HK1162198A - Metal powder containing molybdenum for producing hard metals based on tungstene carbide - Google Patents

Description

Molybdenum-containing metal powder for producing hard metals based on tungsten carbide

Technical Field

The invention relates to the use of a molybdenum-containing binder alloy powder for producing sintered hard metals based on tungsten carbide. Hard metals are composite materials sintered from a hardness imparting material (e.g., carbide) and a continuous binder alloy. Sintered hard metals are very widely used and are used for processing virtually all known materials such as wood, metal, stone and composite materials such as glass-epoxy, cardboard, concrete or asphalt-concrete. Here, local temperatures up to over 1000 ℃ occur as a result of the cutting, deformation and friction processes. In other cases, the deformation process of the metal workpiece is carried out at high temperatures, as is the case, for example, in forging, drawing or rolling processes. In all cases, hard metal tools are subject to oxidation, corrosion and diffusion and adhesive wear and at the same time are under high mechanical stress, which leads to deformation of the hard metal tool. The term "adhesive wear" refers to any phenomenon that occurs under the following circumstances: when two objects are in contact with each other and a weld and a firm connection is formed at least temporarily, it is released again by means of an external force, wherein the material of one object adheres to the other object. The term "diffusion wear" refers to any phenomenon that occurs in the following cases: the two materials are in contact with each other and the composition diffuses from one material to the other, such that pits are formed in the first material.

Background

WO 2007/057533(Eurotungstene Poudres) describes FeCoCu-based alloy powders and which contain 15-35% Cu and 1.9-8.5% Mo for the production of diamond tools. The FSSS value is typically 3 μm. These powders are not suitable for use in the field of hard metals, due to the high FSSS value (measured by the Fisher granulometry or according to ISO standard 10070), and to Cu contents exceeding 500 ppm. Molybdenum is added to the oxide as a water-soluble ammonium salt, the latter being subsequently reduced to metal powder by means of hydrogen.

EP 1492897B 1(Umicore) describes alloy powders based on FeCoNiMoWCuSn for the production of diamond tools, where the sum of the Cu and Sn contents is 5-45%. However, both elements are detrimental to the hard metal because Cu "bleeds" during sintering and Sn causes pore formation. These alloy powders are therefore not suitable for producing hard metals.

EP 0865511B 9 (umcore) describes alloy powders which are based on FeCoNi and have an FSSS value of not more than 8 μm and may also contain up to 15% Mo, although this is at least partly present as an oxide. Furthermore, these powders contain 10-80% Fe, up to 40% Co and up to 60% Ni and are used for the production of diamond tools. In addition, powders are described which are similar but which contain up to 30% each of Co and up to 30% of Ni.

The alloy powders described in WO 98/49361(Umicore), EP 1042523B 1(Eurotungstene Poudres) and KR 062925 are also unsuitable because of the copper content.

EP 1043411B 1 describes carbide-Co- (W, Mo) composite powders in which the binder alloy is produced by pyrolysis of organic precursor compounds. The formation of an alloy of cobalt with Mo and/or W avoids the occurrence of porosity, as occurs by the addition of metals. However, the described method has the disadvantage over the use of the alloy powder according to the invention that the carbon content of the composite powder changes during the pyrolysis of the organic precursor compounds (carbon deposits or is removed by methane formation) and therefore the carbon content has to be re-analyzed and adjusted before sintering. The existence form of Mo or W after sintering is also unclear, because neither comparative test or indication of the state of Mo and W alloy before sintering nor magnetic saturation value is given. The method results in a fixed formulation of carbide and binder alloy phase content and composition and is therefore very inflexible in practice, since simple and rapid changes in the formulation depending on the use of the hard metal to be produced are difficult to use.

Also known are alloy powders based on FeCoMo, FSSS values<8 mu m and the specific surface area of more than 0.5m²(DE 102006057004A 1) and they are used for producing carbon-free high-speed steels via powder metallurgical processes. They may optionally contain up to 10% or 25% Ni, but it is particularly advantageous not to contain any nickel beyond the unavoidable contamination level. They are preferably composed of 20-90% Fe, up to 65% Co and 3-60% Mo. Since pure FeCo alloys without additional Ni alloying are not suitable for hard metals due to their brittleness and poor corrosion and oxidation resistance, these alloy powders clearly do not provide a solution to the problem. In addition, preferred ranges, i.e., high Mo content and for producing liquid-sintered carbon-containing hard metals (which have hard material phases such as carbides as hardness imparting agents) are not described.

Objects of the invention

It is known that metallic cobalt presents a health hazard when used as the sole binder metal, particularly for tungsten carbide. It is therefore an object of the present invention to find a further alloying element and to use it for producing sintered hard metal materials which allows the use of FeNi and FeCoNi binders instead of Co at high operating temperatures of 400 ℃. + 800 ℃ without the disadvantages such as binder precipitations (bindersen), lack of explanation for magnetic saturation or the relative proportion of elements in the binder phase is unknown and the elements involved lead to an increase in hot hardness in the temperature range of 400 ℃. + 800 ℃. On the other hand, the contents of the elements involved should be as low as possible, and for efficiency reasons, the best possible distribution is likewise possible.

This object is achieved by the use of a molybdenum-containing binder alloy powder for the production of cemented hard metals based on tungsten carbide, characterized in that:

a) the FSSS value of the binder alloy powder used is 0.5 to 3 [ mu ] m, measured according to ASTM B330, and

b) the binder alloy powder used comprises iron in an amount of 0.1-65% by weight, cobalt in an amount of 0.1-99.9% by weight and nickel in an amount of 0.1-99.9% by weight, and

c) the binder alloy powder used contains 0.1-10 wt.% Mo in the form of an alloy or a pre-alloy.

The molybdenum is preferably present entirely in metallic form. The binder alloy powder used comprises at least 10 wt% nickel, based on the total binder alloy.

The binder alloy powder used comprises up to 20 wt.%, in particular up to 10 wt.%, of tungsten, based on the total binder alloy.

At least one component of the binder alloy is present as a powdered alloy of at least one metal and molybdenum, the remaining components of the binder alloy being present as elements or alloys (which do not contain any molybdenum), i.e. an alloy is used which is made from a powder mixture of molybdenum-containing alloy powder of at least one alloy or pre-alloy and alloy powder or elemental powder of at least one alloy or pre-alloy, and the powder of the latter contains molybdenum only in the unavoidable impurity range.

The molybdenum-containing binder alloy powder of the invention is used for producing sintered hard metals, and the sintering is carried out in the form of liquid phase sintering.

The molybdenum-containing binder alloy powder of the present invention may contain up to 30 wt% of organic additives.

Drawings

Fig. 1 shows the thermal hardness profile of example 1 with a FeCoNi binder (triangular symbols, solid lines for the "low carbon" variant and dashed lines for the "high carbon" variant) compared to the thermal hardness (square symbols with pointed ends down) of the hard metal of example 2 with a cobalt binder.

Fig. 2 shows the hot hardness profile of the hard metal of example 3(FeCoNi binder, Mo used as elemental powder, round symbol, 1% Mo = dotted line, 3% Mo = solid line) compared to example 4 (FeCoNi binder alloyed with Mo, laid down square symbol) and example 2 (cobalt as binder, square symbol pointed downwards).

Detailed Description

This object is achieved by using an iron-, cobalt-or nickel-containing binder metal powder comprising iron in an amount of 0.1-65% by weight, cobalt in an amount of 0.1-99.9% by weight and nickel in an amount of 0.1-99.9% by weight.

The binder alloy powder used additionally contains 0.1 to 10% by weight of molybdenum in the form of an alloy, based on the total binder metal powder. The binder alloy powder used preferably comprises from 0.10% to 3% by weight of molybdenum, particularly preferably from 0.5% to 2% by weight of molybdenum, very particularly preferably from 0.5% to 1.7% by weight of molybdenum, based in each case on the total binder metal powder.

The FSSS value of the binder alloy powder used is 0.5 to 3 μm, preferably 0.8 to 2 μm, in particular 1 to 2 μm, measured using a "Fisher Sub Siever Sizer" apparatus according to the standard ASTM B330.

The elements Mn and Cr are preferably each present in an amount of less than 1%. The binder alloy powder used preferably comprises molybdenum either completely in non-oxidized form or completely in alloyed metallic form.

The binder alloy powder used preferably comprises at least 20 wt% nickel, based on the total binder alloy. The binder alloy powder used preferably comprises up to 20 wt% of tungsten, more preferably up to 10 wt% of tungsten, based on the total binder alloy. In particular, the preferred alloy powders are substantially free of tungsten and have a tungsten content of less than 1% by weight.

Among the binder alloy powders used, preference is given to introducing at least one component of the binder alloy as a pulverulent alloy of at least one metal with molybdenum, and the respective remaining components of the binder alloy as elements or alloys which do not contain any molybdenum.

According to the invention, the sintering of the binder alloy powder together with the hard material is carried out as liquid phase sintering. This means that the appearance and disappearance of the liquid metal phase is due only to the temperature change used and that the hard material crystallizes upon dissolution in the binder alloy (uml fosten) and thus undergoes a grain size increase (Ostwald ripening). This is in contrast to solid state sintering, in which no melt is formed, nor is any melt temporarily formed due to momentary, local changes in composition, but in which hard materials that may be present, such as diamond, undergo a grain size increase without crystallization after dissolution.

Description of the invention

The hard metals produced by the process of the present invention need to have sufficient stability in terms of plastic deformation and temperature dependent creep behaviour to enable their use in their intended applications. Creep, e.g. plastic deformation, of a material is a major material failure mechanism and must be avoided anyway. The deformation mechanism is a time law of creep that undergoes a known load dependence, and the creep rate depends not only on the load, but also to a large extent on temperature. In addition, the creep mechanism varies in each case predominantly as a function of temperature. In the case of hard metals, it is known that the creep rate at temperatures up to about 800 ℃ is measured mainly by deformation of the metal binder phase, whereas above about 800 ℃ the binder phase is so soft that it has virtually no appreciable creep resistance, i.e. the load-bearing strength of the hard material phase is the determining factor at temperatures above 800 ℃. This load-bearing capacity in turn depends on the grain shape and grain size distribution of the hard material phase and on the proportion of heat-resistant cubic carbides. For this reason, all hard metal materials used for cutting steel contain not only WC but also cubic carbides such as TiC, TaC, NbC, VC, ZrC or mixed carbides such as TaNbC, WTiC or WVC in a certain proportion.

Since the experimental determination of the temperature dependence of the creep behavior at high temperatures is very complicated, hot hardness determination is used instead. The hardness of a material is an indirect measure of its ability to plastically deform. The central idea is that the plastic deformation process is dominant in the formation of the hardness indentation, and therefore the size of the hardness indentation at sufficiently high loads and load durations is a measure of the ability of the material to plastically deform under a given compressive load.

During sintering, hard metals based on WC and having Co, tungsten, carbon as a binder alloy and small amounts of metals (which form cubic carbides, such as V, Ta, Ti and Nb) dissolve into the binder phase during liquid phase sintering. This also applies to Cr if the Cr carbide acts as a so-called "grain growth inhibitor" (i.e. acts as an agent inhibiting grain growth) for inhibiting the growth of WC microstructures occurring during sintering.

The term "liquid phase sintering" refers to sintering at such high temperatures: so that the binder alloy is at least partially melted. The liquid phase during sintering of the hard metal is a result of the sintering temperature, which is typically 1100 ℃ to 1550 ℃. The melt mobile phase (essentially the binder metal used, such as cobalt or one or more binder metal alloys) is in equilibrium with the hard material, consistent with the solubility product principle used. This means that more tungsten is present in the melt and less carbon is dissolved in the melt and vice versa. The tungsten content of the binder alloy is determined by the total W: c ratio, wherein W: c =1, so there is a W in the adhesive metal melt that is not equal to 1: different concentrations of C ratio. When the ratio of tungsten in the melt: carbon-lean carbides, e.g. Co, at a carbon ratio reaching a critically low value₃W₃C is known to precipitate as eta phase (η phase) upon cooling. These eta phases are very hard, but also very brittle and are therefore considered quality defects for hard metals.

It has also generally been found that the lower the content of a particular metal in the binder alloy can be reached, the higher the chemical stability of the corresponding carbide. The chemical stability of the corresponding carbide is known and can be expressed in terms of the free enthalpy of formation of the carbide. If the values are ordered in unconventional representation, i.e. based on a metal content of 1mol, the order is at 1000 ℃:

Cr₃C₂ < Mo₂C < WC < VC < NbC < TaC < ZrC < TiC < HfC。

here it can be seen that as expected, chromium carbide as the first carbide releases metallic chromium in the progressive absence of carbon, which dissolves in the binder alloy, but surprisingly molybdenum is the next least stable carbide, even before tungsten. It is theoretically possible to alloy hard metal binders with relatively large amounts of molybdenum without acting as a binderThe eta phase (eta phase) is formed as a result of the carbon deficiency in the mixture phase. The above metal carbide sequences are also a measure of the affinity of the metal for carbon. For example, titanium and Cr₃C₂Competing for carbon, chromium is preferentially present as a metal and titanium is preferentially present as a carbide. Tungsten carbide must be present in the material as a hardness-imparting agent; all carbides to the left of tungsten carbide in the above sequence (i.e. carbides which are not stabilized by tungsten carbide in terms of liberation of metal from the corresponding carbide) are therefore suitable for increasing the hot hardness, since they are able to enter the metal binder phase without forming carbides which are depleted in carbon, i.e. a so-called "η -phase" occurs.

Since the concentration of all the above-mentioned metals in the binder is governed by the law of the solubility product (the more unstable the carbides, the greater the solubility product) and since there is only one carbon potential in equilibrium, the sequence also shows the order in which the metals dissolved in the binder precipitate in the form of carbides with increasing carbon supply and are therefore no longer able to increase the hot hardness with the binder.

The content of chromium or tungsten is very important for the high temperature properties of the binder alloy, since these elements lead to an increased hot hardness and thus to an increased deformation resistance. For this reason, hard metals of this type, which are intended for use as tools (inserts), for example for turning steels, are sintered with a carbon balance such that the tungsten content of the binder alloy, which usually contains cobalt, is maximized without the formation of eta-phase (η -phase) occurring. Also in the case of tools containing Cr carbides for metal machining by drilling and milling, the carbon content is set so that as much Cr as possible is present in the binder alloy. Since the magnetic saturation of cobalt continuously decreases with increasing Cr and W content, nondestructive testing of the alloy state can be carried out very simply by measuring the magnetic saturation, a method that is standard in the industry.

However, chromium makes it difficult to determine the carbon content of the hard metal and thus the chromium and tungsten content, because the relationship between magnetic saturation and chromium and tungsten content is no longer well-defined, due to its antiferromagnetic properties. As a result, the lack of η phase cannot be determined based on the measurement of magnetic saturation alone.

Due to the health hazards associated with the combination of WC and cobalt as binder alloy, it is interesting to replace cobalt so that it is possible for alloy powders based on FeCoNi or FeNi. Although their suitability for wear parts and tools for machining wood or stone has been demonstrated, their suitability for high temperature-related applications has not been demonstrated. One of the main reasons for this is the lower thermal strength of hard metals with fe (co) Ni binders in the temperature range 400-800 c compared to cobalt.

The thermal hardness of the binder alloy may be increased by precipitation or alloying in of other metals. However, possible alloying elements are only metals which do not form stable carbides (i.e. which do not have more stability than carbides of tungsten carbide) and therefore satisfy the prerequisite for a measurable solubility in the binder alloy. For example, if Ta is alloyed into the binder, this will (depending on the carbon content of the hard metal) be present virtually completely as eta phase or as TaC after sintering, and thus not represent a high-heat-strength binder alloy of a high-quality hard metal, since eta phases are undesirable in hard metals due to their brittleness, which leads to a reduction in strength.

In principle, the metals W, Mn, Cr, Mo, Re and Ru are possible elements in particular for increasing the hot hardness.

The solubility of tungsten in the binder alloy is limited by the solubility product of tungsten carbide in the binder alloy. At the limit of eta phase formation, two cases can be distinguished in terms of tungsten content: a) when the carbon content is reduced and cobalt is used as the binder metal, up to 20 wt% of the tungsten is dissolved in the cobalt binder; b) when the carbon content is reduced and a FeCoNi binder alloy is used, significantly less tungsten (i.e., only up to about 5 wt%) is dissolved into the FeCoNi binder alloy. Thus, the solubility of tungsten in FeCoNi and FeNi alloys is even lower than pure cobalt, which is one reason for the low thermal hardness of hard metals that rely on FeCoNi bonding.

Manganese has a relatively very high vapor pressure and, therefore, by sintering hard metals containing manganese, concentration gradients and precipitates of pyrophoric Mn-metal condensates are obtained. Therefore, the concentration of Mn in the sintered article cannot be set accurately, and is estimated to be lower at the abutment of the workpiece surface than at the workpiece core.

The metals rhenium, osmium and ruthenium have limited availability and are extremely rare, but are suitable in principle. Rhenium is used, for example, in high-heat-strength alloys for aircraft turbines in order to suppress high-temperature creep of the components. Ruthenium and rhenium are used commercially to a limited extent in special hard metals based on cobalt.

Chromium is also suitable and has a high solubility in FeNi and FeCoNi alloys, but has disadvantages due to its antiferromagnetic properties, which make it difficult to interpret magnetic saturation (die interposition der magneticischen S ä ttingerwert). This is a disadvantage because the hard metals used for metal machining cutting are as close as possible to the limit of the formation of the eta phase, but there is no appreciable amount of the latter.

Also, molybdenum (Mo) in the form of added molybdenum carbide₂C, 5 wt%, added as an additive to hard metals containing 10% Fe-based binder) has been shown (article by Prakash) to result in an increase in the hot hardness of FeCoNi alloys. However, since the unknown part of Mo is present in the form of carbides, mixed carbides are formed between WC and the latent modification MoC (cryptomodification) dissolved therein, which results in an unwanted and uncontrolled reduction of the intrinsic strength of the hard material. The formation of mixed carbides in the case of molybdenum can be described by the following reaction equation:

Mo₂C->mo (alloyed in the binder) + (W, Mo) C.

Molybdenum has a higher solubility in alloys containing Fe-and Ni-than tungsten. The curve of the efficacy of Mo in increasing the creep resistance of pure iron at 427 ℃ is clearly steeper than that of Cr (trans. amer. inst. min. met. eng. 162, (1945), 84), and only a very slow increase is observed above 0.5% chromium. Even 1% Mo results in a creep resistance of 38 kpsi (262 MPa), while 1% Cr gives only 16 kpsi (110 MPa), and even 4% chromium does not achieve values in excess of 18 kpsi (124 MPa). The hot hardness-temperature curve of Mn does not have a plateau, but has a significantly lower slope. Mo is therefore a preferred choice element for increasing the thermal hardness, in particular for iron-containing binders in sintered hard metals. L. Prakash found that even a few percent molybdenum was sufficient to achieve a significant effect on the hot hardness of Fe-containing hard metals (article by Leo j. Prakash, university ä t Karlsruhe1979, Fakult ä t f ur masschinenbau, KfK 2984). However, since Mo is used₂C, and thus the proportion of Mo actually present in the binder remains unclear.

The metals that cause the increase in the hot hardness of the binder must be present in the binder, rather than in the hard material, so that they can cause the increase in the hot hardness of the hard metal below 800 ℃. Precautions must be taken to ensure that the metal is actually present in the binder metal alloy and not in the hard material. In the case of W and Cr, the industry standard is to use carbides, metals or nitrides, and the carbon content of the hard metal is set by means of the formulation and measures during sintering, so that the hard metal is at the edge of the area where the eta phase (η phase) is present, and the largest possible proportion of W and Cr is present in the binder. Thus, Cr is usually added as chromium carbide, which disproportionates during sintering, for example according to the following equation:

Cr₃C₂->cr (alloyed in binder) +2 CrC (alloyed in WC)

Thus, only a small portion (i.e., 1/3) of the Cr used is effective in the binder. Mo₂C is also similarIn this case:

Mo₂C->mo (alloyed in the binder) + (W, Mo) C.

Thus, when molybdenum carbide is used, only a maximum of about 50% is effective in the binder alloy; for this reason, instead of Mo, elemental Mo metal powders are used₂C. However, even when very finely dispersed Mo metal powder is used, regions are formed after sintering which consist only of the binder alloy phase and which contain no hard material. This behavior may be attributed to the inability of the Mo metal powder aggregates to effectively pulverize during the mixing grinding process due to the high modulus of elasticity of molybdenum; and the formed amorphous aggregates dissolve during liquid phase sintering of the molten binder alloy, which in turn fills the pores formed by dissolving the Mo particles into the molten binder. This results in the formation of a "binder precipitate," which is a term that refers to a specific region of the binder alloy that is larger in size than the grain size of the hard material phase, but does not contain tungsten carbide or hard material particles.

These are disadvantageous and unacceptable for both strength and local wear resistance. Due to the limited diffusion time (corresponding to the time during which the molten binder phase is present during sintering), it is unclear whether complete dissolution of the Mo metal powder and homogeneous alloying of Mo in the binder alloy has been completely achieved.

If the molten binder does not fill the secondary pores formed in the sintering, they are visible in the sintered body as described in EP 1043411B 1, column 1, line 29/30. These secondary pores reduce the strength.

According to the invention, an iron-, cobalt-or nickel-containing binder metal powder, which contains iron in an amount of 0.1 to 65% by weight, cobalt in an amount of 0.1 to 99.9% by weight and nickel in an amount of 0.1 to 99.9% by weight, is used for producing the sintered hard metal. The percentage data are weight percentages and are in principle based on the binder alloy powder, unless otherwise indicated.

The binder alloy powder used comprises 0.1-10 wt% of molybdenum in the form of an alloy, based on the total binder metal powder. The binder metal powder used preferably comprises from 0.10% to 3% by weight of molybdenum, particularly preferably from 0.5% to 2% by weight of molybdenum, very particularly preferably from 0.5% to 1.5% by weight of molybdenum, based in each case on the total binder metal powder. Too high a molybdenum content leads to too high a strengthening of the binder powder, so that the compression force and the resulting sintering shrinkage in the production of hard metals become too high, while too low a content leads to an insufficient increase in hot hardness.

Preferred hard materials are carbides, in particular tungsten carbide WC. Preferred binders are alloys of iron, cobalt and nickel, especially iron and nickel, iron and cobalt, cobalt and nickel and combinations of iron, cobalt and nickel. Cobalt alone may also be used as a binder.

The binder metal powders, which have been alloyed with molybdenum, exhibit good dispersion behavior in the production of hard metal powders by carbide mixed milling due to their physical properties. The FSSS value (measured according to ASTM standard B330 using the "Fisher Sub Siever Sizer" apparatus) is therefore 0.5 to 3 μm, preferably 1.0 to 2 μm. The finer powder is pyrophoric; coarser powders no longer have satisfactory dispersing behavior and again lead to "binder precipitation". The size distribution of the aggregates is 0.5 to 10 μm for the same reason. The specific surface area is preferably 2.5 to 0.5m for the same reason²(ii) in terms of/g. The oxygen content is preferably less than 1.5%.

The preferred cobalt content in the binder alloy is up to 60 wt%. The preferred nickel content in the binder alloy is 10-80 wt% or 20-60 wt% or 30-75 wt%.

Organic additives which are added subsequently may also be present. In order to determine the above parameters, they must be removed again as necessary, for example by washing with a suitable solvent. The organic additives include waxes, passivation and inhibitors, corrosion protection, compression aids. Possible examples are paraffin and polyethylene glycol. The organic additive has the additional purpose of preventing the powder from ageing, which would lead to an increase in the oxygen content. The additive may be present in an amount of 30 wt% based on the sum of the binder alloy powder and the additive.

The Mo-containing binder powder may include Fe, Ni, and Co. Since the sinterability and the hot hardness decrease with increasing Fe content, the iron content is less than 65%, preferably less than 60%. The balance to 100% is Mo plus Co and/or Ni. Preference is given to alloys in the system FeCoNi which are stable austenite in sintered hard metals, such as FeCoNi 30/40/30 or 40/20/40 or 20/60/20 or 25/25/50 and FeNi 50/50 or 30/70 or 20/80, or CoNi in the proportions 50/50, 70/30 or 30/70, as binder alloys. However, it is also possible to use elemental powders such as Co or Ni, which are alloyed with up to 10% Mo, which thus becomes an alloy powder.

The molybdenum-containing alloy powder is preferably produced by the following method (DE 102006057004 a 1): adding MoO₂Which has been comminuted to reduce the size distribution of the aggregates, acts as a source of molybdenum. Subjecting the MoO to a reaction₂Added to an oxalic acid suspension, as used in EP 1079950B 1, for the preparation of FeNi or FeCoNi mixed oxalate, which is subsequently heated under oxidative conditions and reduced to an alloy powder by means of hydrogen. The alloy powder obtained in this way is reduced sufficiently after reduction with hydrogen, i.e. MoO can no longer be detected by means of X-ray diffraction₂. Optionally, de-agglomeration is relied upon to further reduce the aggregate size in order to improve dispersion in the mixed milling with carbides. The aggregates consist of primary particles aggregated with each other. Aggregate size and aggregate distribution can be measured by means of laser light scattering and sedimentation.

Instead of MoO₂Other particulate Mo compounds that are insoluble in oxalic acid, such as sulfides or carbides, may also be used. They are oxidized to the oxide in an air calcination to precipitate the oxalate. Molybdenum oxides such as MoO₃Are formed during calcination and due to their high vapor pressure, very quickly form a mixture with fe (co) Ni mixed oxidesOxide and exhibit good transport properties, so that in subsequent reduction with hydrogen a FeCoNi alloy powder (which is homogeneously alloyed with a small proportion of Mo) is formed.

However, other known methods are also suitable; for example, ammonium salts of oxalic acid are used instead of oxalic acid, Na or K hydroxide is used, and formic acid and maleic acid are used. In all cases, preference is given to using MoO₂It should be as pure phase as possible and contain only traces of Mo or MoO₃Or Mo₄O₁₁. Using MoO₂The reason for (A) is because of the presence of MoO₃In contrast, it is not soluble in either acid or base and therefore remains completely in the alloy metal powder throughout the process. MoO₃Dissolving into a base for precipitating the Fe (Co) Ni component or into a complexed organic acid; the element Mo will be too coarse and not fully oxidized to MoO in the subsequent calcination₃And thus is not sufficiently alloyed in the reduction process with hydrogen. Fine MoO with high specific surface area₂Complete oxidation to MoO during air calcination of Fe (Co) Ni oxalate₃(which has a high vapor pressure) and, via the gas phase, molybdates and mixed oxides with these metal oxides are formed, which results in a very homogeneous molybdenum distribution which is retained in the subsequent hydrogen reduction.

It is known to use the alloyed Mo containing powder of the invention for the production of sintered parts by means of solid phase sintering, as in the diamond tool industry, but not in the hard metal industry, where a molten phase is formed in-between during sintering.

However, particularly preferred are given Mo-alloyed FeCoNi powders, which contain Mo in completely metallic form. In these powders, Mo oxides can no longer be detected by means of X-ray diffraction, so that the oxygen present has to be present very predominantly on the powder surface. A very particularly useful powder is a powder according to the invention whose FSSS value is from 0.5 to 3 μm, since this improves the dispersibility in the mixed grinding. In this case, they contain as little additional metal as possible in the form of oxides.

Since molybdenum oxide reacts with carbon during sintering of hard metals to form CO and thus leads to local carbon deficiency and thus to local eta phases, the alloy powder described in the preceding paragraph is suitable for hard metal production when precautions are taken during sintering of hard metals to ensure that oxygen, which is released mainly in the form of carbon monoxide, can escape from the sintered body. These powders are suitable for the application of the invention in the following cases: when they have the preferred physical properties of the invention, but only contain the above-mentioned elements Mn, Cr, V, Al and Ti at least partly in oxide form to such an extent that is permissible from the point of view of microstructural defects (pores and binder precipitations) in the hard metal.

According to the invention, a FeCoNi-or FeNi-based Mo-alloyed powder can additionally be alloyed with up to 20% of tungsten, for example to shift the onset of sintering shrinkage to higher temperatures or to induce precipitate formation (which precipitates reinforce the binder phase), but this can only be successful in the case of very coarse tungsten carbide.

According to the invention, the alloy powder used can be FeCoNi in a wide composition range. In the high Fe content range (90-60%), the binder alloy system was found to have a proportion of the martensite phase after sintering, and therefore high hardness and wear resistance at room temperature. Examples are FeNi 90/10, 82/18, 85/15, FeCoNi 72/10/18, 70/15/15 and 65/25/10. However, the above alloys have very low thermal hardness in sintered hard metals. In the range of about 80-25% Fe, the binder alloys were found to be austenitic after sintering and although they have a lower intrinsic hardness, they have high fatigue strength and are capable of undergoing limited plastic deformation. Examples are FeNi 80/20, 75/25, FeCoNi 60/20/20, 40/20/40, 25/25/50, 30/40/30, 20/60/20. In most cases, the thermal hardness of hard metals in 400-600 ℃ is lower than those with pure Co as binder, if Mo or other alloying elements are not additionally mixed into the alloy. Although the object of the invention is particularly preferred to produce hard metals with increased hot hardness, it is also suitable for producing hard metals with other objects, such as hard metals with a molybdenum-containing corrosion-resistant binder alloy system, which are currently produced using elemental or carbonized molybdenum (as described in EP 0028620B 2), or cutter blades for drill bits (as described in US 5305840).

Binder alloys of hard metals present in the sinter may also be obtained according to the invention using a number of different alloy powders and optionally elemental powders (as described in WO 2008/034903), and at least one of these powders is alloyed with molybdenum. The advantages of such a method are the compressibility and the control of sintering shrinkage.

The hard metal parts present after sintering and, if appropriate, as a result of the final machining by grinding or electroerosion have a defined tool geometry. This can be particularly preferably elongate (for example ground from sintered round bars), but also particularly preferably sheet-like, for machining materials such as metals, stone and composite materials by turning or milling. In all cases, the hard metal tool may preferably have one or more coatings selected from the group consisting of: nitrides, borides, oxides and ultra-hard layers (e.g., diamond, cubic boron nitride). They may have been applied by PVD or CVD methods or combinations or variants thereof and their residual stress state after application may have changed. However, they may also preferably be hard metal parts of any other geometry for any other use, such as forging tools, forming tools, countersinking tools, components, milling cutters, scrapers, rollers, punching tools, pentagonal bits for welding-in, mining cutters, milling tools for concrete and asphalt milling, rotating mechanical seals and any other geometry and application.

The present invention is illustrated in detail by the following examples.

Examples

Example 1 (comparative, not according to the invention)

462.5g of tungsten carbide 0.6 μm and 37.5g of FeCoNi alloy powder 40/20/40 (Ampersint)^®MAP A6050; the manufacturer: starck, germany) and 0.57L of 94% strength ethanol were mixed and ground in a ball mill at 63 rpm for 14 hours. 5 kg of hard material balls were used for this purpose. The FeCoNi powder used had the following properties: fe 38.8%, Co 20.22%, Ni 40.38%, O0.71%, specific surface =1.63m²G, FSSS value = 0.90. 2 batches with different carbon contents ("high carbon" and "low carbon") were produced, which resulted in different carbon contents after sintering. The results are shown in the table below.

Ethanol was separated from the resulting suspension by distillation under reduced pressure, and the obtained hard metal powder was uniaxially compressed at 150 MPa and sintered under reduced pressure at 1450 ℃ for 45 minutes. The hard metal pieces in sheet form were ground, polished and examined for their properties. As sintered bodies, neither batch showed eta phase nor carbon precipitation, but relatively small binder precipitates. In both cases, the room temperature hardness and the thermal hardness at the selected temperature of at most 800 ℃ are measured under a protective gas. Figure 1 shows the results: both batches showed a large drop in hot hardness in the region of approximately 600 ℃. The binder alloy is clearly inferior to pure cobalt for producing hard metal tools for metal turning at relatively high stresses due to plastic deformation of the cutting edge due to the expected cutting forces, which is due to low thermal hardness (in particular at 600 ℃).

Carbon (C)	Low carbon "	High carbon "
			Hardness (HV30) (kg/mm)2)	1582	1585
Magnetic saturation (G.cm)3/g)	137	140
			Porosity (ISO4505)	<A02<B02C00	A02B00C00
Fracture toughness (MPa. m)1/2)	9.5	8.2
			Density (g/cm)3)	14.69	14.65

Example 2 (comparative example, WC-Co, not according to the invention)

WC — Co having the same volume fraction of the binder phase as in example 1 was produced in a similar manner to example 1. Since Co has a higher density than FeCoNi 40/20/40, the weight fraction of cobalt is 8 wt.%, based on the total hard metal. Compression and sintering at reduced pressure and 1420 ℃ for 45 minutes produced defect-free magnetic saturation of 133G cm³Hardness per gramMetal, which corresponds to 82% of theoretical magnetic saturation. The room temperature hardness (HV 301597 kg/mm)²) And thermal hardness, and are plotted in fig. 1. It can be seen that Co outperforms FeCoNi binders from 350 up to 800 ℃, above which the carbide backbone determines the main factor of hot hardness. K of hard metals at room temperature₁C (fracture toughness, determined by the crack length at the corner of the hard indentation and calculated by Shetty's formula) is 10.1 MPa m^1/2. The cobalt binder thus additionally has a better hardness/K at room temperature than the binder of example 1₁And C relation.

Example 3 (comparative example, not according to the invention)

Example 1 was repeated and 1 wt% of Mo metal powder was added to the first batch and 3 wt% of Mo metal powder was added to the second batch. (these contents relate to the Mo content of the binder alloy phase). The deaggregated molybdenum metal powder has the following properties: FSSS value: 1.09, O content: 0.36% by weight. The particle size distribution is determined by the following parameters: d₅₀ 3.2 µm，D₉₀ 6.4 mu m. The carbon content was chosen based on the experience of example 1 such that neither eta phase nor carbon precipitation could be expected in the sintered hard metal. For Mo addition, no additional carbon is taken into account, so that the molybdenum is present in the binder alloy as completely as possible in metallic form. The carbon content of the formulation is therefore 5.94 and 5.94% (3% by weight of Mo, based on the binder). The results after sintering at 1420 ℃ are shown in the table below. The hot hardness is measured as before and is represented by a circle in fig. 2:

mo addition amount in the binder	1%	3%
			Hardness (HV30)	1635	1652
Magnetic saturation (G.cm)3/g)	137.5	136.2
			Porosity (ISO4505)	<A02<B02C04	<A02<B02C00
Fracture toughness (MPa. m)1/2)	9.2	9.0
			Microstructure	Many and sometimes large adhesive lakes	Very numerous and sometimes large binder precipitations

Surprisingly, eta phase did not occur at both 1 and 3 wt% molybdenum; rather, carbon porosity occurs even at 1 wt% molybdenum. The hardness is surprisingly increased compared to example 1, while K₁The C value is not reduced, thus a combination of properties is obtained at room temperature which is equal to Co-bonded hard metal and superior to pure FeCoNi-bonded hard metal. Surprisingly, 1 wt% molybdenum in the binder is sufficient; k is observed at 3% by weight of molybdenum compared to 1% of Mo₁C and hardness did not change strongly. Molybdenum alloyed into the binder is therefore notOnly the inherent hardness of the binder is increased and at the same time the fracture toughness is increased. This behavior differs in this respect from the case of alloyed W: in Co-based hardmetals and in FeCoNi based materials, see example 1, while an increase in the intrinsic hardness of the binder is also found here, there is also K at the same time₁And C value is reduced.

However, very many binder precipitations occurred, which are evidence of Mo dissolving into the binder, which then filled the pore volume formed. However, these binders are not acceptable for precipitation in hard metals.

A comparison of the thermal hardness of these of example 2 is shown in fig. 2. The hot hardness at all temperatures up to 800 ℃ is surprisingly even lower than those of example 1.

Example 4 (inventive)

Example 1 was repeated, but wherein a FeCoNi binder alloy alloyed with 1.5 wt% Mo produced by the method described in DE 102006057004 a1 was used. The powder is then deagglomerated. The properties of the powder analyzed were: fe 38.23 wt%, Co 19.96 wt%, Ni 39.10 wt%, Mo 1.55 wt%, O0.8565 wt%, FSSS value: 1.21, specific surface =2.17m²/g，D₅₀ =3.46 µm，D₉₀=5.84 μm. Even after a long irradiation, MoO can no longer be detected by X-ray diffraction at its characteristic diffraction angles₂. 37.5g of this powder was used together with 462.5g of WC for producing hard metals. The carbon content of the hard-metal mixture was 5.92% by weight, which was set by adding 1.14g of carbon black. The compressed body was sintered in both open and closed crucibles. This variation has an effect on the carbon content of the hard metal after sintering. The properties of the sintered hard metal at 1420 ℃ are as follows:

sintering	Crucible with opening	Closed crucible
			Hardness (HV30)	1661	1626
Magnetic saturation (G.cm)3/g)	128.8	134.2
			Porosity (ISO4505)	A02-A04，<B02，C00	A02，<B02，C00
Fracture toughness (MPa. m)1/2)	13.6	7.9
			Microstructure	Without adhesive starch	Without adhesive starch

The hard metal from open sintering is at the low carbon end of the two phase region because it has very low magnetic saturation compared to example 1. But the eta phase could not be detected. The highest possible concentration of Mo in the binder results in a strong strengthening of the binder alloy, which is reflected in a simultaneous increase in hardness and fracture toughness. The hard metal from the closed sintering is also in the carbon content of the 2-phase region, but contains more carbon, as can be seen from the high magnetic saturation. Since significantly more Mo is present as carbides (due to the higher carbon supply) and therefore not in the binder, the fracture toughness (which is decisively dependent on the binder) is greatly reduced to the extent of the "high carbon" variant of example 1. This example demonstrates the theoretical considerations discussed in the specification.

Additional compacts were produced and sintered at 1420 ℃ under reduced pressure, but argon at a pressure of 40 bar was applied until the end of the sintering at the final temperature. The cooling is carried out under pressure. A hard metal block with the following parameters was obtained: hardness of 1643 HV30, fracture toughness of 8.2 MPa m^1/2And magnetic saturation of 123G cm³(ii) in terms of/g. Both room temperature hardness and hot hardness were measured as a function of temperature on a hard metal block on an additional hardness tester. The evaluation results of both the room-temperature hardness and the hot hardness are shown in fig. 2, indicated by squares, and the curves drawn in examples 2 and 3 are compared: the hot hardness at 600 ℃ of the hard-metal of example 4 is significantly lower than those of example 2 compared to cobalt-bonded hard-metal. This hot hardness is now higher than that of the hard metal produced with the binder alloy powder (which was not alloyed with Mo) (example 3). (there is a difference in room temperature hardness due to the other hardness tester).

It can be seen that the use of the binder powder of the invention, which is (pre-) alloyed with molybdenum, enables the production of defect-free hard metals without binder precipitations and with a hot hardness profile practically identical to that of cobalt binders. Specifically, the reduction in hot hardness at around 600 ℃ is substantially eliminated. In addition, when the carbon balance is properly set as compared to example 1, there is a significant increase in both room temperature strength and hardness as compared to example 1, which also provides the advantage of use at or near room temperature. In addition, an improved corrosion resistance over example 1 can be expected, since corrosion on hard metals generally occurs via the binder phase.

The principle of improving the hard metal properties by alloying molybdenum in the binder can be applied not only to the described FeCoNi 40/20/40 binder, but also to pure cobalt and pure Ni as hard metal binders, to CoNi and FeNi alloys and to further FeCoNi alloys.

Claims (12)

1. Use of a molybdenum-containing binder alloy powder for the production of liquid phase sintered hard metals based on tungsten carbide, characterized in that:

a) the FSSS value of the binder alloy powder used is 0.5 to 3 [ mu ] m, measured according to ASTM B330, and

b) the binder alloy powder used comprises iron in an amount of less than 60% by weight, cobalt in an amount of up to 60% by weight and nickel in an amount of 20-60% by weight, and

c) the binder alloy powder used contains 0.1-10 wt.% Mo in the form of an alloy or a pre-alloy.

2. Use according to claim 1, wherein the molybdenum is present entirely in metallic form.

3. Use according to one or both of claims 1-2, wherein the binder alloy powder used comprises at least 10 wt% nickel, based on the total binder alloy.

4. Use according to one or more of claims 1 to 3, wherein the binder alloy powder used comprises up to 20% by weight of tungsten, based on the total binder alloy.

5. Use according to one or more of claims 1 to 4, wherein at least one component of the binder alloy is introduced as a powdered alloy of at least one metal and molybdenum, the respective remaining components of the binder alloy being introduced as elements or alloys which do not contain any molybdenum.

6. Use according to one or more of claims 1 to 5 for the production of sintered hard metals, wherein the sintering is carried out in the form of liquid phase sintering.

7. Use according to one or more of claims 1 to 6, characterized in that the binder alloy powder comprises up to 30% by weight of one or more organic additives.

8. Use according to one or more of claims 1 to 7, wherein the binder alloy powder used comprises not more than 10% by weight of tungsten, based on the total binder alloy.

9. Prealloyed powder containing 0.1-65 wt.% iron, 0.1-60 wt.% cobalt, 10-80 wt.% nickel and 0.1-20 wt.% molybdenum in metallic form, wherein the FSSS value according to ASTM B330 is at most 3 μm and the remaining components of the powder are unavoidable impurities.

10. Prealloyed powder according to claim 9, which additionally contains up to 10% by weight of tungsten in the form of an alloy or prealloy.

11. Prealloyed powder according to one or both of claims 9-10, characterized in that the prealloyed powder contains 0.1-20% by weight of molybdenum.

12. Prealloyed powder according to one or more of claims 9-11, characterized in that it comprises 0.1-65% by weight of iron and 10-60% by weight of nickel.