Study of rail steel alloying by chrome and vanadium effect on dissociation of overcooled austenite

To determine weldability and quality of a rail welded joint, information on kinetics of the rail steel overcooled austenite transformation is high importance. Thermo-kinetic diagrams of overcooled austenite dissociation of steels Э76ХФ, Э76ХАФ and Э76Ф, built based on results of dilatometric, metal science and durometric tests of rail steel samples. It was shown, that increase of chrome content in steels Э76Ф and Э76ХФ composition from 0.09 to 0.39% results in expanding of dissociation area of overcooled austenite at temperature scale for ferrite-cementite mixture and increasing of resistivity of the overcooled austenite against dissociation in the area of ferrite-cementite mixture formation. This can be characterized as decrease of critical quenching velocity from 100 to 30 °С/sec. It enables to obtain structural states of higher hardness at cooling with velocities in the area of 0.1−30 °С/sec. It was established, that increase of vanadium content from 0.04 to 0.07% does not cause quality changes at the thermo-kinetic diagram of overcooled austenite dissociation. However, it known, that vanadium is a strong carbide-formation element, which combines with carbon at low cooling velocities and removes it out of the solid solution. Due to this effect, sample of steel Э76ХФ with lower vanadium content at cooling with velocities from 0.1 to 10 °С/sec, had somewhat higher hardness level comparing with steel Э76ХАФ sample. Increase of chrome content in alloy content results in an increase of temperature of austenite formation completion at heating from 760 to 774 °С, while the temperature of martensitic transformation commencement at that remains practically unchanged at the level of 230 °С. In steels Э76ХФ and Э76ХАФ in the chemistry of which chrome was added in the amount of 0.37−0.39%, after cooling with velocities of 1 °С/sec and lower, apart from ferrite-carbide mixture of perlite type, formation of redundant ferrite with a volume share of 4−5% was observed as a result of overcooled austenite dissociation. However, in a steel Э76Ф sample, the content of which has chrome at the level of 0.09% at close content of carbon, after cooling in an analogue range of velocities, a ferritecarbide mixture of perlite type is formed with a slight trace of redundant ferrite in the structure as a result of overcooled austenite dissociation.

SEARCH FOR THE OPTIMAL VALUE OF THE INITIAL CONCENTRATION OF DIISOBUTYLALUMINUM HYDRIDE FOR THE REGULATION OF THE POLYISOPRENE MOLECULAR MASS ON A NEODYMIUM-CONTAINING CATALYTIC SYSTEM

Статья посвящена нахождению оптимального значения начальной концентрации диизобутилалюминийгидрида для полимеризации изопрена на неодимсодержащей каталитической системе. Для исследования закономерностей протекания процесса полимеризации изопрена построена кинетическая модель, то есть система обыкновенных дифференциальных уравнений, выражающих динамику концентраций частиц, участвующих в процессе. Система дифференциальных уравнений высокой размерности сведена к конечному виду путем применения метода моментов. Приведена в общем виде постановка задачи поиска оптимальных значений начальных концентраций веществ химического процесса. В качестве метода решения поставленной оптимизационной задачи в работе использованы эволюционные методы, в основе одного из которых лежит генетический алгоритм. Проведен вычислительный эксперимент для процесса полимеризации изопрена по поиску оптимального значения начальной концентрации диизобутилалюминийгидрида с целью регулирования молекулярной массы полиизопрена. Показано, что рассчитанное значение концентрации диизобутилалюминийгидрида обеспечивает достижение заданного критерия оптимальности. The article is devoted to finding the optimal value of the initial concentration of diisobutylaluminum hydride for the polymerization of isoprene on a neodymium-containing catalyst system. The authors present a kinetic diagram of the polymerization of isoprene on a neodymium-containing catalyst system using two types of organoaluminum compounds: triisobutylaluminum and diisobutylaluminum hydride. To study the patterns of the polymerization of isoprene, a kinetic model is constructed, that is, a system of ordinary differential equations expressing the dynamics of the concentration of particles involved in the process. The system of high-dimensional differential equations is reduced to the final form by applying the method of moments. The general statement of the problem of finding the optimal values of the initial concentrations of the substances of the chemical process is given. As a method for solving the optimization problem posed, evolutionary methods are used in the work, one of which is based on a genetic algorithm. A computational experiment was conducted for the polymerization of isoprene to find the optimal value of the initial concentration of diisobutylaluminum hydride in order to control the molecular weight of polyisoprene. It is shown that the calculated value of the concentration of diisobutylaluminium hydride provides achievement of the specified optimality criterion.

Approximate Engineering Model of Ferritic-Pearlitic Steel Constructions Resource Evaluation

In the constructions and weld seals microdefects while repeated stress lead to metal structure damage, low-cycle and high-cycle fatigue, and cracks dissemination. The calculation method of operating life estimate presupposes a microcrack development hypothetical speed application. This speed in many cases is unknown. Initial speed determination suggested in this work is based on endurance limit values design analysis when considering a combined problem for the first two sections of fatigue failure kinetic diagram.

FLUCTUATIONS IN NON-EQUILIBRIUM ELECTRON GAS: EFFECT OF QUANTUM STATISTICS

Here we develop a mathematical apparatus to describe quasi-classical fluctuations in a non-equilibrium electron gas with electron-electron collisions. We substantiate the method by deriving, from general principles of quantum kinetics, an equation recently proposed by us for an equal-time electron-electron correlation function. The derivation is performed using the kinetic diagram technique. In degenerate non-equilibrium gas, the theory predicts that there exists a specific equal-time correlation between electrons. Due to the prevalence of small-angle electron-electron scattering, the equation in question takes a rather simple and treatable form (the Coulomb-type electron-electron interaction stands out against the background of all other types of interaction as one that does not generate, in the framework of quasi-classical approach, any direct exchange effects).

Si ADSORPTION ON SiC(0001) SURFACES

Activation energies and atomic geometries associated with Si adsorption and desorption processes on the Si face of SiC (0001) have been obtained from in situ reflection high-energy electron diffraction measurements and ab initio pseudopotential calculations. During the initial stage of Si deposition, the SiC (0001) surface develops a series of superstructures, depending on temperature and Si coverage. A kinetic diagram of the structural transformations during Si exposure or annealing is outlined and the adsorption energy Ea is determined. The latter is found to be strongly affected by the surface structure and coverage, changing from 4.5 eV to 1.6 eV as the geometry of the Si adlayer changes from [Formula: see text] to (3×3). These observations are interpreted with the help of ab initio pseudopotential calculations. It is concluded that [Formula: see text] measured on [Formula: see text] corresponds to adsorption at adatom vacancy while Ea3=1.6 eV measured on (3×3) corresponds to adsorption on top of a capping adatom. Finally, the development of surface structures from the [Formula: see text] surface to silicon-rich and carbon-rich extremes is discussed.

Modeling and simulation of free radical polymerization of styrene under semibatch reactor conditions

The first part of this approach is concerned with the elaboration of a radical polymerization model of styrenne, based on a kinetic diagram that includes chemical and thermal initiation, propagation, termination by recombination and chain transfer to the monomer. Furthermore, volume contraction during polymerization is considered, as well as the gel and glass effects. The mathematical formalism that describes the model in terms of moments is explored in detail. The model was then used to predict the changes in monomer conversion and molecular weight after intermediate addition of initiator and monomer. The results of this operation are dependent on the conditions of the reaction mass, quantity, and moment of substance addition. Therefore, the simulations were performed at different times with respect to the gel effect; before, during and after this phenomenon, and also with respect to different temperatures and initiators. Increasing the initiator concentration before the gel effect leads to an earlier appearance of the phenomenon and to a decrease in molecular weight. The ratio $$\bar M_w /\bar M_n $$ reveals a polydispersity index smaller for the intermediate addition of initiator. No significant changes take place during or after the gel effect. If along with the initiator, unreacted monomver (used to dissolve the initiator) enters the reactor, a small dip in conversion is observed. The general conclusion of this paper reveals the intermediate addition of initiator as a method to control polymer properties and to prevent the “dead-end” polymerization of styrene.