EP1206585B1 - 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

The technical sector of the present invention is that of sintered materials based on tungsten, intended especially in the manufacture of refractory products high purity, greater than 99%, high relative density, greater than 93%, and of more or less complex forms, generally with thin wall as by example the melting furnace crucibles.

It is well known to use materials based on tungsten in fields of application where the intrinsic properties of tungsten are particularly sought after, in particular a high melting point (3,410 ° C.), an excellent compromise between thermal conductivity ( 178 W / m ° C at 25 ° C) and electrical resistivity (5.5 Ω / cm ² / cm at 25 ° C) as well as a strong Young's modulus (406000 MPa). These properties are essential for crucibles intended for preparation in the vapor phase under electron beams, but also for heating resistors, thyristor elements, X-ray anticathodes, welding electrodes, or even pellets for electrical contact.

The manufacture of such products is the most often of powder metallurgy which consists of bring to very high temperature, under hydrogen, simple geometry products obtained by compression of tungsten powder with an average grain size of generally between 2 and 10 µm (measured on Fisher sub sieve sizer).

This treatment at very high temperature or sintering is carried out between 2000 and 2800 ° C during times of isothermal maintenance of the order of 3 to 15 hours. he leads to relatively grain sintered products coarse (20 to 80 µm average size measured by counting on micrograph) with a density varying from 17.4 to 18.5, i.e. a relative density ranging from 90 to 96% of the theoretical density of 19.3 and this with structures all the more heterogeneous as the products sintered are bulky. This heterogeneity is reflected not only by a great dispersion of the sizes of tungsten grains, but also by a great dispersion pore sizes of the residual porosity around the average values which can vary from 3 to 10 µm. Now, these heterogeneities favoring the initiation and development of cracks significantly lower the mechanical properties sintered material and in particular its hardness. In order to reduce or eliminate residual porosity and above all to refine the grain size, the sintered products must be wrought by forging, rolling or hammering and this at high temperature taking into account the brittleness tungsten.

With an already high transformation cost, this metallurgy also does not allow the development of products of small dimensions with more or less geometry complex and thin-walled, unless you have very delicate and costly machining prohibition box for solid tungsten blanks wrought.

To try to improve quality and produce lower cost of tungsten parts, those skilled in the art has a number of ways to limit when from sintering the magnification of the tungsten grains all maintaining relatively densification levels high.

It is indeed possible to reduce the size of grains as well as porosities by addition of dispersoids such as thorine, zirconia, or even silica, introduced into the weight proportion generally of 0.5 to 2% in tungsten powder. We thus obtain with powders of 4 to 5 µm (Fisher) sizes grain sizes from 5 to 20 µm associated with porosities of average size varying from 1 to 3 µm, with balances of acceptable densification up to 93-95% density relative.

However, there is a significant drop in hardness (HV ₃₀ <300) while the sintering temperature level remains very high (2000 to 2400 ° C). In addition, any search for shaped parts with thin walls requiring the use of organic binders to obtain sufficient cohesion of the compressed part, creates additional porosities during sintering and leads to significant density losses which fall between 75 and 85 % of theoretical density.

In order to limit the grain size while maintaining sufficiently high densification and hardness levels, isostatic compression processes are also used, combining for example cold isostatic compression with hot isostatic compression and then heat treatment at medium temperature. Thus from powders with an average diameter of 3 to 5 μm Fisher with cold isostatic compression at 13.10 ⁷ Pa followed by isostatic compression at 1300 ° C at 13.10 ⁷ Pa, then a heat treatment at 1600 ° C, cylindrical blanks were produced by the applicant with a relative density of 97%, but with an average grain size remaining high (50 to 80 μm).

It is also known to those skilled in the art that the sintering of tungsten can be activated by adding elements such as nickel, cobalt, palladium, even iron and platinum. These activating elements are generally mixed in the form of metallic powder with tungsten powder, but can also be introduced in the form of oxides or salts decomposable at low temperature in tungsten powder or tungsten oxide WO ₃ before co-reduction under hydrogen.

Many publications describe the effects of these activators added in weight proportions varying from 0.15 to 4, even 5%, to tungsten powder.

So, the article of Ma Kangzhu published previously in the journal P / M Research Institut, Central South University of Technology, Changsha, Hunan, China, pages 777-782, a very significant lowering of the sintering temperature of tungsten is obtained by activation of tungsten powder with 2 to 3% by mass a mixture of Co + Ni which allows 1350 ° C to limit the magnification of tungsten grains to less than 10 µm, while achieving relative densities greater than 96% as well as acceptable hardness characteristics (HB> 300). But the material obtained is rather an alloy.

The article by Moon and Kim published in 1998 in the journal "Modern Development in Powder Metallurgy", Vol. 19 pages 259-268, also notes a significant lowering the sintering temperature of the tungsten powder activated with only 0.2 to 0.4% nickel when reaching 98% of the density of tungsten at 1400 ° C, with however tungsten grains from 20 to 30 µm and a drop sensitive to hardness and resistance characteristics compared to pure tungsten sintered in the same conditions.

More recently, with the marketing of powder submicron class tungsten (Fisher diameter between 0.1 and 0.8 µm) and presenting, due to their higher specific surface, greater activation during sintering, it was possible to lower significantly the sintering temperatures and durations.

Thus, the article by MM. Blaschtro, Prem and Leichtfried, published in 1996 in Scripta Materialia, Vol. 34, No. 7, pages 1045-1049 concerning a study by changes in porosity during sintering of powders of tungsten of different particle sizes obtain with submicron tungsten powder (0.77 µm) sintered for 1 hour at 1500 ° C the same density 17.8 (i.e. 92% relative density) than with tungsten powder conventional (4.05 µm) sintered for 1 hour at 2400 ° C. However, this study does not provide any details on the evolution and the dispersion of the tungsten grain sizes and pore sizes and by the fact on the evolution of mechanical properties of the material.

The article by Johnson and German published in 1996 in the "Metallurgical and Material Transaction", Vol. 27A, pages 441-450, relates in particular to the study of the influence of Ni, Co, Pd, Fe activators added in the 0.35% by weight proportion to tungsten powder fine (0.49 µm per BET measurement) in the presence of a binder organic, on the densification of tungsten. This one is effective from 1400 ° C with nickel and cobalt; but it results in a significant magnification of tungsten grains especially in the presence of nickel.

We will also note the study by Kayssez and Ahn from the magazine “Modern Developments in Powder Metallurgy ", Vol. 19, pages 235-247, 1988, relating to sintering of coarse tungsten powders (5 µm according to the manufacturer) and fine (0.5 µm according to the manufacturer) doped with 0.15% by weight of nickel (this rate of 0.15% being considered according to the prior art and in particular the Brophy's publications as the minimum threshold effectiveness of nickel as a sinter activator tungsten). It emerges in particular from this study that the rate of temperature rise to the level of sintering set at 1400 ° C has little effect on the densification of tungsten and magnification of grains. On the other hand, this magnification of the grains of tungsten becomes very important as soon as one reaches the 92% relative density with coarse powder and 97% with the fine powder.

The different structural states and densities of previously described materials may prove sufficient for applications where it is not expected specific properties of the sintered product. In however, for applications where compromises of characteristics are sought with including a high density, hardness and chemical purity of tungsten, these materials require, when it is technically feasible, costly operations of additional working, then machining of the blank solid tungsten to modify these structural states which have grain sizes and porosities too important and not uniformly distributed within these materials rendering them unsuitable for this use.

The object of the invention is to define a material tungsten-based sinter capable of satisfying a set of severe physico-chemical characteristics and thereby exhibiting a fine-grained structure and very low porosities evenly distributed, essential for making in particular sintered parts in tungsten of more or less complex or weak form thickness.

Another object of the invention is a method direct production of said sintered material with properties and shape required in the best economic conditions of production, in particular avoiding additional wrought operations and machining in most cases if you don't search an accuracy of more or less 0.5 mm, but also in implementing sintering conditions at sufficiently low temperature (T ≤ 1600 ° C) to allow the use of industrial resources conventionally used for example for manufacturing tungsten alloys with liquid sintering (heavy metal, pseudo alloy).

• du tungstène ayant une pureté supérieure à 99,9%,
• un additif constitué de poudre de nickel et/ou de cobalt selon un pourcentage en masse inférieur ou égal à 0,08%, le reste étant constitué par des impuretés inévitables,
• et en ce que la taille moyenne des grains de tungstène de forme équiaxe comprise entre 2 et 40 µm et uniformément répartie pour une taille moyenne donnée,
• et des porosités résiduelles uniformément réparties avec au moins 85% de la population de ces porosités ayant un volume unitaire inférieur à 4 µm³.

The subject of the invention is therefore a sintered material of tungsten, of relative density relative to that of tungsten greater than 93% and of hardness HV0.3≥400, characterized in that it comprises:

• tungsten having a purity greater than 99.9%,
• an additive consisting of nickel and / or cobalt powder in a percentage by mass less than or equal to 0.08%, the rest being constituted by unavoidable impurities,
• and in that the average size of the tungsten grains of equiaxed form comprised between 2 and 40 μm and uniformly distributed for a given average size,
• and residual porosities uniformly distributed with at least 85% of the population of these porosities having a unit volume of less than 4 μm ³ .

Advantageously, the percentage by mass of cobalt is less than 0.08% and the percentage by mass of nickel equal to zero, and the size of the tungsten grains of equiaxed form is between 2 and 6 μm, with porosities uniformly distributed and with an elementary volume of less than 4 µm ³ for more than 95% of the grain population.

Advantageously, the percentage by mass of nickel is less than 0.08% and the percentage by mass of cobalt equal to zero, and the size of the tungsten grains of equiaxed form is less than 28 μm, with uniformly distributed porosities and of volume elementary less than 4 µm ³ for more than 95% of the grain population.

a) sélection d'une poudre de tungstène de pureté supérieure à 99,9% et de diamètre Fisher moyen compris entre 0,1 et 0,8 µm,

b) mélange de cette poudre avec un liant organique de compression ajouté dans la proportion pondérale inférieure ou égale à 0,4%,

c) ajout au mélange d'un activateur de frittage choisi dans le groupe constitué par le nickel, le cobalt, l'oxyde de nickel, ou un mélange de ceux-ci, dans la proportion pondérale de la part métallique égale ou inférieure à 0,08% de la masse de tungstène et obtention du matériau pulvérulent,

d) mise en forme du matériau par compression entre 10⁸ et 8.10⁸ Pa,

e) frittage du matériau sous hydrogène relativement sec (point de rosée ≤ 15°C), avec une vitesse de montée moyenne en température comprise entre 1 et 15 °C/minute jusqu'à un palier de température compris entre 1 150 et 1600°C, avec un temps de maintien compris entre 10 minutes et 3 heures.

The subject of the invention is also a method of manufacturing a sintered material based on tungsten, characterized in that it comprises the following steps:

a) selection of a tungsten powder with a purity greater than 99.9% and an average Fisher diameter of between 0.1 and 0.8 μm,

b) mixing this powder with an organic compression binder added in the proportion by weight less than or equal to 0.4%,

c) adding to the mixture of a sintering activator chosen from the group consisting of nickel, cobalt, nickel oxide, or a mixture of these, in the proportion by weight of the metallic part equal to or less than 0 , 08% of the mass of tungsten and obtaining the pulverulent material,

d) shaping of the material by compression between 10 ⁸ and 8.10 ⁸ Pa,

e) sintering of the material under relatively dry hydrogen (dew point ≤ 15 ° C), with an average temperature rise rate between 1 and 15 ° C / minute up to a temperature plateau between 1150 and 1600 ° C, with a holding time between 10 minutes and 3 hours.

Advantageously, the sintering is carried out in the absence activator by direct illumination of the material at a bearing temperature between 1,500 and 1,600 ° C, with a holding time between 30 minutes and 3 hours.

Advantageously also, the sintering is carried out in presence of activator by direct lighting at a bearing temperature between 1,150 and 1,500 ° C, with a holding time between 10 and 90 minutes.

Advantageously also, the sintering is carried out in presence of activator by indirect lighting at a bearing temperature between 1,500 and 1,600 ° C, with a holding time between 15 to 30 minutes.

A particular application of the sintered material to tungsten base according to the invention resides in the manufacture of complex or wall-shaped products thin.

Another special application of sintered material based on tungsten obtained according to one of the invention resides in the manufacture of components such as crucibles refractory.

In his search for a sintered material based on tungsten with sufficient characteristics in terms in particular of hardness, density, the applicant has advantageously made a number of observations.

First of all, thanks to the use of powders of submicron tungsten, obtaining a structure suitable is possible with simply sintered material and this contrary to the principle generally accepted in the prior art that a wrought of the sintered material is essential to reach a density close to theoretical density by absorbing the majority of porosities as well as to reduce the grain size to less than 50 µm.

Then, the great compressibility of the powders submicrones allows almost direct shaping of complex or thin products it can also be sintered at temperatures not exceeding not 1600 ° C instead of 2000 ° C, even 2400 ° C of art previous, given a significant activation of the submicron powder linked to its very specific surface high.

These conditions therefore favor obtaining a fine-grained tungsten structure with porosities residual of very small dimensions. The choice of submicron powders is also contrary to principle long advocated in this sector technique, namely that the use of powders too fine tungsten promotes texture coarse of the sintered material.

Finally, the addition of sintering activators such as Ni and / or Co in very small quantities (800 ppm maximum) in submicron tungsten powder allows to further significantly and unexpectedly lower the sintering temperature between 1,150 and 1,450 ° C, with holding time between 10 and 30 minutes. The also, the process differs from the prior art which considers that the contribution of activator, in this case the nickel, must be at least 0.15% by mass in submicron tungsten powder to be effective, this which corresponds substantially to a nickel monolayer whose diffusion at the joints of the tungsten grains is very fast during sintering.

• les figures 1 à 4 sont des micrographies représentatives de structures de matériaux frittés à base de poudre de tungstène élaborés selon l'art antérieur,
• les figures 5 à 14 sont des micrographies représentatives de structures de matériaux frittés à base de poudre de tungstène élaborés selon l'invention.

Other characteristics, details and advantages of the invention will be better understood on reading the additional description which follows of particular embodiments given by way of example with reference to the drawings in which:

• FIGS. 1 to 4 are representative micrographs of structures of sintered materials based on tungsten powder produced according to the prior art,
• Figures 5 to 14 are representative micrographs of structures of sintered materials based on tungsten powder produced according to the invention.

To highlight the materials and the process according to the invention, a set of materials has been prepared in the form of thick tungsten crucibles varying from 1 to 15 mm for heights between 40 and 200 mm and diameters between 20 and 80 mm. We has gathered in table 1 the main physicochemical characteristics of four examples of tungsten powder with different particle sizes: 4 to 5 μm powder A; 2 to 3 µm powder B; 0.5 to 0.8 µm powder C and 0.1 to 0.4 µm powder D. powders A and B are powders conventionally used in the technical sector, while powders C and D are powders selected as part of the invention. These A-D powders are processed in accordance to the process according to the invention and the results indicated below highlight the importance of choice of powder characteristics in setting up work of the invention and obtaining results satisfactory.

Regardless of the Fisher diameter, the powders fine submicron C or ultrafine D are distinguished from powders A and B generally used in the art anterior by a lower particle size spread non-friable agglomerates (measured by diffraction laser), by greater compressibility according to law of Heckel and above all by greater sinterability measured by the relative withdrawal percentage after 1 hour isothermal maintenance under dry hydrogen. We indeed notes with submicron powders shrinkage 2 to 5 times greater than 1100 ° C than those obtained with the powders of the prior art at 1500 ° C.

The data relating to AD powders are given below. POWDER A Fisher particle size 4-5 µm Laser particle size D ₁₀ 3.5-7 µm D ₅₀ 7.5-17.5 µm D ₉₀ 17.5-48 µm Compressibility A (10 ^-2 ) 66-90 K (10 ^-5 ) 52-70 Sintering temperature 1,500 ° C (under hydrogen) Withdrawal 1.5-2.5% Impurities (ppm) C <5, S <5, Na <30, K <5, Ni <10, Fe <40, Co <10 POWDER B Fisher particle size 2-3 µm Laser particle size D ₁₀ 1.2-2.3 µm D ₅₀ 4.6-8 µm D ₉₀ 9-21 µm Compressibility A (10 ^-2 ) 72-80 K (10 ^-5 ) 55-62 Sintering temperature 1,500 ° C (under hydrogen) Withdrawal 5% Impurities (ppm) C <5, S <5, Na <15, K <5, Ni <10, Fe <40, Co <10 POWDER C Fisher particle size 0.5-0.8 µm Laser particle size D ₁₀ 0.9-1 µm D ₅₀ 3-9 µm D ₉₀ 20-25 µm Compressibility A (10 ^-2 ) 55-60 K (10 ^-5 ) 31-34 Sintering temperature 1100 ° C (under hydrogen) Withdrawal 7-13% Impurities (ppm) C <45, S <5, Na <4, K <4, Ni <5, Fe <25, Co <5 POWDER D Fisher particle size 0.1-0.4 µm Laser particle size D ₁₀ 0.1-0.5 µm D ₅₀ 2.5-8 µm D ₉₀ 10-20 µm Compressibility A (10 ^-2 ) 43-50 K (10 ^-5 ) 30-33 Sintering temperature 1100 ° C (under hydrogen) Withdrawal 8-17% Impurities (ppm) C <45, S <5, Na <4, K <4, Ni <5, Fe <20, Co <5

Three materials were prepared in the form of cylindrical tungsten blanks from the same tungsten powder with Fisher diameter 4.3 μm (POWDER A), with and without addition of dispersoid (La ₂ O ₃ powder in the present case), after compression under 2.10 ⁸ Pa in the absence of a binder for different sintering stages and holding time under dry hydrogen. Example 1 dispersoid 0% Sintering temperature 2400 ° C (level) Sintering time 10 hours Density 18.4 Relative density 95.4% Grain diameter 44-62 µm Porosity 3-10 µm ³ Hardness HV ₃₀ 325 HBW 5/250 285 Example 2 dispersoid 0.8% Sintering temperature 2,200 ° C (level) Sintering time 4 hours Density 17.6 Relative density 93% Grain diameter 8-15 µm Porosity 1-3 µm ³ Hardness HV ₃₀ 280 HBW 5/250 206 Example 3 dispersoid 1.6% Sintering temperature 2,200 ° C (level) Sintering time 4 hours Density 17.8 Relative density 94% Grain diameter 6-9 µm Porosity 1-3 µm ³ Hardness HV ₃₀ 280 HBW 5/250 200

We note that to reach an acceptable density (dr = 93%) with a tungsten powder used in prior art while limiting the magnification of tungsten grains, it is necessary in the presence of dispersoid sinter for at least 4 hours at 2200 ° C; but at the same time there is a decrease in the hardness of the material with HV30 <300 due to the presence dispersoid.

If we now refer to Figures 1 and 2 representing micrographs (respectively Gx500 and Gx200) sintered material without dispersoid or binder according to example 1, they appear for a tungsten powder chosen according to the prior art a majority of porosities in this case, with an average diameter of 3µm with heterogeneous distribution for 90% of the population (Figure 1), and a high proportion of grains of tungsten with an average size of around 50 µm after attack (Figure 2).

If we refer to Figures 3 and 4 of micrographs (Gx500 and Gx2000) of sintered material in the presence of 1.8% of dispersoid in accordance with Example 3, they show for a tungsten powder chosen in accordance with art previous existence of small diameter porosities generally between 1 and 3 µm, located at grain boundaries (Figure 3) whose size exceeds rarely 10 µm (Figure 4).

In Figure 3, we can note after attack a homogeneous structure with fine tungsten grains with a distribution of porosities and a lanthanum phase. The HV30 hardness is 228.

In Figure 4, we can note after attack (image SEM in backscattered electrons) of tungsten grains smaller than 10 µm and the porosity at the joints grains are predominantly less than 3 in size µm (dark phase). The lanthanum phase at the joints of grains appear in light gray.

For the implementation of the invention, therefore, as indicated above, a high purity submicron tungsten powder C or D (W> 99.9%) will be chosen which is then compressed as such in a shaping tool (punches). and cylindrical or frustoconical dies for the production of crucibles for example) at pressures preferably between 10 ⁸ and 8.10 ⁸ Pa.

In order to further lower the sintering temperature, the tungsten powder can be according to the invention added by successive dilutions of an activator of very low sintering (<800 ppm) such as iron, palladium, but preferably nickel and / or cobalt.

This sintering activator is generally in the form of a metal powder whose Fisher diameter does not exceed 3 to 4 μm. The supply of activator can also be carried out by mixing the tungsten powder or tungsten oxide WO ₃ with the activator itself in the form of powdery oxide (NiO, CoO) or in the form of a salt in an aqueous medium (Ni (NO ₃ ) ₂ , Co (NO ₃ ) ₂ , NiCl ₂ , CoCl ₂ ) and, after drying, the mixture is reduced under hydrogen at approximately 800 ° C.

To increase the compressive strength of the powder of tungsten intended for the manufacture of shaped parts complex or thin wall (0.4 to 15 mm), it it is advantageous according to the invention to add in the submicron tungsten powder of a binder organic polyethylene based more generally. The amount of binder must remain low and not exceed 0.4% by mass so as not to create over-porosity during its decomposition and thereby alter the characteristics of the material, in particular its density and hardness.

Once compressed to the required shape, the material is sintered under relatively dry hydrogen at speeds of average temperature rise which can vary from 1 to 15 ° C / minute up to and including the desired temperature level between 1,150 and 1,600 ° C, for holding times between 10 minutes and 3 hours.

More specifically, in the absence of activator of sintering, heating by direct illumination of the material to be sintered can be done in preference to bearing temperatures between 1,500 and 1,600 ° C for holding times varying from 30 minutes to 3 hours. These bearing temperatures and these times of support can be further significantly lowered (between 1,500 and 1,150 ° C for holding times between 10 and 90 minutes) with activated tungsten powders.

The following examples illustrate the characteristics results obtained on different series of crucibles made of tungsten made from powder Φ = 0.7 µm Fisher for different sintering conditions.

Example 4 0.7 µm tungsten powder C

Sintering bearing temperature: 1,500 ° C (or 1,550 ° C)

Sintering time: 180 min (or 30 min)

Direct illumination

Binder: none

Activator: none

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density maximum 18.25 minimum 18.04 average 18.15 relative 94% Grain size maximum 6 µm, minimum 2 µm, average 4 µm, Porosity (volume) <4 µm ³ 99%, > 500 µm ³ 1%, Medium hardness HV _0.3N : 450 HV ₃₀ : 400.

Example 5 0.7 µm tungsten powder C

Sintering bearing temperature: 1,500 ° C (or 1,550 ° C)

Sintering time: 180 min (or 30 min)

Direct illumination

Binder: 0.15%

Activator: none

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density maximum 18.1 minimum 17.9 average 18 relative 93.3% Grain size maximum 6 µm, minimum 2 µm, average 4 µm, Porosity (volume) <4 µm ³ 85%, > 500 µm ³ 15%, Medium hardness HV _0.3N : 440 HV ₃₀ : 370.

The above results show according to Example 4 that a first series of 8 crucibles sintered at 1500 ° C for 3 hours has exactly the same structure as a second series of 8 crucibles sintered at 1550 ° C for 30 minutes with a low density dispersion in both cases, a homogeneous distribution of porosities of the order of 1 µm (volume <4 µm ³ ).

The third and fourth series of crucibles according to Example 5 made from the same powder tungsten, but in the presence of 0.15% by mass of binder then sintered respectively at 1500 ° C for 3 hours and at 1550 ° C for 30 minutes, also show structural characteristics very similar to those from previous series. The grain sizes of tungsten does not exceed 6 µm.

The Gx500 micrograph of the sintered material at 1500 ° C. without attack, according to example 4, represented in FIG. 5 shows a low dispersion of the density and a homogeneous distribution of the porosities of size of the order of 1 μm (volume < 4 µm ³ ) for 99% of the population. Note an absence of porosities from 5 to 20 µm.

Gx500 micrograph of the sintered material at 1500 ° C after attack, according to example 4, shown on the Figure 6 shows a homogeneous grain size of 2 to 4 µm.

The Gx500 micrograph of the sintered material at 1500 ° C. without attack, according to example 5, represented in FIG. 7 shows a homogeneous distribution of the porosities of size on the order of 1 μm (volume <4 μm ³ ) for 85% of population. Note however some residual porosities of larger diameter from 5 to 20 µm representing approximately 15% of the population.

Gx500 micrograph of the sintered material at 1500 ° C after attack, according to Example 5, shown on the Figure 8 shows that the size of the tungsten grains is homogeneous from 4 to 6 µm, and does not exceed 6 µm.

Finally, it will be noted that for the two examples 4 and 5 hardnesses remain very high, systematically greater than 400 HV0.3 as long as the amount of binder implementation remains less than 0.4%.

The following examples illustrate other structural characteristics obtained on different series of tungsten crucibles made from activated powder Φ = 0.7 µm Fisher for different sintering conditions.

Example 6 0.7 µm tungsten activated powder C

Sintering bearing temperature: 1,360 ° C (or 1,250 ° C)

Sintering time: 15 min (or 20 min)

Direct illumination

Binder: none

Activator: 660 ppm Ni

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density maximum 19.14 minimum 18.85 average 19 relative 98.4% Grain size maximum 30 µm, minimum 10 µm, average 25 µm, Porosity (volume) <4 µm ³ 100%, > 500 µm ³ 0%, Medium hardness HV _0.3N,: 440 HV ₃₀ : 345 HBW5 / 250: 303.

Example 7 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1420 ° C (or 1360 ° C)

Sintering time: 15 min

Direct illumination

Binder: 0.15%

Activator: 660 ppm Ni

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density maximum 18.67 minimum 18.40 average 18.54 relative 96% Grain size maximum 30 µm, minimum 20 µm, average 25 µm, Porosity (volume) <4 µm ³ 90%, > 500 µm ³ 10%, Medium hardness HV _0.3N : 430 HV ₃₀ : 300.

Example 8 0.7 µm activated tungsten powder C

Sintering temperature: 1,550 ° C (or 1,500 ° C)

Sintering time: 15 min (or 30 min)

Indirect lighting

Binding nil

Activator: 660 ppm Ni

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density maximum 19.04 minimum 18.81 average 18.93 relative 98.1% Grain size maximum 55 µm, minimum 35 µm, average 40 µm, Porosity (volume) <4 µm ³ 95%, > 500 µm ³ 5%, Medium hardness HV _0.3 N: 420 HV ₃₀ : 310.

Thus, after sintering with direct illumination at 1360 ° C for 15 minutes or at 1250 ° C for 20 minutes according to Example 6, the desired structural characteristics are obtained with porosities of the order of 1 μm (<4 µm ³ ). The micrograph represented in FIG. 9 shows the structure of this tungsten material obtained according to Example 6, without attack, having a distribution of the porosity of uniform size and an absence of porosities from 5 to 20 μm. Figure 10 (Gx200 micrograph) shows grain sizes varying from 20 to 30 µm after attack.

For similar sintering conditions, the crucibles produced with activated tungsten powder with 660 ppm of nickel and in the presence of 0.15% of organic binder according to Example 7, very similar structural characteristics are obtained. Figure 11 (Gx500 micrograph without attack) shows a distribution of the porosity with a homogeneous size distribution of the order of 1 µm (<4 µm ³ ) representing 90% of the population and the presence of some residual porosities from 5 to 20 µm representing approximately 10% of the pore population. Figure 12 (Gx200 micrograph after attack) shows a grain size of 20 to 30 µm.

Furthermore, the sintering with indirect lighting (screening by a protective layer of alumina) of the crucibles produced from powder activated by 660 ppm of Ni and in the absence of a binder according to Example 8, requires increasing the temperatures at 1500 ° C-1550 ° C, or even 1600 ° C, for holding times of 15 to 30 minutes if we want to achieve the desired structural characteristics, in particular in density (> 98%) and hardness (> 400 HV0 , 3) with a homogeneous distribution of the porosities, 95% of which consist of pores of the order of 1 μm (<4 μm ³ ) as shown in FIG. 13 (micrograph at Gx500 without attack). Note however a significant increase in the sizes of the tungsten grains centered on 40 μm according to FIG. 14 (micrograph Gx200 after attack).

• palier final à 1300°C - 30 minutes ou
• palier final à 1500°C - 90 minutes.

Finally, the examples below show the structural characteristics of tungsten crucibles doped at the rate of 660 ppm of metal by different activators Ni, NiO, Co, Co + Ni. These crucibles were sintered under hydrogen by direct lighting and in the absence of a binder according to one of the following two thermal cycles:

• final level at 1300 ° C - 30 minutes or
• final level at 1500 ° C - 90 minutes.

Example 9 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,300 ° C

Sintering time: 30 min

Direct illumination

Binding nil

Activator: 660 ppm of active Ni, from NiO.

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 18.92 relative 98.03% Grain size average 23 µm, Porosity (volume) <4 µm ³ 99%, > 500 µm ³ 1%,

Example 10 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,500 ° C.

Sintering time: 90 min

Direct illumination

Binding nil

Activator: 660 ppm of active Ni, from NiO.

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 19.02 relative 98.55%

Example 11 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,300 ° C

Sintering time: 30 min

Direct illumination

Binding nil

Activator: 660 ppm Ni

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 18.94 relative 98.13% Grain size average 28 µm, Porosity (volume) <4 µm ³ 99%, > 500 µm ³ 1%,

Example 12 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,500 ° C.

Sintering time: 90 min

Direct illumination

Binding nil

Activator: 660 ppm Ni

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 19.05 relative 98.70%

Example 13 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,300 ° C

Sintering time: 30 min

Direct illumination

Binding nil

Activator: 330 ppm Ni and 330 ppm Co.

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 18.84, relative 97.62%, Grain size average 15 µm, Porosity (volume) <4 µm ³ 99%, > 500 µm ³ 1%.

Example 14 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,500 ° C.

Sintering time: 90 min

Direct illumination

Binding nil

Activator: 330 ppm Ni and 330 ppm Co.

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 18.74, relative 97.10%.

example 15 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,300 ° C

Sintering time: 30 min

Direct illumination

Binding nil

Activator: 660 ppm Co.

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 15.60, relative 80.83%, Grain size average 1 µm, Porosity (volume) <4 µm ³ 99%, > 500 µm ³ 1%.

Example 16 0.7 µm activated tungsten powder C

Sintering bearing temperature: 1,500 ° C.

Sintering time: 90 min

Direct illumination

Binding nil

Activator: 660 ppm Co.

Diamètre 20 à 80 mm,

Hauteur 40 à 200 mm,

Epaisseur 1 à 15 mm.

A crucible is prepared having the following characteristics:

Diameter 20 to 80 mm,

Height 40 to 200 mm,

Thickness 1 to 15 mm.

The following results are obtained: Density average 18.04, relative 93.47%.

With NiO, Ni and Ni + Co activators, excellent densification is obtained (relative density> 97%), with almost all porosities greater than 2 µm (<4 µm ³ ) disappearing, with grain sizes on average smaller at 30 µm for the least severe sintering conditions, namely 1300 ° C for 30 minutes. The Cobalt activator alone, however, requires the most severe sintering cycle 1,500 ° C for 90 minutes to obtain a relative density greater than 93% while maintaining grain sizes and porosity volumes (<4 µm ³ ) very weak.

If the nature of the weight proportion activator equal has an indisputable impact on the conditions of sintering which must therefore be adapted to obtain the desired structure for the material, there is none even for the grain size of tungsten powder, less on the submicron scale. Indeed, that the powder submicron is activated or not, the incidence of grain size of tungsten powder on structural characteristics of the material, for identical sintering conditions is negligible. In effect, replacing the tungsten powder Φ = 0.7 µm Fisher with a finer powder Φ = 0.4 µm Fisher has not allowed to the plaintiff during the tests carried out note significant structural changes tungsten material and therefore its physicochemical characteristics.

In summary, by its structure and composition, the high purity tungsten material according to the invention provides an excellent compromise between the characteristics of density, hardness, toughness, and as a result to be freed from complementary operations costly wrought and machined. Furthermore, his process of preparation by almost final shaping direct and sintering at temperatures not exceeding 1600 ° C and therefore allowing the use of means industrialists, also contributes to lowering significantly its development cost.

This tungsten material finds its best application in the manufacture of refractory products complex in shape or thin walled (0.4 to 15 mm) such as refractory crucibles.

Claims (10)

1. A tungsten-based sintered material having a high relative mean density at 93% and a hardness at HV_0,3≥ 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%, the rest being represented by unavoidable impurities,

   a mean size for the equiaxed shaped tungsten grains of between 4 and 40 µm 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 µm³.
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 µm, with the porosities evenly distributed and the elementary volume less than 4 µm³ 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 zero, and wherein the equiaxed shaped tungsten grain size is of less than 28 µm, with the porosities evenly distributed and the elementary volume less than 4 µm³ for more than 95% of the grain population.
4. A tungsten-based sintered material according to 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 µm with evenly distributed porosities and the elementary volume less than 4 µm³ for 99% of the grain population.
5. A production process for a tungsten-based sintered material according to Claim 1, wherein it incorporates 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 µm,

   b) mixing this powder with an organic compression agent added in a gravimetric proportion of less than or equal to 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% 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 (dew point ≤ 15°C), with a mean temperature build-up rate of between 1 and 15°C/minute until reaching a temperature stage of between 1,150 and 1,600°C, with a holding time of between 10 minutes and 3 hours.
6. A process according to Claim 5, wherein sintering is 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 is carried out with an activator by direct illumination at a temperature stage of between 1,150 and 1,500°C, 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 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 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.

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