can operate at 620–680°C for 2 × 10⁵ h at a stress of 70 MPa in power units intended for operation at supercritical vapor parameters. The specific features of the fine structures of the steels in the forged and aged states that provide their high high-temperature strength are revealed. The evolution of structural constituents during long-term strength tests is studied, and the role of these constituents in fracture is determined.     

Effect of quenching conditions on the formation of the grain structure and the mechanical properties of high-nitrogen austenitic 02Kh20AG14N8MF and 02Kh20AG12N4 steels

The formation of the grain structure of high-nitrogen 02Kh20AG14N8MF and 02Kh20AG12N4 steels in forging and quenching and their mechanical properties in this state have been studied. It is found that both steels have close mechanical properties under the same quenching conditions. In 02Kh20AG14N8MF steel, a homogeneous structure of primarily recrystallized austenite grains forms under the quenching conditions under study. In 02Kh20AG12N4 steel, the processes of secondary recrystallization and normal grain growth take place.     

Effect of heat treatment on the mechanical properties and the structure of a high-nitrogen austenitic 02Kh20AG10N4MFB steel

The effect of heat treatment on the mechanical properties of a high-nitrogen austenitic 02Kh20AG10N4MFB steel has been studied in the temperature region 550—1200°C. The yield strength and the ultimate tensile strength are shown to change nonmonotonically as a function of the heat treatment temperature. They sharply decrease in the annealing temperature range 850—900°C, which can demonstrate a change in the character of the structure–phase state of this steel. After annealing at 850—900°C, aging occurs with the precipitation of embrittling phases; at higher annealing temperatures, these particles dissolve and austenite recrystallizes. The study of the stress–strain diagrams makes it possible to find the laws of strain hardening of the 02Kh20AG10N4MFB steel as a function of the heat treatment temperature.     

Influence of deformation on the structure and mechanical and corrosion properties of high-nitrogen austenitic 07Kh16AG13M3 steel

The correlation has been studied between the structure of a high-nitrogen austenitic Cr-Mn-N steel formed in the process of combined hardening treatment, including cold plastic deformation (CPD), and its mechanical and corrosion properties. The structure and properties of commercial high-nitrogen (0.8% N) 07Kh16AG13M3 steel is analyzed after rolling by CPD and aging at 500 and 800°C. It is shown that CPD of the steel occurs by dislocation slip and deformation twinning. Deformation twinning and also high resistance of austenite to martensitic transformations at true strains of 0.2 and 0.4 determine the high plasticity of the steel. The contribution of the structure imperfection parameters to the broadening of the austenite lines during CPD is estimated by X-ray diffraction. The main hardening factor is stated to be lattice microdistortions. Transmission electron microscopy study shows that heating of the deformed steel to 500°C leads to the formation of the intermediate CrN phase by a homogeneous mechanism, and the intermtallic χ phase forms along the austenite grain boundaries in the case of heating at 800°C. After hardening by all investigated technological schemes, exception for aging at 800°C, the steel does not undergo pitting corrosion and is slightly prone to a stress corrosion cracking during static bending tests, while aging at 800°C causes pitting corrosion at a pitting formation potential E _pf = ?0.25 V.     

Correlation between the granular structure and the mechanical properties of high-nitrogen austenitic 02Kh20AG10N4MFB steel after annealing

The effect of the annealing temperature and time on the formation of a granular structure in high-nitrogen austenitic 02Kh20AG10N4MFB steel has been studied. The hardness and the strength properties of the steel are shown to be related to the mean grain size by an inverse dependence, according to the Hall–Petch relation, and the impact toughness is proportional to the mean grain size. At annealing temperatures to 1100°C, structure formation is determined by the precipitation of secondary phases; at higher annealing temperatures, it is determined by the recrystallization of austenite grains.     

Structure of a cast high-strength corrosion-resistant 05Kh20AG10N3MF austenitic steel containing 0.40 or 0.53% nitrogen

The phase composition and fine structure of a high-strength corrosion-resistant 05Kh20AG10N3MF austenitic steel containing 0.40 or 0.53% N are studied by X-ray diffraction and electron microscopy. In the as-cast state, this steel has a structure containing austenite, δ ferrite, and dispersed CrV(C, N) carbonitrides. The δ ferrite is represented by layers between austenite grains, the dislocation density in which is lower than in the δ ferrite. After quenching from 1100, 1150, and 1200°C, the structure of the steel with 0.53% N has no δ ferrite and the structure of the steel with 0.40% N has a low δ-ferrite content and χ-phase precipitates.