US3488833A - Copper alloys for vacuum switches - Google Patents

Description

United States Patent Q vs. Cl. 29-527.6 3 Claims ABSTRACT OF THE DISCLOSURE Contacts for electric vacuum switches are manufactured by preparing an alloy body consisting essentially of 1% to 5% of iron and 99% to 95% of copper, by Weight, forming the contacts from the alloy body in the cast state by machining, and annealing the machined contact.

This invention relates to the production of alloys for use in the contacts of vacuum switches.

It is desirable to increase the mechanical strength and springiness in the fingers of the split contrate contacts of an electrical vacuum switch as described in British Patent No. 997,384, dated Oct. 1, 1963. For this purpose the alloy should have the following properties:

(a) Good electrical conductivity, e.g. 50% IACS (International Annealed Copper Standard),

(b) A high strength in bending and appreciable elastic springback after heating to 700 C. and slow cooling during the processing of the vacuum switch, compared with pure copper or 99% copper-1% silver alloy,

(c) Be produced with a very low gas content.

We have found that copper-iron alloys in the range 15% iron by weight are suitable for this purpose, the favoured composition being 2% iron. The alloys are employed in the cast state and are produced by the following method.

A billet of O.F.H.C. (oxygen free, high conductivity) copper together with the required amount of high purity iron is placed in a previously degassed graphite crucible. If the iron were allowed to come into contact with the graphite crucible, before alloying with the copper, it would react with the graphite, causing damage to the crucible inside Wall and resulting in undue carbon pick-up in the melt. To prevent such contact, the iron is placed in a hole drilled in the copper billet. Using this method, no significant reaction occurs. The crucible is placed in a high frequency cvacuable furnace. To deoxidise the metal to the required low level of the order of 0.1 ppm. it is heated to 1350 C. and held at this temperature for one hour in a continuous stream of hydrogen at atmospheric pressure. The temperature is then reduced to 1200 C. whilst the furnace is evacuated to a pressure of about mm. Hg, to degas the molten metal. The metal is then solidified progressively from the bottom upwards by slowly raising the induction heating coil. This method of solidification produces a pipe-free ingot and facilitates the removal of any gas which may remain in the molten metal. To promote uniformity of composition, the graphite crucible wall is made sufliciently thin to be penetrated by the RF. field so that electromagnetic stirring of the molten metal is obtained.

The fingers of a split contrate contact are stressed in bending, and the properties of alloys for this purpose were compared by loading small cantilever strips of the materials. To stimulate the vacuum outgassing of the vacuum switch, the alloy samples are annealed at 700 C. for 2 hours and slowly cooled. After this treatment, the

alloy of copper with 2% iron has an electrical conductivity of 55% IACS. In the cantilever test, the load required to give 0.020 deflection of the 2% iron alloy was 28 /2 lbs. compared with 13 lbs. for the 99% copper-1% silver alloy. Also the elastic springback from the 0.020 deflection was considerably higher for the copper alloy containing 2% iron, i.e., 0.011" compared with 0.005" for the copper alloy containing 1% silver.

A pair of split-contrate switch contacts were made in the usual manner from a copper-2% iron alloy casting, to give a finger length of 1 /2". The machined contacts were annealed at 700 C. for two hours and allowed to cool slowly. The contacts were placed in a mechanical test rig with their fingers in juxtaposition as in normal use. A load of 1000 lbs. was applied to the pair of contacts and the amount of spring-back of the fingers was noted as the load was released.

Spring-back commenced when the load has been reduced to 300 lbs. and proceeded almost linearly down to zero load. The total amount of spring-back was between 0.016" and 0.020", i.e., 0.008" to 0.010" for each contact. Satisfactory contact was thus maintained between all opposing pairs of fingers.

What We claim is:

1. A method of manufacturing a contact for an electric vacuum switch, which comprises providing a charge consisting essentially of 95 to 99% by weight copper and 5 to 1% by weight high purity iron in a previously degassed graphite crucible; heating the said charge in a furnace at a temperature of about 1350 C. in an atmosphere of hydrogen for about one hour to form a homogeneous melt; degassing the molten metal by reducing the temperature of the melt to about 1200 C. while evacuating the furnace to a pressure of about 10 mm. Hg; solidifying the melt to form an alloy body; and forming a contact from the alloy body in the cast state by machining.

2. A method according to claim 1, in which the copper is put into the crucible in the form of a billet, and the iron is placed in a hole drilled in the copper billet.

3. A method according to claim 1, in which the alloy body is solidified from the bottom upwards by slowly raising an induction heating coil surrounding it.

References Cited UNITED STATES PATENTS 1,921,060 8/1933 Williams. 2,048,824 7/1936 Simpson -153 X 2,066,512 1/1937 Archer 75153 2,140,607 12/ 1938 Thompson 164-65 557,182 2/1897 Baron 75153 2,169,187 8/1939 Kelly 75-153 1,144,034 6/1915 Giolitti 164125 X 3,019,102 1/1962 Saarivirta 75135 2,169,189 8/1939 Kelly 75153 184,884 11/1876 Merrell 75153 415,656 11/1889 Schneider 75-153 298,983 5/1884 Keeler 75153 2,576,267 11/1951 Scatf et a1. 164-68 X FOREIGN PATENTS 635,673 1/ 1928 France. 1,480,989 4/1967 France.

I. SPENCER OVERHOLSER, Primary Examiner V. K. RISING, Assistant Examiner US. Cl. X.R.