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.     

SINTERING KINETICS IN IRON-COPPER ALLOYS WITH AND WITHOUT A LIQUID PHASE

Abstract

Sintering of iron-copper alloys has been studied in the temperature range 950–1250°C. The factors involved include compacting pressure, sintering temperature, sintering time, and atmosphere. The results are interpreted as a decrease in pore volume due to the filling of voids between particles by a diffusion mechanism. An empirical equation of the Arrhenius type, based upon volume change as a function of sintering time, has been derived in order to evaluate the rate constant of the sintering process.

Volume diffusion is considered to be the primary mechanism of material transport in alloys containing 0·5–2·0% copper, when sintered in the range 950–1250°C, and in alloys containing 5·0–10·0% copper, when sintered in the range 950–1050°C. The activation energy derived for the sintering process is 53·4 kcal/mole. Surface diffusion appears to be the operative mechanism of material transport in alloys containing 5·0–10·0% copper, when sintered above the melting point of copper. The activation energy for this sintering process is 32·6 kcal/mole.     

WEAR AND FRICTION STUDIES OF NICKEL–TUNGSTEN CARBIDE–GRAPHITE COMPOSITES

Abstract

Wear and friction studies have been carried out on pressed and sintered composites of 75–95% theoretical density. Carbonyl nickel and tungsten carbide, both with a particle size of 5 μm, and natural crystalline flake graphite with an approximate size of 1·1 μm were used. The wear specimens were run against a rotating (100 rev/min) steel cylinder. The wear-resistance correlates well with the ratio of the volume fraction of tungsten carbide to that of graphite (WC/graphite); there is a minimum in weight loss at a ratio near unity. This effect is explained in terms of powder coating with graphite during blending, which affects grain-boundary formation and grain growth during sintering; supporting metallographic evidence is given. The wear-debris particle size produced is related to wear; the wear rate increases with increasing particle size. Residual porosity in the materials reduces wear-resistance. The wear rate does not correlate with hardness or coefficient of friction; the results of wear tests in oil or water are very similar to those for tests run in air.     

CREEP OF SILICON CARBIDE

Abstract

The creep behaviour of high-density silicon carbide has been examined under four-point transverse bending in air at temperatures from 1000 to 1300°C. Stresses in the range 0·207 GN m^?2 (30 000 Ib in^?2) to 0·496 GN m^?2 (72000 lb in^?2) were applied.

In tests lasting up to 3·6 Msec (1000 h) the creep strain (?_p) increased with time (t) as ?_p = At^k where k was between 0·2 and 0·5. The dependence of A on stress (σ) and absolute temperature (T) was found to be with n between 1 and 2 and Q～230 kJ mole^?1 (55 kcal mole^?1).

These results are compared with estimates of creep in SiC obtained by extrapolation of steady-state creep data from higher temperatures.     

EFFECTS OF PARTICLE SIZE ON THE SINTERING KINETICS IN TUNGSTEN POWDER

Abstract

An experimental study has been made of the effects of initial particle size on sintering kinetics in tungsten powder within the temperature range 1100–1500°C. Particle size, compacting pressure, sintering time and temperature all influence the rate of sintering. Isothermal changes in density and volume have been measured. The results indicate grain-boundary diffusion as the mechanism principally responsible for material transport in the case of particle sizes <4 μ Surface diffusion appears to bethe mechanism of material transport in compacts with particle sizes of 14– 16 μ The temperature-dependence of the rate of sintering is characterized by activation energies of 101± 2 and 72± 2 kcal/mole for fine particles (< 4 μ) and coarse particles (14–16 μ), respectively.     

THE OXIDATION OF HOT-PRESSED TITANIUM CARBIDE AND TITANIUM BORIDE IN THE TEMPERATURE RANGE 300°–1000° C.

Abstract

The oxidation characteristics of pure hot-pressed titanium carbide and titanium boride (TiB₂) have been studied in the range 300°–1000° C.

Titanium carbide shows a very marked peak in the rate of oxidation at ～450° C., the oxidation product at this temperature being anatase (TiO₂) which is powdery and non-adherent; the oxidation thus follows a linear law. Above ～700° C. the rate increases rapidly again and is approximately parabolic. The scale in this case consists of a dense crystalline film of rutile.

The oxidation of the boride is less rapid than that of the carbide over the whole temperature range but it, too, shows a slight peak at 450° C. The oxidation tends to be parabolic at higher temperatures. The nature of the oxidation products has not been determined.     

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%.     

LIQUID-FLOW DENSIFICATION IN THE TUNGSTEN CARBIDE–COPPER SYSTEM

Abstract

The fluid-flow stage of densification in two-phase sintering, with minimum contribution from intersolubility effects and change of particle shape, has been studied by selecting the insoluble tungsten carbide-copper system. Density determinations, photography of specimen shrinkage, and microscopic examination were carried out over a range of copper contents, with two carbide particle sizes, for sintering in a hydrogen atmosphere.

When the copper melts it flows into regions of high carbide density to form carbide/copper colonies. If these occupy a minor proportion of the compact, densification is limited and determined by the larger, “rigid” carbide part of the compact, but if the colonies predominate there is massive shrinkage on fluid flow. Overall densification subsequent to fluid flow is unaffected by the presence of the copper. The copper may, however, be redistributed within the compact as hydrogen in pores near the surface diffuses out and the pores shrink, drawing copper from central regions to form a dense skin.

As densification proceeds the carbide particles form a rigid inter-connected framework. On cooling, the copper contracts more than the solid framework so that, even if at high temperatures the compact is fully dense, shrinkage porosity results on solidification.

The structure after the fluid-flow stage is highly dependent on the initial processing. Mixing produces agglomerates of copper that, on melting, flow into the surrounding carbide matrix leaving behind large voids. Ball-milling, in contrast, yields a more uniform green structure and hence a more uniform compact after flow of the copper.     

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.     

VAPOUR-PHASE SYNTHESIS OF SUBMICRON TANTALUM CARBIDE

Abstract

The synthesis of pure, submicron, crystalline TaC powder by the vapour-phase reaction of methane and TaCl₅ has been studied in the range 1400–1600°C (1673–1873 K). Particular attention was given to the effect of process variables on particle characteristics. Dry collection and subsequent acid treatment of the powders were found necessary to yield a pure product. A minimum temperature of 1400°C was required for the reaction, but elevation to 1600°C had no apparent effect on the degree of conversion or the particle-size range. Electron micrographs showed the powders to have a very narrow particle-size range of from 0·02 to 0·07 μm. Over 45% of the particles were in the range 0·04-0·045 μm, and ～80% in the 0·03–0·05 μm range. In general, the particles exhibited equidimensional shapes.     

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.     

THE PREPARATION AND PROPERTIES OF SELF-BONDED SILICON CARBIDE

Abstract

Dense self-bonded silicon carbide, prepared by impregnating with silicon pressed mixtures of silicon carbide and graphite, has been improved by attention to grading and composition. Treatment at 2000°C for ½ h has given a much greater degree of self-bonding. Porous self-bonded silicon carbide with a dense surface has also been prepared.     

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%     

THE INFLUENCE OF PRESSURE,TEMPERATURE, AND TIME ON THE HOT PRESSING OF COMMERCIAL BERYLLIUM POWDER

Abstract

For a particular batch of Brush Super-Pure – 200-mesh beryllium powder the hot-pressability (defined as the length of a standard compact of < 98% theoretical density), increased: (1) continuously with pressure over the range 0–1·25 ton/in² for compacts pressed for 1 h at 1100°C;(2) with temperature from 1000°C to a maximum at 1150°C when pressed for 1 h under 1 ton/in²; and (3) to a lesser extent with time over the range 10–120 min when pressed at 1050°C under 0·25 ton/in² and at 1100°C under 1·0 ton/in². Differences in hot-pressability between various batches of the same powder were small compared with the effects of temperature and pressure.

Compaction during hot pressing occurs in two stages: first, collapse of the powder column causing bulk powder flow; followed, secondly, by sintering of particles forced into close contact. The latter is accompanied by a diminution in both the number of pores and their average size and is associated with grain growth, particularly above 1100°C; after pressing at 1200°C any remaining porosity assumes a thermally stable, spherical configuration.     

Thermodynamic Properties of Dilute Aluminum in Liquid Zinc

Abstract

The thermodynamic properties of the liquid zinc-aluminum system in the range of 0 to 0.36 mole fraction of aluminum and over a temperature range of 430 to 530 °C were investigated by using an electromotive force (EMF) cell of the type

Al(s)|LiCl–KCl–AlCl₃|Zn–Al(l),Mo

The activity of aluminum was found to show a significant positive deviation from Raoultian behaviour. The activity coefficient of aluminum in the dilute solution range (γ°) and the free energy of solution of aluminum based on the one weight percent (1 wt %) standard state were determined as a function of temperature and found to follow the relationships:

logγ°=(1195/T)?0.874

ΔG_s=5457?11.4T (cal/mole)

Over the temperature range investigated, the self-interaction coefficient of aluminum in liquid zinc (e^Al_Al) was found to be approximately constant at ?0.01.

On a étudié les propriétés thermodynamiques du système liquide zinc-aluminium, entre 0 et 0.36 fraction molaire d'aluminium, à des températures de 430 à 530 °C, en utilisant une cellule à force électromotrice (FEM) du type:

Al(s)|LiCl–KCl–AlCl₃|Zn–Al(l),Mo

On a trouve que l'factivite de l'faluminium montrait une deviation positive importante du comportement Raoultien. On a determine le coefficient d'factivite, γ°, de l'faluminium pour une solution diluee ainsi que l'fenergie libre de solution de l'faluminium, base sur l'fetat standard de un pour-cent en poids (1%), en fonction de la temperature. On a trouve que ceux-ci suivaient les relations suivantes:

logγ°=(1195/T)?0.874

ΔG_s=5457?11.4T (cal/mole)

Dans le domaine de température étudié, on a trouvé que le coefficient d'auto-interaction de l'aluminium dans le zinc liquide (e_Al^Al) était approximativement constant à ?0.01.     

Sintering,microstructure, and mechanical properties of PM manganese–molybdenum steels

Abstract

The effects of 0·5 wt-%Mo addition on the processing, microstructure, and strength of PM Fe–3·5Mn–0·7C steel are described. Water atomised and sponge irons, Astaloy 1·5Mo, milled ferromanganese, and graphite were the starting powders. During sintering in 75H₂ /25N₂ or pure hydrogen the dewpoint was controlled and monitored; in particular the effects of improving it from -35 to -60°C were investigated. Faster heating rates (≥20 K min^-1), sufficient gas flowrates, milling the ferro alloy under nitrogen, a low dewpoint (<-60°C), and a getter powder can all contribute to the reduction or prevention of oxidation of the manganese, in particular formation of oxide networks in the sintered steels. For 600 MPa compaction pressure densities up to 7·1 g cm^-3 were obtained; these were not significantly affected by sintering at temperatures up to 1180°C. The sintered microstructures were sensitively dependent on the cooling rate. Irrespective of the presence of Mo, slow furnace cooling at ～4 K min^-1 resulted in mainly pearlitic structures with some ferrite and coarse bainite, whereas fast cooling at ～40 K min^-1 produced martensite and some retained austenite, very fine pearlite, bainite, and some ferrite. Young's modulus, determined by tensile and ultrasonic tests, was in the range 110–155 GPa. Sintering with -60°C dewpoint resulted in tensile and transverse rupture strengths of420 and 860 MPa for the Mn steel, rising to 530 and1130 MPa as a result of the Mo addition. This contrasts with strength decreases observed when processing included use of high oxygen containing ferromanganese and sintering with -35°C dewpoint.     

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 use of nitride intermediates in the preparation of metals. A study of the reduction of Nb2O5 with NH3

A kinetics study of the reduction of Nb₂O₅ with NH₃ was conducted at 600° to 1300°C, using vertical fixed-bed, flow-through reactors, with the goal of using the nitride as an interme-diate in the preparation of niobium (columbium) metal via a thermal decomposition step. The effects of reactor materials (stainless steel, nickel, molybdenum, graphite, alumina, and Vycor) upon ammonia reactivity toward Nb₂O₅ were investigated. At low temperatures, the metal reactor systems were more catalytically reactive, yielding faster rates of reac-tion and a greater degree of nitride conversion, whereas at high temperatures, the non-metal reactor systems performed better. In general, the initial reaction rate-temperature data exhibited a maximum, associated with oxynitride formation, near 700°C for the metal reactor systems and 800° to 900°C for the nonmetal reactor systems, followed by a mini-mum, associated with NbO₂ formation, at 800° to 850°C for the metal reactor systems and 950° to 1000°C for the nonmetal reactor systems where NbN formation commences. A sec-ond maximum, associated with the hexagonal NbN phase, occurred at 1200°C. The ranges of activation energies for these regions were from 15 to 30 kcal/mole for region I, 8 to 22 kcal/mole for region II, and 10 to 22 kcal/mole for region III.     

Effects of cobalt additions on WC grain growth

Abstract

Inhomogeneity in the particle size of the tungsten carbide raw material can result in abnormal WC grain growth in WC–Co cemented carbides. For the preparation of ultrafine tungsten carbide powders and ultrafine cemented carbides, abnormal WC grain growth is the most troublesome issue. This paper deals with the effects of cobalt additions on WC grain growth during the carburisation process of nano- and coarse tungsten powders and the sintering process of ultrafine tungsten carbide powders. For the preparation of tungsten carbide powders, it was shown that through the incorporation of 0·035 wt-%Co into W+C mixtures, a dramatic change in WC grain morphology took place for coarse tungsten raw material, while for nanotungsten raw material, a pronounced WC grain growth took place. Plate-like truncated trigonal and hexagonal WC grains were formed during the carburisation process of coarse tungsten raw material containing 0·035 wt-%Co. For the sintering of ultrafine tungsten carbide powders containing 0·3 wt-%Co, an anisotropic and abnormal WC grain growth took place. The mechanisms for WC grain growth were discussed, and suggestions were made for the quality improvement of nano- and ultrafine tungsten carbide powders and ultrafine cemented carbides.     

Kinetics of Domain Growth in Ordered Ni4Mo

The kinetics of domain growth in Ni4Mo in the temperature range of 600 to 850 °C were investigated using transmission electron microscopy. It was found that domain growth in Ni₄Mo is analogous to metallurgical grain growth and can be described by the expressionD ⁿ =kt, whereD is the average domain size, t is the aging time, k is a constant, and the exponent n is the reciprocal of the slope of the log D vs log t plot. The value of n changed with temperature from 2.0 at 850 and 800 °C to 2.9 at 700 and 600 °C. This change was explained in terms of relative domain orientation effects. The activation energy for domain growth was obtained as 69 Kcal/mole (2.9 × 105 Joules/mole) in the temperature range of 800 to 850 °C and as 92 Kcal/mole (3.85 x 105 Joules/mole) in the temperature range of 600 to 700 °C, which on comparison with available diffusion data established that the growth process was interface-controlled at the higher temperatures and bulk diffusion-controlled at the lower temperatures.