Process for making finished or semi-finished articles of silver alloy

Abstract

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 preferably 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; and reheating the article to effect precipitation hardening thereof. The avoidance of quenching reduces the risk of damage to the article.

Claims

1. 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 92.5-95.44 wt %, copper and an amount of germanium that is at least 0.5 wt % and that 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 600 C. for annealing and/or brazing or to a melting temperature; gradually cooling the article over a period of at least 10 seconds; and reheating the article at 250-300 C. to effect precipitation hardening thereof and achieve an increase in hardness of 15 HV or more compared to the hardness of the alloy at the end of gradual cooling.

2. The process of claim 1, wherein the silver is melted, gradually cooled from molten and reheated to effect precipitation hardening.

3. The process of claim 1, wherein annealing and/or brazing is carried out at a temperature of from 600-650 C.

4. The process of claim 1, wherein germanium is present in the alloy in an amount of 0.8-1.5 wt %.

5. The process of claim 1, wherein 1-40 ppm boron is present in the alloy as grain refiner.

6. The process of claim 1, wherein the article is a ternary alloy of silver, copper and germanium further comprising a grain refiner.

7. The process of claim 6, wherein the alloy comprises, apart from impurities, incidental ingredients and any grain refiner, 92.5-95.44% silver, 0.5-3% germanium and 1-19.9% copper, by weight of the alloy.

8. The process of claim 1, wherein the alloy consists, apart from impurities, incidental ingredients and grain refiner, of 92.5-95.44% silver, 0.8-15% germanium, and 1-7.2% copper, by weight of the alloy, together with 1-40 ppm boron as grain refiner.

9. The process of claim 1, wherein the alloy is an AgCuGeZn alloy containing boron as grain refiner.

10. The process of claim 1, wherein the alloy is an AgCuGeSi alloy containing boron as grain refiner, silicon being present in an amount of up to 0.2 wt %.

11. The process of claim 1, wherein the article is of jewellery or giftware.

Description

EXAMPLES 1-8

(1) 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 4 was obtained.

(2) TABLE-US-00002 TABLE 1 Cooling Cooling Annealed Example method 1* method 2* hardness* No Ag % Zn % Ge % B Cu % HV HV HV 1 95.44 0 1.5 4 ppm Balance 108 115 67 2** 96 0 1.55 Yes Balance 107 110 64 3** 96 0 2 Yes Balance 110 106 63 4** 97.30 0 1 Yes Balance 93 99 40 5** 98.66 0 1.2 Yes 0 28 28 28 No precipitation No precipitation hardening hardening 6** 95 1 1.5 Yes Balance 109 114 74 7** 93.2 0.7 1.3 Yes Balance 113 117 56 8** 92.7 2 1.3 Yes Balance 113 117 72 *Cooling method 1sample annealed at red heat (about 600 C.), air cooled, then heated at 300 C. for 45 minutes. Cooling method 2sample annealed at red heat (about 600 C.), quenched in water, then heated at 300 C. for 45 minutes. Annealed hardnesssample annealed (about 600 C.), air cooled, no further heat treatment. **No final assay results available. Table shows alloy make-up before melting.

EXAMPLES 9-10

(3) Alloys of Examples 9 and 10 are prepared by melting with the following compositions:

(4) TABLE-US-00003 Ex. 9 Ex. 10 Ag 92.5 92.5 Cu 2.35 3.0 Zn 2.82 3.14 Si 0.19 0.15 B 0.01 0.01 In 0.23 0.2 Ge 1.9 1.0

(5) 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 alter the above described annealing and post-treatment without quenching.

EXAMPLES 11-12

(6) Alloys of Examples 11 and 12 are prepared by melting with the following compositions:

(7) TABLE-US-00004 Ex. 11 Ex. 12 Ag 92.5 92.5 Cu 3.25 4.78 Zn 3.75 2.25 Si 0.2 0.2 B 0.01 0.01 In 0.25 0.075 Ge 0.04 0.125 Sn 0.075

(8) 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.

EXAMPLE 13

(9) A master alloy is made by melting together 79 wt % Cu, 18 wt % Ge and 3 wt % of a Cu/B alloy containing 2 wt % boron. The Cu is melted together with the Cu/B master alloy. High temperatures can he used because there are no other elements to damage. The temperature is then lowered and the germanium is added just above the Ge inching point. Melting is therefore in descending order of melting temperatures i.e. copper/copper-boron master alloy/germanium. The resulting master alloy comprises, apart from impurities, and with a 50% boron loss on inching, about 82 wt % Cu, about 18 wt % Ge and about 0.03 wt % boron, together with an impurities.

(10) There is then added 72 g of the above master alloy and 928 g of 9999 purity fine silver which when melted together just above the melting point of the fine silver (e.g. at about 960-1200 C.) with a 50% boron loss gives the desired silver/copper/germanium ternary alloy of composition about 92.8 wt % Ag. 5.90 wt % Cu, 1.30 wt. % Ge and about 11 ppm boron. The master alloy is weighed and placed in a crucible for melting and the fine silver is weighed and placed in the crucible, which is then heated to melt the silver and the master alloy under a protective cover of natural gas to prevent unnecessary oxidation. Silver has a known affinity for oxygen, which affinity increases with temperature. When exposed to air, molten silver will absorb about twenty-two times its volume of oxygen. Like silver, copper also has a great affinity for oxygen, typically forming copper oxide. Thus, forming or re-melting sterling silver and other silver-copper alloys, care must be taken to prevent oxidation. When the mixture becomes molten, it may be stirred e.g. with a carbon rod and poured through a tundish into water, so that the silver becomes solidified into shot-like granules or pellets of diameter about 3-6 mm which is the form in which sterling silver is typically sold.

(11) The resulting alloy granules are used in investment casting using traditional methods 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. 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 a Vickers hardness of about 70 which is approximately the same hardness as sterling silver. The products exhibit excellent tarnish and firestain resistance and have a fine grain structure due to their boron content. 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 90-100 HV. 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 approaching 1250 HV. The germanium content is towards the upper limit of that presently considered desirable in a 0.925 type alloy.

(12) As an alternative, the master alloy and fine silver in the form of granules can be mixed together in a crucible, and poured straight into the investment mould, giving similar results to those described above.

EXAMPLE 14

(13) Alloys were prepared with the compositions and boron contents indicated in Table II below using CuB master alloys the source of boron.

(14) TABLE-US-00005 TABLE II Precip. Precip. Annealed Hardened* Hardened* hardness Sample (air-cooled) (quenched) (air-cooled) ID Ag % Ge % B ppm Cu % HV HV HV 14.1 95.44 1.5 4 3.06 108 115 67 14.2** 96 1.55 Yes 2.45 107 110 64 14.3** 96 2 Yes 2 110 106 63 14.4** 97.30 1 Yes 1.7 93 99 40 14.5** 98.66 1.2 Yes 0.14 28*** 28*** 28*** *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.

EXAMPLE 15

(15) Zinc containing alloys according to the invention are prepared as set out in Table III below and their hardness is measured. In the above table, boron is added as CuB master alloy.

(16) TABLE-US-00006 TABLE III Precip. Hard- Annealed ened* Precip. hardness (air- Hardened* (air- Zinc Ag Ge B Cu Zn cooled) (quenched) cooled) alloys % % ppm % % HV HV HV 15.1** 95 1.5 4-8 2.5 1 109 114 74 15.2** 93.2 1.3 4-8 4.8 0.7 113 117 56 15.3** 93.2 1.1 0 5.2 0.5 101 115 64 15.4** 92.7 1.3 4-8 4 2 113 117 72 In the above, references to Vickers Hardness (HV) is understood in the US to be referred to as Diamond Pyramid Number (DPN).