Process for the manufacture of a pure gold alloy

Abstract: In the process of producing a high-purity gold alloy, (1) trace elements are added and (2) heat treatment is performed, so that the hardness is increased to a level approximately equivalent to that of 18-karat gold at relatively low working ratio, thereby eliminating the drawbacks associated with high-purity gold, that is, improving the workability, heat resistance, flaw resistance, durability, etc. A high-purity gold alloy according to the present invention can be hardened to a level approximately equivalent to that of 18-karat gold at relatively low working ratio, and the high-purity gold alloy thus hardened is not extremely softened by heat treatment performed as a post-treatment, such as brazing or welding. Even a cast article which is not subjected to plastic working has a hardness comparable to that of a hardened article which has been subjected to plastic working.

Description: Technical Field Gold matrices generally used for jewelry include alloys such as 14-karat or 18-karat gold alloy, and Ni, Pd, Zn, etc. are added in large quantities to these alloys to increase their hardness or tensile strength. These alloys cannot therefore be called pure gold in respect of purity. A high-purity gold alloy produced according to the present invention given by claim 1 has a purity of 99.7% or more, and its hardness is increased to a level approximately equivalent to that of 18-karat gold at relatively low working ratio by (1) adding trace elements and (2) performing a heat treatment in the process of a production process, thereby eliminating the drawbacks accompanying the enhancement of purity, that is, improving the workability, heat resistance, flaw resistance, etc. Background Art High-purity gold jewelry is low in hardness and it is extremely difficult to retain its aesthetic value for a long time in daily life. Also, heat treatment performed during the production process, such as brazing, inevitably causes a great reduction in the hardness. The use of high-purity gold as ornaments is therefore limited. Both JP-A-7 070 671 and JP-A-7 070 670 disclose high purity gold alloys containing rare earth metals in small amounts, in some cases also Gd to produce a hard high purity gold alloy resistant to softening during brazing. Members obtained according to the present invention given by claim 1 have a gold content of 99.7% or more and their Vickers hardness (Hv) of 100 or more for cast articles and 150 or more for worked articles. Even with the use of compositions qualifying as pure gold, the hardness Hv was higher than 100 for cast articles and higher than 150 for worked articles (working ratio: 99.6%). In the case where heat treatment was performed with Gd added, the pure gold according to the present invention was remarkably increased in hardness and also improved in heat resistance. The pure gold thus obtained is less liable to be marred or scratched and undergoes less variation with time, and reduction in the hardness due to heat treatment such as brazing is small. To obtain high-purity hardened pure gold capable of retaining high-quality look for a long term, research was conducted and as a result, a member with high hardness was obtained which contained 99.7% by weight or more of gold, to which was added 50 ppm or more of Gd as an alloying component, preferrably along with amount of Ca, Al, Si so that the total amount of the additional elements was 100 to 3000 ppm. Reduction in the hardness of this member due to heat treatment was small. Adding a smaller amount of the elements resulted in lower hardness, and the hardness was nearly proportional to the tensile strength. As the heat treatment for obtaining the above high-purity gold alloy, solution heat treatment, rapid cooling and aging treatment were performed. The resulting alloy was less lowered in hardness by welding, brazing or the like and thus can retain high aesthetic value for a long term, proving to be suitable as a member for use as high-purity gold jewelry. Disclosure of the Invention The ornamental member produced according to the invention according to claim 1 has a gold content of 99.7% by weight or more since, in general, high gold content is preferred because of high-quality look. By adding 50 ppm or more of Gd, the hardness was increased by the heat treatment and working, and reduction in the hardness due to brazing, welding or the like lessened, showing advantageous effects of the additional element. The claimed alloying additions and heat treatment provide a remarkable hardening effect for both cast and worked articles. The hardened high-purity gold alloy had a gentle softening curve and was improved in hardness, tensile strength and heat resistance. By including the optional elements of claim 2 it is possible to select either thermal hardening or work hardening. For cast articles, hardening is achieved by (1) adding the preferrably claimed optional additional elements and (2) hardening by means of heat treatment, and for worked articles, work hardening is also utilized in combination. Since the present invention employs a thermal hardening process, hardening is observed at an initial stage of the production process. The working cost could be greatly cut down and also unnecessary working time could be eliminated. Where Gd and the other element(s) were added in combination so that these components coexisted in a total amount of 100 to 3000 ppm, the hardness was increased at an initial stage of the production process and reduction of the hardness due to application of heat could be lessened. The alloy thus obtained undergoes less variation with time and thus is suitable as a high-purity hardened gold alloy. Brief Description of the Drawings FIG. 1 shows dependence of high-purity hardened gold alloys according to the present invention on heat treatment conditions; FIG. 2 shows dependence of high-purity hardened gold alloys on elements added; FIG. 3 shows dependence of high-purity hardened gold alloys on aging treatment temperature; and FIG. 4 shows dependence of high-purity hardened gold alloys on heat treatment conditions, that is, dependence on heat treatment itself. Best Mode of Carrying out the Invention Members according to the invention will be described with reference to specific examples. Evaluation samples shown in FIGS. 1 and 2 were obtained by melting gold alloys having the respective compositions and pure gold by high-frequency vacuum melting, casting the melt into ingots of 20 mm × 20 mm × 150 mm, and then subjecting the ingots to heat treatment, rolling and dicing to obtain wires of 0.8 mm in diameter Φ. In the case of evaluation samples shown in FIG. 4, wires of 8 mm in diameter Φ were obtained by continuous casting following the high-frequency vacuum melting. After the wires were subjected to solution heat treatment, aging treatment, rolling and dicing, hardness and tensile strength were evaluated and also the elements contained were analyzed. The results reveal that the hardness can be greatly increased by performing the solution heat treatment following the casting and by performing the aging treatment following the working, thus proving high thermal hardening effect. With regard to the gold-alloy ornamental members obtained by the claimed aforementioned process, and pure-gold ornamental members, micro-Vickers hardness (load: 100 g) was measured after the casting, before and after the heat treatment, and before and after the working. The results are shown in FIG. 1. If Gd added is small in quantity, then the effect of the heat treatment as well as the heat resistance lower. On the other hand, if an increased amount of Si is added, a crack is caused during the working. The article containing both Gd and Ca has a hardness Hv as high as 170, which is higher by about 40% than that of the article containing Gd alone and higher by about 25% than that of the article containing Ca alone. Articles containing rare earth elements tend to show high heat resistance, and among them, the article containing Gd exhibits the highest heat resistance, proving a remarkable effect of the heat treatment as shown in FIG. 2. The cast article containing both Gd and Si has a hardness Hv of 100, which is higher by about 64% than that of the article containing Gd alone. The article containing Si alone is extremely low in heat resistance. For the purpose of evaluation, samples were prepared using Gd (rare earth element) showing a high age hardening effect and Ca (alkaline earth metal) showing a high work hardening effect, and excellent results were obtained in both cases. By applying the production process of the present invention using the heat treatment, it is possible to increase the hardness by approximately 30%. Similar results were obtained also in cases where elements were added in combination. The high-purity gold-alloy ornamental member according to the present invention has high hardness and improved heat resistance, as compared with pure-gold ornamental members on the market, and the hardness thereof scarcely lowers due to application of heat. Further, the inspection after a lapse of 10 months revealed no substantial variation with the passage of time in respect of hardness, tensile strength and color tone. Thus, the high-purity hardened gold alloy member according to the present invention can retain these properties for a long term, and accordingly, is highly useful in the industrial field where it is put to practical use in a variety of ornamental articles. Also, the high-purity hardened gold alloy produced according to the present invention may probably be used in other fields, such as in electronic parts, medical parts, etc.

Claims: A process for producing a hard high purity gold alloy, characterized by the sequence of: a) casting a high purity gold alloy consisting of from 50 to 3000 ppm Gd, optionally Ca, Al and Si, and balance gold, said gold having a purity of at least 99.7%. b) solution-heat-treating said alloy at a temperature above 700°C, and c) aging said alloy between 150 to 350°C. A process according to claim 1, characterized in that said alloy consists of one or more elements selected from the group consisting of Ca, Al, and Si, the total amount of Gd, Ca, Al, Si being from 100 to 3000 ppm.

Inventors: OGASA KAZUO, OGASA, KAZUO
Applicants: OGASA KAZUO, OGASA, KAZUO
Application year: 1996
Main classification: C22C5
IPC codes: C22F1, A44C27, C22C5