No 2 (2020)

Ferritic-martensitic steels with a chromium content of 9-12 % are currently considered as the promising structural materials for nuclear power. Interest in steels of this class is caused by their higher resistance to radiation swelling compared with austenitic steels used in the existing fission reactors. The operating temperature range of these steels is limited from below by their tendency to low-temperature embrittlement (cold fracture) under the radiation influences, and from above - by the long-term strength level (heat resistance). The authors studied the features of the microstructure of 12 % Cr ferritic-martensitic EK-181 steel near the neck of the samples deformed by tension at T=20 °С and within the range of temperatures close to the operating temperatures of a nuclear reactor (T=650 and T=720 °C). The authors carried out the comparative study of the materials processed by two methods: traditional and high-temperature treatment. The study showed that plastic deformation at T=20 °C after two treatments is similar in quality and leads to curvature and fragmentation of martensitic lamella, as well as to the formation of new low-angle boundaries. Deformation near the operating temperature range (T=650 and T=720 °C) contributes to the development of the processes of dynamic polygonization, recrystallization, increasing the density, and the size of carbide particles. After high-temperature thermomechanical treatment, these processes are less intensive compared to the state after traditional thermal treatment. After high-temperature thermomechanical treatment, EK-181 steel has an increased level of strength and has a higher resistance to plastic deformation compared to the state after traditional treatment. It is related to the high density of vanadium carbonitride nano-particles V(C, N) and the increased dislocation density after high-temperature thermomechanical treatment.

The paper considers the possibility of conversion applying of a rocket engine as a sandblasting machine for thermo-abrasive treatment. The higher performance characteristics of a treated surface can be achieved through the exposure of the high-temperature two-phase flow accelerated in the device nozzle barrel on the object. The ejection feed of granular abrasive substances determines the relative structural simplicity of the device structure. The authors prove the efficiency of such a device using the gas-dynamic process modeling in the CFD software package, the calculations of which are based on combined equations including the key parameters of both the carrier gas and the solid phase particles. The process modeling considers the influence of the geometry and the specifics equal to the real operating prototype. During further analysis, to determine the optimal mode, the authors investigated the influence of various border conditions on the supersonic two-phase flow. The study considers the mutual influence of gas flow and abrasive solid particles starting from the powder delivery section to the nozzle outlet section. The study presents the comparison of temperature and pressure fields depending on the input values, as well as the fluid velocity fields based on these values. The authors carried out the analysis of the dependence of solid particle motion speed on the coordinate at various initial data of temperature and pressure. The study pays special attention to the consideration of the impact of the k -phase particle size on the speed parameters. During the study, the authors identified the main methods of device adjustment to achieve the required mode parameters. As a result of the analysis, the paper concludes on the efficiency and competitive ability of the thermo-abrasive treatment method under the study.

The authors study the corrosion of AISI 316 stainless steel in 1M perchloric acid at 90 °C, including in the presence of the benzotriazole corrosion inhibitor. Electrochemical experiments were carried out in a three-electrode glass cell with a platinum counter electrode and a saturated silver chloride electrode as a reference electrode. The authors carried out the potentiodynamic measurements at the temperature of (90±2) °C and the potential sweep speed of 1 mV/s; the impedance measurements within the frequency range from 20 kHz to 0.1 Hz at the voltage amplitude of ±10 mV. Cyclic polarization curves show that the cathode direction currents are always lower than the anode direction currents of the potential sweep. Consequently, the curves of anode and cathode directions of the potential sweep are analyzed separately. When analyzing, the authors use the modified Tafel equation, which is linear at any overload that allow determining the corrosion currents more accurately. The study shows that with an increase in the inhibitor concentration, the potentiodynamic curves shift to the cathode side, and the cathode currents decrease more strongly than the anode currents. Therefore, benzotriazole in perchloric acid is an inhibitor of cathodic action, i.e. slows down the cathodic reaction of the perchloric acid anion reduction to chloride ions. The authors identified that benzotriazole inhibits corrosion at concentrations of more than 10^-4 mol/L. At the concentration of 1×10^-3 mol/L, the inhibition efficiency is 33±10 %, and at the concentration of 1×10^-2 mol/L, it is 36±13 %. The inhibiting effect of a benzotriazole molecule in the acidic medium is caused by the possibility of its protonated form to be adsorbed on the metal surface. The protonated form of benzotriazole in acidic medium allows explaining the slow-down of the cathode depolarization reaction as the inhibitor is adsorbed predominantly on metal surface areas charged more negatively. The impedance measurements showed that the corrosion process is modeled by the element parallel circuit with the constant phase shift and corrosion resistance.

Pressure vessels, in particular cylindrical and spherical thin-walled vessels, are widely used in the industry. The aggressive impact of the environment during operation, as well as workloads, lead to the gradual accumulation of defects in structures. Since local defects act as stress concentrators, to ensure the strength and reliability of a structure, it is necessary to take into account the stress concentration near the defects. The paper considers a thin-walled sphere under pressure with the damages on its inner surface. The author modeled the defects as spherical notches immersed to the depth equal to half of their radius. Defects are evenly spaced along one of the circumferences of a large sphere. To estimate the stress state, the author built 3-D models of a spherical vessel with defects. The study considers the different number of defects and various sizes of defects; each parameter value corresponds to its geometry model. With the ANSYS Workbench package of finite element analysis, for each model, the author carried out the application of loads (pressure acts on the inner surface of a vessel), model decomposition into finite elements, and builds the field of maximum normal stresses distribution in a body. Calculations are made in the framework of the linear theory of elasticity. The author carried out a numerical experiment to study the influence of the number of surface defects on the stress state within their neighborhood. The paper studies the dependence of calculated stresses in the body on the depth of defects. The study showed that with an increase in the number of defects, as well as with an increase in their depth, the maximum normal stress increases.

Recently, the interest in copper-based alloys (in particular, CuAlNi alloys containing 10-14 % Al and 4-5 % Ni) having the narrow temperature hysteresis and showing a full return of deformation increased. However, at the moment, there are practically no works dealing with the modeling of the behavior of Cu-based alloys with shape memory, which determines the relevance of this study. The paper considers a microstructural model of the mechanical behavior of the CuAlNi-type alloy, taking into account the reversible martensitic transformation β₁(D0₃)↔β₁'(18R) occurring in this material. An important parameter - deformation matrix - is the basis of this model. The authors carried out necessary calculations in the deformation smallness assumption. The strain tensor matrix for this transformation is calculated based on the available crystallographic data in the literature. The authors used the obtained matrix for further modeling of functional properties of the CuAlNi-based alloys and performed calculations to determine the crystallographic transformation resource, i.e. the maximum deformation of the crystal lattice for given transformation. The simulation of the quasi-elastic behavior of a single CuAlNi crystal was carried out, which identified a certain orientation of a single crystal causing deformation approximately equal to the calculated value of the crystallographic resource. Thereby, the deformation matrix makes it possible to adequately simulate the behavior of the shape memory alloy under the study. The results obtained are in good agreement with the experimental data available in the literature, which suggests that the constructed deformation matrix can be used for further calculations.