Browsing by Author "Krastev D."

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    About the surface hardening of tool steels by electrical discharge treatment in electrolyte
    (2010-01-01) Krastev D.; Yordanov B.
    In this study are given some results about the influence of electrical discharge treatment in electrolyte on the microstructure of high speed steel HS 6-5-2. The metallic surface after electrical discharge treatment in electrolyte has a different structure in comparison with the metal matrix, which determines different properties. It is observed remarkable increasing of hardness, strength and corrosion resistance related to the nonequilibrium phase transformations and the obtained finecrystalline microstructure. © (2010) Trans Tech Publications.
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    Formation of molybdenum carbide during carbidization of molybdenum chips with powdered carbon at electrothermal rotary bed condition
    (2014-02-25) Yordanov B.; Krastev D.; Davidkov A.; Klyavkov K.
    The ability of electrothermal rotary bed (ETRB) which is composed from metal chips and powdered carbon particles as an electroresistant self-heating mixture to accomplish a high temperature carbidization process has been studied. X-ray analysis and microstructure of the molybdenum carbide phase formed during carbidization of Mo-chips with powdered carbon in an ETRB condition process has been determined, and the microhardness of the final product (Hμ = 20050 MPa) was measured. The ability to use scrap-metal or chips from previous work as starting components to obtain molybdenum carbide in an elecrothermal rotary bed is proved.
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    HEAT TREATMENT INFLUENCE ON THE ABRASIVE WEAR RESISTANCE OF HYPOEUTECTIC CROMIUM CAST IRON
    (2022-01-01) Yordanov B.; Krastev D.; Mitov I.
    In the present work the variation of hardness and wear resistance of a machine specially designed to simulate the service conditions of abrasive wear were investigated depending on the type of the heat treatment of a hypoeutectic chromium white cast iron with composition C - 2.19 %, Mn - 0.46 %, Si - 0.51 %, Cr - 11.6 %, P - 0.031 %, S - 0.046 %, Ni - 0.23%, Mo - 0.077 %, V - 0.052 % and Al - 0.006 %. Maximum hardness of HRC 67 after quenching at 980ºC in oil and tempering at 200ºC is obtained which is a heat treatment suitable for workpieces operating under frictional and grinding conditions without high impact and dynamic loads. The wear rate after quenching at 980ºC in oil and subsequent low temperature tempering at 200ºC is V = 2.42.10-5 g m-1 and the abrasive wear resistance E = 41284. In cast condition the values of these indices are V = 1.33.10-5 g m-1 and E = 75157, respectively, and reduced to V = 0.30.10-4 g m-1 and E = 3331 after annealing at 870ºC and 180 min holding.
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    OBTAINING OF IRON AND COPPER BY DIRECT REDUCTION OF THEIR MILL SCALE WASTES AT ELECTROTHERMAL ROTARY BED
    (2021-01-01) Yordanov B.; Krastev D.
    In this work some results of direct reduction process of iron and copper wastes from low carbon steels hot-rolling and ETP copper slabs with a powder graphite as a reducer to obtain a pure iron and copper are presented. The reduction in a furnace operating on the principle of electrothermal rotary bed (ETRB) at temperature range from 600°C to 800°C for copper and from 950°C to 1050°C for iron scale and isothermal hold for 60 min is carried out. Necessary heat for the process and reduction atmosphere are generated directly in the working space of the furnace when electrical current with industrial frequency passes through the reaction mixture which is composed from grinded copper or iron scale and graphite powder. The quantity of the reducer is above the stoichiometrically required for the reduction is taken. Cooling process by passing through reaction mixture of Ar as an inert gas is carried out to the temperatures 80°C - 100°C. The products of iron oxides reduction at 1050°C, size +1 mm, show completely reduced iron powder achieved for the time of isothermal hold of 60 min. The reduction process of copper scale at temperature 800°C, 60 min of isothermal hold for fractions -0.63 + 0.8 mm, is completely finished and the product consists of small globular copper particles with technical purity.
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    Recast layers on high speed steel surface after electrical discharge treatment in electrolyte
    (2016-06-01) Krastev D.; Paunov V.; Yordanov B.
    In this work are discussed some experimental data about the obtaining of recast layers on the surface of high speed tool steel after electrical discharge treatment in electrolyte. The electrical discharge treatment of steel surface in electrolyte produces a recast layer with specific combination of structure characteristics in result of nonequilibrium phase transformations. The modification goes by a high energy thermal process in electrical discharges on a very small area on the metallic surface involving melting, alloying and high speed cooling in the electrolyte. Obtained recast layers have a different structure in comparison with the metal matrix and are with higher hardness, wear- and corrosion resistance.
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    Surface modification of steels by electrical discharge treatment in electrolyte
    (2014-02-25) Krastev D.; Paunov V.; Yordanov B.; Lazarova V.
    In this work some experimental data about the influence of applied electrical discharge treatment in electrolyte on the surface structure of steels are discussed. The electrical discharge treatment of steel surface in electrolyte gives a modified structure with specific combination of characteristics as a result of nonequilibrium transformations. The modification is performed by a high energy thermal process in a very small volume on the metallic surface involving melting, vaporisation, activation and alloying in electrical discharges, and after that cooling of the surface with high rate in the electrolyte. The surface layer obtains a different structure in comparison to the metal matrix and it possesses higher hardness, wear resistance and corrosion resistance.
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    SYNTHESIS OF NOVEL NOCICEPTIN ANALOGUES: INVOLVEMENT IN ANALGESIC EFFECTS OF OPIOID AND NITRIC OXIDEERGIC SYSTEMS AFTER COLD STRESS
    (2021-01-01) Himcheva I.; Angelova N.; Naydenova E.; Stavreva G.; Krastev D.; Kochev D.; Bocheva A.
    Stress is known to exert an influence on neuroendocrine, autonomic, hormonal, and immune functioning. Various stress models have been reported to induce analgesia. This is a phenomenon, referred to as stress-induced analgesia. Nociceptin and analogues are neuropeptides, neuromodulators, which are able to inhibit the expression of some forms of SIA. Nociceptin/Orphanin FQ(N/OFQ) is a heptadecapeptide which has been found to play a direct role on pain perception. Nitric oxide (NO) plays an important role in initiation and maintenance of pain. It is also known that acute and chronic stresses induce biochemical changes affecting both pain threshold and behaviour. Thus, endogenous opioid peptides and NO, mediated a wide variety of physiological processes including pain transmission and SIA. The aim of the present study was to investigate the effects of novel analogues of N/OFQ(1-13)NH2, where Lysine (Lys) at position 9 and/or 13 was substituted by L-ornithine (Orn) on nociception after cold stress and the involvement of the opioid and nitric oxideergic systems in these effects. Analgesic activity was examined by nociceptive test - pawpressure (PP). All novel analogues of N/OFQ were injected at a dose of 10 p.g kg-1; naloxone (Nal, 1 mg kg-1), JTC-801 (NOP receptor antagonist, 0,5 mg kg-1), NG-nitro-L-arginine methylester (L-NAME, 10 mg kg-1) and L-arginine (L-Arg, 1mg kg-1). All drugs were dissolved in saline and were injected intraperitoneally (i.p.). The nociceptive tests were performed 10 min after peptide injection. Antinociceptive effects were statistically accessed by ANOVA. In conclusion we suggest that in analgesic effects of the novel analogues of nociceptin were involved opioid-, nociceptin- and nitric- oxideergic systems after cold stress.
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    Technological research on converting iron ore tailings into a marketable product
    (2021-05-01) Mitov I.; Stoilova A.; Yordanov B.; Krastev D.
    We present three technological scenarios for the recovery of valuable components from gangue, stored in the tailings dam at Kremikovtzi metallurgical plant in Bulgaria, into marketable iron-containing pellets. In the first approach the iron concentrate was recovered through a two-stage flotation process, desliming, and magnetic separation. In the second proposed process, the iron concentrate was subjected to four sequential stages of magnetic separation coupled with selective magnetic flocculation. The third route entails the not very common practice of magnetizing roasting, followed by selective magnetic flocculation, desliming, and magnetic separation. The iron concentrate was pelletized in a laboratory-scale pelletizer. Each technology has been assessed with regard to the mass yield of iron concentrate, the iron recovery. and the iron, lead, and zinc content in order to identify the most effective route.