CA2214613A1 - Coinage material and process for production thereof - Google Patents
Coinage material and process for production thereof Download PDFInfo
- Publication number
- CA2214613A1 CA2214613A1 CA002214613A CA2214613A CA2214613A1 CA 2214613 A1 CA2214613 A1 CA 2214613A1 CA 002214613 A CA002214613 A CA 002214613A CA 2214613 A CA2214613 A CA 2214613A CA 2214613 A1 CA2214613 A1 CA 2214613A1
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- CA
- Canada
- Prior art keywords
- chromium
- coinage
- austenite
- coinage material
- blank
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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- 239000000463 material Substances 0.000 title claims abstract description 38
- 238000000034 method Methods 0.000 title claims description 15
- 230000008569 process Effects 0.000 title claims description 11
- 238000004519 manufacturing process Methods 0.000 title description 5
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims abstract description 42
- 239000011651 chromium Substances 0.000 claims abstract description 42
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims abstract description 38
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 36
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 20
- 229910001209 Low-carbon steel Inorganic materials 0.000 claims abstract description 13
- 230000005291 magnetic effect Effects 0.000 claims abstract description 13
- 230000035699 permeability Effects 0.000 claims abstract description 11
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 16
- 238000005229 chemical vapour deposition Methods 0.000 claims description 16
- 229910000831 Steel Inorganic materials 0.000 claims description 14
- 239000010959 steel Substances 0.000 claims description 14
- 238000009792 diffusion process Methods 0.000 claims description 11
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 9
- 238000009713 electroplating Methods 0.000 claims description 9
- 239000011572 manganese Substances 0.000 claims description 9
- 229910052742 iron Inorganic materials 0.000 claims description 8
- 229910052748 manganese Inorganic materials 0.000 claims description 8
- 229910000599 Cr alloy Inorganic materials 0.000 claims description 6
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 4
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 4
- 229910052799 carbon Inorganic materials 0.000 claims description 4
- 229910002058 ternary alloy Inorganic materials 0.000 claims description 3
- 229910052757 nitrogen Inorganic materials 0.000 claims description 2
- 229910002059 quaternary alloy Inorganic materials 0.000 claims description 2
- 238000001514 detection method Methods 0.000 abstract description 2
- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 abstract 1
- 229910045601 alloy Inorganic materials 0.000 description 13
- 239000000956 alloy Substances 0.000 description 13
- 229910052751 metal Inorganic materials 0.000 description 10
- 239000002184 metal Substances 0.000 description 10
- 238000000576 coating method Methods 0.000 description 8
- 229910001220 stainless steel Inorganic materials 0.000 description 8
- 239000000758 substrate Substances 0.000 description 7
- 238000005254 chromizing Methods 0.000 description 6
- 239000011248 coating agent Substances 0.000 description 6
- -1 ferrous metals Chemical class 0.000 description 5
- 239000000203 mixture Substances 0.000 description 5
- 239000010935 stainless steel Substances 0.000 description 5
- 238000011282 treatment Methods 0.000 description 5
- 238000010200 validation analysis Methods 0.000 description 5
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 4
- 229910052802 copper Inorganic materials 0.000 description 4
- 239000010949 copper Substances 0.000 description 4
- 230000005294 ferromagnetic effect Effects 0.000 description 4
- 238000004381 surface treatment Methods 0.000 description 4
- 235000006679 Mentha X verticillata Nutrition 0.000 description 3
- 235000002899 Mentha suaveolens Nutrition 0.000 description 3
- 235000001636 Mentha x rotundifolia Nutrition 0.000 description 3
- 238000006243 chemical reaction Methods 0.000 description 3
- 238000000151 deposition Methods 0.000 description 3
- 230000008021 deposition Effects 0.000 description 3
- 239000007789 gas Substances 0.000 description 3
- 230000035515 penetration Effects 0.000 description 3
- 229910000990 Ni alloy Inorganic materials 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000001747 exhibiting effect Effects 0.000 description 2
- 239000010410 layer Substances 0.000 description 2
- 239000002344 surface layer Substances 0.000 description 2
- 229910001369 Brass Inorganic materials 0.000 description 1
- CPELXLSAUQHCOX-UHFFFAOYSA-M Bromide Chemical compound [Br-] CPELXLSAUQHCOX-UHFFFAOYSA-M 0.000 description 1
- 229910000906 Bronze Inorganic materials 0.000 description 1
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 1
- 229910017518 Cu Zn Inorganic materials 0.000 description 1
- 229910017752 Cu-Zn Inorganic materials 0.000 description 1
- 229910017943 Cu—Zn Inorganic materials 0.000 description 1
- 229910000640 Fe alloy Inorganic materials 0.000 description 1
- KRHYYFGTRYWZRS-UHFFFAOYSA-M Fluoride anion Chemical compound [F-] KRHYYFGTRYWZRS-UHFFFAOYSA-M 0.000 description 1
- 235000014435 Mentha Nutrition 0.000 description 1
- 241001072983 Mentha Species 0.000 description 1
- 229910000914 Mn alloy Inorganic materials 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 239000010951 brass Substances 0.000 description 1
- 239000010974 bronze Substances 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 150000001844 chromium Chemical class 0.000 description 1
- BCDWXIPZSYBYCG-UHFFFAOYSA-N chromium iron manganese Chemical compound [Mn][Cr][Fe] BCDWXIPZSYBYCG-UHFFFAOYSA-N 0.000 description 1
- BIJOYKCOMBZXAE-UHFFFAOYSA-N chromium iron nickel Chemical compound [Cr].[Fe].[Ni] BIJOYKCOMBZXAE-UHFFFAOYSA-N 0.000 description 1
- KUNSUQLRTQLHQQ-UHFFFAOYSA-N copper tin Chemical compound [Cu].[Sn] KUNSUQLRTQLHQQ-UHFFFAOYSA-N 0.000 description 1
- TVZPLCNGKSPOJA-UHFFFAOYSA-N copper zinc Chemical compound [Cu].[Zn] TVZPLCNGKSPOJA-UHFFFAOYSA-N 0.000 description 1
- 230000002950 deficient Effects 0.000 description 1
- 230000005672 electromagnetic field Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 150000004820 halides Chemical class 0.000 description 1
- 239000012433 hydrogen halide Substances 0.000 description 1
- 229910000039 hydrogen halide Inorganic materials 0.000 description 1
- XMBWDFGMSWQBCA-UHFFFAOYSA-N hydrogen iodide Chemical compound I XMBWDFGMSWQBCA-UHFFFAOYSA-N 0.000 description 1
- 238000003754 machining Methods 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 235000014569 mints Nutrition 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 239000010955 niobium Substances 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 238000007747 plating Methods 0.000 description 1
- 239000002243 precursor Substances 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 230000003019 stabilising effect Effects 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 229910052715 tantalum Inorganic materials 0.000 description 1
- GUVRBAGPIYLISA-UHFFFAOYSA-N tantalum atom Chemical compound [Ta] GUVRBAGPIYLISA-UHFFFAOYSA-N 0.000 description 1
- 239000010936 titanium Substances 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
Classifications
-
- G—PHYSICS
- G07—CHECKING-DEVICES
- G07D—HANDLING OF COINS OR VALUABLE PAPERS, e.g. TESTING, SORTING BY DENOMINATIONS, COUNTING, DISPENSING, CHANGING OR DEPOSITING
- G07D5/00—Testing specially adapted to determine the identity or genuineness of coins, e.g. for segregating coins which are unacceptable or alien to a currency
- G07D5/08—Testing the magnetic or electric properties
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
- C23C10/02—Pretreatment of the material to be coated
Landscapes
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Chemical Vapour Deposition (AREA)
- Electroplating Methods And Accessories (AREA)
Abstract
A coinage material comprises a ferritic blank, for example of mild steel, which has been surface-treated with an austenite-forming element such as nickel or manganese, and which has chromium diffused into the surface, such that a specific, marked difference, for example in electrical conductivity and/or magnetic permeability, which is electronically detectable exists between the surface and the interior. The coinage material may therefore be validated electronically using suitable detection means.
Description
Coinage material and process for production thereof This invention relates to a coinage material and production thereof, particularly a coinage material based on a ferritic material, exhibiting electronically detectable characteristics, in particular electrical and/or magnetic properties detectable by electronic S validation means.
Coinage has traditionally been produced from homogeneous blanks formed entirely from non-ferrous metals such as copper-based alloys. In many instances worldwide, primarily for reasons of manufacturing economy, such homogeneous coinage is now being replaced by coinage based on more widely available and less costly materials such as mild steel. To afford such coinage materials with adequate protection against tarnish and corrosion, and to provide the desired external appearance, the metal substrate is commonly coated on the surface. Typical coating materials include, copper, brass (Cu-Zn), bronze (Cu-Sn), and nickel, which materials are generally applied by electroplating 1 5 techniques.
Another type of coating process, known as "chromising", has now been developed for coinage production. In this process, originally disclosed for example in GB 862 282, chromium is deposited onto mild steel blanks by the technique of chemical vapourdeposition (CVD) and diffused into the surface to generate a layer which is effectively a ferritic stainless steel. Such chromised coinage material may be offered at a substantial price advantage over conventional homogeneous coinage made entirely from stainless steels, for example AISI 430-type stainless steel, which has hitherto been used in certain countries. Furthermore, it can be struck more easily in a coining press because the interior remains as a soft unalloyed iron.
Electronic coin validators are now widely in use to distinguish valid coinage from coinage which is invalid such as counterfeit, defective or foreign coinage. These validators can exploit differences in the properties between the coating and the substrate materials of conventional electroplated coins. Typically, electronic coin validators are able to detect differences in electrical conductivity and magnetic permeability. However, conventional chromised steel coinage, as with the AISI 430-type stainless steel coinage, does not present special security characteristics which might be exploited in electronic coin validators. Indeed, the two materials may appear equivalent in electronic validators.
This is because in a chromised steel blank the stainless steel at the surface and the . unalloyed steel in the interior are both of a ferritic character, and consequently show similar electrical and magnetic properties. Therefore, as with the homogeneous stainless steel coinage, no substantial property differences exist between the surface and the interior of chromised steel coinage which might be detected by electronic coin validators.
It has now been found that this limitation which is intrinsic to chromised steel coinage may be overcome by treating the surface of a ferritic blank with an austenite-forming element. Thus, the surface is transformed to a non-ferromagnetic austenitic structure, in contrast to the interior which remains ferritic. Specific, marked differences in properties generated between the surface and the interior as a consequence may then be detected using conventional electronic validation technology.
Accordingly, the present invention provides a coinage material comprising a ferritic blank which has been surface-treated with an austenite-forming element and which has chromium diffused into the surface, such that a specific, marked difference which is electronically detectable exists between the surface and the interior. The invention further provides a process for producing the coinage material, which comprises sequentially or simultaneously treating the surface of a ferritic blank with an austenite-forming element and diffusing chromium into the surface.
The coinage material according to the invention includes struck as well as unstruck coins, coin blanks, tokens, or the like, such as may be used to operate so-called slot machines, for example automatic vending machines and arcade game machines, or any other such machines providing goods or services.
As ferritic material for the blank to be treated in accordance with the invention, preferably mild steel is used, both on account of its low cost and easy availability, and because of its relative softness whereby coin blanks produced from mild steel may be struck without causing undue wear of the striking die or coin press. By "mild steel" is meant steel which has a low carbon content, generally below 0.25 wt%, for example steel conforming to British Standard BS 1449, Pt 1, 1991. Preferably, mild steel having a carbon content of 0.01-0.1 wt%, in particular 0.03-0.06 wt%, is used. Alternatively, so-called stabilised mild steel is used for the blank, meaning that the steel has been pre-treated, usually at the ladle stage, with an element which binds with interstitial elements such as carbon and nitrogen present in the steel, so as to reduce to a negligible amount the content of free interstitial elements remaining in the steel. Thus, the formation of hard carbides or nitrides of chromium during the subsequent chromising treatmentaccording to the invention may be precluded or reduced, thereby advantageously prolonging die life. Suitable stabilising elements include niobium, tantalum and titanium.
According to the invention, the ferritic blank is treated with an element which causes an austenitic layer to form in the surface. Any suitable austenite-forming elements may be used, for example nickel, m~ng~nese, or mixtures thereof. Both nickel and manganese are well known austenite-formers for steel; for example manganese is present in austenitic stainless steels AISI 202 and 205 in amounts of 9 wt% and 14.5 wt%, respectively. The austenite-forming treatment may be carried out by any suitablemethod, such as by electroplating or by CVD, as appropriate to the particular austenite-forming element used. For example, if nickel is used as the austenite-former, it may suitably be applied by electroplating, for example by the plating process disclosed in GB
1 477 981. If m~n"~nese is used as the austenite-former, then it may suitably be applied using conventional CVD techniques, or by electroplating. In a preferred embodiment, a mild steel blank is surface-treated with nickel by electroplating. For example, the nickel may be deposited to a thickness up to 50 ~lm, in particular 2 - 10 ~lm.
Conveniently, nickel-plated coin blanks are now commercially available from a number of producers across the world, and are supplied to a large number of national mints.
Suppliers include Westaim, The Royal Mint (GB), IMI Birmingham Mint (GB) and theSouth African Mint. Such blanks are all fabricated to withstand the high temperatures required in the chromising process and are therefore suitable for use in the present invention.
-According to the invention, in addition to the austenite-forming surface treatment the blank is subjected to chromium diffusion, suitably using CVD techniques known in the art. Chromium CVD has been used for example to increase the resistance of substrates such as nickel and nickel alloys to high temperature corrosive attack. The application of chromium coatings in the above manner is particularly useful where any advantageous mechanical properties of the base material need to be preserved. This is especially important for highly stressed components that operate at elevated temperatures such as gas turbine blades. For components that are not highly stressed in service, CVD of chromium may also be advantageous. For example, the machining of components madefrom alloys containing nickel and chromium is difficult and rapid tool wear can result.
Therefore, it is in principle easier to machine the component from nickel and apply the chromium coating at a later stage. There is also an economic advantage in the above route.
The basic principles for the chemical vapour deposition of chromium are as follows:-A precursor chemical which is normally a chromium halide is generated at elevated temperature by the reaction of a hydrogen halide (or halooen) gas with metallic chromium. The halides used are typically chloride, bromide, fluoride or iodide. The volatile chromium halide may be produced by the following reaction, for example:-2Cr (metal) + 2HX - 2CrX + H7 (i) in which X represents Cl, I, Br or F
The chromium halide gas is then allowed to come into contact with the substrate to be coated. Deposition can occur via the following reactions:-2Crx - 2Cr(metal~ + Xz (ii) 2CrX + H7 - 2Cr (metal) + 2HX (iii) Once deposition of chromium has occurred onto the substrate, the elevated temperatures used in the process allow subsequent diffusion of the chromium into the surface.Typically a chromium concentration of 40 wt% is achieved at the surface after a coating -operation. The depth of diffusion can be controlled by varying the deposition temperature and the residence time at this temperature.
Where surface treatment to form an austenitic surface layer is carried out by electroplating, for example using nickel or m~ng~nese, then the chromium diffusion must S take place subsequently. Thus, in accordance with the invention, chromium may for example be diffused into the surface of a mild steel blank that has already beenprecoated with an electroplate of nickel. The depth of this chromium coating is typically up to 50 ~um and preferably from 10 - 40 ,um. However, if the surface-treatment is to be effected by CVD of m~ng~nese, then the manganese and chromium treatments may be carried out by CVD either simultaneously in one step, or sequentially in two separate steps. Advantageously, the chromising treatment in accordance with the inventionprovides a coinage material which is resistant to corrosion and discolouration, and of an attractive, lustrous, silver-like appearance.
As a result of the austenite-forming and chromising treatments in accordance with the invention, the ferritic, body-centred cubic crystal structure of the blank is transformed in the surface to an austenitic, face-centred cubic crystal structure, thereby significantly altering the material properties of the blank substrate material. The specific, marked differences in properties thus generated between the surface and the interior may then be detected electronically as a means of validation.
Now, in an electronic validator the coin is made to pass through the magnetic field of one or more electrical coils. A sinusoidally varying magnetic field is set up in each coil;
the presence of the coin disturbs the field, and the instrument then detects the change in the resonant frequency, phase or amplitude as a means of validation or rejection. The dimensions and material properties of the coin are the relevant factors that serve to characterise it in such validators.
In practice, most electronic mechanisms use several coils operating at differentfrequencies. When a coin with a low magnetic permeability and low electrical conductivity passes over a coil operating at a relatively low frequency, the electromagnetic field will induce currents at some distance within the coin itself. By Co~ dsl~ a high frequency coil will tend to induce currents predominantly in the surface of the coin, especially when the coin is made of a material with a high permeability or high conductivity.
The depth (d) of penetration of the magnetic field in the metal is given by the relationship d = \/{P/21lf~o~lr}
where p is the resistivity of the metal, f is the frequency of the sinusoidal variation of the magnetic field, ~uo is the permeability of free space, ll, is the relative permeability of the metal, and d is the penetration depth, being the distance into the magnetic field from the surface at which the magnetic field strength is one half of the field strength at the surface.
Thus, by selecting approp~iate frequencies for the various coils it is possible to "read"
a coin in a vending machine at various depths and to detect the difference in the metal of the core from that of the coating.
The present invention provides a coinage material in which the interior and surface differ very markedly in their relevant properties (resistivity and permeability), and which can therefore be validated with a higher degree of confidence than has hitherto been possible. In particular, the security of chromised coinage is substantially improved.
This may be demonstrated by considering the actual values for the resistivity and relative penneability of the common coinage metals - with reference to the penetration relationship quoted above.
Thus, for homogeneous coinage made from the various copper-based alloys, the relative permeability is close to unity, since, in every case, these alloys are non-ferromagnetic.
Discrimination between these alloys therefore depends principally on their resistivities.
-The most conductive of the copper-based coinage alloys are the so-called coinagebronzes, for which the resistivity is typically about 35 nQ-m.
Stainless steel of the AISI 430 type is also used for homogeneous coinage. The quoted resistivity of this alloy is 620 nQ-m, but because it is a ferromagnetic alloy its relative S perr~leability will be very large, perhaps 1000 or more. Mild steel will similarly exhibit a high relative magnetic permeability, but is of lower resistivity - typically 120 nQ-m.
The austenitic or face-centered cubic alloys of iron-chromium-nickel (or iron-chromium-manganese) are of even higher resistivity, probably in excess of 1000 nQ-m, depending on the exact composition. However, because the alloys in question are non-ferromagnetic, their permeability will again be close to unity.
It can be seen from these figures that the invention offers a coinage material with a low-permeability, high resistivity surface in contrast to the interior which has the opposite properties. These marked differences will greatly assist the task of positive discrimination in coin validators, and thereby offer much increased security.
The surface-treatment of the ferritic blank material with the austenite-forming element and the chromium diffusion results in a complex alloy system in the surface of the coin, comprising the iron, chromium and austenite-forming elements. The composition of this alloy changes continuously across the depth of the surface layer. For example, in a preferred embodiment, if nickel plate is used as austenite-forming element to surface-treat a mild steel blank, followed by chromium CVD and diffusion, then the thickness of the nickel plate may be varied independently of the depth of chromium diffusion.
Thus, chromium may be diffused into the nickel plate to a depth which is less than the thickness of the nicke1 plate, or to a ~reater depth such that chromium dif~uses through the nickel plate completely and into the steel base. Moreover, the solubility of the iron and nickel is such that a solid solution of these elements forms at the original boundary of the electroplate and the steel substrate. Consequently, a complex ternary alloy of iron, nickel and chromium may be created following the chromium diffusion. Similarly, various complex alloys of iron, manganese and chromium can be created by controlling -the CVD diffusions of manganese and chromium. Thus, a ternary alloy of iron, chromium and an austenite-forming element may be formed. ~f more than one austenite-forming element is used, a quaternary alloy of iron, chromium and two austenite-forming elements may be formed. Therefore, by controlling the austenite-forming and chromising treatments, the composition profile of the complex alloy system can be varied. It is thus possible to produce different coin specifications, each exhibiting characteristic properties according to the particular complex composition in the surface.
Any suitable remote detection means which is able to measure the characteristic properties in the coin surface can then be used to identify a particular coin specification, and thus distinguish it from other coin specifications as a means of validation, even if dimensionally similar.
Coinage has traditionally been produced from homogeneous blanks formed entirely from non-ferrous metals such as copper-based alloys. In many instances worldwide, primarily for reasons of manufacturing economy, such homogeneous coinage is now being replaced by coinage based on more widely available and less costly materials such as mild steel. To afford such coinage materials with adequate protection against tarnish and corrosion, and to provide the desired external appearance, the metal substrate is commonly coated on the surface. Typical coating materials include, copper, brass (Cu-Zn), bronze (Cu-Sn), and nickel, which materials are generally applied by electroplating 1 5 techniques.
Another type of coating process, known as "chromising", has now been developed for coinage production. In this process, originally disclosed for example in GB 862 282, chromium is deposited onto mild steel blanks by the technique of chemical vapourdeposition (CVD) and diffused into the surface to generate a layer which is effectively a ferritic stainless steel. Such chromised coinage material may be offered at a substantial price advantage over conventional homogeneous coinage made entirely from stainless steels, for example AISI 430-type stainless steel, which has hitherto been used in certain countries. Furthermore, it can be struck more easily in a coining press because the interior remains as a soft unalloyed iron.
Electronic coin validators are now widely in use to distinguish valid coinage from coinage which is invalid such as counterfeit, defective or foreign coinage. These validators can exploit differences in the properties between the coating and the substrate materials of conventional electroplated coins. Typically, electronic coin validators are able to detect differences in electrical conductivity and magnetic permeability. However, conventional chromised steel coinage, as with the AISI 430-type stainless steel coinage, does not present special security characteristics which might be exploited in electronic coin validators. Indeed, the two materials may appear equivalent in electronic validators.
This is because in a chromised steel blank the stainless steel at the surface and the . unalloyed steel in the interior are both of a ferritic character, and consequently show similar electrical and magnetic properties. Therefore, as with the homogeneous stainless steel coinage, no substantial property differences exist between the surface and the interior of chromised steel coinage which might be detected by electronic coin validators.
It has now been found that this limitation which is intrinsic to chromised steel coinage may be overcome by treating the surface of a ferritic blank with an austenite-forming element. Thus, the surface is transformed to a non-ferromagnetic austenitic structure, in contrast to the interior which remains ferritic. Specific, marked differences in properties generated between the surface and the interior as a consequence may then be detected using conventional electronic validation technology.
Accordingly, the present invention provides a coinage material comprising a ferritic blank which has been surface-treated with an austenite-forming element and which has chromium diffused into the surface, such that a specific, marked difference which is electronically detectable exists between the surface and the interior. The invention further provides a process for producing the coinage material, which comprises sequentially or simultaneously treating the surface of a ferritic blank with an austenite-forming element and diffusing chromium into the surface.
The coinage material according to the invention includes struck as well as unstruck coins, coin blanks, tokens, or the like, such as may be used to operate so-called slot machines, for example automatic vending machines and arcade game machines, or any other such machines providing goods or services.
As ferritic material for the blank to be treated in accordance with the invention, preferably mild steel is used, both on account of its low cost and easy availability, and because of its relative softness whereby coin blanks produced from mild steel may be struck without causing undue wear of the striking die or coin press. By "mild steel" is meant steel which has a low carbon content, generally below 0.25 wt%, for example steel conforming to British Standard BS 1449, Pt 1, 1991. Preferably, mild steel having a carbon content of 0.01-0.1 wt%, in particular 0.03-0.06 wt%, is used. Alternatively, so-called stabilised mild steel is used for the blank, meaning that the steel has been pre-treated, usually at the ladle stage, with an element which binds with interstitial elements such as carbon and nitrogen present in the steel, so as to reduce to a negligible amount the content of free interstitial elements remaining in the steel. Thus, the formation of hard carbides or nitrides of chromium during the subsequent chromising treatmentaccording to the invention may be precluded or reduced, thereby advantageously prolonging die life. Suitable stabilising elements include niobium, tantalum and titanium.
According to the invention, the ferritic blank is treated with an element which causes an austenitic layer to form in the surface. Any suitable austenite-forming elements may be used, for example nickel, m~ng~nese, or mixtures thereof. Both nickel and manganese are well known austenite-formers for steel; for example manganese is present in austenitic stainless steels AISI 202 and 205 in amounts of 9 wt% and 14.5 wt%, respectively. The austenite-forming treatment may be carried out by any suitablemethod, such as by electroplating or by CVD, as appropriate to the particular austenite-forming element used. For example, if nickel is used as the austenite-former, it may suitably be applied by electroplating, for example by the plating process disclosed in GB
1 477 981. If m~n"~nese is used as the austenite-former, then it may suitably be applied using conventional CVD techniques, or by electroplating. In a preferred embodiment, a mild steel blank is surface-treated with nickel by electroplating. For example, the nickel may be deposited to a thickness up to 50 ~lm, in particular 2 - 10 ~lm.
Conveniently, nickel-plated coin blanks are now commercially available from a number of producers across the world, and are supplied to a large number of national mints.
Suppliers include Westaim, The Royal Mint (GB), IMI Birmingham Mint (GB) and theSouth African Mint. Such blanks are all fabricated to withstand the high temperatures required in the chromising process and are therefore suitable for use in the present invention.
-According to the invention, in addition to the austenite-forming surface treatment the blank is subjected to chromium diffusion, suitably using CVD techniques known in the art. Chromium CVD has been used for example to increase the resistance of substrates such as nickel and nickel alloys to high temperature corrosive attack. The application of chromium coatings in the above manner is particularly useful where any advantageous mechanical properties of the base material need to be preserved. This is especially important for highly stressed components that operate at elevated temperatures such as gas turbine blades. For components that are not highly stressed in service, CVD of chromium may also be advantageous. For example, the machining of components madefrom alloys containing nickel and chromium is difficult and rapid tool wear can result.
Therefore, it is in principle easier to machine the component from nickel and apply the chromium coating at a later stage. There is also an economic advantage in the above route.
The basic principles for the chemical vapour deposition of chromium are as follows:-A precursor chemical which is normally a chromium halide is generated at elevated temperature by the reaction of a hydrogen halide (or halooen) gas with metallic chromium. The halides used are typically chloride, bromide, fluoride or iodide. The volatile chromium halide may be produced by the following reaction, for example:-2Cr (metal) + 2HX - 2CrX + H7 (i) in which X represents Cl, I, Br or F
The chromium halide gas is then allowed to come into contact with the substrate to be coated. Deposition can occur via the following reactions:-2Crx - 2Cr(metal~ + Xz (ii) 2CrX + H7 - 2Cr (metal) + 2HX (iii) Once deposition of chromium has occurred onto the substrate, the elevated temperatures used in the process allow subsequent diffusion of the chromium into the surface.Typically a chromium concentration of 40 wt% is achieved at the surface after a coating -operation. The depth of diffusion can be controlled by varying the deposition temperature and the residence time at this temperature.
Where surface treatment to form an austenitic surface layer is carried out by electroplating, for example using nickel or m~ng~nese, then the chromium diffusion must S take place subsequently. Thus, in accordance with the invention, chromium may for example be diffused into the surface of a mild steel blank that has already beenprecoated with an electroplate of nickel. The depth of this chromium coating is typically up to 50 ~um and preferably from 10 - 40 ,um. However, if the surface-treatment is to be effected by CVD of m~ng~nese, then the manganese and chromium treatments may be carried out by CVD either simultaneously in one step, or sequentially in two separate steps. Advantageously, the chromising treatment in accordance with the inventionprovides a coinage material which is resistant to corrosion and discolouration, and of an attractive, lustrous, silver-like appearance.
As a result of the austenite-forming and chromising treatments in accordance with the invention, the ferritic, body-centred cubic crystal structure of the blank is transformed in the surface to an austenitic, face-centred cubic crystal structure, thereby significantly altering the material properties of the blank substrate material. The specific, marked differences in properties thus generated between the surface and the interior may then be detected electronically as a means of validation.
Now, in an electronic validator the coin is made to pass through the magnetic field of one or more electrical coils. A sinusoidally varying magnetic field is set up in each coil;
the presence of the coin disturbs the field, and the instrument then detects the change in the resonant frequency, phase or amplitude as a means of validation or rejection. The dimensions and material properties of the coin are the relevant factors that serve to characterise it in such validators.
In practice, most electronic mechanisms use several coils operating at differentfrequencies. When a coin with a low magnetic permeability and low electrical conductivity passes over a coil operating at a relatively low frequency, the electromagnetic field will induce currents at some distance within the coin itself. By Co~ dsl~ a high frequency coil will tend to induce currents predominantly in the surface of the coin, especially when the coin is made of a material with a high permeability or high conductivity.
The depth (d) of penetration of the magnetic field in the metal is given by the relationship d = \/{P/21lf~o~lr}
where p is the resistivity of the metal, f is the frequency of the sinusoidal variation of the magnetic field, ~uo is the permeability of free space, ll, is the relative permeability of the metal, and d is the penetration depth, being the distance into the magnetic field from the surface at which the magnetic field strength is one half of the field strength at the surface.
Thus, by selecting approp~iate frequencies for the various coils it is possible to "read"
a coin in a vending machine at various depths and to detect the difference in the metal of the core from that of the coating.
The present invention provides a coinage material in which the interior and surface differ very markedly in their relevant properties (resistivity and permeability), and which can therefore be validated with a higher degree of confidence than has hitherto been possible. In particular, the security of chromised coinage is substantially improved.
This may be demonstrated by considering the actual values for the resistivity and relative penneability of the common coinage metals - with reference to the penetration relationship quoted above.
Thus, for homogeneous coinage made from the various copper-based alloys, the relative permeability is close to unity, since, in every case, these alloys are non-ferromagnetic.
Discrimination between these alloys therefore depends principally on their resistivities.
-The most conductive of the copper-based coinage alloys are the so-called coinagebronzes, for which the resistivity is typically about 35 nQ-m.
Stainless steel of the AISI 430 type is also used for homogeneous coinage. The quoted resistivity of this alloy is 620 nQ-m, but because it is a ferromagnetic alloy its relative S perr~leability will be very large, perhaps 1000 or more. Mild steel will similarly exhibit a high relative magnetic permeability, but is of lower resistivity - typically 120 nQ-m.
The austenitic or face-centered cubic alloys of iron-chromium-nickel (or iron-chromium-manganese) are of even higher resistivity, probably in excess of 1000 nQ-m, depending on the exact composition. However, because the alloys in question are non-ferromagnetic, their permeability will again be close to unity.
It can be seen from these figures that the invention offers a coinage material with a low-permeability, high resistivity surface in contrast to the interior which has the opposite properties. These marked differences will greatly assist the task of positive discrimination in coin validators, and thereby offer much increased security.
The surface-treatment of the ferritic blank material with the austenite-forming element and the chromium diffusion results in a complex alloy system in the surface of the coin, comprising the iron, chromium and austenite-forming elements. The composition of this alloy changes continuously across the depth of the surface layer. For example, in a preferred embodiment, if nickel plate is used as austenite-forming element to surface-treat a mild steel blank, followed by chromium CVD and diffusion, then the thickness of the nickel plate may be varied independently of the depth of chromium diffusion.
Thus, chromium may be diffused into the nickel plate to a depth which is less than the thickness of the nicke1 plate, or to a ~reater depth such that chromium dif~uses through the nickel plate completely and into the steel base. Moreover, the solubility of the iron and nickel is such that a solid solution of these elements forms at the original boundary of the electroplate and the steel substrate. Consequently, a complex ternary alloy of iron, nickel and chromium may be created following the chromium diffusion. Similarly, various complex alloys of iron, manganese and chromium can be created by controlling -the CVD diffusions of manganese and chromium. Thus, a ternary alloy of iron, chromium and an austenite-forming element may be formed. ~f more than one austenite-forming element is used, a quaternary alloy of iron, chromium and two austenite-forming elements may be formed. Therefore, by controlling the austenite-forming and chromising treatments, the composition profile of the complex alloy system can be varied. It is thus possible to produce different coin specifications, each exhibiting characteristic properties according to the particular complex composition in the surface.
Any suitable remote detection means which is able to measure the characteristic properties in the coin surface can then be used to identify a particular coin specification, and thus distinguish it from other coin specifications as a means of validation, even if dimensionally similar.
Claims (13)
1. A coinage material comprising a ferritic blank which has been surface-treatedwith an austenite-forming element and which has chromium diffused into the surface, such that a specific, marked difference which is electronically detectable exists between the surface and the interior.
2. A coinage material as claimed in claim 1 wherein the coinage material is a struck or unstruck coin, coin blank or token.
3. A coinage material as claimed in claim 1 or claim 2 wherein the blank consists essentially of mild steel.
4. A coinage material as claimed in claim 3 wherein the steel has a negligible content of free interstitial elements such as carbon or nitrogen, and/or is stabilised for example by the inclusion of carbide- and/or nitride-formers.
5. A coinage material as claimed in any of claims 1 to 4 wherein the austenite-forming element is selected from nickel and manganese.
6. A coinage material as claimed in claim 5 wherein the core has been surface-treated with nickel by electroplating or with manganese by chemical vapour deposition or electroplating.
7. A coinage material as claimed in any of claims 1 to 6 wherein the surface comprises a ternary alloy of iron, chromium and the austenite-forming element.
8. A coinage material as claimed in any of claims 1 to 6 wherein the surface comprises a quaternary alloy of iron, chromium and two austenite-forming elements.
9. A coinage material as claimed in any of claims 1 to 8 wherein the difference is in electrical conductivity and/or magnetic permeability.
10. A process for producing a coinage material as claimed in any of claims 1 to 9, which comprises sequentially or simultaneously treating the surface of a ferritic blank with an austenite-forming element and diffusing chromium into the surface.
11. A process as claimed in claim 10 wherein the surface is treated with nickel or manganese by electroplating and is subsequently subjected to the chromium diffusion.
12. A process as claimed in claim 10 or claim 11 wherein the chromium is deposited onto the surface by chemical vapour deposition and allowed to diffuse therein.
13. A process as claimed in claim 12 wherein the surface is treated with manganese by chemical vapour deposition and is subsequently or simultaneously subjected to the chemical vapour deposition and diffusion of chromium.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GBGB9618460.1 | 1996-09-04 | ||
| GBGB9618460.1A GB9618460D0 (en) | 1996-09-04 | 1996-09-04 | Coinage material and process for production thereof |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2214613A1 true CA2214613A1 (en) | 1998-03-04 |
Family
ID=10799400
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002214613A Abandoned CA2214613A1 (en) | 1996-09-04 | 1997-09-03 | Coinage material and process for production thereof |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0828011A1 (en) |
| KR (1) | KR19980024324A (en) |
| CA (1) | CA2214613A1 (en) |
| GB (1) | GB9618460D0 (en) |
| ZA (1) | ZA977879B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9447515B2 (en) | 2008-06-13 | 2016-09-20 | Royal Canadian Mint | Control of electromagnetic signals of coins through multi-ply plating technology |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2337765A (en) * | 1998-05-27 | 1999-12-01 | Solicitor For The Affairs Of H | Aluminium diffusion of copper coatings |
| GB0419609D0 (en) * | 2004-09-03 | 2004-10-06 | Diffusion Alloys Ltd | Process for the production of coin blanks |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1021039A (en) * | 1950-06-27 | 1953-02-13 | Onera (Off Nat Aerospatiale) | Improvements in processes for the electrolytic deposition of a metal, in particular chromium, on metal parts |
| GB862282A (en) * | 1959-10-16 | 1961-03-08 | Chrome Alloying Company Ltd | A process for the manufacture of metal articles involving die stamping |
| DE1774498C2 (en) * | 1968-06-29 | 1975-06-19 | Vereinigte Deutsche Metallwerke Ag, 6000 Frankfurt | Metal coins or the like made of layers of non-magnetizable and magnetizable metal |
| DE3817657A1 (en) * | 1988-05-25 | 1989-12-07 | Vdm Nickel Tech | LAYER COMPOSITE FOR THE PRODUCTION OF COINS |
-
1996
- 1996-09-04 GB GBGB9618460.1A patent/GB9618460D0/en active Pending
-
1997
- 1997-09-02 ZA ZA9707879A patent/ZA977879B/en unknown
- 1997-09-03 CA CA002214613A patent/CA2214613A1/en not_active Abandoned
- 1997-09-04 KR KR1019970045699A patent/KR19980024324A/en not_active Application Discontinuation
- 1997-09-04 EP EP97306863A patent/EP0828011A1/en not_active Withdrawn
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9447515B2 (en) | 2008-06-13 | 2016-09-20 | Royal Canadian Mint | Control of electromagnetic signals of coins through multi-ply plating technology |
Also Published As
| Publication number | Publication date |
|---|---|
| ZA977879B (en) | 1998-03-02 |
| GB9618460D0 (en) | 1996-10-16 |
| KR19980024324A (en) | 1998-07-06 |
| EP0828011A1 (en) | 1998-03-11 |
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