Effect of particle dispersion on the densification of parts from columbium carbide powder at different sintering temperatures

Summary Attainment of relative densities of up to 97% in parts from columbium carbide powder is only feasible when fine comminution of the powder, cold compacting, and sintering at temperatures not exceeding 0.6 T_melt. are employed.It is possible to calculate, within the limits needed from the practical point of view, the sintering temperature and powder dispersion necessary for the production of parts from columbium carbide powder, having the required volume density.In the production of low-porosity columbium carbide parts, sintering temperature may be substantially lowered if the dispersion of the powder to be sintered is increased, thereby significantly simplifying production technology.The densification process of columbium carbide is analyzed from the point of view of the theory of viscous flow during sintering. On this basis, the feasibility of quantitative evaluation of the minimum degree of powder dispersion necessary for securing intensive sintering of the material at a given temperature is demonstrated. The lattice destruction energy of columbium carbide, calculated from experimental data, is about 410 kJ/mole.     

Sintering of blanks obtained by die pressing powders of refractory compounds

Summary This article describes an investigation of the process of sintering blanks made of refractory compounds by the die pressing method. The effect of the protective atmosphere of the furnace, containing CO, on the growth and shrinkage of samples during sintering is shown. It is found that one of the causes of the increase in size of MoSi₂ and ZrB₂ blanks at temperatures up to 1000C is the separation of free carbon from the gas phase on the grain boundaries.During sintering of blanks made of titanium carbide at temperatures up to 1600C decarbidization occurs, and free carbon is separated out both from the liquid phase and from the solid solution of carbon in titanium carbide. The parameter of the titanium carbide lattice was measured, and it is shown that during decarbidization vacant places in the TiC lattice are occupied by oxygen or nitrogen atoms which get into the furnace from the air. It is also shown that the size of the particles of the initial powders and of the free carbon affects the density of the resulting parts.A rational technology of sintering blanks from powders of refractory compounds is proposed.     

Effect of carbon concentration in binder on the mechanical properties of heat-treatable titanium carbide-steel alloys

Conclusions The optimum amount of carbon in the binder of titanium carbide-steel alloys depends on the carbon content of the carbide employed. In the preparation of alloys from titanium carbide of composition close to stoichiometric and high free-carbon content, alloying with only small amounts of carbon is recommended. When a nonstoichiometric, low-carbon carbide is used, however, larger amounts of carbon can be added to the alloys. The strong influence exerted by the concentration of carbon (combined and free) in the carbide and steel binder is clearly one of the causes of discrepancies in literature data on the properties of titanium carbide-steel alloys.Translated from Poroshkovaya Metallurgiya, No. 3(267), pp. 43–46, March, 1985.     

Sintering kinetics of tantalum carbide

Conclusions Tantalum carbide sinters at a temperature above 2500C. Decreasing the powder particle size activates the sintering process, but even with a powder of 0.17-m particle size specimens sintered at 2700C have a porosity of 11%. Coarse powders (> 7–8 m) sinter, without densification, at 2000–2200C by a surface self-diffusion mechanism. Fine powders (<7–8 m) undergo densification already at temperatures above 1400C by a diffusion-viscous flow and a volume self-diffusion mechanism during long holding periods and also probably by an activated grain-boundary sliding mechanism in the initial stage of sintering after rapid heating.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 16–19, October, 1982.     

Structure of powder brazing alloys based on the system Ni-Cr-Si-Fe-B-C(-Mo)

Conclusions A study was made of the sinterability of PKh12N75S8R brazing alloy powder and a mechanical mixture of this powder with 15% of molybdenum (brazing alloy No. 6MA), which are widely used for the brazing of high-alloy steels. Appreciable shrinkage of compacts begins at 1000°C and steadily increases with rise in sintering temperature. The most intense shrinkage is observed above 1100°C, i.e., at temperature close to the solidus. The structure of brazing alloy No. 6MA produced by sintering at 1175–1200°C or melting consists of a solid solution and molybdenum suicides and chromium borides of various compositions.Translated from Poroshkovaya Metallurgiya, No. 10 (118), pp. 77–84, October, 1972.     

Electric Explosion Comminution of Silicon Carbide in an Elastic Shell

Comminution of silicon carbide powder in an elastic shell by electric explosion is less efficient than powder comminution in an elastic shell by mechanical impact because of its greater energy consumption. The level of silicon carbide powder comminution is the same by both methods and it is 40 m with an original size of _300+160 m. Comparative analysis showed that for more efficient powder comminution in elastic shells by pulsed methods it is desirable to match the duration of applied loads with frequency characteristics of the shell.     

Electric Rolling of Powder Materials with a Dielectric Phase

The effect of dielectric phase content on the properties of powder matrix composites (Kh20N802M, NPG80, Nibon20) prepared by electric-pulse sintering with rolling is studied. The change in properties is most pronounced in composites with a filler that has greater hardness than the matrix material. The highest specific electrical resistivity, ultimate tensile strength, and a reduction in hardness are achieved with identical volume fractions of components and the maximum exponent in the Joule component of electric current. Use of electric-pulse sintering with rolling makes it possible to prepare strips with a large fraction of nonforming phase, especially from highyield powders.     

Properties of R6M5 steel powders of various particle sizes and the structure of the steel in the sintered condition

Conclusions An investigation was carried out into the effect of particle size of powders produced by comminution of R6M5 high-speed steel waste (swarf) on their compressibility and sinterability. It is shown that with decreasing mean particle size the compressibility of such powders deteriorates, but their sinterability improves. The finer the steel powder, the lower is the sintering temperature necessary for the attainment of a given density. Varying the powder particles size brings about marked changes in the microstructure of the steel in the sintered condition. The liquid phase (ledeburitic eutectic) appearing during the sintering of steel powders of different particle sizes varies in form and distribution. With coarse powders (d_m=100 m) the eutectic is similar to that in cast steel, but with powders of small particle size (d_m=22 m) it is finer and evenly distributed throughout the structure in the form of thin lamellae.A 0.85%C-6% W-5% Mo-4% Cr-2% V (nominal composition) high-speed steel — Translator.The authors wish to thank A. N. Nikolaev of the Gorkii Polytechnic Institute for helpful discussion in the experimental part of the work.Translated from Poroshkovaya Metallurgiya, No. 6(234), pp. 9–15, June, 1982.     

The effect of conditions of preparation on the strength of WC — Co hard alloys and the relationship between the strength of these alloys and their structure and composition

Conclusions This paper presents a review of investigations which elucidated the effect of the technological conditions of obtaining the initial tungsten powder and the grinding and sintering conditions on the strength of a hard alloy during bending. An analysis of the results of these investigations shows that the differences in the strength of alloys differing in carbide grain size is primarily due to various manufacturing conditions and not to differences in grain size.On applying various methods of regulating the size of the carbide grain, sharply different changes in alloy strength are observed. The influence of the size of the grain was not noted, since it was suppressed by the greater effect of the technological factors.On excluding the effect of changing the sintering temperature (and other variable technological factors) the dependence of the alloy strength on the cobalt content changes its form: the maximum strength between 15 and 25% cobalt, noted in many researches, is not observed in this case. The strength increases continually up to 50% cobalt.The dependence on grain size with various methods of regulating carbide dispersity, and the dependence of the strength on the cobalt content on excluding the effect of the technological factors differ substantially from the regularities described by Gurland [1].The theoretical theses of Gurland are ungrounded, inasmuch as the strength relations are at variance with them, when the effect of the technological factors is excluded.The relations actually existing may be explained on the basis of the skeleton structure of the carbide and cobalt phases.     

Effect of heat treatment on the strength and tribotechnical properties of material of LNKhB grade

Conclusions The Nichrome alloy was alloyed with niobium to produce the materials of the LNKhB7 and LNKhB8 grades with a heterogeneous structure which consist of an alloyed solid solution and nickel and a high-hardness phase, i.e., molybdenum carbide. Heat treatment (normalizing, aging) greatly hardens the material as a result of the formation of the high-hardness phase (Ni₃Al).The materials hardened with the high-hardness molybdenum carbide and the phase have high and stable strength properties in the temperature range 20–700°C (800°C); at temperatures higher than the temperature of hardening heat treatment the strength properties of the materials rapidly decrease.Alloying the grade LNKhB materials with niobium greatly improves the tribotechnical characteristics as a result of the formation of a surface film in the form of complex compounds on the friction surfaces.Translated from Poroshkovaya Metallurgiya, No. 7(283), pp. 74–77, July, 1986.     

Feasibility of surface carbidization of refractory metals by electric-spark alloying

Conclusions A study was made of the conditions of formation of alloyed layers during the ESA of transition metals with graphite. Negative cathode weight gains were observed together with the formation on the metal specimen surfaces of 5– to 15-m-thick layers with a changed structure, consisting of carbides and oxides of the metals and of the basis material. Carbidization during ESA with graphite was found to be intensified for Group IV metals under soft conditions (process No. 2) and for Groups V and VI metals, under hard conditions (processes Nos. 3 and 6).Translated from Poroshkovaya Metallurgiya, No. 6(222), pp. 42–47, June, 1981.     

Processing properties of reduced nickel powders obtained from Cuban ore

Conclusions Nickel powder obtained by reduction from Cuban production waste (PNVK) exhibits good compressibility. Difficult-to-reduce oxide impurities decrease its low-temperature sintering activity compared with carbonyl powder and powder reduced from pure nickel oxides. As a result, for sintering PNVK powder higher temperatures (1100–1200°C) are required.Translated from Poroshkovaya Metallurgiya, No. 5(317), pp. 4–7, May, 1989.     

Effect of charge fractional composition on high-speed steel powder sintering

The fractional composition of gas-atomized high-speed steel R6M5K5 is studied for its effect on the main features of activated sintering of compacts. A mixture consisting of nickel powder and 30% (wt.) boron is used as an activating addition. It is established that a reduction in average particle size increases shrinkage after sintering, but the dependence of compact shrinkage on fractional composition is different for monodispersed and polydispersed compositions. The maximum size of powder particles for polydispersed compositions has little effect on shrinkage, whereas in the case of monodispersed mixtures there is a marked increase in compact density after sintering with a reduction in particle size. There is a marked increase in density providing closed porosity (90–93%) with a sintering temperature of 1200C, whereas in order to achieve a similar density in the mixture without activator the sintering temperature should be at least 1230C.Translated from Poroshkovaya Metallurgiya, No. 10, pp. 15–20, October, 1992.     

Structure,mechanical properties,and special features of failure of nickel-based detonation coatings

Conclusions Alloying nickel with chrome, aluminum, and boron, as well as the variation of the spraying conditions makes it possible to control purposefully the composition, structure, and properties of the detonation coatings. In alloying nickel with aluminum and chrome, the strength properties of the coatings are fully utilized at thicknesses not greater than 0.25 mm. The adhesion strength of these coatings to the titanium substrates is 35–150 MPa, to steel substrates 20–120 MPa, in the thickness range 0.1–0.3 mm. The ductility properties of the coating are utilized only to 40–60% in comparison with the dense material of the same composition. The ductility of the coatings is reduced most markedly by additions of aluminum and boron, whereas the addition of chrome causes the smallest reduction.Alloying nickel with chrome increases its cohesion strength and heat resistance, alloying with aluminium reduces heat resistance, creep strength, and wear resistance, and combined alloying with aluminum and boron reduces wear and heat resistance.The soft conditions are optimum for depositing coatings on titanium substrates. Under these conditions, particles are only welded without high deformation, whereas medium conditions with moderate temperature and kinetic parameters are optimum for depositing the steel components. This gradation is caused by the susceptibility of the titanium alloys to brittle failure in the presence of surface defects and cracks and also by a large difference in the coefficients of thermal failure of the materials of the coating and the substrate and higher chemical activity of titanium.Failure of coatings of alloyed nickel alloys takes place by means of intergrain delamination which is preceded by plastic deformation of the materials of the coating and the substrate. Only simultaneous alloying with aluminum and boron results in a slightly different fractographic pattern because these additions embrittle the material of the coating.Translated from Poroshkovaya Metallurgiya, No. 7(343), pp. 53–61, July, 1991.     

Preparation and study of rhombic iron carbide

Conclusions The authos investigated the K-spectrum of iron absorption in steel carbides, containing silicon and chromium. It is shown that the K-edge of iron absorption in steel carbide, alloyed with silicon and tempered at a high temperature (650), is similar to the K-edge of iron absorption in the carbide of carbon steek, tempered at 400. The presence of chromium in the carbide changes the form and energetic position of K-edge absorption in it. This change depends on the temperature of tempering and chromium concentration in the steel.Translated from Poroshkovaya Metallurgiya, No. 8(44), pp. 29–32, August, 1966.     

Hot pressing and vacuum sintering of fine titanium nitride powder

Conclusions Increasing the specific surface of titanium nitride from 18 to 90 m²/g lowers the initial recrystallization temperature of loosely poured powder from 1300 to 600°K. The temperature at which blanks attain practically 100% density in the hot pressing of finely divided titanium nitride (a starting powder particle size of 0.05–0.07 m) is 1600°K, which is 500–700°K below the temperature level of full sintering of relatively coarsegrained powders (a particle size of about 0.5 m). At hot-pressing temperatures above 1800°K a fall in the density of sintered compacts is observed, which is apparently attributable to the beginning of nitrogen evolution from the nitride and also to the formation of microcracks. In vacuum sintering without a plasticizer, crack formation lowering the density of specimens by 3–4% is characteristic of the whole sintering temperature range. The grain size in hot pressing and vacuum sintering is practically the same, being determined chiefly by the sintering temperature and time. At the maximum specimen density the maximum grain size is 20 m.Translated from Poroshkovaya Metallurgiya, No. 12(204), pp. 27–32, December, 1979.The authors wish to thank V. I. Berestenko, T. N. Miller, and D. I. Medvedev for the provision of titanium nitride specimens.     

Properties of Highly-Dense Iron‐Base Powder Metallurgy Materials Pressed without Zinc Stearate

The possibility of obtaining highlydense compacts by single pressing and sintering and the use of heatresistant solid lubricants (graphite, talc) instead of conventional lubricant was studied. It was shown that the addition of graphite is most effective to obtain highly-dense powder metallurgy materials since it substantially facilitates ejection of the part from the die and improves the mechanical properties.     

Contact reaction of Kh20N80 alloy with boron nitride

Conclusions A study was made of the causes of the contact reaction between Kh20N80 alloy and boron nitride during the sintering of composite compacts in various environments at temperatures of 1170–1330°C. It was established that the contact reaction between Kh20N80 alloy and boron nitride occurs in the following stages:partial dissociation of the boron nitride, with the evolution of free nitrogen, which then reacts with the chromium;reaction of boron with the nickel and chromium, with the formation of boride phases;enrichment of the nickel-base solid solution in chromium and the appearance of a chromium-base solid solution.In sintering in various gaseous environments the temperature at which partial melting begins during the contact reaction falls in the order argon (1330°C) > hydrogen and cracked ammonia (1250°C) > vacuum (1170°C).A 20% Cr-80% Ni alloy - Translator.Translated from Poroshkovaya Metallurgiya, No. 5(161), pp. 31–39, May, 1976.     

Structural Changes during Milling of Silicon Carbide

The kinetics of milling and structural changes during milling of commercial silicon carbide powder in a ball mill lined with hard alloy are studied. Data are provided for the specific surface, x-ray patterns, and infrared spectra. It is shown that milling kinetics are described best of all by an exponential relationship and that milling of silicon carbide powder is accompanied by changes in polytype composition. The x-ray patterns and infrared spectra obtained make it possible to assume that during milling the 15R polytype content in the powder composition decreases while the content of structures with less layering (the 4H polytype and -SiC) increases; in this case the number of absorption bands in the infrared spectra decreases.     

Regulation and determination of the phase composition of tungsten hard metals

Conclusions The two-frequency magnetic method can be employed for detecting the presence of additional phases (free carbon and the ₁-phase) in tungsten hard metals. The degree of carburization of tungsten hard metals and hence the presence and amount of free carbon and the ₁-phase can be regulated with sufficient accuracy for practical purposes by adding graphite or tungsten powder to electrocorundum packing material and varying the rate of hydrogen flow. Control over this process may be exercised by the magnetic method. The carbon content of tungsten hard metals grows when the concentration of graphite in packing material is raised, the concentration of tungsten in packing material is lowered, and the rate of hydrogen flow is decreased. The hard metals can thus change in structure from three-phase WC + Co + ₁ to two-phase WC + Co and then to three-phase WC + Co + C alloys.Translated from Poroshkovaya Metallurgiya, No. 4(220), pp. 46–50, April, 1981.