4

Consolidation ofCemented Carbides

Consolidation of cemented carbides involves numerous operationsin which almost every factor is critical. Figure 1 gives a general flowsheetof sintered part production. The starting product consists of fine tungstencarbide and metallic cobalt powders. The tungsten carbide is sometimespartially substituted by the carbides of titanium, tantalum, niobium, chro-mium, vanadium, molybdenum, or hafnium, independently or in combina-tion. The binder cobalt may also be alloyed with or substituted by nickel ornickel-molybdenum. The product, after sintering, is not used in as suchcondition, but may be ground, polished, or coated to impart a more accurateform or a still harder surface. In the present chapter, all operations until thesintering stage have been described.

1.0 MILLING OF CARBIDES

Ball milling is the commonly applied comminution method in thecemented carbides industry. This operation is carried out mainly to blendthe carbides with the auxiliary binder metal and the processing aids. Thelatter usually consists of paraffin wax or polyethylene glycol (PEG). Themechanical properties and the elimination of porosity from the sinteredproducts are largely associated with the uniformity of cobalt distribution.

This can be assured only when the ball milled admixture is uniform. Theprime object of ball milling, apart from particle size reduction, is to ensurethat every carbide particle is coated with cobalt. In addition, it creates newactive surfaces and an increased defect structure of both carbides and metalbinder. The new surfaces created during milling are very reactive with thegaseous species in the environment. There is some effect of the process ongrain size also when the grain size of the carbides exceeds 2//m.[1]

Figure 1. General flowsheet of cemented carbide production.

Tungsten Carbide Alloy Carbide Cobalt Wax

Wet Milling

Pressing

Dewaxing/Presintering

Sintering

Post SinteringOperations

Inspection

End Products

Granulation

Extrusion

Shape

Coating

Conventional ball milling of hardmetal powders is carried out insimple cylindrical rotating ball mills. These mills vary in size over a widerange but little difference is observed in their milling action. Larger millsappear to be more effective than small ones. But, from an efficiency pointof view, vibratory milling is far ahead because the balls tend to rotateindividually, as well as in unison. As an illustration, 48 hours of vibratorymilling can be more effective than seven day's rotational ball milling.[2] Inattritor milling, the ball charge is stirred vigorously with rotating paddles.For better results, carbide balls are used.

Ball mills and attritors are preferably lined with carbide and carry acharge of carbide balls of 2.5-3.0 times the weight of the powder charge.However, stainless steel mills with stainless steel balls, porcelain ballmills, and rubber lined types are used in cemented carbide production. Inall cases, care has to be taken so as to minimise the impurities pick up.

Speed of the mill is an important factor during ball milling. At acritical speed, given by the expression

Eq- (D ncrit= ^j-

where n = rpm and D = diameter of cylindrical drum in metres. Theparticles are in equilibrium between the centrifugal force and the gravita-tional force. With a higher rotational speed than critical, a uniform rollingmotion of the milling medium does not occur. An optimised speed should,therefore, be about 70-80% of the critical speed. The optimised charge fora ball mill lies between 30-45% of the drum volume.

As the energy of milling is considerably high, it is necessary to carryout the operation under a protective liquid to minimise temperature rise andalso to prevent oxidation. Wet milling of carbides is generally carried outusing an organic liquid such as acetone, hexane, alcohol, etc. The organicliquid is removed by decantation or a more complete separation can beachieved by a subsequent vacuum filtration. In modern technique, themilled mixture is dried and lubricated in a vacuum planetory mixer with atwo-fold advantage. It provides a uniform lubricating of the powders.Secondly, almost total recovery of the organic liquid can be done. Thetemperature for evaporation of organic liquid is maintained by hot watercirculation through the mixer jacket. With the innovation of spray drying,the task becomes still easier.

The ball milling operation has been studied extensively from theviewpoint of cobalt dispersion, but rather little information is availableabout size reduction during the process. Grain size reduction does takeplace in the milling process, but this is apparent only in the case of coarsergrain hardmetals. The time dependence for the subdivision of tungstencarbides in a ball mill has been studied by Fischmeister et al.[4] Theysuggest that the size reduction can be described by the equation

Eq. (2) S = K Vr

where S is the specific surface (m2/g), t is the milling time (hour) and K isa constant determined by the type of mill and milling condition. Parnamaet al.[5] have found that this relationship is valid for cemented carbidepowders for practically all milling times used in their production.

Lardner[1] studied the effect of milling time, milling liquids, and sizeof balls on the grain size of milled carbides.[1] For a particular mill and millcharge combination, there is a limiting grain size beyond which no furthercomminution takes place (see Fig. 2).

M i l l i n g Time , hr

Figure 2. Effect of milling time on particle size reduction of tungsten carbide powder.[1]

20mm Boils SBP6

20mm Balls water9mm Balls water

Fis

her

Part

icle

Siz

e, p

m

It is not possible to determine the changes in grain size and grain sizedistribution that occur during wet milling with the available FSSS mea-surement. When carbides less than 2 /urn are used, direct microscopicobservations show very little change in particle size, whereas, according toFSSS measurements, there is continued production of fines. However, it ismore likely that during ball milling of such carbides, there is no appreciablereduction of original particle size, but the larger particles breakdownrapidly so that the product becomes more uniform.^ In the case ofcarbides with a powder size more than 2 //m, changes in particle size areobserved during ball milling. When very coarse tungsten powders arecarburised, polycrystalline tungsten carbide particles result. In these poly-crystalline carbides, certain grain boundaries are very weak. During theearly stage of ball milling, fracturing of the particles takes place along theseweak boundaries resulting in particle size reduction. In the later stage ofb< U milling, more fines are produced, mainly due to attrition. The sizedistribution of the tungsten carbide and cobalt phase in sintered hardmetalsare mainly determined by the milling conditions and the initial size distri-bution of the carbide powders.[6] However, the range of distribution can bealtered by mixing powders of different particle sizes.[4][6]

Attritor milling is presently becoming increasingly popular. It givesthe same result as conventional ball milling with the advantage of ashortened milling period. The following are some of the advantages of theattritor milling for the commercial production of hardmetal:

• The attritor's action is much faster than that of theconventional equipment, especially when the particle sizeapproaches /xm range. The rate of grinding is often muchmore than ten times of that in conventional milling.

• The floor space required is only a fraction of the spacerequired for other equipment utilised for the same purpose.

• The attritor has a stationary grinding tank (unlike hugerotating drums as in ball milling). The charges can beinspected continuously and any additions made any timeduring grinding.

• With the attritor circulating chilled water through the jacket,milling can be carried out at lower temperatures, whichcontributes to the prevention of the oxidation of the milledpowder.

• The product discharged from the attritor is in the form ofslurry which reduces health hazards.

Mashl et al.[81] attempted to evaluate the feasibility of accuratelypredicting the median WC particle size resulting from milling to a givenenergy input. They applied Charles equation in the form

Eq. (3) E=A(d~a-d-0a)

where E = the specific energy input, d = milled median particle size, d0 =the initial median particle size, A and aare constants. The values for A anda were 0.349 and 2 respectively in the case of a small size attritor. Forlarger scale industrial attritors, the coefficients in the above equation areexpected to change due to variation in the milling efficiency of a largermill. A similar affect was expected with varying cobalt content due to achange in the level of energy lost to the deformation of relatively ductilecobalt. The coarse type WC (3.6jum) -6 Co blend, in contrast to fine typeWC (1.47 /im) -6 Co, showed a general decrease in both apparent andgreen density upon milling to a median particle size of 0.75 /Jm. It wasnoticed that an increase in agitator arm length resulted in an increased rateof grinding. However, the variation in agitator configuration over thelengths examined, did not change the specific energy input required toattain a given median particle size.

Particle size reduction during attritor milling of two compositions ofcemented carbides: WC -11 Co and WC -11(Fe-Co- Ni), with composi-tion of 14-16% Co, 1-5% Ni and balance Fe, has shown that the major partof the WC particle size reduction takes place within the first two hours ofmilling (Fig. 3), especially in the case of WC-Co system.[73] Considerablepick up of iron during milling (up to 10%) was noticed, mainly due to thefact that martensitic stainless steels balls were used. Similar results wereobserved in the case of attritor milled WC-7 TiC-25 binder (34-36% Co,18-20% Ni and balance iron) cemented carbides with the minor addition ofMo2C, VC and Cr3C2.

[75^ No details on the amount of additives werereported.

Attritor milling in argon atmosphere has been found most effectivefor hardmetals containing less than 3% Co. Increasing the cobalt contentresulted in the formation of r]phase.c64^

Milling Time, hours

Figure 3. Variation in the Fisher sub-sieve size (FSSS) of as milled powder mixtures of (a)WC-Co and (b) WC-Co-Ni, with milling timeJ73^

2.0 GRANULATION/SPRAY DRYING

The powder obtained after drying of milled carbide slurry is veryfine. Consequently, the powder is non-free flowing and has low apparentdensity. Pressing of such powder is hence a major problem, and the finepowder poses an additional problem because of inter-particle friction.Hence, a technique of 'granulation' is adopted, whereby loose agglomer-ates of these fine powders are formed. Such granulated powders obtainedare comparatively coarse, nearly spherical (or rounded), and hence havegood flow and fill properties. Therefore, the final pressing is easier andfaster. The individual nature of each particle is retained in these agglomerates.

During milling, wax gets coated around the particles. Milling sol-vents (acetone, hexane, alcohol) form a suspension of the powder. This

Fis

her

Su

b-s

ieve S

ize,

pm

(a) WC-Co (b) WC-(Fe-Co-Ni)

WC only(binder extracted]As milled powder(with binder)

carbide slurry is to be converted to granulated powder for pressing. In theold conventional method, the slurry is dried in a closed vessel. The solventis extracted by boiling it off for which the vessel and the material within areheld at a temperature slightly above the boiling point of the solvent.Extraction of the solvent is aided by maintaining the vessel at a lowvacuum and the vapour condenses outside the chamber. The dried powderis then prepressed into billets which are later disintegrated to give coarsegranules. The pressure applied during prepressing should be low so as toavoid welding of the particles. The disintegrated billets are initially sievedthrough a coarse sieve, eliminating very coarse granules which need to befurther broken down. The sieved powder is then sized according to therequirements for pressing.

In a modified conventional process, the powder is fed continuouslyon to a rotating inclined granulating table on which it is carried upwardsand falls from the highest point with a rolling motion. The granule size isinfluenced by the quantity and nature of the granulating liquid, the inclina-tion of table, and the holding period.

Another method of granulation which is being widely adopted these daysis spray drying. Here, the carbide slurry is sprayed into a stream of preheatedinert gas and simultaneously dried. Granules so obtained are, unlike conven-tional granulation, uniform in size and nearly spherical. Closed cycle spraydrying (Fig. 4)[8] has been found to be very convenient and economical for theproduction of granulated carbide powders for pressing compacts.

Figure 4. A closed cycle spray dryer systemJ8I

NozzleAtomizer Drying

chamber

Scrubbercondenser

Indirect heating(Liquid phase)

Coolingwater

SolventRecovery

Cyclone

PressPowder

Collection

Feedsystem

The slurry, containing the milled powder, wax, and the hydrocarbonsolvent (usually the same used as the milling medium) is contained in afeed tank. From the feed tank, the slurry is passed to the drying chamber.By applying a pressure of 6-15 kg/cm2 the slurry is sprayed upwards intothe drying chamber through a nozzle. Preheated inert gas (generallynitrogen) passes to the drying chamber from the top through a gas dis-perser. The gas disperser helps to achieve rapid and controlled drying.Due to surface tension forces, the spray of liquid disintegrating in the gasstream assumes a spherical shape. The solvent evaporates on contact withthe gas due to high temperatures (170-21O0C). The dried granules arecollected in the drying chamber. A small percentage of fines is carried outof the drying chamber with the exhaust gas which contains solvent vapoursand the drying medium. These fines are separated in a cyclone separator.The exhaust gas then proceeds to a scrubber-condenser, where the solventis extracted. The drying medium, after controlling the vapour content ofthe same, is recycled back to the drying chamber through a heater.

A special pressure nozzle is used to spray the slurry into the dryingchamber. The slurry comes out of the pressure nozzle as a high energy filmwhich forms a spray in the chamber. The nozzle is positioned upwards sothat the spray, consisting of high specific gravity droplets, is completelydried before depositing in the chamber. Variation of the operating pressure(6-15 kg/cm2) of the pressure nozzle controls the spray characteristics andconsequently the size of granules obtained, e.g., from 20 jum to 200 /am.For bigger sizes, spray drying offers a higher proportion of hollow gran-ules. Pressure applied is determined by the solid content of slurry, thepowder mix, and wax content.

Spray drying is preferred to conventional vacuum drying and granu-lation mainly because of following reasons:

(i) There is less handling, as many of the intermediatesteps between drying and granulation are eliminated.This in turn ensures lower labour costs and betterquality of final powder as contamination is reduced.

(ii) Binder/lubricant distribution is greatly homogenizedas the constantly agitated slurry is instantly dried.

(iii) Spray dried powders are nearly spherical and quitecoarse; hence they have good flow and fill propertieswhich are essential while pressing compacts inautomated presses.

(iv) Uniformity of size and sphericity of granules permits amore flexible die design than conventionally granulatedpowders.

(v) Spray-dried powders exhibit constant shrinkage. Also,in comparison with conventionally granulated powder,shrinkage is low.

(vi) The uniformity of the spray-dried particle size of thepowder helps in better dimensional control and goodsurface finish. Hence, the grinding costs of the sinteredproduct is reduced appreciably.

(vii) Atmospheric pollution is minimized by using a closedchamber and a dry, inert medium for drying the spray.

A drawback to the spray drying technique is that it is economicalonly for large scale production.

In hardmetal production, good granules are difficult to obtain fromspray drying when wax content is lower than 1 wt% and cobalt content ishigher than 15 wt%. Nishii et alJ57] reported a new binderless granulationmethod, known as pressure swing granulation (PSG). In this process, thesurface cohesiveness and agglomeration tendency of fine particles them-selves are utilized, and no special liquid binders are applied. The granulegrowth and their densification are accelerated by the gas pressure compac-tion with downward flow through the powder beds. Size control andshaping into spheres are performed by gas fluidization with upward flow.The granules produced by the PSG method are usually soft, spherical, andhave diameters between 0.1-1.0 mm which provide good flowability andcompressibility. Green density at 50-150 MPa pressure range was foundto be greater than 50% theoretical density. Authors confirmed that Rockwellhardness and transverse rupture strength of the sintered hardmetals satis-fied a cemented carbide industrial standard of Japan Cemented CarbideTool Manufacturer's Association (CIS 019C-1990).

3.0 GREEN CONSOLIDATION

Green compacts are prepared by pressing loose powder mass usingan external pressure. This gives shape to the compacts and also dimensional

control. Generally, compaction pressure in the range of 21^42 kg/mm2 isemployed to impart sufficient green strength to the compacts.

Tsuchiya et al.[80] found that for a constant ball milling condition, thecompactibility of WC-Ni milled powder was generally lower than that ofWC-Co milled powder. They explained this by the fact that the size ofbinder agglomerates formed during milling was larger in the WC-Ni alloy.The refining of the agglomerates by a prolonged ball milling was necessaryto improve compactibility (Fig. 5).

Milling Time, Day

Figure 5. Effects of milling time on the density (a) and compressive strength (b) of WC-15Ni(Co) compacts. Compacting pressure, 1 t/cm2.[80]

After compaction, a green density of 60% of theoretical density canbe generally attained for a given hardmetal composition. It is difficult tocompact the green parts beyond this density level. Irrespective of the greendensity of the compacts, hardmetal parts attain almost complete densifica-tion upon liquid phase sintering. However, for compacts with less than

Density

g/c

m3

Com

pre

ssiv

e S

trength

kgf/cm

2

60% theoretical density, percent shrinkage is high, dimensional control isdifficult, and, hence, one prefers to have a high green density of thecompacts prior to sintering.[9] This leads to lower shrinkage, better controlover sintered dimension, and consequently less grinding on the sinteredpieces (Fig. 6).

Length of Pressing , mm

Figure 6. Variation in contraction along the length of sintered cemented carbide compacts.[9]

For green compaction of hardmetals, either a single or double actingpress may be used. However, double acting presses have an inherentadvantage over single acting presses, since they provide a more uniformdensity distribution and hence more uniform shrinkage. Apart from suchprecautions, authors[9] have observed some gradient in sintered dimensionof the hardmetals in axial direction. Due to the particle size distribution ofthe powder and the non-uniformity of lubrication, a variation of ±1% in

Lin

ea

r C

on

tra

cti

on

, 9Io

shrinkage is possible which is obviously undesirable. Hence, the pressesare designed to press the compacts to a given green density. This is madepossible by having a constant weight of the powder in the die and pressingto a constant volume.

For pressing cemented carbide green parts, either hydraulic or me-chanical presses can be used, the former for bigger size parts. Standardparts, like throw away tool tips, are produced in fully automatic mechanicalpresses. Compaction pressure varies from about 50-150 N/mm2. Athigher compaction pressure, there is uneven density within the pressedcompact, which may lead to cracking after the release of pressure. It is,therefore, advisable to have reasonable pressure. Mechanical pressesusually operate on the eccentric or knuckle drive method and a commonproduction rate is about 25 strokes/minute.

Isostatic pressing of powder is sometime necessary. Since pressureis exerted uniformly from all directions, the density distribution is veryuniform throughout the compact and sintered properties are more uniformthan for those pressed in rigid dies. Occasionally, where the number ofparts are few and of large size, it is economical to carry out isostaticcompaction. Another advantage of isostatic compaction over conventionalcompaction in rigid dies is that no die-wall friction is present and pressureloss is minimized, whereas in compaction using rigid dies, there is aperceptible pressure loss due to die-wall friction. However, all powderswhich are directly isostatically pressed need a presintering cycle, followedby shaping operations and finally sintering. This method is thereforeadopted where the shape of the parts are such that pressing in rigid-dies isnot practical. Occasionally, a duplex process is adopted, where partspressed to exact shape in rigid dies are consequently isostatically pressedand directly sintered. This has been found to give better uniform densitydistribution, lower shrinkage, and closer dimensional control.

Another method of effective green consolidation of powder is throughextrusion. The method is very advantageous where length to diameter ratiois high such as in drills, reamers, boring tools, etc., with working diametersranging from 0.5 mm to 10 mm with lengths varying from 10 mm to 100 mm.In such cases, the conventional pressing process is not of any use as itresults in non-uniform density. Extrusion can be used in such casesprofitably.

The hardmetal powder does not exhibit plastic flow even at highpressure. A plasticizer is used which has a low yield stress. The commonplasticisers used are polyvinyl alcohol, poly ethylene glycol, starch solution,

paraffin wax, synthetic resins, etc. Ideally, each particle is separated by afilm of plasticizer. On application of pressure, the material begins to flowout through the die orifice when the critical yield stress of the mixture isreached. The critical yield stress is a function of the plasticizer material,temperature of extrusion, and the degree of mixing obtained. The plasti-cizer is added in the liquid form and the mixing is carried out in a planetorymixer. For this, a kneading machine is very common where the powderand plasticizer is filled, evacuated, and heated to the melting point of theplasticizer using a hot water system. Screw or piston types of extrusionmachines are common (Fig. 7). The former is employed for production ofsmall diameter solid rods and is a continuous type, while the latter is suitedfor large diameter rods with an internal profile and is a batch type machine.[82]

Screw Extruder Principle

Screw ExtrusionChamber

Die

MtLFlow

Piston Extruder Principle

MtLFlow

DieMaterialCylinder

Piston(Hydr. Controlled)

Figure 7. Screw versus piston extrusion.

Once the part is extruded, the plasticizer has to be removed from theextruded part before sintering. The extruded part is therefore heated untilthe plasticizer melts and seeps out. The removal of plasticizer is carried outslowly to avoid damaging the green extruded part. Sintering is then carriedout in the usual manner. A small amount of microporosity is alwayspresent in such sintered parts. A very minor lowering in porosity isobtained by increasing the degree of compaction.

Variables in the process are:

(i) The extrusion pressure which decreases with increasingtemperature, and increases with decreasing particle sizeof powder.

(ii) The amount of plasticizer material which affects theextrusion pressure (Fig. 8), and porosity in the sinteredproduct (Fig. 9).

(iii)The die configuration.

(iv) Temperature and time for deplasticisation.

(v) Rate of plasticizer removal.

Plosticiser Content, wt.%

Figure 8. Dependence of extrusion pressure on plasticiser content.

CMEu

5<uDl/>COO;acO'i/iV-+*XOi

Plasticiser Content, wt.°/«

Figure 9. Influence of plasticiser content on specimen porosity.

4.0 DEWAXING

In the hardmetal industry, paraffin wax is commonly added to thepowder as a lubricant to facilitate pressing and also to have some greenstrength for preventing damage during handling. Once the pressing opera-tion is completed, the wax is of no further use and has to be removed. Theusual technique is to volatalize the wax by heating the compacts either inhydrogen or in vacuum. It is important that the removal of the wax becompleted without causing any physical or chemical change within thecompacts.

Different types of wax are available depending upon the method ofextraction. But among those, only fully refined waxes are commonly usedas a pressing lubricant in the hardmetals industry. The fully refined waxesare not single compounds but are composed of varying amounts of normalparaffins, isoparaffins, cycloparaffins, and traces of alkyl benzene and oil.This variation in composition determines the physical properties of the

Po

ros

ity ,

vo

l. •/<

»

wax, including the melting and boiling ranges. The removal of the wax fromthe compacts by volatilization is closely associated with the boiling range.

Usually, low melting point waxes in the melting range 40-5O0C areused in the hardmetal industry. Under a protective atmosphere, e.g.,hydrogen, no decomposition occurs until temperatures in excess of 40O0Care reached. Removal of the wax starts at around 15O0C but a temperatureof 250-30O0C is required to remove the last traces (Fig. 1O)J11^ If atemperature in excess of 40O0C is reached, the wax cracks to form lowerparaffins, olefins, and free carbon. Thus, waxes with high molecularweight compounds will decompose at lower temperatures than waxescontaining low molecular weight compounds. In general terms, this meansthat high melting point waxes decompose more readily than low meltingpoint waxes. As the temperature is increased above 40O0C, the probabilityof decomposition to carbon or soot is increased. The reaction is moredependent on temperature than time.

Time at Temperature, hr

Figure 10. The loss of wax on heating in a stream of hydrogen in WC-Co compacts.[11]

Perc

enta

ge

Loss

of

Wax

If the wax is introduced to the powder in a satisfactory manner, it ispresent in the compacts as a very fine uniform dispersion. Wax vaporiza-tion occurs from the exposed surfaces of the compact on heating. As thewax is lost, further wax diffuses to the surface from the inner zone of thecompact and such a process continues until the compact is fully dewaxed.

In an inert- or reducing-atmosphere furnace, an additional factor tobe considered is the relationship of the compacts to the gas flow. Exposureof as much as possible of the compact surfaces to gas flow ensures themaximum removal of wax. Covering the compacts by various packingmedia, such as alumina, introduces a further diffusion path to the waxvapour before it can be carried away. Once the wax is in the gas stream, ithas to be removed from the reaction zone as quickly as possible. In a semi-continuous stoking furnace, this means that the wax vapour is carriedtowards the cool end of the furnace. In this zone, condensation of the waxwill occur and because of the forward stoking action of the boats, the waxwill once again be carried forward. To reduce this to a minimum, thelength of the 'cold' zone is kept to a minimum and the furnace tube issloped to allow the liquid wax to run out; trays and boats are also raised onrunners to permit the molten wax to escape.

The position of the compacts within the boat, and the type of boat,e.g., open-ended with shallow sides, is also of importance if the furnace isa combined dewax/presinter unit. In this instance, the temperature gradientis so arranged that the compacts are gradually introduced into the high-temperature zone, e.g., 800-90O0C. All wax must therefore be removedfrom the compacts before temperatures in excess of 40O0C are reached.Boats with closed ends and deep sides allow the wax vapour to be trappedin the zone close to the walls and this cracks to carbon or soot on reachingthe high temperatures. This can cause splitting of the pressings and thecompacts are ruined. A continuous-stoking furnace with a suitable atmo-sphere which both protects the compacts and acts as a carrier for theremoved wax is used for small components. In the case of large compactswith a low ratio of surface area to volume, it is more usual to employ abatch furnace. With the continuous furnace, the dewaxing process iscontrolled by the temperature gradient, stoking speed, and gas flow; whilein the latter, it is controlled by a programmed heating cycle and gas flow.In both cases, due attention must be paid to the positioning of the compactsin relation to the gas flow.

A vacuum heating cycle can also be used to remove wax. Similarprecautions to those mentioned for operating an atmosphere furnace have

to be taken with this method of dewaxing. Vacuum furnaces are invariablybatch furnaces and the dewax cycle is controlled by the heating rate. Greatcare is necessary to ensure that all the wax is removed from the furnacezone before the temperature exceeds the cracking temperature. It is verydifficult to obtain uniform temperatures at the low levels required fordewaxing. Conditions can arise where wax vapour is being drawn from arelatively cold zone over a hot zone. If the temperature of the latter zone isabove the cracking temperature, soot can be formed on the compacts. Oneof the most difficult operations in this type of furnace is the satisfactorycollection of the wax after removal from the work chamber. Presently, dueattention is being paid for the satisfactory removal of wax from the furnacechamber and a number of furnaces have been developed. A 'Sweep gas'line is provided in almost all the vacuum furnaces in addition to the mainvacuum line. At the same time, an inert carrier gas, e.g., argon is used tosweep out the wax vapour efficiently. Usually, the gas travels through along pipe before entering into the mechanical pump or booster pump.While travelling this distance, the wax vapour is condensed by a watercooling arrangement. Subsequently, an optimum temperature of the insidefurnace wall is maintained by passing hot water through the furnace shellwhich helps in melting of the wax if any deposition takes place. Themolten wax runs to the wax collector attached to the 'Sweep gas' line. Assoon as the dewaxing is over, the vacuum is changed to main vacuum line.

5.0 PRESINTERING

In the production of hardmetal components, it is occasionally neces-sary to carry out a number of shaping operations before final sintering. Theoriginal pressed compacts are produced in simple shapes such as rectangu-lar and round blanks which are subsequently machined by the normaltechniques of turning, drilling, and grinding. Due allowance has, of course,to be made for the shrinkage that occurs on final sintering. This can be ofthe order of 20-25%.

To enable the machining operations to be carried out without breakingthe fragile compacts, they are strengthened by presintering at 750-100O0Cwhen they become strong enough to withstand these operations. Thisapparently simple operation of heating to 750-100O0C, in fact, turns out tobe most complicated. Because of the fineness of the powder giving a high

specific surface, extensive changes in composition can occur duringpresintering.

The cobalt particles begin to weld together at quite a low tempera-tures, e.g., 100-20O0C. These reactions at low temperatures are obviouslydue to the heavily cold-worked condition of the cobalt particles producedby the wet milling process.[12] Further strengthening is obtained byincreasing the temperature, but this is not accompanied by any shrinkage.It can be assumed, therefore, that the strengthening is due to local weldingof cobalt particles.

For straight tungsten carbide-cobalt hardmetals, temperatures in theregion of 80O0C yield compacts with sufficient strength for most machin-ing operations. As the cobalt content increases, the temperature can bereduced to give comparable strength. At such temperatures, a smallamount of contraction occurs which is the result of rearrangement of theparticles within the compacts. As no liquid phase forms, it is probable thatsolid-state sintering of the cobalt and/or tungsten carbide begins.

Lattice distortion, already inherited during wet milling, can result inlocal welding of carbide to carbide contacts. It has been reported^13! thatthe formation of the carbide skeleton starts at this stage particularly inalloys of low cobalt content. There is also a possibility of diffusion oftungsten and carbon into the cobalt leading to the rearrangement of theparticles. Alloys containing titanium and/or tantalum carbide requirehigher presintering temperatures than the straight tungsten carbide-cobaltalloys to give equivalent strengths.

The gaseous atmosphere around the compacts, if not inert, may reactwith the compacts during presintering. Both oxygen and carbon content oftungsten carbide-cobalt hardmetals undergo a change while heating topresintering temperature. If the process is done in hydrogen, any oxides ofcobalt or tungsten are reduced. To achieve 100% reduction, a minimumtemperature of 70O0C is required (Fig. 11).

Tungsten carbide reacts with hydrogen and water vapour in thepresintering temperature range. In the case of pure 'as carburised' tungstencarbide, there is no reaction with dry hydrogen, but when the powder ismilled, either with or without the addition of cobalt, the following reactiontakes place with loss of carbon:

Eq. (4) WC + 2H2 -> CH4 + W

Time, hr

Figure 11. The progressive reduction of oxides within a 6% Co tungsten carbide compacton heating in dry hydrogen.

Hinnuber et al.[14] have attributed this reaction to the lattice strainintroduced in carbide during ball milling. Cobalt is then presumed tocatalyse the reaction. Increase in milling intensity results in an increase inthe reaction rate with hydrogen. On the other hand, as the cobalt contentincreases, the apparent reactivity decreases. The influence of cobalt on thereaction with hydrogen is complex. However, during the reaction theformation of the compounds Co3W3C and Co3W in the compacts wasobserved.[11] These compounds form around the surfaces of the tungstencarbide particles and to some extent reduce the diffusion rate.

Yang[60] reported the effect of presintering temperature/time on theproperties of ready to press mixes of 7.79 Co, 0.335% O2,5.08% combinedC and 0.8% rubber, and the rest tungsten carbide from a Chinese source.The presintering was carried out in hydrogen with flow rates of 1-2.5 m3/h. He confirms that the debonding was completed below 4350C. Weightloss variation with time carried out at 60O0C, showed that when presinteringtime was more than 80 minutes, no increase in weight loss was noticed.Shrinkage during presintering increased with an increase in temperature.At 600°C presintering, the shrinkage increased slowly with presinteringtime. The mean shrinkage was 0.37% for the samples which were presinteredfor 100 minutes. Both presintering temperature and time had a minimum inplot with respect to combined carbon variation.

Per

cent

age

of O

xyge

n

Wattrconttnt