US2224573A - Alloy - Google Patents

Description

Patented Dec. 10, 1940 UNITED STATES ALLOY Matthew A. Hunter, Troy, N. Y., assignor to Driver-Harris Company, Harrison, N. J a corporation of New Jersey No Drawing. Application October 25, 1939, Serial No. 301,159

3 Claims.

This invention relates to alloys and more particularly to an alloy possessing special properties which render it highly useful as an element of a thermocouple.

Thermocouples are now extensively used consisting of an element electropositive to platinum, such as iron or copper and a second element electronegative to platinum consisting of an alloy of copper and nickel. The copper-nickel element contains between 50 and 60 percent of copper, the balance being mostly nickel. In most of the material used today in the manufacture of the electronegative copper-nickel element of thermocouples, minor percentages of manganese and iron are employed. The addition of these elements has a pronounced efiect in lowering the electromotive force of the thermocouple. By controlled additions of these elements alloys for thermocouple materials can be made which will reproduce with exactitude a definite electromotive force at a definite temperature of operation.

While the copper-nickel alloy asmodified by additions of small amounts of manganese and iron has been generally satisfactory, it has been 9 found that when such element is used with iron or copper in a thermocouple, the thermocouple has a tendency to change in calibration when submitted to oxidizing gases or reducing gases for long periods of time or, when in use, it is alternatively subjected to oxidizing gases and reducing gases. This tendency to change in calibration makes such thermocouples unsatisfactory in many uses with the close tolerances that are demanded in industry today.

I have found that this tendency to change in calibration can be overcome by the addition of small amounts of silicon to the alloy. I preferably employ silicon in amounts less than 1.5 percent. The use of greater amounts results in the precipitation of silicides from the metal with a resulting change in electromotive force, in electrical resistance and in resistance to corrosion.

In addition to the above stated advantage of the use of .silicon in thermocouple alloys, I have found that it is also superior to iron and manganese in controlling the electromotive force in relation to temperature. To determine this, various additions of silicon weremade to a copper-nickel base containing approximately 54.2 percent of copper and 45.8 percent of nickel.

Percent silicon added While the present invention is not predecated upon any theory but is based upon the results which I have obtained in the use of the alloy, it is probable that the silicon additions result in the partial or total elimination of oxides in the alloy; In the ordinary methods of melting alloys of this character, they are subjected to the oxidizing action of the air in which they are melted. This results in alloys containing oxides which are not completely reduced by manganese which has heretofore been used for deoxidation. The addition of small amounts of silicon to the molten bath is much more effective in lowering its oxygen concentration and in the production of a better alloy. The result is that the alloys so prepared are less susceptible to change in calibration when used in oxidizing or reducing atmospheres and that the electromotive force in relation to a given temperature can be better controlled. Thus a thermocouple'consisting of an iron or copper element and a second element of a copper-nickel alloy to which manganese has been added increases its electromotive force when heated in hydrogen and conversely its electromotive force is decreased when heated in an oxidizing atmosphere. These changes are probably due to changes in the oxygen content under the conditions of operation whereas when silicon is added to the copper-nickel alloy, these changes are minimized to a very considerable degree and practically eliminated.

A further advantage resulting from the use of silicon in the copper-nickel element is that the alloy so produced is less subject to intercrystalline brittleness in the heating and cooling operations in various oxidizing and reducing atmospheres to which the thermocouple is subjected in industrial use.

The silicon may be added to the alloy as metallie silicon or as an alloy of nickel and silicon or as an alloy of copper and silicon following the usual practice of such additions. As stated, the amount of silicon added is preferably less than 1.5 percent and it may be added in the amounts specifically stated above. While I prefer to employ silicon for this purpose, I can also obtain very satisfactory results by the use of aluminum, titanium, zirconium, magnesium and calcium and any of such metals may be used in whole or in part to replace the silicon. While the silicon or one of the above stated equivalents apparently perform, in a much more satisfactory manner, the functions heretofore performed by manganese and iron in such alloys, I do not exclude in the preparation of the alloy, additions of manganese or of iron in the manner in which these metals have heretofore been employed. I found, however, that even where the copper-nickel alloy from which the thermocouple element is to be prepared contains manganese or iron or both, subsequent additions of silicon or of one of the other metals mentioned above result in material improvement in the functioning of the coppernickel alloy to which manganese or iron or both has been added.

As stated above, the silicon should not be added in amounts which produce precipitation hardening of the alloy when it is heated and quenched. When the silicon is present in amounts which result in precipitation of silicides, the hardened alloy thus obtained is unsuitable for use as an element of a thermocouple.

I claim:

1. A thermocouple comprising an element electropositive to platinum and an element electronegative to platinum, the latter element consisting essentially of nickel, copper and a substantial amount but not more than 1.5 percent silicon.

2. A thermocouple comprising an element electropositive to platinum and an element electronegative to platinum, the latter element consisting essentially of 50 to 60 percent copper, a substantial amount but not more than 1.5 percent silicon, balance nickel.

3. The method of making an electronegative element of a thermocouple which comprises adding a substantial amount but not more than 1.5% of silicon to a molten bath of copper and nickel, and pouring the resulting alloy.

MATTHEW A. HUNTER.