METAL ALLOY MANUFACTURING
Silver alloys containing copper and germanium e.g. about 1 wt % Ge and of very low copper content e.g. about 0.8 wt % Cu can be precipitation hardened to 65 HV or above, whereas alloys of similar copper content and not containing germanium remain soft. In an embodiment, a silver alloy comprises 92.5-97 wt % Ag, 1-4.5 wt % Cu, 0.4-4 wt % Zn, 0.8-1.5 wt % Ge, 0 to 0.2 wt % Si, In or Sn and 0-0.2 wt % Mn, the balance being boron as grain refiner, incidental ingredients and impurities. The said alloy preferably comprises boron as grain refiner added as a boron hydride, e.g. sodium borohydride. A further group of alloys comprises a ternary alloy of silver, copper and germanium containing from more than 93.5 wt % to 95.5 wt % Ag, from 0.5 to 3 wt % Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride. Silicon-containing casting grain that gives rise to bright as-cast products is also disclosed. In a further embodiment, a zinc-containing silver alloy resistant to tarnish under severe conditions e.g. exposure to human sweat or French dressing comprises 1-5 wt % Zn, 0.7-3 wt % Cu, 0.1-3 wt % Ge, 0-0.3 wt % Mn, 0-0.25 wt % Si, B in an amount effective for grain refinement, up to 0.5 wt % incidental ingredients, the balance being Ag in an amount of 92.5-96 wt %, and impurities. A preferred manufacturing method giving an alloy with favourable physical properties involves melting together the ingredients, and incorporating boron by dispersing into molten silver alloy to foirn the whole or a precursor pait of said alloy a compound selecting fiom alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof The alloy is particularly suitable for castings which may be hardened in an oven e.g. at about 300° C. for 30-45 min.
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This application is a continuation-in-part of U.S. patent application Ser. No. 11/628260 having a filing date of 12 Jan. 2006 and which is a 371 of PCT/GB2005/050074 filed 27 May 2005 (International Publication Number WO 2005/118903) which claims priority from UK Patent Application Number 0421172.8 filed 23 Sep. 2004 and UK Patent Application Number 0412256.0 filed 2 Jun. 2004. It is also a continuation-in-part of PCT/GB2006/050116 filed 19 May 2005 (International Publication Number WO 2006/123190) which claims priority from UK Patent Application Number 0523002.4 filed 11 Nov. 2005 and UK Patent Application Number 0510243.9 filed 20 May 2005.The disclosure of each application is hereby incorporated by reference in its entirety where appropriate for teachings of additional or alternative details, features and/or technical background, and priority is asserted from each.
BACKGROUND TO THE INVENTIONThis invention relates to a metal alloy, a method of manufacturing precious metal alloy, and to precious metal products made of the above alloy.
Molten silver and copper are completely soluble in each other in all proportions. However, alloys which have copper contents ranging from about 2% through 27%, when solidified and examined under a microscope, exhibit two discrete constituents: one is nearly 100% silver; the other is a silver-copper “eutectic” (71.9% silver; 28.1% copper), whose melting point is 1435° F. (780° C.). When standard sterling silver is cooled, microscopic analysis shows both of the above constituents to be present in the solidified sterling. The alloy is entirely liquid at 1640° F. (890° C.) and entirely solid at 1435° F. (780° C.) However, 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 above 775° C., (b) holding the alloy at that temperature for 15-30 minutes for annealing thereof (i.e. dissolving all the copper in the silver), (c) quenching rapidly in cold water, which prevents formation of Cu-rich coarse precipitates which are ineffective in bringing about hardening, (d) re-hardening the softened alloy by heating to e.g. 300° C. for 30-60 minutes resulting in the formation of very fine Cu-rich particles which are effective in bringing about hardening and (e) air cooling. The annealing temperatures involved are very high and are close to the onset of melting. Furthermore, there are very few times in practical production that a silversmith can safely quench a piece of nearly finished work because of the risk of distortion of the article being made and/or damage to soldered joints. Silversmiths therefore regard precipitation hardening of sterling silver as of metallurgical interest only. It is too difficult for commercial or industrial production of articles of jewellery, silver plate, hollowware, and the like (see Fischer-Buhner, “An Update on Hardening of Sterling Silver Alloys by Heat Treatment”, Proceedings, Santa Fe Symposium on Jewellery Manufacturing Technology, 2003, 20-47 at p. 29.) and it is unnecessary because sterling silver as produced generally has hardness of 70 Vickers or above. Alloys of higher Vickers hardness are obtained by work hardening rather than precipitation hardening.
Many attempts have been made to produce silver alloys that are resistant to tarnish and/or firestain.
In all but the largest manufacturing companies, most of the annealing and soldering required to assemble finished or semi-finished articles is carried out with the flame of an air-gas blowtorch. The oxidising or reducing nature of the flame and the temperature of the articles are controlled only by the skill of the silversmith. Pure silver allows oxygen to pass easily through it, particularly when the silver is heated to above red heat. Silver does not oxidise in air, but the copper in a silver/copper alloy is oxidised to cuprous or cupric oxide. Pickling of the oxidised surface of the article in hot dilute sulphuric acid removes the superficial but not the deeper seated copper oxide so that the surface consists of fine or unalloyed silver covering a layer of silver/copper oxide mixture. The pure silver is easily permeated during further heating, allowing copper located deeper below the surface to become oxidised. Successive annealing, cold working and pickling produces a surface that exhibits the pure lustre of silver when lightly polished but with heavier polishing reveals dark and disfiguring stains known as ‘fire-stain’ or ‘fire’. Soldering operations are much more productive of deep fire-stain because of the higher temperatures involved. When the depth of the fire-stain exceeds about 0.025 mm (0.010 inches) the alloy is additionally prone to cracking and difficult to solder because an oxide surface is not wetted by solder so that a proper metallurgical bond is not formed.
U.S. Pat. No. 3,811,876 (Harigaya et al. K. K. Suwa Seikosha) discloses a tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10 wt % Sn, 0.5-12 wt % In, and 0.1-5 wt % Zn, the remainder being silver. The Sn, In and Zn are alleged to be synergistic in their effects. Small quantities of Ti, Zn, Be, Cr, Si, Al, Ge and Sb when used in addition to Sn, In and Zn are alleged to further increase resistance to tarnishing by sulphur-containing materials. The alloy is said not to suffer from firestain because of the absence of copper but this property is not confirmed by torch annealing experiments carried out by the present applicants and furthermore the alloy is soft.
U.S. Pat. No. 4,973,446 (Bernhard et al., United Precious Metal Refining) discloses a silver alloy composition of the Sn, In, Zn type that also contains copper and boron. It comprises 89-93.5 wt % Ag, 0.01-2 wt % Si, about 0.001-2 wt % B, about 0.5-5 wt % Zn, about 0.5-6 wt % Cu, about 0.25-2 wt % Sn, and about 0.01-1.25 wt % In. Silicon is added as a de-oxidant. Boron is added to reduce the surface tension of the molten alloy, and to allow it to blend homogeneously. Zinc is added to reduce the melting point of the alloy, to add whiteness, to act as a copper substitute, to act as a deoxidant, and to improve fluidity of the alloy. Copper is added as a conventional hardening agent for silver, as well as acting as the main carrying agent for the other materials. Tin is added to improve tarnish resistance, and for its hardening effect. Indium is added as a grain-refining agent, and to improve the wetability of the alloy. Silver must be present in the necessary minimal percentage to qualify as either coin silver or sterling silver. In the experience of the present inventors, although tarnish resistance is exhibited to some extent, together with some firestain reduction on investment casting, firestain resistance on soldering or annealing is not obtained because of the copper content.
U.S. Pat. No. 5,039,479 (Bernhard, United Precious Metals) discloses a silver alloy composition allegedly exhibiting the desirable properties of reduced fire scale, reduced porosity, reduced grain size and reduced oxide formation when heated. It consists essentially of about 89-93.5 wt % Ag, about 0.02-2 wt % Si, about 0.001-2 wt % B, about 0.5-5 wt % Zn, about 0.5-6 wt % Cu, about 0.25-6 wt % Sn, and about 0.01-1.25 wt % In.
U.S. Pat. No. 5,817,195 (Davitz, Astrolite, Inc) discloses an alloy alleged to be highly tarnish resistant, corrosion resistant and non-brittle, which comprises 90% to 92.5 wt % Ag, 0.25% to 0.5 wt % Ni, 0% to 0.5 wt % In, metal silicate consisting of 5.75% to 7.5% zinc by weight of the silver coloured alloy, 0.25% to <1 wt % copper by weight of the silver coloured alloy and 0.1% to 0.25 wt % silicon by weight of the silver coloured alloy.
U.S. Pat. No. 5882441 (Davitz) discloses a silver coloured allegedly highly tarnish resistant, corrosion resistant and non-brittle free alloy consisting essentially of 90% to 94 wt % Ag, 3.75% to 7.35 wt % Zn, 1% to 3 wt % Cu and 0.1% to 0.25 wt % Si. A preferred alloy formula is 92.5 wt % Ag, 4.5 wt % Zn, 2.9 wt % Cu and 0.1 wt % silicate (sic).
U.S. Pat. No. 6,841,012 (Croce) discloses an allegedly tarnish-resistant silver alloy comprising at least 85 wt % Ag, the balance including zinc, copper, indium, tin and iron, see also WO 04/097056 (Croce, Steridyne Laboratories, Inc). Copper contents of up to 1.5 wt % are disclosed, e.g. an allegedly a tarnish resistant silver alloy including at least 90 wt % Ag, 0.5-1.5 wt % Cu, 2-5 wt % Zn, about 0.1 wt % In and about 0.2 wt % Au.
US 2004/0219055 (Croce) discloses further allegedly anti-tarnish silver alloys of the Zn, Cu, In, Sn family, the alloys having at least 85 wt % Ag and the balance also including Fe. Boron is an optional ingredient.
US-A-2005/0186107 (Davitz, Sterilite LLC) discloses a silver-coloured, tarnish-resistant, corrosion-resistant alloy. It includes 92.5-95 wt % silver combined with a master alloy of 24-34 wt % Zn, 60-74 wt % Cu, 0.5-1.8 wt % Si and 0.0-8.0% Sn or 0.0-1.5 wt % In. The alloy can be used for jewellery items, tableware, dental items or other items that should resist tarnishing or corrosion and which require a non-brittle alloy.
US-A-2005/0211342 (Menon, United Precious Metal Refining, Inc) discloses a manganese sterling silver alloy composition alleged to exhibit the properties of improved hardness and reversible heat treatability, in addition to reduced fire scale formation, reduced porosity, and reduced grain size. It consists essentially of the about 92.5-92.8 wt % Ag, about 2.0-3.0 wt % Cu, about 2.0-3.0 wt % Zn, about 0.03-0.05 wt % In, about 0.01-0.03 wt % Sn, about 0.20-0.50 wt % Cu/B alloy (2.0 wt % B, 98.0 wt % Cu) about 0.50-0.90 wt % Si/Cu alloy (10.0 wt % Si, 90.0 wt % Cu), and 0.01%-0.10 wt % Mn.
Patent GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche) discloses a silver alloy that maintains the properties of hardness and lustre inherent in Ag-Cu alloys while reducing problems resulting from the tendency of the copper content to oxidise The alloys are ternary Ag—Cu—Ge alloys containing at least 92.5 wt % Ag, 0.5-3 wt % Ge and the balance, apait firom 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 exerts a protective fimction that is responsible for the advantageous combination of properties exhibited by the new alloys, and was in solid solution in both the silver and the copper plases. The microstructure of the alloy is constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver and copper. The germanium in the copper-rich phase inhibits surface oxidation of that phase by forming a thin GeO and/or GeO2 protective coating that prevents firestain during brazing and flame annealing. Furthermore the development of tarnish is appreciably delayed by the addition of germanium, the surface turning slightly yellow rather than black and tarnish products being easily removed by ordinary tap water. The alloy is useful inter alia in jewellery and silversmithing.
U.S. Pat. No. 6,168,071 (Johns) describes and claims inter alia a silver/germanium alloy having an Ag content of at least 77% by weight, a Ge content of between 0.5 and 3% by weight, the remainder being copper apart fiom any impurities, which alloy contains boron as a grain refiner at a concentration of up to about 20 parts per million. It further discloses providing the boron content by a master Cu/B alloy having a boron content of about 2 percent by weight. The boron in the copper/boron alloy is elemental boron. Providing the boron content within a Cu/B alloy is stated to overcome the problem of handling pure boron, which typically is a lightweight powder. Such copper/boron master alloys are said to be commonly available, and for example Belmont Metals Inc offers on its website a copper-based master alloy containing 2 wt % boron with any of As, Be, Cd, Cr, Fe, Li, Mg, Ni, P, Si, Te, Ti, Zn or Zr.
U.S. Pat. No. 6,726,877 (Eccles) discloses an allegedly fire scale resistant, work hardenable jewellery silver alloy composition comprising at least 86 wt % Ag, 0.5-7.5 wt % Cu, 0.07-6 wt % of a mixture of Zn and Si wherein 0.02-2 wt % Si and 0.01-2.0 wt % Ge are present. The alloy may also include rheology modifying and other additives to aid in improving the castability and/or wetting performance of the molten alloy. For example, about up to 3.5% by weight of a modifying additive selected fiom In, B or a mixture thereof may be added to the alloy to provide grain refinement and/or provide greater wettability of the molten alloy. For manufacturing jewellers the copper content of a 925 alloy is recommended to be in the range of 2-3 wt %, the amount of zinc being 2-4 wt %, and silicon being added in proportion to the amount of zinc incorporated and being preferably 0.15-0.2 wt %. The compositions iay be formed by the addition of a master alloy to fine silver, the master alloy comprising e.g. 52.5-99.85 wt % Cu, 0.1-35 wt % Zn and 0.05-12.5 wt % Ge. Experiments by the present applicants have not confirmed fire resistance of available embodiments of the alloy, especially during torch annealing.
Silver alloy according to the teaching of GB-B-2255348 and EP-B-0729398 is now commercially available in Europe and in the USA under the trade name Argentium, and the word “Argentium” as used herein refers to these alloys. The 925 grade Argentium alloy comprises 92.5 wt % (minimum) Ag, 1.1-1.3 wt % Ge, 6 ppm B, the balance being copper and impurities. The alloy shows excellent resistance to tarnishing even under very arduous conditions. A passive layer is formed by the germanium, which significantly slows the formation of silver and copper sulphides, the main cause of tarnish on conventional silver alloys. Even in a hydrogen sulphide atmosphere the degree and depth of tarnish is significantly less compared to a conventional silver alloy or a silver plated item. The same mechanism that creates the tarnish resistance also results in the formation of a passive layer which significantly reduces the depth of ‘fire-staining’ or the ‘fire layer’ that is produced in this alloy when torch annealing in air. Trials have shown that Argentium is substantially free from firestain, which reduces the amount of polishing that the alloy requires and can give rise to considerable cost savings. As previously explained, other commercial alloys develop firestain on simple torch annealing, and it has been found that in practice just over 1% Ge in a silver jewellery or silversmithing alloy is desirable to avoid firestain even in subsequently tested alloys containing relatively low copper levels and even with zinc. The Society of American Silversmiths maintains a website for commercial embodiments of the above-mentioned alloys known as Argentium (Trade Mark) at the web address http://www.silversmithing.com/largentium.htm. It discloses that 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. However, there is no suggestion that precipitation hardening is appropriate for nearly finished work and that the problems of distortion and damage to soldered joints can be avoided.
Despite the advantages of existing Argentium alloy grades, in embodiments of the invention there is a need for further improvement of the alloy with respect to its stability under thermal processing and in particular to its resistance to pitting and/or sagging when heated for the purposes of annealing or joining. There is also a need for alloys that combine these favourable propel-ties with hardness and resistance to tarnishing.
U.S. Pat. No. 6,726,877 (Eccles) discloses inter alia 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.
By way of background, U.S. Pat. No. 4,810,308 (Leach & Garner) 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. Preferably. 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. It can then be age hardened by reheating to 300-700° F. (149-371° C.) for a predetermined time followed by cooling of the age hardened alloy to ambient temperature. 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 U.S. Pat. No. 4,869,757 (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 U.S. Pat. Nos. 05,817,195 and 5,882,441 and to exhibit high tarnish and corrosion resistance. The alloy of U.S. Pat. No. 5,817,195 (Davitz) 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 U.S. Pat. No. 5,882,441 (Davitz) 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 U.S. Pat. No. 4,973,446 (Bernhard) and is said to exhibit reduced firestain, reduced porosity and reduced grain scale. None of these references discusses annealing or precipitation hardenining.
WO2004/106567 discloses the desirability of reducing or avoiding the formation and/or melting of the above mentioned binary copper-germanium eutectic which melts at 554° C. During the production of e.g. 925 Argentium silver alloys, the formation of this phase can be avoided by careful control of the casting conditions since under equilibrium cooling conditions the crystallisation is complete at below 640° C. However, this binary phase can create problems during subsequent thermal treatment of the alloys, e.g. using brazing alloys which typically have melting points in the range 680-750° C. and torch annealing which typically involves heating a workpiece to a dull red heat at 700-750° C. On heating the workpiece to or beyond these temperatures incipient melting occurs with a small amount of material corresponding to this binary phase becoming molten while the bulk remains stable. When the workpiece returns to ambient temperature, porosity develops where the alloy has liquefied. This contributes brittleness and e.g. as noted in GB-B-2255348 there is a tendency for the alloy to sag when heated for joining or annealing operations. Although the use of the boron grain refiner of U.S. Pat. No. 6,168,071 and EP-B-0729398 significantly reduces the pitting and sagging consequent on formation and melting of the binary eutectic, the formation and melting of that eutectic is, as previously mentioned, not eliminated and there is still scope for the further development of the ternary alloy to improve its pitting and sagging properties. By increasing the silver content above the level for Sterling but less than that for Britannia (95.84 wt % Ag) it is possible to produce an alloy in which the above binary eutectic either does not form or gives rise to reduced problems in subsequent heat treatment. This provides alloys with a much greater inherent stability under thermal processing. The germanium addition prevents the reduction in hardness that would be seen in a silver-copper alloy of this composition. The alloy also shows resistance to tarnishing, even under very arduous test conditions.
The invention of WO 2003/106567 therefore provides a ternary alloy of silver, copper and germanium containing from more than 93.5 wt % to 95.5 wt % Ag, from 0.5 to 3 wt % Ge and the remainder, apait from incidental ingredients, impurities and grain refiner, copper. A typical alloy that has been found to be suitable contains about 94.5 wt % Ag, about 4.3 wt % Cu and about 1.2 wt % Ge. In the above alloy the weight ratio of Cu to Ge is about 3.6:1 whereas in the existing 925 grade Argentium the ratio can be from 5.8:1 (1.1 wt % Ge) to 4.8:1 (1.3 wt % Ge). The applicants suggested that it is the reduction in the Cu:Ge weight ratio that is responsible for the reduced thermal processing problems, the CuGe eutectic either not forming or forming in a significantly reduced amount during post-melt thermal processing. In particular the ratio is preferably from 4:1 to 3:1, more preferably about 3.5:1, Above 4:1 the alloy is more likely to exhibit firestain, whereas below 3:1 the high germanium content gives rise to formability problems. In the above alloy, preferred Ag contents ranged from about 94.0 to about 95.5 wt %, lower values being preferned for reducing the expense of the silver.
SUMMARY OF THE INVENTIONWe have observed a surprising difference in properties between conventional sterling silver alloys and other silver alloys of the Ag—Cu family on the one hand and silver alloys of the Ag—Cu—Ge family on the other hand. Gradual cooling of e.g. the binary Sterling-type alloys results in coarse precipitates and little precipitation hardening, whereas gradual cooling of Ag—Cu—Ge alloys (including those containing the further additives and incidental ingredients set out above) results in fine precipitates and useful precipitation hardening, especially in those embodiments where the silver alloy contains an effective amount of grain refiner e.g. boron.
Experimental evidence has shown that Ag—Cu—Ge alloys of Ag content at least 92.5 wt % become precipitation hardened following cooling from a melting or annealing temperature by baking at e.g. 200° C.-400° C. even at copper contents below 2 wt %, e.g. below 1.7 wt %, e.g. below 1.5 wt % e.g. below 1 wt % down to e.g. 0.5 wt %, and that baking the alloy can achieve a hardness of 65 or above, preferably 70 HV or above and still more preferably 75 HV or above which is equal to or above the hardness of standard sterling silver used to make jewellery and other silverware. These advantageous properties are believed to be the result of the combination of Cu and Ge in the silver alloy and are independent of the presence and amounts of Zn, In, Sn, Sb, Mn or other incidental alloying ingredients. This behaviour contrasts with that of high silver low copper alloys not containing Ge which do not precipitation harden. Silver alloys of very low copper content can exhibit sweat resistance and can perform well in salt spray tests. A useful precipitation-hardenable silver alloy comprises 96-97.3 wt % Ag, 1-1.55 wt % Ge, balance copper and optionally zinc, and boron as grain refiner.
We have now found that 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. Significantly it has been found that 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. with water which as explained above is required for Ag—Cu Sterling silver, and which as explained above can give rise to distortion or damage to the product, and therefore can be used for nearly finished work. The process is applicable to alloys of the general kind disclosed in GB-B-2255348. It is also believed to be applicable to some or all of the alloys disclosed in U.S. Pat. No. 6,726,877, including those of relatively high germanium content and also those of lesser germanium content and relatively high zinc and silicon content.
From one aspect, the invention provides a shaped object of Ag—Cu—Ge of a silver alloy including at least 92.5 wt % Ag, 0.5-2 wt % Cu and 0.1-3 wt % Ge together with boron as grain refiner, the object being precipitation hardened to at least 65 HV, e.g. to at least 70 HV and preferably to at least 75 HV.
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.
DETAILED DESCRIPTION OF PREFERRED FEATURESWhen applied to finished or semi-finished articles of the alloys disclosed in U.S. Pat. No. 6,726,877, 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 fiom 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.
In another aspect, the 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, an amount of germanium that is at least 0.5 wt % and is effective to reduce tarnishing and or firestain and boron incorporated by dispersing throughout said alloy a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof;
making or processing the finished or semi-finished article of the alloy by heating at least to an annealing temperature;
cooling the article gradually, without an abrupt cooling step, so that cooling to ambient temperature takes more than 10 seconds; and
reheating the article to effect precipitation hardening thereof.
Addition of germanium to sterling silver changes the thermal conductivity of the alloy compared to standard sterling silver. The International Annealed Copper Scale (IACS) is a measure of conductivity in metals. On this scale the value of copper is 100%, pure silver is 106%, and standard sterling silver 96%, while a sterling alloy containing 1.1% germanium has a conductivity of 56%. The significance is that the Argentium sterling and other germanium-containing silver alloys do not dissipate heat as quickly as standard sterling silver or their non-germanium-containing equivalents, a piece will take longer to cool, and precipitation hardening to a commercially useful level (e.g. to about Vickers hardness 70 or above, preferably to Vickers hardness 110 or above, more preferably to 115 or above) can take place during natural air cooling or during slow controlled air cooling.
Furthermore, the ability of the alloys of the Ag—Cu—Ge family to precipitation harden to useful values without the need for quenching is retained to copper contents as low as 1 wt % or even as low as 0.5 wt %, whereas other silver alloys become unacceptably soft at such low copper contents and cannot be hardened sufficiently by heat treatment. The ability of the present silver alloys containing 0.5 wt % Cu or above e.g. 1 wt % copper or above and optionally zinc and/or palladium as well as germanium to precipitation harden makes it practical to reduce the copper content of the alloy. Even though an alloy of lower copper content may be relatively soft as cast, reheating at a low temperature e.g. 150° C. or 200°-400° C. e.g. 300° C. may bring the hardness up to the level of normal sterling silver or better. This is a significant advantage because from the standpoint of corrosion resistance the copper content is the most detrimental part of the alloy, but reduction of copper in a standard Sterling alloy gives rise to unacceptably low hardness. In the present alloys, if the copper content is reduced, the silver content may simply be increased or there may be incorporated zinc e.g. in an amount of 1-2 wt %. Other possibilities include increasing the germanium content or adding further zinc or another alloying element e.g. palladium. Silver alloy of Ag 973 parts per thousand and containing about 1.0 wt % Ge, balance copper, has been successfully precipitation hardened by gradual air cooling from an annealing temperature, and it is believed that Ag—Cu—Ge alloys with silver content above this level are also precipitation hardenable. Significant hardness has been reported for air cooled/quenched Ag—Zn—Cu alloys containing as little as 0.8 wt % Cu.
The benefit of not having to quench to achieve the hardening affect is a major advantage of the present silver alloys. There are very few times in practical production that a silversmith can safely quench a piece of nearly finished work. The risk of distortion and damage to soldered joints when quenching from a high temperature would make the process not commercially viable. In fact standard sterling can also be precipitation hardened but only with quenching from the annealing temperature and this is one reason why precipitation hardening is not used for sterling silver.
In order to distinguish the operations of annealing and precipitation hardening (which are regarded as distinct by silversmiths) annealing temperatures may be defined to be temperatures above 500° C., whereas precipitation hardening temperatures may be defined to be in the range 150° C.-400° C., the lower value of 150° C. permitting embodiments of the alloys of the invention to be precipitation hardened in a domestic oven.
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 1 wt % 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. Other 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. As used herein the expressions “ternary” and “quaternary” refer to the content of principal atomic types e.g. silver, copper and germanium and do not refer to boron which is added in smaller amounts as a grain refiner.
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. For an ‘800 grade’ ternary alloy, 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.
In addition to silver, copper and germanium, and optionally zinc, 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.
In a preferred embodiment, 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. In a more preferred embodiment, 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. In a further preferred embodiment, the alloy is a ternary alloy consisting, apart fiom impurities and grain refiner, of 92.5% to 96% silver, 0.9% to 2% germanium, and 1% to 7% copper, by weight of the alloy, together with 1 ppm to 40 ppm boron as grain refiner. 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.
As previously explained, the alloys disclosed in U.S. Pat. No. 6,726,877 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. In some embodiments 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 (Eccles) 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. In the case of these alloys, 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.
Other alloys of the invention are described below.
Group 1 Alloys
It has been realized that a good way to reduce sag in AgCuGe alloys when heated for soldering and annealing is to remove some of the copper. Ideally the copper needs to be below 3%. Increasing the silver content is effective to a degree, but at 95% and above, the alloys were very soft after annealing or soldering (although they could be hardened by precipitation).
Silver alloys of the first group comprise 92.5-97 wt % Ag, 1-4.5 wt % Cu, 0.4-4 wt % Zn, 0.8-1.5 wt % Ge, 0 to 0.2 wt % Si, In or Sn and 0-0.2 wt % Mn, the balance being boron as grain refiner incidental ingredients or impurities.
Alloys of this group also comprise 92.5-97 wt % Ag, 1-3 wt % Cu, 1-4 wt % Zn, 0.8-1.5 wt % Ge, 0 to 0.2 wt % Si, In or Sn and 0-2 wt % Mn, the balance being boron as grain refiner added as an alkali metal borohydride, incidental ingredients or impurities.
The alloys of this group may be provided as casting grain as aforesaid containing silicon in an amount effective to produce an as-cast silvery appearance and inhibit mould reactions in articles made by investment casting. Such reactions are generally nor detrimental to the properties of finished products, but require processing for their removal and can be disconcerting for those new to the use of the present alloys. The alloys of this group therefore include silver alloy casting grain comprising 92.5-97 wt % Ag, 1-4.5 wt % Cu, 0.4-4 wt % Zn, 0.8-1.5 wt % Ge, 0.05-2 wt % Si, 0 to 0.2 In or Sn and 0-0.2 wt % Mn, the balance being boron as grain refiner, incidental ingredients and impurities.
There is also provided the use in Ag—Cu—Ge silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt % and preferably 1-3 wt %, especially 1-1.5 wt % of 1-3 wt % Zn to reduce or prevent pitting or sagging on heating the alloy and/or to increase annealed hardness.
There is also provided the use in Ag—Cu—Ge silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt % and preferably 1-3 wt %, especially 1-1.5 wt % of 1-3 wt % Zn e.g. 2-3 wt % Zn to reduce or prevent pitting or sagging on heating the alloy and up to 0.2 wt % of Sn, In or Si or a mixture thereof to reduce or prevent zinc oxide formation on heating the alloy.
There is also provided a ternary alloy of silver, copper and germanium containing from more than 93.5 wt % to 95.5 wt % Ag, from 0.5 to 3 wt % Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride.
It has also been noted that when the silver content of the alloy was above 95.5%, firestain surprisingly reappeared. It has now been found that if the content is raised above 92.5% and about 1-2% Zn is added, sagging can be reduced or eliminated, and surprisingly a better annealed hardness is obtained using higher silver in combination with zinc, rather than replacing the copper with silver or zinc alone. This is significant as high silver or high zinc silver alloys are normally too soft to be used as general-purpose alloys. fligher levels of zinc (above 2%) result in heavy zinc oxide when annealing or soldering in air but it has been observed that a small amount of Sn, In or Si (>0.2) will suppress the zinc oxide formation. The resulting alloy also has superior cottosion resistance to attack by acids. The invention therefore contemplates alloys containing Sn, In or Si in an amount effective to suppress zinc oxide formation during torch annealing. Surprisingly, the presence of zinc has not proved detrimental to the ability of Ag—Cu—Ge based silver alloys to precipitation harden using the methods described below. If desired palladium may be added in partial or total replacement of zinc in an amount of up to 3 wt %.
Boron may be added as described below. Grain refining Ag—Cu—Ge-based silver jewellery or silversmithing alloys using sodium borohydride can also improve hardness by about 10 HV. An optimum combination of Ag, Zn and sodium borohydride can produces a sterling silver alloy with improved annealed hardness and superior mechanical properties. Furthermore, sodium from e.g. sodium borohydride could make the alloy useful as an electrical contact material. Sodium at ppm levels has arc-quenching properties, as also does germanium.
If desired, the germanium or copper content may be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected. The weight ratio of germanium to incidental ingredient elements may range from 100:0 to 80:20, preferably from 100:0 to 60:40. The term “incidental ingredients” permits the ingredient to have ancillary functionality within the alloy e.g. to improve colour or as-moulded appearance, and includes the previously mentioned metals or metalloids Si, Zn, Sn or In in amounts appropriate for “deox”.
Silicon, in particular, may be added to silver alloys for casting grain e.g. in an amount of up to 0.5 wvt % more usually 0.1-0.2 wt %, and is conveniently provided in the form of a copper-silicon master alloy containing e.g. about 10 wt % Si. When incorporated e.g. into casting grain of a silver-copper-germanium ternary alloy it can provide bright investment castings immediately on removal from the mold. It may be added to casting grain e.g. before investment casting or it may be incorporated into the silver at the time of first melting to form an alloy.
Group II Alloys
A second group of silver alloys resistant to tarnish under severe conditions e.g. exposure to human sweat or French dressing comprises 1-5 wt % Zn, 0.7-3 wt % Cu, 0.1-3 wt % Ge, 0-0.3 wt % Mn, 0-0.25 wt % Si, B in an amount effective for grain refinement, up to 0.5 wt % incidental ingredients, the balance being Ag in an amount of 92.5-96 wt % and impurities.
The alloys of this second group may be made by
melting together the ingredients; and
incorporating boron by dispersing into molten silver alloy to form the whole or a precursor part of said alloy a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.
Shaped articles or casting grain of the alloys of this group are also within the invention.
There is also provided a method of making a shaped article, which comprises:
providing a molten silver alloy comprising 1-5 wt % Zn, 0.7-3 wt % Cu, 0.1-3 wt % Ge, 0-0.3 wt % Mn, 0-0.25 wt % Si, B in an amount effective for grain refinement, up to 0.5 wt % incidental ingredients, the balance being Ag in an amount of 92.5-96 wt % and impurities;
casting an article in a mould with said molten silver alloy;
allowing said cast article to cool; and
reheating said article to effect hardening thereof.
Silver contents of this group of alloys may be from 92.5-96 wt %, preferably 92.5-95.5 wt %, and most preferably at or closely above 92.5 wt %. For present purposes, it is desirable to maximize the content of zinc which is inexpensive compared to silver.
Copper content is desirably in the range 0.7-3 wt %, preferably about 1-2 wt % and especially about 1.5 wt %. A certain amount of copper is desirable for hardness and ability to precipitation harden, but the proportion of copper is relatively low in order to minimise tarnishing.
Zinc content may be in the range 1-4.5 wt % especially about 2-4 wt %, and should preferably be close to the upper limits of the above ranges. Contrary to the teaching of the prior art, zinc-containing alloys have in the past been relatively soft, and it is the ability of germanium-containing alloys to precipitation harden that has enabled useful hardnesses of the order of 100 HV to be achieved with relatively high zinc content alloys. The very fine grain structure consequent on the addition of sodium borohydride or other decomposable boron compounds also contributes to the achievement of unexpectedly good hardness and/or other physical at high zinc content because of the excellent grain refinement that can be achieved. One unexpected advantage is that the borohydride-treated alloys exhibit relatively high ductility compared to standard material. Items which are liable to breakage on bending e.g. claws for rings which need to bend back and forth when setting stones into the ring can be bent back and forth with relatively few problems using the present alloys.
Manganese has unexpectedly been found to improve tarnish resistance and is incorporated in amounts of e.g. 0.02-0.2 wt %, especially about 0.1-0.2 wt %. An additional or alternative advantage of manganese is that it adds hardness in the annealed state after a slow cool.
It is preferable to use silicon to give brighter castings. Silicon is also believed to inhibit the formation of zinc oxide. Amounts of silicon may be 0.04-0.25 wt % e.g. about 0.1 wt %.
The alloy may contain one or more incidental ingredients known per se in the production of silver alloys in amounts that are not detrimental to the mechanical strength, tarnish resistance and other properties of the material. Cadmium may also be added in similar amounts although its use is presently not preferred. Tin may be beneficial, typically in an amount of 0.5 wt %. Indium may be added in small quantities e.g. as a grain refiner and to improve the wetability of the alloy. Other possible incidental ingredient elements selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected.
Boron is incorporated into the present silver alloys as a grain refiner and may be incorporated as described below.
Group III Alloys
The invention also relates to Ag—Cu—Ge alloys of Ag content at least 92.5 wt % and Ge content 0.1-3 wt %, preferably 0.8-1.5 wt %, more preferably about 0.8-1 wt % that have copper contents below 2 wt %, e.g. below 1.7 wt %, e.g. below 1.5 wt % e.g. below 1 wt % down to e.g. 0.5 wt % (in some embodiments as low as 0.2 wt %), become precipitation hardened following cooling from a melting or annealing temperature by baking at e.g. 200° C.-400° C. and on baking can achieve a hardness of 65 HV or above e.g. 70 HV or above.
The invention also relates to shaped articles of the alloys mentioned above that have been precipitation hardened to 65 HV or above e.g. 70 HV or above.
The alloys of this group in typical embodiments have silver contents of 93-94 wt %. The combined content of Cu and Ge ranges from 0.6 wt % to 5 wt % leaving at least 1 wt % and usually at least 3 wt %, more usually at least 4 wt % of other silver-compatible metals that make up the remainder of the alloy together with incidental ingredients (if any) and impurities. There are various possibilities for making up the balance of the alloy.
One possibility is to avoid other alloying ingredients, except incidental ingredients (if any), and increase the amount of silver e.g. up to 97 wt % to. As previously explained, if the silver content rises above 96 wt %, then the problem of firestain reappears, but in some embodiments this disadvantage can be accepted. In other embodiments up to 4.5 wt % Zn e.g. 3-4 wt %, up to 2.5 wt % Sn e.g. about 1 wt % Sn or up to 3 wt % Pd may be incorporated into the alloy.
Manganese has been found to improve tarnish resistance and may be incorporated in amounts of e.g. 0.02-0.2 wt %, especially about 0.1-0.2 wt %. As previously explained, an additional or alternative advantage of manganese is that it adds hardness in the annealed state after a slow cool.
For casting alloys, it is preferable to use silicon to give brighter castings. Silicon is also believed to inhibit the formation of zinc oxide. Amounts of silicon may be 0.04-0.25 wt % e.g. about 0.1 wt %.
The alloy may contain up to 0.5 wt % of one or more incidental ingredients known per se in the production of silver alloys in amounts that are not detrimental to the mechanical strength, tarnish resistance and other properties of the material. Cadmium may also be added although its use is presently not preferred. Indium may be added in small quantities e.g. as a grain refiner and to improve the wetability of the alloy. Other possible incidental ingredient elements selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected.
Boron may be incorporated into the alloy using the procedures indicated below. With the relatively low quantities of copper present in this group of alloys, it is considered particularly advantageous to incorporate the boron as a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof
General Methods for Adding Boron
A boron compound may be introduced into molten silver alloy in the gas phase, advantageously in admixture with a carrier gas which assists in creating a stirring action in the molten alloy and dispersing the boron content of the gas mixture into said alloy. Suitable carrier gases include, for example, hydrogen, nitrogen and argon. The gaseous boron compound and the carrier gas may be introduced from above into a vessel containing molten silver e.g. a crucible in a silver-melting furnace, a casting ladle or a tundish using a metallurgical lance which may be a elongated tubular body of refractory material e.g. graphite or may be a metal tube clad in refractory material and is immersed at its lower end in the molten metal. The lance is preferably of sufficient length to permit injection of the gaseous boron compound and carrier gas deep into the molten silver alloy. Alternatively the boron-containing gas may be introduced into the molten silver from the side or from below e.g. using a gas-permeable bubbling plug or a submerged injection nozzle. For example, Rautomead International of Dundee, Scotland manufacture horizontal continuous casting machines in the RMK series for the continuous casting of semi-finished products in silver and gold. The alloy to be heated is placed in a solid graphite crucible, protected by an inert gas atmosphere which may for example be oxygen-free nitrogen containing <5 ppm oxygen and <2 ppm moisture and is heated by electrical resistance heating using graphite blocks. Such furnaces have a built-in facility for bubbling inert gas through the melt. Addition of small quantities of thermally decomposable boron-containing gas to the inert gas being bubbled through the melt readily provides a desired few ppm or few tens of ppm boron content The introduction of the boron compound into the alloy as a dilute gas stream over an period of time, the carrier gas of the gas stream serving to stir the molten metal or alloy, rather than in one or more relatively large quantities is believed to be favourable from the standpoint of avoiding development in the metal or alloy of boron hard spots. Compounds which may be introduced into molten silver or gold or alloys thereof in this way include boron trifluoride, diborane or trimethylboron which are available in pressurised cylinders diluted with hydrogen, argon, nitrogen or helium, diborane being preferred because apart from the boron, the only other element is introduced into the alloy is hydrogen. A yet further possibility is to bubble carrier gas through the molten silver to effect stirring thereof and to add a solid boron compound e.g. NaBH4 or NaBF4 into the fluidized gas stream as a finely divided powder which forms an aerosol.
[A boron compound may also be introduced into the molten silver or gold alloy in the liquid phase, either as such or in an inert organic solvent. Compounds which may be introduced in this way include alkylboranes or alkoxy-alkyl boranes such as triethylborane, tripropylborane, tri-n-butylborane and methoxydiethylborane which for safe handling may be dissolved in hexane or THF. The liquid boron compound may be filled and sealed into containers of silver or of copper foil resembling a capsule or sachet using known liquid/capsule or liquid/sachet filling machinery and using a protective atmosphere to give filled capsules sachets or other small containers typically of capacity 0.5-5 ml, more typically about 1-1.5 ml. As an alternative, especially for gold casting, the capsules or sachets may be of a polymer e.g. polyethylene or polypropylene. The filled capsules or sachets in appropriate number may then be plunged individually or as one or more groups into the molten silver or gold or alloy thereof. A yet farther possibility is to atomize the liquid boron-containing compound into a stream of carrier gas which is used to stir the molten silver as described above. The droplets may take the form of an aerosol in the carrier gas stream, or they may become vaporised therein.
Preferably the boron compound is introduced into the molten silver alloy in the solid phase, e.g. using a solid borane e.g. decaborane B10H14 (m.p. 100° C., b.p. 213° C.). However, the boron is preferably added in the form of either a boron containing metal hydride or a boron containing metal fluoride. When a boron containing metal hydride is used, suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof When a boron containing metal fluoride is used, sodium is the preferred metal. Most preferred is sodium borohydride, NaBH4 which has a molecular weight of 37.85 and contains 28.75% boron.
Boron can be added to the other molten components both on first melting and at intervals during casting to make up for boron loss if the alloy is held in the molten state for a period of time, as in a continuous casting process for grain. This facility is not available when using a copper/boron master alloy because adding boron changes the copper content and hence the overall proportions of the various constituents in the alloy.
It has surprisingly been found that when adding a borane or borohydride that more than 20 ppm can be incorporated into a silver alloy without the development of boron hard spots. This is advantageous because boron is rapidly lost from molten silver: according to one experiment the content of boron in molten silver decaying with a half-life of about 2 minutes. The mechanism for this decay is not clear, but it may be an oxidative process. It is therefore desirable to incorporate more than 20 ppm boron into an alloy as first cast i.e. before investment casting or before rolling into strip, and amounts of e.g. up to 50 ppm, typically up to 80 ppm, and in some instances up to 800 or even 1000 ppm may be incorporated. Thus there could be produced according to the present method silver casting grain containing about 40 ppm. boron. Owing to boron loss during subsequent re-melting and investment casting, casting to form strip, rod or wire, strip rolling or other downstream processes, the boron content of finished pieces may be closer to the 1-20 ppm characteristic of the prior art, but the ability to achieve relatively high initial boron concentrations means that improved consistency may be achieved during the manufacturing stages and in the final finished products. Furthermore higher boron content is desirable for master alloys which will be melted with precious metal to make casting grain and then further melted to form rod, wire, or investment casting.
Articles made e.g. by casting the present alloy may be hardened by heating in an oven e.g. at about 300° C. for about 45 minutes.
Shaped or Fabricated Articles
In one embodiment the article is a shaped or fabricated article e.g. of jewellery, woven mesh or chain or mesh knitted from drawn wire, or of hollowware 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. Such conveyors are available from e.g. Lindberg, of Watertown, Wis., USA and Dynalab of Rochester N.Y. as mentioned above. Generally such articles will be a soldered or brazed assembly of two or more components.
When annealing, it is desirable that 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.
When brazing it should be noted that the addition of germanium lowers the melting temperature of the alloy by 59° F. (15° C.) relative to sterling silver. It is recommended that an “easy” or “extra easy” grade of solder should be used. 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 tai-nish protection are described in UK Patent Application 03 26927.1 filed 19 Nov. 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.
[Compared to what is required for sterling silver, it will be noted that what is necessary for Argentium sterling and other germanium-containing silver alloys involves a reduced number of processing steps with avoidance of quenching and only mild reheating to precipitation hardening to a required level.
Investment Cast Articles
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.
Investment casting with air-cooling for 15-20 minutes followed by quenching of the investment flask in water after 15-20 minutes gives a cast piece having a Vickers hardness of about 70, which is approximately the same hardness as sterling silver. Surprisingly it has been found that a harder cast piece can be produced by allowing the flask to cool in air to room temperature, the piece when removed from the flask having a Vickers hardness of about 110. Most standard investment removers will successfully remove the investment powder, as will a pneumatic hammer whose vibration can break up the investment. A water-knife can also be used for removing the investment. The production by casting of pieces that combine this degree of hardness with firestain and tarnish resistance has not been reported.
Even more surprisingly, and contrary to experience with Sterling silver, where necessary, 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.
In particular, as explained by Fischer-Buhner (supra) at p. 41, with conventional sterling silver simple slow cooling of flasks after casting results in growth of coarse Cu-rich precipitates and eliminates the possibility of precipitation hardening during a subsequent aging treatment. Water quenching is required within a narrow and critical range of times after casting, typically 4 minutes after casting, the hardening effect being reduced both by quenching too soon and too late. In the case of pieces cast on a tree different cooling conditions at different places on the tree prior to quenching result in the individual cast pieces differing in their ability to become hardened during the subsequent precipitation hardening step. All these problems of additional processing steps and control difficulties are avoided by the use of Ag—Cu—Ge alloys as described herein.
Polishing
Finished articles are advantageously polished with a protective agent. As protective agent there may be used a compound containing a long chain alkyl group and a —SH or —S—S— group, e.g. an alkanethiol, dialkyl sulfide or dialkyl disulfides in which the chain is preferably at least 10 carbon atoms long and may be C12-C24. The —SH or —S—S— compounds that many be used include straight chain saturated aliphatic compounds containing 16-24 carbon atoms in the chain, for example cetyl mercaptan (hexadecyl mercaptan) and stearyl mercaptan (octadecyl mercaptan) and cetyl and stearyl thioglycollates whose formulae appear below.
Octadecyl mercaptan is a white to pale yellow waxy solid that is insoluble in water and that melts at 30° C. Hexadecyl mercaptan is also a white or pale yellow waxy solid that melts at 15-16° C.
How the invention may be put into effect will now be further described with reference to the following Examples:
EXAMPLE 1A silver-copper-germanium-silicon alloy (Ag=94.7 wt %, Ge=1.2 wt %, Cu=3.9 wt % Si=0.2 wt %, added as a Cu/Si master alloy), is prepared by melting silver, copper, germanium and master alloy together in a crucible by means of a gas-fired furnace which becomes heated to a pour temperature of about 1093° C. (2000° F.). The melt is covered with graphite to protect it against atmospheric oxidation and in addition a hydrogen gas protective flame is provided. Stirring is by hand using graphite stirring rods. When the above ingredients have become liquid, pellets of sodium borohydride to give up to 100 ppm boron e.g. 80 ppm are packaged or wrapped in pure silver foil of thickness e.g. about 0.15 mm. The foil wrapper holds the pellets of sodium borohydride in a single group and impedes individual pellets becoming separated and floating the surface of the melt. The wrapped pellets are placed into the hollow cupped end of a graphite stirring rod and plunged beneath the surface of the melt which at this stage is covered with a ceramic fibre blanket to quench the resulting flame from decomposition of the borohydride. The hydrogen burns off over a period of about 1-2 minutes with a stirring action being applied, after which evolution of hydrogen ceases and the boron content is substantially incorporated into the melt together with at least some of the sodium which is believed innocuous to properties of the resulting alloy.
After boron addition, the crucible pivots to permits the molten alloy to be poured into a tundish whose bottom is formed with fine holes. The molten silver pours into the tundish and runs through the holes in streams which break into fine pellets which fall into a stirred bath of water and become solidified and cooled. The cast pellets are removed from the bath and dried.
The resulting alloy granules are used in investment casting using traditional methods and using a calcium sulphate bonded investment, and are cast at a temperature of 950-980° C. and at a flask temperature of not more than 676° C. under a protective atmosphere. The investment material, which is of relatively low thermal conductivity, provides for slow cooling of the cast pieces. Investment casting with air-cooling for 15-25 minutes followed by quenching of the investment flask in water after 15-25 minutes gives a cast piece having an expected Vickers hardness of about 70, which is approximately the same hardness as sterling silver. The resulting casting has a matt silvery finish when removed from the mold, and an even finer grain structure than when Cu/B master alloy is used, due e.g. to the relatively high boron content permitted by the sodium borohydride and the energetic dispersion of the boron into the molten silver as the borohydride decomposition reaction proceeds. The alloy can be polished easily, is free from boron hard spots, and gives products that exhibit excellent tarnish and firestain resistance. Precipitation hardening to expected hardness values of e.g. about 110 Vickers can be achieved by subsequent torch annealing, quenching and reheating in an oven at about 300° C.
However, a harder cast piece can be produced by allowing the flask to cool in air to room temperature, the piece when removed from the flask having an expected Vickers hardness of about 110 which is similar to the value that can be achieved by the torch anneal/quench/reheat method. Contrary to experience with Sterling silver, where necessary, the hardness can be increased even further by precipitation hardening e.g. by placing castings or a whole tree in an oven set to about 300° C. for 20-45 minutes to give heat-treated castings of an expected hardness approaching 125 Vickers.
EXAMPLE 2A ternary silver-copper-germanium alloy (Ag=94.7 wt %, Ge=1.2 wt %, Cu=4.1 wt %) is prepared by melting silver, copper and germanium and master alloy together and adding sodium borohydride as described in Example 1 and is formed into sheet. Pieces of the sheet are brazed together to form shaped articles by passage through a brazing furnace and are simultaneously annealed. Precipitation hardening develops 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 8-30 minutes in the temperature range 200-300° C. which is most favourable for precipitation hardening. Articles that have been brazed in a furnace in this way and gradually cooled can achieve hardness of 110-115 Vickers. Exceptionally small grain size and good firestain and tarnish resistance is obtained because of the sodium borohydride addition.
EXAMPLE 3 Alloys Were Prepared with the Compositions and Boron Contents Indicated in Table I Below Using CuB Master Alloys the Source of Boron
*Precipitation hardening (air cooled) - sample annealed, air cooled, then heated at 300° C. for 45 minutes. Precipitation hardening (quenched) - sample annealed, quenched, then heated at 300° C. for 45 minutes.
**No final assay results available. Table shows alloy make-up before melting.
***No precipitation hardening.
Further improvements in hardness and greater ease in polishing are obtained by increasing the boron content using sodium borohydride in place of CuB master alloy, melting following the procedure set out in Example 1.
EXAMPLE 4Zinc containing alloys according to the invention are prepared as set out in Table II below and their hardness is measured. In the above table, boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.
EXAMPLE 5Zinc and manganese containing alloys according to the invention are prepared as set out in Table III below and their hardness is measured. In the table III, boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.
It will be noted by comparison of samples 5.4 and 5.8 in table III that the effect of manganese is to increase hardness in the annealed state after a slow cool. Sample 5.4 contains Mn. and has an air cooled hardness of 76 HV. Sample 5.8 does not contain Mn but has an increase in Zinc, otherwise it is the same composition. Its annealed air cooled hardness 64 HV.
EXAMPLE 6The alloys of Tables II and III were tested for resistance to human sweat. Samples were polished with metal polish, ultrasonically degreased in an aqueous detergent solution for 2 minutes, rinsed, dried and wiped with acetone. For test purposes, samples were stood upright in a dish containing about 13 mm depth of an artificial sweat test solution (Stein-Leach, USA). Droplets of the artificial sweat were splashed onto an upper region of each sample and left to dry.
Alloy such as standard Sterling silver with a high copper content and no germanium or zinc exhibited poor resistance to the artificial sweat. A germanium-containing Silver alloy of high copper content with no added zinc or manganese (Argentium Sterling silver) showed improved resistance both to droplets of the artificial sweat and to immersion in the artificial sweat but were subject to some discoloration. Other germanium-containing silver alloys with no zinc content or with added zinc below about 0.8 wt % showed a similar level of sweat resistance. Increase in zinc content permitted germanium content to be somewhat reduced without significant detriment to sweat resistance. Alloys of high silver content, zinc content of 1 wt % or above and germanium content of 1 wt % or above showed good sweat resistance properties. The best sweat resistance amongst the alloys tested is obtained with an alloy containing a high level of zinc with low copper and relatively low germanium content. Addition of Mn is believed to significantly improve sweat resistance. Results are indicated in Table IV.
EXAMPLE 7It is well-known that sterling silver tarnishes faster than pure silver because of corrosion of the copper phase. French Vinaigrette Dressing (pH level 3.5) test has been used to attack the copper phase in a selection of silver alloys as set out in Table V, containing different quantities of copper. Samples of silver alloys were partially immersed in French Vinaigrette Dressing for 24 hours. A round ‘blob’ of vinaigrette is placed on the samples approximately 1 cm above the surface of the liquid. In this test, standard sterling Sterilite B and the ternary alloy all exhibited discoloration, whereas the alloys 5.4 and 5.7 exhibited substantially no discoloration.
EXAMPLE 8The casting grain of the first stage of Example 1 made using sodium borohydride as grain refiner is re-melted with about 2 wt % zinc in a continuous caster and cast to give strip of composition about 95.7 wt % Ag, 2 wt % Zn, 1 wt % Ge, balance copper, grain refiner and impurities. The resulting alloy strip can be made into shaped articles by stamping, further hardens by oven treatment at less than 500° e.g. 300° C. for 30-45 min to >about 100 HV, and exhibits resistance to tarnish by artificial sweat and by French dressing.
EXAMPLE 9 An alloy is made comprising Ag 93.20 wt %, Cu 1.456 wt %, Ge 1.00 wt %, Mn 0.20 wt %, Zn 3.00 wt %, Pd 1.00 wt % and CuB 0.144 wt %. It combines the properties of firestain resistance, high Vickers hardness and resistance to corrosion in a salt spray test.
*Precipitation hardening (air cooled) - sample annealed, air cooled, then heated at 300° C. for 45 minutes. Precipitation hardening (quenched) - sample annealed, quenched, then heated at 300° C. for 45 minutes.
**No final assay results available. Table shows alloy make-up before melting.
An alloy is cast comprising Ag 93.8 wt %, Zn 4.0 wt %, Ge 1.0 wt %, Cu 0.82 wt %, Mn 0.2 wt % and Cu/B 0.18 wt % (Cu 98.2 wt %, B 1.8 wt %). The above alloy has an annealed and air-cooled hardness of about 75 HV, similar to that of standard sterling silver. An alloy developed at Sheffield Hallam University and being sold under the trade name Carrs Lustre Silver by Carrs of Sheffield and which contains 1.6 wt % copper when treated under the same conditions has an HV of 45. For the HV test, the cast samples are air-cooled from annealing temperature for 3 minutes, after which they were water quenched. It gives good performance in sweat resistance and tarnish resistance tests.
EXAMPLES 11-18 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 alloy of Example 14 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, then heated at 300° C. for 45 minutes. Annealed hardness - sample annealed (about 600° C.), air cooled, no further heat treatment.
**No final assay results available. Table shows alloy make-up before melting.
Alloys of Examples 19 and 20 are prepared by melting with the following compositions:
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.
EXAMPLE 21-22 Alloys of Examples 11 and 12 are prepared by melting with the following compositions:
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.
Claims
1. A silver alloy including at least 93.5 wt % Ag, 0.5-4.6 wt % Cu, 0.1-3 wt % Ge and B in an amount effective for grain refinement, and optionally Mn, Si and/or one or more incidental ingredients selected from the group consisting of Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, and Yb, said alloy being precipitation hardened to a hardness of 65 VH or above.
2. The alloy of claim 1, wherein Ge is 0.8-1.5 wt %.
3. The alloy of claim 1, wherein Ge is 0.9-1.2 wt %.
4. The alloy of claim 2, wherein Cu is 0.7-3 wt %.
5. The alloy of claim 2, wherein Cu is 0.7-2 wt %.
6. The alloy of claim 4, which has been precipitation hardened to 70 VH or above.
7. The alloy of claim 4, which has been precipitation hardened to 75 VH or above.
8. The alloy of claim 1, having a content of 1-3 wt % Zn for reducing or preventing pitting or sagging on heating the alloy.
9. The alloy of claim 1, having a content of 1-3 wt % Zn for reducing or preventing pitting or sagging on heating the alloy and to for increasing annealed hardness.
10. The alloy of claim 1 having a content of 0.8-3 wt % of 1-3 wt % Zn to for reducing or preventing pitting or sagging on heating the alloy and up to 0.2 wt % of Sn, In or Si or a mixture thereof for reducing or preventing zinc oxide formation on heating the alloy.
11. 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 in an amount of 0.5-5.5 wt %, an amount of germanium that is at least 0.5 wt % and is effective to reduce tarnishing and/or firestain and boron in an amount effective for grain refinement;
- making or processing the finished or semi-finished article of the alloy by heating at least to an annealing temperature;
- cooling the article gradually, without an abrupt cooling step, so that cooling to ambient temperature takes more than 10 seconds; and
- reheating the article to effect precipitation hardening thereof.
12. The process of claim 11, wherein the boron is incorporated using a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.
13. The process of claim 11, wherein precipitation hardening is at 180-350° C.
14. A precipitation-hardenable silver alloy comprising 96-97.3 wt % Ag, 1-1.55 wt % Ge, balance copper and optionally zinc, and boron as grain refiner.
15. The alloy of claim 14, comprising 01.-0.7 wt % Zn.
16. The alloy of claim 14 comprising 0.3-0.5 wt % Zn.
17. The alloy of claim 24, comprising about 97-97.3 wt % Ag, about 1.2 wt % Ge, balance copper and boron as grain refiner.
18. The alloy of claim 17, precipitation hardened to a hardness of at least 75 HV.
Type: Application
Filed: Nov 20, 2007
Publication Date: Mar 20, 2008
Applicant: MIDDLESEX SILVER CO. LIMITED (London)
Inventor: Peter Johns (Hertfordshire)
Application Number: 11/942,827
International Classification: C22C 5/08 (20060101); C22C 5/06 (20060101); C22F 1/14 (20060101);
