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).     

REINFORCING EFFECTS OF SILICON-NITRIDE WHISKERS IN SILVER AND RESIN MATRICES

Abstract

Whiskers of silicon nitride have been incorporated in two representative matrices-silver and an epoxide resin. The techniques are described and the measured properties of the composite materials discussed in the light of existing theories of reinforcement. Reinforcement has been established both in silver and resin, using the whiskers in random orientation. Increases in Young’s modulus, tensile strength, and hot strength are obtained with relatively small volume fractions of whiskers. The nature of the discrepancies between practical and ideal values of these properties indicates the course of future work.     

THE MECHANICAL PROPERTIES OF SINTERED LOW-ALLOY STEELS

Abstract

An unacceptable degree of scatter in the values obtained in tensile tests of sintered parts may occur if their elongation does not exceed a few percent. To ensure the required elongation, the composition of the material selected must correspond to that of alloys made by melting which exhibit a certain minimum elongation in the forged and annealed condition. This minimum is a function of the density of the sintered compact and a knowledge of its value can be of assistance in choosing the correct composition. Values of Young’s modulus, Poisson’s ratio, and impact-resistance are also given as a function of density.     

OPTIMIZATION OF PROPERTIES OF POWDERED IRON AND STAINLESS-STEEL PARTS BY STATISTICAL DESIGN AND COMPUTER ANALYSIS

Abstract

A statistically designed experiment was formulated to study the effect of several major powder variables on the strength properties of porous iron and stainless-steel parts. The resulting data were analysed by means of a suitable computer programme to develop individual response equations relating the chosen dependent variables with selected independent variables. Computer analysis of the data and the optimization techniques adopted led to an improvement of ～50% in the strength of sintered parts by comparison with those made by conventional processes. A certain set of powder properties and process variables resulted in a tensile strength of 170 MN/m², 11.5% elongation, and very low dimensional change in a sintered iron sample with 25% porosity. In a 316L stainless-steel part with 25% porosity, a tensile strength of 435 MN/m², 0.2% yield strength of 269 MN/m², and 12.6% elongation were reached–values far above those that can be obtained without the benefit of statistical design.     

THE IMPORTANCE OF SURFACE PROPERTIES AND PARTICLE SHAPE IN THE COMPACTION OF GRAPHITE AND MoS2

Abstract

Adsorptive studies of the surfaces of graphite and MOS₂have shown that these consist of two distinct types of site. The sites on the basal-plane surface differ from those on the edge surface with respect to their relative affinities for different organic compounds. These findings led to the development of grinding techniques to produce graphite and MoS₂ powders possessing different ratios of basal-plane:edge-surface area.

Grinding graphite and MoS₂ in the presence of low-viscosity, volatile hydrocarbons produced very thin flake-like powders, consisting predominantly of basal-plane surface. These fine flakes showed a high affinity for long-chain n-paraffins and were therefore termed oleophilic solids. Grinding under reduced pressure also produced very fine powders, having, however, a more granular structure exhibiting a far lower ratio of basal-plane: edge-surface area. These were termed polar solids to distinguish them from the solids ground in liquid hydrocarbons.

The cold-forming properties of the various powders have been compared under uniaxial compaction. The conversion of synthetic and natural graphite powders to the oleophilic form resulted in marked improvements in both compact strength and modulus. Synthetic graphite converted to the polar form would not form a compact at cold-forming pressures up to 800 MN/m².

The cohesive properties of the oleophilic graphite powders were improved by heating to 900°C in hydrogen. Electrical-resistivity measurements showed that cold-formed oleophilic graphite compacts exhibited a marked anisotropy. The improved cold-forming properties of the powders are ascribed directly to improved cohesion via basal-plane site interactions, coupled with the facility of the flake powders to take up a preferential orientation during compaction in order fully to utilize the extensive basal-plane sites available for cohesion.

The differences between the oleophilic and polar forms of MoS₂ were less marked. It is believed that interparticle cohesive junctions are more readily formed via edge/edge interactions, and basal-plane junctions do not play as important a role in the cohesion of MoS₂ as in that of graphite.

The corrosion and abrasion of metal surfaces by graphite and MoS₂ have been examined. In all cases the powders converted to the oleophilic form showed reduced abrasive and corrosive characteristics when compared to similar powders converted to the polar form. These improvements are believed to result from the reduction of the possibilities of edge interactions with the metal surfaces.     

Rotary cold re-pressing and heat treatment of sintered materials from Co–Cr–Mo alloy powder

Abstract

This paper presents the analysis of research results concerning the application of PM technology to produce porous implantation material from Co-Cr-Mo alloy. Rotary cold re-pressing and heat treatment has been used to increase the density and mechanical properties of sintered samples. The microstructure, hardness and compressive properties, and ultrasonic data of the obtained materials were investigated. The material had about 10% of porosity and has higher mechanical properties, i.e. UCS and plastic strain, compared with cast cobalt alloy, and had 25% lower values of Young's modulus and shear modulus.     

THE MECHANICAL PROPERTIES OF CERMETS WITH A METALLIC MATRIX

Abstract

The effective elastic moduli of cermets consisting of ceramic particles located in a metal are discussed and compared with the properties of sintered matrix materials. New experimental work shows that the properties depend significantly on the degree of bonding between the phases. If this is strong, the nominal Young’s modulus increases with concentration; if not (as in uranium oxide-stainless steel) it diminishes, but less rapidly than that of a porous steel. The rigidity modulus falls continuously with the proportion of the ceramic phase present, because local plastic deformation occurs rapidly in shear.

The decrease in tensile strength with concentration has previously been explained in terms of the packing arrangements of spheres, which reduce the load-bearing area of the matrix cross-section. A tentative equation is derived which shows that stress concentrations near the inclusions may also play an important role in determining the effective mechanicalstrength at high ceramic contents.     

The Influence of Ageing Temperature on the “Köster Effect” in Martensite

Abstract

Measurements have been taken which show that the rate s of recovery of internal friction and of modulus of rigidity in freshly quenched steel increase rapidly with the ageing temperature. The experimental results can be at least partly explained by the theory of Granato et al. However this theory cannot account completely for the experimental values of internal friction. A basis for a possible modification of the theory is suggested.     

STRUCTURAL AND MECHANICAL PROPERTIES OF SINTERED MAGNESIUM PRODUCTS AT ELEVATED TEMPERATURES

Abstract

The use of products sintered from pure magnesium or a magnesium alloy is envisaged for certain structural components in the field of nuclear technology. The mechanical properties at elevated temperatures (up to 525°C) of sintered compacts of oxidized Mg-Zr or Mg-MgO of various particle-size ranges (～20–800 μm) are discussed. Comparison of the tensile properties and of the values of the creep parameters in tension and compression emphasizes the direct dependence of the increase in mechanical strength at elevated temperatures on the content and distribution of the oxide and on the particle-size distribution of the oxidized powders. Metallographic examination of the various materials confirms these observations. The ductility varies with the size of the original particles.

Finally, data are presented for comparison on a material obtained by sintering magnesium-zirconium and magnesium-aluminium cuttings. In this case hardening occurs as a result of the formation of intermetallic compounds between the zirconium and the aluminium.     

MECHANICAL AND METALLURGICAL PROPERTIES OF POWDER FORGINGS

Abstract

The properties of a forged powder-metallurgical preform depend on all stages of manufacture from powder production to the heat-treatment of the finished forging. Special reference is made to the effects of process variables on mechanical and metallurgical properties.     

FABRICATION AND PROPERTIES OF SOME DISPERSION-STRENGTHENED COPPER-GLASS ALLOYS

Abstract

The fabrication of dispersion-strengthened copper-glass alloys is described and the effect of Cu₂O and Ti as adherence promoters evaluated. An unusual particle size effect has been observed, and a particle/void interaction model is proposed to account for this behaviour.     

THE MEASUREMENT AND FORM OF POROSITY IN THE LOOSE SINTERING OF COPPER COMPACTS

Abstract

Changes in surface area of specimens of loose-sintered –300-mesh spherical copper powder, measured by the BET gas-adsorption technique, are given for sintering temperatures of 700,800,900, and 1000°C under furnace atmospheres of hydrogen and argon, for times of 0–24 h. Porosity determinations, using a xylene-impregnation technique, show that the porosity is composed entirely of interconnected pores at 700, 800, and 900°C, connected porosity occurring only after 14 h sintering at 1000°C. Determinations of pore-size distribution are also given, measured by a technique based on a “capillary rise of a liquid in a porous material”. Results indicate that for specimens sintered under a hydrogen atmosphere, an overall increase in pore size occurs, whereas for specimens sintered at 800 and 900°C under an argon atmosphere the size of the majority of the pores remains constant, whilst a small percentage of extremely large pores is developed. Permeability coefficients calculated from surface-area and pore-size distribution data are compared with the experimental values.     

THE HEAT -TREATMENT AND MICROSTRUCTURE OF A SINTERED NICKEL-MOLYBDENUM STEEL

Abstract

The effects of post-sintering heat-treatment on the properties and microstructure of a 5Ni–0·5Mo–0·5C sintered steel have been studied in detail. It has been demonstrated that by using a relatively high tempering temperature after oil-quenching, UTS and elongation can be simultaneously raised above their as-sintered values, while impact-resistance is at least maintained. By tempering at 650–675°C a UTS of 900 N/mm² can thus be combined with 4% tensile elongation and with a Charpy unnotched impact value of 26 J. Properties close to these can be achieved even if oil-quenching is replaced by cooling in a furnace cold zone. Alternatively, by quenching and then tempering at 200°C the UTS can be raised to ～1400 N/mm² with 1 ½ elongation. The constituents of the microstructures, which are generally not homogeneous, have been identified for each heat treatment condition. It is concluded that for this steel, which is predominantly martensitic on quenching, the properties are controlled by the tempering of the martensite, and the heterogeneity generally does not have any pronounced effect on properties. Studies of the effect of raw material variables have indicated: (a) that iron powder compressibility and even the final density of the steel are not a safe guide to properties after high-temperature sintering; (b) that costs may be reduced without detriment to properties by using ferromolybdenum powder instead of elemental molybdenum; (c) that the use of ferronickel powder or codecomposed iron/nickel gives results inferior to those achieved with elemental carbonyl nickel powder.     

THE STRUCTURE AND PROPERTIES OF POROUS NICKEL PLAQUES

Abstract

The structure of porous nickel plaques produced by the following routes has been examined using scanning electron microscopy: (a) pressure-less sintering of nickel powder; (b) slurry sintering of powder; (c) roll-compaction and sintering of powder; (d) pyrolysis of cloth impregnated with nickel chloride. The observations show that measurements of pore-size distribution with a mercury porosimeter can be misleading, and also reveal the existence of fine closed porosity in plaques made from impregnated cloth. The relation of the observed porosity characteristics and inter-particle bond diameters to the mechanical and electrical properties of the plaques is discussed.     

PRODUCTION AND PROPERTIES OF GRAIN-STABILIZED PLATINUM AND PLATINUM ALLOYS

Abstract

The production techniques used in the manufacture of grain-stabilized platinum and platinum alloys are discussed. Room-temperature tensile and stress-rupture results covering the temperature range 1250–1460°C for lives of 10–1000 h, indicating the improved strengths that are obtained with thoria-stabilized platinum-l0% rhodium alloy and “Fibro” thermo-element platinum, are quoted, and comparison is made with the results obtained for the equivalent melted materials. The structural properties of the grain-stabilized materials have been studied     

THE PROPERTIES AND SOME APPLICATIONS OF CARBONYL-NICKEL POWDERS

Abstract

Nickel carbonyl, formed by the reaction of carbon monoxide with a nickel concentrate, can be decomposed in a heated space to produce pure nickel in powder form. By varying the decomposition conditions it is possible to achieve a wide range of powders, differing in particle shape and size distribution.

The properties of carbonyl-nickel powders now commercially available or in an advanced stage of development are discussed, and it is shown how such powders can be used for some applications in which specific properties are required.     

DETERMINATION OF STATIC AND FATIGUE COMPRESSIVE STRENGTH OF HARD-METALS

Abstract

Procedures for the assessment of the very high static and fatigue compressive strength of hard-metals are briefly reviewed. A test-piece with enlarged ends has been developed. Using this, stress/strain curves and fatigue–strength data have been obtained on WC–Co hard-metals with various cobalt contents and carbide grain sizes. The results can be rationalized by being plotted against the mean free path in the binder metal.     

METHOD OF PRODUCTION AND SOME PROPERTIES OF A NEW TYPE OF ULTRAFINE PARTICLES

Abstract

A micro-rolling method for the production of fine metallic particles in the size range 0.05–5μm is described, together with ancillary equipment for the separation and collection of predetermined size fractions. The material produced is a flake form of high (up to 100: 1) breadth :thickness ratio and is suitable for applications such as dispersion-hardening and for many operations that involve pressing and sintering procedures.

Details are given of particle shape and size distribution for two typical products, aluminium flake and tungsten flake. Some physical properties of specimens produced from these materials are also cited. Reference is made to other products of the mill, both metallic and non-metallic, and to future development work in relation to these materials.     

THE EFFECT OF MINOR ADDITIONS OF SULPHUR ON PORE CLOSURE AND CASE-HARDENABILITY OF SINTERED IRON

Abstract

Interconnected porosity in sintered iron structural parts can have a detrimental effect on case-hardening by permitting penetration of the carburizing gases to the interior of the compact. Experiments have shown that small additions of sulphur to the iron powder may provide a means of effecting the desired pore closure, though this method has still to be proved applicable on an industrial scale.     

THE FABRICATION AND PROPERTIES OF CHROMIUM–ALUMINA AND MOLYBDENUM–CHROMIUM–ALUMINA CERMETS

Abstract

After a brief review of the literature, various methods for preparing chromium–alumina and molybdenum–chromium–alumina cermets are described. The properties of such materials are discussed and an indication is given of their possible applications.