EP2097549A2

EP2097549A2 - Metal powder

Info

Publication number: EP2097549A2
Application number: EP07847463A
Authority: EP
Inventors: Bernd Mende; Gerhard Gille; Ines Lamprecht
Original assignee: HC Starck GmbH
Current assignee: HC Starck GmbH
Priority date: 2006-12-02
Filing date: 2007-11-28
Publication date: 2009-09-09
Also published as: DE102006057004A1; ZA200903076B; WO2008065136A2; US20120132033A1; RU2468111C2; RU2009125183A; WO2008065136A3; UA95824C2; US20100061879A1; JP2013224491A; NO20092142L; JP2010511782A; IL198570A0; US8133297B2; MX2009005542A

Abstract

The invention relates to novel pre-alloyed metal powders a method for production and use thereof.

Description

description

Metal powder technical field

Alloy powders have many applications for the production of sintered moldings by powder metallurgy. The main feature of powder metallurgy is that corresponding powdered alloy or metal powders are pressed and then sintered at a sufficiently high temperature. This method is introduced on an industrial scale for the production of complicated moldings which can otherwise only be produced with a high degree of complex finishing or, as in the case of liquid phase sintering, e.g. Hard or heavy metals where no technological alternatives exist.

Generally, as the sintering temperature increases, the porosity decreases, i. E. the density of the sinter is approaching its theoretical value. For reasons of strength, therefore, the sintering temperature is chosen as high as possible and also the adjustment of certain phases, compositions, etc. requires correspondingly high sintering temperatures and times. On the other hand, however, the hardness of the metallic matrix drops above an optimum temperature again, since it comes as a result of grain growth to a coarsening of the structure (Ostwald ripening). For these reasons, such powders are advantageous for sintered bodies which already achieve their theoretical density and suitable phase formation at the lowest possible sintering temperatures.

State of the art

So there are also a number of proposals, metallic

Alloy powder by precipitation, z.T. in the presence of organic phases, and subsequent reduction (WO 97/21 844, US 5,102,454, US 5,912,399, WO 00/23 631).

The object of the invention is to provide alloy powder, ie pre-alloyed metal powder containing at least the metals iron, cobalt and molybdenum, which meet the stated requirements for sintered materials available. It has long been known that FeCoMo-based steels within certain compositional ranges, with appropriate heat treatment, form the intermetallic compounds (FeCo) 7Mθ6, which provide very high hardnesses and strengths and are an alternative to the steels produced by certain applications carbidic precipitates are hardened (Köster _f W .: Mechanical and magnetic precipitation hardening of the iron-cobra-Woifram- and iron-cobalt-molybdenum alloys, Archive for the iron and steel industry, 1932, No. 1 / JuIi, p 17-23). However, the production of such steels by melting the components and casting in an inert atmosphere is complicated and has hitherto found no entry into industrial practice.

Recently, there have been successful investigations to prepare such FeCoMo steels by powder metallurgical means by mixing the individual powders and to test their properties (Danninger, H. et al., Heat Treatment and Properties of preconditioned hardened carbon-free PM Tool Steeis, Powder Metallurgy Progress, VoS.5 (2005), No.2, pp. 92-103). The disadvantage here is that the formation of intermetallic compounds requires several heat treatments at high temperatures and zeitiichem effort to achieve a uniform diffusion of metals into one another and subsequent formation and finely dispersed distribution of these phases in the structure as a carrier of the desired properties.

Successful commercialization depends on whether the production of such sintered pieces can be realized at a reasonable cost. A decisive advantage would therefore be very sinter-active powders which contain all components in homogeneous form with a uniform microstructure and isotropic properties and avoid cost-intensive temperature treatments.

Presentation of the invention

In the previously known Puivern is particularly disadvantageous that they contain the components in inhomogeneous distribution and therefore require high temperatures to homogenize the components to achieve diffusion. In addition, the sintering behavior depends on the heating rate (rapid heating requires higher final temperatures or longer holding times) and the sintered bodies only achieve insufficient sintering densities, ie they have a corresponding porosity.

It has surprisingly been found that in the presence of molybdenum in a hydrometallurgically produced FeCo matrix mixed oxides, such as CoMoθ4 and FeMoO ₄ already arise upon quenching in air, from which in a subsequent reduction under hydrogen at moderate conditions very fine-grained and high sinterable metal powders are formed, which contain the Mo in dissolved, homogeneous form. Surprisingly, the FeCoMo alloy powders thus produced differ from the prior art in that their sintering behavior is almost independent of the heating rate during sintering (FIG. 5) and significantly higher sintering densities and thus lower porosities are achieved under comparable sintering conditions than when mechanically mixed powders are used , In addition to the advantageous sintering behavior, noticeably higher hardnesses are also achieved (Table 2).

The invention is first a method for producing the pre-alloyed metal powder by mixing aqueous Metallallsalziösungen with a Fäilungsmittel, preferably a carboxylic acid solution, separating the precipitate of the mother liquor and reduction of the precipitate to the metal, wherein advantageously the Fäliungsmittel superstoichiometric and as concentrated aqueous Solution is used. The metal salt solutions and / or the aqueous solution of the precipitating agent may additionally contain dispersed solid compounds. As precipitant, the aqueous solution or suspension of a carboxylic acid, a hydroxide, carbonate or basic carbonate can be used.

In this case, the metal salt solution can be mixed with the precipitant, but advantageously the metal salt solution is added to the Fäilungsmittel. Preferably, the precipitate is washed after separation from the mother liquor with water and dried.

The reduction of the precipitate is preferably carried out in a hydrogen-containing atmosphere at temperatures between 600 ⁰ C and 850 ° C. The reduction can be carried out in indirectly heated rotary kilns or in push-through furnaces. Other ways to carry out the reduction are readily apparent to those skilled in the art, such. B. in deck ovens or fluidized bed ovens. This is surprising, since Mo oxides can be reduced with hydrogen only above 1000 ° C to Mo metal powders with sufficiently low oxygen contents, which are necessary for a powder metallurgical further processing and sintering.

At these relatively high reduction temperatures inevitably occurs a greater grain size, which reduces the sintering activity.

According to a preferred embodiment of the invention, the moist or dried precipitate is calcined before reduction in an oxygen-containing atmosphere at temperatures between 250 ⁰ C and 600 ⁰ C. On the one hand, the calcination causes the precipitate product consisting of polycrystalline particles or agglomerates to be comminuted by decrepitation by the gases released in the decomposition (of the carboxylic acid residue), so that larger surfaces and shorter diffusion paths are available for the subsequent, diffusion-controlled gas phase reaction (reduction) and a finer-grained end product is obtained. On the other hand, a pre-alloyed metal powder with significantly reduced porosity is obtained. In the further processing of the precipitated product [(mixed) metal carboxylic acid salt] to the pre-alloyed metal powder, a significant volume reduction of the particles continues to occur, which leads to the formation of pores. Through the intercalated calcination step in an oxygen-containing atmosphere, the precipitated product is first transferred into the (mixed) rnetalloxid and annealed, so that a pre-compression takes place with the healing of defects clusters and micropores. In the subsequent reduction in a hydrogen-containing atmosphere, accordingly, only the volume shrinkage from the oxide to the metal has to be overcome. By the intermediate calcination step accordingly a stepwise volume shrinkage is achieved, each taking place with structural stabilization of the intermediate crystals.

Suitable precipitants are carboxylic acids, but also hydroxides, carbonates or basic carbonates, in particular of the alkali or alkaline earth metals, advantageously of sodium or potassium. These are in particular hydroxides of the alkali or alkaline earth metals, very particularly sodium or potassium hydroxide.

Suitable carboxylic acids are aliphatic or aromatic, saturated or unsaturated mono- or dicarboxylic acids, in particular those having 1 to 8 carbon atoms. Due to their reducing action, formic acid, oxalic acid, acrylic acid and crotonic acid are preferred, and because of their availability, especially formic and oxalic acid, most preferably oxalic acid are used. The carboxylic acids can be used as aqueous solution or suspension, but also in pure form as precipitant, when the carboxylic acid is liquid.

An excess of reducing carboxylic acids prevents the formation of Fe (III) ions, which would lead to a reduction in the yield.

Preferably, the carboxylic acid with 1.1 to 1, 6-fold stoichiometric excess, based on the metals used. Particularly preferred is a 1, 2-1, 5-fold excess.

According to a further preferred embodiment of the invention, a carboxylic acid solution is used as the suspension agent as a suspension containing undissolved carboxylic acid (as a suspension). The preferred carboxylic acid suspension contains a depot of undissolved carboxylic acid, from which the carboxylic acid removed by precipitation of the solution is replenished, so that a high concentration of carboxylic acid is maintained in the mother liquor during the entire precipitation reaction. Preferably, the concentration of dissolved carboxylic acid in the mother liquor at the end of the precipitation reaction is still at least 10% of the saturation concentration of the carboxylic acid in water, in particular 20% of the saturation concentration of the carboxylic acid in water. This will be a complete and largely identical precipitation of the MetaJisalze ensured. The alloy composition of the pre-alloyed powders can thus be determined by the choice of the composition of the metal salt solution.

Other suitable precipitants are hydroxides, carbonates or basic carbonates. These are, in particular, hydroxides of the alkali metal or alkaline earth metals, with sodium or potassium hydroxide being advantageous. These can be used analogously to carboxylic acids, also with respect to their use in the form of a solution or suspension as described above for the carboxylic acid.

Although the precipitant may indeed be added to Metailsalzlösung, but advantageously the solution or suspension of the precipitant is presented and added the metal salt solution.

According to the particularly preferred embodiment of the method according to the invention, the addition of the Metallisäiziösung to the carboxylic acid suspension is carried out gradually, in such a way that the content of dissolved carboxylic acid in the mother liquor during the feeding of Metailsalzlösung a value of 50% of the solubility of the Carboxylic acid in water does not fall below. In particular, the addition of the metal salt solution is particularly preferably carried out gradually so that the concentration of dissolved carboxylic acid does not fall below 80% of the solubility in water until the suspended carboxylic acid is dissolved. The rate of addition of the metal salt solution to the carboxylic acid suspension thus takes place in such a manner that the removal of carboxylic acid from the mother liquor, including concentration reduction by dilution by the water supplied with the metal salt solution, is compensated by the dissolution of undissolved, suspended carboxylic acid.

As a metal saiz for the preparation of Metailsalzlösung all water-soluble compounds can be used. Preferably, the chlorides or sulfates of the metals are used, so that in each case a metal chloride or sulfate solution is used. It is also possible to use mixed chlorides and sulfates, in which, for example Ferric chloride and cobalt sulfate are used to prepare the metal salt solution. Preferably, the concentration of the metal salt solution is about 1.6 to 2.8 moles of metal per liter.

Preferably, the metal salt solution has a content of 20 wt .-% to 90 wt .-% iron, based on the Gesamtmetallgehatt, and the elements cobalt and molybdenum. The content of iron in the metal salt solution is particularly preferably between 25% by weight and 85% by weight, very particularly preferably more than 30% by weight to 70% by weight, in each case based on the total metal content.

More preferably, the metal salt solutions contain up to 65 wt .-% cobalt, based on the Gesamtmetallgehait, advantageously 5 wt .-% to 50 wt .-%, in particular 10 wt .-% to 30 wt .-%. The molybdenum content of the metal salt solution is 3% by weight to 60% by weight, preferably 4% by weight to 50% by weight, in particular 5% by weight to 40% by weight, particularly advantageously 6% by weight to 35% by weight, 9% by weight to 30% by weight, 12% by weight to 20% by weight or 14% by weight to 19% by weight.

As Molybdänsaiz molybdenum dioxide MoOΣ is advantageously used. For example, since MoO 2 is insoluble, it can be suspended in the metal salt solution. However, it may also be suspended in the solution or suspension of the precipitant to which the metal salt solution is preferably added as described above.

With regard to the precipitation of the metal salts, a concentrated carboxylic acid solution has the "activity 1", a half-concentrated carboxylic acid solution has the "activity 0.5". Accordingly, the activity of the mother liquor should preferably not fall below 0.8 during the addition of the metal salt solution.

For example, the solubility is preferably used

Oxalic acid in water about 1, 1 moles per liter of water (room temperature), corresponding to 138 g of oxalic acid (with 2 moles of water of crystallization). According to the preferred method according to the invention, the oxalic acid should be presented as an aqueous suspension containing from 2.3 to 4.5 moles of oxalic acid per liter of water. This suspension contains about 1.2 to 3.4 moles of undissolved oxalic acid per liter of water. After initiation of the Metal salt solution and completed precipitation should be the content of oxalic acid in the mother liquor still 15 to 30 g / l. During the introduction of Metailsalziösung in the oxalic acid suspension, the precipitated oxalic acid is constantly replaced by dissolution of suspended oxalic acid. For homogenization of the mother liquor it is constantly stirred. According to the preferred embodiment, the addition of the metal salt solution is carried out gradually such that the oxalic acid concentration in the mother liquor during the addition does not fall below 69 g, more preferably not below 110 g per liter of mother liquor. This causes a sufficiently high supersaturation is constantly achieved during the addition of the metal salt solution, which is sufficient for nucleation, ie for the production of additional precipitation particles. As a result, on the one hand, a high nucleation rate, which correspondingly only leads to small particle sizes, is guaranteed and, on the other hand, owing to the small concentration of metal ions present in the mother liquor, agglomeration of the particles by anionic substances is largely prevented.

The inventively preferred high carboxylic acid concentration during the precipitation also causes the precipitate with respect to the relative contents of metals has largely the same composition as the metal salt solution, d. H. a homogeneous precipitate with respect to its composition and thus alloy metal powder is produced.

The invention furthermore relates to pre-alloyed metal powders which contain the elements iron, cobalt and molybdenum and which advantageously have an average particle size according to ASTM B330 (FSSS) of less than 8 μm, advantageously from 0.1 μm to 8 μm, in particular 0, 5 μm to 3 μm,

The BET surface area of the pre-alloyed powders is generally more than 0.5 m ² / g, advantageously 0.7 m ² / g to 5 m ² / g, in particular 1 m ² / g to 3 m ² / g ,

The alloy powders contain 20 wt .-% to 90 wt .-% iron, preferably 25 wt .-% to 85 wt .-% and particularly preferably 30 wt .-% to 70 wt .-% iron, based on the total metal content. More preferably, the pre-alloyed Metaüpulver contain up to 65 wt .-% Co ₁ advantageously 5 wt. ~% wt to 50 wt .-%, in particular 10. ^'% to 30 wt .-%. The molybdenum content of the metal powders is 3 wt .-% to 60 wt .-%, preferably 4 wt .-% to 50 wt .-%, in particular 5 wt .-% to 40 wt .-%, particularly advantageously 6 or 7 wt. % to 35% by weight, 9% by weight to 30% by weight, 12% by weight to 20% by weight or 14% by weight to 19% by weight, further constituents of the alloying powder be unavoidable impurities.

The present invention therefore also relates to alloy powder containing

From 20% to 90% by weight of iron,

Up to 65% by weight of cobalt,

3 wt .-% to 60 wt .-% molybdenum;

Or

From 20% to 90% by weight of iron,

Up to 65% by weight of cobalt,

9 wt .-% to 30 wt .-% molybdenum;

Or

From 20% to 90% by weight of iron,

To 65% by weight of cobalt,

From 12% to 20% by weight of molybdenum;

Or

From 20% to 90% by weight iron,

Up to 65% by weight of cobalt,

14 wt .-% to 19 wt .-% molybdenum.

Advantageously, alloy powder containing

From 25% to 85% by weight of iron,

From 5% by weight to 50% by weight of cobalt,

4 wt .-% to 50 wt .-% molybdenum;

Or

From 25% to 85% by weight iron,

From 5% by weight to 50% by weight of cobalt,

9 wt .-% to 30 wt .-% molybdenum;

Or

From 25% to 85% by weight of iron, From 5% by weight to 50% by weight of cobalt,

From 12% to 20% by weight of molybdenum;

Or

From 25% to 85% by weight of iron,

From 5% by weight to 50% by weight of cobalt,

14 wt .-% to 19 wt .-% molybdenum.

Particularly advantageous are alloy powder containing

From 30% to 70% by weight of iron,

10% by weight to 30% by weight of cobalt,

6 wt .-% to 35 wt .-% molybdenum;

Or

From 30% to 70% by weight of iron,

From 10% by weight to 30% by weight of cobalt,

From 9% by weight to 30% by weight of molybdenum;

Or

From 30% to 70% by weight of iron,

From 10% by weight to 30% by weight of cobalt,

From 12% to 20% by weight of molybdenum;

Or

From 30% to 70% by weight of iron,

10% by weight to 30% by weight of cobalt,

14 wt .-% to 19 wt .-% molybdenum.

Very particularly advantageous are containing alloy powder

From 45% to 70% by weight of iron,

From 16% by weight to 26% by weight of cobalt,

From 10% to 38% by weight of molybdenum;

Or

From 45% to 60% by weight of iron,

20% by weight to 26% by weight of cobalt,

15 wt .-% to 25 wt .-% molybdenum.

Also particularly advantageous are alloy powders in which the molybdenum content is less than 25 wt .-%, when the iron content is greater than 50 wt .-%; and or Alloy powder in which the cobalt content at 10 wt .-% to 30 wt.

% is when the sum of the molybdenum content and the iron content is less than 90% by weight. Advantageously, the remaining components of the

Alloy powder around unavoidable impurities. Alloy powders of the compositions according to Table 1 are particularly advantageous. Table 1: Advantageous compositions of the pre-alloyed metal powders according to the invention

Table 1

The X-ray diffractograms of the pre-alloyed powders according to the invention differ markedly from those prepared from purely mechanically mixed elemental powders. Advantageously, the molybdenum reflex is absent at 2Theta = 40.5 ° (CuKα radiation). Advantageously, the X-ray diffractogram has a reflection for (FeCo) ₇ MOe at 2Theta = 37.5 °.

After the sintering, the prealloyed metal powders according to the invention achieve a higher hardness than metallic powder mixtures of the same chemical composition (see Table 2). The sintered bodies obtained from the pre-alloyed metal powder have densities of at least 97%, advantageously greater than 98.5%, but in particular values greater than 99% of the theoretical density. These values can rarely be achieved in powder metallurgy processes. The shaped articles obtained by sintering the pre-alloyed powder have high Rockwell hardnesses of more than 50 HRC, in particular more than 55 HRC, and very particularly preferably after sintering more than 60 HRC. Depending on subsequent heat treatments ("tempering"), in particular high Rockwell hardnesses of generally greater than 60 HRC are achieved, because of the achievable high sintering density, they can be immediately sintered close to the final shape, so that little or no post-processing is required According to the invention, they are free from fracture generated by milling and are available immediately after reduction with this grain size, ie the fineness of the primary grains is determined by the chemical production process but not by mechanical processes such as mahal, sifting, screening etc. The metal powders pre-alloyed according to the invention have a low carbon content of less than 0.04% by weight, preferably less than 0.02% by weight and most preferably less than 0.005% by weight to the temperature carried out between precipitation and reduction treatment in an oxygen-containing atmosphere, in which the organic carbon present after the precipitation is removed. Preferred pre-alloyed metal powders furthermore have an oxygen content of less than 1% by weight. However, the composition of the powders according to the invention is not limited to the elements iron, cobalt and molybdenum. Although the alloy powders according to the invention advantageously contain only these metals and unavoidable impurities, additional metals M selected from the group consisting of tungsten, copper, nickel, vanadium, titanium, tantalum, niobium, manganese, and aluminum may additionally be contained. Advantageously, tungsten or copper may be contained in amounts of up to 25 wt .-%. Copper is advantageously contained in amounts of up to 10% by weight, in particular 6.5 to 10% by weight. Nickel may also be present in amounts of up to 10% by weight, advantageously from 1% by weight to 10% by weight, in particular from 6.5 to 10% by weight. The alloy powder according to the invention particularly advantageously contains no nickel except for unavoidable impurities. Further Constituents of the alloy powder may still be unavoidable impurities

Further, the alloy powder according to the invention may contain vanadium, titanium, tantalum, niobium, manganese and aluminum. These additives are advantageously up to a maximum of 3 wt .-%, in particular 0.5 wt .-% to 3 wt .-% contained. Thus, a targeted adjustment of mechanical, thermal or electrical properties is possible. Advantageously, however, the alloy powder is free of the metals M selected from the group consisting of vanadium, titanium, tantalum, niobium, manganese and aluminum. Advantageously, the remaining components of the alloy powder are unavoidable impurities.

[099] The pre-alloyed metal powders are excellently usable for the powder metallurgical production of components. The invention therefore also relates to shaped articles obtainable by sintering a pre-alloyed metal powder according to the invention. These molded objects are suitable for applications that require high-temperature resistant components {mechanical stress at a continuous temperature greater than 500 ⁰ C) and are characterized by a high hot hardness (even at temperatures greater than 600 ⁰ C), high creep resistance, good heat conduction and good chemical Corrosion resistance. Therefore, these shaped articles are particularly well suited as cutting tools for austenitic steels or for parts of internal combustion engines, turbines, turbochargers, jet engines and the like.

Brief description of the drawings

The figures illustrate the properties of the alloy powders according to the present invention.

FIG. 1 shows an X-ray diffractogram of the mechanical powder mixture according to Example 5.

FIG. 2 shows an X-ray diffractogram of the powder according to the invention according to Example 4. FIG. 3 compares the results of thermodilatometric measurements during sintering of the mechanical powder mixture and of the

Alloy powder according to the invention. FIG. 4 shows the results of thermodiamtometric measurements in FIG

Sintering the mechanical powder mixture according to Example 5 in

Dependence on the heating rate. FIG. 5 shows the results of thermodilatometric measurements in FIG

Sintering the alloy powder according to the invention of Example 4 in

Dependence on the heating rate. FIG. 6 compares the sintering behavior of the mechanical powder mixture according to Example 5 and of the alloy powder according to the invention

Example 4 with repeated heating. FIG. 7 compares the behavior with repeated cooling of the mechanical powder mixture according to Example 5 and FIG

Alloy powder according to the invention after Example! 4. FIG. 8 compares the residual porosities of the sintered bodies ex mechanical

Powder mixture according to Example 5 and the alloy powder according to the

Invention according to Example 4. Figure 9 compares the hot strengths of the alloy powder after the

Examples 6, 7 and 8, wherein improved hot curing at

Molybdenum contents of greater than 6 or 7 wt .-% can be seen. Way (s) for carrying out the invention The invention will be explained below by means of examples.

Example 1

In a stirred vessel 90 L of deionized water were introduced and with constant stirring 33.95 kg of iron! L-ch! orid with 4 moles of crystal water (FeCi2 * 4H ₂ O) and 17.82 kg of CobaJt-II sulfate with 7 moles of water of crystallization (CoSO ₄ THaO) dissolved therein and homogenized. In parallel, 114 L of deionized water were placed in a second stirred tank and 32.51 kg of oxalic acid with 2 moles of water of crystallization {(COOH) ₂ * 2H ₂ O} and 3 kg of molybdenum dioxide (MoO ₂ ) dissolved or dispersed therein and homogenized with vigorous stirring , The following became The FeCo mixed salt solution is pumped into the original of oxalic acid and molybdenum dioxide with a dosing pump with a volume flow of approx. 2 L / min. After completion of the precipitation, the precipitation suspension was stirred for a further 30 min to adjust the equilibrium and then filtered to separate the precipitate from the mother liquor through a suction filter and washed free of deionized water of chloride and sulfate ions.

The nutschfeuchte precipitated product was calcined in a push-through furnace at about 550 ⁰ C with air in countercurrent and reduced in a subsequent push-through furnace at 750 ⁰ C in a hydrogen atmosphere to the metal powder. The analytical test gave the following values: 58.38 wt.% Fe / 24.65 wt.% Co / 15.27 wt.% Mo / 0.63 wt.% Oxygen. The content of carbon was 17 ppm. The grain size, measured with FSSS (ASTM B 330), was found to be 0.85 μm and the specific surface area (ASTM D 4567) measured to be 1.46 m ² / g.

Example 2

In a stirred tank 93 L of deionized water were introduced and with constant stirring 32.79 kg of iron! l-chloride with 2 moles of crystal water (FeCl2 * 2H ₂ O) and 12.83 kg of Co-ll sulfate with 7 moles of water of crystallization (CoSO4 ^* 7H2O) dissolved therein. In parallel, 120 L of deionized water were placed in a second stirred tank and 34.29 kg of oxalic acid with 2 moles of water of crystallization {(COOH) ₂ * 2H ₂ O} and 2.40 kg of molybdenum dioxide dissolved or dispersed therein. Subsequently, the FeCo mixed salt solution was pumped with a metering pump with a Voiumenstrom of about 2 L / min in the template of oxalic acid and Moiybdändioxid. After completion of precipitation, the precipitate suspension was stirred for a further 30 min to adjust the equilibrium, then filtered to remove the precipitate from the mother liquor through a suction filter and washed free of deionized water of chloride and sulfate ions.

The nutschfeuchte precipitated product was calcined in a push-through furnace at about 550 ⁰ C with air in countercurrent and in a subsequent Push-through furnace at 750 ° C in a hydrogen atmosphere reduced to metal powder. In the analytical test, the following values were measured: 69.11% by weight of Fe / 17.73% by weight of Co / 12.21% by weight of Mo / 0.46% by weight of oxygen. The content of carbon was 21 ppm. The grain size, measured with FSSS (ASTM B 330), was determined to be 0.97 ppm, the specific surface area (ASTM D 4567) was 1.08 m ² / g.

Example 3

20.8 L of deionized water were initially introduced into a stirred vessel and, with constant stirring, 5.91 kg of Fe (II) chloride (FeCl ₂ .2H ₂ O), 2.33 kg of Co-Il suifate (CoSO ₄ .7H ₂ O) and 1, 09 kg of Cu sulfate (CuSO ₄ * 5H ₂ O) dissolved therein. In the clear solution, 436 g of molybdenum dioxide were homogeneously dispersed with vigorous stirring. In parallel, 23.9 L of deionized water were placed in a second stirred tank and 6.83 kg of oxalic acid {(COOH) ₂ ^* 2H ₂ O} dissolved or suspended therein. Subsequently, the FeCoCu mixed salt solution with the dispersed MoO _{2 was} pumped into the original of oxalic acid with a metering pump. After completion of precipitation, the precipitation suspension was stirred for a further 30 min, then filtered through a suction filter and washed free of deionized water of chloride and sulfate ions.

The wet-moist precipitate was calcined in a chamber furnace in the presence of air at 550 ⁰ C and reduced in a second chamber furnace in H2 atmosphere at 725 ⁰ C to MetallpuSver.

In the analytical test, the following values were determined: 61, 57 wt .-% Fe / 16.34 wt .-% Co / 11, 30 wt .-% Mo / 9.98 wt .-% Cu / 0.647 wt .-% oxygen. The content of carbon was 14 ppm. Grain size (ASTM B 330) was measured to be 1.35 μm and the specific surface area (ASTM D 4567) to be 1.41 m ² / g.

Example 4

After the procedure and conditions of Example 1 was a

Metal powder with the composition 58.02% by weight Fe / 24.64% by weight Co / 15 _f 03% by weight Mo / 0.774% by weight oxygen and a residual content produced by 26 ppm carbon. The grain size was determined to be 0.67 μm and the specific surface area to be 2.15 m ² / g. This prealloyed powder is hereinafter called "powder according to the invention".

Comparative Example! 5

To compare the properties of the individual powders of the elements involved a mechanical Puivermischung was produced in largely the same composition. 1800 g of carbonyl iron powder ex BASF (5-9 μm), 750 g of cobalt metal powder ex Umicore, grade SMS (0.9 μm) and 450 g of molybdenum metal powder ex HCStarck (1.3 μm) in a Turbula mixer with the addition of beads for 60 min mixed intensively. The resulting powder mixture is hereinafter referred to as "mechanical powder mixture".

The analytical control gave 59.94 wt.% Fe / 24.80 wt.% Co / 14.46 wt.% Mo / 0.61 wt.% Oxygen and 141 ppm carbon. The grain size was measured at 1.88 μm (FSSS) and the specific surface at 0.78 m ² / g.

In comparison, the powders according to Examples 4 and 5 show very different X-ray diffraction patterns. The mechanical powder mixture according to Example 5 shows distinct, separate reflections for the components Fe, Co and Mo (see FIG. 1), which are virtually no longer detectable in the prealloyed inventive powder according to Example 4 (FIG. 2); obviously, the components Co and Mo are dissolved in the Fe matrix.

The different structure of the mechanical powder mixture and the powder according to the invention leads to different sintering behavior, which is shown in the thermo-parametric analysis, see Figure 3. For testing, green bodies were prepared by cold isostatic pressing at 221 MPa and in dilatometer 402 E Netzsch apparatus construction GmbH sintered under hydrogen atmosphere.

The mechanical powder mixture shows several shrinkage levels

(See length change rates - dashed line), which complicate the design sintering of dense and fabric optimized components. The powder according to Example 4 according to Example 4 shows in addition to the α / γ phase transition {cubic body-centered in the cubic surface-centered crystal lattice) of FeCo matrix at about 900 ° C, only one, very sharply defined, shrinkage level between 1000-1200 ⁰ C. Der Phase transition occurs in the mechanical powder mixture with a widening of the volume, in the powder according to the invention, however, with a shrinkage.

The sintering behavior of the mechanical powder mixture according to Example 5 depends on the heating rate, see FIG. 4. Faster heating rates shift the shrinkage in the direction of higher temperatures. Surprisingly, the shrinkage of the powder according to the invention according to Example 4, in contrast, practically independent of the heating rate, see Figure 5.

The different Sinterverhaiten remains even with repeated heating (Figure 6) and cooling (Figure 7) is obtained, so is a reproducible property of the respective powder.

The sintering intervals from the dilatometric investigations at different heating rates and uniform cooling rate were investigated for sintering density, porosity and hardness. The results are summarized in Table 2. Surprisingly, despite significantly lower green densities after pressing, the powder according to the invention systematically achieves higher sintering densities with correspondingly lower porosity and significantly higher hardnesses compared to the mechanical powder mixture according to Example 5. The residual porosity remaining after sintering for the mechanical powder mixture according to Example 5 and the powder according to the invention Example 4 shows FIG. 8 (each 10 K / min heating rate) [0132]

Table 2

Table 2: sintered densities and curing of the thermodilatometric

sintered samples

Mechanical powder mixture according to the invention powder

Example 5 according to Example 4 theoretical density: 8.40 g / cm ³ theoretical density: 8.40 g / cm ³

Green density% of theor. Density Green density% of theoretical density in one case

Pressing pressure of 221 of 221

MPa MPa

(g / cm ³ ) (g / cm ³ )

5.59 66.2 4.6 54.4

Heat Sinter% of HärStandard Sinter% of HärStandard rate density theor. s density theor. te abw. s

K / min (g / cm ³ ) Density HRC (g / cm ³ ) Density HRC

1 8,20 97,6 49,4 0,95 8,37 99,6 56,1 0,76

3 8.23 98.0 47.7 0.31 8.34 99.3 56.2 0.31

10 8,20 97,6 49,6 3,59 8,32 99,0 56,1 0,21

20 8.23 98.0 51.7 7.68 8.39 99.9 56.3 0.46

Sintering temperature: 1370 ⁰ C, isothermal for 60 min

Cooling rate>: uniform 1C) K / min

Examples 6, 7 and 8.

Analogously to Examples 1 to 4, alloy powders were prepared by the process of Example 1 via the Oxalatfällung, calcination and reduction under hydrogen, whose compositions are listed in Table 3 and wherein all alloying elements are present in pre-alloyed form. The molybdenum content was as in the Salt solution suspended molybdenum dioxide introduced during the oxalate precipitation. The alloy powders became "as produced" below to rectangular

Chopsticks pressed and sintered. The green bodies were produced uniaxially with a pressure of 374 MPa. Green densities were realized by 5 g / cm ³ , corresponding to about 60% of the theoretical density. Sintered in the belt furnace under hydrogen, the sintering time was a total of 8h including preheating and cooling time. The

Sintering temperatures and results are summarized in Table 4. Table 3: Composition of the produced alloys

Table 3

Table 4: Sintering temperatures and results

Table 4

To determine the heat curing, rods with a width of about 8 mm were used. The heat-hardening tests on the sintering rods were carried out according to the method Vickers hardness HV5 (Vickers hardness with 5 kg load) with a calibrated micro-macro hardness test system. Prior to the measurement, the samples were ground plane-parallel and the measuring surface was polished. The hardnesses were determined at 22 ° C (room temperature), 500 ⁰ C, 700 ° C and 900 ⁰ C. The Haitezeit the samples at the specified temperatures was 10 minutes, the holding time during impression setting for 20 seconds there were 5 measurements made. Table 5 and Figure 9 show the results achieved, s denotes the standard deviation. It is clear that materials having a molybdenum content of more than 6 wt .-% lower Hersteliungs- and measurement conditions considerably higher Vickers hardnesses HV5 in the temperature range from room temperature up to 700 ⁰ C having hardness of the sample of Example 6 is almost twice as high as the hardnesses of the samples of Examples 7 and 8 (159 in Example 6 versus 84 and 82 in Examples 7 and 8). Table 5: Heat curing of the alloys

Table 5

Meßtemp. Example 6 Example 7 Example 8

CO Hardness S Hardness S Hardness S

HV5 HV5 HV5

22 515 14 295 11 185 5

500 499 20 262 16 130 4

700 159 5 84 2 82 5

900 31 1 26 1 23 1

Claims

claims

1. Pre-alloyed metal powder containing the elements iron, cobalt and molybdenum.

2. Pre-alloyed metal powder according to claim 1 containing 20 wt .-% to 90 wt .-% iron, up to 65 wt .-% cobalt and 3 wt .-% to 60 wt .-% molybdenum.

3. Pre-alloyed meta powder according to claim 1 or 2 having an average particle size according to ASTM B 330 of less than 8 microns and a BET specific surface area greater than 0.5 m ² / g.

4. Pre-alloyed metal powder according to one of claims 1 to 3 having a carbon content of less than 0.02 wt .-%.

5. Pre-alloyed metal powder according to one of claims 1 to 4, containing up to 25 wt .-%, in particular 6.5 to 10 wt .-% molybdenum and / or copper,

6. Pre-alloyed metal powder according to one of claims 1 to 5, containing 1 wt .-% to 10 wt .-% nickel.

The pre-alloyed metal powder according to any one of claims 1 to 6, containing up to 3% by weight of a metal selected from the group consisting of titanium, niobium, vanadium, tantalum, manganese and aluminum

8. A process for the preparation of pre-alloyed metal powders containing the elements iron, cobalt and molybdenum, by mixing aqueous Metailsalzlösungen with a precipitant, separating the precipitate from the mother liquor and reducing the Fäliungsproduktes to metal.

9. The method according to claim 8, characterized in that the precipitate prior to the reduction to the metallic alloy powder of a thermal decomposition at 200 ⁰ C to 1000 ⁰ C in an oxygen-containing atmosphere is subjected.

10. The method according to claim 8 or 9, characterized in that a saturated aqueous Garbonsäurelösung is used as precipitant.

11. The method according to claim 10, characterized in that the aqueous carboxylic acid solution contains solid carboxylic acid in an amount such that the mother liquor after completion of the precipitation is still saturated to at least 10%, based on metal salt-free aqueous solution.

12. The method according to any one of claims 8 to 11, characterized in that the metal salt solution is introduced into the initially introduced aqueous carboxylic acid solution.

13. The method according to any one of claims 8 to 12, characterized in that aqueous metal salt solution and carboxylic acid are introduced continuously into a precipitation reactor and continuously from the Fällunsgprodυkt containing mother liquor is withdrawn.

14. Use of a prealloyed metal powder according to one of claims 1 to 7 for the powder metallurgical production of components.

15. A molded article obtainable by sintering a pre-alloyed metal powder according to any one of claims 1 to 7.