Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/14—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of noble metals or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C5/00—Alloys based on noble metals
  • C22C5/06—Alloys based on silver
  • C22C5/08—Alloys based on silver with copper as the next major constituent

Definitions

• the present invention relates to a process for making finished or semi-finished articles of silver alloy and to articles made by the above process.
• the alloy is entirely liquid at 1640°F (890°C.) and entirely solid at 1435°F (780°C.)
• the degree of copper solubility in the solid alloy depends on the heat treatment used, and the overall physical properties of the sterling can be materially affected, not only by heating the silver to different temperatures, but also by employing different cooling rates.
• Silver alloys are normally supplied soft- for easy working. Heat treatment can be used to increase hardness (and decrease ductility).
• the process known as precipitation hardening involves heating and cooling the silver in such a way as to cause copper to precipitate out of solid solution, thereby producing a fine binary structure. This type of structure is hard, but it is also difficult to work, and has a tendency to crack.
• Precipitation hardening of conventional sterling silver can be achieved by (a) heating the alloy to or above775°C, (b) holding the alloy at that temperature for 15-30 minutes for annealing thereof (i.e.
• the alloys are ternary Ag-Cu-Ge alloys containing at least 92.5 wt% Ag, 0.5-3 wt% Ge and the balance, apart from impurities, copper.
• the alloys are stainless in ambient air during conventional production, transformation and finishing operations, are easily deformable when cold, easily brazed and do not give rise to significant shrinkage on casting. They also exhibit superior ductility and tensile strength. Germanium was stated to exert a protective function that was responsible for the advantageous combination of properties exhibited by the new alloys, and was in solid solution in both the silver and the copper phases.
• the microstructure of the alloy was said to be constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver in copper which itself contains a few intermetallic CuGe phase dispersoids.
• the germanium in the copper-rich phase was said to inhibit surface oxidation of that phase by forming a thin GeO and/or GeO 2 protective coating which prevented the appearance of firestain during brazing and flame annealing.
• tarnish was appreciably delayed by the addition of germanium, the surface turned slightly yellow rather than black and tarnish products were easily removed by ordinary tap water. It is explained that increased hardness can be developed by de-tensioning the alloy e.g.
• Patents US-A-6168071 and EP-B-0729398 (Johns) disclosed a silver/germanium alloy which comprised a silver content of at least 77 wt % and a germanium content of between 0.4 and 7%, the remainder principally being copper apart from any impurities, which alloy contained elemental boron as a grain refiner at a concentration of greater than 0 ppm and less than 20ppm.
• the boron content of the alloy could be achieved by providing the boron in a master copper/boron alloy having 2 wt % elemental boron.
• Argentium (Trade Mark) sterling comprises Ag 92.5 wt % and Ge 1.2 wt %, the balance being copper and about 4 ppm boron as grain refiner.
• Argentium Sterling is precipitation hardenable (i.e. by heating to an annealing temperature and quenching), that a doubling in final hardness can be achieved by reheating at temperatures obtainable in a domestic oven e.g. 450°F (232°C) for about 2 hours or 570°F (299°C) for about 30 minutes. It further discloses that the hard alloy can be softened by conventional annealing (i.e. heating to an annealing temperature and quenching) and then hardened again if required.
• precipitation hardening is appropriate for nearly finished work and that the problems of distortion and damage to soldered joints can be avoided.
• US-A-6726877 discloses inter alia an allegedly fire scale resistant, work hardenable jewellery silver alloy composition
• an allegedly fire scale resistant, work hardenable jewellery silver alloy composition comprising 81-95.409 wt % Ag, 0.5-6 wt% Cu, 0.05-5 wt% Zn, 0.02-2 wt% Si, 0.01-2 wt % by weight B, 0.01-1.5 wt% In and 0.01- no more than 2.0 wt% Ge.
• the germanium content is alleged to result in alloys having work hardening characteristics of a kind exhibited by conventional 0.925 silver alloys, together with the firestain resistance of allegedly firestain resistant alloys known prior to June 1994.
• Amounts of Ge in the alloy of from about 0.04 to 2.0 wt% are alleged to provide modified work hardening properties relative to alloys of the firestain resistant kind not including germanium, but the hardening performance is not linear with increasing germanium nor is the hardening linear with degree of work.
• the Zn content of the alloy has a bearing on the colour of the alloy as well as functioning as a reducing agent for silver and copper oxides and is preferably 2.0-4.0 wt%.
• the Si content of the alloy is preferably adjusted relative to the proportion of Zn used and is preferably 0.15 to 0.2 wt%. Precipitation hardening following annealing is not disclosed, and there is no disclosure or suggestion that the problems of distortion and damage to soldered joints in nearly finished work made of this alloy can be avoided.
• US-A-4810308 discloses a hardenable silver alloy comprising not less than 90% silver; not less than 2.0% copper; and at least one metal selected from the group consisting of lithium, tin and antimony.
• the silver alloy can also contain up to 0.5% by weight of bismuth.
• the metals comprising the alloy are combined and heated to a temperature not less than 1250-1400°F (676- 760°C) e.g. for about 2 hours to anneal the alloy into a solid solution, a temperature of 1350° (732°C) being used in the Examples.
• the annealed alloy is then quickly cooled to ambient temperature by quenching.
• the age-hardened alloy demonstrates hardness substantially greater that that of traditional sterling silver, typically 100 HVN (Vickers Hardness Number), and can being returned by elevated temperatures to a relatively soft state.
• the disclosure of US-A-4869757 (Leach & Garner) is similar. In both cases the disclosed annealing temperature is higher than that of Argentium, and neither reference discloses firestain or tarnish-resistant alloys. The inventor is not aware of the process disclosed in these patents being used for commercial production, and again there is no disclosure or suggestion that hardening can be achieved in nearly finished work.
• a silver alloy called Steralite is said to be covered by US-A-05817195 and
• the alloy of US-A-5817195 contains 90-92.5 wt % Ag, 5.75-5.5 wt % Zn, 0.25 to less than 1 wt % Cu, 0.25- 0.5 wt % Ni, 0.1-0.25 wt % Si and 0.0-0.5 wt % In.
• the alloy of US-A-5882441 contains 90-94 wt % Ag, 3.5-7.35 wt % Zn, 1-3 wt % Cu and 0.1-2.5 wt % Si.
• a similar high zinc low copper alloy is disclosed in US-A-4973446 (Bemhard) and is said to exhibit reduced firestain, reduced porosity and reduced grain scale. None of these references discusses annealing or precipitation hardenining.
• Ag-Cu-Ge alloy workpieces heated to an annealing temperature can be hardened by gradual cooling followed by mild reheating to effect precipitation hardening, and that products of useful hardness can be obtained.
• the use of reheating to e.g. 180-350°C, and preferably 250-300°C, to develop precipitation hardness is typical.
• over-aging of Ag-Cu-Ge alloys during precipitation hardening does not cause a significant drop-off of the hardness achieved.
• the new method of processing workpieces is applicable, for example as part of soldering or annealing in a mesh belt conveyor furnace or in investment casting, eliminates quenching e.g.
• the present invention provides a process for making a finished or semi- finished article of silver alloy, said process comprising the steps of: providing a silver alloy containing silver in an amount of at least 77 wt%, copper and an amount of germanium that is at least 0.5 wt% and is effective to reduce tarnishing and/or firestain; making or processing the finished or semi- finished article of the alloy by heating at least to an annealing temperature; gradually cooling the article to ambient temperatures; and reheating the article to effect precipitation hardening thereof.
• the above process is based on a surprising difference in properties between conventional Sterling silver alloys and other Ag-Cu binary alloys on the one hand and Ag-Cu-Ge alloys on the other hand, in which gradual cooling of the binary Sterling-type alloys results in coarse precipitates and only limited precipitation hardening, whereas gradual cooling of Ag-Cu-Ge alloys results in fine precipitates and useful precipitation hardening, particularly where the alloy contains an effective amount of grain refiner.
• Gradual cooling includes the avoidance of any abrupt cooling step as when an article is plunged into water or other cooling liquid, and normally implies that cooling to ambient temperatures takes more than 10 seconds, preferably more than 15 seconds.
• Control can be achieved during the mesh belt conveyor furnace treatment of workpieces to be brazed and/or annealed by gradual cooling as the workpiece is moved towards the discharge end of the furnace. Control can also be achieved during investment casting if the piece being cast is allowed to air-cool to ambient temperature, the rate of heat loss being moderated by the low conductivity investment material of the flask.
• said process comprises the steps of: providing a silver alloy comprising at least 86 wt% Ag, 0.5-7.5 wt% Cu, 0.07-6 wt % by weight of a mixture of Zn and Si wherein said Si is present in an amount of from about 0.02 to about 2.0 wt%, and from about 0.01 to no more than 3.0 wt% by weight Ge (prefereably no more than 2.0 wt% Ge), making or processing the finished or semi- finished article of the alloy by heating at least to an annealing temperature; gradually cooling the article; and reheating the article to effect precipitation hardening thereof.
• DESCRIPTION OF PREFERRED EMBODIMENTS DESCRIPTION OF PREFERRED EMBODIMENTS
• the alloys that may be treated according to the invention include an alloy of at least 77 wt% silver containing copper and an amount of germanium that is effective to reduce firestain and/or tarnishing.
• the inventor considers that 0.5 wt% Ge provides a preferable lower limit and that in practice use of less than lwt% is undesirable, amounts of 1 - 1.5 wt% being preferred.
• the ternary Ag-Cu-Ge alloys and quaternary Ag-Cu-Zn-Ge alloys that can suitably be treated by the method of the present invention are those having a silver content of preferably at least 80 wt%, and most preferably at least 92.5 wt %, up to a maximum of no more than 98 wt%, preferably no more than 97 wt%.
• the germanium content of the Ag-Cu-(Zn)-Ge alloys should be at least 0.5%, more preferably at least 1.1%, and most preferably at least 1.5%, by weight of the alloy, up to a maximum of preferably no more than 3%.
• Major alloying ingredients that may be used to replace copper in addition to zinc are Au, Pd and Pt.
• alloying ingredients may be selected from selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb Si, Sn, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly adversely affected.
• the weight ratio of germanium to incidental ingredient elements may range from 100: 0 to 60: 40, preferably from 100: 0 to 80: 20. In current commercially available Ag-Cu-Ge alloys such as Argentium incidental ingredients are not added.
• the remainder of the ternary Ag-Cu-Ge alloys, apart from impurities, incidental ingredients and any grain refiner, will be constituted by copper, which should be present in an amount of at least 0.5%, preferably at least 1%, more preferably at least 2%, and most preferably at least 4%, by weight of the alloy.
• copper for example, a copper content of 18.5% is suitable. It has been found that without the presence of both copper and germanium, hardening upon re-heating may not be observed.
• the remainder of a quaternary Ag-Cu-Zn-Ge alloys, apart from impurities and any grain refiner, will be constituted by copper which again should be present in an amount of at least 0.5%, preferably at least 1%, more preferably at least 2%, and most preferably at least 4%, by weight of the alloy, and zinc which should be present in a ratio, by weight, to the copper of no more than 1: 1. Therefore, zinc is optionally present in the silver-copper alloys in an amount of from 0 to 100 % by weight of the copper content. For an '800 grade' quaternary alloy, for example, a copper content of 10.5% and zinc content of 8% is suitable.
• the alloys preferably contain a grain refiner to inhibit grain growth during processing of the alloy.
• Suitable grain refiners include boron, iridium, iron and nickel, with boron being particularly preferred.
• the grain refiner, preferably boron may be present in the Ag-Cu- (Zn)-Ge alloys in the range from 1 ppm to 100 ppm, preferably from 2 ppm to 50 ppm, more preferably from 4 ppm to 20 ppm, by weight of the alloy and very typically in the case of boron 1-10 ppm ,e.g. 4-7 ppm..
• the alloy is a ternary alloy consisting, apart from impurities and any grain refiner, of 80% to 96% silver, 0.1 % to 5% germanium and 1 % to 19.9% copper, by weight of the alloy.
• the alloy is a ternary alloy consisting, apart from impurities and grain refiner, of 92.5% to 98% silver, 0.3% to 3% germanium and 1% to 7.2% copper, by weight of the alloy, together with 1 ppm to 40 ppm boron as grain refiner.
• the alloy is a ternary alloy consisting, apart from impurities and grain refiner, of 92.5% to 96% silver, 0.
• a particularly preferred ternary alloy being marketed under the name Argentium comprises comprises 92.5-92.7 wt% Ag, 6.1-6.3 wt% Cu and about 1.2 wt% Ge.
• the alloys disclosed in US 6726877 comprise at least 86 wt% Ag, 0.5-7.5 wt% Cu, 0.07-6 wt % by weight of a mixture of Zn and Si wherein said Si is present in an amount of from about 0.02 to about 2.0 wt%, and from about 0.01 to no more than 3.0 wt% by weight Ge, preferably no more than 2.0 wt% Ge.
• at least 92.5 wt % of silver is present, 2-4 wt% Cu may be present, 2-4 wt% Zn is preferably present, 0.02-2 wt% Si is present and 0.04-3.0 wt% Ge is present.
• the alloys may also contain up to 3.5 wt% of at least one additive selected from the group consisting of In, B and a mixture of In and B, e.g. up to about 2 wt% B and up to 1.5 wt% In, and they may also contain 0.25-6 wt% Sn.
• One particular species of alloys comprises 81-95.409 wt% Ag, 0.5-6 wt% Cu, 0.05-5 wt% Zn, 0.02-2 wt% Si, 0.01-2 wt% B, 0.01- 1.5 wt% In and 0.01-3 wt% Ge.
• a second species of alloy comprises 75-99.159 wt% Ag, 0.5-6 wt% Cu, 0.05-5 wt% Zn, , 0.02-2 wt% Si, 0.01-2 wt% B, 0.01-1.5 wt% In, 0.25-6 wt%Sn and 0.01-3 wt% Ge.
• High copper alloys according to WO9622400 may also be used, and these are based on 2-5-19.5 wt% Cu, 0.02-2 wt% Si, 0.01-3.3 wt% Ge, the balance being silver, incidental ingredients and impurities.
• Examples of such alloys comprise (a) 92.5 wt% Ag, 7.0 wt% Cu, 0.2 wt% Si and 0.3 wt% Ge, (b) 92.5 wt% Ag, 6.8 wt% Cu, 0.3 wt% Si and 0.2 wt% Ge and 0.2 wt% Sn, (c) 83.0 wt% Ag, 16.5 wt% Cu, 0.2 wt% Si and 0.3 wt% Ge.
• the combination of the germanium and copper content is believed to give rise to an ability to harden on heating to an annealing temperature, gradually air cooling and reheating under mild conditions to effect precipitation hardening.
• the article is a shaped or fabricated article e.g. of jewellery, woven mesh or chain or mesh knitted from drawn wire, or of hollo wware spun from sheet or tube made of the above alloy and is treated by heating to a soldering or annealing temperature by passage through a continuous mesh belt conveyor brazing or annealing furnace.
• a continuous mesh belt conveyor brazing or annealing furnace Such conveyors are available from e.g. Lindberg, of Watertown, WI, USA and Dynalab of Rochester NY as mentioned above.
• Such articles will be a soldered or brazed assembly of two or more components.
• the furnace gas although protective, should not deplete the surface layer of germanium, as this will reduce the tarnish resistance of the alloy and its resistance to firestain.
• Atmospheres may be of nitrogen, cracked ammonia (nitrogen and hydrogen) or hydrogen.
• the annealing temperature should preferably be within the range 620 - 650°C. It is desirable not to exceed a maximum temperature of 680°C. The annealing time for this temperature range is 30 to 45 minutes.
• the brazing temperature is preferably not more than 680°C, and preferably in the range 600-660°C.
• a low-melting solder (BAg-7) which may be used contains 56% silver, 22% copper, 17% zinc, and 5% tin.
• the BAg-7 solder (an international standard) melts at 1205°F (652°C). Solders containing germanium, which will give better tarnish protection are described in UK Patent Application 03 26927.1 filed 19 November 2003, the contents of which are incorporated by reference.
• a suitable solder which melts in the range 600-650°C comprises about 58 wt % Ag, 2 wt % Ge, 2.5 wt % Sn, 14.5 wt % Zn 0.1 wt % Si, 0.14 wt % B, and the balance Cu., a practically used variant of that solder having the analysis 58.15 wt % Ag, 1.51 wt % Ge, 2.4 wt % Sn, 15.1 wt % Zn, 0.07 wt % Si, 0.14 wt % B, and the balance Cu.
• Articles that are brazed by passage through a brazing furnace will, of course, have simultaneously been annealed. It has been found that precipitation hardening can develop without a quenching step by controlled gradual air-cooling in the downstream cooling region of the furnace. For this purpose, it is desirable that the material should spend at least about 10-15 minutes in the temperature range 200-300°C which is most favourable for precipitation hardening. Articles which have been brazed in a furnace in this way, gradually cooled and then re-heated at 300°C for 45 minutes have achieved hardness of 110-115 Vickers.
• Argentium casting grain is melted using traditional methods (solidus 766°C, liquidus 877°C) and is cast at a temperature of 950-980°C and at a flask temperature of not more than 676°C under a protective atmosphere or with a protective boric acid flux. Flask temperatures during investment casting may be e.g. 500-700°C and it has been found that sound castings are relatively insensitive to flask temperature.
• the investment material which is of relatively low thermal conductivity provides for slow cooling of the cast pieces.
• the hardness can be increased even further by precipitation hardening e.g. by placing the castings or the whole tree in an oven set to about 300°C for 45 minutes to give heat-treated castings of approaching 125 Vickers.
• Examples 1-8 The alloys indicated in the table below were prepared by melting together the listed constituents, and were subjected to the tests indicated below. Compositions where boron is indicated to be present are believed to contain about 4 ppm boron, but were not separately assayed. It will be noted that a very significant hardness increase was noted for the germanium-containing alloys, except where there was no copper content, in which case no hardening was observed. It is surprising that useful hardening of the initially very soft allow of Example 4 was obtained.
• Cooling method 1 sample annealed at red heat (about 600°C), air cooled, then heated at 300°C for 45 minutes.
• Cooling method 2 sample annealed at red heat (about 600°C), quenched in water, 10 then heated at 300°C for 45 minutes.
• Annealed hardness - sample annealed (about 600°C), air cooled, no further heat treatment.
• the two alloys are cast and are tested for Vickers Hardness as cast and when annealed at red heat (about 600°C), air cooled, then heated at 300°C for 45 minutes. The hardness rises to over 100 Vickers after the above described annealing and post-treatment without quenching.
• the above alloys are cast and are tested for Vickers Hardness as cast and when annealed at red heat (about 600°C), air cooled, then heated at 300°C for 45 minutes. The hardness rises significantly after the above described annealing and post-treatment without quenching.

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