A PRELIMINARY STUDY OF THE REACTIONS BETWEEN TUNGSTEN CARBIDE AND TUNGSTEN CARBIDE - 6% COBALT POWDERS AND WET AND DRY HYDROGEN

Abstract

The apparent activation energy for the following reactions has been determined by measuring the amount of carbon removed from samples of powder heated to various temperatures in an atmosphere of either dry or wet hydrogen:

Dry Hydrogen

“As-carburized” tungsten carbide: no reaction up to 900°C.

Water–milled tungsten carbide: 8·38 kcal/mole over the range 600–900°C.

Water–milled tungsten carbide+ 6% cobalt: complex reaction over the range 600–900°C.

Wet Hydrogen

“As-carburized” tungsten carbide: 58·1 kcal/mole over the range 800–900°C.

Water–milled tungsten carbide: 34·8 kcal/mole over the range 700–900°C.

Water–milled tungsten carbide+ 6% cobalt: 38·4 kcal/mole over the range 825–900°C; 7·38 kcal/mole over the range 600–800°C.

It has been shown that ball-milling in water changes both the physical and chemical properties of tungsten carbide powder. These changes are discussed in relation to the marked differences in reaction rates. A number of possible reasons for these differences are given.     

A STUDY OF THE FACTORS CONTROLLING GRAIN SIZE IN SINTERED HARD-METAL

Abstract

Previous experimental work concerning the grain growth observed during the sintering of tungsten carbide–cobalt alloys is reviewed. Particle-sizing methods suitable for the examination of hard-metal powders are described, and techniques for the evaluation of the carbide grain size in the sintered compacts are discussed.

By using a Model A Coulter Counter to examine the size distribution of the carbide grains (obtained from the milled hard-metal powders by dissolution of the cobalt with hydrochloric acid), and by counting techniques on electron photomicrographs of carbon replicas of the sintered compacts, it has been established that the increase in grain size during sintering is quantitatively related to the carbon content of the material after pre-heating. The results presented indicate that the cobalt content exerts little influence on the average grain size of the sintered structures A cobalt content >10% by weight is shown to exert a strong damping effect on the rate of comminution during milling.

The linear relationships between the specific surface area of the carbide grains in milled powders (obtained using a Perkin–Elmer Sorptometer) and the specific surface of the carbide phase in sintered compacts are given. The influence of sintering temperature and time on average grain size and contiguity in a commercial alloy is shown. Some preliminary work indicates that the morphologies of the initial carbide powders may be important factors with respect to the grain size of sintered hard-metal.     

DENSIFICATION PROCESSES IN THE TUNGSTEN CARBIDE-COBALT SYSTEM

Abstract

The tungsten carbide-cobalt system is one in which the carbide matrix phase is to some extent soluble in the cobalt binder and which readily sinters to give 100% density. The characteristics of this system have been examined by determination of densification curves and metallographic examination and compared with a previous investigation on the behaviour of the insoluble tungsten carbide-copper system.

It is found that the good densification characteristics of tungsten carbide-cobalt are due to the initial solution of the carbide by the cobalt. The cobalt diffuses in the solid state into interfaces between carbide particles and as the temperature increases dissolves carbide from the adjacent particle surfaces. Forces arising from minimization of surface energy then act to bring about close packing of the carbide. By this means 100% density is achieved with 8 wt.-%, or more, of cobalt. With between 5 and 8 wt.-% cobalt 100% density is obtained by initial solution followed by a few minutes further sintering during which a solution/reprecipitation mechanism is operative.

If sintering is prolonged, a rigid carbide network is formed and this then determines the contraction of the compact on cooling. Even when 100% density is attained at sintering temperature the cobalt contracts on solidification and cannot fill the intervening space between the carbide particles, so that shrinkage-pipe-porosity forms.

Porosity seals off from the surrounding atmosphere when ～90% density has been reached. If the gas entrapped is insoluble, then the pores shrink until the balance is attained between the surface energy and the excess pressure in the pores and this results in a porosity level of the order of 0·2%     

An x-ray diffraction investigation of heat-treated Wc-Co sintered carbides

Conclusions Hardening of sintered carbides from 1300°K and more may lead to a change in the phase composition of the alloy under conditions of long holds at the hardening temperature. With an increase in hardening temperature there is an increase in the solubility of tungsten in the binder phase and an increase in stresses in the carbide and cobalt phases. An increase in hold time causes practically no change in the tungsten content in the binder but causes an increase in stresses in the phases, which may cause the appearance of some quantity of -Co.The optimum heat-treatment cycle for sintered carbides corresponds to the maximum solubility of tungsten in cobalt with the minimum change in phase composition and stressed state of the phases.Translated from Poroshkovaya Metallurgiya, No. 5(281), pp. 93–98, May, 1986.     

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; secondly, the clusters and filaments contract. The solubility of the tungsten carbide in the cobalt is important, since densification occurs far less rapidly when copper is used as the binder phase. Shrinkage can virtually cease before the compact is fully dense, either because voids form as a result of unsatisfactory mixing, or, with a small amount of cobalt, because the periphery of the compact sinters to full density before the interior, preventing further overall densification.     

Structure and Magnetic Properties of WC–50% Co Model Alloys Containing TaC Additives

The structure and magnetic properties of model high-cobalt WC–50% Co alloys with different carbon contents and TaC additions in the amount of 1.6–5.6 wt % are studied. Model alloys are fabricated by the liquid-phase sintering of powder mixtures at 1420°C, and their composition is described by the formula 50% Co + 50% WC + x% TaC + y% C, where x = 0, 1.6, 2.6, 3.6, 4.6, and 5.6 wt %; y = 0, 0.2, and 0.5 wt %. It is shown that precipitations of the (Ta,W)C phase are present in all studied alloys and (Ta,W)C precipitations are needle-shaped at a TaC concentration up to 3.6 wt %, while the (Ta,W)C grains become spherical at =3.6 wt %. The (Ta,W)C precipitations are arranged both in a binding phase and along the WC grain boundaries. The lattice parameter of the (Ta,W)C phase in alloys with a low carbon content lies in a range from 0.4438 nm for the alloy with 1.6% TaC to 0.4451 nm for the alloy with 4.6% TaC. It is established by the EDX analysis that the concentration of dissolved tungsten in a cobalt phase is independent of the TaC content and strongly depends on the total carbon content; it is 7, 12, and 17 wt % for alloys with high, elevated, and low carbon contents, respectively. The TaC addition in alloys with a low and elevated carbon content leads to an increase in the coercive force up to 875 A/m and a decrease in the magnetic saturation by 5–10 G m³/g. The experimental results made it possible to put forward a hypothesis on the possibility of formation of dispersed tantalum-containing precipitates in a binder phase.     

STUDIES OF THE BINDER PHASE IN WC-Co CEMENTED CARBIDES HEAT-TREATED AT 650°C

Abstract

The binder phase of WC-Co cemented carbide is a Co-rich alloy in which tungsten and carbon are in solid solution. The binder phase is supersaturated with respect to tungsten even after slow cooling from the sintering temperature. In this study the binder phase contained 6% W in solid solution before heat-treatment at 650°C (923 K). Transmission electron microscopy on thin foils of binder phase showed that a finely dispersed. phase, α′, precipitated in the cubic binder phase. After long ageing times Co₃W could be identified by X-ray diffraction methods. As no discontinuous , ‘cells’ of Co₃W and ?-Co could be observed, the following reaction is suggested.

α-Co(W, C)→α-Co+α′→α-Co (C)+Co₃W (needles)

The precipitation of α′ was accompanied by an increase in hardness and a decrease in transverse rupture strength. The effects observed are consistent with those found during annealing of Co-rich cobalt-tungsten-carbon alloys (> 85% Co).     

Ultrafine and nanoscaled tungsten carbide synthesis from colloidal carbon coated nano tungsten precursor

Abstract

A new process for synthesising homogeneous ultrafine and nanoscaled tungsten carbide with good stability in air from well dispersed colloidal carbon coated nano tungsten precursor with highly agglomerated nanoscaled tungsten powder as starting material in a cost effective way is introduced. It is shown that hydrogen atmosphere facilitates the carbon and tungsten reaction process. Inheritance character in grain size distribution of tungsten carbide from tungsten starting material with BET calculated grain size of 46·1 nm has been observed. When the carburisation temperature increases from 1000 to 1300°C, the Brunauer–Emmett–Teller calculated grain size of tungsten carbide powder increases from 68·6 nm to 339·4 nm and the oxygen content decreases from 0·44 to 0·10%.     

High-temperature strength and refractoriness of doped WC-Co-Ni-Re(Mn) hard alloys

The regularities of oxidation of hard alloys (HA) in the “tungsten carbide-metal binder based on cobalt, nickel, rhenium, and manganese” system are established. It is revealed that the introduction of Ni-Re or Ni-Mn into the cobalt binder increases the temperature of the onset of active oxidation by 20–70°C and decreases its rate in the following order: Co-Ni, Co-Mn → (Co-Ni-Mn, Co-Re, Co-Ni) → Co-Ni-Re → Ni-Re. The character of the temperature dependence of the bending strength of the HA with Co-Ni and Ni-Mn binders is almost indistinguishable from that for the VK alloy, while the addition of Re into the binder abruptly changes this character. The HA strengthens somewhat (up to 20%) up to temperatures of 600°C for the Ni-Re binder and 800°C for the Co-Re binder; then its strength decreases but remains higher than for materials with cobalt and nickel binders. The maximal strength of the HA of the WC-Co-Ni-Re systems is attained in the range t = 600–800°C, which corresponds to the temperature of the working edge of the cutting material.     

SLIP-CASTING TUNGSTEN CARBIDE/COBALT POWDER MIXTURES

Abstract

By utilizing a correctly constituted suspending medium and binder, stable slip-casting powder mixtures of tungsten carbide and cobalt were prepared. The stability of the suspension was governed by the pH of the medium; casting rate was dependent on the formulation of the slip, and particularly on the solid concentration. Close control of these variables permitted the use of plaster of Paris moulds for casting sound, dense bodies of the powder mixtures, which were sintered for final densification without the appearance of any defects.     

AN ELECTRON-MICROSCOPE AND X-RAY INVESTIGATION OF THE MILLING OF TUNGSTEN CARBIDE/COBALT MIXTURES

Abstract

It is well known that some interaction takes place between the two components when mixtures of cobalt and tungsten carbide are milled. To gain a further insight into this phenomenon, the milling process has been studied by means of the electron microscope and by the BET and X-ray methods.

During the milling of cobalt powder the number of stacking faults and the amount of the hexagonal phase both increase. On milling 80:20 tungsten carbide/cobalt mixtures increasing agglomeration of cobalt and fine tungsten carbide particles was found with rise in milling intensity. The larger tungsten carbide particles appeared to have a smooth surface. However, if the cobalt was dissolved in hydrochloric acid, the true surface of the tungsten carbide particles was revealed. This became rougher with increasing milling intensity. From this it can be concluded that, during milling, cobalt settles between the surface irregularities.

The variation in distribution of the cobalt that results from different milling conditions leads to a difference in sintering behaviour. A dilatometric study has been made of this aspect.

The original particle size of the cobalt used for hard-metal mixtures does not affect the properties of the sintered product, if the mixtures are milled very intensively.     

Recycling of WC–Co from scrap materials

Abstract

WC-Co scrap generated by the cutting tool industries was electrochemically broken down to cobalt, which was deposited at the cathode and a mixture of tungsten oxide and tungstic acid was collected at the anode with an overall recovery efficiency of about 90%. The tungsten oxide/tungstic acid was reduced to produce nanostructural tungsten powders, which were subsequently carburised and chemically coated with cobalt to produce WC-Co powders. The powders synthesised were characterised for purity and size.The WC-Co powders, thus obtained were consolidated to near theoretical densities using a novel plasma pressure compaction (P²C) technique. The microhardness of the consolidated sample was measured to be 2200 HV, which is 20% higher than the reported literature values.     

Use of x-ray photoelectron spectroscopy for the investigation of the surfaces of hard-alloy parts machined with polycrystalline superhard materials

Conclusions The thin (<600 Å) surface layers forming on VK20 and VK20K hard alloys during machining with polycrystalline superhard tool materials and diamond grinding under recommended conditions consist chiefly of tungsten and cobalt oxides, cobalt containing dissolved tungsten and carbon, and pure tungsten (or the compound W₂C). The formation of such layers must be regarded as a beneficial phenomenon, because the presence on the surface of a tool material of a thin layer of oxides of the elements of which the material is composed helps reduce tool wear.Translated from Poroshkovaya Metallurgiya, No. 11(191), pp. 73–79, November, 1978.     

Effects of strain aging on the structure and mechanical properties of PM 92·5W–5Ni–2·5Fe heavy alloys

Abstract

This paper describes the effects of strain aging on the mechanical properties and the microstructure of forged 92·5W–5Ni–2·5Fe and its heavy alloys microalloyed with cobalt. The investigation was performed on cold rotary forged rods deformed 15, 20 and 30% and strain aged at temperatures from 673 to 1273 K for 1·8–32·4 ks. The results show that for these alloys, there is a temperature range from 773 to 873 K in which maximum ultimate strength and hardness can be attained. Furthermore, the strain aged alloys have shown strength and hardness increase at a temperature of 973 K in a time period of 10·8 ks. The fracture analysis has shown the presence of predominant transgranular fracture of the tungsten phase and γ-phase in the strain-aged alloys in comparison with the forged alloys. The results indicate that interface and tungsten phase strengthening are predominant mechanisms of strain aging.     

Microstructure evolution of TiC cermets with ferritic AISI 430L steel binder

From the outlook of healthcare, economic importance and supply risk, utilisation of raw materials like tungsten, cobalt and nickel should be reduced or replaced with other metals. Nontoxic titanium carbide and iron are the top-of-the-line solution for displacing these materials. Our focus was on conventionally fabricated titanium carbide-based cermets with a chromium ferritic steel binder. To study microstructural evolution, specimens were sintered at different temperatures (600–1500°C). We used a scanning electron microscopy, X-ray diffraction and differential scanning calorimetry to analyse the microstructure and phase formation of the cermets. Our results showed that during the solid and liquid phase sintering of the TiC–FeCr cermet, chromium ferrous complex carbides M₇C₃ are formed and as a result, chromium content in the binder phase is decreased. Alloying TiC–FeCr cermets with strong carbide formers improves the structural homogeneity of the cermets. Also, mechanical characteristics (hardness, fracture toughness) were evaluated.     

A STUDY OF THE THERMAL FATIGUE CRACK PROPAGATION IN WC-Co CEMENTED CARBIDE

Abstract

In WC-Co cemented carbides operating under conditions of cyclic temperature variations the nucleation and propagation of thermal fatigue cracks are caused by high stresses that arise due to the great difference in thermal expansion coefficients between the WC grains and the cobalt binder phase. The present investigation, which is based on laboratory tests where a ceramic pin is pressed against a rotating carbide ring, has been carried out with the aim of elucidating the influence of the cobalt content and the WC grain size on the propagation rate of thermal fatigue cracks. The rate-controlling factor is shown to be the width of the binder phase interlayers between adjacent carbide grains, and this is explained by taking into account the distribution of stacking faults and partial dislocations in the binder phase in the as-sintered condition.     

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.     

Electrodeposition of cobalt–tungsten alloys from alkaline citrate containing bath as alternative for chromium hexavalent replacement

Abstract

Cobalt–tungsten alloys have been considered as potential materials for various applications. This work investigates the wear and corrosion resistance behaviour of cobalt–tungsten–phosphorus alloys electroplated from alkaline citrate containing baths. The wear rates of the cobalt–tungsten alloy electrodeposits were lower than those of the unalloyed cobalt electrodeposits. The wear rate was also lower than those of chromium plated from hexavalent chromium plating bath. It was also demonstrated that the addition of small quantities of phosphorus into Co–W alloys improved the corrosion resistance of the alloy. In fact, the addition of phosphorus further improved the corrosion of the alloy exposed to immersion test in comparison to cobalt–tungsten. The cobalt–tungsten alloys without having any phosphorus component were subjected to further oxidation of tungsten component followed by dissolution in deionised water during the immersion tests.

On a considéré des alliages de cobalt et tungstène comme matériaux potentiels pour diverses applications. Ce travail examine le comportement d’usure et de résistance à la corrosion d’alliages de cobalt, tungstène et phosphore déposés par électrolyse, à partir de bains alcalins contenant du citrate. Les taux d’usures des dépôts de l’alliage de cobalt et tungstène étaient plus faibles que ceux des dépôts non alliés de cobalt. Le taux d’usure était également plus faible que ceux de chrome déposés à partir d’un bain de dépôt électrolytique de chrome hexavalent. On a également démontré que l’addition de petites quantités de phosphore dans les alliages de Co–W améliorait la résistance à la corrosion de l’alliage. En fait, l’addition de phosphore améliorait davantage la corrosion de l’alliage exposé à un essai d’immersion par rapport à l’alliage au cobalt et tungstène. Les alliages de cobalt et tungstène, sans la composante de phosphore, étaient sujets à davantage d’oxydation de la composante de tungstène suivie par la dissolution dans de l’eau déionisée lors des essais d’immersion.     

Structural investigations of detonation-deposited tungsten carbide-cobalt coatings

Conclusions The phase composition of detonation-deposited VK type coatings differs from that of the starting mixtures. A deposited layer contains about 40–50% of the tungsten carbide WC present in the starting mixture, metallic tungsten and cobalt, the intermetallic compounds Co₇W₆ and Co₃W, traces of W₂C, and a small amount of complex carbides. The amount and particle size of the tungsten carbide in a coating are determined by the particle size distribution of the powder being deposited and detonation process parameters. Our investigations have shown that the optimum powder particle size range for the process of detonation deposition of hard-metal mixtures may be taken to be 5–40 m. During detonation deposition under the conditions investigated the usual hard-metal structure and composition are not obtained. Most of the starting tungsten carbide decomposes with the formation of tungsten and cobalt intermetallic compounds. Attempts should now be made to improve deposition conditions so as to increase the phase and structural homogeneity of layers being deposited. In coatings deposited by an optimum method the ductile Co binder will be present in an amount close to that in a sintered VK type hard metal. The fracture of coatings and basis metal on the antivibration shelves of compressor blades has a fatigue character. Crack initiation takes place in the surface and inner layers of detonation-deposited coatings as a result of their considerable brittleness, which is due to the presence of metastable intermetallic phases as well as of defects in the form of pores and blowholes.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 24–29, October, 1982.     

A REVIEW OF PARAMETERS INFLUENCING SOME MECHANICAL PROPERTIES OF TUNGSTEN CARBIDE–COBALT ALLOYS

Abstract

The paper reviews experimental results relating to the influence of composition, structure, and testing conditions on the hardness, compressive strength, and transverse rupture strength of sintered tungsten carbide–cobalt alloys.