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

Definitions

• 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 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 techniques.
• chromising Another type of coating process, known as "chromising”, has now been developed for coinage production.
• chromium is deposited onto mild steel blanks by the technique of chemical vapour deposition (CVD) and diffused into the surface to generate a layer which is effectively a ferritic stainless steel.
• CVD chemical vapour deposition
• 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.
• 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.
• 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.
• 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.
• 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.
• mild steel having a carbon content of 0.01-0. 1 wt%, in particular 0.03-0.06 wt%, is used.
• stabilised mild steel is used for the blank, meaning that the steel has been pretreated, 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.
• interstitial elements such as carbon and nitrogen present in the steel
• 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, manganese, 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 suitable method, such as by electroplating or by CVD, as appropriate to the particular austenite-forming element used.
• 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 manganese is used as the austenite-former, then it may suitably be applied using conventional CVD techniques, or by electroplating.
• a mild steel blank is surface-treated with nickel by electroplating.
• the nickel may be deposited to a thickness up to 50 ⁇ m, in particular 2 - 10 ⁇ m.
• 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 the South 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.
• the blank 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.
• CVD of chromium may also be advantageous.
• the machining of components made from 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.
• a precursor chemical which is normally a chromium halide is generated at elevated temperature by the reaction of a hydrogen halide (or halogen) 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 + H 2 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) + X 2 2CrX + H 2 ⁇ 2Cr (metal) + 2HX
• the elevated temperatures used in the process allow subsequent diffusion of the chromium into the surface.
• 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.
• chromium may for example be diffused into the surface of a mild steel blank that has already been precoated with an electroplate of nickel.
• the depth of this chromium coating is typically up to 50 ⁇ m and preferably from 10 - 40 ⁇ m.
• the surface-treatment is to be effected by CVD of manganese, then the manganese and chromium treatments may be carried out by CVD either simultaneously in one step, or sequentially in two separate steps.
• the chromising treatment in accordance with the invention provides a coinage material which is resistant to corrosion and discolouration, and of an attractive, lustrous, silver-like appearance.
• 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.
• the coin 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.
• 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.
• the security of chromised coinage is substantially improved.
• 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 coinage bronzes, for which the resistivity is typically about 35 n ⁇ -m.
• Stainless steel of the AISI 430 type is also used for homogeneous coinage.
• the quoted resistivity of this alloy is 620 n ⁇ -m, but because it is a ferromagnetic alloy its relative permeability 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 n ⁇ -m.
• the austenitic or face-centered cubic alloys of iron-chromium-nickel are of even higher resistivity, probably in excess of 1000 n ⁇ -m, depending on the exact composition. However, because the alloys in question are non-ferromagnetic, their permeability will again be close to unity.
• the invention offers a coinage material with a low-permeability, high resistivity surface in contrast to the interior which has the opposite properties.
• 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.
• 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.
• chromium may be diffused into the nickel plate to a depth which is less than the thickness of the nickel plate, or to a greater depth such that chromium diffuses through the nickel plate completely and into the steel base.
• 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.
• various complex alloys of iron, manganese and chromium can be created by controlling the CVD diffusions of manganese and chromium.
• a ternary alloy of iron, chromium and an austenite-forming element may be formed. If more than one austenite-forming element is used, a quaternary alloy of iron, chromium and two austenite-forming elements may be formed.
• 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.

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)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10799400

Family Applications (1)

Country Status (5)

Cited By (2)

Families Citing this family (1)

Citations (4)

• 1996
  • 1996-09-04 GB GBGB9618460.1A patent/GB9618460D0/en active Pending
• 1997
  • 1997-09-02 ZA ZA9707879A patent/ZA977879B/xx unknown
  • 1997-09-03 CA CA002214613A patent/CA2214613A1/en not_active Abandoned
  • 1997-09-04 KR KR1019970045699A patent/KR19980024324A/ko not_active Application Discontinuation
  • 1997-09-04 EP EP97306863A patent/EP0828011A1/en not_active Withdrawn

Patent Citations (4)

Also Published As

Similar Documents

Legal Events