THE STRUCTURE AND PROPERTIES OF SOME INTERNALLY OXIDIZED ALLOY POWDER COMPACTS

Abstract

The advantages of internally oxidized structures produced by powder-metallurgical techniques are briefly reviewed and discussed. A metallographic survey of the structures of some internally oxidized copper alloy and nickel alloy powder compacts is presented, and the effect of pressing and sintering variables upon the density and hardness of the product is established. Hot-hardness data up to 800°C, and also some tensile data up to 620°C, are presented for certain alloys.

It is concluded that although dispersion-hardened structures can be prepared by the pressing, sintering, and internal oxidation of appropriate alloy powders, and although the methods described offer a valid comparison of the properties of the various alloys studied, optimum mechanical properties are undoubtedly developed only after the powders are hot worked. The latter treatment densifies the product, removes any porosity in the structure, and increases the stored energy in the material.     

MECHANICAL PROPERTIES OF SINTER/FORGED LOW-ALLOY STEELS

Abstract

The paper describes preliminary work on sinter/forged low-alloy steels. The mechanical properties and structures of both atomized and blended alloys were investigated. By using a good-quality atomized powder of the SAE 4600 type, tensile and fatigue properties equivalent to those of wrought steels could be obtained. Atomized alloy powders with higher oxygen contents had poor ductility and impact values because of surface oxides on the powder particles.

Blended iron alloys gave tensile strengths up to 72 tonf/in² (1112 MN/m²) with much higher ductility and impact-resistance than would be obtained with conventionally pressed and sintered alloys.     

Processing and properties of sinter hardened Fe–Cr–Mo steels with admixed extra-fine nickel

Abstract

The processing and properties of chromium–molybdenum, powder metallurgy steels with admixed extra-fine nickel (XF Ni) were investigated. Prealloyed Fe–1·5Cr–0·2Mo powder was blended with different quantities of XF Ni, while a hybrid steel with lower Cr content was prepared by blending Fe–1·5Cr–0·2Mo and Fe–0·5Mo prealloyed powders, with additions of XF Ni and copper powders. These steels were compacted into different part shapes in order to evaluate the effect of part thickness on sinterhardening behaviour. These parts were also subjected to different cooling rates after sintering. This study showed that additions of XF Ni improve the compressibility, densification behaviour and mechanical properties of Cr–Mo steels. Furthermore, the properties of the hybrid steel were shown to be either equal to or greater than those of the reference material. Hardenability of all steels was sufficiently high such that part thickness was seen to have negligible impact. Higher cooling rates generally resulted in improved mechanical properties.     

POSSIBLE FAST-REACTOR CANNING MATERIAL STRENGTHENED AND STABILIZED BY DISPERSION

Abstract

The high-temperature irradiation embrittlement of steels and nickel alloys is probably due to helium precipitation in grain boundaries. It would be desirable to retain the helium as fine bubbles evenly distributed within the grains. A fine dispersion in the metal matrix may act as nucleation site for helium bubbles and it is proposed to produce by powder-metallurgy techniques afine dispersion of a ceramic oxide in a steel matrix. Ferritic chromium steel was chosen because it is known to be less susceptible to embrittlement than austenitic alloys and in this case the oxide dispersion must also improve the high-temperature mechanical properties.

Methods of preparation are discussed and results of preliminary tensile tests performed on iron containing alumina, magnesia, and titania dispersions indicate the feasibility of the proposed solution.     

METHODS OF EXTENDING THE APPLICABILITY OF SINTERED STEELS

Abstract

It is a common opinion among users of structural parts that applications for sintered steels are limited to those where requirements for strength are low to moderate. Furthermore, sintered steels of moderate strength are thought to be very brittle. It is the object of this paper to draw attention to significant improvements which have been achieved in the last few years. These are basically a result of powder developments which are based partly on traditional alloying additions, such as Cu, Ni, Mo, and C, and partly on unique combinations of iron powders and phosphorus or on combinations of iron powders, phosphorus, carbon, and/or copper. Unusually favourable combinations of strength and ductility can be achieved with diffusion-alloys based on iron and phosphorus. Components of high-duty sintered steels capable of replacing components of conventional wrought steels can be produced from partially prealloyed combinations of iron, copper, nickel, molybdenum, and carbon. For many applications these materials can also be an alternative to powder-forged steels. All the above powder combinations show consistent and low dimensional changes during sintering so that close tolerances of intricately shaped components can be maintained. Material and processing costs are such that the improved properties can be achieved economically.     

THE FABRICATION OF HIGH-SPEED TOOL STEEL BY ULTRAFINE POWDER METALLURGY

Abstract

Various powder-metallurgy techniques have been developed during recent years to avoid segregation effects associated with the conventional methods of casting and forming high-speed steels. These techniques have generally involved the consolidation of hot working or hot pressing of 50–500μm prealloyed powders into dense billets or rods.

The work described has demonstrated that much finer, 0·5–5μm, powders of M2 and M50 steels may be cold pressed and sintered to produce bodies with densities of 99% theoretical containing uniformly distributed 1–2μm particles of carbides. It is anticipated that the method will have application for the manufacture of complex-shaped parts with very small material losses and little machining.

An account is given of the preparation of the fine powders by ball-milling and their subsequent compaction, sintering, and microstructure. The control of carbon and oxygen levels by carbon addition to the powders is described.     

Properties of iron-copper master alloy powder

The advantages of low-alloy sintered materials produced from iron—copper master alloy are confirmed. Physical and technological properties of the master alloy powders are determined. Experiments show that mixtures of iron, graphite, and iron—copper powders compact well to produce high-quality articles. Sintered materials have strength values (σ_f ～ 345 MPa, HB ～ 1430 MPa) comparable with those of similar materials produced using electrolytic copper.     

THE PROPERTIES OF PLATINUM-COBALT MAGNETS PRODUCED BY POWDER METALLURGY

Abstract

Improved and more consistent magnetic properties can be obtained from platinum-cobalt alloys produced by powder-metallurgy techniques. The importance of heat-treatment is emphasized, and for optimum permanent-magnet properties it is shown that the equiatomic stoichiometric alloy should be employed. Magnetic measurements demonstrate that energy products < 10 × 10⁶ gauss-Oe, and coercive forces up to 4900 Oe can be achieved.     

Effects of adding overspray powder on microstructures and mechanical properties of spray deposited 8009 aluminium alloy

Abstract

Under normal condition the cooling rate in a spray deposition process is too low to form 8009 aluminium alloy. In order to increase the cooling rate, overspray 8009 aluminium alloy powders were added. The effects of adding overspray powder on microstructures and mechanical properties of the spray deposited 8009, with and without addition of the overspray powders, were studied. It is shown that adding proper content of overspray powders significantly improves the mechanical properties of the spray deposited 8009 alloy. The mechanism involved in modification of the microstructures and the mechanical properties of the spray deposited 8009 alloy produced with the overspray powder addition approach was discussed.     

A REVIEW OF SINTERED METAL BEARINGS: THEIR PRODUCTION,PROPERTIES, AND PERFORMANCE

Abstract

Five classes of sinter-based bearings are identified: steel-backed materials with compacted non-porous sintered linings for engine bearings and wrapped bushes; steel-backed materials with porous sintered linings impregnated with a plastic, intended for operation dry or with sparse lubrication; unbacked porous sintered-metal parts impregnated with oil to form self-lubricating bearings; unbacked non-porous sintered metal parts incorporating graphite; sintered polytetrafluoroethylene parts incorporating powdered metals. Methods of manufacture are outlined.

Techniques of bearing evaluation are described, the most useful being sophisticated, flexible test-rigs which approach closely the conditions of actual bearing applications.

The relative fatigue strengths of sintered engine-bearing linings are given and sintered copper-lead is shown to be equivalent in fatigue strength to cast linings.

The impregnation of sintered bronze linings with PTFE and lead yields a material with good un lubricated wear-resistance. The effect of load, rubbing speed, and other variables is described. A polyacetal lining bonded to a steel backing via a porous bronze interlayer gives a bearing material with good performance under conditions of sparse lubrication.

The relative merits of oil-impregnated porous metal bearings obtained by the pressing and sintering of copper, iron, or aluminium alloy powders are described. For optimum performance the bearing conditions should favour the formation of a hydrodynamic film of oil over the bearing surface.

The incorporation of graphite into fully compacted powder-metallurgy parts gives improved wear-resistance under dry and sparsely lubricated rubbing conditions.

PTFE parts incorporating metal powders can be moulded to finished size by powder-metallurgy techniques.     

THE HOT EXTRUSION OF METAL POWDERS

Abstract

The procedure of consolidating metal powders by isostatic compaction, canning, and hot extrusion is described. The properties of a number of nickel. base alloys and stainless steels prepared by this method are given.

It is concluded that, even when no precautions are taken to eliminate air (oxygen) from the canned billet, the resulting oxide content of the extruded bar is insufficient to exert a serious influence on the mechanical properties. The corrosion-resistances of certain stainless-steel samples, extruded from powders, were found to be somewhat lower than the corrosion-resistance of equivalent conventionally wrought products.     

On the Optimization of Compressibility and Hardenability of Sinter-Hardenable PM Steels

Sinter-hardenable steel powders eliminate the extra steps normally required for heat treating since they allow for direct quenching of components at the end of the sintering cycle with a forced convection cooling unit. The current article presents the results of the effect of the alloying method on the optimization of compressibility and sinter-hardenability of sinter-hardenable PM steels. Water-atomized steel powders were produced. Two successive designs of experiments were used to optimize the chemical composition with prealloyed (nickel, chromium, molybdenum, and manganese) and admixed elements (nickel, chromium, manganese, and copper). Static mechanical properties were also characterized. Results show that among all of the combinations of chemical elements and within the range of concentrations studied, the optimum sinter-hardenable powder had the following prealloyed chemistry: 1.5 wt pct Ni, 1 to 1.25 wt pct Mo, and 0.40 to 0.55 wt pct Cr.     

FAST DIFFUSION ALLOYING FOR POWDER FORGING USING A LIQUID PHASE

Abstract

Prealloyed powders are dominant items in the economics of powder forgings today. Present estimates predict a maximum powder consumption that is too low to bring about essential reductions in the cost of these materials. Long homogenization discourages the use of plain iron powders with admixed alloying elements, especially where induction heating is employed, and the use of cheap alloying elements such as manganese is hampered by oxidation problems. A way of overcoming these difficulties is described. If the alloying additions needed for hardenability are made in the form of a low-melting master-alloy powder, diffusion times can be very much reduced. A condition is that the molten prealloy wets the iron particles, reducing the diffusion distance to the order of one particle radius. It is also desirable that the master alloy should penetrate quickly along the grain boundaries of the iron, further reducing the diffusion distance. Low-melting alloys of manganese with copper satisfy these conditions. Forgings can be produced from plain iron powders with copper-borne manganese additions without excessive oxidation of the manganese and preforms can be sufficiently homogenized within a heating time of a few minutes to give hardenability and tensile properties similar to those of conventional quenched and tempered steels. Since only small amounts of master-alloy powder are needed, ‘solid-liquid alloyed’ plain iron powders appear to offer great flexibility in alloy composition at a cost substantially below that of conventional prealloyed powders.     

CONTINUOUS COMPACTION BY CYCLIC PRESSING

Abstract

A process recently developed produces wide, thick bars of unlimited length from powder, granular, or sponge raw materials by a cyclic pressing operation. The process can be adapted readily for laboratory study or for large-scale production.

Many materials have been pressed by this technique, and in all cases compaction was similar to that experienced in conventional pressing. Work on the pressing, sintering, and subsequent working of various nickel powders and of a nickel-iron-molybdenum magnetic alloy has been carried out. Carbonyl nickel powder produces nickel strip of high quality. A high-permeability alloy containing nickel 79, iron 17, and molybdenum 4% can be made into strip with good magnetic properties by compacting the powders with the cyclic-pressing technique and then sintering and rolling the pressed bar into strip.     

INFLUENCE OF THIN OXIDE FILMS ON THE MECHANICAL PROPERTIES OF SINTERED METAL-POWDER COMPACTS

Abstract

The influence of thin oxide films, in the range 200–1200Å thick, on the mechanical properties of sintered iron, copper, and nickel powder compacts has been investigated. As the thickness of the oxide film on the metal powders increased, the properties studied, namely, densification parameter, hardness, and tensile strength improved and attained a maximum at a critical oxide-film thickness, the value of which was ～ 625 Å for iron and nickel and ～ 500 Å for copper. Further increase in thickness to ～ 1200 Å led to a gradual decline in the properties. The improvement in the properties obtained with powders having the optimum oxide thickness was independent of the sintering atmosphere. A probable explanation in terms of activated sintering is given.     

Development of forging quality high strength low alloy steels

Abstract

The effect of alloy composition and heat treatment on the structure and properties of a set of high strength low alloy (HSLA) steels has been investigated. By addition of a relatively high dose of niobium (0·17–0·23%)along with nickel (0·2%), chromium (0·4–0·6%), and manganese (1·5–1·8%) to 0·2%steels, it is possible to develop high strength forging grade steels having baintic or autotempered martensitic matrixes. The microstructure and mechanical properties of these steels are sensitive to cooling rate and heat treatment.     

THE EFFECTS OF IRON-POWDER CONTAMINATION ON THE PROPERTIES OF POWDER-FORGED LOW-ALLOY STEEL

Abstract

Contamination of low-alloy steel powder intended for powder-forging purposes occasionally occurs by virtue of processing through plant normally producing plain iron sintering powders. This paper shows that the effects of moderate amounts of plain iron-powder contamination on the tensile properties of a through-hardened low-alloy steel powder are not significant, provided that the carbon content is maintained at the design level. The results also show that the fatigue endurance limit is lowered although the ratio of fatigue endurance to elastic limit remains constant. In carburizing-grade material, however, there is a significant reduction in the tensile and fatigue strength attainable; and the presence of ‘soft spots’ may have a deleterious effect upon the performance of case-hardened components     

High performance sinter-hardening materials for synchronising hubs

Abstract

The development of synchronising hubs is driven by the need for more complex shapes and higher performance; at the same time as there is a strong pressure on costs. Many high performance synchronising hubs are currently produced using diffusion bonded materials with relatively high alloy contents, but there is a desire to move to materials and processes with better cost/performance ratios. This implies a move from nickel and molybdenum, whose costs are high and unstable, to pre-alloyed materials, preferably based on chromium with no or small additions of plain nickel. Pre-alloyed powders with 1·8%Cr provide a suitable basis for sinter-hardening. By carefully selecting the additions of copper or nickel and the carbon content, properties can be tailored for different strength requirements. The usage of CCT diagrams can assist in optimising composition depending on which cooling rate that can be applied.     

Solid-State Sintering: Critical Assessment of Theoretical Concepts and Experimental Methods

Abstract

Prealloyed Ti–6Al–2Sn–4Zr–2Mo titanium alloy powders made by a rotating electrode process were consolidated by three techniques: hot isostatic pressing (HIP), hot explosive compaction (HEC), and hot rolling (HR), to optimize the processing parameters in order to obtain fully dense compacts and the desired microstructures and mechanical properties. Room-temperature tensile properties show that all three techniques are capable of giving full density and mechanical properties equivalent to or better than wrought properties. Control of processing parameters which may require another step of deformation such as rolling gives more reliability for HIP and HEC compacts. Metallography and scanning electron microscopic studies on fractured tensile specimens revealed the presence of both metallic and non-metallic inclusions in the compacts. The control of such contaminations during manufacturing as well as processing is necessary for good mechanical properties. PM/0181     

Influence of powder characteristics on sintering behaviour and properties of PM Ti alloys produced from prealloyed powder and master alloy

Abstract

The use and development of titanium and titanium alloys have been strongly correlated to high technology industries where costs are not the most important aspect. Titanium could see its market grow by the application of lower cost and more efficient processing methods such as powder metallurgy. This work deals with the characterisation of two types of powders: commercial prealloyed powder and powder produced from master alloy combining mechanical milling and conventional blending to adjust the particle size. The characteristics of the powders, sintering behaviour and final properties of the parts indicate that the master alloy approach leads to better compressibility than the prealloyed powders and, therefore, to lower dimensional change during sintering. The most important result is that it is possible to obtain Ti alloys with properties similar to or better than alloys from prealloyed powders and to obtain homogeneous microstructures, which allows the composition to be adjusted to requirements.