Investigation of the trends in air flow through porous materials made of dendritic powders

Conclusions We performed investigations for determining the hydraulic resistance coefficients of porous cermet materials made of stainless steel powder with dendritic particles, screened into fractions of –0.38, +0.28 mm and –0.16+0.10 mm. The specimens had a thickness of 2–8 mm and a porosity of 30–55%, while the temperatures of the air forced through were in the range of up to 200°C.A qualitative dependence of the viscous hydraulic resistance coefficient on the size of the initial particles was established; the thickness of the specimens did not affect the viscous coefficient.The inertial hydraulic resistance coefficient is virtually independent of the size of the initial particles; however, this coefficient is materially affected by changes in the thickness of specimens.The air flow through the investigated materials is adequately described by Eq. (8).It was found that the determining dimension depends on the size of the initial powder particles and the specimen's thickness; no appreciable effect of porosity on the determining dimension was observed. However, this observation presently applies only to the materials investigated. Further experimental investigations are necessary before this conclusion can be extended to other materialsTranslated from Poroshkovaya Metallurgiya, No. 12, pp. 52–57, December, 1967.     

Effect of Structural Anisotropy on Contact Properties in a Silver - Graphite Composite

We have studied the effect of structural anisotropy on the contact properties of silver –5 mass% graphite composite. We have established that the highest erosion resistance is typical of contacts with a structure in which the flake-shaped graphite particles are located perpendicular to the working surface. Welding resistance is higher in contacts with a structure in which the graphite particles are located parallel to the working surface of the contact. We have shown that for any arrangement of the graphite particles in the silver matrix, contacts with an anisotropic structure made from bars obtained by extrusion are distinguished by a higher erosion resistance compared with contacts prepared by single pressing - sintering - final pressing - annealing. The results obtained are of practical importance for designing electrical contacts to be used in low-voltage automatic switches. They make it possible to improve the operating characteristics of silver - graphite composite materials.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 101–105, January–February, 2005.     

Relaxation of diffusion stresses in a contact during sintering of various crystals

Specific dislocation structures and cracks formed near a contact during sintering of different kinds of crystals have been observed. Stress field interaction in groups of particles was considered.Kharkov University. Translated from Poroshkovaya Metallurgiya, Nos. 3–4, pp. 11–16, March–April, 1994.     

A rapid method for determining the particle-size distribution of micropowders

Conclusions Use of a flow-type laser analyzer in an investigation of the particle size distribution of a powder markedly accelerates analysis and enables the latter to be performed automatically, with the recording of numerical characteristics and forms of distribution functions for particles ranging in size from a few hundreds of angstrom units to some tens of micrometers. It is shown that the distribution functions of the particles of titanium carbide powders produced by comminution in a ball mill and subsequent liquid centrifugal classification into 3–5-, 2–3-, and 1–3-m fractions obey the logarithmic normal law. The numerical characteristics of these distributions are given.Translated from Poroshkovaya Metallurgiya, No. 11(263), pp. 10–14, November, 1984.     

Some features of the formation of A1N during the reduction-nitriding of ultrafinely divided alumina

Conclusions A study was made of the process of carbothermic reduction-nitriding of ultrafinely divided alumina in the temperature range 1200–1450C. It was established that the carbothermic reduction-nitriding of ultrafinely divided alumina is accompanied by the formation of intermediate oxycarbide and oxynitride phases. This is a result of the alumina reduction process having a stepwise character and of the formation of solid solutions in the AI-O-N system. It is shown that the resultant aluminum nitride is in the form of polycrystalline particles. The sizes of the particles and of the grains in them are 1–3 and 0.01–0.2 m, respectively. With rise in nitriding temperature, the particles decrease and the grains increase in size.Translated from Poroshkovaya Metallurgiya, No. 11(263), pp. 14–19, November, 1984.     

Effect of the amount of electrolytic nickel in an iron charge on its distribution on the particle surfaces and on the powder particle size

Conclusions After the passage of 0.16 A · h of electricity per 1 g of the charge investigated, the amount of nickel in the powder was 13–14 wt.%. The surfaces of the powder particles, which had retained their original shape and dendritic structure, were covered with fairly even, 5- to 8-m-thick nickel layers. During nickel plating the powders coarsened as a result of coalescence of separate particles, which was especially pronounced with charges containing large nickel additions (up to 30–50%). Additional comminution of nickel-plated powders increased their fineness. The milling operation virtually restored the original fine particle size of powders containing 13–14% of nickel.Translated from Poroshkovaya Metallurgiya, No. 1(229), pp. 1–4, January, 1982.     

Reactions of antimony-doped SnO2 powder with molten glass in resistive composite formation

Conclusions Heat treatment of (Sn-Sb-0)-glass composites causes antimony and tin to diffuse into the glass. Around the particles, the glass acquires a diffusion profile. The diffusion coefficients of those ions at 1173 K are, respectively, 10^–15 and 10^–16 m²/sec.Translated from Poroshkovaya Metallurgiya, No. 11, pp. 28–32, November, 1990.     

Microstructure formation in wurtzite BN (Geksanit-R) sinterings

Conclusions The loading of wurtzite boron nitride to high static pressures (55–100 kbar) under room-temperature conditions is accompanied by the processes of comminution of polycrystalline powder particles and their monocrystalline grains. Comminution of wurtzite boron nitride particles and grains has been found to take place also during sintering at high pressures and temperatures. The intensity of particle and grain comminution during loading and compression grows with increasing size of the particles in the starting condition. The comminution of particles and grains has a marked effect on structure formation in polycrystalline Geksanit-R alloy sintering.Translated from Poroshkovaya Metallurgiya, No. 8(236), pp. 32–37, August, 1982.     

Rheological characteristics of magnetic-abrasive powders in a magnetic field

The size of particles of magnetic-abrasive powders, magnetic induction in the working zones of magnetic gaps, and the size of working zones have been studied for their effect on tangential and normal stresses which develop in magnetic-abrasive tools during polishing. Friction coefficients for the powder in magneticabrasive tools and in a tool—titanium pair are determined.Kiev Polytechnic Institute. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 62–66, January–February, 1994.     

Particle interaction in the atomization of high-speed steel

Conclusions Liquid, solid, and semisolidified particles in an atomized metal-gas spray experience numerous collisions, which exert a strong influence on the end structure of the bulk of the resultant powder. Very fine, rapidly solidified particles colliding with larger drops act as foreign inner centers of solidification. Collisions between previously solidified particles and drops of smaller weight are accompanied by a sharp increase in cooling rate during solidification (to 10⁶–10⁷ deg C/sec) and the formation on the particles of shells with a very fine-columnar structure (without any visible-signs of primary carbide evolution).Translated from Poroshkovaya Metallurgiya, No. 3(255), p. 7–13, March, 1984.     

Influence of the aggregability of powders on the coagulative structure and inhomogeneity of the mass in disperse systems

The density of a randomly formed coagulative structure is shown to depend on the number of primary particles in the aggregates. The influence of the particle roughness, the thickness of equilibrium interlayers of disperse medium between the particles, and the anisometry of the particles on the density, of the coagulative structure the inhomogeneity of the mass in disperse systems has been assessed.Lvov University. Translated from Poroshkovaya Metallurgiya, Nos. 1/2(383), pp. 11–16, January–February, 1996. Original article submitted April 11 1994.     

Hard facing alloys obtained from charges containing refractory compounds

Conclusions On the basis of results of investigations and laboratory tests, a composite material produced by hard facing with a charge consisting of tungsten carbide particles and a Ni-Cr-B-Si alloy has been chosen as the most suitable for the surface reinforcement of parts operating under intense abrasive-gas wear conditions. To improve the processing qualities of such a charge, a flux should be added to it. The recommended charge composition for induction hard facing is: Relit 56–61%, PG-KhN80SR4 alloy powder 33–38%, boric anhydride 2.8–3%, dehydrated borax 1.5–1.6%, and silicocalcium 1.7–1.0%. Charges of this composition are being used for hard facing various parts of metallurgical plant units (small bells and charge distribution hoppers, valves of blast furnace charging devices, blades of sintering machines, and the like).Translated from Poroshkovaya Metallurgiya, No. 10(178), pp. 92–98, October, 1977.     

Mechanism of additional reduction of metallic powders in a falling layer

Conclusions The specific rate of additional reduction of the oxides films on the particles of iron in a falling layer of the powder in the temperature range 800–1000°C calculated for the initial specific surface was 0.036–0.078 g[O]/m²· sec. The activation of the reduction process was equal to 54.16 kJ/mole.The rate of additional reduction of the iron powder in the falling layer is high. For example, in the temperature range 800–1000°C it is 1.2–3.1% [O]/sec.The experimental results as well as the comparison with the available data make it possible to conclude that the process of reduction of the surface oxides in the falling layer in the temperature range 800–1000°C takes place in the kinetic range of reaction and is limited by the reaction of interaction of hydrogen with the surface oxides of iron.Translated from Poroshkovaya Metallurgiya, No. 9-(345), pp. 1–5, September, 1991.     

Heat treatment of highly dispersed electrolytic iron and Fe-Co alloy powders with a composite coating

Conclusions A necessary condition for improving the magnetic properties of powers is improvement of the metal phase structure, elimination of nonmagnetic impurities, and destruction of a chemisorbed coating. Annealing in hydrogen at 633–653 K makes it possible to retain the main geometric size of particles within the limits 0.8–1.0 m, to reduce dendrites and to increase the coercive force by a factor of 1.5–2 and the residual specific magnetization of iron and Fe-Co alloy powders up to 95 and 105 A·m²/kg, respectively.Translated from Poroshkovaya Metallurgiya, No. 4(340), pp. 74–78, April, 1991.     

Ceramic coatings of high thermal-cycling resistance

Conclusions A single-component ceramic coating produced by plasma-spraying a powder mixture consisting of 85% of 5–30- particles and 15% of 70–110- particles exhibits improved thermal-cycling resistance, because the dispersed inclusions of the coarse fraction present in the coating hinder crack propagation and coating rupture. The chemical identity of the fine and coarse fractions has a beneficial effect on the properties of the coating.Translated from Poroshkovaya Metallurgiya, No. 10 (106), pp. 46–48, October, 1971.     

Purification of metal powders from oxide films in a jet mill

Conclusions During the jet comminution of metal powders covered with oxide films the latter shear off, and their particles detach themselves from the main volume of material. The amount of oxygen decreases during purification by a factor of 2–17 depending on the oxygen content of the starting material and the latter's hardness and particle size. The removal of oxide films is virtually completed in 5–30 min depending on the properties of the starting powder. For the purification of powders low-pressure (0.5-MPa) compressed air can be employed. During the comminution of powders covered with oxide films, in certain circumstances detached film particles may find their way into the comminuted material.Translated from Poroshkovaya. Metallurgiya, No. 2(230), pp. 1–4, February, 1982.E. A. Filler, of the Special Design and Technology Bureau of the Chemical Engineering Industry, also took part in this work.     

Microporosity of powders produced by centrifugal sputtering

Conclusions The nature of the liquid-metal film flow about the end of the pulverized billet is defined by its rotational velocity. In the case of a turbulent film flow regime gas is captured and pores are formed in individual powder particles.The greatest number of porous particles produced in a single pulverization regime is found in a large-fraction powder while the smallest quantity is found in the finely dispersed powder with grain sizes less than 100n.Increasing the helium content in the pulverization medium to 100% produces a discontinuous increase in the number of porous particles as a result of the reduced viscosity of the gaseous medium and the high penetration power of the helium. The smallest number of porous particles is formed when pulverization takes place in a medium with a volumetric argon content in excess of 30%.The pore size in the powder particles is virtually independent of the production regime and usually amounts to about 25–35 % of the particle diameter. The average gas-pore volume is 2–3% of the volume of the powder particle.To achieve minimum microporosity in compacted billets fabricated by the methods of powder metallurgy, it is expedient to use 100m powder fractions, produced at a billet rotation velocity of 35–40 m/sec.Translated from Poroshkovaya Metallurgiya, No. 12(348), pp. 1–7, December, 1991.     

Some features of the sintering of nickel fibers extruded from viscose-base suspensions. Part I

Conclusions The sintering of fibers extruded from a viscose-base nickel powder suspension is characterized by volume changes whose intensity varies with temperature: At 200–400°C the fibers undergo densification under the action of capillary forces set up by liquid fractions of the polymer binder being removed during the latter's thermal destruction, the mechanism of the process being analogous to that of high-intensity drying; at 400–800°C the magnitude of shrinkage is independent of temperature because the carbon-metal skeletons of the fibers are stable in hydrogen. Local agglomeration of particles in the temperature range 600–800°C too has no effect on the magnitude of the volume changes of the fibers because of the discreteness of this process; at 800–1200°C the metallic phase sinters by a diffusion-viscous flow mechanism. To the characteristic temperature ranges of densification of fibers extruded froma viscose-base nickel powder suspension there correspond characteristic temperature ranges of polymer binder removal, which points to the existence of a relationship between these two processes.Translated from Poroshkovaya Metallurgiya, No. 6(210), pp. 28–32, June, 1980.     

Effect of surface-active agents on the properties of a copper powder produced by the autoclave method

Conclusions Carboxyl-containing water-soluble polymers have the strongest influence on the properties of copper powders and are the most effective in reducing their deposits on the inside reactor surface. The optimum SAA consumption rate is 0.003–0.007 g per 1 g of copper. Higher consumption rates intensify the flocculatton of powder particles and increase the carbon content of the powder, which is undesirable, since it makes the latter's subsequent processing more difficult. The carboxyl-containing substances currently produced by industry can be used as SAAs, but, because of their low carboxyl-group content (not more than 45–60%), powders produced in their presence become fairly severely contaminated with carbon owing to destruction of inert radicals. By suitable choice of type of carboxyl-containing SAA it is possible to vary the properties of powders in the following ranges: specific surface 0.02–0.18 m²/g, apparent density 0.9–2.9 g/cm³, mean particle size 20–42m, and flowability 0–2.6 g/sec.Translated from Poroshkovaya Metallurgiya, No. 7(283), pp. 5–8, July, 1986.     

Effect of porosity on the ductility and ductile failure mechanism of powder iron

Conclusions Formation of the ductility characteristics of the powder materials based on iron is associated with the transformation of the porous structure during deformation. The model proposed by Gerland [1] for materials with particles describes accurately the behavior of the material containing pores and links the true strain to fracture with porosity.The experimental dependence e_r=F(O) explains satisfactorily the formation of certain important processing characteristics of the material, such as cracking resistance and impact toughness. Since the yield stress of the material produced from various powders differs only slightly in a wide range of porosity, the main contribution to formation of cracking resistance and impact thoughness comes from the true strain to fracture (e_r).The difference in the content of the second phase particles in the iron powders of variousggrades leads to differences in the e_r values for the dense states; e_r decreases with increasing porosity. Thus, the high values of the impact toughness for the materials produced from WPL-200 and O Cher MK powders are explained by the low content of the second phase particles (1–2%) in comparison with the other materials, where the volume fraction of the particles was 3–5%.Translated from Poroshkovaya Metallurgiya, No. 10(298), pp. 90–96, October, 1987.