US2793951A - Powder metallurgical process for producing dense tungsten alloys - Google Patents

Description

2,793,951 POWDER METALLURGICAL PROCESS FOR PRO- DUCING DENSE TUNGSTEN ALLOYS Edward Charles Green, London, David John Jones,

Harrow, and Wilfred Robarts Pitkin, Belvedere, England, assignors to The General Electric Company Limited, London, England No Drawing. Application June 18, 1954, Serial No. 437,868 Claims priority, application Great Britain June 19, 1953 7 Claims. (Cl. 75-214) This invention relates to dense alloys of which the main constituent consists of tungsten and/ or molybdenum, and to the manufacture of such alloys.

The term dense alloy, as used in this specification, means an alloy which consists structurally of relatively large grains of the main constituent embedded in a matrix composed of a solid solution and/or dispersion of the main constituent in one or more other metals forming a minor constituent of the alloy; such an alloy is substantially free from voids, so that its density approaches the theoretical density calculated from the proportions and densities of the components of the alloy on the assumption that there are no voids and no interpenetration.

The class of alloys with which the invention is concerned includes those known as heavy alloys of which the main constituent is tungsten and which have densities greater than gms./ml., and also includes similar alloys of moderately high density in which the tungsten is wholly or partly replaced by molybdenum, the density of course being reduced in proportion to increasing molybdenum content. Heavy alloys of the kind referred to, containing nickel and copper as minor constituents, have been described, for example, in Great Britain patent specifications Nos. 447,567 and 497,747, and dense alloys containing molybdenum as a main constituent in addition to tungsten, and also containing, as the minor constituent, nickel and copper, have been described in Great Britain patent speci fications Nos. 517,442 and 531,117.

These dense alloys are manufactured by methods, described in the above-mentioned specifications, which cutploy the techniques of powder metallurgy, that is to say the alloys are made by mixing the metal powders in the required proportions, pressing the mixture of powders to form a compact, and heating the compact at a temperature and for a time such that it is sintered to form an alloy of density approaching the theoretical density. The alloys so formed have been found to consist structurally, as described in specifications Nos. 497,747 and 531,117, of large grains of tungsten and/or molybdenum embedded in a matrix consisting of tungsten and/or molybdenum, nickel, and copper. This structure, and the fact that the density of the alloy approaches the theoretical density, are due to the presence in the alloy-forming mixture of a metal whose melting point is below that of the main constituent, composed of tungsten and/ or molybdenum, and in which, when molten, the main constituent is soluble to a limited extent, the heating being carried out at a temperature, above the melting point of the solution so formed, such that a minor proportion of liquid phase is present throughout the sintering process: the smaller particles of the main constituent dissolve in the liquid phase, and part of the tungsten and/ or molybdenum thus dissolved is redeposited on the larger particles, this process being continued for the duration of the sintering and resulting in growth of the grains of the main constituent and the reduction or substantial elimination of voids in the alloystructure by the formation of a matrix as aforesaid in the interstices between the grains. In the heavy alloy compositions described in the above-mentioned specifications,

nickel is the metal of lower melting point than tungsten 2,793,951 Patented May 28, 1957 and in which tungsten is soluble to a limited extent: tungsten is insoluble in copper, but dissolves to a limited extent in the copper-nickel alloy formed on sintering.

The eifect described above is obtained even when the proportion of liquid phase present during sintering is very small, say 1% or less of the composition.

Dense alloys of the type described above are particu larly useful for various engineering purposes; in particular the heavy alloys are suitable for the manufacture of parts which are required to be of considerable mass but to occupy a small volume, for example balance weights for various types of engines. These alloys are especially valuable for such purposes because they are capable of being machined and worked and usually have good mechanical properties such as tensile strength. For some purposes, however, it is desirable that the metal part used should possess an elongation of at least 1%, that is to say the total extension produced in a tensile test at room temperature should be at least 1%, of the initial length of a body of the alloy, in all dimensions, since this property results in an improved impact value and improved resistance to fracture under the application of a bending stress. The known heavy alloys have at times been produced with an elongation exceeding 1%, but it has not been found possible to obtain such a result consistently with the known alloys.

It is an object of the present invention to provide dense alloys of which the main constituent consists of tungsten and/ or molybdenum, which are of novel composition and many of which can be so manufactured as consistently to have an elongation of at least 1%.

According to the invention a dense alloy is composed of a main constituent consisting of tungsten and/or molybdenum and a minor constituent consisting of one or more of the metals iron, nickel, cobalt, chromium, the proportion of the main constituent being not less than by weight of the alloy, and the proportion of chromium, if present, being not greater than 15% by weight of the alloy.

The alloys in accordance with the invention are manufactured by sintering compacted mixtures of the metal powders in the requisite proportions.

The main constituent may consist wholly of tungsten, wholly of molybdenum, or of mixtures or alloys of tungsten and molybdenum in all proportions. Tungsten and molybdenum are both soluble to a limited extent in each of the metals which may form the minor constituent, producing in each case a solution of melting point below that of the main constituent. During sintering of the compacted mixture of metal powders in the manufacture of the alloy a liquid phase composed of such a solution is present: the minimum temperature at which a compacted mixtureof the metal powders can be heated in order that sintering shall take place, which temperature is herein referred to as the sintering temperature, is the lowest temperature at which the liquid phase is in equilibrium with the solid phase, that is to say, the lowest. temperature at which, by virtue of the process of solution 7 molybdenum"to settle out during sintering, so that a homogeneous distribution of the solid in the liquid phase will be maintained and the alloy produced will be of sub- 5 stantially homogeneous composition throughout its mass. Furthermore, these alloys will in general not be subject to deformation during sintering, although the minimum;

proportion of tungsten and/or molybdenum which is permissible in order that the mass will retainits shape throughout the sintering, when not supported by a containing vessel, will, of course, vary according tothe metal or metals forming the minor constituent,.since the proportion of liquid phase formed at thesintering temperature for any given composition will depend upon'the solubility of the main constituent in-the -said metal or metals.

If the minor constituent is formed .of two or more metals, these are preferably usedinsuch proportions that they form a solid solution withone another.

The minor constituent preferably consists of iron and nickel. The preferred range of compositions of alloys consisting of tungsten and/or molybdenum, iron and nickel is 80% to 96% tungsten and/or molybdenum, and 4% to 20% iron plus nickel in any proportions, by weight, and for obtaining the most advantageous mechanical properties we have-found that .the ratio of nickel to iron is preferably from 3:2 to 3:1 by weight. An example of a particularly useful series of alloys of high density and goodmechanical properties,-especially in respect of elongation, is the series of alloys consisting of 90% tungsten, 6% to 7.5% nickel, and 4% to 2.5% iron.

If chromium is incorporated in the minor constituent,

of the metals iron, nickel, cobalt, since if the minor con-.

stituent consists entirely of chromium, which has a relatively high melting point, the liquid phase is formed at a temperature considerably higher than that at which it is formed when iron, nickel or cobalt is present, so that the sintering temperature is inconveniently high. Moreover, in some respects the mechanical properties .of alloys in which the minor constituent consists-only of chromium are not so good as those of alloys which include one or more of the metals iron, nickel, cobalt. The inclusion of chromium in an alloy in accordance with the invention may, however, sometimes be advantageous since it results in improving the hardness of the alloy.

For the manufacture of an alloy in accordance with the invention, we have used the following method: a

mixture of the required metal powders in the appropriate proportions is subjected to a pressure between .5 and 40 tons per square. inch, and the compact thusformed is heated at or above the sintering temperaturein a reducing or inert atmosphere, for a length of-,time such that an alloy of density approaching the theoretical density is formed; If desired, a presintering heating stepmay be carried out at a temperature below the sintering temperature, to enable .the compact to be handled or machined, prior to sintering, without breakingor crumbling. The sintering temperature will be difierent for different alloys, depending upon the constituent metals employed andupon the relative proportions of these metals. The optimum sintering time, for producing an alloy having the desired mechanical properties, will also vary to some extent with the constituent metals employed and their relative proportions, and with the temperature at .which the sintering is carried out. Asian example ofthe details ofprocedure, for the production ofa tungsten-nickel-iron alloy containing from 80% to'96-% by weight of tungsten and 4% to 20% by weight-of nickel and iron in any proportions, the sintering iscarried out aha-temperature in the range of 1420 C. to 1480" C. for A to 1% 1 hours.

For the'preparationof the initial mixture of metal. powders for compaction andsinteringasdescribed :above,

tiOXIJOf :a thoroughly .intimatemixture of--the =metal powders, but is not usually essential, as will be seen from the method of manufacturing a specific alloy in accord ance with the invention, which will now be described by way of example.

The alloy of the example consists of tungsten, 7% nickel and 3% iron, by weight. The three'metals are mixed in these proportions, each in the form of a,

finely divided powder, mixing being partially effected in a paddle type mixer, and then completed by milling'for 24 hours in a stainless steel ball mill with carbide balls. The ball-milled powder is mixed with a solution of parafiin wax in benzol, the proportion of wax being approximately 30 ml. to 1000 gms. of powder, and the benzol is evaporated ofi during continuous stirring of the mixture.

The Waxed powder is compacted at a pressure of 5 tons per square inch, and the compacts are placedin a mufile furnace and first heated to 350 C. to 400 ,C.;for de-waxing, then to 950 C. in a presinterin'g operation, and are finally sintered at 1440 C. to 1460" C. for A to 1 hour, all these heating steps being carried out in hydrogen or in an inert atmosphere.

Themethod described above by way of example may be modified insome of the details -for themanufacture. of an alloy of the stated composition or of alloys of" different compositions. Thus'ball-millingfor a shorter period may in some cases besufiicient; the'waxing,lwhich is merely for facilitating pressing in well-known manner, may be omitted if desired; the pressure employed maybe increased if desired; the presintering operation may be omitted; andthe sintering time and temperaturemay be varied as aforesaid for the manufacture of :ditterent alloys; finally, the rate of-cooling of the alloy from the sintering temperature maybe varied according tothe mechanical properties required, as will befurther explainedbelow.

- Most of the alloys in accordance with the invention, otherthan those containing relatively high proportions of chromium, are characterised by possessingelongations'of at least 1%, in some cases the elongation being considerably higher than this, for example up to about 20%. This improved elongation is, moreover, in general not accompanied by any appreciable corresponding-decrease in tensile strength. The alloys also often possess im proved ductility, and more consistenttensile strength, in comparison with previously known dense alloys containing.metals,-such as copper, in whichtnngstemand molybdenum are insoluble. Moreoventhe alloys can in general readily be machined, and usually canalso beureadily cold worked: the attainment of such cold working properties represents an important improvement over previously known heavy alloys, which can only be worked at temperatures above 300- C., as clescribed in Great Britain patent specificationNo. 5 2l,0l2.

The actual mechanical properties of any specificalloy in accordance with the'invention, inparticular the elongation, are dependent upon a number of factors: thusthese properties vary according to the compositionzof'the alloy and according to certain details of the process of manufacture of the alloy.

,With regard to the effect of variations, in'composition on the mechanical properties,it mayfirst be noted that-the elongation decreases as the proportion oftungstenrand/ or molybdenum in the alloy increases: if thepro-portion of tungsten and/or molybdenum is greater ;than 96% by weight, the elongation is-usually less-thanl%, and accordingly this figure of 96% is the upper limit of the proportion of the main constituent in the preferred range of compositions. The elongation is also decreased by the incorporation of chromiumlin the.alloy,'andxbecomes 7 lower as theproportionof chromium is increased: therefore, when it is desired to'include chromium in an alloy for improving its hardness, the undesirable. effect of diminished elongation must be counterbalanced against the advantage of improved hardness.

Referring now to the effect of variations inthe conditions of manufacture, the temperature used 'for the sinteringprocess 'has relatively little influence on the 'mechanical properties, provided that it is sufiiciently high to ensure that a liquid phase is present in equilibrium with the solid phase. The sintering time has a more important effect, the elongation usually Showing a marked increase and cooled from'the sintering. temperatures, by either the quick method or the slow method as shown in the tables.

As implied above, and as shown in Tables 2 to 4, the

with increasing time; the yield stress, which is defined 5 alloys in accordance with the invention can be manuas the stress applied in a tensile test at which a substantial factured so as to have a density which closely approaches amount of plastic deformation takes place under constant the theoretical density; this is the case even when the load, does not appear to vary in a regular manner with proportion of tungsten and/or molybdenum, as the case variations in sintering time, but the ultimate tensile may be, is very high, for example 96% or higher, the strength, which is the load applied at the maximum point attainment of such high densities being facilitated by of the stress-strain curve, usually'increases with increasthe use of one or more of iron, nickel, cobalt, or ing sintering time. chromium as the minor constituent.

The density of an alloy according to the invention in- Table 2 creases markedly with increase in the temperature at which the compact is heated, as is usual in the manufacgg ggiigi hgggfigf mz 'z ture of dense alloys, up to the sintering temperature (as hereinbefore defined); however, provided that the tem- Conditions of Ultimate perature is sufiiciently high to ensure the presence of a Sintering D it s'gensili Sliield Per- R ens y rengt ress, ceri ate at l quid phase in equilibrium with the solid phase, varia gm/cc. Tons/Sq T0ns/Sq E101} Cooling tions in sintering temperature and variations in sintering T211113. Time inch inch gatiori time over the range A to 1 /2 hours, have only a rela- (mm) tively small eifect on the density, which in all cases under these conditions is close to the theoretical density. The g 8? $2 $318 if? 5 size of the particles-of tungsten and/ or molybdenum em- 60 17.03 50.4 43.2 4.35 D0. ployed in the manufacture of the alloy also affects the 1,3 33 it}; igjg 23 8: density: these particles are preferably mainly in the range 60 17.03 56.8 43. 2 7.1 D5. of l to 3 microns in diameter. The effect of the tem- 8 8: g3 :32 H B8: perature at which the compact is heated on the density 60 17.03 53.6 41.6 7.0 D0. of an alloy of composition 90% tungsten, 7% nickel, 3% $3 8: 321i 1%: ai 33: iron, is indicated in the following table, in which the densi- 1 60 17.05 44.8. 40.4 1.70 Do. ties of alloys heated at temperatures over a range of 5,3 1%; :31; 13;; $1 33; 1000 C. to 1500 C. are given: 60 17.03 55.2 44.4 7.5 Do.

ii iii? it?) it? 2'8 it 0. Table 1 60 17.03 59.6 42.6 16.0 Do. 15 17.02 52.0 38.8 4.7 Do. 85 30 17. 04 70. 4 52.8 10. 4 Do. Temperature, O. Density, 60 17.05 60. 8 43. 6 22. 2 Do. gin/cc. I5 17. 03 60. 0 44. 0 3. 9 Do. 30 17. 03 55. 2 40. 0 7. 9 Do. 60 17.05 v 59.2 43.2 18.8 Do. 1,000. 9. 67 15 17.08 56. o 45. 2 7. 75 Do. 1,200 11.72 30 17.01 60.4 40.2 20.40 Do. 1 am 13.19 60 17.05 60.4 35.8 20.90 D0. 1 406 16.33 1,440 1,460. 1,500 16. 90 Table 3 Composition of alloy: W=98%, Ni=4.9%, Fe=2.1%. Theoretical density: 17.75.

The mechanical properties of the alloys, especially the elongation, are affected to a considerable extent by the rate at which the alloys are cooled from the sinter- OOHditIQBS of intering Tensile Yield Pei-- ing temperature to room temperature. The coohng of Density, Strength, Stress, cent Rate of the alloy after completion of the sintering process may Temp Time -l 23251- Tqgi q E1 1; Cooling be carried out either by transferring the sintered alloy m g bodies to a cold part of the furnace which may be provided with a water-cooled jacket, or by simply allowing 15 17.58 43.60 43. 60 1.32 Quick. the bodies to remain in the position in the furnace in 30 17-58 50154 t- 60 17.60 51. 52 3D. 80 3. 52 D0. which they have been sintered, so that they cool down 15 17,60 0,0 44, D with the furnace: the former method is herein referred 30 17-53 47-60 43-76 60 17.56 40.24 33.00 1. 05 Do. to as quick cooling, and the latter method as slow 15 17.17 28.72 27.20 0.74 Slow.

30 17.57 48. 00 40.43 2. 41 D0. coolin We have found that the elongation of any 60 17.63 5M0 4088 208 Do given alloy sintered at a given temperature and for a 15 17.60 50.00 46.40 3.39 D0. given time is considerably higher if it is cooled by 30 17-65 55-20 44-00 17.58 57.60 43.36 1 th slow method than if it is cooled by the quick method, 7 5 Do this effect being particularly marked when the alloy has T bl 4 been sintered for a fairly long period of time, for example a e an hour or more ggmposititliriioi 8:1l0yl1Vi{=95%,Ni=3.5%, FG=1.5%.

The effect of sintering temperature and time and rate eoreflca enmy' of cooling on the density and mechanical properties of Conditions of Ultimate some alloys in accordance with the invention is indi- 60 Sintering D it s iiensiilg sigield Pei; Rt f a ens y. reiig ress, con a co cated in Tables 2 to 4 below, the composition and theogmJcc Tons/sq. TOHSML Elem Cool-mg retical density of the alloy in each case being indicated Tgmn, Tirne inch inch gation at the top of each table. The samples employed for (mm') the tests, the results of which are given in these tables, were tensile test rods of gauge length 0.447 inch and di- 8g 3% :38 :32 'fig fameter 0.1260 inch to 0.1265 inch, the cross-sectional g8 1;.30 9. 68 44.80 2.26 Do. area being 3/30 of a square inch. These rods were obtained 15 ,3 21% g: g: g; by machining rectangular bars of the respective alloys 30 18.00 59.20 46.00 11.03 Do. produced by sintering in hydrogen for the periods of 30 $182 251%3 $133 $1? g8: time and at the temperatures indicated in the tables,

The cold working properties of 'an' alloy in accordance with the invention-will now be 'describedbyway of example. The alloy concerned is that consisting of 90% tungsten, 7% nickel and 3% iron, manufactured by a method similar to that described above by way of example. Some specimens of this .alloy were produced in the-form of rectangularplates with a final sintered thickness of about A; inch and of area 6 inches by 1 /2 inches. These plates, some of which were subjected to quick cooling and some'to slowcooling'after sintering, could all be cold rolled to form sheets either 18-24 inches long by 22 /z inches wide, or '6 inches'long by 4-6 inches wide, according to whether therolling was applied lengthwise or breadthwise. During the rolling thevhardness of the alloy increased from 330 V. P. N. (Vickers Pyramid Number) to 520 V. P. N. Rolling was carried out in several stages, and after each;stage an intermediate anneal was carried out by heating the alloy at a temperature of 1440 C. to 1460 C., a reduction of the crosssectional area by 60% being found possible in each such stage. t

In further cold working tests carried out on the alloy consisting of 90% tungsten, 7% nickel and 3% iron, it was found that a bar of'breadth 0.325 inch and thickness 0.137 inchcould be bent completely round a rod of 0.5 inch diameter, and that a bar. of thickness 0.137 inch could be cold rolled to thickness 0.048 inch without any edge cracking taking place. This alloy was also easy to machine and had a good finish.

Cold rolling tests carried out on a series of alloys of tungsten content varying from 75% to-95%, showed that all alloys containing 90% tungsten orless were readily rolled with reductions of 60% in cross-sectional area between intermediate anneals, but the alloy containing 95% tungsten could onlybe given a reduction of 30% without the appearance of cracks in the specimen.

The etfect of the incorporation ofchromium in a series of tungsten-nickel-iron alloys is shown iuthe following table (Table from which itis apparent thatthe ing chromium content; the chromium may be regarded as replacing part of the tungsten in the alloy composition, the nickel and iron contents beingkept constant throughout the series of alloys. The table includes an alloy containing no chromium for the sake of comparison ,with the remaining three alloys, which contain increasing prohardness of the alloys increasesmarkedly with increasportions of chromium. These alloys were prepared by a pressing the mixed metalpowders under five 'tons per square inch pressure, and sintering in a furnace 'at 1460 C. to 1480 C. for 1 hour, the four samplesbeing sintered simultaneously in the same furnace to ensurethat there were no differences in'the sintering conditions.-

We claim:

1. A method of manufacturing a dense alloy which includes the steps of forming amixture of metal powders composed of a main constituent, in a proportion not less than by weight of the mixture, consisting t.

of at least one member of the group consisting of tungsten and molybdenum, and a minor constituent consisting of iron and at least one member of the group consisting of nickel and cobalt in proportions such that the ratioof the total weight of nickel and cobalt to the weightrof iron is from 3:2 to 3:1, subjecting the mixtureto a pressure between 5 and-40 tons/sq. inch, heating the-compact thus formed at or above the sintering temperature -in;a non-oxidizing atmosphere in a furnace, for a period of A to 1V2 hours, and subjecting the sintered compact to slow cooling by allowing it to remain, during cooling of the furnace, in the position in the furnace inwhich it has been sintered.

2. -A method according to claim 1 wherein the said minor constituent includes also chromium in a proportion not greater than 15%, by weight, of the total metal powder.

'3. A method according to claim 1 wherein the proportions of the metal powders, and the conditions of sintering and cooling thereof, are such that the resulting alloy has an elongation of at least 1%.

4. A method of manufacturing a dense alloy which includes the steps of preparing a mixture of metal powders consisting of to 96%, by Weight, of tungsten and 4%J to 20%, by weight, of nickel and iron in proportions such that the ratio of nickel to iron, by Weight, is from 3:2 to 3: 1, subjecting the mixture to a pressure between 5 and 40 tons/ sq. inch, heating the compact thus formed at a temperature in the range of 1420" C. to 1480 C. in a non-oxidizing atmosphere in a furnace, for a period of a A to 1 /2 hours, and subjecting the sintered compact to slow cooling by allowing it to remain, during'cooling of the furnace, in the position in the furnace in which it has been sintered.

5. A method of manufacturing a dense alloy which includes the steps of forming a mixture consisting of tungsten'powder, 7% nickel powder and 3% iron powder, by weight, mixing said metal powder mixture with 3% of its weight of paraflin wax, compacting the waxed powder at a pressure of 5 tons/ sq. inch, heating the compacts thus produced in hydrogen in a muflietfurnace, first at 350 .C. to 400 C. for removal of the wax, then at 950 C. for

the position in the furnace in which they have been sintered.

6. A method of manufacturing a dense alloy which includes the step of preparing a mixture consisting of a 93% tungsten powder, 4.9% nickel powder and 2.1%

'iron powder, by weight, mixing said metal powder mixturewith 3 :of its weight of paraflin wax, compacting the waxed powder at a. pressure of 5 tons/sq. inch, heating the compacts thus produced in hydrogen 'in a mutfie furnace, first at350 C. to 400 C. for removal of the wax, then at950 Cfifor pre-sintering, and finally at 1440 C. to, 1460" .C. for'l5 'to 60 minutes to effect sintering of the compacts, and subjecting the sintered compacts to slow cooling by allowing them .to remain during cooling of thefurnacefln'the position in the furnace in which they have'zbeen sintered.

7. A method of manufacturing a dense alloy which includes the step of preparing a mixture consisting of 95% tungsten powder, 3.5% nickel powder and 1.5% iron powder, by weight, mixing said metal powder mixture with.3% of-its weight of paraffin wax, compacting the waxed .powder at a pressure of 5 tons/sq. inch, heating the compacts thus produced in hydrogen in a muflle furfurnace, in the position in the furnace in which they have been sintered.

UNITED STATES PATENTS References Cited in the file of this patent 1,110,303 Kreusler Sept. 8, 19.1 4 2,153,390 Pirani Apr. 4, 1939 2,466,992 Kurtz Apr. 12, 1949

Claims (1)

1. A METHOD OF MANUFACTURING A DENSE ALLOY WHICH INCLUDES THE STEPS OF FORMING A MIXTURE OF METAL POWDERS COMPOSED OF A MAIN CONSTITUENT, IN A PROPORTION NOT LESS THAN 75% BY WEIGHT OF THE MIXTURE, CONSISTING OF AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF TUNGSTEN AND MOLYBDENUM, AND A MINOR CONTITUENT CONSISTING OF IRON AND AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF NICKLE AND COBALT IN PROPORTIONS SUCH THAT THE RATIO OF THE TOTAL WEIGHT OF NICKLE AND COBALT TO THE WEIGHT OF IRON IS FROM 3:2 TO 3:1, SUBJECTING THE MIXTURE TO PRESSURE BETWEEN 5 AND 40 TONS/SQ. INCH, HEATING THE COMPACT THUS FORMED AT OR ABOVE THE SINTERING TEMPERATURE IN A NON-OXIDIZING ATMOSPHERE IN A FURNACE, FOR A PERIOD OF 1/4 TO 11/2 HOURS, AND SUBJECTING THE SINTERED COMPACT TO SLOW COOLING BY ALLOWING IT TO REMAIN, DURING COOLING OF THE FURNACE, IN THE POSITION IN THE FURNACE IN WHICH IT HAS BEEN SINTERED.