GB2102708A - Process for producing coin blanks - Google Patents

Abstract

A process for producing blanks suitable for minting into coins includes providing a plurality of appropriately disc-shaped metallic cores, heating the cores to decrease their hardness to less than about 65 of the Rockwell 30T hardness scale, and cooling the heated cores to provide cooled cores with a hardness less than about 65 on the Rockwell 30T hardness scale. A plurality of cooled cores are loaded into an electrically non-conducting perforated container, which is then placed in an electroplating bath. A metallic cladding is electroplated onto the cores, while moving the container angularly about a horizontal axis, until a plating thickness of at least about 0.01 mm has been deposited on each face of each core and a thickness of from about 2 to about 4 times the face thickness has been deposited on the circumference of each core. The cladded core pieces are then removed from the container.

Description

SPECIFICATION Process for producing coin blanks This invention relates to the production of coin blanks suitable for minting into coins, the term "coins" being intended to cover not only coins used as currency but also similar disc-like articles such as medals and medallions upon which insignia is imprinted.

Because of the escalating cost of metals nprmally used for coins, attempts have been made to develop satisfactory coins which are made of less expensive materials. Prior proposals in this respect are disclosed in United States patent No. 3,940,254 issued February 24, 1976, United States patent No. 4,089,753, issued May 16, 1978, United States patent No. 4,176,014 issued March 27, 1979 and United States patent No. 4,247,274 issued January 27, 1981.In these prior proposals, a metal such as nickel or copper is electroplated onto a disc-shaped steel core to produce a nickel or copper cladding with a thickness of at least about 0.03-0.05 mm on each opposed face of the core and a thickness on the circumference of the core in the range of from about 2 to about 4 times the face thickness, and the cladded core is heated to form a metallurgical bond between the nickel or copper cladding and the core and to reduce the hardness to less than about 65 on the Rockwell 30T hardness scale. The resultant blanks are then imprinted to form coins.

In the heating step in such prior proposals, care must be taken to ensure that the smooth external surface appearance of the cladded core does not deteriorate. It is therefore an object of the invention to provide a process which overcomes this problem.

The present invention is based on the discovery that satisfactory coin blanks can be produced by annealing a metal core to reduce its hardness to less than about 65 on the Rockwell 30T hardness scale, and electro-plating a metallic cladding onto the annealed core to produce a thickness of at least about 0.01 mm on each opposed face of the core and a thickness on the circumference of from about 2 to about 4 times the face thickness. In other words, the metallurgical bond formed between the cladding and the core in the prior proposals can be omitted, and the annealing of the core can be carried out before instead of after the electroplating operation.

Accordingly, the present invention provides a process for producing blanks suitable for minting into coins, comprising providing a plurality of appropriately disc-shaped metallic cores, heating the cores to decrease their hardness to less than about 65 on the Rockwell 30T hardness scale, cooling the heated cores to provide cooled cores with a hardness less than about 65 on the Rockwell 30T hardness scale, loading a plurality of cooled cores into an electrically non-conducting perforated container, placing the container in an electroplating bath, electroplating a metallic cladding onto the cores, while moving the container angularly about a horizontal axis, until a plating thickness of at least about 0.01 mm has been deposited on each face of each core and a thickness of from about 2 to about 4 times the face thickness has been deposited on the circumference of each core, and removing the cladded core pieces from the container.

The metallic cladding may be nickel, a nickel alloy, copper or a coppy alloy, silver or gold, and the cores may be of steel, nickel, zinc, zinc alloys or other commercial coinage metals such as cupronickel. The steel may have a carbon content in the range of from about 0.005 to about 0.1% by weight. It has been found that, where the steel has a carbon content of less than about 0.01% the heated cores may be cooled by immersion in water. However, when the steel has a carbon content higher than about 0.01%, it has been found that the heated cores should be cooled at a much slower rate, for example about 20"C/min, to provide the resultant required hardness.

Advantageously, the cooled cores have a hardness less than about 50 on the Rockwell 30 T hardness scale. The annealing step will usually be carried out at a temperature of from about 800 to about 1000"C in a non-oxidizing atmosphere, for example a reducing atmosphere such as hydrogen.

The metallic cladding may be applied in the manner described in previously mentioned United States patent No. 4,176,014, with a face thickness of at least about 0.01 mm, preferably between about 0.01 mm and 0.1 mm.

Examples of the heating and cooling steps will now be described.

Several disc-shaped steel cores of different carbon content were heated to 900"C and retained at that temperature for 30 minutes in a nitrogen atmosphere. After this heat treatment, the cores were cooled by plunging them into water to give an estimated cooling rate of more than 1000"C/min. The core hardness was then measured.

A similar number of fresh cores were heated as before, with the cooling in this test being slow cooling effected by merely allowing the furnace to cool down, the cooling rate in this case being less than 1 C/min. Other cores were then subjected to a further test in which after the heating step, the cores were cooled under a nitrogen atmosphere at a rate of about 20"C/min. The hardness of the cooled cores was measured in each case.

The results of the tests are shown in the following table, the hardness being measured on the Rockwell 30 T scale.

Hardness Hardness Hardness Carbon Content after water after slow after 20 > C/min (w/o) cooling cooling cooling 0.0523 57 31 46 0.0758 63 31 46 0.0814 66 32 46 0.105 79 35 49 0.008 32 31 23 It will be seen from the above table that, where the carbon content was less than 0.008%, water cooling was satisfactory but that, where the carbon content was greater than 0.05%, the cooled cores had a hardness which was too high. The very slow cooling was satisfactory for the cores of high carbon content, but such a cooling rate is rather slow for a commercial operation.

Cooling at a rate of about 20"C/min is more satisfactory for the cores of higher carbon content.

As mentioned earlier, the metallic cladding may be of nickel, nickel alloys, copper, or copper alloys. A nickel alloy may be nickel-iron and a copper alloy may be copper-tin (i.e. bronze), with the metallic cladding of such an alloy being obtained by co-depositing from an electroplating solution containing the two alloy constituents. As also mentioned, the core may be of zinc or zinc alloy, and in the latter case an alloying element may for example be cadmium, copper or magnesium.

Suitable metallic cladding and metallic core combinations which can be used with the process of the present invention are nickel on steel, silver on nickel, copper on nickel, nickel on copper, nickel on cupronickel, gold on copper on nickel, bronze on steel, bronze on nickel, copper on steel and nickel-iron on steel.

Other embodiments and examples of the invention will be readily apparent to a person skilled in the art.

Claims (10)

1. A process for producing blanks suitable for minting into coins, comprising providing a plurality of appropriately disc-shaped metallic cores, heating the cores to decrease their hardness to less than about 65 on the Rockwell 30T hardness scale, cooling the heated cores to provide cooled cores with a hardness less than about 65 on the Rockwell 30T hardness scale, loading a plurality of cooled cores into an electrically non-conducting perforated container, placing the container in an electroplating bath, electroplating a metallic cladding onto the cores, while moving the container angularly about a horizontal axis, until a plating thickness of at least about 0.01 mm has been deposited on each face of each core and a thickness of from 2 to 4 times the face thickness has been deposited on the circumference of each core, and removing the cladded core pieces from the container.

2. A process according to claim 1 wherein the metallic cladding is selected from the group consisting of nickel, nickel alloys, copper and copper alloys.

3. A process according to claim 1 or 2 wherein the cores are of steel.

4. A process according to claim 3 wherein the steel has a carbon content in the range of from 0.005 to 0.1 % by weight.

5. A process according to claim 4 wherein the steel has a carbon content of less than about 0.01% and the heated cores are cooled by immersion in water.

6. A process according to claim 4 wherein the steel has a carbon content higher than about 0.01 % and the heated cores are cooled at a rate to provide a hardness of less than about 50 on the Rockwell 30T hardness scale.

7. A process according to claim 6 wherein the heated cores are cooled at a rate of about 20"C/min.

8. A process according to claim 1 wherein the cooled cores have a hardness less than about 45 on the Rockwell 30T hardness scale.

9. A process according to claim 1 wherein the cores are of nickel.

10. A process according to claim 1 wherein the cores are of cupronickel.