N. V. Kataeva

Comparative analysis of corrosion cracking of austenitic steels with different contents of nitrogen in chloride- and hydrogen-containing media

The structural state and the resistance to stress-corrosion cracking (SCC) at constant loads have been studied using samples with a grown crack by the method of the cantilever bending on quenched austenitic stainless steels of the 20Cr-6Ni-11Mn-2Mo-N-V-Nb (Kh20N6G11M2AFB) type, with different contents of nitrogen (0.17, 0.34, 0.43, and 0.50 wt % N). The tests were conducted in a 3.5% aqueous solution of NaCl (without providing polarization) and in a similar solution under cathodic polarization, which causes the formation of hydrogen. It has been shown that, in a chloride solution without polarization, the steels do not undergo SCC for 2000 h. In the case of significant cathodic polarization via employment of a magnesium protector, there was revealed a brittle character of fracture upon SCC in all steels. It has been shown that steel with a nitrogen content of 0.43 wt % possesses the maximum absolute values of rupture stresses under the conditions of cathodic polarization.

Structural transformations in hull material clad by nitrogen stainless steel using various methods

Specimens of a 10N3KhDMBF shipbuilding hull steel were clad by a 04Kh20N6G11M2AFB nitrogen austenitic steel using various treatment conditions, which included hot rolling, austenitic facing, and explosive welding followed by hot rolling and heat treatment. Between the base and cladding materials, an intermediate layer with variable concentrations of chromium, manganese, and nickel was found, in which a martensitic structure was formed. In all the cases, the strength of bonding of the cladding layer to the hull steel (determined in tests for shear to fracture) was fairly high (σsh = 437–520 MPa). The only exception was the specimen produced by unidirectional facing without subsequent hot rolling (σsh = 308 MPa), in which nonfusions between the faced beads of stainless steel were detected.

Structural mechanism of reverse α → γ transformation and strengthening of Fe-Ni alloys

Fe-32% Ni alloy subjected to slow heating to a temperature below As at a rate of 0.01 K/min demonstrates the untwinning and appearance of an intermediate ɛ phase with an hcp lattice and lattice parameters a = 2.535, c = 4.132 Å, and c/a = 1.63. Slow heating to 430–490°C leads to the formation of nanocrystalline austenite enriched in nickel, which substantially increases the hardness of martensite. The formation of austenite in the Fe-32% Ni alloy, which is a mixture of martensite with 20–30% nanocrystalline austenite, during its rapid heating to 600°C occurs via the bulk mechanism with short-range atomic diffusion. In this case, the diffusion does not eliminate the concentration micro-inhomogeneity of the alloy in nickel but leads to the reorientation of γ-phase nanocrystals, almost eliminates the dislocation structure, and removes the strengthening by phase hardening.

Comparative analysis of the kinetics of dissolution of oxides Y2O3 and Fe2O3 in the iron matrix upon mechanical alloying

The kinetics of low-temperature dissolution of oxides Y2O3 and Fe2O3 in an iron matrix during mechanical alloying has been studied using electron microscopy. It has been shown that the dissolution rate upon deformation of primary coarse oxides Fe2O3 in α iron (and, hence, saturation of the α matrix with oxygen) during treatment in a ball mill for up to 10 h is several times higher than the dissolution rate of Y2O3 oxides. The high-temperature (1100°C) annealing of a mechanoalloyed mixture of Fe + 1.5% Y + 1.35% Fe2O3 leads to the precipitation of 60% (of the total number of particles) secondary oxides 2–5 nm in size and only of 5–7% secondary nanooxides in a mechanoalloyed mixture of Fe + 2% Y2O3.

Solid-state mechanical synthesis of austenitic Fe-Ni-Cr-N alloys

The possibility of producing high-nitrogen nanostructured austenitic Fe-Ni-Cr-N alloys using solid-state mechanical synthesis in a ball mill has been studied. It has been shown that, in the matrices of iron and Fe-xNi (x = 6–20 at %) alloys with chromium nitrides, bcc and fcc Fe-Ni-Cr-N solid solutions are formed. With an increase in the concentration of nickel in the matrix, the amount of mechanically synthesized austenite grows. The subsequent heating of the mechanically synthesized specimens into the austenitic field of the phase diagram of Fe-Ni alloys leads to the α → γ transformation with the retention of the nanostructured Fe-Ni-Cr-N solid solution and precipitation of secondary nitrides CrN, which stabilize the structure.

Acoustic detection of stress-corrosion cracking of nitrogen austenitic steels

Structural changes and resistance to the stress-corrosion cracking of the nitrogen-bearing austenitic steels 04Kh20N6G11M2AFB and 09Kh20N6G11M2AFB (with 0.04 and 0.09 wt % C, respectively) after different treatments, including thermomechanical action, quenching from 1200°C, and aging at 700°C for 2 and 10 h, have been studied. It has been shown that aging at 700°C of the air-melted austenitic steel 09Kh20N6G11M2AFB leads to a decrease in the strength of samples with an induced crack upon the cantilever bending in air and in a 3.5% aqueous solution of NaCl as compared to the strength of the steel 04Kh20N6G11M2AFB-EShP with a smaller carbon content after high-temperature mechanical treatment or quenching from 1200°C. The smallest resistance to stress-corrosion cracking is observed in the samples of 09Kh20N6G11M2AFB steel after 10 h of aging, which is accompanied by the most intense acoustic emission and by brittle intergranular fracture. This is explained by the high rate of the anodic dissolution of the metal near chromium-depleted grain boundaries due to the formation of continuous chains of grain-boundary chromium-containing precipitates of carbides and nitrides.

Nitrogen distribution in austenitic high-nitrogen chromium-manganese steel under friction and high-pressure torsion

Mössbauer spectroscopy and electron microscopic analysis were used to investigate the precipitation of products of cellular decomposition and their dissolution in high-nitrogen chromium-manganese steel FeMn22Cr18N0.8 under room-temperature severe deformation via dry sliding friction and high pressure torsion in Bridgman anvils. It has been established that the nitrogen content increases in interstitial positions in the quenched and pre-aged alloy due to the strain-induced dissolution of chromium nitrides, which are contained in the products of decomposition. Mössbauer analysis showed that the friction-induced dissolution of chromium nitrides occurs at a depth of more than 10 μm. Aging reduces the amount of nitrogen that occurred in the solid solution upon deformation. This is explained by the additional energy consumed in grinding the decomposition products.

Dynamic aging in an Fe–Ni–Al alloy upon megaplastic deformation. Effect of the temperature and deformation rate

The method of Mössbauer spectroscopy has been used to investigate the effect of the temperature and the rate of megaplastic deformation on the processes of dissolution–precipitation of intermetallic compounds in aging austenitic alloy with a composition of Fe–36Ni–9Al. It has been established that, upon deformation in revolving Bridgman anvils, in the temperature range of cryogenic temperatures (liquid nitrogen) up to 573 K, a change occurs in the character of phase transitions from atomic disordering and the dissolution of intermetallic compounds to their additional accelerated precipitation. The factor that affects the kinetics of the processes of dissolution–precipitation of intermetallic compounds in the metallic matrix is dynamic aging. Dynamic aging is activated with an increase in the temperature and a decrease in the deformation rate.

Shape memory effect in Mn-V-C austenitic steels involving deformation reorientation of ɛ-martensite

The shape memory effect with a reversible deformation of 1.6–1.7% in the metastable steels like 20G20S2F with the ɛ-martensitic original structure is obtained as a result of the shear repeated twinning of ɛ-martensite during cold deformation and the subsequent ɛ → γ transformation during heating.

Structure, properties, and resistance to stress-corrosion cracking of a nitrogen-containing austenitic steel strengthened by thermomechanical treatment

The results of comparative studies of the structure, mechanical properties, and resistance to stresscorrosion cracking (SCC) in the chloride solutions of a Cr–Mn–Ni austenitic nitrogen-containing steel (20Cr–6Ni–11Mn–1.5Mo–N–V–Nb) produced with the use of different regimes of the high-temperature thermomechanical treatment (HTTMT) have been presented. An unfavorable effect of the grain-boundary precipitates of the nitride phase on the impact toughness and resistance to SCC has been found. It has been shown that the strengthening of nitrogen-containing steel upon HTTMT, which ensures an increase in the yield stress by 1.8 times compared to the austenitized state, does not decrease the resistance to SCC.