US1906455A - Salt bath for annealing metal articles - Google Patents

Description

Patented May 2, 1933 UNITED STATES PATENT OFFICE FAIR-LENDER, OF IFORZHEIM, GERMANY SALT BATH FOR ANNEALING METAL ARTICLES 1T0 Drawing. Application filed June 28, 1930, Serial No. 463,373, and in Germany January 30, 1930.

The salt baths, serving for annealingarticles made of precious metals and their alloys, should have the following properties:

The melting point should as much as pos- 5 sible not exceed 600 0., in order for example to prevent baking together of the thin silver sheeting, when it is annealed in roll shape.

The bath liquor must further possess good covering power, that is the annealed article must be covered on withdrawal with a thin and at the same time dense protecting layer, so that access of air is excluded on the way to the cooling liquid into which the article annealed is to be dipped, and this article cannot tarnish.

Finally the bath liquor must not act chemically on the articles being annealed, as otherwise discoloring of the surface and the like will occur, which is of great disadvantage', especially in the case of plated 3.1? ticles. v

The salt baths hitherto known do not possess these properties, and they cannot be used without disadvantage for annealingarticles made of precious metals and of the alloys of the same.

It has definitely the necessary thin liquidity necessary for an- 0 nealing such articlesby a ding cyanide compounds, but the use of cyanide compounds is open to the following objections:

They have a great tendency to decompose,

so that the composition of the bath does not remain constant. The temperature of the bathchanges with the alteration of the potassium, for example of cyanide content,if

the bath has to be maintained at a uniform degree of thin liquidity. Moreover, owing to the presence of cyanide compounds vapors are produced when quenching the ar-, ticles, which have a great tendency to cause sneez' There are also difliculties in get-. tingri of the waste water containing cyanide of potassium. H

been possible, attain The salt bathmixtures with a high percentage of common salt are certainly .remarkable for their cheapness, but they possess very little or no covering power. If they contain soda or potash, they give for example'a greenish color to plated articles.

' It has been found, that the discolorings and the other objections to the known salt baths do no longer occur, if to the mixture of common salt and potash, to which larger quantities of potassium sulphate are added, the proportion of soda to the potassium is not greater than 1. If the proportion is inverse, the melting point of the mixture rises in a very short time, that is, if the temperature of the bath is maintained constant, it will quickly become viscous and useless.

Suitable compositions for the bath are:

. Percent I. Common salt 46 Potassium sulphate 43 Potassium carbonate" 11 For improving the covering power of I Percent II. Common salt 30 Potassium sulphate 44 Potassium carbonate 21 BOI'flX 5 0 III. Common salt ,.;i 30 Potassium sulphate 44- Potassium carbonate 21 Sodium phosphates; 5

Baths having a very good covering power, which are not affected by the material being annealed and are therefore particularlysuitable for plated articles, are obtained from the above mixtures by the admixture of mineral alkaline carbonates or oxides up to The following are examples:

Percent 5 IV. Common salt 28 Potassium sul hate 41. 5 Potassium car onate 21 Borax 4. 5

Magnesium carbonate 2. 5

m Calcium carbonate 2. 5

V. Common salt 25 Potassium sulphate 54 Potassium carbonate Magnesium carbonate 3 15 Calcium carbonate 3 VI. Common salt 25 Potassium sulphate 54 Potassium carbonate 15 Magnesium oxide 3 Calcium oxide 3 The proportion of pure sodium to pure potassium is in the Examples I to VI:1: 1.33; 112.55; 122.5; 1:1.22;1:3.2 and 123.2,

that is not greater than 1.

It may happen, that a bath of this kind becomes black, especially when heated from above, if this is carried out for protecting the crucible, owing to the flame coming directly into contact with the salt mixture.

In order to overcome this objection 0.1 to 0.2% of saltpeter may be added to the bath.

The annealing baths are suitable also for many nonprecious metals and alloys.

I claim l. A salt bath for annealing metal articles, consisting in a mixture of common salt, potash and potassium sulphate, the proportion of thesodium to the potassium being not greater than 1, with addition of mineral alkaline oxides including magnesium to a maximum of 10%.

2. A salt bath as specified in claim 1, comprising in admixture 25% common salt,

5 1% potassium sulphate, 15% potassium carbonate, 3% magnesium oxide and 3% calcium oxide. Y I In testimony whereof I afiix my signature.

ERWIN FAHRLANDER.