Browsing by Author "Yankova S."
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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 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 Influence of the transversal displacements of metal on the camber formation of hot-rolled strip(2017-01-01) Belskiy S.; Yankova S.; Mazur I.; Stoyakin A.Transversal displacements of metal in the hearth of deformation reduce unevenness of elongations across strip width, wherein the camber curvature becomes smaller, and its radius is larger than that calculated on the assumption of plane deformation scheme. The investigations are based on the variational principle of possible changes of strain state (Jourdain principle). It is assumed that the input velocity unevenness of the metal flow in the hearth of deformation across the strip width passes into an output one proportionally to reductions. Further, velocities and powers of deformation and displacements of metal in the hearth of deformation during hot rolling and corresponding powers are calculated. Solving Euler-Poisson's equation, we receive the expression for coefficients, which take into account the influence of the transversal displacements of metal on decreasing the output unevenness and the radius of camber curvature for every harmonic.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 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 Temperature field of stripes under hot rolling(2015-01-01) Belskiy S.; Yankova S.; Chuprov V.; Bakhaev K.; Stoyakin A.The study of temperature distribution along width of stripes in the Hot Rolling Mills HRM 2000 line was carried out in collaboration of NLMK Hot Rolled Production specialists with students and lecturers of the Lipetsk State Technical University. This study was made using a portable thermal scanner BALTECH TR-1400. It was marked that the surface temperature increased from the right edge of the strip to the left in relation to rolling direction. The temperature distribution asymmetry along the width of the strip is one of the reasons for camber, wedging and flatness of hot-rolled strips.Item Thermal behavior and melt fragility number of Cu100-x Zr x glassy alloys in terms of crystallization and viscous flow(2009-01-01) Russew K.; Stojanova L.; Yankova S.; Fazakas E.; Varga L.K.Six Cu100-xZrx amorphous alloys (x in the range 35.7 - 60 at. percent) were prepared via chill block melt spinning (CBMS) method under low pressure Helium atmosphere. Their crystallization and viscous flow behavior was studied with the aid of Perkin Elmer DSC 2C and Perkin Elmer TMS 2 devices, respectively. The viscous flow temperature dependencies at a heating rate of 20 K min-1 were interpreted on the basis of the f ree volume model. The DSC and TMS data were used to determine the fragility number m of Angell in three different ways as a function of alloy composition. It has been shown that the fragility number goes over a maximum and has a minimum at x very near to the alloy composition Cu64Zr36 in good agreement with the results of Donghua Xu et al. and Wang D et al. The experimental techniques and model interpretation used provide a tool for understanding the glass forming ability (GFA) and relaxation phenomena in metallic glasses. © 2009 IOP Publishing Ltd.