Browsing by Author "Yordanova R."
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Item Analysis of fatigue behaviour of stainless steels under hydrogen influence(2016-06-01) Angelova D.; Yordanova R.; Yankova S.Three stainless steels – ASTM 304, 316 and 316L - used in hydrogen utilization equipment are under investigation at conditions of tension-compression, rotating-bending and fretting fatigue. Fatigue tests are carried out with hydrogen charged and uncharged specimens. Hydrogen charging includes cathodic type of charging and exposure to high pressure hydrogen gas. The experiments under rotating bending and tensioncompression fatigue are conducted under different frequencies in three different laboratories: at The University of Chemical Technology and Metallurgy, Sofia, Bulgaria; at Sandia National Laboratory, California and The University of Tufts, Medford, Massachusetts, USA; The HYDROGENIUS Institute at Kyushu University, Japan. The fretting fatigue tests are presented by The HYDROGENIUS Institute at Kyushu University, Japan. The obtained results are presented in Wöhler curves complemented by plots ``Short fatigue crack length–Number of cycles`` and “Tangential force coefficient–Stress amplitude”. The found fatigue characteristics are analyzed and compared at different loading conditions, showing the best performance of Steel 316L.Item COMPUTER MODELING AND INVESTIGATION OF THE DEFORMATION PROCESS IN CONVEX-CONCAVE AND TRAPEZOIDAL STRIKERS(2023-01-01) Volokitin A.; Naizabekov A.; Panin E.; Tolkushkin A.; Nurakhmetov D.; Yordanova R.; Volokitina I.; Arbuz A.The article presents the results of a comparative analysis of the deformation process in convex-concave and trapezoidal strikers using computer modeling by the finite element method in the Deform program. In both designs, two variants are considered - with an equal and unequal ratio of the protrusion to the depression. For comparative analysis, equivalent stresses and strains, energy - power parameters are considered. Also, on the basis of computer modeling, the analysis of the microstructure evolution during the deformation in convex - concave and trapezoidal strikers was carried out. In the course of computer modeling, some advantage of the design of convex - concave strikers in comparison with trapezoidal strikers was proved.Item DESIGN IMPROVEMENT OF THE STEP-WEDGE DIES AND EXPERIMENTAL STUDY OF THE FORGING PROCESS OF 45 STEEL BILLETS(2024-01-01) Tolkushkin A.; Naizabekov A.; Panin E.; Yordanova R.; Shvarts D.; Tsyba P.; Volokitina I.; Rovin S.; Esbolat A.In this paper, a forging tool of a new design is proposed, which allows to implement shear and alternating strain in the entire volume of the deformed billet with a smaller change in its initial dimensions. To confirm the effectiveness of the proposed new forging tool in obtaining billets and forgings of the plates and slabs type of the required quality, a laboratory experiment was conducted. During the laboratory experiment, the forging of billets made of the steel 45 was carried out in such forging tools as step-wedge dies of two different variants (proposed and previously known) and in step dies. Analysis of the shape change of billets and the evolution of the microstructure of steel after two deformation cycles showed that the use of the step-wedge dies with a wedge protrusion on the upper die and a wedge cavity on the lower die is more promising for forging billets and forgings of the plates and slabs type with a given level of mechanical properties, since it allows to obtain a fine-grained structure with a smaller change in the initial dimensions of the billet.Item Development and Computer Simulation of the New Combined Process for Producing a Rebar Profile(2023-01-01) Lezhnev S.; Naizabekov A.; Panin E.; Tolkushkin A.; Kuis D.; Kasperovich A.; Yordanova R.The study presents results of computer simulation by finite elements method of a new metal forming process combining the deformation of a billet with round cross-section on a radial-shear rolling mill and subsequent billet twisting in a forming die with a specific design. To analyze the efficiency of metal processing, the main parameters of the stress-strain state are considered: effective strain, effective stress, average hydrostatic pressure, and Lode-Nadai coefficient. The maximum value of effective strain up to 13.5 is achieved when a screw profile on the billet in the die is forming, which indicates an intensive refinement of the initial structure of the billet. During combined process, the nature of the deformation changes in the transverse direction from the axis of rotation to the surface. The central area of the billet is under the action of tensile stresses. In the peripheral part, compressive stresses grow. In the surface area, Lode-Nadai coefficient is 0.1 approximately, which indicates the high level of shear strain.Item EFFECT OF THE LIMITING DEFORMATION ZONE UNDER CONDITIONS OF ASYMMETRIC LOADING DURING ROLLING OF MEDIUM THICKNESS STRIPS(2024-01-01) Chigirinsky V.; Naizabekov A.; Lezhnev S.; Kuzmin S.; Panin E.; Tolkushkin A.; Volokitina I.; Yordanova R.; Knapinski M.The aim of the work is to develop a physical and mathematical model of the process under complex asymmetric loading in conditions of single-area and two-area deformation zone during plastic processing of medium-thickness strips. The stress state in case of loss of stability during rolling of strips of medium thickness was investigated. The patterns of changes in the stress state of the strip under conditions of reach of the limiting deformation zone, as well as the effects of plastic shaping determined by a decrease in contact stresses under conditions of increasing deformation loading, are revealed. The described method is a visual approach to assessing the stress state of a plastic medium under conditions of complex interaction and asymmetric loading.Item Fatigue crack development in a low-carbon steel. Microstructure influence. Modelling(2016-01-01) Angelova D.; Yordanova R.; Yankova S.Fatigue in a low-carbon steel with ferrite and pearlite microstructure is investigated through testing of three groups of specimens. Two of the groups consist of cylindrical specimens subjected to tension-tension and rotating-bending fatigue; in this case surface microstructurally-short crack propagation is monitored by acetate-foil replica technique. The third group of specimens includes flat specimens preliminary notched by FIB-technique and then subjected to pure-bending fatigue. The study is focused on examining of crack paths in terms of interaction between the propagating short cracks and the microstructure. The obtained data for pure-bending fatigue show higher crack growth rates and shorter fatigue lifetimes than those found for rotating-bending fatigue. In comparison, the registered tension-tension fatigue data present the lowest crack growth rates, due to much lesser loading than that applied at rotating-bending and pure-bending fatigue. Based on data obtained, a Parabolic-linear model ``Crack growth rate - Crack length`` is used for describing and predicting adequately short crack propagation under the specified three types of fatigue. The model is supported by a comparison between the predicted and the actual fatigue lifetimes.Item Finite Element Modeling of ECAP-Linex Combined Process of Severe Plastic Deformation(2023-01-01) Panin E.; Volokitina I.; Volokitin A.; Naizabekov A.; Akhmetova G.; Lezhnev S.; Tolkushkin A.; Esbolat A.; Yordanova R.The paper presents theoretical studies of a new deformation process combining the stages of equal-channel angular pressing (ECAP) and the ``Linex``scheme. For correct finite element modeling of the process, a technique with sequential input for the calculation of conveyor links is presented. To analyze the efficiency of metal processing, the main parameters of the stress-strain state are considered: equivalent strain, equivalent stress, and average hydrostatic pressure, as well as the deformation force on the main elements of the combined process: pulley, matrix, and conveyor link. To analyze the resulting deformation forces, the stages of pressing in a matrix and compression by a chain conveyor were separately considered. Equations for determining the forces acting on the drive pulley, ECA matrix, and the chain element link were obtained. Comparison of values showed that the force values in the calculation and simulation have a high level of convergence. In all three considered details, the difference value did not exceed 10%. The variational modeling allowed to determine the optimal values of the main technological and geometric parameters of the process.Item INFLUENCE OF Fe-Si-Mg-REM-INOCULANTS ON THE MICROSTRUCTURE OF IRON CASTINGS WITH VERMICULAR GRAPHITE(2022-01-01) Boldyrev D.; Nefedyef S.; Dema R.; Kharchenko M.; Chernyi S.; Suvorova E.; Mihailov E.; Latypov O.; Yordanova R.Complex additives, inherently, are not only inoculants, but also complex alloyed products having their own specific structure, whose influence on the crystallization processes of an alloy is actually not taken into account or studied. The work investigates inoculants made by two technologies. According to the first technology, an inoculant is made in the form of an ingot by crystallizing melt in a mold, according to the second technology, an inoculant is made in the form of an extra-thin ingot (0.5 - 3 mm thick plates) by continuous casting and freezing on water-cooled molds (drums). A phase electron and X-ray spectral analysis of the inoculants’ structure are carried out to produce vermicular graphite iron. Thus, the paper presents a research on the microstructure of CGI castings “Countershaft” molded in a ladle with the use of inoculants and describes specific features of their influenceItem INVESTIGATION OF THE POSSIBILITY OF DETERMINING THE THERMAL CONDUCTIVITY COEFFICIENT OF A356 FOAM BY MATHEMATICAL MODELING AND IDENTIFICATION PROCEDURE(2022-01-01) Georgiev G.E.; Velikov A.; Krastev B.; Popov S.; Stanev S.; Yordanova R.; Manolov V.A method for determining the thermal conductivity coefficient of a foam material of A356 aluminum alloy has been proposed. The foam material has been produced by decomposition of titanium hydride introduced into the melt and subsequent cooling and crystallization. During the process, the measurement of temperature dependences on time has been performed at two selected points of the foam casting. The problem of heat transfer and crystallization of the casting and mold is solved by means of mathematical model proposed. The experimental data and the corresponding solutions of the mathematical model have been used for the realization of an identification procedure for determining the coefficient of thermal conductivity of the foam material and the coefficient of heat transfer at the casting-mold boundary. Through the represented methodology new data for important thermophysical characteristic are obtained.Item Investigation on fatigue behavior and fatigue crack growth of spring steel. Part I. Wöhler curve and fracture surfaces(2014-02-25) Nikolova L.; Yordanova R.; Angelova D.Fatigue behavior of spring steel at different stress ranges was investigated. The specimens were machined in hour-glass shape with polished surface and were subjected to symmetric cyclic rotating-bending fatigue in air and room temperature to fracture. Fatigue life time data was obtained and Wöhler curve of the investigated steel at different conditions is plotted. A mathematical description of the obtained results is presented. The fatigue test results are filled out with images and analysis of the fracture surface made with scanning electron microscope. A replication technique is used for short fatigue-crack growth monitoring for two specimens. The experimental data are presented in plots ``Crack lengths, a - cycles, N``.Item Modelling of the mechanical properties of steel 316L after EB treatment(2018-10-19) Dzharov V.; Koleva E.; Vassileva V.; Yordanova R.; Yankova S.Investigation of the variation of the mechanical properties of 316L steel after electron beam surface modification is performed. The samples were processed at an installation in IE BAS - ELIT - 60. The irradiated plates were with thickness of 0.5 mm. The experiments were carried out with a change in the beam power in the range of 0.6 to 0.9 kW and different irradiation times between 1 to 120 seconds. Regression analysis was performed on the obtained results.Item On factors influencing fatigue process in steel 316L used in hydrogen energy technologies(2014-02-25) Angelova D.; Yordanova R.; Lazarova T.Investigations of fatigue in steels exposed to hydrogen media is extremely important problem. In this work, an austenitic stainless steel ASTM 316L resistant to hydrogen destructive influence is examined. The experiments presented have used hydrogen charged and uncharged specimens and were carried out under rotating bending and tension-compression fatigue in three different laboratories: at The University of Chemical Technology and Metallurgy, Sofia, Bulgaria; at Sandia National Laboratory, California and The University of Tufts, Medford, Massachusetts, USA; The Institute Hydrogenous at Kyushu University, Japan. The results are presented in Wöhler curves complemented by ``Short fatigue crack length - Number of cycles`` curves and ``Frequency - Lifetimes`` plots, and compared respectively.Item On fatigue behavior of two spring steels. Part I: Wöhler curves and fractured surfaces(2016-06-01) Angelova D.; Yordanova R.; Lazarova T.; Yankova S.Symmetric fatigue in two spring steels is investigated in three groups of specimens. One of the groups (Steel EN10270-1SH/ DIN 17223C – C 0.82%, Mn 0.76%, Si 0.26%) has experienced rotating-bending fatigue in air, and the other two groups (Steel BS250A53/ DIN 55Si7 – C 0.56%, Mn 0.81%, Si 1.85%), torsion fatigue in-air and corrosion environment. Fatigue life-time data are obtained for both spring steels, and their corresponding Wöhler curves plotted and mathematically described. Surface short fatigue cracks are observed from origin to fracture by using acetate-foil replication technique, and their length, a, measured at the corresponding number of fatigue cycles, N. Those data are presented in plots “Crack lengths, a – Cycles, N” and a comparison made between both the steels. The fractured surfaces of all specimens have been studied and analyzed by the scanning-electron microscopy methods.Item On fatigue behaviour of two spring steels. Part II: Mathematical models(2016-06-01) Angelova D.; Yordanova R.; Lazarova T.; Yankova S.Symmetric fatigue in two spring steels is investigated in three groups of specimens. One of the groups (Steel EN10270-1SH/ DIN 17223C – C 0.82%, Mn 0.76%, Si 0.26%) has experienced rotating-bending fatigue in air, and the other two groups (Steel BS250A53/ DIN 55Si7 – C 0.56%, Mn 0.81%, Si 1.85%), torsion fatigue in-air and corrosion environment. All experiments include testing to fracture, applying acetate-foil replication technique, replica monitoring of short crack surface growth, length measuring of propagating cracks, a, at the corresponding number of fatigue cycles, N. Data obtained from replica monitoring are presented in plots “Crack lengths, a – Cycles, N”, and used for calculating fatigue crack growth rates, da/dN, and graphical presentations “Crack growth rates, da/dN – Crack lengths, a”. A mathematical description of da/dN – a is presented by introducing a parabolic-linear model in different versions for each of the steels. The model versions are verified through comparing the experimental fatigue lifetimes with those calculated by the proposed model version.Item On monitoring mechanical characteristics of rolled electrolytic copper(2016-01-01) Angelova D.; Yankova S.; Yordanova R.; Atanasova G.Rolling technology for producing high-electrical-conductivity-copper strips and their quality are under investigation. The technology is designed for and used in the Bulgarian Metallurgical Plant SOFIA MED SA, Sofia. The copper strips are produced in three different tempers - soft, half hard and hard - under different rolling and heat-treatment conditions. The copper strips from different tempers are subjected to specialized testing and their mechanical and high-electrical-conductivity characteristics analyzed. On the basis of mechanical rolled-strip parameters - yield strengths, R e , ultimate tensile strengths, R m , Vickers hardness, HV, and elongation after fracture, A, - some Stress-hardness and Stress-hardness-elongation spaces have been plotted. These Spaces can be used as an instrument for general evaluation of the applied rolling technology and for prediction of copper-strip mechanical behaviour under given exploitation conditions.Item On monitoring of mechanical characteristics of hot rolled S355J2 steel(2016-01-01) Angelova D.; Yordanova R.; Georgiev A.; Yankova S.Hot rolling normalization technology for producing sheets from low-carbon Steel S355J2, used in the Bulgarian Metallurgical Plant ``STOMANA Industry SA`` is under investigation. A newly introduced automatic application optimization procedure to this technology is an important step that leads to avoiding traditional heat treatment, improving steel mechanical characteristics, increasing production efficiency, all resulting in high quality final products. On the basis of the final mechanical rolled-sheet characteristics - yield strengths, Re, ultimate tensile strengths, Rm, absorbed energies in impact tests, K, and elongations after fracture - some energy-stress and energy-stress-elongation constructions-spaces have been plotted. These spaces can be used for general evaluation of the applied rolling technology and for prediction of steel-sheet mechanical behaviour.Item On monitoring of mechanical characteristics of hot rolled S355J2 steel(2016-06-01) Angelova D.; Yordanova R.; Georgiev A.; Yankova S.Hot rolling normalization technology for producing sheets from a low-carbon steel, Steel S355J2, used in the Bulgarian Metallurgical Plant “STOMANA Industry SA” is under investigation. A newly introduced automatic application optimization procedure in this technology is an important step that leads to avoiding of traditional heat treatment, improving of steel mechanical characteristics, increasing of production efficiency, all resulting in high quality final products. On the basis of the final mechanical rolled-sheet characteristics - yield strengths, Re, ultimate tensile strengths, Rm, absorbed energies in impact tests, K, and elongations after fracture - some energy-stress and energy-stress-elongation constructions-spaces have been plotted. These spaces can be used for general evaluation of the applied rolling technology and for prediction of steel-sheet mechanical behaviour.Item Recycling of metal scrap of ferrous metals using mini-mill of radial-shear rolling and drawing mill(2020-01-01) Lezhnev S.; Naizabekov A.; Panin E.; Volokitina I.; Turabaeva M.; Yordanova R.; Sal'ko O.Re-processing of scrap metal is one of the most important directions in the production of metals and alloys, as well as in the manufacture of various metal products. This activity is called the recycling. Recycling of scrap metal can be carried out, both by melting it, or by technology, currently gaining distribution in some metallurgical and engineering plants and which includes the processing of end-of-life metal products by hot forming. In this paper, the technology of recycling of ferrous metal scrap in the conditions of Akmola branch of JSC ``Kazvtorchermet``, in order to obtain high-quality commercial product in the form of bars of circular cross-section or wire, is proposed.Item REDUCTION OF METAL CONSUMPTION OF ROLLED PRODUCTS(2023-01-01) Chigirinsky V.; Volokitina I.; Bozhko L.; Ovchynnykov O.; Yordanova R.Weight significantly affects the strength of products and technological parameters of production. Section weight determines temperature mode, power parameters of rolling, conditions of thin-walled section formation. In our solution, the central zone was lightened by transverse stiffeners. This made it possible to reduce the weight of the product while ensuring reliable performance characteristics. Wheels made of sections rolled in Rolling Mill 550 conditions successfully passed road and strain gauge tests.