JP2008538387A - Method of adding boron to the alloy - Google Patents

Abstract

  A method of atomizing and deoxidizing a noble metal alloy or master alloy includes: (a) producing a precursor melt consisting essentially of noble metal alloy or master alloy components and inevitable impurities; and (b) a precursor. Dispersing a compound selected from the group consisting of boron-containing metal hydrides, boron-containing metal fluorides, and mixtures thereof throughout the melt; and (c) a noble metal alloy of the melt containing boron. Or a step 20 of solidifying the master alloy. One suitable compound is solid sodium borohydride (sodium tetrahydroborate). In order to minimize the evaporation of boron in contact with the precursor alloy melt, the sodium borohydride can be encased in a metal foil formed from a noble metal alloy or master alloy component. Cast noble metal alloys or master alloys have been found to reduce hard spot count and silicon contamination compared to conventional casting methods.

Description

  The present invention relates to a method for producing a noble metal alloy and a master alloy containing boron. More particularly, a solid compound, either a metal hydride containing boron, preferably a solid tetrahydroborate, or a metal fluoride containing boron, is dispersed throughout the molten noble metal alloy or master alloy. It is something to be made.

  Precious metal alloys for gemstones are often processed into complex decorative shapes. In order to withstand a wide range of processing without breaking, noble metal alloys need to have high ductility and high strength. By reducing the oxygen content of the alloy and making it a fine grain structure, it becomes easy to increase ductility and strength.

  Boron is known to both deoxidize noble metal alloys and refine grain. Boron cleans the surface of the metal by removing oxygen from the melt and removing other oxides in the melt. U.S. Pat. No. 5,384,089 issued to Diamond discloses the use of boron as a deoxidizer for gold-base alloys. This patent discloses that boron produces hard spots. US Pat. No. 6,168,071 to Jones discloses a diffusion-bondable silver-copper-germanium alloy that can contain up to 20 ppm boron as a grain refiner. Boron is disclosed as being added as a component of a 2% by weight copper-boron master alloy. Unless otherwise specified, all percentages are weight percentages throughout the patent application.

The usual method of introducing boron into a noble metal alloy or master alloy is by use of a master alloy of 98% copper-2% boron. However, the use of such master alloys often results in hard spots in the product. These hard spots are believed to be non-equilibrium phase CuB ₂₂ particles formed as boron saturated copper when cooled from the liquid phase to the solid phase. Hard spots can also be caused by other metal borides such as iron borides (eg, Fe ₅ B ₂ and FeB ₂ ). Often, hard spots may not be detected until after the precious metal alloy gemstone is ground and inspected, which ultimately results in an unnecessary expense to handle defective products.

  The copper-boron 2% master alloy is often contaminated by silicon. Contamination with silicon can lead to embrittlement because it produces brittle intermetallics, oxides and low melting eutectic mixtures. High purity white gold and silver are required to have a low silicon content.

  Another disadvantage associated with the use of a 2% copper-boron master alloy is that it may not be desirable for the manufacture of the alloy to have a high copper weight percentage. Excess copper can cause discoloration and / or firestain in the silver-based alloy.

  Therefore, there remains a need for more effective methods for introducing boron into a noble metal melt as a grain refiner and oxygen / oxide scavenger.

  According to the present invention, a method for producing a noble metal alloy or master alloy is provided. The method includes (a) forming a molten precursor alloy of a noble metal alloy or master alloy, (b) distributing a boron-containing compound throughout the molten precursor alloy, and (c) boron. Solidifying a precursor alloy containing.

  A feature of the present invention is that the compound containing boron is a metal hydride containing boron or a metal fluoride containing boron. In the case of a metal hydride containing boron, the metal can be sodium, lithium, potassium, calcium, zinc and mixtures thereof. In the case of metal fluorides containing boron, the metal is sodium. Most preferred as the compound containing boron is sodium borohydride.

  Another feature of the present invention is that more than 20 ppm of boron can be incorporated into silver-based alloys or other noble metal alloys without hard spot growth.

  The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

  Like reference numbers and designations in the various drawings indicate like elements.

  The following definitions are used throughout this patent application.

  Master alloy: A component of a precious metal alloy that omits major precious metals. For example, a yellow 10 carat, 14 carat or 18 carat alloy can contain both silver and gold, and only the gold is omitted from the master alloy. Silver will be present. In the case of a sterling silver alloy, silver is omitted and no precious metal component is present. The master alloy is usually sent to the end user, who adds the required amount of precious metal.

  Noble metal alloy: An alloy having a desirable composition for jewelry applications. The alloy contains the required amount of gold, silver, palladium and / or platinum.

  Precursor alloy: A composition that deviates slightly from the specification for the desired master or noble metal alloy A metal foil containing a boron compound is added to obtain a composition as specified. If the boron compound is not wrapped in a metal foil, for example, if it is wrapped in paper or not at all, the composition of the precursor alloy is in accordance with the desired master alloy or noble metal alloy specifications.

  The method of the present invention is useful for adding boron to precious metal alloys and master alloys while minimizing hard spot formation. Examples of noble metal alloys include sterling silver alloys and silver alloys that contain more than 75% silver with the balance being, but not limited to, alloying elements and inevitable impurities including copper and zinc. Silver alloys and sterling silver alloys with 80% to 97% silver benefit most from the method of the present invention.

  The method of the present invention has a gold alloy for gemstones having at least 33% by weight gold (8 carats), the balance being alloy elements including but not limited to silver, nickel, copper and zinc, and inevitable impurities. Is also useful. Most benefited by the method of the present invention is such a gold alloy having between 37.5% and 77% gold.

  FIG. 1 shows the initial processing procedure for the alloy of the present invention as a flow chart. A precursor melt of a noble metal alloy or master alloy is produced by melting (10) appropriate amounts of noble metal and alloy elements in a suitable crucible. As described below, the boron-containing compound can be wrapped in a metal foil formed from one of the noble metals or alloy elements and added to the precursor melt. Therefore, considering the metal content of the added foil, the composition of the precursor melt is usually slightly different from the desired final product composition.

  The alloy is melted in a suitable crucible (10). In the case of silver alloys, one suitable crucible is one that is formed from clay-graphite, and in the case of gold alloys, one suitable crucible is one that is formed from ceramic. Other suitable crucibles for silver and gold based alloys include clay-graphite, fused quartz, silicon carbide, graphite and zirconia. Metals up to temperatures effective for the mixture to become fully liquid and flow, typically in the range of 1066 ° C. (1950 ° F.) to 1260 ° C. (2300 ° F.) (the nominal temperature is about 1177 ° C. (2150 ° F.)) Heat. The melting temperature affects the boron evaporation rate, which determines the final boron concentration in the cast noble metal alloy or master alloy. The selected temperature should be well above the liquidus temperature of the alloy to prevent solidification in the die during continuous casting or in the grain box during grain production. is there. Although the alloy is easily cast at atmospheric pressure, higher or lower pressures do not affect the benefits of the present invention, but the rate of boron evaporation is affected.

  In order to reduce the formation of oxide slag, the molten precursor alloy should be coated to isolate the metal surface from oxygen. Suitable gas coatings include, but are not limited to, carbon monoxide flame, forming gas flame, argon, nitrogen, hydrogen flame and natural gas flame. Suitable powdered solid coatings include, but are not limited to, borax, boric acid, graphite and charcoal.

When the precursor melt is at the desired melting temperature, a boron-containing compound is added to the precursor melt (12). Boron is incorporated into the noble metal alloy as an oxygen scavenger, or in the case of a silver alloy, as an additional or alternative grain refiner. Blowing gaseous borane, such as diborane, into the alloy as a mixture with a non-reactive gas such as argon, borane that is solid at room temperature, such as decaborane B ₁₀ H ₁₄ (melting point = 100 ° C., boiling point = 213 ° C.) Boron was melted by introduction into the alloy or by addition of alkylated boranes such as triethylborane or tri-n-butylborane (but the latter reagent is pyrophoric and requires care) It may be added to silver.

  When adding boron compounds to the noble metal in the gas phase, it is advantageous to cause the molten alloy to agitate and mix with the carrier gas to help disperse the boron contained in the gas mixture in the alloy. It is. Suitable carrier gases include hydrogen, nitrogen and argon. Gaseous boron compounds and carrier gases can be introduced into a vessel containing molten silver or a silver-based alloy by using a metallurgical lance. The metallurgical lance may be an elongate tubular body of refractory material (eg, graphite), or may be a metallic tubular cladding within the refractory material. The lance is preferably of sufficient length to allow the gaseous boron compound and carrier gas to be injected deep into the molten alloy.

  Alternatively, the boron-containing gas can be introduced into the molten alloy from the side surface or bottom surface of the container containing the alloy through a gas transporting means such as a gas-permeable blowing plug or a dip injection nozzle. Routomead International, Dundee (Scotland) manufactures, as the RMK series, a horizontal continuous casting machine for continuous casting of semi-finished silver and gold based products. For example, a hard graphite crucible protected by an inert gas atmosphere, which can be nitrogen-free (containing less than 5 ppm oxygen and less than 2 ppm water), put the alloy to be heated and use a graphite block Heat by electrical resistance heating. Such furnaces have built-in means for blowing inert gas into the melt. When a gas containing a small amount of pyrolyzable boron is added to the inert gas blown into the melt, several ppm or tens of ppm of the desired boron is easily contained. From the point of view of avoiding the occurrence of boron hard spots in the metal or alloy, rather than introducing a relatively large amount in one or more times, the boron compound is introduced into the alloy as a dilute gas stream over a period of time. It can be preferred that the carrier gas in the gas stream acts to stir the molten metal or alloy.

Compounds that can be introduced as a gas into molten silver or gold or their alloys include boron trifluoride and diborane that can be used in pressurized cylinders diluted with hydrogen, argon, nitrogen or helium Alternatively, although trimethylboron is included, diborane is preferable because the only other element introduced into the alloy in addition to boron is hydrogen. Yet another possibility is that the carrier gas is blown into the molten silver and stirred, and a solid boron compound such as NaBH ₄ or NaBF ₄ is fluidized as a finely divided powder that forms an aerosol. Is to add to.

  The boron compound can also be introduced in the liquid phase into the molten silver alloy or molten gold alloy as a liquid phase or an inert organic solvent. Compounds that can be introduced in this manner include alkylboranes such as triethylborane, tripropylborane, tri-n-butylborane and methoxydiethylborane, or alkoxy-alkylboranes, which can be used for safe handling. It can be dissolved in hexane or tetrahydrofuran (THF). Use well-known liquid / capsule or liquid / sachet filling devices, and capsules, sachets or other volumes typically 0.5-5 ml, more typically about 1-1.5 ml Using a protective atmosphere to fill small containers, liquid boron compounds can be filled and sealed in containers made of silver-based or copper-based foils resembling capsules or sachets. Alternatively, particularly in the case of gold casting, the capsule or sachet can be of a polymer film, such as polyethylene or polypropylene. The appropriate number of filled capsules or sachets can then be poured into molten silver or gold, or alloys thereof, either individually or in groups. Yet another possibility is to spray the liquid boron-containing compound into the carrier gas stream used to stir the molten silver as described above. The droplets may take the form of an aerosol in the carrier gas stream or may evaporate therein.

  Boron is preferably added as a metal boron hydride, such as, for example, an alkali metal, pseudo-alkali metal or alkaline earth metal boron hydride, such as lithium borohydride. Sodium borohydride is particularly preferred because it is widely available commercially and is available in the form of relatively large pellets that are convenient for handling during the precious metal melting operation.

  Advantageously, the boron is a solid, for example a higher borane such as a metal borohydride or decaborane, and is in the form of pellets or particulates. It is also advantageous to wrap them in a noble metal foil layer and throw them into the molten metal as a group. If the alloy is kept in a molten state for some time, such as in a continuous casting process for granules, the other materials that have melted boron both during the initial melting and between castings to compensate for the loss of boron. It can be added to the compound. Adding boron to a molten copper-based mother alloy is not recommended because adding boron changes the copper content and thus the overall proportion of the various components in the alloy.

Boron is added in the form of a metal hydride containing boron, preferably solid tetrahydroborate, or a metal fluoride containing boron. For metal hydrides containing boron, suitable metals include sodium, lithium, potassium, calcium, zinc, and mixtures thereof. Sodium is a preferred metal as the metal fluoride containing boron. Most preferred are sodium borohydride NaBH _4, also known as sodium tetrahydroborate. Sodium borohydride has a molecular weight of 37.85 and contains 28.75% boron.

  For effective grain refinement and deoxygenation, sufficient boron is added so that an effective amount remains in the cast noble metal alloy or master alloy. It is effective if 1 ppm to 1600 ppm of boron remains. The boron content is preferably 100 ppm to 1600 ppm for the master alloy and 1 ppm to 1000 ppm for the silver-based noble metal alloy or gold-based noble metal alloy. Most preferably, the nominal boron content in the cast noble metal alloy or master alloy is about 250 ppm. Typically, it is effective to add about 0.001% to 0.16% boron to the precursor alloy melt.

  Boron produces a gas that evaporates at a high temperature by the reaction. To maintain an appropriate concentration for grain refinement, it may be necessary to add boron sequentially as described below. The boron compound can be wrapped in a thin metal foil to allow it to be better mixed into the precursor alloy. The foil can be any component of the underlying melt or an inert material such as paper, but is preferably a ductile metal capable of forming a relatively thin foil. Preferred metals for the foil include silver, copper and gold. Before the foil melts during the release of the boron compound, the foil is about 0.01 millimeters to about 0.3 millimeters so that the boron compound wrapped in the foil can be sufficiently immersed in the underlying melt. Have a thickness. After being released, each component of the boron compound combines with the oxygen in the precursor melt to effectively deoxidize the melt, and boron reacts with several elements in the melt to react with the entire matrix. It forms discrete insoluble particles that are dispersed, which act as nucleation sites that promote the formation of fine grains of uniform size and inhibit growth.

The initial reaction when sodium borohydride is first added to the base melt is believed to be the decomposition of the grain refiner containing boron.
(1) NaBH _{4 (s)} → Na _(g) + B _(s) + 2H _{2 (g)}
The decomposition reaction when diborane is first added to the molten metal is considered as follows.
(2) B ₂ H ₆ → 3B _(s) + 3H _{2 (g)}
Hydrogen is effective in deoxidizing the melt.

After decomposition, sodium, hydrogen and boron are all effective in deoxidizing the melt as follows.
(3) Na _(g) + 0.5O _{2 (g)} → Na ₂ O _(s)
(4) H _{2 (g)} +0.5 O _{2 (g)} → H ₂ O _(g)
(5) B _(s) + 0.5O _{2 (g)} + 0.5H _{2 (g)} → HBO _(g)

  In order to perform uniform casting, boron is dispersed throughout the precursor melt by stirring (14). It is preferred that the boron is stirred for more than 1 minute, typically 1-5 minutes (14). Stirring can be by any means that does not contaminate the precursor melt, such as using a graphite stirrer.

  The molten noble metal alloy or master alloy is then cast (16) by a method suitable for forming the desired final product.

  One such useful end product is a casting granule. Casting granules are generally spherical grains that are sold to jewelry manufacturers who perform investment casting to form the desired jewelry. After stirring (14), the molten metal alloy is poured into the granule box (FIGS. 2, 18). The granule box is a container having an opening on the bottom surface, and a liquid metal flows through the opening to produce a granule having a desired shape and size. The granule box is made from a material similar to the crucible, such as, but not limited to, graphite, clay / graphite, ceramic and silicon carbide. When the molten noble metal alloy flows through the opening, it becomes droplets separated in the granule box, and then solidifies in the granule tank (20) into substantially spherical particles. The granule tank (20) contains water and the droplets fall into it and solidify.

  The granules are then removed from the granule tank (20) and dried by centrifugal force or hot air (22). The typical diameter of a substantially spherical particle is about 0.1 millimeters to about 5 mm.

  According to the second embodiment of the present invention, continuous casting can be used to form wrought materials such as sheets, tubes and wires that are later made into final products such as jewelry. The agitated, boron-containing molten metal alloy is transferred to the die (FIGS. 3 and 24) and partially solidified in the die so that the aggregated structure is recovered from the die and sprinkled or cooled. The secondary cooling (26) can be received due to the effect of passing through the coil. The continuously cast structure is then finished (28), such as by passing through a rolling mill and shear, to obtain the desired cross-sectional shape and surface finish, and then coiled for shipment to a jewelry manufacturer. (30). Continuously cast sheets have a more consistent boron content than that obtained by prior art processes, thereby allowing a more consistent weld to the tube.

In a well-stirred batch process, shown as a combination of FIGS. 1 and 2, the boron concentration gradually decreases according to the following equation:
(5) C _B = C _{B, 0} exp (−kρt / m)
Where
C _B = Current boron content (ppm)
C _{B, 0} = initial boron content (ppm)
k = rate constant depending on alloy composition, temperature, gas coating and melt coating expressed in units of cubic inches / minute ρ = alloy density (troy ounces / cubic inch)
t = hours (minutes)
m = weight of the melt (troy ounce)
It is.

  Equation (5) predicts that the smaller the melt size, the faster the boron evaporation rate, which has been observed in practice. FIG. 4 shows a graphical representation of the loss rate of boron in a batch melt process under exemplary conditions with CO gas coating and graphite powder solid coating.

In the continuous casting process shown as a combination of FIGS. 1 and 3, the mass balance must take into account the mass change in the casting crucible over time. The amount of boron present can be calculated by the following equation:
(6) C _B = C _{B, 0} (m ₀ / (m ₀ −F ₀ t)) exp− (kρ / F ₀ )
Where
C _B , C _{B, 0} , t, k and ρ are already defined,
m ₀ = [at time = 0] initial mass of alloy in crucible (troy ounce)
F ₀ = Casting speed (troy ounces / minute)
It is.

Each time boron is added gradually, the time t is reset to zero and the initial mass m _{0 of the} alloy is recalculated.

  The above process can be used to produce alloys of any noble metal that is required to incorporate boron, in particular pure silver, pure gold and alloys thereof. In particular, it has an Ag content of at least 77% by weight, a Ge content of 0.5 to 3% by weight, an effective amount of boron to enhance ductility and strength, the balance being copper, and the accompanying components and It can be used to produce a silver / germanium alloy that is an impurity. If required, the germanium content can be partially reduced to Al, Ba, Be, Cd, Co, Cr, Er, if the effect of germanium that provides resistance to flames and discoloration is not unduly affected. One or more incidental constituent elements selected from Ga, In, Mg, Mn, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, Yb and Zr can be substituted. The weight ratio of germanium to the accompanying component elements can be in the range of 100: 0 to 60:40, preferably 100: 0 to 80: 0. The term “collateral component” is intended to allow the component to have an auxiliary function within the alloy, for example to improve the color or as-molded appearance, and is suitable for deoxygenation. Amounts of metals or metalloids, Si, Zn, Sn and In are included. The present invention can also be applied to the production of master alloys such as Cu / Ge / B and CuB.

  Alloys that can be manufactured according to the process of the present invention include monetary grades, 800 grades (80 wt% silver) (including 830 and 850 grades (83 wt% and 85 wt% silver, respectively)), and Standard sterling silver and silver alloys containing an amount of germanium effective to reduce flame and / or discoloration are included. The Ag—Cu—Ge ternary alloy and the Ag—Cu—Zn—Ge quaternary alloy that can be appropriately manufactured have a minimum silver content of 80% by weight, more preferably a minimum silver. The content is 92.5% by weight. The maximum silver content is 98% by weight, but the maximum silver content is preferably 97% by weight. The germanium content is at least 0.1 wt%, preferably at least 0.5 wt%, more preferably at least 1.1 wt%, and most preferably at least 1.5 wt%. The maximum germanium content is preferably 6.5% by weight, more preferably 4.0% by weight.

  Silicon can be added to the silver alloy in an amount up to 5% by weight, preferably 0.5-3 weight percent, most preferably 0.1-0.2 weight percent. When silicon is taken into the casting particles of an Ag—Cu—Ge ternary alloy, a glossy investment casting can be provided immediately after removal from the mold. Silicon may be added to the casting granules prior to investment casting, for example, or may be incorporated into the silver during the initial melting for alloy formation.

  The process of the present invention has at least 33% by weight gold (8 carats) and the balance is for, but not limited to, alloy elements including silver, nickel, copper and zinc, and gemstones that are inevitable impurities It can be used for the production of gold alloys. Specific gold alloys that can be processed according to the present invention include:

  24K gold: gold is at least 99.7% by weight, and the balance is crystal grain refining agent, curing additive and impurities.

  22K gold: As an example, by weight, Au 91.67%, Ag 5%, Cu 2% and Zn 1.33%.

  18K gold: for example, by weight, Au 75%, Ag 20% and Cu 5%; Au 75%, Ag 15% and Cu 10%; Au 75%, Ag 13% and Cu 12%; Au 75%, Ag 5% and Cu 20%; Au 75%, Ag 2.75% and Cu 22.25%; Au 75% and Cu 25%; Au 80% and Al 20%; Au 75%, Pt, Pd or Ag 25%; Au 75%, Pd 10%, Ni 10% and Zn 5%; Au 75%, Fe 17% and Cu 8%; and Au 75%, Cu 23% and Cd 2%.

  14K gold: As an example, by weight%, Au 58.33%, Cu 24.78% and Zn 0.14%; Au 58.33%, Ag 4.00%, Cu 31.24%, Zn 6.43%, Ni 0.10% Fe 0.05% and Si 0.01%; and Au 58.33%, Ag 2.08% and Cu 39.59%.

  10K gold: As an example, by weight%, Au 41.70%, Ag 11.66%, Cu 40.81%, Zn 5.83%, Si 0.03% and B 0.02%; Au 41.70%, Ag 5.50% Cu 43.80% and Zn 9.00%; and Ag 41.70%, Ag 2.82% and Cu 55.48%.

  The present invention is better understood with reference to the following examples.

Example 1 (Prophytic) Ag-Cu-Ge-Si alloy 93.2 weight percent pure fine silver casting granules, 1.3 weight percent germanium in the form of small debris, silicon A silver alloy is produced by melting together 2 weight percent (added as a Cu / Si master alloy containing 10 weight percent silicon) and the remainder copper particulates. Melting is performed in a gas combustion furnace heated to a casting temperature of about 1093 ° C. (2000 ° F.). The melt is coated with graphite to protect it from atmospheric oxidation. In addition, a protective flame for hydrogen gas is provided. Stirring is performed manually using a graphite stir bar.

  As the alloy components liquefy, 20.2 grams (0.65 troy ounces) of sodium borohydride per 46.7 kilograms (1500 ounces) of melt is wrapped in a pure silver foil about 0.15 mm thick. The wrapping foil retains the sodium borohydride and prevents it from floating on the surface of the melt. Wrapped sodium borohydride is placed in the hollow cup-shaped end of a graphite stirrer and placed below the surface of the melt. In order to extinguish the flame caused by the decomposition of the boron hydride, the melt is coated with a blanket of ceramic fibers. Hydrogen and sodium burn out in 1-2 minutes with a bright yellow flame, while constantly stirring the melt. When the evolution of hydrogen is over, boron is subsequently taken into the melt together with at least some sodium.

  After adding boron, the crucible is rotated so that the molten alloy can be poured into a tundish with very fine holes formed in the bottom. The molten silver flows into the tundish, passes through the holes as a thin stream, the flow is divided into fine pellets, falls into a stirred water tank, and is solidified and cooled. The cast pellets are then removed from the vessel and dried.

  The pellets are tested by investment casting using an investment combined with calcium sulfate. The resulting casting is finished like matte silver when removed from the mold, has a fine grain structure, and can be easily polished. It does not have boron hard spots and was ductile as shown by the ability to stretch 4-6 size rings, but similar materials made with copper / boron Can only stretch about 2 sizes.

Example 2 (working) Production of sterling silver casting granules 6.11 kilograms (200 troy ounces) of sterling silver precursor melt was melted in a clay-graphite crucible. did. The precursor melt had a nominal composition of 93% silver, 5.7% copper and 1.3% germanium by weight. The components of the precursor melt were mixed together, heated under a carbon monoxide flame, and covered with a layer of borax salt that was 2.54 cm (1 inch) thick. When the temperature of the precursor melt reaches flow temperature, it was added 0.0125% boron as NaBH _4. The boron compound was wrapped in a 0.15 mm silver foil for introduction into the underlying melt. Sufficient energy was applied to maintain the temperature of the molten noble metal alloy at the flow temperature. The molten noble metal alloy was then stirred for 3.7 minutes with a graphite stir bar and poured into a granule box. During the injection at the flow temperature, the molten noble metal alloy was protected by a reducing atmosphere. After about 0.25 minutes, all of the molten noble metal alloy was converted to casting granules.

  The casting granules were analyzed and found to have 13.8 ppm boron. In order to examine the crystal grain structure and hard spots, the grains were attached, polished and etched. The resulting crystal grain structure was fine and contained no boron hard spots. Even if the thickness was reduced by 75% with a rolling mill, the material did not become brittle. Investment-cast rings were formed from casting granules that did not contain fire scale or hard spots. The ring was stretched to 3.25 size without heat treatment before failure.

"Example 3" (with data support) Production of sterling silver granules by batch process Table 1 shows that the process of the present invention is effective for adding boron to sterling silver precursor alloys and noble metal alloys It shows that the melt coating appears to be more effective than the injected gas for the boron content in it. In either case, no hard spot was detected on the casting particles.

Example 4 (with data support) Manufacture of a continuous casting of sterling silver 140 kg (4500 troy ounces) of sterling silver precursor melt was melted in a clay-graphite crucible. The underlying melt had a nominal composition of 93% silver, 5.7% copper and 1.3% germanium by weight. The components of the precursor melt were mixed together, heated under a natural gas flame, and coated with a layer of charcoal. When the temperature of the precursor melt reaches flow temperature, it was added 0.0020% boron as NaBH _4. The boron compound was wrapped in 0.15 mm silver foil for introduction into the precursor melt. Sufficient energy was applied to maintain the temperature of the molten noble metal alloy at the flow temperature. Taking into account the change in melt weight over time and boron evaporation, NaBH ₄ was added gradually to maintain an appropriate boron concentration. A timer was used to add boron as planned below. At the times shown in Table 2, additional boron was added to the graphite plungers and mixed into the molten noble metal alloy.

  During transfer to a continuous casting die, the molten noble metal alloy is heated to the transfer temperature and cast into a 5.08 cm (2 inch) diameter cylindrical rod continuous under a natural gas flame and charcoal coating at the casting temperature. did. The boron level of the cast noble metal alloy sample was analyzed. Table 3 summarizes the analysis results.

Example 5 (with data support) Production of 18 carat white gold casting granules 3.89 kilograms (125 troy ounces) of a noble metal alloy of white gold was melted in a ceramic crucible. The noble metal alloy melt had a nominal composition of 75% gold and the balance 6% nickel, 14% copper and 5% zinc. The components of the noble metal alloy were melted together and heated to the flow temperature under a carbon monoxide flame, at which time 0.08% boron was added as NaBH ₄ . The boron compound was wrapped in a paper wrap for introduction into the melt. Sufficient energy was applied to maintain the temperature of the molten noble metal alloy at the flow temperature. After adding boron, the molten noble metal alloy was stirred for 1.75 minutes and then poured into the granule box. The gas coating during the injection into the granule box was a reducing atmosphere. After about 0.25 minutes, all of the molten noble metal alloy was converted to casting granules. Analysis of the casting granules showed that the surface was clean, there were no hard spots, and the particle size was fine.

  One or more embodiments of the present invention have been described. However, it will be understood that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

FIG. 3 is a flow chart showing initial processing procedures for an alloy of the present invention. 1 is a flow chart showing subsequent processing of an alloy of the present invention according to a first embodiment of the present invention. 4 is a flow chart showing subsequent processing of an alloy of the present invention according to a second embodiment of the present invention. The graph which shows the loss rate of the boron in the batch process of this invention.

Claims (38)

In a method of casting a noble metal alloy or a master alloy,
(A) forming a precursor alloy melt substantially consisting of components of the noble metal alloy or the master alloy and unavoidable impurities;
(B) dispersing (14) a compound selected from the group consisting of boron-containing metal hydrides, boron-containing metal fluorides, and mixtures thereof throughout the underlying melt. ,
(C) A step (20) of solidifying the molten noble metal alloy or master alloy containing boron.   The metal component of the metal hydride containing boron is selected from the group consisting of sodium, lithium, potassium, calcium, zinc and mixtures thereof, and the metal component of the metal fluoride containing boron is sodium. The casting method according to claim 1.   The casting method according to claim 2, wherein solid sodium borohydride (sodium tetrahydroborate) is selected as the compound.   Before the boron-containing metal hydride or the boron-containing metal fluoride is dispersed (14) in the precursor alloy melt, of the components of the noble metal alloy or master alloy. 3. A casting method according to claim 2, characterized in that it is wrapped in a metal foil selected to be one.   The casting method according to claim 4, wherein the metal foil is selected to have a thickness of 0.01 millimeters to 0.3 millimeters.   The casting method according to claim 4, wherein the noble metal alloy or the master alloy contains silver, and silver or a silver-based alloy is selected as the metal foil.   The casting method according to claim 4, wherein the noble metal alloy includes gold, and copper or a copper-based alloy is selected as the metal foil.   5. Casting according to claim 4, wherein said dispersing step (b) (14) comprises stirring for a time effective to disperse boron throughout said noble metal alloy or said master alloy. Method.   The casting method according to claim 1, wherein the noble metal alloy or the master alloy is transferred to a granule box.   The precious metal alloy is transferred to a continuous casting die (24), and after the solidifying step (c), the noble metal alloy is recovered as an extension of a desired cross-sectional shape (28). Casting method.   11. The casting method according to claim 10, wherein the dispersing step (b) (14) is repeated a number of times in order to maintain a desired boron content.   5. Casting method according to claim 4, characterized in that sufficient boron is added (12) to obtain a noble metal alloy or master alloy having 1 ppm to 1600 ppm boron by weight.   The casting method according to claim 12, wherein the boron content is 100 ppm to 1600 ppm in the master alloy and 1 ppm to 100 ppm in the noble metal alloy by weight.   A silver-based or gold-based alloy or master alloy containing 1 ppm to 1600 ppm by weight of boron and substantially free of silicon and copper.   15. The silver-based or gold-based alloy according to claim 14, wherein the boron content is 100 ppm to 1600 ppm for the master alloy and 1 ppm to 100 ppm for the noble metal alloy.   A casting granule formed from the silver-based or gold-based alloy or master alloy according to claim 14.   By weight, silver 93%, copper 5.7% and germanium 1.3%, gold 74.8%, nickel 12.2%, copper 9.9% and zinc 3.1%, copper 81.4 And 18.6% germanium, all having a nominal composition selected from the group comprising inevitable impurities.   15. A stretch of desired cross-sectional area formed from a silver-based or gold-based alloy according to claim 14.   19. The extension of claim 18 formed from an alloy having a nominal composition of 93% silver, 5.7% copper, 1.3% germanium and unavoidable impurities by weight percent.   19. An extension as claimed in claim 18 which is a sheet formed into a product selected from the group consisting of wires, chains, sheets and welded tubes. In a method of casting a noble metal alloy or a master alloy,
(A) forming a molten alloy in a crucible (10), wherein the molten alloy consists essentially of components of the noble metal alloy or the master alloy and unavoidable impurities;
(B) injecting boron-containing gas into the molten alloy melt (12, 14);
(C) A casting method characterized by solidifying the molten alloy containing boron (20).   The casting of claim 21, wherein the boron containing gas is mixed with a carrier gas selected from the group consisting of hydrogen, nitrogen, argon, helium, and mixtures and compounds thereof. Method.   The casting method according to claim 22, wherein the boron-containing gas is selected from the group consisting of diborane, boron trifluoride, and trimethyl boron.   23. The casting method according to claim 22, wherein the boron-containing gas is an aerosol containing particles containing dispersed boron. The casting method according to claim 24, wherein the boron-containing particles are selected from the group consisting of NaBH ₄ and NaBF ₄ .   23. A casting method according to claim 22, including the step (12) of inserting a lance into the molten alloy and flowing a gas containing boron through the lance.   23. A casting method according to claim 22, comprising the step of forming the crucible having a gas transport member and flowing the boron-containing gas through the gas transport member.   A casting granule selected from the group consisting of a silver-base alloy and a gold-base alloy produced by the casting method according to claim 22.   23. A stretch of desired cross-sectional area selected from the group consisting of silver-based alloys and gold-based alloys produced by the casting method of claim 22. In a method of casting a noble metal alloy or a master alloy,
(A) forming a precursor alloy melt substantially consisting of components of the noble metal alloy or the master alloy and unavoidable impurities;
(B) introducing a liquid containing boron into the precursor alloy melt (12);
(C) A step (20) of solidifying the molten noble metal alloy or master alloy containing boron.   The boron-containing liquid comprises selecting from the group consisting of alkylboranes, alkoxy-alkylboranes, triethylborane, tripropylborane, tri-n-butylborane, methoxydiethylborane, and mixtures thereof. 30. A casting method according to 30.   The casting method according to claim 31, comprising mixing the boron-containing liquid with an organic solvent.   The casting method according to claim 32, wherein the organic solvent is selected from the group consisting of hexane, tetrahydrofuran, and a mixture thereof.   32. The casting method according to claim 31, wherein the boron-containing liquid is sealed.   The casting method according to claim 34, wherein the liquid containing boron is sealed with a material selected from the group consisting of a silver-based foil, a copper-based foil, and a polymer film.   32. A casting method according to claim 31, wherein said boron-containing liquid droplets are mixed with a carrier gas and introduced as an aerosol into said molten alloy melt.   32. A casting granule selected from the group consisting of a silver-based alloy and a gold-based alloy produced by the casting method according to claim 31.   32. A stretch of desired cross-sectional area selected from the group consisting of silver-based alloys and gold-based alloys produced by the casting method of claim 31.

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