DK2082072T3 - EXTRAORDINARY METAL POWDER, PROCEDURE FOR GETTING THEREOF AND CREATED CUTTING TOOLS - Google Patents

Description

PREALLOYED METAL POWDER, PROCESS FOR OBTAINING IT, AND CUTTING TOOLS PRODUCED WITH IT

Description

The invention relates to the field of prealloyed metal powders, from which diamond cutting tools are made such as segments for saws and beads for the production of wires for cutting hard materials such as granite.

The metal powders used to produce diamond beads are usually made from granules containing approximately 20% of tungsten carbide and approximately 80% of cobalt. These granules are mixed with diamonds and compressed into ring shapes, and the green parts are sintered in one of two ways.

In a first case, graphite molds are filled with the green parts each equipped with a steel cover, then the sintering is done under pressure in classic hot presses. But owing to the particular shape of the diamond beads: - the graphite molds have a complex shape and are expensive, even more so because they have to be replaced periodically; - since filling the molds with the green parts and the rings is difficult, it must be done manually, which causes high manpower costs; - to produce homogeneous sintered diamond beads, the number of beads sintered in each mold is limited to a few tens of parts, which implies low productivity.

In a second case, natural sintering, also called “free sintering”, is done (without a mold) of the green parts with their steel covers, in a static or continuous furnace. But after this sintering, the beads based on cobalt and tungsten carbide are not sufficiently densified. A second heat treatment is indispensable; it must be done in a furnace operating at a high pressure of between 150 and 200 MPa (between 1,500 and 2,000 bars), to make a hot isostatic compression of the beads. This furnace is expensive to buy and maintain.

These processes are therefore very expensive anyway, both in terms of raw materials and of production process. The article AMARAL P M ET AL: “Evaluation of metallic binder systems used in diamond tools for stone cutting” POWDER METALLURGY WORLD CONGRESS & EXHIBITION (PM2004) EUROPEAN POWDER METALLURGY ASSOC SHREWSBURY, UK, 2004, discloses metal powders for the production of cutting tools comprising a mixture of a prealloyed iron and cobalt powder with a copper powder. The objective of the invention is above all to provide prealloyed metal powders whose cost would be relatively affordable, and that would be compatible with production processes of diamond beads noticeably less expensive than the existing processes, particularly because natural sintering, conducted without a mold, would nevertheless produce sufficiently well performing products, particularly for cutting granite. Also, these powders ought to be compatible with the production of other types of cutting tools intended for less demanding applications.

For this purpose, the invention relates to a prealloyed metal powder, in particular for the production of cutting tools by sintering, characterized in that its composition in percentage by weight is: *Fe = 48 - 52% *Co = 14- 19% *Cu = 32 - 37% *0 < 1.2% the remainder being impurities resulting from its production.

Preferably, the Fisher diameter of its particles is from 1 to 3 pm.

It is preferably constituted by a mixture of such a powder and at least one sintering aid additive in a ratio of 80 to 90% by weight of powder and 10 to 20% by weight of additive.

The sintering aid additive is preferably an iron, nickel, copper or cobalt phosphide, or a mixture of at least two of these phosphides, or a mixed phosphide of at least two of these metals.

The powder is preferably obtained by mixing a first powder and a second powder, and an optional sintering additive, where said first and second powders have the respective characteristics: - for the first powder *Fe = 27 - 32% *Co = 24 - 28% *Cu = 42 - 47% *0 < 1 % the remainder being impurities resulting from its production. - for the second powder *Fe = 75 - 80% *Co < 5% *Cu = 17 - 22% *0 < 1 % the remainder being impurities resulting from its production.

Preferably, the Fisher diameter of the particles of the first powder is from 0.8 to 1.5 pm, the Fisher diameter of the particles of the second powder is from 3.0 to 4.0 pm, and the Fisher diameter of the powder obtained after mixing is from 1 to 3 pm.

The invention also relates to a process for the production of a diamond cutting tool, comprising a step in which a prealloyed metal powder and diamonds are mixed, a step in which the mixture is cold-pressed, and a step in which said compressed mixture is sintered, characterized in that said metal powder is of the preceding type.

The sintering is preferably natural sintering.

Said tool can be a cutting segment for a diamond saw.

Said tool may be a diamond bead for a cutting wire.

Said powder can be of the previously cited type.

The invention also relates to a diamond saw of the type including cutting segments attached to the periphery of a metal disc, characterized in that said segments were obtained by the previous process.

The invention also relates to a cutting wire of the type including diamond beads threaded onto a cable, characterized in that said beads have been obtained by the preceding process.

As will be understood, the invention relies on the use of a prealloyed powder of precise composition, based on iron, cobalt and copper. This powder, which does not use very expensive elements in high proportions, produces very impressive diamond cutting tools (saws and beads) by simple natural sintering, therefore by a cheap process that can be executed with high productivity. A process of obtaining the powder, producing, from said powder, sintered products with particularly high characteristics, is also proposed.

The invention will be better understood upon reading the description that follows.

The prealloyed powder according to the invention must in particular meet the following imperatives.

The relative density of the green parts obtained with it must be at least 60% for a maximum cold pressure of 700 MPa.

It must preferably be easily granulable into the particle size fraction inclusively between 63 and 450 pm, which is the best suited for filling steel cold-compression molds, intended for the production of beads for diamond wires.

After free sintering at 850-1100°C in a continuous furnace (for continuous production) or in a static furnace (for batch production), the relative density of the obtained part must be able to be preferably at least 97%.

The powder must be able to be used to produce parts whose hardness after sintering would be at least 220 HB, so that they can be used to cut granite.

It has appeared that these objectives, and those previously described, are reached, according to the invention, using a prealloyed powder having the following characteristics.

Its composition is (in weight percentages): - Fe = 48 - 52% - Co = 14-19% - Cu = 32 - 37% - 0<1.2% the remainder being impurities resulting from its production.

The average Fisher diameter of the particles (measured according to Standard 150 10070 by determination of the specific surface area of the envelope from the measurement of the air permeability of a bed of powder in permanent flow conditions) is preferably from 1 to 3 pm.

Its typical theoretical density is preferably 8,400 kg/m3 (8.4 g/cm3)

The ratio between the iron and cobalt contents is deliberately adjusted so as to prevent the formation of a hard and fragilizing a’ phase, which forms when the Fe/(Fe + Co) mass ratio is inclusively between 30 and 70%. According to the invention, this ratio is inclusively between 72 and 78%, and the a’ phase is therefore avoided.

The quantity of copper added is what is sufficient to procure good sintering.

The oxygen content is maintained at a maximum of 1.2% to avoid the presence of oxides that would not be completely reduced by hydrogen during natural sintering. Such non-reduced oxides would reduce the sinterability of the green parts, would cause heterogeneities in the structures of the sintered parts, would increase the hardness, therefore the fragility of the parts, and would react with the diamonds by the destroying them at least on the surface. Accordingly, the cutting performance of the tools would be reduced.

This powder may be obtained in particular in two different ways.

According to a first method, a powder having the targeted composition and morphology characteristics is prepared directly by the classic hydro metallurgical route.

This hydro metal I urgic route consists of firstly making metal hydroxides by precipitation with sodium hydroxide of a mixture of metal chlorides via the reaction:

where x + y + z = 1, and t is the stoichiometric excess NaOH content, x, y and z are in ratios corresponding to atomic ratios that we want to see in the final powder between the respective Cu, Fe and Co contents.

Solid-liquid separation is then conducted, followed by washing the hydroxide cake with demineralized water to remove the NaCI. Then the cake is passed into a drier to produce a powder of co-precipitated hydroxides with a residual water content of a few %.

Then the hydroxide powder is reduced, to be transformed into prealloyed metal powder. This reduction is preferably conducted in a continuous furnace and under H2 according to:

After reduction, the prealloyed powder is ground under inert gas in a grinder, then sieved at 90 pm.

According to a second method, the powder of the invention is made by mixing two powders of different compositions, also obtained separately by hydro metallurgy. Table 1 shows the compositions of the two powders to be used:

Table 1: Characteristics of Powders I and II used.

In a surprising manner, as will be seen, during sintering better results are obtained from several points of view when the powder of the invention is made by mixing these two powders I and II, in proportions such that globally a powder having the specified characteristics is obtained, than when a powder obtained directly by a hydro metal I urgic process according to the first production mode described is used.

Generally, a mixture of powders I and II in relative proportions of approximately 60 - 40% by weight produces the powder of the invention.

After obtaining the powder of the invention, it can be used directly, or shaped in the form of granules by a classic process that will be described shortly. These granules can then serve to produce specific diamond tools, such as diamond wires and diamond segments with low thickness.

The prealloyed powder to be granulated is mixed with an organic binding powder at 2 to 3% by weight of the quantity of powder to be granulated and with an organic solvent, in a high shear granulator. After the granulation step, the solvent is removed by evaporation.

Finally, the granules are sieved continuously over vibrating sieves including two superimposed cloths, with different mesh openings (450 pm for the first, 63 pm for the second for example). In this way, the fraction of diameter inclusively between 63 pm and 450 pm is selected. The smallest and largest granules are recycled into the next granulation operation.

It is also advised to add, to the powder, one or more additives that increase the hardness of the sintered parts. Classic additives known for this purpose, such as tungsten carbide, did not work in the scope of the invention, because they reduced the densification during sintering, therefore the hardness of the parts, the opposite of the desired result. Tungsten carbide is insoluble in the powder of the invention and therefore does not bind metallurgical^ to the metal matrix. However, iron phosphide produces remarkable results from this point of view; nickel, copper and cobalt phosphides are also interesting.

Natural sintering tests were conducted on the powders of the invention, which demonstrated the superiority of the powders obtained by a mixture of powders I and II described above on the powders obtained directly by a single hydrometallurgical treatment.

The powder obtained directly (“direct powder”) was prepared by the hydrometallurgical process described previously, i.e. by addition of NaOH to a mixture of Co and Fe chlorides, drying the resulting hydroxide in a microniser drier, reduction at 660 °C and grinding in a nitrogen-jet grinder. Its composition was Fe = 48.8%; Co = 16.0%; Cu = 34.4%; O = 0.8%. Its Fisher diameter was 1.3 pm.

The powder obtained by mixing (“mixture powder”) was mixed in a mixer first put under CO2, from 60% of powder I and 40% of powder II, where these powders have previously been prepared separately by hydrometallurgy. The mixing operations lasted 50 minutes. The resulting powder had the composition:

Fe = 49.1 %; Co = 16.0%; Cu = 34.4%; O = 0.6%. Its Fisher diameter was 1.74 pm.

The “direct” and “mixture” powders were then compressed at 200 MPa, to make parts of type PS 21, whose green density was calculated from their scores and weights. The direct powder had a density of 58.0% of the theoretical density; the mixture powder had density of 55.2% of its theoretical density.

As a reminder, conventionally, parts PS21 are parallelepipeds parts obtained by cold-pressing at 200 MPa of 6 g of powder in a steel matrix with dimensions 24.48 x 7.97 mm. The height of the obtained green part depends on the compressibility of the powder, and is generally of the order of 5 to 6 mm.

Then sintering took place in a static laboratory furnace under H2 at temperatures ranging from approximately 850 to 1000°C. In all cases, the rate of temperature rise was 150°C/h, the plateau at the sintering temperature was 1 h and the cooling was natural, lasting approximately one night. The sintered parts were measured for density as a % of the theoretical value (8,350 kg/m3 (8.35 g/cm3)), FIB hardness and FIRB hardness. The results are summarized in Table 2.

Table 2: Results of sintering of direct powders and mixtures.

The test results show that the direct powder has better cold compressibility than the mixture powder. It will therefore be the easier of the two to shape before sintering.

However, the mixture powder has better densification upon sintering and better hardness after sintering.

Similar tests were conducted on direct and mixture powders to which, before sintering, iron phosphide was added at 10% by mass of P provided by BASF. The mixing took place in a Gericke mixer under CO2 for 50 min, at 85% of powder and 15% of FeP (% by mass).

Cold-pressing tests were conducted under the same conditions as previously. It was found that the direct powder with added FeP had a density of 59.1% of the theoretical density; the mixture powder had density of 53.1% of its theoretical density.

Next, these powders were sintered, in the same conditions as previously, and the densities and HB and HRB hardnesses of the parts obtained were measured. The results are summarized in Table 3.

Table 3: results of sintering of direct powders and mixtures with added

FeP.

The performance ranking of the direct and mixture powders with added FeP is the same as for the pure powders (without additives). The mixture powder has the best results after sintering.

Additivation produces sintered parts with appreciably higher hardness than those of the parts obtained under the same conditions from powders without additives, as can be seen by comparing the results of Tables 2 and 3.

As an indication, a powder of the invention to which FeP would be added at 85% powder and 15% FeP would have approximately the following characteristics: - typical theoretical density 8,210 kg/m3 (8.21 g/cm3) - Fe = 54 - 58% - Co = 12-16% - Cu = 27 - 31% - P = 1 -2% - OS 1.5% - Fisher ø = 2-5 pm.

Sintering tests were also conducted on a mixture powder with added Ni phosphide containing 8.8% by mass of P, at 85% of mixture powder and 15% of NiP. The results are summarized in Table 4.

Table 4: Results of sintering of the powder mixture with added NiP.

Additivation with NiP in the conditions described therefore cause remarkable results in terms of density and hardness of sintered parts.

As an indication, a powder of the invention to which NiP would be added at 85% powder and 15% NiP would have approximately the following characteristics: - typical theoretical density 8,370 kg/m3 (8.37 g/cm3) - Fe = 40 - 44% - Co = 11 -17% - Cu = 27 - 31% - Ni = 13- 15% - P = 1 -2% - 0<1.5% - Fisher ø 1 - 4 pm.

Additivation may also be conducted using copper or cobalt phosphide. Also, a mixture of at least two of iron, nickel, copper and cobalt phosphides, or a mixed phosphide of at least two of these metals.

Granite cutting tests conducted with parts made using the powders of the invention and a reference powder have given the following results.

Granite cutting tests were conducted with diamond saws with diameter 500 mm whose cutting segments were made by natural sintering, using to produce the segments: - a reference powder known in the prior art (Cobalite® CNF) with composition (mass percentages):

Co = 0%; Cu = 26%; Fe = 68.4%; Ni = 0%; Sn = 3%; W = 2%; Y203 = 0.6% - the powder of the invention with added FeP (85% -15%) as previously described.

Both powders were used to produce diamond segments forming saw teeth. These segments were of the “segments sandwich” type, i.e. they had a higher diamond concentration around their periphery (1.1 carat/cm3 of segment) than in their center (0.8 carat/cm3 of segment). Standard diamonds and titanium-coated diamonds were used. This type of segment was chosen because they are particularly complex and expensive to make by the classic hot-pressing process in graphite molds.

The segments made with the reference powder and with the powder of the invention were sintered by natural sintering in a continuous furnace at 940 °C for the powder of the invention and 980 °C for the reference powder, then brazed on steel discs 500 mm in diameter to make the saws. Different categories of granites were then cut with the saws. For each type of powder, three types of diamond mixtures were tested, made from diamonds from ELEMENT SIX, whose references will be indicated.

After each cutting test, the cutting rate (in cm2 of granite cut per minute) and the saw lifetime (in m2 of granite cut per mm of segment height) were calculated. The higher these values, the better the quality of the saw.

Using a mixture of SDB VB 0.400 to 0.297 mm (40 to 50 mesh) (30%) and SDB LBW 0.297 t- 0.250 mm (50 to 60 mesh) (70%) diamonds, the results were as follows:

The reference saw had a lifetime of 4.4 m2/mm and a cutting rate of 520 cm2/min).

The saw of the invention had a lifetime of 4.8 m2/mm and a cutting rate of 620 cm2/min.

Using a mixture of SDB VB 0.595 to 0.400 mm (30 to 40 mesh) (10%) and SDB LBW 0.400 to 0.297 mm (40 to 50 mesh) (40%) diamonds, and SDB LBW 0.297 to 0.250 mm (50 to 60 mesh) (50%) the results were as follows:

The reference saw was incapable of cutting the granite.

The saw of the invention had a lifetime of 3 m2/mm and a cutting rate of 620 cm2/min.

Using a mixture of SDB VB 0.595 to 0.400 mm (30 to 40 mesh) (10%) and SDB TMF 0.400 to 0.297 mm (40 to 50 mesh) (40%) and SDB TMF 0.297 to 0.250 mm (50 to 60 mesh) (50%) diamonds, the results were as follows (TMF diamonds are coated with titanium).

The reference saw had a lifetime of 4.1 m2/mm and a cutting rate of 600 cm2/min.

The saw of the invention had a lifetime of 6.7 m2/mm and a cutting rate of 900 cm2/min.

The test results of the saws of the invention are therefore excellent in absolute terms, and systematically better from all points of view than those of the reference saws. The production process for segments of saws of the invention, coupling a natural sintering of the segments with a precise composition of the prealloyed powder used, therefore gives satisfactory results for a very moderate price relative to the known processes using molds.

It was also checked that the powder of the invention could be used for the production of diamond beads usable to produce cutting wires for cutting granite, which are the preferred application envisaged for the invention.

These beads had exterior diameters of 7.2 mm (beads intended for multi-wire machines) and 11 mm (beads intended for one-wire machines) and were produced by the following process: - production of granules by the previously described process, from a powder of the invention with added FeP (85% /15%); - mixture of granules with standard or titanium-coated diamonds according to the test; - cold-pressing of the granule/diamond mixture, causing a density of green parts of approximately 65% of the theoretical density; - elimination of the granulation binder at 590 °C; - sintering at 900 °C; - brazing at 900 °C using a brazing containing 72% Ag and 28% Cu to ensure sufficient grip on the steel cover that serves as support.

The debinding, sintering and brazing operations were conducted in a continuous furnace under H2.

The beads obtained were threaded on steel cables at 37 beads/linear meter, then the whole was plasticized to strengthen it.

The wires were tested on different machines for cutting varied granites. The test results are summarized in Table 5.

Table 5: results of tests conducted on cutting wires (beads made from powder according to the invention with added FeP).

These results are absolutely satisfactory, and show that the invention allows the production of impressive diamond beads, for a cost appreciably lower than by the classic processes. As a comparison, the lifetime of usual wires, using titanium-coated diamonds, is of the order of 28 m2/linear m.

Generally, the powder of the invention, used pure, has good cold compressibility and densities very well from 900 °C (97% of its theoretical density), particularly when it is obtained by mixing powders I and II as previously defined. The hardness obtained after sintering can be considered as insufficient for cutting granite, but would be sufficient for cutting marble. But the addition of 15% of iron or nickel phosphide increases the densification and hardness of the sintered parts in a manner that makes it perfectly suited to cutting granite.

As a comparison, to show that the invention requires the use of a prealloyed powder or a mixture of such powders to produce the desired results, the following tests were conducted. A mixture called “Mixture 1” was prepared under CO2 for 50 min, from commercial Fe, Co and Cu powders, as indicated in Table 6:

Table 6: Characteristics of Mixture 1

The weight percentages of the metals are expressed without oxygen content.

This composition is in the middle of the range of the prealloyed powder of the invention.

To 85% by mass of this mixture, was added (using the same protocol as previously) 15% of iron phosphide FeP 10% of BASF, of the same quality as that of the tests conducted previously. The composition of this Mixture 2 is then (Table 7):

Table 7: Composition of Mixture 2

The weight percentages of the metals and phosphorus are expressed without oxygen content.

For both mixtures, parts of type PS21 were compressed under 200 MPa.

The average green density of the parts, calculated from the scores and the weight, was (Table 8):

Table 8: Percentages of theoretical densities of green parts

These green parts are then sintered at 850, 900, 950 and 1000°C. These parts were measured for density as a % of the theoretical density, HB hardness, and HRB hardness, according to the protocol described previously for the parts made according to the invention.

After sintering, the following results were obtained (Tables 9 and 10)

Table 9: Results of sintering Mixture 1.

Table 10: Results of sintering Mixture 2.

All of the results show that these two mixtures of commercial metal powders, in comparison with the prealloyed powders of the invention of comparable compositions, have: - very similar particle size - better cold compressibility - clearly poorer densification after sintering and lower HB and HRB hardnesses - a structure of clearly much larger sintered parts because of the high initial particle size of the constituent parts.

Under these conditions: - the diamond tools made from these commercial mixtures of metal powders will have a lower diamond retention (i.e. lower diamond grip in the metal matrix), among other things because of their high porosity; - this will cause clearly worse tool performance (cutting rate and lifetime) than the prealloyed powder (pure or with added iron phosphide) of the invention, with comparable composition and particle size.

The powder according to the invention, particularly in its additive version, is easily granulable, which allows low thickness segments and diamond wires to be made by inexpensive processes. It is easy to sinter in the presence of diamonds, whether in a static furnace or a continuous furnace, both in powder state and in granule state. It therefore solves the posed problems well.

Of course, the powder of the invention could also be used profitably to produce cutting tools by processes different than those described.

PATENTKRAV 1. Forlegeret metalpulver, især til fremstilling af skæreværktøjer ved sintring, hvilket metalpulver er kendetegnet ved, at sammensætningen i vægtprocent er: * Fe = fra 48 til 52 % * Co = fra 14 til 19% * Cu = fra 32 til 37 % * O < 1,2 %, hvor resten er urenheder som følge af fremstillingen deraf. 2. Forlegeret metalpulver ifølge krav 1, der er kendetegnet ved, at Fisher-diameteren på partiklerne er fra 1 til 3 pm. 3. Forlegeret metalpulver, der er kendetegnet ved, at det udgøres af en blanding af et pulver ifølge krav 1 eller 2 og mindst ét sintringshjælpetilsætningsstof i et forhold på fra 80 til 90 vægtprocent pulver og fra 10 til 20 vægtprocent tilsætningsstof. 4. Forlegeret metalpulver ifølge krav 3, der er kendetegnet ved, at sintringshjælpetilsætningsstoffet er et jern-, nikkel-, kobber- eller cobaltphosphid eller en blanding af mindst to af disse phosphider eller et blandet phosphid af mindst to af disse metaller. 5. Forlegeret metalpulver ifølge et hvilket som helst af kravene 1 til 4, der er kendetegnet ved, at det opnås ved at blande et første pulver og et andet pulver og valgfrit et sintringshjælpetilsætningsstof, idet det første og andet pulver har de respektive egenskaber: - for det første pulver * Fe = fra 27 til 32 % * Co = fra 24 til 28 % * Cu = fra 42 til 47 % * O < 1 %, hvor resten er urenheder som følge af fremstillingen deraf, for det andet pulver * Fe = fra 75 til 80 % * Co < 5 % * Cu = fra 17 til 22% * O < 1 %, hvor resten er urenheder som følge af fremstillingen deraf. 6. Forlegeret metalpulver ifølge krav 5, der er kendetegnet ved, at Fisher-diameteren på partiklerne i det første pulver er fra 0,8 til 1,5 pm, at Fisher-diameteren på partiklerne i det andet pulver er fra 3,0 til 4,0 pm, og at Fisher-diameteren på det pulver, der opnås efter blanding, er fra 1 til 3 pm. 7. Fremgangsmåde til fremstilling af et diamantskæreværktøj, idet fremgangsmåden omfatter et trin, hvor et forlegeret metalpulver og diamanter blandes, et trin, hvor blandingen koldpresses, og et trin, hvor den komprimerede blanding sintres, hvilken fremgangsmåde er kendetegnet ved, at metalpulveret er af typen ifølge et hvilket som helst af kravene 1 til 6. 8. Fremgangsmåde ifølge krav 7, der er kendetegnet ved, at sintringen er naturlig sintring. 9. Fremgangsmåde ifølge krav 7 eller 8, der er kendetegnet ved, at værktøjet er et skæresegment til en diamantsav. 10. Fremgangsmåde ifølge krav 7 eller 8, der er kendetegnet ved, at værktøjet er en diamantperle til en skæretråd.

Claims (3)

Process according to any one of claims 7 to 10, characterized in that the powder is of the type according to claim 5 or 6. A diamond saw of the type comprising cutting segments attached to the periphery of a metal disk, said diamond saw characterized in that the segments have been obtained by the method of claim 9. A cutting thread of the type comprising diamond beads threaded on a cable, characterized in that the beads have been obtained by the method of claim 10.