Formation Of Needle-like Structures Research Articles

INVESTIGATION OF THE AUSTENITE DECAY KINETICS DURING CONTINUOUS COOLING OF STEEL WITH 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са FOR RAILWAY RAILS OF THE NEW GENERATION

Purpose. Investigation of the supercooled austenite decay kinetics during continuous cooling of steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca for railway rails of a new generation with increased operational properties to develop parameters of differential cooling. Method. The investigation of the austenite decay kinetics was carried out using differential thermal analysis. Samples were cooled at different cooling rates, temperature control was performed using chromel-aluminum thermocouples installed in the center of the sample. Microstructural investigations are performed using optical (AxioVert 200M mat) and electronic (SEM-106) microscopes. The interplate distance in pearlite is determined by the linear method, the diagonals are located perpendicular to the plate packages. Vickers hardness at a load of 10 kg. The number of structural components is recognized on microstructural images using the imageJ software complex. Results. The TKD analysis showed that at cooling rates of 0,06...5,96 °C/s, the structure of the investigated steel consists of pearlite. As the cooling rate increases, the morphology changes and the dispersion of pearlite increases: from medium pearlite to sorbitol-like. It should be noted that in the cooling rate range of 0,06...0,07°С/s, tertiary cementite is formed from ferrite. The diagram shows that at a cooling rate of 5.96 °С/s the hardness is 468 НВ (432 НВ), and the structure does not contain bainite. At a cooling rate of 1.47 °С/s, the hardness is 356 НV (345 НВ). This is slightly below the minimum allowable value (370 HV), but by interpolation it can be determined that the cooling rate must be at least 2.5 °C/s to achieve a hardness of at least 393 HV (370 HV). That is, when the rolling surface of the rail is cooled at a rate of 5.96 °C/s, the central volumes of the rail head at a rate of at least 2.5 °C/s, high-strength rails of the following categories can be manufactured: higher (DSTU (National Standard of Ukraine) 4344:2004) and R400HT (EN 13674-1:2011) from steel with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca. Scientific novelty. Constructed thermokinetic diagram of steel austenite decay with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са. Practical value. It was established that with the use of experimental steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca it is possible to achieve a hardness above 400NV without the formation of needle-like structures, therefore this is recommended chemical composition for the manufacture of high-strength rails of the R400NT category according to EN 13674-1-2011.

ВИЗНАЧЕННЯ ГРАНИЧНО ДОПУСТИМИХ ШВИДКОСТЕЙ ОХОЛОДЖЕННЯ ОБОДУ ЗАЛІЗНИЧНИХ КОЛІС ДЛЯ ФОРМУВАННЯ ОДНОРІДНОЇ СТРУКТУРИ

Compliance with the new more stringent requirements for railway wheels is possible only with an integrated approach to improving their production technology with a reasonable choice of heat treatment modes, taking into account the chemical composition of steel. The purpose of the study is the experimental and analytical determination of the maximum permissible cooling rate of the rolling surface and the minimum required cooling rate of the central sections of the rim of a railway wheel to achieve a high level of hardness of the ferrite-pearlite structure. The studies were carried out on steels of grades ER8 and ER7 according to EN 13262, grade 2 according to GOST 10791: 2016, steel of class C according to AAR M-107 / M-208. Based on the hardenability test (GOST 5657) by the end hardening method (according to the Jomeni method), the distance from the surface from which one-sided cooling was performed, where the hardness met the requirements of the normative documentation for wheels made of the steel under study, the distance where the formation of needle-like structures did not occur, was determined, including bainite and widmanstatt according to the results of metallographic studies and determination of hardness. Based on the results of temperature changes during the cooling process, the cooling rates were determined in places that met the specified conditions. In the work, modeling was applied in the QForm VX 8.2 software package, as a result, a model was developed for the adaptation of which the results of the experiment were used. The high convergence of the results of calculation and experiment was confirmed. At the same time, the model made it possible to obtain the instantaneous cooling rate in a form that is more consistent with the physical meaning of the process and to avoid the scatter of actual values associated with the discreteness of data recording. It was found that the instantaneous cooling rate changes during the continuous uniform supply of the coolant. In some cases, a significant slowdown in cooling was recorded, or even negative values of the instantaneous cooling rate. This is due to the release of the heat of phase transformation, which occurs during the decomposition of austenite by the diffusion mechanism. The developed model can be used to construct thermokinetic diagrams during continuous cooling and to develop recommendations for heat treatment modes to achieve the specified mechanical properties at a certain structural state.

Nanostructured Ceramics of Potassium Sodium Bismuth Titanate: Hydrothermal Synthesis and Piezoelectric Response at Morphotropic Phase Boundary

Potassium Sodium Bismuth Titanate (KNBT) ceramics, with the general formula (1 - x)K0.5Bi0.5TiO3 -xNa0.5Bi0.5TiO3, have been synthesized following hydrothermal route, starting with solid solutions of pure perovskite nanoceramics of KBT and NBT in desired stoichiometric weight ratios, followed by sintering between 850°C and 1000°C for few hours. Pure KNBT nanoceramics with perovskite structure, having mean particle size around 30 nm, could be obtained. Morphology of the samples is found to depend strongly on composition. A change of composition results in a phase change, as evident from X-ray structure analysis. This phase change is a result of rhombohedral to tetragonal morphotropic phase boundary (MPB) in the sample with x around 0.80. Composition dependent occurrence of MPB leads to formation of needle like structures with micrometer length scales. These are typical of tetragonal lamellar structures, suggesting partial induction of tetragonal polar order from rhombohedral structure at MPB. Dielectric and piezoelectric properties, such as dielectric constant and loss, piezoelectric coefficients and figures of merit, exhibit threshold maxima in their values at the composition corresponding to MPB. These values reported for a lead-free piezoceramic, synthesized by a comparatively simple hydrothermal route, are highly promising, and comparable to well-known PZT.

Enhanced Photoelectrochemical Performance of Nanostructured Zinc Oxide Photoelectrodes Via Morphology Control

Photoelectrochemical solar cells (PEC) provide both harvesting and storage of the solar energy by converting it to chemical energy. In other words PEC systems convert sunlight into fuels by means of electrochemical reactions [1]. Total system efficiency of the PEC is directly affected by the performance of the working photoelectrode and the counter electrode. Here we investigated the influence of the morphology of the zinc oxide (ZnO) nanostructured photoelectrodes on the efficiency. ZnO photoelectrodes with nano-film, nanowire, and urchin-like morphologies have been used as working electrode in the three-electrode in one compartment cell configuration. Nano-films (ZnO-NF), 60 nm thick, have been deposited on the indium tin oxide (ITO) coated glass via physical vapor deposition. These films were used as the seeding layer to grow the ZnO nanowire arrays (ZnO-NW) by chemical bath deposition (CBD). Finally we fabricated the urchin-like nanostructures (ZnO-UL) on fluorine doped tin oxide coated (FTO) glass substrates by again CBD technique. In order to increase the coverage and the adhesion of the urchin-like structures we modified the surface of the FTO via hydroxylation process [2]. These electrodes were labeled as ZnO-H-UL. As expected, the light absorption of the nanowire and urchin-like structures was superior to the nanofilms. Therefore, nanowire formation slightly increased the current density under illumination at VAg/AgCl=0V bias. On the other hand, current density increased approximately 103 times and reached to 3 mA/cm2 for the ZnO-H-UL samples compare to the nano-films. Another drastic change was observed for the photoresponse, which is a measure of photo-generated current in the system. In other words photoresponse of the electrode can be calculated from the ratio of Jill-Jdark to Jdark, where Jill is the current density under illumination and Jdark is the current density at dark. The maximum photoresponse of the ZnO-NF was 4.5 indicating almost no current change in dark and under illumination. However, this value was 55, 7.9x102, and 1.2x103 for ZnO-NW, ZnO-UL and ZnO-H-UL electrodes, respectively. Enhancement in the photo generated current density for the urchin like structure could be related with the better light absorption and longer electron lifetime. According to our best knowledge this is the highest photoresponse reported in literature. Efficiency of the ZnO electrodes was calculated using the well known applied bias photon-to-current efficiency (ABPE) definition [3]. ABPE of the ZnO-NF and NW electrodes was 0.09 and 0.32 %, respectively. Efficiency of the ZnO electrodes increased with formation of needle like structures on the spherical bases. 0.65 and 0.87 % ABPE was observed for ZnO-UL and H-UL nanostructures, respectively. One can determine the overall PEC device efficiency by the ratio between the energy of the produced hydrogen and the total energy spent to accomplish the water splitting reaction [4]. PEC solar cells produced using ZnO-H-UL photoanode had the overall device efficiency of the 2.5%. Better photoresponse, photo current density, and the device efficiency of the ZnO-H-UL electrodes compare to the other nanostructures made them very favorable in PEC systems as working electrodes.

Viscous fingering: A singularity in Laplacian growth models.

The \ensuremath{\eta} model in the linear or radial geometry is investigated numerically. It turns out that the main characteristics of the viscous fingering instability (\ensuremath{\eta}=1) at vanishing capillary number such as the \ensuremath{\lambda}=1/2 limit are not recovered. For \ensuremath{\eta}\ensuremath{\ne}1, the selected finger width decreases with the capillary parameter, indicating the formation of needlelike structures.