Titanium. Problems of production. Prospects. Analytical review. Part 3.
In melting in the induction furnace heating and melting of the metal occurs due to induction of alternating electromotive force and creation of eddy currents, which are transformed into heat directly released in the metal. The melted metal is intensively mixed due to occurring electrodynamic forces in it. That's why within the whole mass of the melt the required temperature is maintained with the least melting loss in comparison with different methods of arc and electron beam melting.
The most important advantage of the induction furnaces, stipulated by generation of heat inside the melted metal, turns into disadvantage in case of their application for the refining melting. Slags with a comparatively low electric conductivity get heat in the induction furnaces from the molten metal, which hinders processes of the metal refining. This stipulates application of the induction melting furnaces mainly in foundries.
Vacuum-induction furnaces are mainly used for melting of the charge, protection of the melt from influence of atmosphere, bringing it up to the necessary condition (in general case the area of the interphase surface being the same, the content of impurities in the melt is proportional to duration of seasoning in vacuum), and for casting of metals and alloys in vacuum.
Vacuum-induction melting, as well as other methods, is used in production of titanium ingots . In addition to triple VAR of a consumable electrode, it is possible to use the following schemes with participation of the vacuum-induction melting as VIR + ESR + VAR or VIR + VAR + EBR. Here vacuum-induction melting is the process used for preliminary refining of the metal and compaction of the charge materials into the consumable electrode, which has maximal chemical homogeneity, for subsequent remelting by <<quasi-equilibrium>>processes--VAR, ESR, and EBR.
Designs of the vacuum induction furnaces are simpler than of the vacuum arc ones [2, 3]. As a rule, in the jacket there are a chamber with a melting inductor and a crucible, a mechanism for its turning, and a casting chamber, in which one or several moulds are located. Because of interaction of the metal with material of the melting crucible lining, duration of the melt seasoning in VIR is limited by several minutes. Interaction of the metal being melted with the crucible material in VIR is more intensive than in open induction melting . In the induction melting processes requirements to the crucible material are practically the same as in the skull melting processes.
The need in a fully chemically inert unit for melting of active metals, in particular titanium, enabled development of vacuum-arc, electroslag, electron beam and plasma-arc processes of remelting in copper water-cooled moulds. The induction method was not exception either.
As far back as in 1920s design of the induction furnace with a water-cooled crucible from electricity conducting metal was suggested. Later such units started to be called cold crucibles . In order to ensure possibility to transfer energy from the inductor to the melt, the wall of the cooled crucible was made in the form of separate sections, electrically insulated from each other. Molten metal from the cold crucible was poured into moulds.
Development of the process of ingot production in cold crucibles with application of the induction heating is described in . In this work the conclusion was drawn on principal possibility and prospects of using process of the titanium waste remelting in a sectional mould.
Induction remelting in the sectional mould has its peculiarities. In contrast to vacuum-induction and induction melting in conventional or cold crucibles, when an ingot is produced by casting the whole accumulated mass of the metal into the mould, in remelting in a sectional mould, like in all other kinds of remelting in a water-cooled mould, the process of simultaneous melting of the fed metal and solidification of the produced melt takes place. The ingot may be formed by means of relative movement of the inductor and the sectioned mould. Metal pool in the latter acquires because of influence of the electromagnetic forces and pressing-out of its upper part from the mould walls a shape of a cupola. Both consumable electrodes and different lumpy and loose materials may be subjected to remelting in such mould. The process may proceed under a layer of slag and in the atmosphere of inert gases or vacuum.
In first designs of the induction sectioned moulds special measures for isolation of the sections from each other were no envisaged. Into the interspaces between sections of the induction mould aluminous isolating caps for thermocouples were placed, which also acted as seals that prevented pouring out of the melt through the interspaces between the segments . Such designs did not allow melting titanium ingots of more or less significant diameter.
Increase of the installation power and diameter of the mould from 63.5 to 355.6 mm caused constant emergencies because of occurrence of arc discharges in the interspaces between sections of the mould and short-circuiting of the latter by the molten metal, which showed impossibility of the process performance without reliable isolation of the sections from each other. Experience of the electroslag remelting, in which an ingot is formed in a thin crust of the slag skull, stipulated application for this purpose of the slag that allowed avoiding mentioned problems. Molten slag filled interspaces between the sections and, having solidified, acted as a reliable isolation of the section and the metal of the ingot being formed.
Induction-slag remelting was used in melting of titanium ingots of small diameter. It should be noted that presence of slag does allow performing remelting at reduced pressure. At the pressure below 75 mm Hg strong evaporation of slag was registered, that's why removal of hydrogen in this method of remelting occurs less efficiently than in VIR or VAR. In addition, it is inexpedient to use in induction-slag remelting the schemes with drawing of an ingot from the mould . During movement of an ingot relative the mould wall tear of the slag crust is possible, due to which short-circuiting of the section by molten metal occurs.
State-of-the-art designs of the sectioned moulds envisage isolation of the section by a layer of glass cloth. In addition, external surface of the mould sleeve is covered by the glass cloth, which imparts to it additional rigidity and insulates the sleeve from the inductor. Prevention of break-downs between the sections because of their short-circuiting by melt of the metal being remelted is achieved by application of the matching high-frequency transformers, maintaining on the inductor of a reduced (80-100 V) voltage, and application of a big number of sections. While in design of the mould, described in , there are four such sections, in state-of-the-art sectioned moulds there may be 15, 20, and more sections . In case of increase of the number of sections, voltage between them significantly (by one order) reduces. Moreover, the process is performed in such way that solidification front of the melt to be at the lower turn of the inductor. Molten metal, located above this level, is pressed-out from the mould walls and does not contact with them, forms a "cupola", and the ingot being formed finds itself located outside the area of the inductor influence and does not represent hazard from the viewpoint of a break-down between separate sections due to their short-circuiting by the ingot metal at voltage 4-5 V. Such approach allows refusing from application of slag as an insulator and using possibility of operation at reduced pressure or in shielding gases. Because of a characteristic "cupola", area of the metal pool surface in induction remelting in the sectioned mould exceeds section of the ingot being melted, i.e. the metal pool has a developed surface. Intensive mixing of the melt and developed surface of the metal pool enable intensification of the melt degassing process.
As far as possibility of the induction remelting of titanium in a sectioned mould is concerned, remelting of the consumable electrodes, pressed from titanium sponge, is not used anymore because of clear advantage of the technology with application of the loose titanium sponge, when it is fed into the mould from aside. A promising direction is considered remelting of different kinds of wastes--chips, cuttings, parts, which got unfit for service, for example, flanges of the casting fitting-out of spun casting machines , and gas-saturated titanium sponge [8, 9].
By the induction remelting method in the sectioned mould ingots of small and medium diameter (up to 300-350 mm) are, as a rule, produced. Production of bigger ingots is inexpedient because of the need to use powerful generators supplying the induction remelting installations. And, while application of such generators in VIR, when capacity of the furnace constitutes dozens tons and melting of the metal is performed within a comparatively short time interval, is justified, in remelting in the sectioned mould in case of formation of an ingot at rates not more than several millimeters per minute, increase of the generator power above a certain value becomes economically disadvantages.
Big titanium ingots may be produced by the method of electron beam remelting, in the process of which heating and melting of the metal proceeds, like in VIR, due to generation of heat directly in the metal being remelted, but in this case owing to treatment of the surface by a flow of electrons, kinetic energy of which during their collision with surface of the material being radiated transforms into heat energy of movement of the particles.
Flow of electrons is produced due to the effect of thermoelectron emission of the heated cathode. As a rule, tungsten and tantalum are used as the cathode material of the electron beam gun. For imparting to the electrons necessary kinetic energy acceleration voltage from several kilowatts to several dozens kilowatts (the higher is voltage, the higher is energy of the electrons) is used, whereby positive lead-out of the high-voltage power source is electrically connected either with the material being radiated, or with a non-melting cooled anode.
For efficient application of the produced flow of electrons (the electron beam) high vacuum is required, as far as because of small mass of the electrons their flow on the way to the material being radiated dissipates as a result of collision with other particles. In the course of collision of the electrons with molecules of the residual gases a portion of the energy is lost, whereby the processes of ionization and recombination of the particles occur, which create visible radiation. Way of the beam gets visible.
Interaction of the electrons with ionized gas enables additional dissipation of the beam. All this takes 1-15 % of the beam energy. More significant (from 6 to 25 %) losses of energy occur in direct collision of the electrons with surface of the metal, and they are stipulated by reflection of a portion of the beam electrons from the metal surface and occurrence of secondary electron emission and X-ray radiation .
Power of X-ray radiation in EBR is low (several shares of a percent of the installation power), but its biological action represents a serious hazard for the operation personnel. That's why range of the acceleration voltage values is limited by 35-40 kV. Thickness of steel walls of the installation is selected, taking into account reliable shielding of the personnel against radiation (as a rule 10-15 mm).
At acceleration voltage up to 40 kV depth of penetration of the electrons into metals does not exceed several micrometers. But because of high density of the electron beam energy (by three orders higher than that of the electric arc) the radiated metal not just melts but intensively evaporates, due to which on way of the beam a thin channel is formed, depth of which exceeds several thousands times depth of the electron diffusion.
In electron beam melting and remelting of metals such effect is harmful. That's why for dispersion of the electron beam energy all over the treated surface electromagnetic systems are used, which deviate the beam according to the preset program and ensure necessary trajectory of the heat spot movement. For uniform heating of the surface and ensuring minimum melting loss it is necessary to scan the beam at the frequency above 50 and sometimes more than 100 Hz [11, 12], whereby overheating of the metal pool surface is approximately by 100-150 [degrees]C higher than in VAR .
Two principally different systems of electron guns for melting of metal are known: without an acceleration anode (with a ring cathode) and with it. In case of melting by the electron guns without the acceleration anode, when voltage is applied between the cathode and the material being heated, electron beam is formed in the space immediately adjacent to the surface being heated. This allows obtaining a powerful electron beam at relatively low acceleration voltage. Low values of the acceleration voltage (several kilowatts) determine lower levels of X-ray radiation and cost of the power source. The electron beam guns without the acceleration anode have simple design and are easy in servicing. In guns of this type distance between the cathode and surface of the billet being melted or of the metal pool in the mould should be comparatively small and correspond to the acceleration voltage.
Close location of the cathode unit to the surface of the metal being remelted causes getting on the cathode and the focusing electrode of the molten metal vapors and spatter [10, 11]. Presence of the electrical field in the area of melting causes intensified ion bombardment of the cathode unit and frequently occurrence of glow discharges. As a result stability of the process is disturbed, and service life of the cathode usually does not exceed several hours.
Common for formation of the beam and vacuum treatment of the melt chamber stipulates necessity to maintain in it significantly lower values of the residual pressure of the gas-vapor phase than it is necessary in performance of the degassing and melt distillation processes. For ensuring of stable operation of the electron beam guns one should not allow even for a comparatively short time increase of pressure in the chamber above 1 x [10.sup.4] mm Hg . That's why invariable attribute of EBR is a highly productive vacuum system, consisting of a great number of valves, gates, and pumps of different types (diffusion, booster, and roughing-down ones). As power of the electron beam guns and productivity of the melting installation grows up, requirements to productivity of the vacuum system sharply increase. An important parameter in this case is ratio of the rate of pumping out of the diffusion pumps of the installation to power of the latter. For installations of different types this ratio constitutes 50-200 l/(s-kW).
Application of a more perfect system of the guns with the acceleration anode, which makes it possible to use so called system of differential pumping-out, at which degassing of a small space before the acceleration anode and of the melting chamber proper (behind the anode) is performed separately, allows reducing load on the vacuum system. Pressure levels in the melting chamber and in the chamber, in which the cathode is located, may differ by 2-3 orders .
For efficient application of the produced electron beam and vacuum treatment of the melt, pressure in the melting chamber of the installations with differential pumping-out may be significantly higher than in the installations without the acceleration anode, however in this case it should not exceed 1 x [10.sup.-2] mm Hg either.
Electron beam guns with the acceleration anode may be placed at a significant distance from the melting metal (usually at 800-1500 mm). It enables reduction of the amount of vapors and spatter of the molten metal getting on the anode unit. Absence of electric field in the melting zone and significant reduction of influence of sudden gas releases on operation of the cathode--anode system significantly reduce influence of the glow discharge occurrence. As a result service life of the cathode of such guns equals 100 h and more .
In the space behind the anode one can easily control the electron beam by means of electromagnetic fields (focalize, deviate, move it according to a preset program) and ensure in this way required conditions of heating. There are three varieties of electron beam guns with the acceleration anode: annular, axial, and flat-beam ones. Their principal difference consists in the form of a generated beam. Electromagnetic systems of such guns deviate the electron beam, when it is directed at the metal, at the angle 45, 60 and sometimes up to 180-270 [degrees]. This makes it possible to place guns sideways and even from below from the heated surface, protecting the cathode unit from vapors and spatter of the metal. It should be noted that axial and flat-beam guns as the most perfect units allow implementing to the greatest degree principle of the heat source independence, which radically distinguishes EBR from VAR.
By means of the independent heat source one may regulate duration of the melt stay in the molten state that affects efficiency of its refining. Moreover, the melt is subjected in EBR to the action of a deeper vacuum and higher temperature than in VAR. Big depth of the furnace melting space pumping-out in EBR improves conditions of the degassing, distillation and dissociation processes of the non-metal inclusions. Increase of the melt temperature significantly affects mainly processes of evaporation of the elements. However, overheating and long seasoning of the melt parallel with removal of harmful impurities causes losses of the metal (the base and the alloying elements). Dependence of these losses upon power of the electron beam installation and specific consumption of electric power is practically a linear one .
Electron beam melting of the titanium sponge does not render significant influence on content of main impurities . Concentration of oxygen and nitrogen is at the level of the initial metal . This is connection with high pressure of the titanium vapor and thermodynamic stability of titanium oxides. Hydrogen, weight share of which in EBR reduces 3-6 times, i.e. significantly higher than in VAR, is the exclusion. As a result impact toughness of the EBR metal is 2 times higher than that of the VAR metal. In melting of the titanium alloys significant reduction of content of the alloying components with a higher pressure of vapor than that of titanium, for example, manganese, aluminium, chromium, etc. occurs, especially in melting of the lumpy charge. First of al it concerns aluminium, because it is an alloying element of practically all titanium alloys. The evaporation loss may be reduced by application of a consumable billet instead of the charge or addition of respective hardeners in melting.
Intensification of the processes of titanium refining from nitrogen and oxygen is enabled by increase of the reduced reaction surface that is implemented in the remelting schemes with an intermediate unit. Application of the latter envisages longer stay of the melt in molten state, whereby averaging of the chemical composition, removal of the refractory inclusions from the melt due to their deposition on bottom of the unit, floatation of the non-metal inclusions of low density and their dissociation under the electron beam action take place. In addition, such scheme allows using as an initial charge the scrap (up to 100 %) and spongy titanium of somewhat poorer quality and melting of ingots-slabs [13, 14] and hollow ingots .
Application of such complex and expensive process as EBR should be justified either by impossibility of producing high-quality metal by some other method or achievement of advantages at the process stage . As one can see, there are many other methods besides EBR for production of titanium, but the latter one showed nowadays its efficiency in reprocessing of the titanium scrap and production of secondary titanium . Possibility of refusing from application of the consumable electrodes (which would require for establishment of the respective production) has predetermined predominated role of this process in titanium industry of Ukraine.
Comparative characteristic of the parameters and qualitative estimation of different methods of melting and remelting of metals and alloys, presented in , showed that the most productive are vacuum-induction and plasma-arc skull melting processes (up to 5 t/h); the least productive are plasma-arc and electron beam remelting processes (up to 1.2 t/h); electroslag and vacuum-arc remelting have intermediate value (up to 2 t/h). From the viewpoint of power consumption the most economical is the process of vacuum-arc remelting (about 1 kW x h/kg), while vacuum induction melting is the most power consuming (about 4 kW x h/kg). Effect of evaporation of the melt components was registered in all processes, whereby intensity of evaporation increases along the chain VIR-VAR-EBR.
Selection of a process for production of titanium ingots is rather individual and depends upon many factors (existing infrastructure of the accompanying production; requirements to quality of the metal; designation of the latter, required volumes of its production, etc.).
Reduction of cost of the titanium products and wider application of titanium in the industry can occur due to the following factors:
* perfection and development of new technological processes of titanium processing (new low-cost methods of the titanium sponge production and reprocessing of the gas-saturated, having increased content of technogenic impurities, sponge, and new methods of melting and remelting);
* organizational-technological measures connected with unification of different processing stages, organization of the vertically integrated production, and involvement into reprocessing of different wastes of the titanium industry;
* respective quality of the metal and its designation.
Establishment of a big titanium company VSMPO-Avisma in Russia is a bright example. Unification of two stages of the metallurgical processing allowed reducing production cost of the products by 10-12 % . The VSMPO-Avisma and Boeing companies announced in 2006 about establishment of the joint venture, which will perform machining of the stamped items from titanium for their subsequent application in the <<Boeing>> passenger aircraft . Waste metal of the production (titanium chips) will be returned to VSMPO-Avisma for their subsequent reprocessing and involvement into the production cycle.
For ensuring of the required properties content of impurities in titanium and its alloys is limited. So, weight share of oxygen in the deformable titanium alloys does not exceed 0.30, of nitrogen 0.15 and of hydrogen 0.015 %. The most gas-saturated is VT1-2 alloy (general content of these impurities in it equals 0.46 %) designed for fabrication of different bars. Because of high content of impurities this alloy has approximately 2 times higher value of ultimate tensile strength than similar to it VT1-00 and VT1-0 alloys (commercial titanium). Lately it is sometimes required to supply to the consumers ingots of titanium alloys with increased content of oxygen, i.e. with somewhat increased due to this strength of the alloy (Figure).
Evidently, both producers and consumer of titanium started to understand necessity of the compromise between purity of the alloy and its designation and, therefore, the method of production and cost. High cost of the titanium products at low cost of the initial feedstock is determined by expenses of the process stages. And, probably, there is no need at early stages of production to achieve high purity of the metal, spending for this purpose significant resources, and then to get in the ready metal increased contents of various kinds of impurities. It is more expedient to arrange the production chain in such way that required quality of the metal, for example, increased content of oxygen, be achieved at once at minimal expenses.
Such situation exists in production of steel, when after development of the blast furnace and the acid Bessemer converter, terminated thousand-year epoch of "pure" steel. Replacement of charcoal for coke caused contamination of the cast iron and, therefore, produced from it steel by sulfur; blowing of the molten metal by air caused in addition its contamination with nitrogen . One learned to impart necessary properties to "dirty" steel by means of various alloying additives, because the consumer is interested not in the composition of a produced alloy, but in its properties. Content of harmful impurities in steels, as well as in titanium alloys, is limited. So, for example, for ensuring high reliability and long service life of ball bearings the content of sulfur in the steel, from which they are manufactured, was brought to ten thousand's shares of a percent. It needed high expenses. But it turned out that not just content of sulfur itself was important, but the ratio of oxygen and sulfur in the metal.
Metallurgy of titanium, in comparison with metallurgy of steel, is still very young, and, possibly, in future low-cost titanium alloys will appear, in which negative influence of increased content of impurities will be compensated by a respective system of alloying. Evidently, alloying metals contained in VT3, VT3-1 and VT4 titanium alloys, affect their mechanical properties to greater extent than hydrogen . At present one has to pay attention to classification of new titanium alloys, which are being developed, depending upon their possible application. Of course, in military and airspace fields, as ever, high-purity alloys are needed. Their cost may be reduced only due to new technological solutions. But in other fields, for example, jewelry business, manufacturing of rims for glasses, substrates of hard disks for computers, housings of watches, etc., "space" purity, evidently, is not required, but just high specific strength or just corrosion resistance, or absence of magnetism, etc. That's why such titanium alloys may be produced according to a simplified technology (without application of vacuum or from off-grade sponge, waste, etc.), which will allow significant reducing of their cost. The need in compromise between purity of the alloy and its designation and in ensuring of minimal production cost had, evidently, given impetus to companies <<Allegheny Technologies>> (USA) and VSMPO (Russia) to establish the joint venture UNITI, which is specialized in production of titanium alloys ofjust civil designation. UNITI will be engaged in development of the titanium markets of non-airspace and non-military designation .
Role of Ukraine and its prospects in world titanium industry. In addition to production, fabrication and supply of the titanium feedstock (the concentrates and the sponge), Ukraine started to produce titanium ingots. Establishment of production of titanium ingots became a first step on the way to increase of Ukraine's part in the world titanium industry. This production is mainly based on electron beam remelting, which is rather expensive method. Despite the possibility of melting by this method ingots from the charge materials containing up to 100 % of the titanium waste, such approach to reduction of the production cost is not always justified. Contained in the charge oxygen and nitrogen impurities, which are not removed in EBR, do not allow producing high-quality titanium ingot. The highest efficiency of this method of production of the ingots may be achieved in reprocessing of the high-quality sponge for production of the airspace and military designation titanium.
The next step could be increase of the volumes of production with simultaneous reduction of cost of titanium products due to establishment of the vertically integrated chain for production of titanium. However, attempts to establish such chains for the time being have failed, and without necessary investments one should not count on them in near future. Share of Ukraine in world production of titanium ingots is still small and constitutes approximately 1 %. While volumes of production gradually increase, consumption of the titanium ingots in Ukraine is insignificant and has lately reduced. Produced ingots go mainly for export. That's why the way of the titanium cost reduction through organizational-technological measures and unification of different technological process stages is in the meanwhile closed.
Reduction of cost of the titanium products due to application of cheaper methods of production of the ingots also envisages certain worsening of the titanium alloy purity. So, electroslag method, which did not find practical application in production of titanium and its alloys in airspace and military fields, is successfully used for production of ferrotitanium, because requirements to ferrotitanium, designed for deoxidizing and alloying of steel, alloys and cast iron, as wel as for production of welding materials, do not at all regulate content in it of such harmful impurities as hydrogen, oxygen, and nitrogen (Table 1).
Ferrotitanium and titanium alloys for airspace industry occupy two opposite positions in regard to purity of the titanium alloys. However, certain compromises between them are possible relative price and quality of titanium for so called non-special purpose designation, i.e. used in jewelry business, manufacturing of rims for glasses, substrates of hard disks for computers, housings of watches, sport items, etc. This, for the time being insufficiently developed segment of the world titanium market, can give Ukraine a real chance to increase its role in the world titanium industry. Ukrainian industry, which has high capacities for production of the ESR metal, can relatively quickly reorient itself at production of low-cost titanium alloys of non-special purpose designation. It should be noted once more that quality of titanium, produced by the ESR method, is sufficiently high. At the beginning of development of the titanium market in former USSR a struggle was waged between supporters of the ESR and the VAR processes for possession of the aviation market. Of course, the ESR method practically excludes possibility of removal of gases from titanium and in this respect can not compete with any vacuum process. However, as it is noted in many investigations, the main factor, which determines purity of the ESR ingot metal, is purity of the initial material to be remelted (electrode, sponge, and other charge materials). Content of impurities in the electroslag titanium, depending upon the used slag, kind and polarity of current, and composition of the remelting atmosphere, may noticeably vary . So, minimal content of oxygen in the electroslag titanium is achieved in remelting in the stagnation helium atmosphere or in remelting at reverse polarity current; minimal amount of nitrogen--at reduced pressure or at alternative current. In addition, in case of use of unrefined fluoric calcium as a slag, content of nitrogen in the ESR ingot is even lower than in the VAR ingot (Table 2). Depending upon allowable contamination of titanium by various impurities one can select respective technological scheme of ESR.
Comparative investigations of quality of the cast and deformed ESR and double VAR titanium showed that both in cast and deformed state the ESR and the VAR titanium has rather close mechanical characteristics. Only in case of application as an initial material of high-quality sponge, subjected to vacuum separation, the ESR titanium is noticeably inferior to the VAR titanium in respect to such parameter as impact toughness. Exactly this fact did not allow the ESR process occupying its niche in the titanium industry of former USSR, where practically the whole production went for needs of the defense complex. However, high values of toughness characteristics are needed not in all cases; for example, in jewelry business they are not of great significance.
After development of new equipment and technological solutions (a current-conducting mould, double-circuit scheme of remelting, magnet-controlled melting, etc.) possibilities of ESR as a whole and ESR of titanium, in particular, have expanded. And these possibilities have to be used with maximum efficiency. For development of market of titanium of the non-special purpose parts, which may consume up to 45 % of titanium products, coordination of efforts of many specialists will be needed. This will include development of new titanium alloys, respective slag systems, estimation of applicability of a specific alloy for various fields, development of respective standards, and many other activities. In order to move forward one has to make steps in the chosen direction. Ukraine, having big feedstock resources and means of production, will try to occupy worthy position in the world titanium industry.
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E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine
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|Title Annotation:||GENERAL PROBLEMS OF METALLURGY|
|Publication:||Advances in Electrometallurgy|
|Date:||Jul 1, 2007|
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