CN102187005A - Molybdenum-contained alloy powders used to produce sintered hard metals based on tungsten carbide - Google Patents

Abstract

The invention relates to the use of binder alloy powders containing molybdenum to produce sintered hard metals based on tungsten carbide, wherein the binder alloy powder used has an FSSS value of 0.5 to 3 [mu]m measured with the "Fisher Sub-Sieve Sizer" device according to the ASTM B330 standard, and comprises 0.1 to 65 wt % of iron, 0.1 to 99.9 wt % of cobalt, and 0.1 to 99.9 wt % of nickel, and contains 0.1 to 10 wt % of Mo in alloyed form.

Description

Molybdenum-containing metal powders for the production of tungsten carbide-based hard metals

technical field

The present invention relates to the use of molybdenum-containing binder alloy powders for the production of sintered hard metals based on tungsten carbide. Hard metals are composite materials sintered from hardness-imparting materials such as carbides and a continuous binder alloy. Sintered hard metals are very versatile and are used for processing virtually all known materials such as wood, metal, stone and composite materials such as glass-epoxy resin, cardboard, concrete or asphalt-concrete. Here, local temperatures of up to more than 1000° C. arise as a result of cutting, deformation and friction processes. In other cases, deformation processes of metal workpieces are carried out at high temperatures, for example during forging, wire drawing or rolling. In all cases, hard metal tools are subject to oxidation, corrosion and diffusion as well as adhesive wear and at the same time are under high mechanical stress, which can lead to deformation of the hard metal tool. The term "adhesive wear" refers to any phenomenon that occurs when two objects come into contact with each other and at least temporarily form a weld and a firm connection, which is re-released by external force, wherein the material of one object sticks to the other. attached to another object. The term "diffusion wear" refers to any phenomenon that occurs when two materials come into contact with each other and components diffuse from one material to the other such that dimples are formed in the first material.

Background technique

WO 2007/057533 (Eurotungstene Poudres) describes alloy powders based on FeCoCu and containing 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 method or according to ISO standard 10070), and due to the Cu content exceeding 500 ppm. Molybdenum is added as a water-soluble ammonium salt to the oxide, which is then reduced to a metal powder by means of hydrogen.

EP 1492897 B1 (Umicore) describes alloy powders based on FeCoNiMoWCuSn for the production of diamond tools, where the sum of Cu and Sn contents is 5-45%. However, both elements are detrimental to hard metals because Cu "bleeds out" and Sn causes porosity during sintering. These alloy powders are therefore not suitable for the production of hard metals.

EP 0865511 B9 (Umicore) describes alloy powders, which are based on FeCoNi and have an FSSS value not greater than 8 µm, which 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, similar powders are described, but containing up to 30% Co and up to 30% Ni each.

WO 98/49361 (Umicore), EP The alloy powders described in 1042523 B1 (Eurotungstene Poudres) and KR 062925 are also unsuitable because of the copper content.

EP 1043411 B1 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 appearance of porosity, as would occur by adding metals. However, the described method has the disadvantage compared with the use of alloy powders according to the invention that the carbon content of the composite powder is changed during the pyrolysis of the organic precursor compound (carbon is deposited or removed by the formation of methane), so The carbon content must be re-analyzed and adjusted prior to sintering. The presence of Mo or W after sintering is also unclear, since neither comparative tests nor indications of the state of Mo and W alloys before sintering, nor magnetic saturation values are given. The method produces a fixed recipe of carbide and binder alloy phase content and composition and is therefore very inflexible in practice, since the recipe depending on the use of the hard metal to be produced is simple and fast Variations are hard to use.

Also known are alloy powders based on FeCoMo with FSSS values <8 μm and a specific surface area greater than 0.5 m ² /g (DE 102006057004 A1) and which are used for the production of carbon-free high-speed steels via powder metallurgy methods. They may optionally contain up to 10% or 25% Ni, but are particularly advantageous not to contain any nickel beyond unavoidable contamination levels. They preferably consist of 20-90% Fe, up to 65% Co and 3-60% Mo. Since pure FeCo alloys without additional Ni alloying are unsuitable for hard metals due to their brittleness and poor corrosion and oxidation resistance, these alloy powders clearly do not offer a solution to the problem. In addition, there is no description of a preferred range, ie, a high Mo content and for producing a liquid phase-sintered carbon-containing hard metal having a hard material phase such as carbide as a hardness-imparting agent.

invention goal

Metallic cobalt is known to be a health hazard when used as the sole binder metal, especially for tungsten carbide. It is therefore an object of the present invention to discover additional alloying elements and use it for the production of sintered hard metal materials which allow the use of FeNi and FeCoNi binders to replace Co at high operating temperatures of 400-800°C without Disadvantages such as binder lakes (Binderseen), lack of interpretation of magnetic saturation or the relevant element ratios in the binder phase are unknown and the elements involved lead to an increase in hot hardness in the temperature range of 400-800 °C . On the other hand, the content of the elements involved should be as low as possible, and in order to increase efficiency, the best possible distribution is likewise possible.

This goal is achieved through the use of molybdenum-containing binder alloy powders for the production of tungsten carbide-based sintered hard metals, characterized by:

a) the FSSS value of the binder alloy powder used is 0.5-3 µm as measured in accordance with ASTM B 330, 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% by weight of Mo in alloyed or pre-alloyed form.

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

The binder alloy powder used contains up to 20% by weight, in particular up to 10% by weight, 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, and the respective remaining components of the binder alloy are present as elements or alloys, neither of which contains any molybdenum , that is, use is made of an alloy made of a powder mixture of at least one alloyed or pre-alloyed molybdenum-containing alloy powder and at least one alloyed or pre-alloyed alloy powder or elemental powder, and the latter powder contains only Molybdenum in the range of unavoidable impurities.

The molybdenum-containing binder alloy powder of the present invention is used to produce 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% by weight of organic additives.

Description of drawings

Figure 1 shows the hot hardness of the hard metal of Example 2 with a cobalt binder (square symbols pointing down), Example 1 with FeCoNi binder (triangular symbols, solid line indicates " Low carbon" variant, dashed line indicates the hot hardness curve of the "high carbon" variant).

Fig. 2 shows the comparison of Example 3 with Example 4 (FeCoNi binder alloyed with Mo, square symbols brought down) and Example 2 (cobalt as binder, square symbols pointing down). Hot hardness curves of hard metals (FeCoNi binder, Mo used as elemental powder, circle symbols, 1% Mo = dashed line, 3% Mo = solid line).

Detailed ways

This goal 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 cobalt in an amount of 0.1-99.9% by weight amount of nickel.

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

The binder alloy powder used has an FSSS value of 0.5-3 µm, preferably 0.8-2 µm, especially 1-2 µm, measured according to standard ASTM B 330 using a "Fisher Sub Siever Sizer" apparatus.

The elements Mn and Cr are preferably present in amounts of less than 1% each. The binder alloy powder used preferably contains molybdenum either entirely in non-oxidized form or entirely in alloying metal form.

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

In the binder alloy powder used, it is preferably given that at least one constituent of the binder alloy is introduced as a powdery alloy of at least one metal with molybdenum, and the respective remaining constituents of the binder alloy Introduced as elements or alloys neither containing 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 liquid metal phases is solely due to the temperature changes used and that the hard material crystallizes (umlösen) after dissolution in the binder alloy and thus undergoes an increase in grain size (Ostwald ripening). This differs from solid-state sintering, in which no melt is formed, nor is any temporary formation due to instantaneous, local changes in composition, but hard materials that may be present, such as diamond, do not occur in the melt. Crystallizes upon dissolution to experience an increase in particle size.

Description of the invention

Hard metals produced by the method of the invention need to have sufficient stability in terms of plastic deformation and temperature-dependent creep behavior to enable their intended use. Creep of materials, such as plastic deformation, is a major material failure mechanism and must be avoided at all costs. The deformation mechanism is subject to a known load-dependent creep time law, and the creep rate depends not only on the load but also to a large extent on the temperature. In addition, the creep mechanism varies in each case primarily as a function of temperature. In the case of hard metals, it is known that the creep rate at temperatures up to about 800°C is primarily measured by the deformation of the metallic binder phase, while above about 800°C the binder phase So soft that it has virtually no appreciable creep resistance, ie, at temperatures above 800°C, the load-bearing strength of the hard material phase is decisive. This load-bearing capacity in turn depends on the particle shape and particle size distribution of the hard material phase and on the proportion of heat-resistant cubic carbides. For this reason, all hard metal materials for cutting steel contain not only WC, but also a proportion of cubic carbides such as TiC, TaC, NbC, VC, ZrC or mixed carbides such as TaNbC, WTiC or WVC.

Since the experimental determination of the temperature dependence of the creep behavior at high temperatures is very complex, a hot hardness determination is used instead. Material hardness is an indirect measure of its ability to deform plastically. The central idea is that the plastic deformation method is dominant in hardness indentation formation, so that the size of the hardness indentation at sufficiently high loads and durations of load is a measure of the material's ability to deform plastically 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 that form cubic carbides such as V, Ta, Ti and Nb are in the liquid phase sintering process dissolves into the binder phase. This also applies to Cr if the carbonization of Cr is used as a so-called "grain growth inhibitor" (ie, as an agent for inhibiting grain growth) for inhibiting the growth of the WC microstructure that occurs during sintering.

The term "liquid phase sintering" refers to sintering at such a high temperature that the binder alloy is at least partially melted. The liquid phase during hard metal sintering is a consequence of the sintering temperature, which is typically 1100°C - 1550°C. 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 in accordance with the solubility product principle used. This means that the more tungsten is present in the melt, the less carbon is dissolved in the melt and vice versa. The tungsten content of the binder alloy is set by the total W:C ratio in the hard metal, where W:C = 1 is always satisfied for the hard material phase, so that in the binder metal melt there is Different concentrations of W:C ratios not equal to 1. When the tungsten:carbon ratio in the melt reaches critically low values, carbon-depleted carbides such as Co ₃ W ₃ C, known as the eta phase (η phase), precipitate on cooling. These η phases are very hard, but also very brittle and are therefore considered quality defects of hard metals.

It has also generally been found that the lower the achievable content of a particular metal in the binder alloy, 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 an unconventional manner, i.e. based on 1 mole of metal content, the order at 1000°C is:

Cr ₃ C ₂ < Mo ₂ C < WC < VC < NbC < TaC < ZrC < TiC < HfC.

Here it can be seen that, as expected, chromium carbide as the first carbide releases metallic chromium upon gradual carbon starvation, which dissolves in the binder alloy, but surprisingly, molybdenum is next most unstable carbides, even before tungsten. It is therefore theoretically possible to alloy hard metal binders with relatively large amounts of molybdenum without eta phase (η phase) forming as a result of carbon deficiency in the binder phase. The metal carbide sequence above is also a measure of the affinity of the metal for carbon. For example, titanium competes with _Cr3C2 for carbon, so chromium is preferentially present as _a metal, while 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 without tungsten carbide stabilization in terms of releasing metal from the corresponding carbides) are therefore suitable for increasing thermal hardness because they are able to enter the metal binder phase without forming carbon-poor carbides, i.e. the so-called "eta phase" occurs.

Because the concentrations of all of the above metals in the binder are governed by the solubility product law (the more unstable the carbide, the larger the solubility product), and because there is only one carbon potential in equilibrium, the sequence also expresses A sequence in which the metal dissolved in the binder precipitates out in the form of carbides as the carbon supply gradually increases and thus no longer enables the binder to increase the hot hardness.

The content of chromium or tungsten is very important for the high temperature performance of the binder alloy, since these elements lead to an increase in hot hardness and thus in deformation resistance. For this reason, hard metals of this type (which are intended to be used as tools (blade blades (Schneidplatte)), for example for turning steel, are sintered with such a carbon balance that the binder alloy (which usually contains The tungsten content of cobalt) is maximized without the formation of eta phase (η phase). Also in the case of tools containing Cr carbides for metal machining by drilling and milling, the carbon content is set such that as much Cr as possible is present in the binder alloy. Because the magnetic saturation of cobalt decreases continuously with increasing Cr and W content, non-destructive testing of the state of the alloy can be performed very simply by measuring the magnetic saturation, which is an industry standard.

However, due to its antiferromagnetic properties, chromium makes it difficult to determine the carbon content in hard metals, thus making the determination of chromium and tungsten content difficult, because of the relationship between magnetic saturation and chromium and tungsten content is no longer explicit. As a result, the absence of an η phase cannot be judged based solely on measurements of magnetic saturation.

Due to the health hazards associated with the combination of WC and cobalt as a binder alloy, it is interesting to substitute cobalt, making it possible for FeCoNi or FeNi based alloy powders. While their suitability for wear parts and tools for working wood or stone has been proven, their suitability for high temperature related applications has not been proven. A major reason for this is the lower hot strength of hard metals with Fe(Co)Ni binders in the temperature range of 400°C - 800°C compared to cobalt.

The hot hardness of the binder alloy can be increased by precipitation or alloying of other metals. However, possible alloying elements are only metals that do not form stable carbides (i.e., carbides that are no more stable than tungsten carbide), and thus satisfy the prerequisite of measurable solubility in the binder alloy. condition. For example, if Ta is alloyed into the binder, this will (depending on the carbon content of the hard metal) be present substantially entirely as eta phase or as TaC after sintering and thus does not represent a high quality High hot strength binder alloys for hard metals because 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 especially possible elements 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 with respect to the tungsten content: a) when the carbon content is reduced and cobalt is used as binder metal, up to 20% by weight of tungsten is dissolved in the cobalt binder; b) When the carbon content is reduced and a FeCoNi binder alloy is used, significantly less tungsten (ie only up to about 5% by weight) dissolves into the FeCoNi binder alloy. Thus, the solubility of tungsten in FeCoNi and FeNi alloys is even lower than that of pure cobalt, which is one reason for the low hot hardness of hard metals relying on FeCoNi bonding.

Manganese has a relatively very high vapor pressure, and thus, by sintering manganese-containing hard metals, concentration gradients and precipitations of pyrophoric Mn-metal condensates are obtained. Therefore, the concentration of Mn in the sintered part cannot be precisely set and is presumed to be lower abutting 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 used in aircraft turbines to inhibit high-temperature creep of components. Ruthenium and rhenium are used commercially to a limited extent in cobalt-based specialty hard metals.

Chromium is also suitable and has high solubility in FeNi and FeCoNi alloys, but has disadvantages due to its antiferromagnetic properties, which make it difficult to interpret magnetic saturation (die Interpretation der magnetischen Sättigung erschwert). This is a disadvantage because hard metals for metal machining cuts are as close as possible to the limit of eta phase formation, but no appreciable amount of the latter is present.

Likewise, molybdenum in the form of added molybdenum carbide ( _Mo2C , 5 wt%, added as an additive to hard metals with 10% Fe-based binder) has been shown (Prakash's paper) to lead to FeCoNi alloys increase in hot hardness. However, because the unknown part of Mo exists in the form of carbides, mixed carbides are formed between WC and the recessive modification (Kryptomodification) MoC dissolved in it, which leads to the limitation of the intrinsic strength of this hard material. Unwanted and uncontrolled reduction. The formation of mixed carbides in the case of molybdenum can be described by the following reaction equation:

Mo ₂ C -> Mo (alloyed in binder) + (W, Mo)C.

Molybdenum has a higher solubility than tungsten in alloys containing Fe- and Ni-. The curve of the effectiveness of Mo in increasing the creep resistance of pure iron at 427°C is significantly steeper than that of Cr (Trans. Amer. Inst. Min. Met. Eng. 162, (1945), 84) and higher than 0.5% Only a very slow increase was observed for chromium. Even 1% Mo resulted in a creep resistance of 38 kpsi (262 MPa), while 1% Cr gave only 16 kpsi (110 MPa), and even 4% Cr did not achieve more than 18 kpsi (124 MPa ) value. The hot hardness-temperature curve of Mn does not have a plateau, but has a significantly lower slope. Mo is therefore the preferred element of choice for increasing the hot hardness, especially of ferrous binders in sintered hard metals. L. Prakash found that even a few percent of molybdenum is sufficient to achieve a noticeable effect on the hot hardness of Fe-containing hard metals (paper by Leo J. Prakash, Universität Karlsruhe 1979, Fakultät für Maschinenbau, KfK 2984). However, since _Mo2C is used, the proportion of Mo actually present in the binder remains unclear.

Metals that cause an increase in the hot hardness of the binder must be present in the binder, not in the hard material, so that they can cause an increase in the hot hardness of the hard metal below 800°C. Precautions must therefore 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 rely on recipes and measures during sintering to set the carbon content of the hard metal so that the hard metal is in the eta phase (η phase ) exists at the edge of the region and the largest possible proportion of W and Cr is present in the adhesive. Therefore, Cr is usually added as chromium carbide, which disproportionates during sintering, for example according to the following equation:

Cr ₃ C ₂ -> Cr (alloyed in binder) + 2 CrC (alloyed in WC)

Therefore, only a small fraction (ie, 1/3) of the Cr used is effective in the binder. Mo ₂ C is also similar to this case:

Mo ₂ C -> Mo (alloyed in binder) + (W, Mo)C.

Therefore, when molybdenum carbide is used, only a maximum of about 50% is effective in the binder alloy; for this reason, elemental Mo metal powder is used instead of _Mo2C . However, even when very finely divided Mo metal powders were used, after sintering, regions were formed that consisted only of the binder alloy phase and contained no hard material. This behavior can be attributed to the inability of the Mo metal powder aggregates to pulverize effectively during mixing and grinding because of the high elastic modulus of Mo; Dissolved during the binder alloy process, which in turn filled the pores formed by dissolving the Mo particles into the molten binder. This results in the formation of "binder lakes", which is a term for specific regions of the binder alloy that are larger in size than the grain size of the hard material phase, but do not contain tungsten carbide or hard material particles.

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

If the molten binder does not fill the secondary pores formed during sintering, they are visible in the sintered body, as described in col. 1, line 29/30 of EP 1043411 B1. These secondary holes reduce strength.

According to the invention, 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 is Used in the production of sintered hard metals. The percentage data are percentages by weight and are based in principle on the binder alloy powder unless otherwise indicated.

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

Preferred hard materials are carbides, especially 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. It is likewise possible to use cobalt alone 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 co-grinding due to their physical properties. The FSSS value (measured using a "Fisher Sub Siever Sizer" device, according to ASTM standard B 330) is thus 0.5-3 µm, preferably 1.0-2 µm. Finer powders are pyrophoric; coarser powders no longer have satisfactory dispersion behavior and lead to "binder lakes" again. The size distribution of the aggregates is 0.5-10 µm for the same reason. The specific surface area is preferably 2.5-0.5 m ² /g for the same reason. The oxygen content is preferably below 1.5%.

The preferred cobalt content in the binder alloy is up to 60% by weight. Preferred nickel contents in the binder alloy are 10-80% by weight or 20-60% by weight or 30-75% by weight.

Organic additives added subsequently may also be present. In order to determine the above-mentioned parameters, they have to be removed again, if necessary, for example by washing with a suitable solvent. The organic additives include waxes, passivators and inhibitors, corrosion protection, compression aids. Possible examples are paraffin and polyethylene glycol. An additional purpose of organic additives is to prevent aging of the powder, which would lead to increased oxygen content. The additive may be present in an amount of 30% by weight, based on the sum of the binder alloy powder and the additive.

The Mo-containing binder powder may contain Fe, Ni and Co. Since sinterability and 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 austenite stable in sintered hard metals, for example 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 ratios of 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 become alloy powders.

The molybdenum-containing alloy powder is preferably produced by (DE 10 2006 057 004 A1) MoO ₂ , which has been pulverized to reduce the size distribution of the aggregates, as molybdenum source. This _MoO2 is added to an oxalic acid suspension, as used in EP 1079950 B1, for the preparation of FeNi or FeCoNi mixed oxalates, which are subsequently heated under oxidative conditions and reduced to alloy powders by means of hydrogen. The alloy powders obtained in this way are sufficiently reduced after reduction with hydrogen, ie MoO ₂ can no longer be detected by means of X-ray diffraction. Optionally, the aggregate size is further reduced by means of deagglomeration, with the aim of improving dispersibility in mixed milling with carbides. The aggregate consists of primary particles agglomerated with each other. Aggregate size and aggregate distribution can be measured by means of laser light scattering and sedimentation.

Instead of MoO ₂ it is also possible to use other particulate Mo compounds which are not soluble in oxalic acid, for example sulfides or carbides. They are oxidized to oxides during air calcination of precipitated oxalates. Molybdenum oxides such as _MoO3 are formed during calcination and, due to their high vapor pressure, form mixed oxides with Fe(Co)Ni mixed oxides very quickly and exhibit good transport properties , thus forming a FeCoNi alloy powder (which is homogeneously alloyed with a small proportion of Mo) in the subsequent reduction with hydrogen.

However, other known methods are also suitable; for example, precipitation with ammonium salt of oxalic acid instead of oxalic acid, precipitation with Na or K hydroxide, precipitation with formic acid and maleic acid. In all cases, preference is given to using MoO ₂ , which should be the purest possible phase and contain only traces of Mo or MoO ₃ or Mo ₄ O ₁₁ . The reason for using _MoO2 is because it is neither acid nor alkali soluble compared to _MoO3 and thus remains completely in the alloy metal powder throughout the process. _MoO3 will dissolve into the base used to precipitate the Fe(Co)Ni component or into the complexed organic acid; elemental Mo will be too coarse and will not be fully oxidized to _MoO3 in the subsequent calcination, so in Insufficient alloying during reduction with hydrogen. Fine _MoO2 with high specific surface area is completely oxidized to _MoO3 (which has a high vapor pressure) during the air calcination of Fe(Co)Ni oxalate, and via the gas phase, molybdates and these metal oxides are formed. mixed oxides, which produced a very uniform distribution of molybdenum, which was preserved in the subsequent hydrogen reduction.

It is already known to use the alloyed Mo-containing powders according to the invention for the production of sintered parts by means of solid phase sintering, as in the diamond tool industry, but not for hard metals in which a molten phase is formed in the middle during the sintering process industry.

However, particular preference is given to Mo-alloyed FeCoNi powders which contain Mo in entirely metallic form. In these powders, Mo oxides can no longer be detected by means of X-ray diffraction, so the oxygen present must be very predominant on the powder surface. Very particularly useful powders according to the invention are powders whose FSSS value is from 0.5 to 3 μm, since this increases the dispersibility in the mixing mill. In this case, they contain as little as possible of additional metals in the form of oxides.

Because during the sintering of hard metals molybdenum oxide reacts with carbon to form CO and thus leads to local carbon deficiency and thus local eta phase, precautions are taken during hard metal sintering to ensure that mainly carbon monoxide The alloy powders described in the previous paragraph are suitable for hard metal production when oxygen released in the form is able to escape from the sintered body. These powders are suitable for use according to the invention 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 The stated extent is permissible from the point of view of microstructural defects (pores and binder lakes) in the hard metal.

According to the invention, Mo-alloyed powders based on FeCoNi or FeNi can additionally be alloyed with up to 20% tungsten, for example to shift the onset of sintering shrinkage to higher temperatures or to induce the formation of precipitates (which enhance binder phase), but this is only possible with 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%), it was found that the binder alloy system has a certain proportion of martensitic phase after sintering, so it has 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 a very low hot hardness in sintered hard metals. In the range of about 80-25% Fe, the binder alloys are found to be austenitic after sintering, and although they have 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 hot hardness of hard metals at 400-600° C. is lower than those with pure Co as binder, if no Mo or other alloying elements are additionally mixed into the alloy. Although a particularly preferred objective for use of the present invention is the production of hard metals with increased hot hardness, it is also suitable for the production of hard metals with other objectives, such as molybdenum-containing molybdenum currently produced using elemental or carbide molybdenum. Hard metals for corrosion resistant binder alloy systems (as described in EP 0028620 B2), or cutting machine inserts for drill bits (as described in US5305840).

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

After sintering and, if appropriate, final machining by grinding or electro-erosion, the hard metal part has a defined tool geometry. This can particularly preferably be elongated (for example ground from sintered round rods), but also particularly preferably sheet-shaped, 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 superhard layers (eg 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, stamping tools, Pentalobe bits for weld-in, mining cutting machines, milling tools for concrete and asphalt milling, rotary mechanical seals and any other geometry and application.

The invention is illustrated in detail by the following examples.

Example

Embodiment 1 (comparative example, not the present invention)

462.5 g of tungsten carbide 0.6 µm were mixed with 37.5 g of FeCoNi alloy powder 40/20/40 (Ampersint ^® MAP A6050; manufacturer: HC Starck, Germany) and 0.57 L of 94% ethanol in a ball mill at 63 rpm Grind for 14 hours. 5 kg of hard material balls are used for this purpose. The FeCoNi powder used has the following properties: Fe 38.8%, Co 20.22%, Ni 40.38%, O 0.71%, specific surface area=1.63m ² /g, FSSS value=0.90. 2 batches were produced with different carbon contents ("high carbon" and "low carbon"), which resulted in different carbon contents after sintering. The results are shown in the table below.

Ethanol was separated from the resulting suspension by vacuum distillation, and the obtained hard metal powder was uniaxially compressed at 150 MPa and sintered under reduced pressure at 1450 °C for 45 minutes. The pieces of flake hard metal were ground, polished and checked for their properties. As sintered bodies, both batches exhibited no eta phase and no carbon precipitation, but relatively small binder deposits. In both cases, the room temperature hardness and the hot hardness at selected temperatures up to 800° C. were measured under protective gas. Figure 1 shows the results: both batches exhibit a large drop in hot hardness in the region of about 600°C. So this binder alloy is significantly inferior to pure cobalt for the production of hard metal tools for metal turning under relatively high stresses due to the plastic deformation of the cutting edge due to the expected cutting forces , which is due to the low hot hardness (especially at 600°C).

carbon "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

Embodiment 2 (comparative example, WC-Co, not the present invention)

WC—Co with 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% by weight, based on the total hard metal. Compression and sintering at reduced pressure and 1420°C for 45 minutes produced a defect-free hard metal with a magnetic saturation of 133 G·cm ³ /g, which corresponds to 82% of the theoretical magnetic saturation. Room temperature hardness (HV30 1597 kg/mm ² ) and hot hardness were measured and plotted in FIG. 1 . It can be seen that Co is superior to the FeCoNi binder from 350 to 800 °C, above which the carbide skeleton is the main factor determining the hot hardness. The K ₁ C (fracture toughness, determined by the crack length at the corner of the hardness indentation and calculated by Shetty's formula) of hard metal at room temperature is 10.1 MPa·m ^1/2 . The cobalt binder thus additionally has a better hardness/K ₁ C relationship than the binder of Example 1 at room temperature.

Embodiment 3 (comparative example, not according to the present invention)

Example 1 was repeated and 1% by weight of Mo metal powder was added to the first batch and 3% by weight of Mo metal powder was added to the second batch. (These contents are related to the Mo content of the binder alloy phase). The deagglomerated molybdenum metal powder had the following properties: FSSS value: 1.09, O content: 0.36% by weight. The particle size distribution is determined with the following parameters: D ₅₀ 3.2 µm, D ₉₀ 6.4 µm. The carbon content was chosen based on the experience of Example 1 such that neither eta phase nor carbon precipitation can be expected in the sintered hard metal. No additional carbon is taken into account for the Mo addition, so that the molybdenum is present as completely as possible in metallic form in the binder alloy. The carbon content of the formulation is thus 5.94 and 5.94% (3% by weight Mo, based on binder). The results after sintering at 1420°C are shown in the table below. Hot hardness was measured as before and is represented by circles in Figure 2:

Mo addition in 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 binder lakes Very profuse and sometimes large binder lakes

Surprisingly, the eta phase does not occur at both 1 wt% and 3 wt% Mo; more precisely, carbon porosity occurs even at 1 wt% Mo. Compared to Example 1, the hardness is surprisingly increased without a decrease in the K ₁ C value, thus obtaining at room temperature a combination of properties equal to Co-bonded hard metals and superior to pure FeCoNi-bonded hard metal. Surprisingly, 1 wt% molybdenum is sufficient in the binder; at 3 wt% molybdenum, no strong change in K ₁ C and hardness compared to 1% Mo is observed. Molybdenum alloyed into the binder thus not only increases the intrinsic hardness of the binder, but also simultaneously increases the fracture toughness. The behavior differs in this respect from the case of alloyed W: in Co-based hard metals and in FeCoNi-based materials, see Example 1, although an increase in the intrinsic hardness of the binder is also found here, but here at the same time There is also a decrease in K ₁ C value.

However, a very high binder lake appeared, evidence of Mo dissolution into the binder, which then filled the pore volume formed. However, these binder deposits are unacceptable in hard metals.

A comparison of these hot hardnesses of Example 2 is shown in FIG. 2 . The hot hardness at all temperatures up to 800° C. is surprisingly even lower than that of Example 1.

Embodiment 4 (the present invention)

Example 1 was repeated, but in which a FeCoNi binder alloy alloyed with 1.5% by weight of Mo produced by the method described in DE 102006057004 A1 was used. The powder is then deagglomerated. The properties of this powder analyzed are: Fe 38.23 wt%, Co 19.96 wt%, Ni 39.10 wt%, Mo 1.55 wt%, O 0.8565 wt%, FSSS value: 1.21, specific surface area = 2.17 m ² /g, D ₅₀ =3.46 µm, D ₉₀ =5.84 µm. Even after prolonged exposure, MoO ₂ can no longer be detected by X-ray diffraction at its characteristic diffraction angle. 37.5 g of this powder were used together with 462.5 g of WC to produce hard metal. The carbon content of the hard metal mixture was 5.92% by weight, which was set by adding 1.14 g of carbon black. The compressed bodies were sintered both in open and closed crucibles. This variation has an effect on the carbon content of the sintered hard metal. The properties of hard metals sintered at 1420°C are as follows:

sintering open crucible 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 Binder-free starch Binder-free starch

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

A further compact body was produced and sintered at 1420° C. under reduced pressure, but applying argon at a pressure of 40 bar until the end of sintering at the final temperature. Cooling is done under pressure. A hard metal block was obtained with the following parameters: hardness 1643 HV30, fracture toughness 8.2 MPa·m ^1/2 and magnetic saturation 123 G·cm ³ /g. Both room temperature hardness and hot hardness are measured as a function of temperature, on a hard metal block, on a separate hardness testing machine. The results of the evaluation of both room temperature hardness and hot hardness are shown in Figure 2, represented by squares, and the curves of Examples 2 and 3 are drawn for comparison: the hardness of Example 4 compared with the cobalt-bonded hard metal The hot hardness of the metal at 600°C is significantly lower than those of Example 2. The hot hardness is now higher than that of the hard metal (Example 3) produced with binder alloy powder which was not alloyed with Mo. (Due to belonging to another hardness testing machine, there is a difference in room temperature hardness).

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

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

Claims (12)

1. Use of molybdenum-containing binder alloy powder for the production of hard metals based on liquid phase sintering of tungsten carbide, characterized in that: a) the FSSS value of the binder alloy powder used is 0.5-3 µm as measured in accordance with ASTM B 330, 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% by weight of Mo in alloyed or pre-alloyed form. 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% by weight of 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 1-4, wherein at least one constituent of the binder alloy is introduced as a powdered alloy of at least one metal with molybdenum, the binder alloy's The remaining constituents are introduced as elements or alloys, none of which 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. Pre-alloyed powders containing 0.1-65% by weight of iron, 0.1-60% by weight of cobalt, 10-80% by weight of nickel and 0.1-20% by weight of molybdenum in metallic form, wherein the FSSS according to ASTM B 330 The value is a maximum of 3 µm, and the rest of the powder is unavoidable impurities. 10. The pre-alloyed powder according to claim 9, which additionally contains up to 10% by weight of tungsten in alloyed or pre-alloyed form. 11. Pre-alloyed powder according to one or both of claims 9-10, characterized in that the pre-alloyed powder contains 0.1-20% by weight of molybdenum. 12. Pre-alloyed powder according to one or more of claims 9-11, characterized in that it contains 0.1-65% by weight of iron and 10-60% by weight of nickel.

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