EP1153150B1 - Method for producing hard metal mixtures - Google Patents

Abstract

The invention relates to a method for producing a homogeneous mixture of hard material powders and binder metal powders without using grinding bodies, liquid grinding auxiliary agents and suspending media. According to the invention, the mixture components are mixed at close range while generating a high shearing collision velocity of the powder particles and are remotely mixed by rotating the mixing bed without resulting in a particle size reduction of the hard material powders.

Description

Hard metals are materials made from hard materials and binder metals. They have meaning as wear-resistant materials and are of cutting and non-cutting shape accessible.

Hard materials are carbides or nitrides or carbonitrides of the refractory metals of the IV. V. and VI. Subgroup of the Periodic Table of the Elements, with titanium carbide (TiC), titanium carbonitride (Ti (C, N)) and especially tungsten carbide (WC) the largest Have gained importance.

Cobalt is used in particular as binder metal. However, however also mixed metal powder or alloy powder made of cobalt, nickel and iron as well if necessary, further ingredients used in minor amounts.

For the production of hard metals, hard materials and binder metal, each in Powder form, intimately mixed, pressed and then sintered, the Binder metal by forming a melt during sintering is a very extensive densification and the construction of a multi-phase structure with more favorable Bending strength and fracture toughness should enable. The effect of Binder metal is optimal when complete wetting of the hard material phase is achieved, the solubility of the hard material depending on the sintering temperature causes a partial redissolution and rearrangement of the hard material in the binder, so that a microstructure structure is achieved which has a large crack propagation Opposed resistance. The sintering result can be in the form of the residual porosity represent. In order to achieve sufficient fracture toughness, this is not the case a prerequisite for a certain residual porosity.

Hard materials are usually preferred with average particle sizes of 3 to 20 microns 3 to 10 µ according to ASTM B 330 used. It should be very fine Hard material portions are avoided, as these are used during the liquid phase sintering Recrystallization (Ostwald ripening). The crystallites grown in this way show multidimensional point defects, which for certain performance characteristics of the Carbide, especially in steel cutting, mining and striking tools are disadvantageous. For example, tungsten carbide can be made to a certain extent Plastic degrees deform when multi-dimensional point defects at high Temperatures above 1900 ° C can be cured. The carburizing temperature, at The tungsten carbide obtained is therefore essential for the performance properties of hard metal. The one at sintering temperature, typically between 1360 and 1450 ° C, redissolved portion of the tungsten carbide phase in the hard metal is in view qualitatively inferior to the unresolved portion on these performance characteristics. A further embrittlement can occur in that grown up by redeployment WC shares may have incorporated binder metals in the lattice.

The binder metal is typically used with a smaller particle size, typically about 1 to 2 µ according to ASTM B 330.

The binder metal is used in such an amount that it is about 3 to 25% by weight of the hard metal.

Up to 50% recycled, recyclable sinterable hard metal powder can be advantageous can also be used.

In addition to the selection of the most suitable hard material (particle size, particle size distribution, Crystal structure) and the binder metal (composition, amount, Proportion of hard metal) and the sintering conditions play a more suitable role Hard metal mixtures, i.e. the mixing of hard material and binder before sintering an outstanding role with regard to the later hard metal properties.

Because of the electrostatic repulsive forces between the fine powder particles (this always means low bulk densities for finer powders), the different Particle size and density as well as the unfavorable quantity relation of both Components are separated by dry mixing according to the current state of the art out. With a dry grinding of the two components, the electrostatic repulsive forces between the particles can be overcome can, however, this would lead to a particle reduction especially of the Lead hard material, in which a large number of fines would be produced. Furthermore, the unavoidable abrasion of the grinding tools an as yet unsolved problem.

Accordingly, the wet grinding has taken place in the attritor or in a ball mill Using an organic grinding fluid and using grinding balls than the industrially used process for the production of hard metal mixtures enforced. By using a grinding fluid, the electrostatic Rejection forces additionally suppressed effectively. It succeeds through the wet mixed grinding in the attritor the grain crushing of the hard material in still keeping reasonable limits, however, the mixed grinding is a very complex Process that requires a large amount of space due to the required volume ratio from grinding media to grinding stock of about 6: 1 and on the other hand Meals take from 4 to 48 hours. There is also the requirement after the mixed grinding, the grinding balls of the hard metal mixture by sieving and the organic grinding liquid by evaporation. A certain amount of grind and a certain size reduction is also with the to accept wet mixed grinding. WC powders are particularly affected, that were carburized at at least 1900 ° C, narrow grain size distribution have no fines and therefore no redeployment in top quality Hard metals should be transferred.

According to a very old proposal (GB Patent 346 473) the problems of Mixture of hard materials and binder metal can be solved in that the hard materials be coated electrolytically with the binder metal. This procedure has however, cannot prevail. According to recent proposals (US-A 5 505 902 and US-A 5 529 804) the binder metal, in particular cobalt, is chemically reacted to the Hard material particles applied. Organic liquid phases are used should not remain without influence on the carbon balance of the hard metal.

In US-A-4 320 156 a mix for the production of Tungsten carbide bodies fluidized by a swirling dry fluid and mixed.

The object of the present invention is to provide a method for producing hard metal mixtures specify which avoids the disadvantages of the prior art, is in particular technically less complex and moreover due to the Homogeneity of the mixture and the avoidance of grain crushing of the hard material after sintering hard metals with excellent performance properties by minimizing the redeemed portion of the WC phase.

The object is achieved by the method according to claim 1.

It has been found that the object is achieved in that the mixing in Close range of the components of the mixture due to the generation of high shear impact speed the powder particles and in the far range by circulating the Mixed goods is carried out.

In this way, dry mixing of hard material and binder metal powders is possible without the use of grinding media or liquid grinding aids or liquid Suspension media essentially without grain size reduction.

According to the invention, "mixing in the close range" is the mixing of a Part of the mix understood in itself, whereas long-range mixing the mixing of the bulk of the mix batch in itself, i.e. the Subsets among themselves, designated.

The method according to the invention therefore consists in the one hand in the short-range mixing with high input of mixed energy (based on that of the Mixing element detected amount of powder) to overcome the electrostatic repulsive forces the powder particles with each other and on the other hand with the mixing of mixing areas with low energy input to homogenize the powder mixture is mixed.

Are preferred according to the invention for short-range and long-range mixing different mixing units used.

The majority of the mix is in the area of long-range mixing by circulating the mixture bed. For example, a Rotary tube, a ploughshare mixer, a paddle mixer or a cone screw mixer.

A subset of the mixture is in the area of short-range mixing, a mixing unit producing high mutual impact speeds. Aggregates suitable for short-range mixing are in particular fast rotating mixing elements. Those with peripheral speeds are preferred according to the invention from 8 to 25 m / s, particularly preferably 12 to 18 m / s. The mix is preferably at least in the area of short-range mixing fluidized in the gas atmosphere of the mixing container, the gas through the Mixing element is strongly swirled and the powder particles due to the in the vortex prevailing shear rates collide. A suitable mixing element is, for example, a quick one with wall-mounted stirring blades running agitator, leaving a gap between the container wall and the agitator blade, the width of which is at least 50 times the particle diameter. The gap width is preferably 100 to 500 times the particle size.

For example, aggregates that are also suitable for short-range mixing are made of US-A 3 348 779, US-A 4 747 550, EP-A 200 003, EP-A 474 102, the EP-A 645 179 and DE-U 29 515 434 under the name micro vortex mill known. Such mills consist of a stator in the form of a cylindrical Housing in which a rotor is axially arranged, the one or more circular disks arranged one above the other on a common drivable axis has, the circular discs on their circumference a variety of im have grinding plates arranged essentially radially and parallel to the rotor axis, which protrude beyond the circular disks, with a gap between the stator and grinding plates remains, the "shear gap". The rotor becomes too high speed of rotation driven, typically 1000 to 5000 U / Min., experienced in the micro vortex mill located gas-dispersed particles due to the imprinted on the gas Shear speed between rotor and stator high acceleration forces, see above that the particles collide while overcoming the electrostatic repulsive forces. When the particles impact, there is a charge exchange or a dielectric recharge instead, so that the repulsive forces of the particles with each other stay lifted after the impact.

According to the invention, the shear gap between the rotor and the stator should preferably be a clear one Have width that is at least 50 times the average diameter of the Particle size with the larger mean diameter, i.e. the hard particles, equivalent. Preference is given to a shear gap with a clear width that is 100 to Corresponds to 500 times the average diameter of the hard material particles. typically, Accordingly, the shear gap can have a clear width of 0.5 to 5 mm, preferably 1 up to 3 mm.

Preferably, the shear rate in the shear gap should be expressed as the ratio circumferential speed of the rotor and gap width at least 800 / s, are particularly preferably 1000 to 20,000 / s.

The dwell time in the short-range mixing is selected so that the Temperature of the powder mixture as it passes through the close-range mixture does not rise above 300 ° C. In the event that in an oxygen-containing atmosphere, in particular Air that is mixed is preferably around a lower temperature Avoid oxidation of the powder particles safely. In the event that the mixing in Protective gas atmosphere, for example argon, takes place, where appropriate, temperatures permissible up to 500 °. The dwell time is typically in the short-range mixing in seconds.

The total mixing time is preferably 30 to 90 minutes, particularly preferably more than 40 minutes, and more preferably less than 1 hour.

According to a preferred embodiment of the invention, the powder mixture recirculated between short-range and long-range mixing, i.e. subsets of the powder mixture are used as a continuous substream of the long-range mixing removed, added to the short-range mixing and back into the Long-range merging initiated.

The circulation speed of the powder mixture is preferably determined by the short-range mixing chosen so that on average during the total mixing time at least 5 passes, most preferably at least 10 passes each Powder particle is ensured by the short-range mixing.

If the process is carried out continuously, the two powder components can or a raw mixture of the powder components continuously on one Be fed at the end of the circulation mixing unit and at the other end homogeneously mixed powder can be continuously discharged.

An alternative continuous implementation of the method is in one to produce a raw mixture of the powder components in the first circulation mixing unit, the raw mix continuously from the first circulation mixing unit remove, insert into the micro vortex mill, and then one to supply the second circulating mixing unit, it may be expedient afterwards to the second circulation mixing unit a further short-range mixing in a micro vortex mill and then another long range mixing in a circulating mixer.

According to a further preferred embodiment of the invention, the mix fluidized both in the short-range and in the long-range mixing. On A suitable method for this includes, for example, a floor and wall accessible Rotor with shear gap to the container wall, the radial rotor blades against the Are vertical, so that the fluidized regrind in the container peripheral is promoted upwards and centrally promoted downwards. Preferably the angle of attack is less than 25 °, particularly preferably 10 to 20 °. This Circulation of the mix for long-range mixing can be done by a opposed coaxial rotor with only half the cross-section of the container limited diameter can be intensified. It was found that in such an aggregate still achieved excellent carbide mixtures if the container contains up to 7% by volume of mix (weight of the mix divided by the density of the powder material).

For the further processing of the powder mixtures from the Additives used in the carbide industry, such as organic adhesion promoters, oxidation inhibitors, Granule stabilizers and / or pressing aids, e.g. on paraffin or Polyethylene glycol base mixed together with the hard material and binder powder and distributed homogeneously. The pressing aids melt due to the generated heat during the mixing process, so that a uniform surface coverage is effected. If the mixtures produced in this way are not yet sufficient A granulation step can have flowability or pressability downstream.

The hard metal mixtures according to the invention and their granules are for Production of hard metal moldings by axial pressing, isostatic pressing, Suitable for extrusion or injection molding and sintering.

Fig. 1

zeigt schematisch eine erste Ausführungsform der Erfindung

Fig. 2

zeigt schematisch eine zweite Ausführungsform der Erfindung

Fig. 3

zeigt schematisch eine dritte Ausführungsform der Erfindung

Fig. 4

zeigt den prinzipiellen Aufbau einer Mikrowirbelmühle als Schnittbild

Fig. 5

zeigt eine erfindungsgemäß geeignete Mischvorrichtung als Schnittbild

Fig. 6.

zeigt eine weitere erfindungsgemäß geeignete Mischvorrichtung.

Fig. 7

zeigt die REM-Aufnahme des in Beispiel 1 eingesetzten Wolframcarbidpulvers

Fig. 8

zeigt die REM-Aufnahme einer Wolframcarbid-/Kobalt-Pulvermischung

Fig. 9

zeigt die REM-Aufnahme des in Beispiel 2 eingesetzten Wolframcarbid

Fig. 10

zeigt die REM-Aufnahme einer Wolframcarbid-/Kobalt-Pulvermischung gemäß Beispiel 2

Fig. 11

zeigt das Schliffbild eines gemäß Beispiel 2 hergestellten Hartmetalls

Fig. 12, 13 und 14

zeigen entsprechende Aufnahmen mit Bezug auf Beispiel 3.

The invention is explained in more detail with reference to the following figures:

Fig. 1

shows schematically a first embodiment of the invention

Fig. 2

shows schematically a second embodiment of the invention

Fig. 3

shows schematically a third embodiment of the invention

Fig. 4

shows the basic structure of a micro vortex mill as a sectional view

Fig. 5

shows a mixing device suitable according to the invention as a sectional view

Fig. 6.

shows a further mixing device suitable according to the invention.

Fig. 7

shows the SEM image of the tungsten carbide powder used in Example 1

Fig. 8

shows the SEM image of a tungsten carbide / cobalt powder mixture

Fig. 9

shows the SEM image of the tungsten carbide used in Example 2

Fig. 10

shows the SEM image of a tungsten carbide / cobalt powder mixture according to Example 2

Fig. 11

shows the micrograph of a hard metal produced according to Example 2

12, 13 and 14

show corresponding recordings with reference to Example 3.

Fig. 1 shows schematically a long-range mixer A in the two Powder P1 and P2 can be introduced continuously or discontinuously. From the Long-range mixing unit A is constantly a partial flow of the powder mixture in the Short-range mixing unit B transferred and returned to the family zone mixing unit A. The long-range mixing unit A finally becomes the finished one Powder mixture PM taken continuously or discontinuously.

Fig. 2 shows a particularly for the continuous execution of the invention Basic arrangement suitable for the method. The powders P1 and P2 will be into a first long-range mixing unit, in particular, for example, a rotary tube introduced. You get out of the rotary tube into a first micro vortex mill B1 and are then transferred to a second long-range mixing unit A2. Possibly can a further short-range mixing B2 and a not shown Long range mixing A3 can be connected.

Fig. 3 shows an arrangement, in particular for batch batch mixing suitable is. The micro vortex mill B is a short-range mixing element arranged within the zone mixing element A.

Fig. 4 shows the structure of a micro vortex mill 1. This consists of a cylindrical housing 2, the inner wall of which forms the stator. The inside wall of the cylindrical housing 2 can be covered with abrasion-resistant material. Within the cylindrical housing 2, an axis drivable for rotation is provided on the Axis 3 are one or more, in particular 2 to 5, drivable with the axis Circular disks 4.1, 4.2 and 4.3 are provided, each of which has a large number on its circumference have grinding plates 5.1, 5.2 and 5.3 arranged radially and parallel to axis 3. The outer edges of the grinding plates 5.1, 5.2 and 5.3 form together with the Inner wall of the cylindrical housing 2 from the shear gap 6. Will the micro vortex mill arranged within a long-range mixer below the filling level, the micro-vortex mill also preferably has a conical cover 7, which is provided with openings 8 through which the free-flowing powder well into the cylindrical housing 2 trickles. An additional circular disc with axis 3 9 can be provided as a distributor plate.

5 shows a device which can be used according to the invention, as shown schematically in FIG Fig. 3 is shown. This consists of a mixing drum 10, which is connected via the axis 11 for rotation with low rotation speed, for example 1 to 2 revolutions per minute, can be driven. The drum is due to the non-rotating Cover cap 12 closed. The micro-vortex mill is located inside the drum 10 1, as shown in Fig. 4. Within the drum 10 can also Baffles 13 may be arranged. The filling level of the drum 10 is due to the dashed line 14 indicated. The method according to the invention now consists in that the powder mixture continuously through the openings 8 in the micro vortex mill 1 occurs where the short-range mixing takes place, and through the below open cylinder is returned to the long-range mixing.

6 shows a device that can be used according to the invention, in which the mixed material both for short-range mixing and for long-range mixing is fluidized. In the container 10 is located on a drivable axis 3 Floor and wall accessible rotor with 4 rotor blades 5a, 5b, 5c and 5d, which for Container wall form the shear gap 6. The rotor blades are at an angle of α = 23 ° against the plane perpendicular to the rotor axis. Above the rotor 5 is a Opposed rotor 20 provided on the axis 3, its diameter corresponds to approximately half the container diameter.

When the axis 3 rotates in the direction of the arrow 21, the mix is fluidized and in addition to rotation about axis 3 as circulated by arrow 22. A part of the fluidized mix enters the shear gap 6, where the high shear rate of the fluid cause a strong particle acceleration.

The invention is illustrated by the following examples:

example 1

There are 13.6 kg of a cobalt powder with an average grain size of 1.55 microns (FSSS, ASTM B 330) and 122.4 kg of a slightly agglomerated tungsten carbide powder an average grain size of 3 µm (FSSS, ASTM B 330) in principle Filled mixing unit shown in Fig. 5. 7 shows an SEM image of the tungsten carbide powder before mixing.

After 20, 30 and 40 minutes of mixing, samples of the powder mixture are made taken. 8 shows a SEM image of the mixture obtained after 40 minutes of mixing Powder mixture. The oxygen content before mixing is 0.068% by weight, after mixing 0.172% by weight.

The samples are pressed and then sintered at 1380 ° C over 45 Minutes into carbide test specimens.

For comparison, a corresponding powder mixture is used in a ball mill 20 Hours with hexane. The comparison powder mixture becomes the same as a carbide test body made.

The density in g / cm ³ , the magnetic coercive force H _C in kA / m, the magnetic saturation in µTm ³ / kg (each with Foerster Koerzinat 1.096), the Vickers hardness at 30 kg load in kg are measured on the hard metal test specimens / mm ² and the A porosity measured according to ISO 4505. The results are shown in Table 1.

Example 2

11.9 kg of a cobalt metal powder with an average grain size of 1.5 µm and 122.4 kg of a slightly agglomerated tungsten carbide powder with a medium Grain size of 6 microns (FSSS, ASTM B 330) are mixed as in Example 1. The Oxygen content before mixing is 0.058% by weight after 40 minutes Mixing time 0.109% by weight.

Furthermore, a comparison mixture (Example 2f) in a ball mill as in Example 1 prepared.

9 shows an SEM image of the starting tungsten carbide powder. Fig. 10 shows the powder mixture after 30 minutes of mixing.

Hard metal samples are produced as in Example 1. The test values obtained are shown in Table 1.

11 shows the micrograph of a hard metal according to Example 2d).

Example 3

13 kg of a cobalt metal powder with an average grain size of 1.55 µm, 117 kg a less agglomerated tungsten carbide powder (Fig. 12) as in Example 1 mixed. 13 shows an SEM image of the powder mixture obtained. The oxygen content before mixing is 0.065% by weight after mixing 0.088% by weight.

14 shows a micrograph of the hard metal produced as in Example 1. The carbide test results are shown in Table 1. E.g. mixing time
(Min) density
(g / cm ³ ) H _c
(came) 4πσ
(µTm ³ / kg) HV ₃₀
(kg / mm ² ) A porosity
ISO 4505 1a 20 14.47 9.4 18.8 1226 Better A02 1b 30 14.52 9.2 18.1 1274 Better A02 1c 40 14.58 9.4 18.7 1311 Better A02 1d 1200
(See.) 14.52 10.4 18.4 1345 Better A02 2a 10 14.56 6.7 18.8 1198 Better A02 2 B 15 14.56 6.7 18.7 1203 Better A02 2c 20 14.51 6.4 17.8 1190 Better A02 2d 30 14.55 6.5 18.1 1203 Better A02 2e 40 14.59 6.5 18.5 1203 Better A02 2f 1200
(See.) 14.55 7.3 18.0 1261 Better A02 3 40 14.51 6.9 18.6 1203 Better A02

Example 4

2.6 kg cobalt metal powder 1 µm FSSS according to ASTM B 330, 23.26 kg WC 0.6 µm FSSS (according to ASTM B 330), and 0.143 kg Cr ₃ C ₂ 1.6 µm according to ASTM B 330 and 375 g paraffin wax Melting point 54 ° C in a mixer (according to FIG. 6) at 1000 U / min. mixed until a temperature of 80 ° C is reached. the hard metal mixture thus obtained is pressed at 1.5 to / cm ² to test specimens. These are first waxed in a sintering oven and then at 1380 ° C for 45 min. sintered at a pressure of 25 bar. The hard metal obtained has a density of 14.45 g / cm ³ , a coercive force of 20.7 kA / m, a magnetic saturation of 15.14 µT m ³ / kg, a Vickers hardness of HV = 1603 kg / mm ² and a better porosity A02 B00 C00. The hard metal has a good structure and a good binder distribution.

Example 5

2.57 kg cobalt metal powder 1 µm FSSS according to ASTM B 330; 26 kg of WC 6 µm FSSS according to ASTM B 330 are mixed as in Example 4 until a temperature of 80 ° C. has been reached. The hard metal mixture thus obtained is pressed at 1.5 t / cm ² to test specimens and then at 1400 ° C for 45 min. sintered in a vacuum. The hard metal obtained has a density of 14.65 g / cm ³ , a coercive force of 5.5 kA / m, a magnetic saturation of 17.11 µTm ³ / kg, a Vickers hardness of HV ₃₀ = 1181 kg / mm ³ and one Residual porosity of A00 B00 C00. The hard metal has a good structure and a good binder distribution.

Claims (10)

1. Process for producing a homogenous mixture of a mix comprising hard material power and binder metal powder without use of milling media and liquid milling auxiliaries and suspension media, characterized in that a partial amount of the mix is mixed within itself with generation of high shearing impingement velocity of the powder particles in a vessel equipped with rotor and stator elements and having a shear gap between the elements and the main amount of the mix is mixed by circulation of the mix.
2. Process according to Claim 1, characterized in that the partial amount of the mix is fluidized in itself during mixing and the high impingement velocity is generated by swirling of the fluid.
3. Process according to Claim 1, characterized in that the shear gap has a width which corresponds to at least 50 times the mean diameter of the type of particles having the greatest mean diameter.
4. Process according to Claim 1, characterized in that the ratio of relative velocity of rotor and stator to shear gap width is at least 800/s.
5. Process according to any of Claims 1 to 4, characterized in that the rotor has a circumferential velocity of from 12 to 20 m/s.
6. Process according to any of Claims 1 to 5, characterized in that the mixing of the main amount of the mix is carried out in a stirred vessel having slowly rotating stirring elements.
7. Process according to any of Claims 1 to 6, characterized in that the mix is fluidized both during mixing of the partial amount in itself and during mixing of the main amount of the mix.
8. Process according to any of Claims 1 to 7, characterized in that the total mixing time is less than 1 hour.
9. Process according to any of Claims 1 to 8, characterized in that the mix additionally contains pressing aids.
10. Process according to any of Claims 1 to 9, characterized in that the powder mixture is granulated.

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