DE102014215337B4 - Powder metallurgical process for producing a workpiece and powder metallurgically manufactured workpiece - Google Patents

Abstract

Powder metallurgical method for producing a coin blank (7), in which the coin blank (7) is produced by sintering a powder composition (2) which contains at least two different powder components, at least one powder component of the powder composition being magnetic and at least one powder component of the powder composition not is magnetic, wherein a melting temperature of the at least one non-magnetic powder component is below or not more than 900 ° C above a melting temperature of the at least one magnetic powder component, characterized in that a mass fraction of the at least one magnetic powder component in the powder composition is between 2% and 40%, the at least one magnetic powder component being selected from one or more of the following material groups: material group A, consisting of ferritic steels, in particular comprising iron-chromium alloys, material ff group B, consisting of austenitic-ferritic steels, especially comprising duplex steels and lean duplex steels, - material group C, consisting of martensitic steels, particularly comprising cobalt steel, - material group D, consisting of ferro-alloys, in particular comprising ferrosilicon, ferromanganese and ferrochromium, - Material group F, consisting of hard magnetic steels, such as in particular cobalt alloyed steels, as well as aluminum-nickel-cobalt alloys, copper-manganese-aluminum alloys, silver-manganese-aluminum alloys, cobalt-samarium alloys, neodymium-iron-boron -Alloys, and wherein the at least one non-magnetic powder component is selected from material group H, comprising austenitic steels, in particular comprising austenitic chromium-nickel steels.

Description

The invention relates to a powder-metallurgical method for producing a workpiece in the form of a coin blank and a coin-produced coin blank and a correspondingly produced coin.

Powder-metallurgical manufacturing processes can be used to produce workpieces with special properties that cannot be produced in any other way and only with great effort. The properties of a workpiece produced by powder metallurgy, as well as that of a finished product which has been produced from such a workpiece, depend on the materials processed in the production and on the setting of the process parameters. Due to the large number of possible combinations of materials and process parameters, it can be very difficult or practically impossible to draw conclusions based solely on the special properties of a workpiece or finished part produced by powder metallurgy about the materials and process parameters required for its manufacture. For example, from the published patent application DT 26 33 323 A1 a powder metallurgical method for the production of coins is known, which pursues the goal of a special material marking of the coins, so as to improve the counterfeit security of the coins. The material labels described there can be checked, for example, by means of X-ray methods, neutron activation methods or metallographic methods. A powder metallurgical manufacturing process according to the preamble of the main claim as well as a workpiece produced by the process and a corresponding coin are described in the document US 4,592,780 A described. Another example of a powder metallurgical manufacturing process for a coin is from the publication JP H03- 258 206 A. known.

It is the object of the present invention to propose a method for producing a material which is characterized by the highest possible level of security against forgery, that is to say by properties which should be as simple as possible to verify but as difficult to reproduce as possible. The method should be carried out as simply and inexpensively as possible. Accordingly, a workpiece that is as forgery-proof as possible and that is as simple and inexpensive to produce as possible should also be proposed.

This object is achieved according to the invention by a powder metallurgical manufacturing method according to the main claim and by a workpiece in the form of a coin blank and a coin according to the subclaims. Further developments and preferred embodiments of the invention result from the features of the remaining claims. The features and advantages of the described method and its configurations are reflected in the workpiece produced accordingly and the coin formed therefrom.

Workpieces produced in the proposed manner, in particular coin blanks and coins made therefrom, are distinguished by an unusual and therefore easily verifiable combination of comparatively low electrical conductivity and comparatively high magnetic permeability, which is difficult to reproduce. This provides the desired security against counterfeiting in a simple manner, making these workpieces particularly suitable for the production of coins.

In the powder metallurgical process proposed here, a workpiece in the form of a coin blank is thus produced from a powder composition which contains at least two different powder components by sintering. Each of the powder components consists of a large number of powder particles or particles. It typically applies to each of the powder components that the particles of this powder component consist of the same material or material and thus in particular have the same electrical and magnetic properties. Therefore, when we speak of the (electrical or magnetic) properties of a powder component, this also means the (electrical or magnetic) properties of the powder particles of the powder component or of the material or material from which these powder particles are made.

According to the invention, the workpiece, also referred to as a sintered part, is a coin blank. The method can accordingly comprise further method steps in which the coin blank is further processed into a coin, in particular a course or circulation coin for public payment transactions, for example by embossing the coin blank. Since the production method proposed here and the workpiece proposed here, that is to say the coin blank, are particularly suitable for coin production, reference is made to this in particularly great detail. However, all information and descriptions can principally transferred to other areas of technology and used there to achieve corresponding advantages.

At least one of the powder components of the powder composition is magnetic, which means that this powder component has ferromagnetic or ferrimagnetic properties. The particles of these at least one powder components thus consist of a ferromagnetic or ferrimagnetic material. The materials and materials referred to here and below as magnetic have permanent magnetic dipoles as ferromagnetic or ferrimagnetic materials, which are below a material-specific temperature, e.g. the Curie temperature, spontaneously (i.e. without external fields) aligned relative to each other. Within so-called Weiss districts, the magnetic dipoles then each have an ordered relative orientation. In the case of a ferromagnet, the magnetic dipoles within the Weiss districts are typically aligned parallel to one another. In the case of a ferrimagnet, the orientation of the dipole moments can be anti-parallel (with dipole moments of different sizes) or with a fixed angle between the dipole moments. Magnetic materials also have a so-called (material-specific) hysteresis, which is characterized in particular by a so-called remanence (also referred to as residual magnetization or remanence magnetization) and a so-called coercive force (also referred to as coercive field strength). Depending on the hysteresis, a distinction is made between "soft magnetic" (small remanence and small coercive force) and "hard magnetic" materials (large remanence and large coercive force).

In addition, the powder composition contains at least one non-magnetic powder component, which is therefore not magnetic in the above sense and is therefore in particular neither ferromagnetic nor ferrimagnetic. For example, the powder particles of the at least one non-magnetic powder component can be diamagnetic or paramagnetic. Examples of non-magnetic materials and materials are given below.

The proposed method provides that a mass fraction of the at least one magnetic powder component in the powder composition is between 2% and 40%. This mass fraction is typically above 3%, preferably in a range between 10% and 40%. The mass fractions stated here and below always relate to the total mass of the powder composition. In the following, the mass fraction is also indicated as a percentage by weight, in short% by weight, again based on the total mass of the powder composition. The mass fraction of the at least one magnetic powder component in the powder composition is the sum of the mass fractions of the individual magnetic powder components in the powder composition in the case of several magnetic powder components. The same applies to the mass fraction of the at least one non-magnetic powder component.

The mass fraction of the at least one magnetic powder component is selected to be so large that the magnetic properties of the workpiece or the finished part produced therefrom, for example the coin, are so pronounced that they can be measured or ascertained in the simplest possible way, for example by means of electromagnetic Test methods, for example by means of (eddy current) induction, as will be described below with the aid of a concrete example, and at the same time not too strong in order not to cause disturbances, for example an undesirable magnetic adhesion of the workpieces or finished parts or coins to or on one another other components.

In order to ensure that the magnetic properties of the at least one magnetic powder component in the workpiece are not lost due to the sintering, it is also provided that a melting temperature of the non-magnetic powder component is so low compared to the melting temperature of the at least one magnetic powder component that the magnetic Powder particles do not completely disintegrate or do not completely dissolve during sintering of the powder composition, but remain intact completely or at least as far as is necessary to achieve the desired magnetic properties of the workpiece.

This is achieved in that the melting temperature of the at least one non-magnetic powder component is not more than 900 ° C. above the melting temperature of the at least one magnetic powder component. Depending on the respective powder composition, smaller melting temperatures of the at least one non-magnetic powder components can also be provided, so that the desired magnetic properties can be obtained after sintering. For example, it is possible that the melting temperature of the at least one non-magnetic powder component does not exceed 850 ° C, does not exceed 800 ° C, does not exceed 600 ° C, does not exceed 400 ° C, does not exceed 300 ° C, is not more than 200 ° C or not more than 100 ° C above the melting temperature of the at least one magnetic powder component. It is also possible that the melting temperature of the at least one non-magnetic powder component is lower than the melting temperature of the at least one magnetic powder component.

If the powder composition contains several non-magnetic powder components, the condition formulated for the melting temperatures should also apply to the highest melting of these non-magnetic powder components. If the powder composition comprises more than one magnetic powder component, then this condition must apply to at least one or more of these magnetic powder components, with a mass fraction of this at least one magnetic powder component in the powder composition requiring that it be at least 2%, preferably at least 3% , is, this mass fraction will typically even be at least 10%. If alloys with a melting interval are involved, the solidus temperature is always meant here as the melting temperature.

During sintering, the powder composition is heated to a sintering temperature so that the diffusion processes typical for sintering can take place, in which microscopic components (typically atoms) of the powder particles diffuse at least one of the powder components of the powder composition and mechanically after sintering (by formation of sinter necks) create stable connections between the powder particles and in this way bring about a cohesion of the powder composition (add the powder composition). The above-mentioned relations between the melting temperatures of the magnetic and non-magnetic powder components mean that the magnetic powder particles remain sufficiently stable during the (diffusion) processes taking place during sintering and are therefore largely retained. After sintering, the magnetic powder particles are typically present in their original grain size or at least in a grain size that is only slightly reduced in comparison to this. Therefore, even after sintering, it is still possible for white areas to form in the magnetic powder particles which are sufficiently large to achieve the desired magnetic properties.

The magnetic and electrical properties of the workpiece and thus also that of the finished part typically also depend on the grain size and the grain shape of the powder particles of the powder components. For example, the magnetic powder particles (as well as the non-magnetic powder particles) can be spherical or non-spherical, such as elongated or acicular. The average grain size of the magnetic powder particles is preferably in a range between 20 μm and 100 μm. The average grain size of the non-magnetic powder particles is preferably in a range between 20 μm and 100 μm. The mean grain size and / or the grain shape of the magnetic powder particles can be different from the mean grain size or the grain shape of the non-magnetic powder particles. For example, in order to achieve the desired electrical and magnetic properties, it may be helpful if the magnetic powder particles are larger than the non-magnetic powder particles, for example in order to ensure that the magnetic powder particles are still after sintering at similarly high melting temperatures of the magnetic and non-magnetic powder components have a sufficiently large grain size to be able to produce the desired magnetic and electrical properties in the workpiece or in the finished part.

Additionally or alternatively, it is also possible to provide the powder particles of the magnetic powder component with a stabilizing coating in order to increase the stability of the magnetic powder particles during sintering and thus to ensure a sufficiently large particle size of the magnetic powder parts after sintering. For example, a stabilizing coating can be made from a refractory ceramic, such as a thin Al ₂ O ₃ layer through a sol-gel. A stabilizing coating can also be produced by preoxidation of the powder particles of the magnetic powder component. For example, the oxidation of chromium in chrome steel creates a very stable thin oxide layer that reduces or completely prevents diffusion during sintering. By sintering the surrounding non-magnetic material, the at least one magnetic powder component, which is the minority component of the powder composition, is nevertheless integrated or included.

The material properties, such as the magnetic and electrical properties and the structural structure of the workpiece, can be influenced and varied by changing one or more of these process parameters. The process parameters include in particular the compression pressure with which the powder composition is compressed before, during and / or after sintering, the sintering temperature to which the (compressed) powder composition is heated during sintering, the duration of the sintering at the sintering temperature (sintering duration) and the heating rate and the cooling rate.

The compressed powder composition before sintering is often referred to as a pressed blank, green body or green body, as will also be described below. Compression before sintering is typically done by cold pressing. Typical values for the compression pressure (often also referred to as compacting pressure) with which the press blank is produced are in a range between 200-800 MPa. The sintering temperature is typically in a range between 60% and 90% of the melting temperature of the at least one non-magnetic powder component. The properties of the powder components make advantageous embodiments of the method possible, in which the highest sintering temperature reached during the production of the workpiece, to which the powder composition and the resulting workpiece are heated during sintering, does not exceed the melting temperature of the at least one magnetic powder component. The sintering temperature can e.g. are in a range between 500 ° C and 1400 ° C. The sintering time in turn can e.g. range from 20 minutes to 2 hours. After sintering, the powder composition is also referred to as a sintered part.

The workpiece or finished part produced using the method proposed here can have a unique signature, such as a special combination of electrical conductivity and magnetic relative permeability. This signature can be easily measured and identified (for example by impedance measurements), but is not reproducible or can only be reproduced with great effort without precise knowledge of the powder components and process parameters. In addition to this very easily verifiable signature, the material also has other specific properties, such as, for example, a special microstructure that, for example, cannot generally be reproduced by conventional casting processes.

In the case of the coin proposed here, the electromagnetic signature of the coin can be checked in a relatively simple manner by means of a coin testing device of known design and mode of operation. For example, the coin validator can be set up to measure the (AC) conductivity or the impedance of the coin by means of an induction system. For example, the induction system can be used to generate electromagnetic test signals with different frequencies in order to induce alternating currents within different areas (on the surface or inside) of the coin. The interaction of the test signals with the coin (in particular the depth of penetration of the test signals into the coin) and thus the eddy currents generated thereby depend in particular on the electrical conductivity and the relative magnetic permeability of the coin. The induced alternating currents generate electromagnetic signals in the coin which are characteristic of the special electrical and magnetic properties of the coin and thus enable their identification, for example by a correspondingly configured sensor system of the coin validator which detects these signals (for example by means of electrical contacting elements or by means of an antenna) and evaluates.

The relative magnetic permeability of the workpiece or of the finished part, also referred to below as μ, for example of the coin, preferably has a value of between 60 and 240 or between 70 and 240. The relative magnetic permeability of the workpiece or of the finished part, for example the coin, is preferably smaller than that of cobalt (Co) or lies between the values of cobalt (Co) and nickel (Ni) or lies between the values of nickel (Ni ) and iron (Fe). Here the relative magnetic permeabilities are given assuming the same measurement conditions.

The relative magnetic permeability of a magnetic material is, as is generally customary, defined by µ = B / µ ₀ H, where B denotes the magnetic flux density (colloquially magnetic field), H the magnetic field strength and µ ₀ the magnetic field constant. Due to the hysteresis of magnetic materials already mentioned, there is no clear connection between the field strength H and the flux density B. For example, magnetic materials are sufficiently saturated at sufficiently high field strength values and then assume maximum magnetization, so that the flux density B (at least approximately) assumes a saturation value B _S. The relative magnetic permeability of a magnetic material is preferably defined under the measurement condition that the material is magnetized to saturation, so that the flux density B then measured has assumed the associated saturation value B _S or at least 0.98 times this value. When calculating μ, the value of the magnetic field strength H that was present when the saturation value B _{S of} the magnetic flux density (or alternatively 0.98 times the saturation value B _S ) was reached is then preferably used. The measurement is preferably carried out statically or quasi-statically, ie the magnetic field strength H preferably has a frequency of approximately 0 Hz. Furthermore, the material preferably has a temperature of approximately 21 ° C. during the measurement. For example, the magnetic flux density or the magnetization of the Materials are measured using a so-called saturation magnetometer, such as in Steingroever, Dr. E .: Magnetic measurement technology; Edition 1833/97; © MAGNET-PHYSIK Dr. Steingroever GmbH; Cologne, 1997 is described.

In order to achieve a suitable strength of its magnetic properties (for example a suitable relative magnetic permeability) for the intended use of the workpiece or finished part, the mass fraction of the at least one magnetic powder component is typically dependent on the remanence and / or on the coercive field strength of the material or material of the respective magnetic powder component, and in particular depending on whether the material or the material of the respective magnetic powder component is soft magnetic or hard magnetic. For example, those materials and materials whose coercive field strength (H _C ) is greater than 1000 A / m can be considered to be hard magnetic. Accordingly, those materials and materials whose coercive field strength (H _C ) is less than 1000 A / m can be regarded as soft magnetic, for example.

The electrical conductivity of the workpiece and / or the prefabricated part made therefrom (for example the coin) is preferably less than 5.8 MS / m (i.e. less than 10% IACS).

The powder particles are typically produced by comminuting the respective material, a typical comminution process being atomizing the material. The materials of the powder components are typically metallic materials, such as alloys or elemental metals, see material groups A to J below. The materials can also be semiconductors, such as silicon from material group K below.

• - Werkstoffgruppe A, bestehend aus ferritischen Stählen, insbesondere Eisen-Chrom-Legierungen und Heizleiterlegierungen (mit relativ hohem spezifischen elektrischen Widerstand) wie etwa Eisen-Chrom-Aluminium-Legierungen umfassend,
• - Werkstoffgruppe B, bestehend aus austenitisch-ferritischen Stähle, insbesondere Duplexstähle und Lean-Duplexstähle umfassend,
• - Werkstoffgruppe C, bestehend aus martensitischen Stählen, insbesondere Cobalt-Stahl umfassend,
• - Werkstoffgruppe D, bestehend aus Ferrolegierungen, insbesondere Ferrosilizium, Ferromangan und Ferrochrom umfassend,
• - Werkstoffgruppe F, bestehend aus hartmagnetischen Stählen, wie insbesondere Kobaltlegierte Stähle, sowie aus Aluminium-Nickel-Cobalt-Legierungen, Kupfer-Mangan-Aluminium-Legierungen, Silber-Mangan-Aluminium-Legierungen, Cobalt-Samarium-Legierungen und Neodym-Eisen-Bor-Legierungen.

According to the invention, the magnetic powder component or at least one of the magnetic powder components is selected from one of the following material groups:

• Material group A, consisting of ferritic steels, in particular iron-chromium alloys and heating conductor alloys (with a relatively high specific electrical resistance), such as iron-chromium-aluminum alloys,
• Material group B, consisting of austenitic-ferritic steels, in particular comprising duplex steels and lean duplex steels,
• Material group C, consisting of martensitic steels, in particular comprising cobalt steel,
• Material group D, consisting of ferroalloys, in particular comprising ferrosilicon, ferromanganese and ferrochromium,
• - Material group F, consisting of hard magnetic steels, such as in particular cobalt alloyed steels, as well as aluminum-nickel-cobalt alloys, copper-manganese-aluminum alloys, silver-manganese-aluminum alloys, cobalt-samarium alloys and neodymium iron - boron alloys.

• - Werkstoffgruppe E, bestehend aus elementaren Ferromagnete, Eisen, Nickel und Cobalt umfassend,
• - Werkstoffgruppe G, bestehend aus Ferriten, insbesondere Weichferrite und Hartferrite, wie insbesondere Magnetit, umfassend.

Examples of material groups for additional magnetic powder components are:

• - material group E, consisting of elementary ferromagnets, including iron, nickel and cobalt,
• - Material group G, consisting of ferrites, in particular soft ferrites and hard ferrites, such as magnetite in particular.

The powder composition can contain exactly one magnetic powder component with exactly one magnetic material, for example a material from one of the above material groups A to D and F. However, the powder composition can also contain several different magnetic powder components. Each of these magnetic powder components then consists of exactly one material, which can be selected from one of the above material groups A to G.

According to the invention, the non-magnetic powder component or at least one of the non-magnetic powder components is selected from a material group H consisting of austenitic steels, in particular comprising austenitic chromium-nickel steels.

• - Werkstoffgruppe I, bestehend aus Chrom-Nickel-Legierungen und Kupfer-Nickel-Legierungen, sowie
• - Werkstoffgruppe J, bestehend aus Titan, Titan-Legierungen, Aluminium, Aluminium-Legierungen, Kupfer und Kupfer-Legierungen.

Examples of material groups for additional non-magnetic powder components are:

• - Material group I, consisting of chrome-nickel alloys and copper-nickel alloys, as well
• - Material group J, consisting of titanium, titanium alloys, aluminum, aluminum alloys, copper and copper alloys.

The powder composition can contain exactly one non-magnetic powder component with exactly one magnetic material from the above material groups H. However, the powder composition can also contain several different non-magnetic powder components. Each of these non-magnetic powder components then consists of exactly one material, which can be selected from one of the above material groups H to J.

In some cases, it is possible that, in order to produce a magnetic powder component and a non-magnetic powder component at the same time, a material is shredded which has both magnetic and non-magnetic components (such as structural components or phase components), for example if the material is in the form of a multi-phase structure with a magnetic phase and a non-magnetic phase. In this case, the powder particles, which only have non-magnetic components, form the non-magnetic powder particles. A concrete example of such an overall (ferro-) magnetic material is, for example, a ferritic-austentic steel (material group B) with a structure that has (ferro-) magnetic ferritic phase components and non-magnetic austenitic phase components, such as duplex steel.

• - Ferritbildner, umfassend Chrom, Molybdän, Silizium, Niob und Titan, und
• - Austenitbildner, umfassend Nickel, Kohlenstoff und Mangan.

The steels mentioned in material groups A to C and H are each preferably stainless steels, that is to say typically chromium steels, that is to say iron-chromium alloys, such as, for example, stainless steels. In particular, elements from the following two material groups are possible as alloy elements in the steels:

• - Ferrite formers including chromium, molybdenum, silicon, niobium and titanium, and
• - Austenite formers, including nickel, carbon and manganese.

In general, the ferrite formers lead to a higher ferritic phase component in the structure of the steel and thus generally also to more pronounced ferromagnetic properties of the steel, whereas the austenite formers lead to a higher austenitic phase component in the structure of the steel and thus the ferromagnetic properties of the steel in the Weaken the rule (and rather lead to paramagnetic properties). This relationship is schematically represented, for example, by the so-called Schaeffler diagram of the steel phases, in which the so-called chrome equivalent is plotted on one axis, which is a measure of the proportion of ferrite formers in the steel, and the so-called nickel equivalent on the other axis is plotted, which is a measure of the proportion of austenite formers in the steel. With the method proposed here, it is possible in particular that the powder composition used contains one or more powder components, each consisting of a steel material, for example selected from material groups A to C and H, these steel materials being characterized by their phase fractions of ferrite, martensite and Can distinguish austenite. In addition, the powder composition can contain one or more powder components which are selected from the group of materials of the ferrite formers. Furthermore, the powder composition can contain one or more powder components which are selected from the group of materials of the austenite formers. In addition or as an alternative to the powder components made of the steel materials, one of the powder components can be an iron powder. In order to be able to more easily reach desired ranges of electrical and magnetic properties, the nickel equivalent and the chromium equivalent can be calculated according to the Schaeffler diagram, for example, or other thermodynamic calculations can also be carried out.

By sintering and optionally also by additional heat treatments, such as, for example, annealing treatments, such as solution annealing, and quenching treatments, depending on the particular process parameters selected (in particular the sintering temperature and possibly the temperature of the heat treatment, heating rates, cooling rates, compression pressure of the powder composition), martensitic phase fractions , ferritic phase fractions and / or austenitic phase fractions are influenced, and in addition to thermodynamic equilibrium states (according to the Schaeffler diagram), even metastable phase states can also be produced. A targeted adjustment of the degree of homogenization of the resulting material structure is also possible. The degree of homogenization is determined by the particle size (smaller particles dissolve faster and cause faster homogenization) and by the homogenization temperature and the holding time at the homogenization temperature. To some extent, the homogenization also depends on the heating and cooling rates. Overall, due to the combination of materials and process parameters described, a very large number of steel materials with specific properties can be produced that cannot be reproduced or can only be reproduced with great effort without knowledge of the selected materials and process parameters.

The following table shows some examples of powder compositions according to the invention and, for illustration, also some examples of non-inventive powder compositions, each with a non-magnetic powder component (left column) and a magnetic powder component (middle column). For each powder component, a typical value or range of melting temperature (referred to as T _S ) and a specific indication of the magnetic property of the respective material are given. The right column shows the difference, referred to as ΔT _S , between the melting temperature of the material of the respective non-magnetic powder component and the melting temperature of the material of the respective magnetic powder component. This difference is defined such that positive values of this difference mean that the melting temperature of the non-magnetic powder component is higher than the melting temperature of the magnetic powder component. Non-magnetic powder component Magnetic powder component ΔT _S Copper-nickel (75/25) cobalt T _S : 1180 ° C paramagnetic T _S : 1493 ° C ferromagnetic - 313 ° C Copper-nickel (75/25) iron T _S : 1180 ° C paramagnetic T _S : 1536 ° C ferromagnetic - 356 ° C Aluminum (alloy) cobalt T _S : 660 ° C (570 ° C) paramagnetic T _S : 1493 ° C ferromagnetic - 833 ° C to - 923 ° C Aluminum (alloy) iron T _S : 660 ° C (570 ° C) paramagnetic T _S : 1536 ° C ferromagnetic - 876 ° C to - 966 ° C austenitic Cr-Ni steels ferritic chrome T _S : 1400 ° C paramagnetic steels T _S : 1450 - 1500 ° C ferromagnetic - 50 ° C to -100 ° C austenitic Cr-Ni steels iron T _S : 1400 ° C paramagnetic T _S : 1536 ° C ferromagnetic - 136 ° C Copper-nickel (75/25) (Soft) ferrite T _S : 1180 ° C paramagnetic T _S : 1300-1500 ° C ferrimagnetic - 120 ° C to - 320 ° C Copper-nickel (75/25) Cobalt samarium T _S : 1180 ° C paramagnetic T _S : 1250 - 1300 ° C ferromagnetic (hard magnetic) - 70 ° C to -120 ° C Copper-nickel (75/25) neodymium iron boron T _S : 1180 ° C paramagnetic T _S : 1100 - 1150 ° C ferromagnetic (hard magnetic) 80 ° C to 30 ° C aluminum neodymium iron boron T _S : 660 ° C paramagnetic T _S : 1100 - 1150 ° C ferromagnetic (hard magnetic) - 440 ° C to - 490 ° C austenitic Cr-Ni steels Cobalt steels T _S : 1400 ° C paramagnetic T _S : 1450 ° C ferromagnetic (hard magnetic) - 50 ° C Copper-nickel (75/25) (Hard) ferrite T _S : 1180 ° C paramagnetic T _S : 1300 - 1500 ° C ferromagnetic (hard magnetic) - 120 ° C to - 320 ° C Titanium (alloys) Ferritic Cr steel 218 ° C to T _S : 1668 ° C (1500 ° C) paramagnetic T _S : 1450 - 1500 ° C ferromagnetic 168 ° C (50 ° C to 0 ° C) Titanium (alloys) cobalt T _S : 1668 ° C (1500 ° C) paramagnetic T _S : 1493 ° C ferromagnetic 175 ° C (7 ° C)

An exemplary embodiment of a powder composition according to the invention contains, as non-magnetic powder component, austenitic stainless steel with 60% to 90% by mass, such as austenitic chromium-nickel steel, and as magnetic powder component, ferritic stainless steel with 10% to 40% by mass, such as ferritic chromium steel , In this case, the difference between the melting temperatures is relatively small, so that the grain size of the ferritic stainless steel is preferably chosen to be larger than the grain size of the austenitic stainless steel. In order to achieve a workpiece that is as quasi-isotropic and homogeneous as possible, a corresponding stabilization can be provided, for example by a stabilizing coating of the magnetic powder particles, as has been described above. Cost advantages can be achieved if a ferro alloy is used instead of ferritic stainless steel. In the case of a heating conductor alloy, for example the FeCrAl system, as ferritic stainless steel, particularly low electrical conductivities can be achieved. Instead of a soft magnetic ferritic stainless steel, a hard magnetic material, such as cobalt steel with a mass fraction between 3% and 20% or a material from the hard magnetic material group F with a mass fraction between 3% and 20%, such as an aluminum-nickel Cobalt alloy, a cobalt-samarium alloy, or a neodymium-iron-boron alloy (hard magnet) can be used. If a copper-nickel alloy is used instead of the austenitic stainless steel, for example an aluminum-nickel-cobalt alloy, a copper-manganese-aluminum alloy or a silver-manganese can be used as the hard magnetic powder component with a mass fraction between 3% and 20%. Aluminum alloy can be used.

In particular in the case of exemplary embodiments in which the powder composition contains a magnetic powder component made of a ferritic steel or ferritic stainless steel, the powder composition can also include, for example, at least one further powder component which is selected from the material group K, consisting of silicon, manganese and aluminum.

A powder component from material group K can be used to create electrically insulating grain boundary phases in the (steel) structure of ferritic steel and thus have a strong influence on its electrical conductivity. It is also possible to oxidize (pre-oxidize) the powder particles of this powder component (s) by a heat or annealing treatment in an oxygen-containing atmosphere, such as air, in order in this way to produce a semiconducting phase on the grain boundaries of the structure of the ferritic steel , The semiconducting grain boundary phase can increase the electrical resistance of the workpiece without significantly influencing the magnetic properties of the workpiece. The mass fraction of this at least one further powder component of material group K in the powder composition is a maximum of 10% in total.

In embodiments in which the powder composition contains a magnetic powder component made of a ferritic steel or stainless steel, which contains chromium, nickel, manganese and / or silicon as alloy components, it is also through a heat or annealing treatment of these powder component (s) in an oxygen-containing atmosphere , such as air, possible to (partially) oxidize these alloy components. The oxides produced in this way can act as resistors in the structure of the steel and thus reduce the electrical conductivity of the workpiece. Magnetic (ferritic) phases can also be generated by the oxides that were generated on the powder surfaces before sintering. Furthermore, the color of the workpiece can be changed by the oxide formation (by glowing in air) after sintering.

In a further possible embodiment variant, the material of an additional non-magnetic powder component made of aluminum or copper is selected.

Magnetic powder components selected from material groups A to D and F are particularly suitable for setting the desired magnetic properties, for example with a total mass fraction of 10% to 40%. In the case of aluminum, there is also the possibility of additionally adding Heusler phases by alloying manganese and copper, which are ferromagnetic without even containing ferromagnetic elements.

• - Werkstoffgruppe L, umfassend Mangan, Aluminium, Titan, Silizium, und den Werkstoffgruppen A bis G.

In a further possible embodiment variant, the material of an additional non-magnetic powder component is copper. In this example, additional powder components can be used to set the desired material properties, such as conductivity and color, which can be selected, for example, from:

• - Material group L, including manganese, aluminum, titanium, silicon, and material groups A to G.

Before the powder composition is sintered, the powder particles of the powder components can be mixed homogeneously in the powder composition, so that the powder composition is then a homogeneously mixed powder mixture. However, it is also possible that not all or none of the powder components are mixed before sintering. The powder components are then only mixed during the sintering by the diffusion processes already described. In particular, the powder components in the powder composition can be arranged in layers one above the other. It may be necessary here to compact the powder composition again after each production of one of the layers. Instead of such a layer-by-layer compression of the powder composition, a layer structure of the powder composition can be produced using printing processes. For example, it is possible to disperse one (or more) of the powder components in a flowable or pasty auxiliary medium and then to produce a layer of this dispersion (paste) by means of a printing process. For example, it is possible to disperse one (or more) of the powder components in a flowable or pasty auxiliary medium and then to produce a layer of this dispersion (paste) by means of a printing process. It is possible, for example, to first compress the other powder component (s) into a press blank and then to introduce the dispersion with the powder particles dispersed therein into the pore spaces of the press blank by means of the printing process, by printing the dispersion blank onto the press blank , for example by means of a pad printing process, or an ink-jet printing process or by means of a screen printing process, for example by means of a (continuous) rotary screen printing process. The dispersion (ie the color in the case of a printing process) can penetrate deeply into the porous framework and thus achieve a gradient effect, which leads to a good connection with the base material.

Depending on the layer structure, a concentration gradient of the materials of the respective (possibly dispersed) powder components can be generated within the powder composition or the press blank. For example, the concentration of the printed material can decrease from the surface of the powder composition or the press blank to the center thereof. Accordingly, the concentration gradients of the materials can also be wholly or partly retained in the workpiece or in the finished part, for example in the coin, even after sintering, for example after the powder particles from the dispersion (paste) have combined with the other powder particles. In this way, attributes such as multicolor (visible features) and local material properties on the surface and / or inside the workpiece or the finished part can also be set.

In particular, by means of the printing method described above, areas with a different color (production of visible markings) and / or with different electrical and / or magnetic properties can be produced with a correspondingly designed printing pattern on the surface of the workpiece, these areas for example as a pattern or decor, for example can be designed as concentric rings, or as characters, letters or numbers (numbers). For example, areas can be produced which are both recognizable as a colored label with the naked eye and which can be checked electromagnetically.

For the production of surface areas with different colors, for example, a powder component can be printed on, the material of which has a color that differs from the color of the press blank or by a corresponding reaction with the other materials of the press blank or with an atmosphere surrounding the press blank, for example in the case of Sintering or, if necessary, during a further heat treatment (for example annealing), cause a locally different color of the workpiece within the respectively printed surface area of the pressed blank or of the finished part, for example the coin. For example, the powder component to be printed can be a metal or an alloy. Metals that are particularly suitable for this purpose are, for example, non-ferrous metals, such as copper, and titanium, with particular titanium also being compatible with high sintering temperatures (such as for stainless steel). With titanium, for example, blue shades (oxides) can be produced by heat treatment in an oxygen-containing atmosphere and gold-yellow shades (titanium nitride) by heat treatment in a nitrogen-containing atmosphere. Colored alloys such as bronzes, brasses and Nordic gold are particularly suitable as suitable alloys. It is also possible, on a coin, the powder composition of which contains one or more non-ferrous metals, or which have a non-ferrous metal surface by means of a corresponding coating, by printing, for example, nickel, stainless steel powders, silver, nickel silver and many other metals, which one other non-"colorful" colors (such as tin, palladium, iron and aluminum) are applied with high-contrast decors and patterns, which also change the electromagnetic properties locally in addition to the optical characteristics.

Other materials which can be printed on in the manner described for producing color codes are aluminum and aluminides, such as, for example, the iron aluminides FeA and FeAl ₃ , the color of the aluminides preferably being changed by anodic oxidation in order to generate visible features.

Due to the described concentration gradients of different materials inside and / or on the surface of the powder composition or the pressed blank, in the case of the mentioned steel materials and the above-mentioned austenite formers and / or ferrite formers, corresponding concentration gradients of different steel phases, for example the ferromagnetic ferritic steel phase and the non-magnetic austenitic steel phases, are produced on the surface and / or inside the workpiece or the finished part. If the material or the materials of the powder particles of the powder component (s) to be printed are from the material groups of the ferrite formers and the austenite formers, areas can be produced on the surface of the workpiece or the finished part (e.g. the coin) that differ from one another by their steel phase concentrations distinguish and have, for example, different sizes of ferritic phase components and different sizes of austenitic phase components. Due to the layer structure described above, in the case of appropriately selected austenite formers or ferrite formers for layers that adjoin layers made of a steel material, (layer-shaped) areas with locally different steel phase fractions and corresponding concentration gradients of the steel phases can also be generated inside the workpiece or finished part. The resulting gradients in the electrical and magnetic properties can be measured and evaluated for example by means of electromagnetic test signals with different measuring frequencies.

Advantageously, the workpieces produced with the method proposed here are practically free of textures or (crystalline) preferred orientations and can therefore be brought into a desired shape or shape particularly easily by forming processes and without disruptive bulges. Often, this enables particularly dimensionally accurate production, in which only minor rework is required. Since the workpieces typically have a relatively low deformation resistance, it is advantageously also possible to process a particularly wide range of different materials, including materials with relatively high strength.

• 1: eine perspektivische Ansicht einer zu einem Pressrohling verdichteten Pulverzusammensetzung gemäß einem speziellen Ausführungsbeispiel des hier vorgeschlagenen pulvermetallurgischen Herstellungsverfahrens,
• 2: einen Teilbereich des in 1 gezeigten Pressrohlings in einer stark vergrößerten Ansicht,
• 3: eine perspektivische Ansicht eines Münzrohlings, der durch Sintern des in 1 gezeigten Pressrohlings hergestellt worden ist,
• 4: einen Teilbereich des in 3 gezeigten Münzrohlings in einer stark vergrößerten Ansicht,
• 5: Pressrohling zur Herstellung einer Umlaufmünze hier vorgeschlagener Art in einer Ansicht von oben,
• 6: eine Ansicht eines Querschnitts durch den in 5 gezeigten Pressrohling entlang der in 5 gekennzeichneten Schnittlinie,
• 6A-6C: Teilbereiche des in 6 gezeigten Querschnitts, jeweils in einer stark vergrößerten Ansicht,
• 7: Umlaufmünze, hergestellt durch Sintern des in 5 gezeigten Pressrohlings in einer Ansicht von oben,
• 8: eine Ansicht eines Querschnitts durch die in 7 gezeigte Umlaufmünze entlang der in 7 gekennzeichneten Schnittlinie,
• 8A-8C: Teilbereiche des in 8 gezeigten Querschnitts, jeweils in einer stark vergrößerten Ansicht.

The invention is explained in more detail below on the basis of special exemplary embodiments. In the 1 to 8C some of these embodiments are shown schematically as well as further illustrative examples that are not part of the claimed invention. It shows:

• 1 1 shows a perspective view of a powder composition compressed into a pressed blank according to a special exemplary embodiment of the powder metallurgical production method proposed here,
• 2 : a part of the in 1 shown press blank in a greatly enlarged view,
• 3 : a perspective view of a coin blank obtained by sintering the in 1 press blank shown has been produced,
• 4 : a part of the in 3 shown coin blank in a greatly enlarged view,
• 5 : Press blank for the production of a circulation coin of the type proposed here in a view from above,
• 6 : a view of a cross section through the in 5 press blank shown along the in 5 marked cutting line,
• 6A-6C : Subareas of the in 6 shown cross section, each in a greatly enlarged view,
• 7 : Circulating coin made by sintering the in 5 shown press blank in a view from above,
• 8th : a view of a cross section through the in 7 circulation coin shown along the in 7 marked cutting line,
• 8A-8C : Subareas of the in 8th shown cross section, each in a greatly enlarged view.

Reference symbols used repeatedly in the figures denote the same or corresponding features.

1 shows a schematic representation of a press blank 1 compacted powder composition 2 according to a special embodiment of the powder metallurgical manufacturing process proposed here. A subarea 3 of the blank is in 2 greatly enlarged and shown schematically. Powder particles are shown therein 4 a magnetic powder component of the powder composition 2 , for example consisting of a ferritic stainless steel or (as an illustrative example not according to the invention) iron, and powder particles 5 a non-magnetic powder component of the powder composition 2 , consisting for example of an austenitic stainless steel or (as illustrative examples not according to the invention) consisting of copper or aluminum. The melting point of the magnetic powder components is so far above the melting point of the non-magnetic powder component that the magnetic powder particles 4 remain during sintering and do not dissolve through diffusion processes. For example, the sintering temperature is 60% to 90%, for example 70%, of the melting temperature of the non-magnetic powder component.

In 3 is a workpiece 6 shown by sintering the in 1 shown blank 1 has been produced, and which is a coin blank 7 acts, which can be further processed into a circulation coin in further steps, for example by producing a coin minting. But it is also possible that the in 1 shown press blank 1 has already been compacted to a final use density and a coin motif has already been applied during the compacting process.

In 4 is a sub-area 3 of the workpiece 6 greatly enlarged and shown schematically. The powder particles are shown again 4 the magnetic powder component and the powder particles 5 the non-magnetic powder component. Due to the sintering and the diffusion processes taking place, the magnetic and the non-magnetic powder particles have become 4 and 5 firmly connected so that the workpiece 6 overall has a sufficiently high strength for the intended use. It can be seen in particular that the magnetic powder particles 4 are still present and have practically not reduced in size. In this way, the magnetic powder particles 4 the workpiece 6 impart the desired magnetic properties.

Three special variants of this exemplary embodiment are explained in more detail below.

The first illustrative variant, but not according to the invention, is designed to produce a special aluminum-stainless steel coin with ferromagnetic properties. The powder composition 2 contains in this example to a mass fraction of 79% by weight a non-magnetic powder component made of atomized pure aluminum powder, its powder particles 4 have a particle size of less than 150 microns. This powder component is mixed with a ferromagnetic powder component with a mass fraction of 20% by weight, the powder particles thereof 5 are made of ferritic stainless steel X6Cr17 and have a particle size of less than 45 µm. The powder composition 2 also contains 1% by weight of a lubricant (not shown). The powder composition 2 is compacted with a compression pressure of 450 MPa in a steel die. The press blank produced in this way 1 then becomes the workpiece in a protective gas sintering furnace under a nitrogen atmosphere 6 , the coin blank 7 , sintered. The heating rate up to the sintering temperature of 580 ° C is 5 K / min. The holding time at the sintering temperature is 45 min. The cooling takes place at 10 K / min until the furnace cools down freely at 300 ° C and the sintered part continues to cool (uncontrolled) until room temperature. The workpiece 6 has the required durability and hardness after sintering and can be lifted, for example, with a magnet with less attractive force (e.g. a pinboard magnet).

The second illustrative variant, which is also not according to the invention, is used to produce a copper-stainless steel coin with ferromagnetic properties. According to this variant, the powder composition includes 2 to 69 , 5% by weight of a non-magnetic powder component made of atomized copper powder with uneven, non-round powder particles 5 which have a particle diameter of less than 100 microns. In this example, the ferromagnetic powder component has a mass fraction of 30% by weight and is due to powder particles 4 consisting of ferritic stainless steel X6Cr13 (410L) with a particle size of less than 63µm. The powder composition 2 contains 0.5% by weight of a lubricant (press wax). The powder composition 2 is compressed uniaxially in a steel die with a compression pressure of 650MPa. The press blank thus produced 1 is sintered in a protective gas furnace under a nitrogen atmosphere at a sintering temperature of 950 ° C. for a period of 40 min. The heating rate up to the sintering temperature is 10 K / min, with a holding phase of 30 minutes being maintained at 450 ° C. in order to eliminate the press wax. The workpiece made in this way 6 is again a coin blank 7 and has good strength and high density, shows ferromagnetic properties and can be attracted by a permanent magnet.

The third illustrative variant, again not according to the invention, is suitable for producing a stainless steel pure iron coin. Here, the powder composition 2 contains 74.2% by weight of a non-magnetic powder component from a water-atomized, austenitic stainless steel powder of the type 316 L (according to AISI; material number 1.4404), its powder particles 5 have a particle size of less than 150 μm, and 25% by weight of a ferromagnetic powder component of pure iron (water-atomized iron powder which has been preheated in a reducing manner) mixed with powder particles 4 which have a particle size below 100 µm. In addition, the powder composition 2 0.8% by weight of a lubricant added. The powder composition 2 becomes the press blank in a steel die 1 pressed. The pressure is 700 MPa. The press blank 1 is sintered under flowing hydrogen gas at atmospheric pressure at a sintering temperature of 1250 ° C. for a period of 30 min. The heating rate up to the sintering temperature is 5 K / min. The cooling rate is 10 K / min. The workpiece made in this way 6 exhibits a ferromagnetic behavior after sintering and can be attracted by a magnet. The workpiece shows through the content of pure iron 6 or the coin blank has a conductivity that differs from that of pure stainless steel.

A very large number of other variants are also possible, which differ from one another in particular by their powder composition and their process parameters, so that each of the variants gives a characteristic set of material properties, such as electrical conductivity, magnetic permeability, microstructure, strength, mass density, etc. , can be achieved.

As in 5 to 8C is shown, it is also possible in a special embodiment of the method proposed here, a sintered coin 9 to produce, see 8th , on their surface 8th a visible mark 10 as well as an area 11 which is distinguished from the rest of the coin by the prevailing electrical conductivity, magnetic permeability and structural structure 9 different.

In the exemplary embodiment shown is used to produce a powder composition 2 a magnetic powder component, for example a ferritic chrome steel with a mass fraction of about 20% by weight, and a non-magnetic powder component, for example made from an austenitic chromium-nickel steel with a mass fraction of about 80% by weight, are mixed and these powder components into a pressed blank1 pressed, see 5 , The compression pressure is selected so that the press blank 1 between the powder particles 4 . 5 of these powder components pore spaces 12 has, see 6 and 6A to 6C ,

A first further powder component of the powder composition 2 whose powder particles 13 consist for example of a ferrite former, such as chromium, is mixed with a first auxiliary medium to form a first dispersion (not shown). A second further powder component of the powder composition 2 whose powder particles 14 consist for example of titanium, is mixed with a second auxiliary medium to form a second dispersion (not shown). The first dispersion and the second dispersion are applied to the surface by means of an ink jet process or alternatively by means of a pad printing process or by means of a screen printing process, for example by means of a continuous rotary screen printing 8th of the press blank 1 printed so that the powder particles 13 . 14 the first and the second further powder component into the pore spaces 12 of the press blank. The powder particles 13 the first further powder component in a surface area, which is indicated by the visible mark to be produced 10 is defined, printed, and the powder particles 14 the second further powder component in the area 11 the surface 8th printed as in 6 and 6A to 6C is shown schematically and enlarged.

The coin blank 7 is sintered in a protective gas furnace under a nitrogen atmosphere at a sintering temperature of about 950 ° C. for a period of 40 min, whereby the powder particles 4 . 5 . 13 . 14 are firmly connected by diffusion processes and the formation of sinter necks. In addition, the titanium reacts to titanium nitride and in this way assumes a golden yellow color, so that the mark 10 on the surface 8th the coin 9 becomes clearly visible. The chrome within the area 10 leads to the formation of further ferromagnetic ferritic phases in the structure of the austenitic powder particles 5 so this in this area 11 also become ferromagnetic, causing a relative magnetic permeability of the coin 9 within the range 11 increases locally.

In a subsequent step, the coin 9 embossed, whereby next to an embossing (not shown here) on the surface 8th the coin 9 also a final use density of the coin 9 will be produced.

In principle, it is already possible when the press blank is manufactured 1 before the sintering, to create a coin motif, so that the pressed blank 1 already has this coin motif and preferably also the final (preferably highest possible) usage density. A further embossing and / or compression step can then advantageously be dispensed with after the sintering.

LIST OF REFERENCE NUMBERS

1

Press ingot

2

powder composition

3

subregion

4

Powder particles of a magnetic powder component

5

Powder particles of a non-magnetic powder component

6

workpiece

7

coin blank

8th

surface

9

coin

10

visible indicator

11

Area

12

pore space

13

Powder particles of a first further powder component

14

Powder particles of a second further powder component

Claims (16)

Powder metallurgical method for producing a coin blank (7), in which the coin blank (7) is produced by sintering a powder composition (2) which contains at least two different powder components, at least one powder component of the powder composition being magnetic and at least one powder component of the powder composition not is magnetic, wherein a melting temperature of the at least one non-magnetic powder component is below or not more than 900 ° C. above a melting temperature of the at least one magnetic powder component, characterized in that a mass fraction of the at least one magnetic powder component in the powder composition is between 2% and Is 40%, the at least one magnetic powder component being selected from one or more of the following material groups: material group A, consisting of ferritic steels, in particular comprising iron-chromium alloys, W Material group B, consisting of austenitic-ferritic steels, in particular comprising duplex steels and lean-duplex steels, - Material group C, consisting of martensitic steels, in particular comprising cobalt steel, - Material group D, consisting of ferro-alloys, in particular comprising ferrosilicon, ferromanganese and ferrochrome, - Material group F, consisting of hard magnetic steels, such as in particular cobalt alloyed steels, as well as aluminum-nickel-cobalt alloys, copper-manganese-aluminum alloys, silver-manganese-aluminum alloys, cobalt-samarium alloys, neodymium-iron-boron -Alloys, and wherein the at least one non-magnetic powder component is selected from material group H, consisting of austenitic steels, in particular comprising austenitic chromium-nickel steels. Powder metallurgical process according to Claim 1 , characterized in that the melting temperature of the at least one non-magnetic powder component is not more than 300 ° C, preferably not more than 100 ° C, above the melting temperature of the at least one magnetic powder component. Method according to one of the preceding claims, characterized in that a highest sintering temperature to which the powder composition is heated during the manufacture of the coin blank (7) does not exceed the melting temperature of the at least one magnetic powder component. Method according to one of the preceding claims, characterized in that the mass fraction of material group F in the powder composition is not more than 20% overall. Method according to one of the preceding claims, characterized in that the magnetic powder component has a larger grain size than the non-magnetic powder component. Method according to claim according to one of the preceding claims, characterized in that the mass fraction of material groups H, I and J in the powder composition is overall between 60% and 98%. Method according to one of the preceding claims, characterized in that at least one of the powder components consists of a steel with martensitic phase components and / or ferritic phase components and / or austenitic phase components, at least one further of the powder components being selected from one of the following material groups: comprising chromium, molybdenum, silicon, niobium and titanium, - austenite formers comprising nickel, carbon and manganese. Method according to one of the preceding claims, characterized in that the powder composition contains a magnetic powder component made of a ferritic steel, the powder composition for the formation of electrically insulating or semiconducting grain boundary phases within the ferritic steel also comprising at least one further powder component which is selected from the group of materials K, consisting of silicon, manganese and aluminum. Method according to one of the preceding claims, characterized in that the powder composition contains a magnetic powder component made of a ferritic steel which has chromium and / or nickel and / or manganese and / or silicon as alloying elements, with oxides comprising at least one of these under an oxygen-containing atmosphere Alloy components are produced, (a) electrically insulating phases and / or magnetic phases being generated by the oxides within the ferritic steel, and / or (b) a color of the coin blank (7) being set by the oxides. Method according to one of the preceding claims, characterized in that one of the powder components is dispersed in an auxiliary medium for producing a dispersion, at least one further of the powder components being compressed in a press blank (1), the dispersion being carried out in pore spaces (12) of the press blank ( 1) is introduced. Procedure according to Claim 10 , characterized in that the press blank (1) is printed with the dispersion, in particular by means of a pad printing process or by means of an ink jet printing process or by means of a screen printing process, in particular by means of a rotary screen printing process. Procedure according to one of the Claims 10 or 11 , characterized in that the introduction of the dispersion into the pore spaces (12) of the pressed blank produces a mark (10) visible to the naked eye on a surface (8) of the coin blank (7). Procedure according to one of the Claims 10 to 12 , characterized in that the powder component contained in the dispersion is a ferrite former or an austenite former and that the at least one further powder component compressed in the press blank consists of iron or of a steel with martensitic phase components and / or ferritic phase components and / or austenitic phase components. Method according to one of the preceding claims, characterized in that the coin blank (7) is further processed into a coin (9), in particular by embossing the coin blank (7). Coin blank (7) or coin (9) produced by a powder metallurgical process according to one of the preceding claims. Coin (9), produced by embossing a coin blank (7) Claim 15 ,

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