KR20070060107A - Process for the production of coin blanks - Google Patents

Abstract

The present invention provides a process for applying a diffusion coating to steel coin blanks. The process comprises the steps of (i) charging a metallising reactor with a plurality of coin blanks and a chromising compound comprising ferrochromium granules, an energiser and a refractive diluent, (ii) heating the chromising compound to generate a chromising vapour for deposition on to the coin blanks, and (iii) removing the coin blanks from the reactor. The reactor rotates at a rate of 0.5-50 rpm. A process for preparing coins is also disclosed.

Description

Process for the production of coin blanks

TECHNICAL FIELD The present invention relates to a process for producing coin blanks, and more particularly, to a process for producing chromised calcination.

Stainless steel is used for coinage in some countries of the world. While stainless steel provides acceptable corrosion resistance for these applications, the use of this material has several disadvantages. First, it is a relatively expensive material. Secondly, it is difficult to strike without causing excessive die wear. Molds are expensive and provide a significant cost to the overall cost of cast coins. Third, it is difficult to achieve good definition on the cast coin and therefore the design of the stainless steel sintered phase tends to lack detail.

One alternative to stainless steel sinters is to use mild steel sinters electroplated with nickel. However, there are commercial disadvantages in using such metals as well as environmental issues associated with the disposal of waste electroplating solutions.

It has been proven in the past that chromed iron is a successful alternative to stainless steel. There are several advantages to making money from chromed iron. As in the case of stainless steel, it is economical to have chromium on the surface (typically 20-50 μm) only on the surface and not over the complete section of the field. The chromising process completely anneals the substrate steel so that a blank can be cast without the large wear on the mold typically seen when casting stainless steel. The chromed layer can be easily finished with a satisfactory appearance by mechanical methods when the coin is in the form of calcination. The softness of the base material also allows complex patterns to be applied to the anterior chamber. This is not the case when casting stainless steel, the design of which must be simple.

A common method of chromizing iron is to pack the components to be chromed into a retort with a chromising compound. Suitable containers are filled with chromium treated compounds comprising the following components: chromium metal powder; Diluents such as titania, alumina, or magnesia; And activators which are generally ammonium halides. The component to be chromed is packed into this compound in layers. The components must be surrounded by a sufficient amount of chrome treatment compound such that a satisfactory coating is applied to the components so that the individual components do not touch each other or the walls of the container. A suitable atmosphere is provided in the vessel to allow the chemical reaction to proceed in the desired manner. The vessel is then loaded into a furnace and treated at an increased temperature, generally 1100 ° C., but is not limited to this temperature. This creates a chromium halide atmosphere by the reaction of an ammonium halide and chromium source material, which is then contacted with the component to be coated. When the base material is low-carbon steel, chromium deposits on the surface to form a solid solution alloy of iron and chromium. Typically this layer will have 20-45% by weight of chromium on its surface and will be about 25-125 μm deep. This will only be the case if iron is substantially free of carbon. The presence of more than 0.05% by weight of carbon tends to form a chromium-iron carbide layer on the surface of the solid solution coating. The ingredients are held at this temperature for 6-18 hours, depending on the size of the container. After soaking at this temperature for this time, the vessel is removed from the furnace and left to cool to ambient temperature.

However, this standard method has some disadvantages for chrome treatment. In particular, bulk chrome treatment of sinters is not useful because manual packing of sinters into chromed powder is labor intensive and inexpensive. One proposed solution is the use of robotic packing, although it is problematic because careful control is required to avoid contacting each other in the vessels. Distortions that occur in a container during successive firings make it difficult to use robotics because the robot must be able to detect different shapes for the individual containers. In practice, this is almost impossible without complicated and expensive control equipment.

Thus, there is a need for an alternative, less expensive method of making this type of coin.

Accordingly, the present invention provides a method of applying a diffusion coating to steel sintering, which comprises: (i) chromium treatment comprising a plurality of sintering and ferrochromium particles and an activator; Charging a compound into a metallising reactor, (ii) heating the chromium treatment compound to produce a chromising vapour for deposition on the pyroelectric phase, and (iii) from the reactor Removing the reactor, and the reactor is rotated at a speed of 0.5-50 rpm.

This method can efficiently diffuse coat the metal matrix with chromium vapor without the aforementioned related disadvantages of the prior methods.

The invention will now be described with reference to the accompanying drawings.

Figure 1 shows a (a) side view, (b) end view, and (c) top view of the reactor used in the present invention.

2 is a graph showing the concentration of chromium and iron in steel sinters coated using the method of the present invention.

FIG. 3 shows a metallographic section of a steel field coated using the method of the present invention.

4 shows the distribution of the chromed layer around the coated steel casting using the method of the present invention.

The present invention relates to a successful method of overcoming the disadvantages of using conventional pack chromising methods for this particular product. Rotary furnaces into which sinters can be introduced are used. 1 shows a (a) side view, (b) end view, and (c) top view of a rotary furnace suitable for use in the present invention. 1 shows a reactor 1 contained in a furnace 2. The furnace 2 generally uses a gas burner, but an electrical resistance heater or an induction heater may also be used. A motor 3 is attached to the reactor to rotate the furnace. The furnace is generally rotated at a speed of revolution per minute (rpm) but other rotational speeds may be used. The rotation should not be too slow to prevent the shunt from attaching to the other shunt. The minimum rotational speed is 0.5 rpm, preferably 1 rpm and most preferably 2 rpm. In addition, the rotation should not be too fast to avoid damaging the cast iron and to prevent the rotating furnace from becoming dangerous. The maximum rotation speed is preferably 10 rpm, more preferably 20 rpm, more preferably 25 rpm and most preferably 50 rpm.

In the chromium treatment method, volatile chromium compounds, generally chromium halides, are produced in situ by heating the chromium treatment compound comprising ferrochrome particles in the presence of an activator and a refractory powder. The use of powdered chromium or ferrochrome as a raw material has proved to be unsatisfactory since particulate adhesion occurs during the chromium treatment which has proven to be nearly impossible to remove. In addition, the use of chromium in particle form was also unsuccessful because chromium produced shards that were strongly adsorbed during chromium treatment. The use of ferrochrome particles has proven successful in this form because this metal is more ductile and less easily broken during processing. Ferrochrome particles can be of any size as long as they are large enough to prevent particulate adsorption to the cells. Preferably, the ferrochrome particles are 2-8 mm in diameter, most preferably 4-6 mm.

Ferrochrome particles generally comprise 40-80% by weight of chromium, 0.05-2.5% by weight of silicon, 0.025-0.25% by weight of carbon, and a balance which is a necessary impurity with iron.

Activators used in chromium treatment methods generally include halide components such as bromide, chloride, or fluoride. Preferred halides are sodium halides, potassium halides, and ammonium halides, with ammonium chloride being particularly preferred.

In some cases, calcinations after chromium treatment have been observed to attach to each other, presumably due to diffusion bonding. In order to eliminate this phenomenon, refractory powder is added. The refractory powder is preferably Al ₂ O ₃ (alumina), TiO ₂ (titania), MgO or Cr ₂ O ₃ . The most preferred fire resistant powder is alumina.

The chromium treatment compound preferably comprises 15-90% by weight, more preferably 50-80% by weight of ferrochrome particles and 0.1-10% by weight, more preferably 1-5% by weight of the activator and the rest Consists of a refractory powder which should be present at least 1% by weight, more preferably at least 5% by weight, more preferably at least 10% by weight, and most preferably at least 15% by weight.

Sintering is composed of steel, usually low carbon steel (typically 0.25 wt% or less carbon). The steel is preferably substantially free of carbon. That is, the steel is low enough carbon to avoid the formation of chromium carbide on the surface of the calcined metal. Alternatively, or in addition, intersitial free steel, ie low carbon steels comprising other metals that can form carbides by being bonded to carbon present in the steel can be used. Such strong carbide formers are known in the art and include titanium, niobium, tungsten, vanadium tantalum, chromium, and molybdenum. Such metals in the steel can be chemically bonded to interstitial carbon, thus reducing the interaction between these carbons and the diffusion chromium to avoid the formation of chromium carbides.

Formation of chromium carbide in the coating should be avoided for two important reasons. First, chromium carbide is a hard, wear-resistant material and, consequently, present in the coating (on the surface or in the coating), causes excessive mold wear when casting is cast. Secondly, chromium carbide has a hazy gray appearance that reduces the aesthetic properties of the chromised blank after finishing. The use of chromed sinters in titanium-stabilised steels (or other carbide formers-stabilized steels) allows them to be made cheaper than stainless steel, and the process for their manufacture is electroplated with nickel. It also has commercial advantages because it does not have environmental problems when making it. While the materials used to apply the chromed layer are all reusable, the waste electroplating solution must be properly disposed of for disposal.

Examples of steels suitable for use as embers consist of:

C 0.001-0.025 wt%

Mn 0.05-0.35 wt%

P 0.005-0.05 wt%

S 0.005-0.05 wt%

Si 0.004-0.1 wt%

N 0.002-0.03 wt%

Al 0.01-0.25 wt%

Ti 0.004-0.1 wt%

Nb 0.01-0.25 wt%

Fe remainder (iron will contain inevitable impurities).

Clearly, the size and shape of the calcination will depend on the size and shape of the coin being manufactured. In general, calculi will be circular, about 15-30 mm in diameter and about 1-4 mm thick.

The total weight of calcination present is preferably 5-75% by weight, more preferably 5-50% by weight, and most preferably 40-50% by weight, based on the total weight of the calcination and chromium compounds.

Somatic and chromium compounds (ferrochrome particles, activators, and optionally refractory powders) are introduced into the reactor. The temperature of the furnace is increased and the reactor contents are sufficiently heated to form chromed vapor coating the sinter. The temperature is preferably 800-1150 ° C., preferably 950-1100 ° C., and most preferably 1000 ° C. The furnace preferably reaches this temperature over 10 minutes to 3 hours, more preferably 1-1.5 hours. The temperature is maintained at this value for a time sufficient to substantially coat all calcinations. For example, from 10 minutes to 12 hours, more preferably from 30 minutes to 2 hours. After this time, when the method is operated as a batch process, the furnace is turned off and the sinter is left to cool, for example to ambient temperature, for removal. It is a simple matter to separate the coated calcination from the chromising medium by using a sieve of the appropriate size. During the chrome treatment process the chromium compound should preferably be protected from attack by atmospheric oxygen. Protection may require an inert gas, which may be produced by ammonium salts present in the compound that decompose at increased temperatures. Alternatively, the protection may be hydrogen, or a mixture of hydrogen and argon, for example Hygon having 10% or less hydrogen, preferably 5% or less hydrogen, more preferably 1-5% hydrogen, By a reducing atmosphere such as a hydrogen-containing gas mixture such as 5% hydrogen in argon).

As an alternative to the batch process, the plant can be automated to allow continuous flow of calcination and chrome treatment media through the furnace. When the method is performed as a continuous process, the reactor is adapted to move the shunt from the first location where the shunt is charged to the reactor to a second position where the shunt is removed from the reactor. This may be accomplished by an Archimedean screw in the reactor, or by some other device for advancing the coins and powder through the hot zone. In this case, the furnace is not powered off but the coated calciner is allowed to move to the cooling section of the reactor prior to removal. The continuous process is particularly preferably combined with the inert gas described above.

The invention also provides relief on one or both sides of the coin by manufacturing the sinter using the method described above, finishing and polishing the sinter, and casting the sinter. a method of making a coin comprising the step of providing an image). An ember is finished using a physical or chemical method to remove asperities from its surface, for example using a high-energy centrifuge that includes an abrasive compound. The surface of the calcined ground is then polished again using physical or chemical methods, for example by performing a rinse and rinse in the presence of a ball bearing. The creation of an embossed image on one or more sides of the sinter, generally on both sides, by casting the sinter is made to impress the specific design or pattern required for the sinter on both sides of the sinter when produced on both sides. Achieved by simultaneously priced tool steel dies. During this operation, the calcination is held in a collar that can be used to create knurled edges if necessary.

Example

Example  One

The following ingredients were charged into a coating retort.

70% by weight of low carbon ferrochrome particles having a particle size of 4-6 mm and the following composition:

65% by weight of chromium

Silicon 1% by weight

0.05 wt% carbon

Remaining iron and inevitable impurities

15% by weight of alumina

3% by weight of ammonium chloride

12 weight percent of sinters about 19 mm in diameter and about 2.3 mm thick. The steel composition in the said casting was as follows.

C 0.003 wt%

Mn 0.15 wt%

P 0.012 wt%

S 0.010 wt%

Si 0.02 wt%

N 0.005 wt%

Al 0.05 wt%

Ti 0.06 wt%

Nb 0.015 wt%

Fe remainder and inevitable impurities.

A protective gas of 5% hydrogen in argon was passed into the vessel at 2 L / min while the vessel was rotating at 8 rpm. The furnace temperature was increased to 1000 ° C. over 1-1.5 hours. The temperature was kept at this value for 1 hour. After this time, the furnace was turned off and the argon / hydrogen gas flow was increased to 10 l / min. When the sour reached the ambient temperature it was removed from the vessel. It was a simple matter to separate the subfield from the chromium treatment medium by using a sieve of a suitable size. The cast iron was bright in appearance, metallic, smooth, and had no adhered chrome treatment medium. Also, there were no coins attached to each other. Metallurgical examination revealed that the ante has a 20-26 micron chromium diffusion coating. The concentration of chromium found in the diffusion coating is shown in FIG. 2.

The metallurgical section shown in FIG. 3 showed that there was no carbide in the chromed layer. The base material was etched away with 20% nitric acid and the chromium diffusion coating stood as its own free standing. This coating was ductile and confirmed the absence of any significant amount of carbide. The distribution of the chromed layer around the calcination is shown in FIG. 4. It can be seen that the layer is uniform and unaffected at the ends or edges of the chamber. This is advantageous compared to sinters coated by nickel plating, which tends to impart a coating thickness of two to three times the thickness of the sinters relative to the center of the sinter's face.

Sinters processed in this way could be finished with high gloss and were therefore satisfactory in producing money castings by casting the finished sinter.

Example  2

The following components were filled into the coating vessel.

39.8 weight% of low carbon ferrochrome particles having a particle size of 4-6 mm and the following composition:

65% by weight of chromium

Silicon 1% by weight

0.05 wt% carbon

Remaining iron and inevitable impurities

8.5 wt% alumina

1.7 weight% ammonium chloride

50 wt% of sinter having the same diameter, thickness, and steel composition as in Example 1.

The procedure of Example 1 was repeated. The resulting cast iron was bright in appearance, metallic, smooth and free of chromium-treated media. Also, there were no coins attached to each other. Metallurgical examination revealed that the ante has a 20-25 micron chromium diffusion coating. The concentration of chromium found in the diffusion coating was 25.4-25.7 wt%.

Sinters processed in this way could be finished with high gloss and were therefore satisfactory in producing money castings by casting the finished sinter.

Example  3

The sorrel of the present invention can also be produced in a continuous process. In a continuous process, the following components were charged into the reactor through a hopper:

70% by weight of low carbon ferrochrome particles having a particle size of 4-6 mm and the following composition:

65% by weight of chromium

Silicon 1% by weight

0.05 wt% carbon

Remaining iron and inevitable impurities

15% by weight of alumina

3% by weight of ammonium chloride

12 wt% of sinter with the same diameter, thickness, and steel composition as in Example 1.

A protective gas of 5% hydrogen in argon was passed into the reactor at 2 l / min while the reactor was rotated at 2 rpm and maintained at a temperature of 1000 ° C. Cast iron and chromium compounds were introduced from the hopper into one end of the reactor. The reactor has an internal spiral with a pitch of 2 inches (50.8 mm) and an upstand of 1 inch (25.4 mm), and the ignition through the heating section of the reactor over a period of 30 minutes by the spiral pin. Will move. It is a simple matter to separate the calcination from the chromium treatment medium by using a sieve of a suitable size when the shunt exits the other end of the reactor.

Sinters processed in this way could be finished with high gloss and were therefore satisfactory in producing money castings by casting the finished sinter.

Claims (14)

As a method of applying a diffusion coating on steel mills, (i) filling a metallising reactor with a chromium treatment compound comprising a plurality of soot and ferrochromium particles, an activator, and a refractive diluent, (ii) Heating the chromium compound to produce a chromising vapour for deposition on the pyroelectric phase, and (iii) removing the pyroelectric from the reactor, the reactor being at a rate of 0.5-50 rpm. How to Rotate The method of claim 1, wherein the ferrochrome particles are characterized in that it comprises 40-80% by weight of chromium, 0.05-2.5% by weight of silicon, 0.025-0.25% by weight of carbon, and balance iron and consequent impurities Way. The method of claim 1 or 2, wherein the ferrochrome particles are 2-8 mm in diameter. The method of claim 1, wherein the activator is bromide, chloride, or fluoride of sodium, potassium or ammonium. The method of any one of claims 1 to 4, wherein the chromium-treated compound further comprises a refractory powder. The method of claim 5 wherein the refractory powder is alumina. The method according to any one of claims 1 to 6, wherein the calcination is made of interstitial free steel. The refractory powder according to any one of claims 1 to 7, wherein the chromium treatment compound comprises 15-90% by weight of ferrochrome particles and 0.1-10% by weight of activator, the remainder being present at least 10% by weight. Method consisting of. The method according to any one of claims 1 to 8, wherein the total weight of the existing calcined iron is 5-75 wt% based on the total weight of the calcined calcined and chromium treated compounds. 10. The method of any one of the preceding claims, wherein the reactor rotates at 1-10 rpm. The process of claim 1, wherein the method is performed as a continuous process, and wherein the reactor is in a second position, wherein the soot is removed from the reactor from a first position where the soot is charged to the reactor. And adapted to move the blast furnace. 12. The method of any one of the preceding claims, further comprising finishing and polishing the calcination. A method of manufacturing a coin, comprising the steps of: preparing a sinter by using the method of claim 12, and providing a relief image on one or both sides of the sinter by casting the sinter. 14. The method of claim 13, wherein the edge of the calcined is knurled.

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