US1731267A - Resistance alloy - Google Patents

Description

Patented Oct. 15, 1929 UNITED, STATES PATENT OFFICE MALCOLM N. RICH, OF EAST ORANGE, NEW JERSEY, ASSIGNOR T0 WESTINGHOUSE LAMP COMPANY, A OORPORATION OF PENNSYLVANIA RESISTANCE ALLOY No Drawing.

This invention relates to refractory metal alloys and more specifically to an alloy useful for electrical ligating elements in fiat-irons, electrical stoves, toasters, etc., and for use in electric furnaces of the resistor type.

An object of the invention is to provide a resistance element which will be comparatively inexpensive to manufacture, which will have a comparatively high melting point and a long life and which may be used under oxidizing or reducing conditions as desired.

A further object of this invention is to provide an alloy of the character specified which is capable of being readily worked into desired forms, such as filaments, ribbons, and the like.

Other objects of the invention will become apparent as the following detailed description is read.

N ichrome, chromel, and similar alloys have been extensively used as resistance wires in heating devices, such as furnaces, flat-irons, toasters, soldering irons, etc., however, such alloys are not suitable for all kinds of work, particularly where higher temperatures are desirable because of the limitations imposed by the melting points of such alloys.

A satisfactory resistance wire may be made by alloying chromium with a metal, such as molybdenum or tungsten, and when the amount of chromium employed predominates so that the resultant alloy possesses the properties of melting at a temperature around 2000 C. and also has an electrical resistance several times that of chromium, it is uteful for open air wire wound furnaces and the like. In addition, the alloy does notoxidize on its surface to a detrimental degree even when heated in the air. Such an alloy may be fashioned by well known metal working processes into suitable forms, such as wire, ribbon and the like.

In the past, the highly refractory nature of the elements, chromium, molybdenum and tungsten, have made them extremely difficult to handle metallurgically. Chromium may be successfully alloyed with metals, such as nickel and steel, but the resultant alloy has a melting point within the temperature range of the usual metallurgical furnace and fur- Application filed March 1,

nace materials. At these temperatures, because of the high melting points of molybdenum and tungsten and to their high atomic welghts, the solubility of these pure metals, or of intermetallic compounds of these metals, is rather low and therefore, difliculty is experlenced in obtaining satisfactory and homogeneous melts or alloys of these metals.

In practicing the present invention to obtain satisfactory alloys of chromium with molybdenum and tungsten, the pure metal powders of these elements are first prepared in as fine a state of subdivision as possible. In the preparation of chromium, the process employed in copending application of Marden and Rich, Serial No. 104,917, filed April 27, 1926, and assigned to the same assignee as the present application, is preferred. Molybdenum or tungsten may be similarly prepared or may be prepared by the usual reduction of the oxide in hydrogen and subsequently the desired amount added to the chromium met-a1 powder, or if desired a suitable mixture of' the reducible compounds of chromium and molybdenum or tungsten may be simultaneously reduced as by the above mentioned application, so as to yield a metal powder containing approximately the alloyed proportions desired. In either of these methods, the admixed metal powders are ball-milled for several hours to ensure thorough mixin of the powders, after which they are pressed up into suitable bars or rods for sintering and heat-treating.

In the sintering and treating of the bars, care must be exercised otherwise good results are not obtained. Very puredry hydroen freed from moisture, oxygen, nitrogen, hydrocarbons, and the like, should be used, otherwise active absorption of these impurities by the metal ingot will result and the metal ingot will be brittle. The time interval employed in the treatingand sintering process must be gaged according to the percent of alloy desired.

It has long been known that metals belonging to the same family group, similar to the Cr, Mo, W group, will form a series of Solid solutions, but no definite compounds with each other. Were it commercially and practheir solution in chromium in this manner is 5 slow and requires a prolonged time interval to accomplish maximum solubility at constant temperatures. For this reason, -it can be seen that variable alloys may be obtained by varying the temperature and time factors during treating.

Under the best supervision variations in temperature and time will produce ingots containing variable amounts of undissolved molybdenum and tungsten metal powder distributed throughout. It has been found good practice to further consolidate and distribute these particles by a retreating process after reduction in area has proceeded approximately 30% by mechanically working. The time interval of this retreat period is dependent upon the particular alloy used, but in general, will increase with the increase in alloyed constituents. Following retreating,,the usual metal working methods are continued.

The following specific example of a suitable alloy for the purposes indicated and a method of preparing the same is given so as to enable those skilled in the art to practice this invenvention.

A mixture of the pure powders of chromium and tungsten in the proportions of 70 and 30% (by weight), respectively, is prepared. This mixture is placed in a mold and subjected to pressure sufficient to form a compact rod capable of being handled. The rod is inserted in a suitably heated furnace and heated (to about 1000 C.) sufliciently to effect the sintering of the particles of chromium and tungsten. A stream of pure hydrogen is constantly flowed through the furnace. The sintered bar is then further heated by the passage of an electrical current therethrough until the temperature thereof is approximately that of the melting point of chromium, usually 90 to 95% of the fusion current. At this temperature the highly heated chromium dissolves the tungsten. This temperature is maintained until the alloy has been completely formed. During the latter heat treatment it is preferable to pass hydrogen in a pure state through the furnace, although it may be desirable to practice the last sta e of heating in an extremely high or non- 10y thus formed, may be subjected to well known mechanical working methods and thereby shaped into desirable forms.

Although I have specifically defined an alloy of chromium and tungsten and have given definite proportions therefor, it is obvious to those skilled in the art that variations in the proportions may be made. Furthermore, it is contemplated substituting for tungsten, a metal, such as molybdenum, to form alloys thereof with chromium as the base material, the primary object of the invention bein to increase the electrical resistance and me ting point of the final product above that which would be obtained if pure chromium were employed, it being appreciated that the characteristic of chromium respecting its resistance to oxidation when heated in the air is maintained.

What is claimed is:

1. An alloy containing at least 60% of chromium and at least 40% of tungsten.

2. An alloy substantially resistant to corrosion at elevated temperatures comprised of chromium approximately 60 to 80 per cent and tungsten 20 to 40 per cent.

3. An electrical resistance alloy containing chromium 60 to 80 per cent and tungsten 20 to 40 per cent.

In testimony whereof, I have hereunto subscribed my name this 28th day of February MALCOLM N. RIoH.

strl 'ng vacuum, of the order of .6 of a micron.

The bar or ingot of chromium-tungsten al-