EP1144702A1

EP1144702A1 - Hard material sintered compact with a nickel- and cobalt-free, nitrogenous steel as binder of the hard phase

Info

Publication number: EP1144702A1
Application number: EP99962163A
Authority: EP
Inventors: Werner Hesse; Hans Wohlfromm; Peter Uggowitzer; Markus Speidel
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1998-12-01
Filing date: 1999-11-25
Publication date: 2001-10-17
Also published as: KR100674048B1; AU1860100A; IL143348A; RU2001118043A; JP2002531693A; KR20010080641A; CN1333839A; TW509726B; IL143348A0; US6641640B1; CN1113968C; DE19855422A1; WO2000032828A1

Abstract

The invention relates to hard material sintered compacts with a nickel- and cobalt-free, nitrogenous steel as binder of the hard phase. The invention further relates to a method for producing said hard material sintered compacts by methods of powder metallurgy, especially by powder injection molding, and to powder injection molding masses for producing said hard material sintered compacts by powder injection molding.

Description

Hard material sintered molded part with a nickel and cobalt-free, nitrogen-containing steel as a binder of the hard material phase

description

The present invention relates to hard material sintered molded parts, as well as feedstocks and methods for their production.

In the context of this invention, hard material sintered molded parts are referred to as sintered materials which consist of a hard material phase and a metallic phase as a binder of the hard material phase. Hard material sintered parts, starting materials and processes for their production are well known. Hard sintered molded parts are usually very hard and have a high melting point, but are also resistant to temperature changes and therefore represent a valuable group of materials. They are used, for example, for combustion chamber or nozzle linings, cutting, drilling, milling, grinding, crushing, Digging or pressing tools, sealing or bearing rings, welding electrodes, thread guides or the like are processed. Among the known hard material sintered molded parts, those are sought as materials whose hard material portion consists of ceramic hard materials such as oxides, nitrides or carbides. The most commonly used hard materials are tantalum and tungsten carbide. As the metallic binder, a metal is to be selected which can be processed well into the hard material sintered molded part, does not impair the required properties of the material and binds the hard material phase in a suitable manner. The most commonly used metals by far are nickel and cobalt, but occasionally other metals are used that meet the required properties. For example, JP-A 63-317 601 discloses the use of a cobalt-nickel alloy as a metallic binder. US Pat. No. 3,964,878 teaches hard material sintered molded parts with metal carbides, the metallic binder of which consists of the metal also contained in the carbide and an additional 0.5 to 1.5% by weight of iron, copper or Nikkei. EP-A 169 292 and FR-A 1 475 069 teach hard material sintered molded parts with a metallic binder made of iron, nickel and / or cobalt, the metallic binder of the hard material sintered molded parts disclosed in EP-A 365 506 additionally contains a special high-speed steel , and the metallic binder of the hard material sintered parts disclosed in JP-A 58-031 059 contains iron, nickel, cobalt and / or molybdenum. US-A 4,308,059 teaches a ruthenium bonded hard material sintered molding. EP-A 46 209 discloses a hard material sintered molded part with steel as a metallic binder. Hard material sintered molded parts often also show color properties that lead to an attractive exterior of the workpieces made from them and are therefore not only used as a material for purely functionally determined components, but also as a material in decorative applications such as watch cases, jewelry,

Writing utensils or the like used. An example of a known hard material sintered molded part is JP-A 48-018 109, which discloses hard material sintered molded parts consisting of TaC and a metallic binder containing nickel, molybdenum and chromium and having a gold-like surface and their use in watch cases.

Hard sintered molded parts are usually produced using powder metallurgy. For this purpose, a mixture of the hard material powder and a metallic powder is brought into a mold, usually pressed, and then sintered, the metal and hard material powders combining to form the hard material sintered molded part. The sintered molded part can then be further processed as such, for example post-treatment, or used, but also ground and applied as a hard material sintered molded part powder as a surface layer on a workpiece. For example, DE-A 40 37 480 teaches the production of a sintered body from tungsten, titanium, tantalum or niobium carbide and cobalt as a metallic binder.

A major disadvantage of simple powder metallurgy shaping processes, such as pressing into a mold, is that only shaped bodies with a comparatively simple outer shape can be produced with them. Another known powder metallurgical process, which is particularly suitable for the production of sintered bodies with complex geometry, is powder injection molding. For this purpose, a sinterable powder is mixed with a thermoplastic, which is usually also called "binder" in powder injection molding technology (but must not be confused with the metallic phase referred to as "binder" of the hard material in the technology of hard material sintered molded parts), and if appropriate mixed with other auxiliaries, so that a thermoplastic injection molding compound (“feedstock”) is formed. This is injection molded into a mold using the injection molding technology known from the processing of thermoplastic materials, and the injection molded body (“green body”) is then used to produce the thermoplastic powder injection molding. Binder removed ("debinding"), and the body freed from this binder ("Braunling") sintered into the finished sintered molding. The main problem with these processes is the debinding, which is usually carried out thermally by pyrolysis of the thermoplastic, which often causes cracks in the workpiece. It is therefore advantageous if one becomes catalytically removable at low temperatures Thermoplastic used. EP-A 413 231 teaches such a catalytic debinding process, EP-A 465 940 and EP-A 446 708 disclose feedstocks for the production of metallic moldings, and EP-A 444 475 discloses a feedstock for the production of ceramic moldings. It is also known from EP-A 443 048 to produce hard material sintered molded parts by powder metallurgy, EP-A 800 882 teaches an improved method for producing hard material sintered molded part feedstocks.

US Pat. No. 5,714,115 describes a special nickel-free austenitic steel alloy with a maximum of 0.3% by weight of carbon, 2 to 26% by weight of manganese, 11 to 24% by weight of chromium, 2.5 to 10% by weight of molybdenum , as well as a maximum of 8% by weight of tungsten, the austenitic structure of which is stabilized by 0.55 to 1.2% by weight of nitrogen. This alloy is used for workpieces that are or may come into contact with the human body in order to avoid the allergic reactions to nickel or cobalt, which have recently become increasingly worrying. W.-F. Bahre, PJ Uggowitzer and MO Speidel: "Competitive Advantages by Near-Net-Shape-Manufacturing" (ed. H.-D. Kunze), Deutsche Gesellschaft für Metallurgie, Frankfurt, 1997 (ISBN 3-88355-246-1) and H. Wohlfromm, M. Blömacher, D. Weinand, E.-M. Langer and M. Schwarz: "Novel Materials in Metal Injection Molding", Processes of PIM-97 - Is European Symposium on Powder Injection Molding, Munich Trade Fair Center, Munich, Germany, October

15-16, 1997, European Powder Metallurgy Association 1997, (ISBN 1-899072-05-5) describe powder injection molding processes for the production of nickel-free nitrogen-containing steels with embossing during the sintering process.

Despite the well-developed state of the art, given the importance of the material class, the task is to find new hard material sintered parts with improved or new properties and widened or new areas of application.

Accordingly, hard-material sintered molded parts with a nickel- and cobalt-free, nitrogen-containing steel were found as a binder in the hard material phase. Furthermore, a process and feedstocks for the production of the hard material sintered molded parts according to the invention were found.

The sintered molded parts according to the invention have excellent mechanical, thermal and magnetic properties. They are hard, have a high melting point and high thermal shock resistance, are non-magnetic in preferred embodiments, and also do not cause any nickel or cobalt allergies. They also show no giant grain growth during sintering, and they can be polished very well. They can be produced in a simple manner using the method according to the invention; in particular, only a comparatively low sintering temperature is required when producing the hard material sintered molded parts according to the invention, compared to the use of Nikkei or cobalt binders.

The hard material sintered molded parts according to the invention contain at least 50% by weight, preferably at least 70% by weight and in a particularly preferred manner at least 80% by weight of hard material. They also contain at most 99% by weight, preferably at most 97% by weight and in a particularly preferred manner at most 95% by weight of hard material. Accordingly, the hard material sintered molded parts according to the invention contain at least 1% by weight, preferably at least 3% by weight and in a particularly preferred manner at least 5% by weight and at most 50% by weight, preferably at most 30% by weight and in particular preferably at most 20% by weight of metallic binder.

The metallic binder of the hard material sintered molded parts according to the invention, its precursor or its constituents, and the hard material are used in the form of fine powders. The average particle sizes used are usually in the range below 100 micrometers, preferably below 50 micrometers, and in a particularly preferred form below 20 micrometers, and generally above 0.1 micrometer. Such powders are commercially available or can be produced in any known manner, for example by precipitation and calcining, grinding, and the metallic powders, in particular by water or gas atomization.

A hard material is contained in the hard material sintered molded parts according to the invention. All known ceramic materials or hard metals that have already been used as hard materials in known hard material sintered molded parts can be used as hard material individually or as a mixture, for example the oxides such as aluminum oxide, cadmium oxide, chromium oxide, magnesium oxide, silicon dioxide, thorium oxide, uranium oxide and / or zirconium oxide , the carbides such as boron carbide, zirconium carbide, chromium carbide, silicon carbide, tantalum carbide, titanium carbide, niobium carbide and / or tungsten carbide, the borides such as chromium boride, titanium boride and / or zirconium boride, the silicides such as molybdenum silicide and / or the nitrides such as silicon nitride , Titanium nitride and / or zirconium nitride, and / or mixed phases such as carbonitrides, oxicarbides and / or sialones. The use of tantalum carbide, tungsten carbide, niobium carbide, titanium nitride and / or zirconium nitride is preferred; the use of tantalum carbide is particularly preferred. carbide and / or tungsten carbide. These hard materials are well known and common commodities.

The metallic binder of the hard material sintered molded parts according to the invention is a nickel and cobalt-free, nitrogen-containing steel. Freedom from nickel and / or cobalt is understood to mean the absence of intentionally added portions of these elements. The permissible upper limit for nickel and / or cobalt in the metallic binder of the hard material sintered molded parts according to the invention is generally 0.5% by weight, preferably 0.3% by weight and in a particularly preferred manner 0.05% by weight. At these levels, the usual limits for the release of nikkei and / or cobalt ions when using the workpiece on or in the human body (as a watch, ear plug, implant, etc.) are normally not reached. In a very particularly preferred manner, the metallic binder contains nickel and / or cobalt exclusively as inevitable impurities. The steel used as the metallic binder contains nitrogen, preferably in an amount of at least 0.3% by weight and at most 2% by weight.

The metallic binder is preferably a non-ferromagnetic and in particular an austenitic steel. Austenitic steels are known to be those in which there is a face-centered cubic lattice of the iron atoms. The austenite structure in the iron / carbon system is a high-temperature modification that is stabilized by certain alloy additives at low temperatures. Depending on the requirement profile, additional alloy additives give the austenitic steels toughness, corrosion resistance, hardness or other desired properties. The manufacture, processing and properties of austenitic steels are well known to the material expert.

In a particularly preferred form, the metallic binder is an austenitic iron alloy which contains at most 0.5% by weight of carbon, 2 to 26% by weight of manganese, 11 to 24% by weight of chromium, 2.5 to 10% by weight. % Molybdenum, a maximum of 8% by weight tungsten and 0.55 to 1.2% by weight nitrogen. In addition to the elements mentioned, it preferably contains no other, with the exception of inevitable impurities. Examples of impurities which can usually be tolerated in the hard material sintered molded parts according to the invention are up to 0.5% by weight of nickel and / or cobalt, up to 2% by weight of silicon, up to 0.2% by weight Sulfur, up to 5% by weight bismuth and up to 5% by weight copper.

The very particularly preferred metallic binder of the hard material sintered molded parts according to the invention is austenitic and contains at most 0.3% by weight of carbon, preferably it contains at most 0.1% by weight of carbon. It contains at least 2% by weight of manganese, preferably at least 6% by weight, and at most 26% by weight of manganese, preferably at most 20% by weight. It contains at least 11% by weight of chromium and at most 24% by weight of chromium, preferably at most 20% by weight. It also contains at least 2.5% by weight of molybdenum and at most 10% by weight of molybdenum, preferably at most 6% by weight. If particularly high corrosion stability is required, the metallic binder of the hard material sintered molded parts according to the invention contains tungsten in an amount of at most 8% by weight, preferably at most

6% by weight. It also contains at least 0.55% by weight of nitrogen, preferably at least 0.7% by weight, and at most 1.2% by weight of nitrogen, preferably at most 1.1% by weight. This metallic binder also contains iron in addition to the elements mentioned, preferably the entire rest is 100% by weight, with the exception of impurities, iron.

Alloys of this type are known to the person skilled in the art, are commercially available or can be produced in a simple manner by known metallurgical processes. Since the nitrogen content of these alloys is above 0.8 to 0.9% by weight higher than the nitrogen solubility in the molten alloy, the alloy must be melted under increased nitrogen pressure, for example with the pressure electro-slag remelting process. It is equally possible to introduce the nitrogen content into the metallic binder of the otherwise finished sintered molded part in a nitridation step (“nitriding”) by heat treatment in a furnace atmosphere containing nitrogen. However, the nitrogen content is preferably obtained by nitridation during the sintering or immediately before or after this, without interim removal of the sintered molded part from the sintering furnace or cooling below the sintering or nitriding temperature, such sintering and nitriding processes are known to the person skilled in the art.

In the case of a subsequent nitridation, the corresponding nitrogen-free or a low-nitrogen alloy is to be used as the precursor of the actual metallic binder, which alloy then converts to the metallic binder of the hard material sintered molded part according to the invention in the course of the nitridation process. These alloys are also commercially available or can be melted in a known manner. In the case of the preferred austenitic binder, the corresponding nitrogen-free precursor is a ferritic steel, which is converted into an austenitic steel by the nitriding. It is also possible to make the metallic binder or its nitrogen-free precursor according to the “master-alloy” technique known to the person skilled in the art from a master alloy or several master alloys which essentially contain or contain elements other than iron and, if appropriate, a proportion of iron , and to produce pure iron, so that the metallic binder according to the invention only forms during the sintering and / or nitriding process by diffusion of the alloying elements, possibly including nitrogen.

The hard material sintered molded parts according to the invention are produced by powder metallurgy. For this purpose, the hard material and the binder or its precursor are mixed in powder form and brought into a shape with a shaping tool which comes as close as possible to the desired final geometric shape in order to avoid any time-consuming finishing of the finished hard material sintered molded part. The shaping step is carried out using a conventional shaping tool, for example a press. As is well known, the workpieces shrink during sintering, which is usually compensated for by correspondingly larger dimensioning of the molded parts before sintering. The molding is then sintered in a sintering furnace to form a hard material sintered part and, if a precursor of the metallic binder which is free of nitrogen or low in nitrogen is used, the desired nitrogen content is set by nitriding.

The optimal composition of the furnace atmosphere for sintering and optionally nitriding and the optimal temperature control depend on the exact chemical composition of the metallic binder or its precursor used, in particular its nitrogen solubility, on the desired nitrogen content of the metallic binder and on the grain size of the powder used from. In general, both the increase in the nitrogen partial pressure in the furnace atmosphere and the decrease in the temperature are directly correlated with higher nitrogen contents in the metallic binder. However, since the lowering of the temperature not only slows down the sintering process itself, but also reduces the rate of diffusion of nitrogen in the metallic binder of the hard material sintered part, the sintering and / or nitridation process takes correspondingly longer at a lower temperature. The optimum combination of furnace atmosphere, in particular the nitrogen partial pressure, temperature and duration of sintering and / or nitriding to achieve a certain desired nitrogen content in a homogeneous, dense sintered molded part, can easily be determined in individual cases using a few routine tests. Such sintering processes are for sintered molded parts made of metal in a particularly preferred form Binder used steel without a hard material phase, for example in the publications by Bahre et al. and Wohlfromm et al. described. We expressly refer to these two publications. The properties of the steel 5 do not change due to the presence of the hard material phase, so that the measures described there bring about the same effects in the method according to the invention.

Nitrogen partial pressures are usually in the furnace atmosphere.

10 re of at least 0.1, preferably at least 0.25 bar, applied. This nitrogen partial pressure is generally at most 2 bar, preferably at most 1 bar. The furnace atmosphere can consist of pure nitrogen or contain inert gases such as argon and / or reactive gases such as hydrogen. Most of the time it is

It is advantageous to use a mixture of nitrogen and hydrogen as the furnace atmosphere in order to remove potentially disruptive oxidic impurities from the metals. The proportion of hydrogen, if present, is generally at least 5% by volume, preferably at least 15% by volume, and generally at most

20 50 vol .-%, preferably at most 30 vol .-%. If desired, this furnace atmosphere can also contain inert gases, for example argon. The oven atmosphere should preferably be largely dry, generally a dew point of - 40 ° C is sufficient.

25

The (absolute) pressure in the sintering and / or nitridation furnace is usually at least 100 mbar, preferably at least 250 mbar. It is also generally at most 2.5 bar, preferably at most 2 bar. In a particularly preferred manner

30 worked at normal pressure.

The sintering and / or nitridation temperature is generally at least 1000 ° C., preferably at least 1050 ° C. and in a particularly preferred manner at least 1100 ° C. It is also

35 generally at most 1450 ° C, preferably at most 1400 ° C and in a particularly preferred manner at most 1350 ° C. The temperature can be varied during the sintering and / or nitridation process, for example in order to completely or largely densely sinter the workpiece only at a higher temperature

40 and then set the desired nitrogen content at a lower temperature.

The optimal heating rates are easily determined by a few routine tests, usually they are at least 1 ° C. per 45 minutes, preferably at least 2 ° C. per minute and in a particularly preferred manner at least 3 ° C. per minute. Economic considerations generally result in the highest possible heating rate in order to avoid a negative influence on the quality of the sintering and / or nitridation, however, a heating rate below 20 ° C per minute will usually have to be set. Under certain circumstances, it is advantageous to heat up the sintering and / or or the nitriding temperature to maintain a waiting time at a temperature which is below the sintering and / or nitriding temperature, for example over a period of 30 minutes to two hours, for example one hour, a temperature in the range from 500 ° C. to 700 ° C., for example 600 ° C.

The sintering and / or nitriding time, that is to say the holding time at the sintering and / or nitriding temperature, is generally set so that the sintered molded parts are both sufficiently densely sintered and sufficiently homogeneously nitrided. At usual sintering and / or nitridation temperatures, nitrogen partial pressures and molded part sizes, the sintering and / or nitridation time is generally at least 30 minutes and preferably at least 60 minutes. This duration of the sintering and / or nitridation process also determines the production rate, which is why the sintering and / or nitridation is preferably carried out in such a way that the sintering and / or nitridation process does not take an unsatisfactorily long time from an economic point of view. In general, the sintering and nitriding process (without the heating and cooling phases) can be completed after a maximum of 10 hours.

The sintering and / or nitridation process is ended by cooling the sintered molded parts. Depending on the composition of the binder, a specific cooling process may be necessary, for example, cooling as quickly as possible in order to maintain high-temperature phases or to prevent the components of the steel from segregating. In general, it is also desirable for economic reasons to cool down as quickly as possible in order to achieve a high production rate. The upper limit of the cooling rate is reached when sintered molded parts occur in economically unsatisfactorily large quantities with defects such as cracking, tearing or deformation due to rapid cooling. The optimal cooling rate is therefore easily determined in a few routine tests. In general, and particularly in the preferred compositions of the metallic binder, it is advisable to use cooling rates of at least 100 ° C per minute, preferably at least 200 ° C per minute. The sintered molded parts can be quenched, for example, in cold water or oil. Subsequent to sintering and / or nitriding, any desired aftertreatment, for example solution annealing and quenching in water or oil or hot isostatic pressing of the sintered molded parts can be carried out. The sintered moldings are preferably solution-annealed by being at a temperature of at least 1000 ° C., preferably at least 1100 ° C. and at most 1250 ° C., preferably over a period of at least 5 minutes, preferably at least 10 minutes and at most 2 hours, preferably at most one hour at most 1200 ° C under inert gas, for example under nitrogen and / or argon, are heat treated and then quenched, for example in cold water.

The hard material sintered molded parts according to the invention are preferably produced using the powder injection molding process and in particular for producing geometrically complex shaped workpieces. This differs in the implementation of conventional powder metallurgical processes such as pressing and sintering by the type of shaping and an additional step required to remove the thermoplastic powder injection molding binder used for shaping. However, what has been said above applies to sintering and nitriding.

To carry out this process, the hard material and metal powders are mixed with a thermoplastic, non-metallic material as a powder injection molding binder, and the powder injection molding composition is thus produced. Suitable thermoplastics for the production of injection molding compositions are known. Thermoplastic materials are mostly used, for example polyolefins such as polyethylene or polypropylene or polyethers such as polyethylene oxide

("Polyethylene glycol"). Preference is given to the use of thermoplastics which can be removed catalytically from the green compact at a comparatively low temperature. A polyacetal plastic is preferably used as the base of the thermoplastic, and in a particularly preferred form polyoximethylene ("POM", paraformaldehyde , Paraldehyde) is used. The injection molding compound is optionally admixed with auxiliaries to improve its processing properties, for example dispersing aids. Comparable thermoplastic compositions and processes for their production and processing by injection molding and catalytic debinding are known and are described, for example, in EP-A 413 231, EP-A 465 940, EP-A 446 708, EP-A 444 475 and EP- A 800 882 described, to which reference is hereby expressly made. The metallic or ceramic powders specified there must be replaced accordingly by a powder mixture of the hard material and the metallic binder or its precursor. A preferred injection molding composition according to the invention consists of:

a) 40 to 65% by volume of a mixture of al) 50 to 99% by weight of a hard material in powder form with an average particle size of at least 0.1 micrometer and at most 100 micrometer and a2) 1 to 50% by weight of one nickel- and cobalt-free, nitrogen-containing steel or a precursor of such steel in powder form with an average particle size of at least 0.1 micrometer and at most 100 micrometer; b) 35 to 60% by volume of a mixture of bl) 70 to 90% by weight of a polyoxymethylene homo- or copolymer with up to 10 mol% of comonomer units and b2) 10 to 30% by weight of a polyoxymethylene copolymer a comonomer content of 20 to 99 mol% of poly-1, 3-dioxolane, poly-1, 3-dioxane or poly-1, 3-dioxepane, or one homogeneously dissolved in bl) or with an average particle size of less than 1 micrometer in bl) dispersed polymer or mixtures thereof as thermoplastic powder injection molding binder of the powder mixture a), and c) 0 to 5 vol .-% of a dispersing aid.

The known ceramic materials or hard metals used in known hard material sintered molded parts are used individually or as a mixture as hard material al), for example oxides such as aluminum oxide, cadmium oxide, chromium oxide, magnesium oxide, silicon dioxide, thorium oxide, uranium oxide and / or zirconium oxide, carbides such as boron carbide, zirconium carbide, chromium carbide, silicon carbide, tantalum carbide, titanium carbide, niobium carbide and / or tungsten carbide, borides such as chromium boride, titanium boride and / or zirconium boride, silicides such as molybdenum silicide and / or nitrides such as silicon nitride, titanium nitride and / or zirconium nitride / or mixed phases such as carbonitrides, oxicarbides and / or sialones. The hard material is preferably tantalum carbide, tungsten carbide, niobium carbide, titanium nitride and / or zirconium nitride, and in a particularly preferred manner the hard material is tantalum carbide and / or tungsten carbide.

An iron alloy which contains at most 0.5% by weight of carbon, 2 to 26% by weight of manganese, 11 to 24% by weight of chromium, is preferably used as the nickel- and cobalt-free, nitrogen-containing steel or precursor of such a steel a2). 2.5 to 10% by weight of molybdenum and a maximum of 8% by weight of tungsten. In a particularly preferred manner, an iron alloy is used as component a2) which contains at most 0.3% by weight, advantageously at most 0.1% by weight carbon, at least 2% by weight, advantageously at least 6% by weight , Manganese, at most 26% by weight, advantageously at most 20% by weight, manganese, at least

11% by weight of chromium and at most 24% by weight, advantageously at most 20% by weight, chromium, and furthermore at least 2.5% by weight of molybdenum and at most 10% by weight, advantageously at most 6% by weight , Contains molybdenum. If a particularly high corrosion stability of the hard material sintered molded parts ultimately to be produced from the injection molding compound is required, component a2) additionally contains tungsten in an amount of at most 8% by weight, preferably at most 6% by weight.

Component a2) contains iron in addition to the elements mentioned. A certain content of impurities in a2), which go beyond the level of unavoidable impurities, is usually tolerable depending on the application of the hard material sintered molded part. Examples of impurities that can usually be tolerated are up to 0.5% by weight of nickel and / or cobalt, up to 2% by weight of silicon, up to 0.2% by weight of sulfur, up to 5% by weight bismuth and up to 5% by weight copper. However, the entire rest of component a2) is preferably 100% by weight of iron, with the exception of inevitable impurities.

However, it is not necessary to use homogeneous alloys as component a2), rather the alloy elements can also be present as a mixture of different alloys and / or pure elements, from which, according to the “master alloy” technique, an alloy of the desired gross composition is formed by diffusion during the sintering process For example, component a2) can be a mixture of pure iron powder and an alloy powder which contains the other alloy elements and optionally also iron.

The hard material (component a1)) is preferably contained in a) in an amount of at least 70% by weight and in a particularly preferred manner in an amount of at least 80% by weight. Its amount is also preferably at most 97% by weight and in a particularly preferred manner at most 95% by weight. The metallic binder or its precursor (component a2)) is accordingly in a) preferably in an amount of at least 3% by weight and in a particularly preferred manner of at least 5% by weight and preferably of at most 30% by weight and in particularly preferably contain at most 20% by weight. The average particle sizes of hard material a1) and metallic powder a2) are preferred wise at most 50 microns and, in a particularly preferred form, at most 20 microns.

The polyoxymethylene homo- and copolymers used as components bl) and b2) and the polymers optionally used as component b2) homogeneously dissolved or dispersed in component bl) are known and, for example, as components B1) and B2), respectively, in EP-A 444 475. The homopolymerizates are usually produced by polymerizing (mostly catalyzed polymerisation) formaldehyde or trioxane. To produce polyoxymethylene copolymers, a cyclic ether or a plurality of cyclic ethers is or are conveniently used as comonomer (s) together with formaldehyde and / or trioxane in the polymerization, so that the polyoxymethylene chain with its sequence of (-0CH ₂ ) units is interrupted by units in which more than one carbon atom is arranged between two oxygen atoms. Examples of cyclic ethers suitable as comonomers are ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,3-dioxolane, dioxepane, linear oligo- and polyformals such as polydioxolane or polydioxepane and oximeethylene terpolymers. A polymer can also be used as component b2), for example an aliphatic polyurethane, aliphatic uncrosslinked polyepoxides, poly (C ₅ -C ₅ ) alkylene oxides, aliphatic polyamides, polyacrylates, polyolefins and mixtures thereof.

Components bl) and b2) can be identical except for a different comonomer (s) content.

Component c) is a dispersing aid. Dispersing aids are widespread and known to the person skilled in the art. In general, any dispersing aid can be used which leads to the improvement of the homogeneity of the injection molding compound. Preferred dispersing agents are oligomeric polyethylene oxide with an average molecular weight of 200 to 400, stearic acid, hydroxystearic acid, fatty alcohols, fatty alcohol sulfonates and block copolymers of ethylene and propylene oxide. A mixture of different substances with dispersing properties can also be used as the dispersing aid.

The injection molding compositions according to the invention are deformed in a conventional manner with conventional injection molding machines. The moldings are freed from the thermoplastic powder injection binder in the usual way, for example by pyrolysis. The powder injection molding binder is preferably removed catalytically from the preferred injection molding composition according to the invention by the green compacts in a known manner with a gaseous acid Atmosphere to be heat treated. This atmosphere is created by evaporating an acid with sufficient vapor pressure, conveniently by passing a carrier gas, in particular nitrogen, through a storage vessel with an acid, advantageously nitric acid, and then introducing the acidic gas into the debinding furnace. The optimal acid concentration in the debinding furnace depends on the specific material and the dimensions of the workpiece and is determined in individual cases through routine tests. In general, treatment in such an atmosphere at temperatures in the temperature range from 20 ° C. to 180 ° C. over a period of from 10 minutes to 24 hours will suffice for the debinding. After the debinding process, any residues of the thermoplastic powder injection binder and / or of the auxiliaries which are still present are pyrolyzed during heating to the sintering temperature and thereby completely removed.

The debindered injection-molded articles are then sintered and, if appropriate, shaped, which originate from other shaping processes, for example pressing, and optionally nitrided.

Examples

example 1

3600 g of tantalum carbide powder (average particle diameter 0.9 micrometer), 400 g of a ferritic alloy powder composed of 17% by weight of chromium, 3.4% by weight of molybdenum, 12.1% by weight of manganese, the rest being iron, with impurities still unavoidable , in particular 0.035% by weight of nickel and 0.6% by weight of silicon were present (average particle size 8 micrometers), 64 g of polyethylene glycol with an average molecular weight of approximately 800 g / mol, 43 g of polybutanediol formal with an average molecular weight Approx. 30,000 g / mol and 312 g of polyoxymethylene with a proportion of 2% by weight of butanediol formal were placed in a heatable kneader, melted by heating to 175 ° C. and homogenized for one hour by kneading. The mixture was then cooled and granulated. The granules were injection molded into molded parts, which were then catalytically debindered at 120 ° C. in a nitrogen atmosphere containing nitric acid. The moldings were then sintered in a sintering furnace at 1350 ° C. for one hour and then at 1280 ° C. for 5 hours in a furnace atmosphere composed of 75% by volume nitrogen and 25% by volume hydrogen. The sintered moldings were then still very weakly magnetic and were subjected to a subsequent heat treatment for 10 minutes at 1150 ° C. under nitrogen and subsequent sealing. scare completely non-magnetic in water. The density of the pale gold sintered molded parts was 13.2 g / ml (theoretical density 13.3 g / ml).

The sintered molded parts had a hardness HV 0.5 of 1400, a four-point bending strength according to DIN EN 843 (in the "as fired" state) of 774 MPa and a fracture toughness Kι _c according to DIN 51109 of 12 MPa (m ) ⁰ - ^5. The parts were ground and polished, the microstructure was homogeneous and showed no crystals with a diameter above 5 micrometers.

Comparative Example 1

3600 g tantalum carbide powder (average particle diameter 0.9 micrometer), 400 g nickel powder (average particle size less than 10 micrometers), 56 g polyethylene glycol with an average molecular weight of approx. 800 g / mol, 41 g polybutanediol formal with an average molecular weight of approx. 30,000 g / mol and 303 g of polyoxymethylene with a proportion of 2% by weight of butanediol formal were placed in a heatable kneader, melted by heating to 175 ° C. and homogenized for one hour by kneading. The mixture was then cooled and granulated. The granules were injection molded into molded parts, which were then catalytically debindered at 120 ° C. in a nitrogen atmosphere containing nitric acid. The moldings were then sintered in a sintering oven at 1500 ° C. for one hour under nitrogen. The density of the sintered molded parts was 13.6 g / ml (equal to the theoretical density). Lower sintering temperatures were used in parallel experiments, but these did not lead to a satisfactory sintering density.

The sintered molded parts had a hardness of HV 0.5 of 950, no further mechanical properties were determined. The parts were ground and polished. The microstructure was practically non-porous, however giant grain growth was observed, crystallite sizes up to more than 100 micrometers in diameter were created, which were visible to the naked eye and significantly impaired the optical appearance of the polished parts.

The comparison of Example 1 with Comparative Example 1 shows that the sintered molded parts according to the invention not only have excellent mechanical properties, but also have advantages in applications in which the optical impression of the sintered molded part is decisive.

Claims

claims

1. Hard material sintered part with a nickel and cobalt-free, nitrogen-containing steel as a binder of the hard material phase.

2. Hard material sintered part according to claim 1, wherein the binder is a non-ferromagnetic steel.

3. Hard material sintered molding according to claim 1, wherein the binder is an austenitic steel with at most 0.5% by weight of carbon, 2 to 26% by weight of manganese, 11 to 24% by weight of chromium, 2.5 to 10% by weight % Molybdenum, a maximum of 8% by weight tungsten and 0.55 to 1.2% by weight nitrogen.

4. hard material sintered part according to claim 1, wherein the hard material is tantalum carbide, tungsten carbide, niobium carbide, titanium nitride and / or zirconium nitride.

5. Hard material sintered part according to claim 3, wherein the hard material is tantalum carbide and / or tungsten carbide.

6. A process for the production of a hard material sintered part with a nickel- and cobalt-free, nitrogen-containing steel as a binder of the hard material phase, which involves the production of a molding from a powder mixture, a hard material powder and a nickel- and cobalt-free, nitrogen-containing steel or a precursor of one contains such steel, and comprises the sintering of the molded article, and, if a precursor of a nickel- and cobalt-free, nitrogen-containing steel is contained in the powder mixture, the conversion of this precursor into such a steel by nitriding.

7. The method according to claim 6, characterized in that a molding from a powder mixture containing a hard material - powder and a ferritic, nickel- and cobalt-free, nitrogen-free precursor of an austenitic, nickel- and cobalt-free, nitrogenous steel contains, and the precursor of the austenitic steel is converted into the austenitic steel by embroidery before, during or after the sintering.

8. The method according to claim 7, characterized in that the shaping by powder injection molding of a thermoplastic injection molding compound, in addition to the metallic and ceramic Powder contains a thermoplastic, and then carries out debinding.

9. Thermoplastic injection molding compound, which contains a metal powder, consisting of a nickel- and cobalt-free, nitrogen-containing steel or a precursor of such a steel, a hard material powder and a thermoplastic.