Effect of conditions of preparation of nickel fibers extruded from powders on their strength properties. Part II

Conclusions In the manufacture of extruded fibers the main process parameters determining their strength properties are sintering temperature and sintering time up to 1 h. Structural factors adversely affecting the strength of extruded nickel fibers are porosity, interfaces between blocks, and ash residue inclusions. Raising the sintering temperature from 1200 to 1280°C decreases the porosity and amount of ash residue inclusions and smoothens out the fiber surface profile. Under these conditions the strength of the fibers rises from 0.5–0.7 to 0.7–0.8 of the ultimate strength of the cast metal annealed at the same temperature.Translated from Poroshkovaya Metallurgiya, No. 7(211), pp. 41–44, July, 1980.     

Effects of oxide particles on properties of sintered nickel

Conclusions The effect was investigated of the addition of aluminum, zirconium, silicon, and hafnium oxides on the hot hardness and rupture strength of nickel at temperatures of 600–800°C. The rupture strength of nickel is shown to increase with increasing amount of oxide introduced into it and with decreasing oxide particle size. In tests below the temperatures at which transformations take place, the nature of the oxides investigated does not affect the properties of nickel.Translated from Poroshkovaya Metallurgiya, No. 6 (78), pp. 81–86, June, 1969.     

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.     

Activated graphitization of carbon fibers in a composite material with a nickel matrix

Conclusions Activated graphitization of nickel-plated carbon fibers commences at the instant when diffusional penetration of the nickel into the fibers begins. As nickel-activated graphitization advances, the amount of nickel in the fibers rises, reaching some hundredths of one weight percent in fully graphitized fibers. A dominant role in the initiation and propagation of nickel-activated graphitization of carbon fibers is played by the high dispersion of their internal structure, which ensures effective penetration of nickel into the fibers at anomalously low temperatures. A hypothesis is advanced according to which the graphite phase appearing in carbon fibers during their nickel-activated graphitization forms through the packing of structural complexes — defect-free portions of the (002) basal planes of the graphite structure.Translated from Poroshkovaya Metallurgiya, No. 4(208), pp. 57–62, April, 1980.     

High-porosity permeable materials from metal fibers

Conclusions Experimental values of strength of fine copper and nickel fiber materials sintered, respectively, in a hydrogen atmosphere at 1000°C and in a vacuum at 1400°C are in agreement with calculated results. By subjecting sintered, metal fiber felts to free compression it is possible to produce materials with reductions of up to 75% and porosities of more than 60% without inducing contact phenomena which have a deleterious effect on their properties. The thermal conductivity of fiber materials with porosities of 65–90% can be correctly calculated with Nekrasov's formula.Translated from Poroshkovaya Metallurgiya, No. 8(224), pp. 44–47, August, 1981.     

Some mechanical properties of silicon carbide fibers

Conclusions Electroerosion of silicon carbide fibers in contaminated mercury electrical contacts decreases their strength by between one-half and two-thirds. During prolonged holding under load at room temperature the creep of silicon carbide fibers is close to zero, while at 1173°K l/l 10^–7 l/day. X-ray structural analysis of silicon carbide fibers annealed for (1.8–10.8)·10³ sec in a hydrogen atmosphere at 1273–1773°K and for (3.6–18)·10³ sec in a nitrogen atmosphere at 1773–1973°K revealed no recrystallization in (4–5)·10^–7-m-thick layers of polycrystalline SiC deposited from a gaseous phase. A correlation was found between the strength of silicon carbide fibers and the number of flashes forming during their fracture.Translated from Poroghkovaya Metallurgiya, No. 1(253), pp. 55–59, January, 1984.     

Structural changes accompanying the sintering of ultrafine aluminum nitride powders

Conclusions An electron-microscopical investigation has established that the sintering of compacts from an ultrafine plasma-chemical AlN powder is determined by the following processes: coagulation, with the formation of strong polycrystalline aggregates (1000–1400°C); coalescence, made possible by a geometric grain boundary match inside the aggregates, and formation of high-angle boundaries between aggregates (1200–1600°C); and collective recrystallization (above 1600°C). In an x-ray diffraction study annealing at temperatures above 1700°C was found to decrease the parameter c of AlN by 0.002 Å as a result of dissolution of oxygen in the lattice without affecting its parametera. The results of both electron-microscopical and x-ray diffraction studies show that annealing at temperatures above 1600°C increases the defectiveness of the wurtzite structure of AlN, brings about twinning and the formation of polytypal interlayers in recrystallized grains, and raises the concentration of other defects causing a broadening of lines in x-ray diffraction photographs. As a result of these phenomena the material experiences a strengthening at sintering temperatures of 1700–1800°C.Translated from Poroshkovaya Metallurgiya, No. 10(226), pp. 35–40, October, 1981.The authors wish to thank A. N. Pilyankevich for helpful discussion.     

Heat treatment of exteuded corrosion- and oxidation-resistant alloy fibers and their strength properties

Conclusions It is shown that fibers of corrosion- and oxidation-resistant nickel-base alloys can be obtained by subjecting viscose fibers filled with nickel, chromium, and molybdenum powders to heat treatment. It has been established that alloy formation in such fibers and the mechanical properties attained in them as a result of this operation depend on heat-treatment parameters controlling the amount of residual carbon in the fibers after the thermal destruction of the polymer binder (cellulose) and also on the final sintering temperature. After two-stage heat treatment — preoxidation in air at 400–450°C and final sintering in hydrogen at 1200°C — extruded Ni-20 Cr, Ni-30 Mo, and M-15 Cr-15 Mo alloy fibers (of 12- to 24-m diameters) were found to have ultimate tensile strengths of 40–52 kgf/mm².Translated from Poroshkovaya Metallurgiya, No. 2(218), pp. 1–5, February, 1981.     

Structural studies of ceramics based on detonation-wave-treated graphite-like boron nitride

The formation of a structure during pressureless sintering of graphite-like boron nitride (BN) powders pretreated in detonation waves was studied. The treated powders contained 10–12% wurtzite BN phase and 2–3% sphalerite phase. During sintering at just 1200 °C a reverse transformation from dense BN phases to graphite-like phases and primary recrystallization of highly fragmented BN_g occur as a result of the pretreatment. Accretive recrystallization of BN_g begins at 1500 °C and at T=1700–1950 °C platy grains with a size of 1–3 µm are formed in the developed surface. At 1700 °C the density of the specimens reach 0.95 of the theoretical value. The specimens produced at 1950 °C have a higher compressive strength and radiation resistance than do those made without pretreatment.Deceased.Institute of Materials Science, Ukrainian Academy of Science, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5/6, pp. 75–80, May–June, 1995.     

Dependence of the mechanical properties of a P/M nickel alloy on deformation temperature and rate

Conclusions At room temperature the strength of ZhS6 nickel alloy produced by the granule metallurgy method is 1.5 times higher and its plasticity 3–4 times higher than those of the same alloy produced by the standard casting process. The P/M alloy remains superior in strength to the cast alloy up to temperatures of the order of 750–800°C, above which the strength characteristics of the P/M alloy fall below the level of properties of the cast material. The deformation of the P/M alloy varies in character depending on testing temperature, and three types of specimen fracture are possible: brittle fracture without waist formation in the temperature range 20–700°C, fracture with waist formation at 700–1000°C, and plastic deformation along the whole specimen length above 1000°C. At temperatures of 1000–1150°C the P/M alloy exhibits superplasticity: At low rates of deformation its elongation attains several hundred percent when the load is about 0.1_0.2. The elongation of the specimen, load applied, and type of stress-strain diagram depend to a large extent on the rate of deformation: As the latter is decreased, _e sharply grows and the load diminishes.Translated from Poroshkovaya Metallurgiya, No. 1(205), pp. 76–80, January, 1980.     

Effect of annealing on the structure and properties of fused titanium carbide

Conclusions It has been established that, under the action of thermal stresses set up during rapid cooling after high-temperature heating, titanium carbide undergoes plastic deformation. This manifests itself in the appearance of slip bands, on which excess carbon is precipitated. With increase in annealing time, these bands decrease in number and increase in size. At the points of intersection of slip bands, flocculent graphite precipitates are formed.Annealing at 1400–1600°C leads to the formation of large equilibrium titanium carbide grains and precipitation of excess carbon on their boundaries. Annealing at 2000–2200°C produces a microstructure with large equilibrium grains, excess carbon in this case being precipitated at the points of intersection of linear defects. Stabilization of thermoelectrical characteristics is achieved after annealing for 4 h at temperatures of 1600–2200°C.Translated from Poroshkovaya Metallurgiya, No. 10 (94), pp. 78–82, October, 1970.     

Investigation of the magnetic properties of strip made from iron powder

Conclusions Strip with a porosity of 25%, when sintered at a temperature of 1200°C in hydrogen with a dewpoint of –30°C, is refined with respect to carbon and oxygen. At the lower sintering temperatures of 1000 and 1100°C, the carbon content is reduced to 0.03–0.02%; the quantity of not easily reducible oxides remains unaltered.With regard to the kinetics of grain growth, nonporous strip made from iron powder may be classed with steels which are coarse-grained by their prehistory.Porosity substantially affects the value of coercivity. Each 2% of pores increases the coercivity approximately by 0.1 oersted.If temperatures of 1200°C and above, with holding times of 2–3 hr are used for presintering, and temperatures of 900–1000°C for the final heat treatment of the nonporous strip, the magnetic properties of the strip can satisfy the requirements of GOST 3836-47 for low-carbon, electrical engineering thin sheet steel.     

High-temperature friction of alloys of the TiNx-TiB2 system

Conclusions It has been established that the highest transverse rupture strength and wear resistance combined with the lowest coefficient of friction are shown by alloys of eutectic composition. In alloys of the TiN_0·9-TiB₂ system higher ductility during friction is exhibited by the boride phase, whose substructure experiences greater changes compared with the nitride phase. The greatest deformation during friction characterizes the phases in the TiN_0·73-TiB₂ alloy. The strengthening and strength loss processes at temperatures of 20–400°C are determined by the strength loss processes occurring in the boride phase, and those above 400°C, by the strengthening and strength loss processes taking place in the nitride phase.Translated from Poroshkovaya Metallurgiya, No. 2(242), pp. 70–76, February, 1983.     

Properties and structure of niobium carbide heating elements

Conclusions In operation at 1900°C in a carbon-free atmosphere at a residual pressure of 10^–2-2· 10^–3 Pa a heating element of composition NbC + Nb₂C + Nb experiences a steady loss of carbon at a rate of 0.5–1 m/h from its surface, with the formation of a metallic phase. Heating elements of this composition may be recommended for service in vacuum resistance furnaces at temperatures of up to 1900°C, where they can be expected to have a useful life of the order of 1000 h. Niobium monocarbide heating elements operating at 2100–2500°C in a carbon-containing atmosphere under a pressure of 10^–2-2·10^–3 Pa experience no change in phase composition or shape and only minor changes in structure, and possess good thermal fatigue resistance. They can thus be employed in vacuum electric furnaces under these conditions. Long experience (more than two years) with the operation of a laboratory vacuum electric furnace with a niobium carbide heating element in the temperature range 1000–1500°C has demonstrated that the use of niobium carbide heating elements widens scope for new electro-thermal equipment.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 92–97, March, 1982.     

Diffusion Interaction in Fiber-Reinforced Composites of the Chromium Alloy – Silicon Carbide System

Diffusion interaction in the Cr – ZrC and Cr – HfC systems has been studied in the temperature range 1200-1400°C. The impurities contained in chromium or carbides were found to affect the interaction in the systems. Almost no interaction was observed when the systems were held at temperatures up to 1300°C for 100 h. At higher temperatures the carbides were reduced to ZrO₂, HfO₂, and Cr₂₃C₆ because of the presence of a slight amount of oxygen. Thermodynamic calculations indicated no interaction in these systems at temperatures up to 1600°C. A study of the interaction in the systems SiC – ZrC and SiC – HfC showed that a transition zone formed already in the stage of sample preparation by diffusion welding in vacuum (1300°C, vacuum of 10^_3 Pa, with a load applied for 20 min). During annealing (1300°C, 50 h) the transition zone stratified, forming a solid solution of silicon in Zr(Hf)C and SiC inclusions in the SiC – Zr(Hf)C solid solution. A transition zone formed on the zirconium carbide side when SiC interacted with Zr(Hf)C. The interaction in SiC – Zr(Hf)C casts doubt on the use of them as a barrier without antidiffusion layers.     

Sinterability of porous compacts from molybdenum fibers with small nickel additions

Conclusions The results are presented as an experimental investigation into the sinterability of molybdenum fibers with small nickel additions. The possibility is shown of activating the sintering of molybdenum fibers by the addition of a small (up to 1 wt.%) amount of nickel. The presence of nickel sharply decreases the impact strength of sintered fiber compacts.Translated from Poroshkovaya Metallurgiya, No. 8 (68), pp. 23–25, August, 1968.     

Oxidation of Cr-Ni coatings on diamond

The kinetics of the oxidation of chromium and chromium-nickel coated diamond powders in air at 750 and 850°C was studied by means of interrupted weighing. The coatings and metallized powders were subjected to x-ray phase analysis. Decrease of the powder mass confirms that permeation of the mass with volatile oxides predominates over the increment due to the weight of the absorbed oxygen in the oxides NiO and Cr₂O₃. The weight loss is caused by the burning away of both the coating (oxidation of chromium and its carbides to CrO with formation of CO and CO₂) and the diamond. Carbon from the diamond is also removed with CO and CO₂ as a result of successive reduction of chromium oxide by carbon, which diffuses to the oxidized layers, and further reduction of the carbides by the same mechanism. The higher the oxidation temperature and the thinner the coating, the greater the contribution of these reactions is to the processes responsible for the decrease in weight of the metallized diamonds. It is also encouraged by diffusion of carbon to the scale formed and by simultaneous diffusion of oxygen to the coating. The reactions of primary oxidation of chromium contained in the nickel solid solution, reduction of nickel by carbon and chromium, and dissolution of chromium in the reduced nickel contribute significantly to the oxidation of chromium-nickel coatings.Translated from Poroshkovaya Metallurgiya, No. 7(367), pp. 62–66, July, 1993.     

Some characteristic features of the production of extruded nickel fibers

Conclusions The catalytic action of nickel on the process of carbon removal during the heat treatment of viscose filaments filled with nickel powder enables virtually nonporous metal fibers to be obtained. In the sintering of nickel-containing viscose fibers the fiber preoxidation operation can be dispensed with, which simplifies the process of production of such fibers.Translated from Poroshkovaya Metallurgiya, No. 2(206), pp. 1–4, February, 1980.     

Effect of Manganese and Niobium and Rolling Conditions on the Properties of Low-Alloy Steel

A series of studies was conducted to determine the optimum manganese and niobium contents in steels alloyed with the system C–Mn–Nb. Researchers also wanted to find the starting and finishing temperatures for rolling on a 2800 mill that would be best in order to obtain the necessary mechanical properties when the metal is deformed in the austenite range with small reductions.The studies established the following:– a greater improvement in strength properties is obtained from an increase in the mass content of manganese if the carbon content is reduced (0.09–0.10%) rather than increased (0.12–0.13%);– other conditions being equal, an increase in the niobium content of the steel from 0.02–0.03% to 0.04–0.06% leads to a substantial increase in strength and toughness properties and a decrease in ductility;– it was established that in terms of its effect on the yield point, microalloying steel with 0.01% niobium is equivalent to alloying with 0.30–0.40% manganese;– the percentage of the tough B component in the fracture of IPG specimens of low-alloy steel with a niobium content of 0.02–0.03 mass % can be increased from 30 to 100% by decreasing the temperature of the slab after the second pass through the two-high stand from 1020 to 980°C. For steels with a higher niobium content (0.05–0.06%), the temperature of the metal when it enters the roughing stand can be increased to 1010–1040°C without adversely affecting the standard characteristic B (%);– a decrease in the finishing temperature is accompanied by an increase in the strength and toughness properties of the metal and a decrease in ductility. Reducing the finishing temperature to below 760°C makes it possible to increase strength properties but does not change impact toughness.     

Production of powders of cast iron shavings and special features of production of components from these powders. III. Structure and strength of heat-treated alloys

Conclusions Quenching the specimens from 800°C does not lead to the redistribution of the carbide in the volume of the alloys of the iron-cast iron system. The most homogeneous structure of the alloys is formed only in quenching from 900°C. However, quenching from 1000°C increases the size of voids and causes supinating of the alloys, although their homogeneity increases.Heat treatment increases the tensile strength of the alloys which contain 10–50% of low-alloy cast iron. The alloys quenched from 800°C have the maximum strength after tempering of 300°C, whereas the alloys quenched from 900 and 1000°C with a maximum strength at tempering temperatures of 250 and 200°C, respectively.The highest hardness was recorded for the alloys after quenching from 900°C. All the alloys quenched from 800, 900, and 1000°C are characterized by sufficiently higher hardenability to a depth of 4mm.An increase of the tempering temperature of the alloys in the range 200–300°C reduces their impact toughness. This fact is not in agreement with the generally recognized interpretations. However, an increase of the cast iron content usually greatly reduces the KC value of the specimens, regardless of the quenching and tempering temperatures.Translated from Poroshkovaya Metallurgiya, No. 1(325), pp. 47–53, January, 1990.