US2396977A - Electrode alloys - Google Patents

Description

.nickel base alloy for fine wire electrodes no. cease a pics. (or. id-dill) invention relates to nickel base alloys. particularly to alloys in which nickel is predominant and which have characteristics such that the alloys i e good electrodes for electron discharge devices and the like, and which are ductile enough to be drawn into fine wire. This application isa continuation in part of my copending application Serial No. 392,947, filed May 10, 1941, now U. 8. Patent 2,323,173, issued June29, 1943.

Nickel is used extensively for the electrodes of radio tubes. To increase the strength of the nickel for fine wire electrodes various metals have been alloyed with the nickel. The addition of such metals as silicon, magnesium, manganese,

chromium and even the more refractory metals, such as zirconium, titanium, and tungsten, added separately as well as in various combinations will in most cases increase the tensile strength of the nickel. However, this wire, when used as electrodes in some radio tubes, fails at elevated temperature even though the tested cold tensile strength may be comparatively-mgh. Fine grid wires, for example, may sag during high temperature exhaust and filaments operated under tension at cathode temperatures may pull apart as,

or burn out after a short life.

An object of my invention is an improved for radio tubes and the. like.

Another object of my invention is an improved nickel base alloy for wire that may be drawn to very small sizes and which has high hot strength.

'A more specific object of my invention is a nickel alloy for filamentary cathodes. 1

Another specific object o! my invention is a nickel alloy for fine wire grids.

The characteristic features'of my invention are defined in the appended claims and preferred embodiments thereof are described in the following specification.

My invention is best embodied .in a ductile nickel base alloy which when drawn into fine ware has high hot strength andis well adapted for use asa base for oxide coated cathodes and for fine wire grids. The alloy consists of a base of nickel which contains cobalt in amounts ranging from the fraction of a percent found in substantially \pure commercial nickel to an amount which is substantial, but is less than the amount of nickel and not great enough to impair seriously the ductility of the alloy. a small amount or aluminum and a small quantity of tungsten and carbon, which I believe are present as submicroscopic crystals of tungsten carbide. eith r W or W. i n:

drawn into a wire that has a cold tee strei of more than 100 grams gram per 200 millimeters compared to the tie strength of only 45 grams per'am per 200 millimeters for one commercial nickel alloy widely used for electrodes and consisting essentially of nickel, 1% aluminum and .4% silicon, but no tungsten and carbon, and also has a hot or burn-gout strength, especially in fine wire sires much greater than the same size wire made oi this commercial nickel alloy.

The presence in the alloy of a small amount of 7 aluminum makes my improved-alloy particularly good for electrodes that must operate for long periods of time at high temperature, and conditions the alloy for use as a core for oxide coated cathodes. One alloy, embodying my invention. which I have found to have the high hot strength characteristics above entioned and, which is particularly well adapt for grid wire as well as for core metal of filamentary cathodes consism of, by I Per cent Aluminum 1,0 Carbon .15 Tlmgsten 2.0 Nickel Remainder Some or the aluminum in the wire appears to I so mists-to to the surface of the alloy when heated.

40 as silicon.

and when coated with the usual barium-strontium oxides for filament use, the surface aluminum reacts with the barium oxide of the coating to produce. on the surface of the wire, a hard adherent crust orlayer light gray in color. The color of this layer, commonly called the "interface" in the radio tube art, is to be distinguished from the dark gray layer produced on the nickel cathode alloys containing such reducing agents The lighter colored inter-face reduces the black body heat radiation and permits operation of a cathode at emission temperature with less power input. Yet the surface'of the grid wire, which presumably is aluminum oxide.

efleotively reduces primary and secondary emissionfromthe grldascomparedtoagridmade or wire of the same alloy without the aluminum.

, 'nis quantity of aluminum with which the best results-have been obtained is from 1% to 2% by weight oi the alloy. Aluminum in quantities of more than 2% of the core reduces the strength of the wire because of the aluminum bulk.

While aluminum combines well with the alloy of nickel-tungstcn-carbon and is the preferred alloyisductiieandcan reducingsgentforslkalineearthmetsiosides.

such as barium-strontium oxides coated on the core of the cathodes, tantalum, titanium, silicon.

zirconium and magnesium singly and in variouscombinations may be substituted in whole or in nickel in the alloy may be varied within limits without adversely affecting the properties of the alloy. The addition of more than 2 or 3% tungsten does not materially improve the hot strength or emission properties of the alloy while tungsten in quantities less than about .66% does not produce the desired high hot strength of the' wire. The total carbon, combined and free,

should not be present in the finished wire in quanties less than .05% by weight of said nickel-tungsten-carbon aluminum alloy in order that sufficient of tr a tungsten may be converted to tungsten carbide, and should not exceed .5% because of the difficulty of drawing the wire. The usual methods of chemical analysis do not reveal the proportion of free and combined carbon, although the total may be determined.

In preparing my nickel alloy, tubes of commercial electrolytic nickel, which by the usual methods of commercial analysis are substantially pure, tree from sulphur, and have only traces of such metals as iron, cobalt, magnesium, silicon and copper, are melted in a magnesia crucible at 1560 to 1600* G. Then pieces of nickel rod containing 3% carbon, called the master alloy,

are added to the melt until the melt becomes completely deoxidized and is quiescent. Alternatively the melt may be deoxidized by stirring with a carbon rod. Additional carbon is then added to solved in the nickel. Then the aluminum, in the form of powder or fine strip, is added and the melt held tor about three minutes for the complete diflusion of the aluminum through the nickel. The melt is then promptly poured into molds to form the usual slugs, which may be swaged and drawn into fine wires by the conventional methods for working nickel. By this method a true alloy is formed and not merely a sintered mixture and consistently good results have been obtained in drawing thousands of meters of wire only eight ten -thousandths of an inch, or slightly less than one .nil, in diameter.

' Cobalt may be used to replace part of the nickel in the alloy, and improves the non-sag and hot strength properties of my alloy. Because of the hardness of cobalt-nickel alloys the cobalt should not exceed about 40 percent by weight of the alloy when the alloy is to be drawn into fine wire. The increased electrical resistance due to cobalt in the alloy is desirable when, for a given current and voltage rating, a filament with a slightly greater diameter or length is required.

The hot strength of my improved alloy may be tested by operating a filament of the alloy in a radio tube at a filament voltage twice the ratin voltage, and noting the time required for the filament to burn out. The time required for burn-out of my alloy was more than twice the time required for burn-out of a similar nickel alloy cathode containing .2% magnesium, .2% silicon and .15% carbon, indicating that my alloy has greater hot strength.

My improved alloy has high hot and cold strength, and may be drawn to very small wire sizes.

I claim:

1. An alloy consisting of by weight of the total alloy from about 1% to 2% aluminum, from about 40 .66% to 3% tungsten, from about .05% to .5%

carry the nickel through the swaging hammers I and through the rolls and wire dies and leave the wire with a final carbon content of, preferably, about 15%. Experience has shown that, after deoxidation of the melt, about 20%, by weight, of the melt of additional carbon should be dropped into the melt from the above mentioned rods to produce a final wire with .15% carbon. the amount of carbon being dependent somewhat on the rolling, swaging and drawing technique. Then the tungsten, which may for convenience be in the form of broken pieces or ends of a pressed unsintered bar of pure tungsten powder, is added to the melt, which is held at about 1600 C. Usually in about ten minutes the tungsten is completely alloyed with or is completely discarbon, balance substantially nickel. I 2. A drawn fine wire electrode for radio tubes consisting by weight of 1% to 2% aluminum, .66% to 3% tungsten, .05% to .5% carbon, and the balance nickel and wire having a greater hot strength and resistance to burn-out by electric heating in vacuum than a wire of the same size and of the same alloy without the tungsten and carbon and with the addition of about one-half percent silicon.

3. An electrode wire for electron discharge devices of a ductile alloy consisting by weight of the alloy of about 15% carbon, about 2% tungsten, about 1% aluminum, and the balance nickel,

said wire being characterized by tensile strength while hot and resistance to burn-out by electric heating in vacuum greater than that of a wire of the same alloy without tungsten and carbon and with the addition of 11% silicon.

EMIL GIDEON WIIJELL.