Showing papers in "Powder Metallurgy in 1968"

Densification processes in the tungsten carbide-cobalt system*

Abstract: Densification of the tungsten carbide-cobalt system has been investigated by determining the effect of the principal sintering variables-composition, temperature and time of sintering, particle size, ball-milling-and by studying the processes that occur. Considerable shrinkage takes place during heating, before the eutectic temperature is attained. A 9% cobalt alloy sintered entirely in the solid state to give comparable density and mechanical properties to those attained by liquid-phase sintering, but the sintering time was increased by a factor of 10. Densification proceeds from nuclei created by ball-milling, which packs the porous cobalt agglomerates with tungsten-carbide particles; if the cobalt particles are only mixed with the tungsten carbide, then on sintering they flow out into the matrix leaving behind voids that do not fill. Densification is characterizedby two features: first, tungsten-carbide particles cement together with cobalt between grains to form clusters and filaments; secondl...

The peace river process for the production of iron powder

Abstract: This hydrometallurgical iron powder process is a joint development by the Research Council of Alberta and Peace River Mining and Smelting, Ltd., in Canada, and Woodall-Duckham, Ltd., in England. A 5 tons/day pilot plant has been in operation for 2 years and work has started on a 50,000 tons/year commercial plant.Essentially, the method consists of leaching an iron-bearing feedstock with hydrochloric acid, crystallizing the resultant liquor to pure ferrous chloride, reducing the chloride to sponge iron, and grinding the sponge to powder. The chemical properties of the product are controlled at the leaching and crystallization stages and the physical properties during reduction and grinding.A wide range of raw materials has been tested, including pyrites, ilmenite, pickle liquor, scrap, and low-grade ores. Purification is good, and where applicable, a second product can be recovered, e.g. sulphur from pyrites and TiO2 from ilmenite. Alloy powders can be made with slight process modifications.The var...

Lattice strain in tungsten carbide

Abstract: X-ray-diffraction measurements have shown that ball-milling of tungsten carbide powder gives rise to considerable line broadening. Analysis of this broadening by integral-breadth and Fourier methods indicates that the main cause of the broadening is lattice microstrains. X-ray studies have been made on tungsten carbide powders, sintered blocks, and tool tips with cobalt binding, to determine how this strain varies at different stages of treatment of the material. It was found that, after sintering, the tungsten carbide was strain-free, but diamond polishing and mechanical work induced strain in the surface layers.

Direct roll compacting sheet from particles

Abstract: The direct roll compacting process for the consolidation of particulate material into sheet form is discussed. Comparisons are made between the compacting characteristics of fine and coarse particl...

The flow of iron powder during roll compaction

Abstract: An experimental study has been made of the direct rolling of several iron powders to form strip. The variables connected with the powder feed have been related to the flow properties of the powders and to the thickness, density, and strength of the green strip. The effect of speed of rolling on these properties has also been determined.

The fabrication of components from aluminium reinforced with silica fibres

Abstract: The salient fabrication parameters involved in the hot pressing of composites of aluminium reinforced with silica fibres have been studied and their effects evaluated. The roles of temperature, atmosphere, and time in promoting bonding are much as would be anticipated from general powder-metallurgy considerations. The effect of pressure is modified by mechanical damage to the fibres at high pressures. This damage can be minimized by ensuring good fibre alignment.Methods have been developed, analogous to those used with fibre reinforced plastics, for the fabrication of simple filament-wound shapes, such as rings, and of more complex shapes. A few trials have been made on pressure-welding and soldering techniques for forming lap-joints between pieces of aluminium-silica.

The production of stainless-steel strip from powder

Abstract: If the market for stainless steel is to be increased substantially in the near future, the material must be made more competitive with others. With this in view alternative production methods for stainless-steel strip, initially in the size range 0·080–0·010 in., have been examined.The production of Type-304L strip direct from water-atomized powder without the addition of bonding agents is described. The production cycle for powder-rolled strip consists of roll compaction using a saturated feed, sintering in line with the compaction mill, conventional cold reduction, and a final annealing treatment. Each stage is discussed, together with the final properties achieved.The main advantages of this production route lie in the elimination of ingot casting and hot rolling and their attendant cleaning and rectification processes. The ability to produce a range of green-strip thicknesses enables the amount of deformation to be reduced to that necessary to obtain the desired surface finish and mechanical p...

Effect of porosity and particle size on the mechanical strength of sintered iron

Abstract: The flexural and tensile strengths and the fatigue limits of sintered iron materials have been measured as a function of porosity and pore size. The resulting experimental values were lower than the theoretical. This discrepancy is attributed to a pore-size effect, analogous to the results of earlier work in which the influence of pore size on elastic modulus was studied (G. Artusio, V. Gallina, G. Mannone, and E. Sgambetterra, Powder Met., 1966, 9, (17), 89).

Metal powder production by the gel-precipitation process*

Abstract: A novel process, termed gel precipitation, has been developed for producing gels of insoluble metallic compounds from aqueous solution. Controlled additions of a solution of the metal, which also contains the dissolved gelling agent, are mixed with a solution of the precipitating reagent, e.g. ammonium hydroxide. The gel is formed by the association of the gelling agent, usually a polymeric organic material, with the phase normally precipitated and in its truest form probably consists of a molecular complex between the organic constituent and the metal compound. The gel forms rapidly on bringing the two solutions into contact and there is no tendency for the discrete particles to adhere or coalesce; all the metal ion is normally confined to the gel phase.The outstanding characteristics of the process are its simplicity and versatility; it operates at normal temperature and requires only conventional chemical plant. It is particularly applicable to precipitating conditions that yield gelatinous pre...

Thin steel strip from powder

Abstract: A process for rolling thin strip from powder is described which overcomes the disadvantages normally associated with powder rolling. Strip samples have been made from both iron and stainless-steel powders and an experimental plant has been built for continuous strip production. An economic assessment of the manufacture of mild steel and stainless-steel strip by the powder route shows possible cost savings when compared with existing production methods.

Factors affecting the radial and axial shrinkage in soft-metal powder compacts

Abstract: The variables affecting the radial: axial (R/A) shrinkage ratio in copper-powder compacts have been investigated. The value of R/A is linearly dependent on compacting pressure, green density, and sintering temperature, and also increases with decrease in the particle size of the powder. The observed variation of R/A is attributed to the differences in density in the green compacts, which result in anisotropic stresses in sintering. Surface-tension forces or residual stresses introduced during compaction cannot alone be regarded as the main driving forces responsible for shrinkage; anisotropic stresses also play an important role in the densification of metal-powder compacts. By proper control of these variables, parts can be produced from the compacts to close dimensional tolerances.

Dispersions of boron carbide with barrier layers in stainless steel

Abstract: The possibility has been studied of preventing reaction between boron-carbide particles and an austenitic stainless-steel matrix by means of a a barrier layer on the particles. Silicon carbide is compatible with boron carbide up to 2000°C but reacts extensively with austenitic stainless steel above 1000°C and is thus ineffective as a barrier layer. Titanium carbide deposited from the vapour phase, although it reacts with B4C above 1300°C and under certain conditions with austenitic stainless steel at 1100°C, was the most suitable material considered.

The economics of stainless-steel strip production by roll compaction

Abstract: While production of a material by a powder-metallurgy route is often necessary to obtain desired properties, the potential economic advantages of such a method may also be sufficiently attractive to justify development on a large scale. This particularly applies, in some instances, to the production of thin strip by roll compaction.The economic attraction of the roll-compaction process lies in the combination of the potentially high utilization of material, low processing costs, and modest demands for capital equipment. With certain materials, powder is readily available at a price that enables these advantages to be realized. However, with others, such as mild steel, new techniques of low-cost powder production must be developed to ensure survival of the method.The more highly alloyed materials, such as stainless steel, generally show a low yield by conventional processing routes and thus the scope for improvement is greater. The addition of alloying elements in the solid state raises certain pro...

A mechanism for the pressure-sintering (hot pressing) of beryllium

Abstract: Previous work on the vacuum hot pressing of beryllium powders is reviewed to illustrate the dominant role of powder chemistry in consolidation behaviour. New electron-microscope evidence is advanced to show that oxides are present on the surface of beryllium powder particles but, in contrast, are not found predominantly at the grain boundaries of hot-pressed compacts.Finally, a model is proposed incorporating the macro and micro features of consolidation, which involves sintering of the compact followed by recrystallization.

Iron powder: prices, costs, and possibilities

Abstract: Alternative feedstocks are reviewed in terms of availability, quality, and order of cost. The principal methods of processing these feedstock—spractised and proposed—are listed, with comments on significant features. Particular attention is directed to the energy requirements for reduction in the case of low-temperature chemical processes, conventional powder processes employing direct reduction, and other methods. Approximate capital costs are provided to form the basis of estimates regarding the economy to be expected at different scales of operation. From these estimates the changing significance of the various components of cost is examined. By means of an empirical correlation derived from market behaviour, possible selling prices are associated with possible output levels.

The properties of cobalt–iron alloys prepared by powder rolling

Abstract: A series of cobalt–iron alloys containing 0–15% iron was prepared in the laboratory by using powder-metallurgical techniques. The alloys were made by mixing commercially pure cobalt and iron powders, roll compacting the blended powders into strip, and directly hot rolling the roll-compacted strip into fully dense alloy strip. The procedures and equipment used to prepare the alloys are described and the structural and mechanical properties of the resulting strip are discussed as a function of alloy composition. The study indicated that a 7% iron addition suppressed the α (f.c.c.) ⇌ ϵ (h.c.p.) transformation in cobalt. The ductility of the alloy series increased with increasing iron content and X-ray and dilatometric analysis related this improvement to the elimination of the ϵ (h.c.p.) phase. One striking feature of the experimental results was that changes in the structural and mechanical properties were achieved without alloy homogenization.The forming characteristics of the alloys were optimized...

Tensile evaluation of unworked tungsten-25% rhenium alloys consolidated by sintering elemental powders*

Abstract: The use of refractory metals in the as-sintered condition has been precluded by the belief that useful engineering properties cannot be obtained in the unwrought alloys.A comparison of tensile data for commercial wrought tungsten-25% rhenium rod in the recrystallized condition and unwrought as-sinteredrod directly converted from elemental powders is presented, to show that at comparable grain size there need be no marked difference in properties. After exposure to very high temperatures the sintered material is markedly superior to the wrought recrystallized alloy as a result of the differences in grain-growth characteristics.

Chromium and Nickel Powders Made by Reduction of Their Oxides with Magnesium, Lithium, or Sodium Vapors

Abstract: Chromium powder separable into 0·0068–μm colloidal and 0·16–μm coarse powder fractions was obtained by reduction of Cr2O3 with Mg vapour in a vacuum furnace. The coarse powder fraction was calculated to contain 8 vol.-% residual MgO plus MgO. Cr2O3 dispersoids, probably suitable for dispersion-strengthening. Other chromium powders, of 0·08–0·60–μm particle size and with oxygen: surface area ratios of ~0·002 g/m2, were made by reduction of Cr2O3 with Li and Na vapours. Nickel powder, of 1·25–μm particle size and with 0·114% oxygen, was made by reduction of NiO with Li vapour.

The production of porous nickel sheet by direct rolling of powder

Abstract: Porous nickel sheet may be produced by the direct roll compaction of nickel powder. An experimental mill used for the production of thin strip is described, and a review of the process is given. Methods of controlling the porosity of the product and the maximum pore size are discussed and some of the problems associated with the production of highly porous material indicated. The air permeability, maximum pore size, and tensile properties of porous nickel strip of a variety of porosities are reported upon, as is the reproducibility of the process.