AU6574800A - High density tungsten material sintered at low temperature - Google Patents

Abstract

The invention concerns a tungsten-based sintered material, with relative mean density higher than 93 % and HV0.3 hardness >/= 400. It comprises: tungsten having a purity higher than 99.9 %, an additive consisting of nickel and/or cobalt powder in a mass percentage not more than 0.08 %, an average particle size of tungsten grains of equiaxial shape ranging between 2 and 40 mu m and uniformly distributed for a given average size; and uniformly distributed residual porosity with less than 85 % of the population of pores having a unit volume less than 4 mu m<3>.

Description

HIGH DENSITY TUNGSTEN MATERIAL SINTERED AT LOW TEMPERATURE The technical scope of the present invention is that of tungsten-based sintered materials intended notably for the 5 manufacture of refractory products having a purity of over 99%, high relative density of over 93%, and more or less complex shapes, generally with walls of reduced thickness such as smelting furnace crucibles. It is well known to use tungsten-based materials in 10 fields of application where the intrinsic properties of tungsten are more particularly sought after, notably a high melting point (3,4100C), an excellent trade-off between heat conductivity (178 W/m.

0 C at 250C) and electrical specific resistance (5.5 Q//cm 2 /cm at 250C) as well as a 15 strong Young's modulus (406000 MPa). These properties are essential for crucibles intended for electron beam vapour phase smelting but also for heating resistors, thyristor elements, x-ray cathodes, welding electrodes, or else electrical contact dots. 20 The manufacture of such products is generally the province of powder metallurgy that consists in bringing to a very high temperature, in hydrogen, simple geometry products obtained by the compression of tungsten powder whose average grain size is generally of between 2 and 10 25 ptm (measured using Fisher Sub Sieve Sizer apparatus). This high temperature processing or sintering is carried out at a temperature of between 2,000 and 2,8000C for isotherm holding times of around 3 to 15 hours. This results in relatively large grain sintered products (20 to 30 80 ptm on average measured by micrograph counting) having a density varying from 17.4 to 18.5, that is to say a relative density of 90 to 96% of the theoretical density of 19.3 and this with structure all the more heterogeneous in that the sintered products are voluminous. This 35 heterogeneity is translated not only by a large dispersion of the tungsten grain sizes, but also by a large dispersion of the residual porosity pore size around mean values that may vary from 3 to 10 pm. However, since these 2 heterogeneities facilitate the onset and development of cracks they notably reduce the mechanical properties of the sintered material and in particular its hardness. So as to reduce or eliminate the residual porosity and above all to 5 reduce the grain size, sintered products must be welded by forging, rolling or swaging and this at high temperature given tungsten's fragility. At a transformation cost that is already high, this metallurgy does not offer the possibility of smelting small 10 products of a more or less complex geometry and having walls of a reduced thickness without requiring a machining operation that is complicated and prohibitively expensive on previously-welded solid tungsten blanks. In an attempt to improve quality and to reduce the 15 production cost of tungsten parts, the expert is able to employ a certain number of means to limit the enlargement of the tungsten grains during sintering whilst retaining relatively high redensification levels. In fact, it is possible for the grain size as well as 20 the porosity to be reduced through the addition of dispersoids such as throia, zirconium, or even silica, introduced in gravimetric proportions of 0.5 to 2% in the tungsten powder. With powders of 4 to 5 tm (Fisher) mean grain sizes of 5 to 20 pm associated with mean porosity 25 sizes of 1 to 3 pim are obtained, with acceptable redensification balances reaching 93 to 95% of relative density. Having said this, there is a significant reduced in hardness (HV 3 0 < 300) whereas the sintering temperature 30 remains very high (2,000 to 2,4000C). Moreover, the production of thin-walled parts requiring the use of organic binders to obtain sufficient cohesion of the compressed part creates additional porosities during sintering thereby leading to a substantial loss of density, 35 which drops to between 75 and 85% of the theoretical density. So as to limit grain size whilst retaining sufficiently high levels of redensification and hardness, isostatic 3 compression processes are also used that combine, for example, cold isostatic compression with hot isostatic compression followed by thermal treatment at average temperature. Thus, using powders of a mean diameter of 3 to 5 5 pm Fisher with cold isostatic compression at 13.107 Pa followed by isostatic compression at 1,300'C at 13.107 Pa, then a thermal treatment at 1,600'C, cylindrical blanks were obtained by the applicant having a relative density of 97%, but with a high mean grain size (50 to 80 pLm). 10 Moreover, it is known to the expert that tungsten sintering may be activated by the addition of elements such as nickel, cobalt, palladium, or even iron and platinum. These activating elements are generally mixed in the form of a metallic powder to the tungsten powder, but may also 15 be introduced in the form of oxides or salts that are decomposable at low temperatures in the tungsten powder or tungsten oxide W03 before co-reduction in hydrogen. Numerous publications describe the effects of these activators added in gravimetric proportions varying from 20 0.15 to 4, or even 5%, to the tungsten powder. Thus, the article of Ma Kangzhu published at an earlier date in the review P/M Research Institute, Central South University of Technology, Changsha, Hunan, China, pages 777-782, where a very marked lowering of the tungsten 25 sintering temperature was obtained by activating the tungsten powder by 2 to 3% in mass of a mixture of Co + Ni, which, at a temperature of 1,350'C, enables the enlargement of the tungsten grains to be limited to less than 10 p.m whilst reaching relative densities of over 96% as well as 30 acceptable hardness properties (HB>300). However, the material obtained is more an alloy. The article by Moon and Kim, published in 1998 in the review "Modern Development in Powder Metallurgy", Vol. 19, pages 259-268, also notes a marked lowering of the 35 sintering temperature for tungsten powder activated with only 0.2 to 0.4% of nickel whilst reaching a tungsten density of 98% at 1,400'C, however, the grains are of a size of 20 to 30 pLm and there is a marked reduction in 4 hardness and resistance properties with respect to pure tungsten sintered in the same conditions. More recently, with the availability of submicronic class tungsten powder (Fisher diameter of between 0.1 and 5 0.8 ptm) that, because of its high specific surface, has a greater activation upon sintering, the sintering temperatures and times have been able to be significantly reduced. Thus, the article of Messrs. Blaschtro, Prem and 10 Leichtfried, published in 1996 in "Scripta Materialia", Vol. 34, No 7, pages 1045-1049 related to a study on the evolution of the porosity during the sintering of tungstens of different granulometries using a submicronic (0.77 pim) tungsten powder sintered for 1 hour at 1,500 0 C, the same 15 density 17.8 (that is a relative density of 92%) was obtained as for a classical (4.05 gim) tungsten powder sintered for 1 hour at 2,400 0 C. However, this study does not give any details of the evolution and dispersion of the tungsten grain size and the pore size and by extension of 20 the evolution of the material's mechanical properties. The article by Johnson and German, published in 1996 in the review "Metallurgical and Material Transaction", Vol. 27A, pages 441-450, relates to the study notably of the influence of activators Ni, Co, Pd, Fe, added in the 25 gravimetric proportion of 0.35% to fine tungsten powder (0.49 pm by BET measurement) in the presence of an organic binder on the re-densification of tungsten. This is effective from a temperature of 1,400 0 C with nickel and cobalt; but it causes a significant enlargement of the 30 tungsten grains, notably in the case of the addition of nickel. Reference will also be made to the Kayssez and Ahn study taken from the review "Modern Developments in Powder Metallurgy", Vol. 19, pages 235-247, 1988, related to the 35 sintering of large-grain (5 ptm according to the manufacturer) and fine-grain (0.5 pm according to the manufacturer) tungsten powders doped with 0.15% in weight of nickel (this rate of 0.15% being considered according to 5 prior art and notably by the Brophy publications as the minimum efficiency threshold for nickel as a tungsten sintering activator) . This study notably brings to light the fact that the temperature build-up rate to the 5 sintering stage fixed at 1,400'C has little incidence on the re-densification of tungsten and the enlargement of the grains. On the contrary, this enlargement of the tungsten grains becomes substantial as soon as the relative density of 92% is reached with the large-grained powder and of 97% 10 with the fine-grained powder. The different structural states and densities of the materials described above may prove sufficient for applications where the sintered product does not require specific properties. On the other hand, for applications 15 where a trade-off in properties in required with, notably, high density, hardness and chemical purity of the tungsten, these materials impose, where technically possible, costly additional welding operations, then the solid tungsten blank must be machined to modify those structural states 20 having grain sizes and porosities that are too substantial and are not evenly distributed throughout these materials making them inapt for this use. The aim of the invention is to define a tungsten-based sintered material able to satisfy a set of strict physico 25 chemical characteristics and therefore has a fine-grained structure with very reduced evenly distributed porosities, essential to the production, notably, of sintered tungsten parts that are more or less complex in shape or are of reduced thickness. 30 A further subject of the invention is a direct production process for said sintered material giving the required properties and shape at the most economical production conditions, in particular by avoiding additional welding and machining operations in most cases where a 35 level of accuracy of plus or minus 0.5 mm is considered acceptable, but also by implementing sintering conditions at sufficiently low temperatures (T ; 1,6000C) to allow the use of industrial means classically used, fot example, to i 6 produce tungsten alloys where sintering is carried out in the liquid phase (heavy metal, pseudo alloy). The subject of the invention is thus a tungsten-based sintered material having a high relative mean density at 5 93% and a hardness at HVO,3 400, wherein it comprises: - tungsten of a purity of over 99.9%, - an additive constituted by nickel powder and/or cobalt according to a percentage in mass that is equal to or less than 0.08%, 10 - a mean size for the equiaxed shaped tungsten grains of between 4 and 40 pLm and evenly distributed for a given mean size, - evenly distributed residual porosities with at least 85% of the total of these porosities having a unit volume 15 of less than 4 pm 3 . Advantageously, the percentage in mass of cobalt in less than 0.08% and the percentage in mass of nickel is equal to zero, and the equiaxed shaped tungsten grain size is of between 2 and 6 pm, with the porosities evenly 20 distributed and the elementary volume less than 4pm 3 for more than 95% of the grain population. Advantageously, the percentage in mass of nickel is less than 0.08% and the percentage in mass of cobalt is equal to zero, and the equiaxed shaped tungsten grain size 25 is of less than 28 ptm, with the porosities evenly distributed and the elementary volume less than 4 tm 3 for more than 95% of the grain population. The invention also relates to a production process for a tungsten-based sintered material, wherein it incorporates 30 the following. steps: a) selecting a tungsten powder of a purity of over 99.9% and a mean Fisher diameter of between 0.1 and 0.8 pm, b) mixing this powder with an organic compression agent added in a gravimetric proportion of less than or equal to 35 0.4%, c) adding to a sintering activator to the mixture selected from the group constituted by nickel, cobalt, nickel oxide, or a mixture of these, in a gravimetric 7 proportion of the metallic part equal to or less than 0.08% of the mass of tungsten and obtaining a pulverulent material, d) shaping of the material by compression at between 5 108 and 8.108 Pa, e) sintering of the material in relatively dry hydrogen (dew point 150C), with a mean temperature build-up rate of between 1 and 15' 0 /minute until reaching a temperature stage of between 1,150 and 1,6000C, with a holding time of 10 between 10 minutes and 3 hours. Advantageously, sintering is carried out without an activator by direct illumination of the material at temperature stage of between 1,500 and 1,6000C, with a holding time of between 30 minutes and 3 hours. 15 Advantageously again, sintering is carried out with an activator by direct illumination at a temperature stage of between 1,150 and 1,5000C, with a holding time of between 10 and 90 minutes. Advantageously again, sintering is carried out with an 20 activator by indirect illumination at a temperature stage of between 1,500 and 1,6000C, with a holding time of between 15 and 30 minutes. One particular application of tungsten-based sintered material according to the invention lies in the manufacture 25 of products that are complex in shape or have walls of reduced thickness. A further specific application of tungsten-based sintered material obtained according to the invention lies in the manufacture of components such as refractory 30 crucibles. When researching a tungsten-based sintered material having sufficient performances notably in terms of hardness, density, the applicant made a certain number of advantageous observations. 35 First of all, thanks to the use of submicronic tungsten powders, it is possible for an adapted structure to be obtained using a simply sintered material and this in contradiction to the generally accepted principle in prior 8 art whereby the welding of a sintered material is essential to reach a density close to the theoretical density by resorbing most of the residual porosities and to reduce the grain size to less than 50 pim. 5 Then, the great compressibility of submicronic powders also the quasi-direct shaping of complex-shaped or thin walled products that may, moreover, be sintered at temperatures not exceeding 1,600 0 C instead of 2,000'C, or even 2,4000C according to prior art, given a substantial 10 activation of the submicronic powder linked to its very high specific surface. These conditions thus facilitate the obtaining of a fine-grained tungsten structure with residual porosities of very small dimensions. The choice of submicronic powders is 15 also contrary to the principle accepted for a long time in this technical field, that is that the use of over-fine tungsten powders facilitate the obtaining of a rough texture of the sintered material. Lastly, the addition of sintering activators such as Ni 20 and/or Co in very small quantities (maximum 800 ppm) in the submicronic tungsten powder once again makes it possible to significantly and surprisingly lower the sintering temperature between 1,150 and 1,4500C, with holding times of between 10 and 30 minutes. Once again, the process can 25 be differentiated from that of prior art which considers that the addition of an activator, in fact nickel, must be of at least 0.15% in mass in the submicronic tungsten powder in order to be efficient, which substantially corresponds to a unilayer of nickel whose diffusion to the 30 tungsten grain boundaries is very rapid during sintering. Other characteristics, particulars and advantages of the invention will become more apparent after reading the additional description that follows of the particular embodiments, given by way of example and in reference to 35 the appended drawings, in which: - Figures 1 to 4 are micrographs showing tungsten powder-based sintered material structures smelted according to prior art, 9 - Figures 5 to 14 are micrographs showing tungsten powder-based sintered material structures smelted according to the invention. To highlight the materials and process according to the 5 invention, a set of materials was prepared in the form of tungsten crucibles of a thickness varying from 1 to 15 mm for a height of between 40 and 200 mm and a diameter of between 20 and 80 mm. Table 1 collates the main physico chemical- characteristics of four examples of tungsten 10 powder having different granulometries: 4 to 5 ptm, powder A; 2 to 3 pm, powder B; 0.5 to 0.8 pm powder C and 0.1 to 0.4 pm, powder D. Powders A and B are powders classically used in the technical field, whereas powders C and D are powders selected within the scope of the invention. These 15 powders A-D are treated in accordance with the process according to the invention and the results given hereafter highlight the importance of the choice of characteristics of the powder in implementing the invention and obtaining satisfactory results. 20 Regardless of the Fisher diameter, the fine submicronic C or ultra-fine D powders are differentiated from powders A and B, generally used in prior art, by a weaker granulometric dispersion of non friable agglomerates (measured by laser diffraction), by a greater 25 compressibility according to Heckel's law and above all by a greater sinterability measured by the percentage of relative retraction after isothermic holding of 1 hour in dry hydrogen. We observe, in fact, that with submicronic powders retractions are 2 to 5 times greater at 1,100'C 30 than those obtained with powders according to prior art at 1,500 C. The data related to powders A-D is given below. POWDER A Fisher granulometry: 4-5 pm 35 Laser granulometry: D 10 3.5-7 pm

D

50 7.5-17.5 pm

D

90 17.5-48 pm Compressibility: A(10- 2 ) 66-90 10 K(10- 5 ) 52-70 Sintering temperature: 1,500'C (in hydrogen) Retraction: 1.5-2.5% Impurities (ppm): C<5, S<5, Na<30, K<5 5 Ni<10, Fe<40, Co<10 POWDER B Fisher granulometry: 2-3 tm Laser granulometry: D" 0 1.2-2.3 pLm 10 D50 4.6-8 pim

D'

0 9-21 pm Compressibility: A (10 2 ) 72-80 K (10 5 ) 55-62 Sintering temperature: 1,500 0 C (in hydrogen) 15 Retraction: 5% Impurities (ppm): C<5, S<5, Na<15, K<5, Ni<10, Fe<40, Co<10 POWDER C 20 Fisher granulometry: 0.5-0.8 pm Laser granulometry: D 10 0.9-1 pm

D

5 o 3-9 pm

D

90 20-25 pm Compressibility: A (10- 2 ) 55-60 25 K(10-5) 31-34 Sintering temperature: 1,1000C (in hydrogen) Retraction: 7-13% Impurities (ppm): C<45, S<5, Na<4, K<4, Ni<5, Fe<25, Co<5 30 POWDER D Fisher granulometry: 0.1-0.4 pm Laser granulometry: D 10 0.1-0.5 pm

D

5 o 2.5-8 pm 35 D 90 10-20 pm Compressibility: A (10~ 2 ) 43-50 K(10-5) 30-33 Sintering temperature: 1,1000C (in hydrogen) 11 Retraction: 8-17% Impurities (ppm): C<45, S<5, Na<4, K<4, Ni<5, Fe<20, Co<5 5 The materials in the form of tungsten-based cylindrical blanks were prepared using the same tungsten powder with Fisher diameter 4.3 pm (POWDER A), with and without the addition of dispersoids (La 2 0 3 powder in the present case), after compression at 2.108 Pa without a binder for 10 different sintering stages and holding times in dry hydrogen. Example 1 Dispersoid: 0% 15 Sintering temperature: 2,400*C (stage) Sintering time: 10 hours Density: 18.4 Relative density: 95.4% Grain diameter: 44-62 pm 20 Porosity: 3-10 pm3

HV

30 hardness: 325 HBW 5/250: 285 Example 2 25 Dispersoid: 0.8% Sintering temperature: 2,2000C (stage) Sintering time: 4 hours Density: 17.6 Relative density: 93% 30 Grain diameter: 8-15 pm Porosity: 1-3 pm3

HV

30 hardness: 280 HBW 5/250: 206 35 Example 3 Dispersoid: 1.6% Sintering temperature: 2,2000C (stage) Sintering time: 4 hours 12 Density: 17.8 Relative density: 94% Grain diameter: 6-9 pm Porosity: 1-3 ptm3 5 HV 3 0 hardness: 280 HBW 5/250: 200 We observe that to obtain an acceptable density (dr=93%) with a tungsten powder used in prior art whilst limiting the enlargement of the tungsten grains, sintering 10 with a dispersoid must be carried out for at least 4 hours at 2,2000C; but in parallel to this we record a reduction in the hardness of the material with HV 30 <300 imputable to the presence of the dispersoid. If we now refer to Figures 1 and 2 showing 15 micrographies (respectively x500 and x200) of the sintered material with neither dispersoid nor binder in accordance with example 1, for a tungsten powder selected according to prior art they show a majority of porosities in this case, of a mean diameter of 3 pm heterogeneously distributed for 20 90% of the population (Figure 1), and a strong proportion of tungsten grains of a mean size of 50 pm after attack (Figure 2). If we now refer to Figures 3 and 4 showing micrographies (respectively x500 and x2000) of the sintered 25 material with 1.8% of dispersoid in accordance with example 3, for a tungsten powder selected according to prior art they show the existence of small porosities of a diameter that is generally between 1 and 3 ptm located at the grain boundaries (Figure 3), whose size rarely exceeds 10 pm 30 (Figure 4). In Figure 3, we can observe a homogeneous structure of fine grained tungsten after attack with a distribution of porosities and a lanthanide phase. The HV 30 hardness is of 228. 35 In Figure 4, we can observe after attack (backscattered electron MEB image) tungsten grains of a size that is less than 10 pm and the porosities at the grain boundaries are of a size that is mainly below 3 pm (dark phase) . The 13 lanthanide phase at the grain boundaries appears in light grey. To implement the invention, a high purity (W>99.9%) submicronic tungsten powder C or D will be selected as 5 indicated above that is then compressed as it is in shaping tooling (punches and cylindrical or tapered dies to makes crucibles, for example) at pressures preferably between 108 and 8.108 Pa. So as to further reduce the sintering temperature, 10 according to the invention the tungsten powder can receive successive dilutions of a sintering activator in very low proportions (<800 ppm) such as iron, palladium, but preferably nickel and/or cobalt. This sintering activator is generally in the form of a 15 metallic powder whose Fisher diameter does not exceed 3 to 4 pLm. The addition of the activator may also be carried out by mixing the tungsten power or tungsten oxide W0 3 powder with the activator itself in the form of a pulverulent oxide (NiO, CoO) or in the form of a salt in an 20 aqueous medium (Ni(N0 3

)

2 ; Co(N0 3

)

2 , NiCl 2 , CoCl 2 ) and, after drying, the mixture is reduced in hydrogen at approximately 800 0 C. To improve the compression strength of the tungsten powder intended to make parts of a complex shape or having 25 walls of reduced thickness (0.4 to 15 mm), it is advantageous according to the invention to add an organic binder, generally polyethylene-based, to the submicronic tungsten powder. The quantity of binder must remain low and must not exceed 0.4% in mass so as not to create excess 30 porosities during its decomposition and thereby alter the characteristics of the material, notably its density and its hardness. Once compressed into the required shape, the material is sintered in relatively dry hydrogen at mean temperature 35 build-up rates varying between 1 to 15 0 C/minute until reaching the required temperature stage of between 1,150 and 1,6000C, for holding times of between 10 minutes and 3 hours.

14 More specifically, without a sintering activator, heating by direct illumination of the material to be sintered may be carried out preferably at temperature stages of between 1,500 and 1,600 0 C for holding times 5 varying between 30 minutes and 3 hours. These temperature stages and these holding times may still be significantly reduced (to between 1,500'C and 1,150'C for holding times of between 10 and 90 minutes) with activated tungsten powders. 10 The following examples illustrate the structural characteristics obtained on different set of tungsten crucibles obtained using a CD = 0.7 pm Fisher powder for different sintering conditions. 15 Example 4 0.7 pim tungsten powder C Sintering temperature stage: 1,500 0 C (or 1,550 0 C) Sintering time: 180 mn (or 30 mn) Direct illumination 20 Binder: none Activator: none A crucible is prepared that has the following characteristics: Diameter: 20 to 80 mm, 25 Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: Density: maximal 18.25 minimal 18.04 30 mean 18.15 relative 94% Grain size: maximal 6 pm minimal 2 pm mean 4 pm 35 Porosity (volume) : <4 pm 3 99% >500 pm 3 1% Mean hardness: HVO.3N: 450

HV

3 0 : 400 15 Example 5 0.7 pim tungsten powder C Sintering temperature stage: 1,5000C (or 1,5500C) Sintering time: 180 mn (or 30 mn) 5 Direct illumination Binder: 0.15% Activator: none A crucible is prepared that has the following characteristics: 10 Diameter: 20 to 80 mm, Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: Density: maximal 18.10 15 minimal 17.90 mean 18.00 relative 93.3% Grain size: maximal 6 pim minimal 2 pim 20 mean 4 ptm Porosity (volume) : <4 pm 3 85% >500 pm 3 15% Mean hardness: HV 0.3N 440

HV

30 : 370 25 The above results show that according to example 4 a first set of 8 crucibles sintered at 1,5000C for 3 hours has exactly the same structure as a second set of 8 crucibles sintered at 1,5500C for 30 minutes with a low dispersion of the densities in both cases, a homogeneous 30 distribution of porosities of around 1 pm (volume <4 jim 3 ). The third and fourth sets of crucibles according to example 5 smelted from the same tungsten powder, but with 0.15% in mass of binder then respectively sintered at 1,5000C for 3 hours and at 1,5500C for 30 minutes also show 35 very similar structural characteristics to those of the previous set. The tungsten grain size does not exceed 6 pm. The Gx500 micrography of the material sintered at 1,5000C without attack, according to example 4, shown in 16 Figure 5 shows a low dispersion of the density and a homogeneous distribution of the porosities of a size of around 1 pm (volume < 4 pim 3 ) for 99% of the population. We note an absence of porosities of 5 to 20 pim. 5 The Gx500 micrography of the material sintered at 1,500 0 C after attack, according to example 4, shown in Figure 6 shows a homogeneous grain size of 2 to 4 pim. The Gx500 micrography of the material sintered at 1,500 0 C without attack according to example 5 shown in 10 Figure 7 shows a homogeneous distribution of the porosities of a size of round 1 pm (volume <4 pim 3 ) for 85% of the population. We note however that some residual porosities of a larger diameter of between 5 to 20 pim represent around 15% of the population. 15 The Gx500 micrography of the material sintered at 1,500'C after attack according to example 5 shown in Figure 8 shows that the size of the tungsten grains is homogeneous at 4 to 6 jim, and does not exceed 6 pim. Lastly, we note that for both examples 4 and 5 the 20 hardness remains very high, systematically greater than 400

HVO.

3 , whereas the quantity of binder implemented is less than 0.4%. The following examples illustrate other structural characteristics obtained on different sets of tungsten 25 crucibles smelted using an activated (D = 0.7 jim Fisher powder for different sintering conditions. Example 6 0.7 jim tungsten 30 powder C Sintering temperature stage: 1,360'C (or 1,250 0 C) Sintering time: 15 mn (or 20 mn) Direct illumination Binder: none 35 Activator: 660 ppm of Ni A crucible is prepared that has the following characteristics: Diameter: 20 to 80 mm, 17 Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: Density: maximal 19.14 5 minimal 18.85 mean 19.00 relative 98.4% Grain size: maximal 30 ptm minimal 10 pm 10 mean 25 pm Porosity (volume) : <4 im 3 100% >500 pm 3 0% Mean hardness: HVO.3N: 440

HV

3 0 : 345 15 HBW5/250: 303 Example 7 0.7 pim tungsten powder C Sintering temperature stage: 1420'C (or 1,3600C) 20 Sintering time: 15 mn Direct illumination Binder: 0.15% Activator: 660 ppm of Ni A crucible is prepared that has the following 25 characteristics: Diameter: 20 to 80 mm, Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: 30 Density: maximal 18.67 minimal 18.40 mean 18.54 relative 96% Grain size: maximal 30 pm 35 minimal 20 pm mean 25 pm Porosity (volume) : <4 gm3 90% >500 ptm 3 10% 18 Mean hardness: HVO.3N: 430

HV

3 0 : 300 Example 8 5 0.7 pm tungsten powder C Sintering temperature stage: 1,5500C (or 1,5000C) Sintering time: 15 mn (or 30 mn) Indirect illumination Binder: none 10 Activator: 660 ppm of Ni A crucible is prepared that has the following characteristics: Diameter: 20 to 80 mm, Height: 40 to 200 mm, 15 Thickness: 1 to 15 mm, The following results are obtained: Density: maximal 19.04 minimal 18.81 mean 18.93 20 relative 98.1% Grain size: maximal 55 pm minimal 35 pm mean 40 pm Porosity (volume): <4 pm3 95% 25 >500 pm3 5% Mean hardness: HVO.

3 N: 420

HV

3 0 : 310 Thus, after sintering with direction illumination at 1,3600C for 15 minutes, or at 1,2500C for 20 minutes 30 according to example 6, the required structural characteristics are obtained with porosities of around 1 pm (<4 pm 3 ) . The micrography shown in Figure 9 shows the structure of this tungsten material obtained according to example 6, without attack, having a homogeneously sized 35 porosity distribution and an absence of porosities of 5 to 20 pm. Figure 10 (Gx200 micrography) shows grain sizes varying from 20 to 30 pm after attack.

19 For similar sintering conditions, the crucibles smelted with the activated tungsten powder with 660 ppm of nickel and with 0.15% of organic binder according to example 7, very similar structural characteristics are obtained. 5 Figure 11 (Gx500 micrography without attack) shows a distribution of the porosity with a homogenous distribution of a size of around 1 pim (<4 pm 3 ) representing 90% of the population and the presence of some residual porosities of 5 to 20 ptm representing around 10% of the pore population. 10 Figure 12 (Gx200 micrography after attack) shows a grain size of 20 to 30 pim. Moreover, sintering with indirect illumination (shielding by a protective layer of aluminium) of crucibles smelted using a powder activated by 660 ppm of Ni and 15 without binder according to example 8 requires temperatures to be increased to 1,500 0 C-1,550 0 C or even 1,600 0 C for holding times of 15 to 30 minutes if the desired structural characteristics are to be attained, notably with respect to the density (>90%) and hardness (>400 HVo.

3 ) with a 20 homogenous distribution of the porosities, of which 95% are constituted by pores of around 1 ptm (<4 pim 3 ) such as becomes apparent from Figure 13 (Gx500 micrography without attack). We observe, however, a substantial increase of the tungsten grain size centred on 40 pm according to Figure 14 25 (Gx200 micrography after attack). Lastly, the following examples show the structural characteristics of crucibles made of tungsten doped with 660 ppm of metal by different activators Ni, NiO, Co, Co Ni. These crucibles were sintered in hydrogen by direct 30 illumination and without binder according to one of the two following thermal cycles: - final stage at 1,3000C - 30 minutes, or - final stage at 1,5000C - 90 minutes. 35 Example 9 0.7 pm tungsten powder C Sintering temperature stage: 1,3000C Sintering time: 30 mn 20 Direct illumination Binder: none Activator: 660 ppm of active Ni, from NiO. A crucible is prepared that has the following 5 characteristics: Diameter: 20 to 80 mm, Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: 10 Density: mean 18.92 relative 93.03% Grain size: mean 23 pm Porosity (volume): <4 Rm3 99% >500 pn3 1% 15 Example 10 0.7 pm tungsten powder C Sintering temperature stage: 1,500'C Sintering time: 90 mn 20 Direct illumination Binder: none Activator: 660 ppm of active Ni, from NiO. A crucible is prepared that has the following characteristics: 25 Diameter: 20 to 80 mm, Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: Density: mean 19.02 30 relative 98.55% Example 11 0.7 pm tungsten powder C Sintering temperature stage: 1,300'C 35 Sintering time: 30 mn Direct illumination Binder: none Activator: 660 ppm of Ni 21 A crucible is prepared that has the following characteristics: Diameter: 20 to 80 mm, Height: 40 to 200 mm, 5 Thickness: 1 to 15 mm, The following results are obtained: Density: mean 18.94 relative 98.13% Grain size: mean 28 pm 10 Porosity (volume) : <4 pm 3 99% >500 pm3 1% Example 12 0.7 pm tungsten powder C 15 Sintering temperature stage: 1,5000C Sintering time: 90 mn Direct illumination Binder: none Activator: 660 ppm of Ni 20 A crucible is prepared that has the following characteristics: Diameter: 20 to 80 mm, Height: 40 to 200 mm, Thickness: 1 to 15 mm, 25 The following results are obtained: Density: mean 19.05 relative 98.70% Example 13 30 0.7 pm tungsten powder C Sintering temperature stage: 1,3000C Sintering time: 30 mn Direct illumination Binder: none 35 Activator: 330 ppm of Ni and 330 ppm of Co. A crucible is prepared that has the following characteristics: Diameter: 20 to 80 mm, 22 Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: Density: mean 18.84 5 relative 97.62% Grain size: mean 15 pm Porosity (volume): <4 [tm3 99% >500 [m3 1% 10 Example 14 0.7 n tungsten powder C Sintering temperature stage: 1,500'C Sintering time: 90 mn Direct illumination 15 Binder: none Activator: 330 ppm of Ni and 330 ppm of Co. A crucible is prepared that has the following characteristics: Diameter: 20 to 80 mm, 20 Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: Density: mean 18.74 relative 97.10% 25 Example 15 0.7 tm tungsten powder C Sintering temperature stage: 1,3000C Sintering time: 30 mn 30 Direct illumination Binder: none Activator: 660 ppm of Co. A crucible is prepared that has the following characteristics: 35 Diameter: 20 to 80 mm, Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: 23 Density: mean 15.60 relative 80.83% Grain size: mean 1 pm Porosity (volume) : <4 pm 3 99% 5 >500 pm 3 1% Example 16 0.7 pm tungsten powder C Sintering temperature stage: 1,500'C 10 Sintering time: 90 mn Direct illumination Binder: none Activator: 660 ppm of Co. A crucible is prepared that has the following 15 characteristics: Diameter: 20 to 80 mm, Height: 40 to 200 mm, Thickness: 1 to 15 mm, The following results are obtained: 20 Density: mean 18.04 relative 93.47% Using activators of NiO, Ni and Ni = Co, we obtain excellent redensification (relative density >97%), with the disappearance of almost all porosities over 2 pm (<4 pm 3 ), 25 with mean grain sizes below 30 pm for the least severe sintering conditions, that is 1,300'C for 30 minutes. Using a Cobalt activator alone, however, requires the implementation of a much more sever sintering cycle of 1,500'C for 90 minutes to obtain a relative density of over 30 93% whilst retaining very low grain sizes and porosity volumes (<4 pm 3 ). If the nature of the activator at equal gravimetric proportions has an incontestable incidence on sintering conditions, which must be adapted so as to obtain the 35 structure required for the material, it is not the same for the tungsten powder grain size, at least on a submicronic scale. Indeed, whether the submicronic powder is activated or not, the incidence of the tungsten powder grain size on 24 the structural characteristics of the material, for identical sintering conditions, is negligible. Indeed, the replacement of the c = 0.7 pim Fisher tungsten powder by a finer (D = 0.4 pim Fisher powder during tests carried out by 5 the applicant did not reveal any significant modifications to the tungsten-based material and thus to its physico chemical characteristics. To sum up, by its structure and composition, the high purity tungsten-based material according to the invention 10 provides an excellent trade-off between the characteristics of density, hardness, toughness and thereby makes additional costly welding and machining operations unnecessary. Moreover, its smelting process by quasi-direct shaping and sintering at temperatures that do not exceed 15 1,6000C and thus allow the use of conventional industrial means, also contributes to a significant reduction in production costs. This tungsten-based material is best applied to the manufacture of refractory products having complex shapes or 20 walls of reduced thickness (0.4 to 15 mm), such as refractory crucibles.

Claims (4)

1. A tungsten-based sintered material having a high relative mean density at 93% and a hardness at HVO, 3 > 400, wherein it comprises: 5 - tungsten of a purity of over 99.9%, - an additive, constituted by nickel powder and/or cobalt according to a percentage in mass that is equal to or less than 0.08%, - a mean size for the equiaxed shaped tungsten grains 10 of between 4 and 40 pm and evenly distributed for a given mean size, - evenly distributed residual porosities with at least 85% of the total of these porosities having a unit volume of less than 4 pm 3 . 15 2. A tungsten-based sintered material according to Claim 1, wherein the percentage in mass of cobalt in less than 0.08% and the percentage in mass of nickel is equal to zero, and wherein the equiaxed shaped tungsten grain size is of between 2 and 6 ptm, with the porosities evenly 20 distributed and the elementary volume less than 4pm 3 for more than 95% of the grain population.

3. A tungsten-based sintered material according to Claim 1, wherein the percentage in mass of nickel is less than 0.08% and the percentage in mass of cobalt is equal to 25 zero, and wherein the equiaxed shaped tungsten grain size is of less. than 28 ptm, with the porosities evenly distributed and the elementary volume less than 4 pim 3 for more than 95% of the grain population.

4. A tungsten-based sintered material according to 30 Claim 3, wherein it comprises 660 ppm of nickel and wherein it has a mean density close to 18.9, a relative density close to 98.1%, a mean grain size of 28 pm with evenly distributed porosities and the elementary volume less than 4 pm 3 for 99% of the grain population. 35 5. A production process -for a tungsten-based sintered material according to Claim 1, wherein it incorporates the following steps: 26 a) selecting a tungsten powder of a purity of over

99.9% and a mean Fisher diameter of between 0.1 and 0.8 pim, b) mixing this powder with an organic compression agent added in a gravimetric proportion of less than or equal to 5 0.4%, c) adding to a sintering activator to the mixture selected from the group constituted by nickel, cobalt, nickel oxide, or a mixture of these, in a gravimetric proportion of the metallic part equal to or less than 0.08% 10 of the mass of tungsten and obtaining a pulverulent material. d) shaping of the material by compression at between 108 and 8.108 Pa, e) sintering of the material in relatively dry hydrogen 15 (dew point 150C) , with a mean temperature build-up rate of between 1 and 15'C/minute until reaching a temperature stage of between 1,150 and 1,6000C, with a holding time of between 10 minutes and 3 hours. 6. A process according to Claim 5, wherein sintering is 20 carried out without an activator by direct illumination of the material at temperature stage of between 1,500 and 1,600'C, with a holding time of between 30 minutes and 3 hours. 7. A process according to Claim 5, wherein sintering 25 is carried out with an activator by direct illumination at a temperature stage of between 1,150 and 1,5000C, with a holding time of between 10 and 90 minutes. 8. A process according to Claim 5, wherein sintering is carried out with an activator by indirect illumination at a 30 temperature stage of between 1,500 and 1,600'C, with a holding time of between 15 and 30 minutes. 9. Application of tungsten-based sintered material according to any one of Claims 1 to 3 in the manufacture of products that are complex in shape or have walls of reduced 35 thickness. 10. Application of tungsten-based sintered material obtained according to any one of Claims 4 to 7 in the manufacture of components such as refractory crucibles.