US2030112A - Alloy - Google Patents

Description

Patented Feb. 11, 1936 PATENT OFFICE ALLOY Erwin F. Lowry, Forest Hills, Pa., assignor to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania No Drawing. Application October 30, 1929, Serial No. 403,664

4 Claims. (Cl. 75-1) My invention relates to alloys and more particularly to alloys containing cobalt and nickel that have superior physical and chemical properties and to processes of preparing the same, this ap- 5 plication being a continuation, in part, of my copending application Serial No. 144,911, filed on 0ctober28, 1926.

An object of my invention is to provide an alloy that has a high tensile strength and a high proportional (elastic) limit at elevated temperatures.

Another object of my invention is to provide an alloy that has a relatively high specific resistance, that does not readily oxidize at elevated temperatures, that is sufficiently ductile to 'be drawn into wire, and that possesses other properties which render it especially suitable as a basemetal filament in thermionic devices.

A further object of my invention is to provide an alloy containing cobalt and nickel to which is added'the ferrous alloy of an additional ingredient for the purpose of increasing the tensile strength of the alloy and improving its forging properties.

26 Another object of my invention is to provide a process of preparing my improved alloy from the raw materials.

It has heretofore been customary to utilize a noble metal, such as platinum or a platinum- 30 iridium alloy, as the base or core metal for the oxide-coated filaments of electron-emissive devices. It has been considered necessary to em ploy such metals or alloys because the impression has prevailed among workers in the art that it was necessary to utilize a metal which would not react with the oxides of the alkaline-earth metals at elevated temperatures. I

Another requirement for filament material is that it shall have a high specific resistance.

Some of the possible high-resistance alloys that are extensively utilized in high-temperature work, such as nichrome, chrome] and karma, are of the type which form acid oxides. They have, consequently, been considered unsuitable for use as a filament material.

Other characteristics necessary in a suitable filament material are a reasonable tensile strength at 1000 C. and sufiicient ductility to enable it to be drawn readily into wire less than 10 mils in diameter.

No single material, with the possible exception of platinum, and no heretofore employed alloy, except platinum-iridium, has all the necessary requirements mentioned. It is, accordingly, one of the principal objects of my invention to produce and temperature at which power-plant apparatus, 10

such as superheaters, steam boilers or turbines, can operate, are governed largely by the proportional limit of the material of which they are composed.

My improved alloy has a proportional limit, at 15 elevated temperatures, that is superior to that of steel or other ferrous alloys previously utilized in such apparatus, and an ultimate tensile strength equally as high or higher. The fact that it has a high proportional limit at elevated temperatures 20 also renders it especially suitable for use as the contacting portions of the valves and valve seats of internal-combustion engines or for valves of other machines or apparatus which are operated at comparatively high temperatures. Its high 25 tensile strength and resistance to oxidation, at comparatively high temperatures, also permit its usein springs and for other applications.

As stated in my copending application, I have made numerous experiments with alloys of dif- 30 ferent metals and have discovered that the metals of the iron group are apparently the only ones which meet all the above requirements.

My improved alloy comprises nickel, cobalt, iron and an additional ingredient capable of 35 increasing the mechanical properties which may also act as a deoxidizer, such as titanium, tungsten, manganese, vanadium or molybdenum. I have utilized alloys containing nickel and cobalt in the aggregate of to in which the nickel 40 and cobalt were present in proportions ranging from 95% to 5% nickel to 5% to 95% cobalt. The remainder of the alloy consists of iron and one or more metals which increase the proportional limit and ultimate tensile strength. of the 45 nickel-cobalt alloy at normal and elevated temperatures and which may also serve to remove the oxides and render the alloy forgeable and duetile. The additional ingredient is preferably added to the cobalt-nickel in the form of aferrous al- 50 loy, the total amount of which must be maintained below 30%. The additional strengthen.- ing material constitutes from one-third to onefourth of the ferrous alloy, so that its content will not exceed 10% of the final material.

Per cent Nickel 40 to 85 Cobalt 10 to 40 Iron plus additional ingredient 5 to 20 I have found that an alloy containing nickel, cobalt, iron and titanium is especially satisfactory because all of the ingredients are in solid solution with each other. alloy which I have prepared is given below:

. Parts Nickel 80 Cobalt 20 Iron 7 V Titanium 2 V the iron and titanium being employed in the form of ferrotitanium. A typical analysis of the complete alloy is'as follows:

Per cent Nickel '70 Cobalt 19.5 Iron 7.4 Titanium 2.8

The following specific example will illustrate the preferred method of preparing my improved alloy, although it will be understood that the example should be considered as an exemplification and not as a limitation of my process.

Nickel and cobalt are first melted together in an electric furnace in a reducing or inert atmosphere and ferrotitanium is then added, care being exercised to prevent access of oxygen. After the melt is completed and just before casting, a small amount, say from 1 to 2 grams of a deoxidizer, such as aluminum or magnesium, is attached to a silica rod and plunged into the molten bath. Care should be exercised in introducing the deoxidizer as it causes a rapid reaction of almost explosive violence.

While titanium has been specifically mentioned as the additional alloying ingredient, it will, of course, be understood that it is within the scope of my invention to employ the ferrous alloys of other metals which increase the mechanical properties of the alloy, such as ferrovanadium, ferromolybdenum, ferromanganese, ferrotungsten or a combination of two or more of these alloys.

The metal may be cast in steel molds which are preferably coated with heavy oil to prevent oxidation. If the metal is poured rapidly but not too hot, an ingot having a smooth surface may be produced which is practically free from pipe. The ingot, thus formed, may be subsequently forged. During the primary forging operation, the temperature should not exceed 1100 0., but, during subsequent operations, it may be raised to a temperature as high as 1150 C. or the forging operations may be conducted at lower temperatures. An ingot which is forged into a square bar having a cross section /2" x /2" may be rolled readily at 700 or 800 G. into a bar 54 x $4 Such a bar may be subsequently cold rolled and An example of suchan meter of 50 mils. The annealing should be conducted in a reducing or in an inert atmosphere, such as hydrogen, or nitrogen, to prevent the formation of an oxide coating which would otherwise become imbedded in the surface during subsequent rolling operations. The 50 mil. diameter wire may then be drawn down to the required size through diamond dies of successively smaller diameters, with occasional annealing, until a diameter of 10 mils is obtained, after which the annealing step may be discontinued.

Finished filaments prepared by the above process have resistances of approximately 40 ohms per mil-inch, as compared with 22 ohms per mil-inch for a platinum-iridium alloy. This feature is distinctly advantageous because'it permits the use of filaments having substantially two times as large a cross section as a platinumfilament having the same electrical and thermionic characteristics.

This high specific resistance has a further advantage in that it will permit filaments of the oxide-coated type to be made having certain filament voltage-current relations which have been heretofore commercially impracticable when employing a platinum-iridium alloy. For example, a filament. .75 inch long may be made from my improved nickel-cobalt alloy which will reach the proper operating temperature at 3 volts and .06 ampere. This feature will enable the thoriated tungsten filament in tubes, such as a small UV-199, to be replaced by oxide-coated filaments.

Furthermore, it has been found thatfilaments made of certain of these nickel-cobalt alloys have the very advantageous property that, when heated to 1000 0., they have an elongation under strain amounting to as much as 10 to 12% of their length before breaking. This stretching, when heated, obviates the necessity for a critical adjustment of a spring support to prevent sag caused by thermal expansion. The take-up of the spring must be only slightly greater than the thermal expansion expected, and the rather critical adjustment of the filament hooks to take careof the expansion that takes place. in platinum-iridium filaments is unnecessary. For this reason, a filament comprising my new alloy will be nonmicrophonic since it will stretch under tensionof the springs until the tension is relieved at the operating temperature. The limits thus set for the spring take-up are, accordingly, not less than the thermal expansion of the filament and not more than 10% of the filament length.

My improved nickel-cobalt alloy has a high proportional limit and a high ultimate tensile strength at elevated temperatures. This property is advantageous because it enables the wire or ribbon to resist the strain incident to its manufacture into filaments and also enables the alloy to be utilized in the applications previously specified.

An investigation of the characteristic properties of my improved alloys indicates that the iron, as well as the titanium is beneficial, although it is not as effective in increasing the physical properties of the alloy as titanium. However, it is highly desirable to add the titanium in the form of a ferrous alloy because titanium in the form of an alloy is considerably less expensive than pure titanium. As previously stated, however, too large a porportion of ferrotitanium should not be added, otherwise the alloy becomes unforgeable. It is, therefore, essential that this in gredient should be maintained below 30%. The

following table gives the principal physical characteristics of a representative alloy containing 17.5% cobalt, 72.5% nickel and 10% of a ferrotitanium alloy containing 25% titanium:

The results obtained were on samples which had been annealed at 900 C. for two hours. The physical properties of my improved alloy are characterized by the fact that they are comparatively stable at elevated temperatures which is not true of ferrous alloys which owe their high mechanical strength to heat treatment. The oxidation of my improved alloy is only slight at a temperature of 600 C. and compares favorably in this respect with stainless steel.

While I have described my invention in considerable detail and given numerous illustrations, it will be understood that the examples should be construed as illustrative and not by way of limitation and, in view of the numerous modifications which may be effected therein without departing from the spirit and scope of my invention, it is desired that only such limitations shall be imposed as are indicated in the appended claims.

I claim as my invention:

1. An improved alloy of nickel-cobalt base having a. high mechanical, electrical and oxidation resistance consisting of about to 85% nickel, and about 40 to 10% cobalt, their sum being about 80 to 95%, and the balance being approximately 20% to 5% iron plus titanium, the titanium ranging from about 10% to 1% of the total alloy.

2. A nickel-cobalt alloy of superior mechanical properties at approximately red heat combined with superior electrical properties, the alloy being approximately nickel, 20% cobalt, 7% iron and 3% titanium.

3. The process of preparing an alloy of nickelcobalt base having high mechanical, electrical and oxidation resistance comprising melting together nickel and cobalt in from approximately equal parts to about eight parts nickel to one part cobalt adding approximately ten percent of a ferrous alloy containing about A; to of metal selected from a group consisting of vanadium,

molybdenum, manganese, tungsten and titanium,

and maintaining reducing conditions in the molten metal.

4. A metallic structure adapted to withstand high mechanical stress at elevated temperature approximating red heat, comprising an alloy containing approximately 50% to nickel and 15% to 40% cobalt, their sum being about to of the alloy, and 1% to 5% titanium, and the balance primarily iron.

ERWIN F. LOWRY.