Solid-phase prereduction of iron-vanadium concentrates and liquid-phase separation of the products of their reduction

The transformations that occur in ore grains during solid-phase carbon reduction of the metals from the iron-vanadium concentrates formed upon the beneficiation of the titanomagnetite ores from Southern Ural deposits are studied. Upon heating to 1000°C, the solid solution in titanomagnetite grains decomposes with the formation of magnetite and ilmenite; the reduction of iron begins in the temperature range 1080–1110°C, and the reduction of titanium begins at above 1215°C. The reaction mixture should be held at 1250°C for 45 min to ensure almost complete iron reduction and the minimum degree of titanium reduction. For rapid separation melting, this procedure results in vanadium-containing cast iron (0.43–0.5% V) with <0.15% Ti and a slag with 42–43% titanium oxides.     

Deformation of materials based on thermally expanded graphite

Strain and strength in uniaxial compression have been examined for specimens with various densities made by uniaxial pressing of thermally expanded graphite powders, which have been produced by the heat treatment of intercalated graphite compounds. Low-density graphite specimens formed at 600°C have the highest strain resistance. Surface Chemistry Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurigya, Nos. 5–6, pp. 18–23, May–June, 1998.     

Synthesis and properties of ceramics in the SiC — B4C — MeB2 system

The effect of impurities and additives of titanium and zirconium borides on the structure and mechanical properties of SiC — B₄C ceramics over a broad temperature range has been investigated. The ceramics was fabricated by hot pressing without a protective medium. Introduction of borides is accompanied by improvement in all the studied mechanical properties at room temperature, and the nature of hardening of the ceramics is practically independent of the type of SiC powders used. At high temperatures, the mechanical behavior of the ceramics is determined by the impurity composition: the ceramics obtained using abrasive powders loses strength beginning at 600°C, while using powders with decreased impurity content makes it possible to preserve the strength of the material up to a temperature of 1400°C. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 29–42, May–June, 2000.     

Magnetizing roasting of leucoxene concentrate

The phase transformations occurring during magnetizing roasting of leucoxene concentrate in the temperature range 600–1300°C are studied. It is demonstrated, that in the temperature range 600–800°C, only iron oxides are reduced to a metallic state; at temperatures above 800°C, combined reduction of iron and titanium oxides takes place. At 1050°C, reduced specimens are represented by the Ti₅O₉ and Ti₆O₁₃ Magnéli phases. The formation of iron metatitanate (FeTiO₃), under reduction conditions and the existence of ferrous iron ions in the Magnéli phases slightly degrade the magnetic properties of the products of magnetizing roasting. In high temperature region (1200–1300°C), a similar effect is exerted by the formation of iron dititanate or anosovite in the system. The possibilities of eliminating the undesired factors decreasing the magnetic properties of the products of magnetizing roasting are determined.     

Phase equilibria in titanium-rich alloys of the Ti — Fe — Nb — Al system

Isothermal cross sections at temperatures of 750°C and 550°C of the phase diagram of the quaternary system Ti — Fe — Nb — Al in the region of titanium-rich alloys for a constant aluminum content of 5 mass % were plotted using metallography, x-ray diffraction, and local x-ray spectral analysis. In the temperature range 550–750°C in alloys with 5 mass % aluminum, the maximum solubility of iron in α-titanium reaches ∼2 mass % for a niobium content of 3 ± 0.5 mass %. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(412), pp. 27–32, March–April, 2000.     

Mechanical activation of the reactive sintering of titanium carbide

This paper describes the mechanical alloying of Ti and C powders by intensive grinding in a planetary mill. The effect of milling conditions and initial composition on the compositional homogeneity and properties of powders and samples consolidated by hot-pressing was studied by x-ray diffraction, scanning electron microscopy, and surface area measurement. The diffraction patterns indicated that a powder mixture ground for 24 hours transforms to TiC after sintering for only one minute at 600°C under a pressure of 3 GPa. Materials Science Instiute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 32–38, May–June, 1998.     

Effect of enriching the blast with oxygen on the production cost of pig iron

Conclusion By mathematically modeling the blast-furnace smelting of conversion pig iron with a change in the oxygen content of the blast from 21 to 30% and a change in blast temperature from 800 to 1400°C, it was possible to determine how blast temperature affects the increases that occur in furnace productivity, coke rate, and pig-iron cost when blast oxygen content is increased by 1% within the ranges from 21 to 25% and from 25 to 30%. Under the furnace operating conditions that were examined, the savings in coke realized when the blast is enriched with oxygen decrease as blast temperature increases. In fact, coke rate increases at blast temperatures above 1100°C when the blast is enriched with oxygen in the range 25–30%. The effect of oxygen enrichment on pig-iron cost within this concentration range is negative throughout the range of blast temperatures examined. Adding more oxygen to the blast reduces the production cost of the pig iron only when blast oxygen content is within the range 21–25% and blast temperature is no greater than 1000–1100°C. At higher temperatures, adding more oxygen to the blast is economically inexpedient even within the lower ranges of oxygen content. Moscow State Institute of Steel and Alloys. Translated from Metallurg, No. 5, pp. 43–44, May, 1999.     

Oxidation of porous nanocrystalline titanium nitride. II. Scale structure and composition

We have used x-ray phase analysis to study the composition of the products of reaction between oxygen and nanocrystalline powders with particle sizes 15, 40, 55, and 80 nm, and also specimens pressed (and sintered) from them. The powders were oxidized in air at 100°C (400 h) to 500°C (5 min), while the sintered specimens were oxidized at 600–900°C for 15, 120, and 240 min. In all cases, in the initial oxidation step the oxynitride Ti(O_xN_y) is formed, which over time is oxidized to TiO, Ti₂O₃, Ti₃O₅, TiO₂ (anatase) and TiO₂ (rutile). In the range 600–800°C, formation of a continuous oxide layer and conversion of anatase to rutile slows down diffusion of oxygen in the scale. We have established that at 900°C, the growth rate of the scale thickness increases and so the reflections from the oxynitride are barely noticeable on the diffraction patterns taken from the surface of the oxidized specimen. In these diffraction patterns, along with strong reflections from the rutile, we also observed weak reflections from lower oxides and anatase, which may be due to reaction between oxygen and the titanium ions diffused to the scale surface. We have concluded that at T > 850°C, the mechanism for oxidation of TiN changes. This is due to superposition of counterdiffusion of titanium ions on the diffusion of oxygen. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 72–78, March–April, 2006.     

Oxidation of porous nanocrystalline titanium nitride. I. Kinetics

We used the continuous weighing method to study the oxidation kinetics in air for TiN specimens pressed and sintered from nanocrystalline powders with particle size ≤55 nm. Oxidation was carried out at 500–1000 °C for 240 min. By comparing with the oxidizability of compact titanium, we estimated the total reaction surface S of the porous specimens as a function of their oxidation conditions. The mass of absorbed oxygen Δm was calculated from the mass gain ΔP, taking into account the volatile component N₂. We have shown that the maximum mass gain Δm at 600 °C is due to reaction of oxygen with the largest reaction surface. Within 120 min, external pores close up, S decreases, and then a continuous oxide layer forms in which diffusion of oxygen is slowed down. At 700–800 °C, the process of closing up of the pores is activated, and S decreases by an order of magnitude compared to 600 °C. After the first 40–50 min, a continuous oxide film forms and virtually no further mass gain occurs. As the temperature increases, the oxidation rate increases. At 900 °C, the reaction surface becomes equal to the external surface of the specimen, but the thickness of the scale increases linearly. We hypothesize that for T > 850 °C, counterdiffusion of titanium ions is superimposed on diffusion of oxygen. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 98–103, January–February, 2006.     

Phase composition and structure of composite powders based on solid solutions of SiC and AlN

The structure of SiC–AlN powders is investigated by x-ray diffraction and transmission electron microscopy methods. The powders were produced by joint carbon reduction and nitriding of silicon and aluminum oxide mixtures. The results show that a mixture of solid solutions forms during joint SiC and AlN synthesis at 1700°C, with SiC forming β (3C) and α (2H) modifications with different grain morphology. The fiber form is characteristic of β-SiC, whereas the grains of the solid solution based on SiC have a predominantly equiaxed form. α-SiC grain dimensions are considerablys smaller than those of AlN. Institute of Materials Science, National Academy of Sciences of Ukraine. Kiev Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 81–86, July–August, 1997.     

Phase and structural transformations in production of powders of intermetallic compounds

The mechanism and kinetics for the formation processes of powders of intermetallic compounds when the mixtures of oxides, metals, and calcium hydride are heated in the range t = 900–1200°C (the calcium hydride method) are investigated. It is established experimentally that the compounds are synthesized in two stages. At the first stage, metal oxides are reduced by the calcium melt (starting from ∼840°C); at the second stage, diffusion interactions of the components proceed. It is shown that, for the effective development of processes of alloy formation, metal components should be dissolved in liquid calcium or to form the eutectics between one another at a value of t not exceeding the working temperature of the process. As the latter increases, the reduction of oxides and diffusion interaction between the components are intensified and finished at t < 1200°C with the formation of homogeneous powders of intermetallic compounds. The possibility of forming a wide nomenclature of powders for intermetallic compounds of different applications is shown.     

Structuring on sintering in the presence of liquid phase in Cr-Cu-iron group metal systems. I. Cr-Cu system

Structuring has been examined for Cr-Cu composites under conditions of impregnation and subsequent liquid-phase sintering at 1200°C in a vacuum of (2–4) · 10⁻³ Pa with reduced and electrolytic chromium powders. The size distribution for the particles of the refractory component in the microstructure containing the reduced chromium on liquid-phase sintering for 60 min corresponds to a logarithmic normal distribution; the distribution parameters are sensitive to the volume fraction of refractory particles. The calculated values for the dihedral angle are close to one of the modes of the distribution for the dihedral angles in the microstructure for specimens made of electrolytic chromium (115°). At 1200°C, the equilibrium Cr_s-Cu_l system obeys the condition . This indicates the probability of formation or preservation of framework structure elements during the liquid-phase sintering, which are observed by experiment in specimens containing reduced chromium. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 3–9, May–June, 2006.     

Kinetics of spreading in initial stages in metallic systems with different types of interaction between components. IV. Effect of chemical reaction on the kinetics of spreading in tin—Iron group metal systems

A droplet flowing over onto a plate introduced from above has been used to study the kinetics of spreading and to describe the observable characteristics of spreading of tin on iron, cobalt, nickel, and the intermetallic compounds Ni₃Sn, Ni₃Sn₂ under a vacuum of (2–4)·10⁻³ Pa at 400–1000°C, droplet mass 0.01–0.06 g. We show by a formal kinetic analysis of experimental data that in the low-temperature range (400–500°C) the kinetic regime dominates, and in the high-temperature range (600–1000°C) the inertial—kinetic regime dominates. In spreading of tin on iron, cobalt, nickel, and the intermetallic compounds Ni₃Sn and Ni₃Sn₂, the nature of the interaction corresponds to the phase equilibrium in the studied systems. The results for the kinetics of spreading of tin on nickel and the intermetallic compound Ni₃Sn showed that spreading of the main bulk is preceded by spreading of a precursor film. Deceased. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 65–72, July–August, 1998.     

Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si₃N₄−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m²/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si₃N₄, β-Si₃N₄, β-SiC, and X-ray amorphous Si₃N₄. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Salaspils. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 7–12, January–February, 1999.     

Investigation into the Influence of the Structure Dispersion and Homogeneity on the Properties of Powder Metastable Austenitic Carbide Steels and Diamond Tools

Diffusion and homogenization in “iron (5 μm–nickel (5 μm or 50 nm)” powder systems of various degrees of dispersion during sintering (900 and 1000°C), as well as spark plasma sintering, are investigated using the Matano–Boltzmann method. Calculated diffusivities in pairs of micron powders sintering without applying pressure (900°C, 6 h) and by the spark plasma method (900°C, 5 min) in these systems are equal to 7 × 10–10 cm2/s. It is shown that the use of nanodispersed nickel powder in diffusion pairs based on finely dispersed iron powder promotes a twofold increase in diffusivity at 900°C in contrast to the pair with the microdispersed nickel powder. Constants in the Ivensen sintering kinetics equation are calculated for the “iron–nickel” powder systems, by which the factors activating sintering of these systems are established. The dependences of the structural phase composition and physicomechanical properties of carbide steels of the Fe(base)–14 wt % Ni–8 wt % TiC system on the sintering temperature in range t = 900–1200°C and structure dispersity and homogeneity are determined. The dependences of the grain size, porosity, hardness, microhardness, fracture toughness, and bending ultimate strength on the sintering temperature are shown. Dependences of tribotechnical properties on the degree of homogeneity of the solid solution and volume of the phase transformation of metastable austenite into deformation martensite during abrasive friction turn out similar for carbide steels and diamond tools based on carbide steel. Optimal values of the variation coefficient of the nickel concentration in austenite and carbide steels of the same chemical composition but with different degrees of dispersity, which provide the maximal volume of the austenite decomposition and high values of the diamond-tool grinding coefficient, turn out equal to 5 in both systems, but the sintering parameters are different. It is shown that the physicomechanical properties of the studied systems depend on the structure porosity and dispersity, while tribotechnical properties depend on the structural homogeneity of steels.     

Thermal stability of hydride phase obtained by mechanical treatment of Mg-10 mass% Fe in hydrogen under pressure

The formation of hydride phases under high-energy ball-milling of Mg and Mg + 10 mass% Fe powders in hydrogen under a pressure of 1.2 MPa has been studied. The influence of iron content and dispersion degree of the Mg-alloys upon thermal stability and decomposition temperature of the hydride phase of the alloys was investigated by thermal desorption, x-ray diffraction, and differential thermal analyses. It was established that addition of 10 mass% of iron to magnesium contributes to higher dispersion degree of the magnesium hydride obtained via milling in hydrogen under pressure. The hydride contains maximum amount of weak-and medium-bound hydrogen. The latter is mainly concentrated in the region of grain boundaries, the quantity of which increases with increase in dispersion degree. The formation of new boundaries and accumulation of defects in these regions are accompanied by an increase in thermodynamic potential, which causes a decrease in both the temperature of decomposition of the hydride phase by 100 °C and its thermal stability. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 99–106, July–August 2006.     

Enhanced Densification of Carbonyl Iron Powder Compacts by the Retardation of Exaggerated Grain Growth through the Use of High Heating Rates

An investigation of the effect of heating rates on the densification behavior of carbonyl iron powder compacts, particularly on the exaggerated grain growth during the α-γ phase transformation, was carried out in this study. Compacts heated at 1200 °C/min and then sintered for 90 minutes at 1200 °C attained 7.14 g/cm³, while those heated at 10 °C/min reached only 6.61 g/cm³. Dilatometer curves using heating rates of 2 °C/min, 5 °C/min, 10 °C/min, 30 °C/min, and 90 °C/min demonstrate that 90 °C/min yields the highest sintered density. The microstructure analysis shows that high heating rates inhibit exaggerated grain growth during the phase transformation by keeping the interparticle neck size small and pinning the grain boundaries. This explanation is supported by the calculation that shows that the energy barrier preventing the grain boundary from breaking away from the neck is reduced hyperbolically as the neck size and the amount of shrinkage increase. The high heating rate, however, shows little beneficial effect for materials that have no allotropic phase transformation or have less drastic grain growth during heating, such as nickel and copper. Thus, bypassing the low temperatures to suppress the surface diffusion mechanism, which does not contribute to densification, is ruled out as the main reason for the enhanced densification of carbonyl iron powders.     

Ferritic Fine Structure Characteristics in Hot Die Forged Powder Steels. I. Effect of Heating Temperature during Die Forging on Ferritic Fine Structure in Specimens of Powder Technical Grade Iron and Carbon Steels

We have studied the effect of the composition and heating temperature during die forging on the microstructure and fine structure of ferrite in powder iron and powder carbon steels. From the structural characteristics obtained for different planes of the blank, we used harmonic analysis of the x-ray line shape to establish the nonuniformity of deformation over the volume. We have established the heating temperature during die forging of powder technical grade iron and carbon steels, for which the deformation is close to uniform over the die forging volume: 1050 °C for technical grade iron and low-carbon steel, 1100 °C for carbon steel. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 108–119, May–June, 2005.     

Behavior of direct reduced iron and hot briquetted iron in the upper blast furnace shaft: Part I. Fundamentals of kinetics and mechanism of oxidation

It is considered that the use of prereduced ferrous materials and sources of metallic iron such as direct reduced iron (DRI) or hot briquetted iron (HBI) improves the productivity of the blast furnace (BF). However, oxidation of DRI/HBI can occur in the upper zone of the BF, which may increase the content of the reducing gases but may not decrease the coke rate substantially. The behavior of DRI and HBI was investigated by measuring the rate of oxidation of the materials in CO₂ gas in a temperature range of 400 °C to 900 °C. In addition, the microstructure of “as-received” and oxidized materials was examined. The iron oxide phases formed due to oxidation were determined using X-ray diffraction (XRD) and a vibrating sample magnetometer. The results of isothermal experiments indicated that the kinetics of oxidation of metallic iron is slow at 400 °C. In DRI samples, the initial rate is controlled by the limited mixed control of chemical kinetics at the iron/iron oxide interface and pore mass transfer, whereas gas diffusion in pores is the rate governing step during the final stages of oxidation. The oxidation of wustite from iron is found to be faster than the oxidation of the former to magnetite. The structure of DRI after oxidation resembled a “reverse topochemical-oxide on the surface metal in the center” structure at 600 °C to 700 °C. The final iron oxide phase formed in DRI after oxidation was magnetite and not hematite. The oxidation of HBI was limited to the surface of the samples at lower temperatures; at 900 °C, moderate oxidation was observed and a topochemical iron oxide layer was formed.