US3415643A - High temperature ferromagnetic cobalt-base alloy - Google Patents

Description

United States Patent 3,415,643 HIGH TEMPERATURE FERROMAGNETIC COBALT-BASE ALLOY John C. Freche, Fairview Park, Richard L. Ashbrook, Berea, Gary D. Sandrock, Cleveland, and Robert Lewis Dreshfield, Parma Heights, Ohio, assignors t0 the United States of America as represented by the Administrator of the National Aeronautics and Space Administration N0 Drawing. Filed May 31, 1966, Ser. No. 554,950

9 Claims. (Cl. 75-170) The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to improved alloys having high strength at temperatures up to 1500 F. while exhibiting high magnetic induction at low applied fields. The invention is particularly concerned with providing improved materials for use in electrical power generating equipment that is subjected to the environment of liquid metal vapor or to the vacuum of space.

Present day aerospace vehicles require space power systems having high efiiciency. In order to increase the chiciency of a space power system, it is necessary to decrease the amount of power expended on cooling generators and motors. This can be accomplished by using rotor materials that have high Curie temperatures, high magnetic induction for low applied fields, and high strength at high temperatures thereby accommodating high operating temperatures.

Commercially available materials for space power system rotors include steels, such as AISI 4340, and high strength tool steel, such as H-ll. Each of these steels has certain limitations. While at low temperatures, these materials have high magnetic induction for low applied fields. However, the strength of these steels falls off rapidly at about 1000 F.

One of the most desirable structural magnetic materials for use up to ll00 to 1150 F. is a cobalt-base alloy called Nivco. However, this material rapidly loses strength above 1200 F., and its magnetic induction falls 01f rapidly above 1300 F.

These problems have been solved by the present invention which provides a cobalt-base alloy for use as a material for rotors of electrical generators or motors to be operated at temperatures up to axxproximately 1400 F. These alloys can be either used as solid rotors or rolled and fabricated into laminated rotors.

It is, therefore, an object of the present invention to provide a ferromagnetic cobalt-base alloy having high strength at the elevated temperatures of liquid metal vapor in Rankine cycle turbine alternator-s.

Another object of the invention is to provide improved alloy materials having high Curie temperatures, high magnetic induction for low applied fields, and high strength at high temperatures, thereby permitting high operating temperatures for use in space power systems.

A still further object of the invention is to provide improved alloy materials for use in aerospace applications which are subjected to the environment of liquid metal vapor or to the hard vacuum of space.

These and other objects and advantages of the inven- Patented Dec. 10, 1968 tion will be apparent from the specification which follows. The present invention is embodied in alloys having the following nominal composition range:

A preferred embodiment of the invention includes a1- loys having the following nominal composition range:

Cobalt From 82.9 percent to 87.9 Tungsten From 7.5 percent to 10 Titanium 1 Zirconium 0.5 Carbon 0.6 Iron From 2.5 percent to 5 An alloy composition used where high strength is of paramount importance has the following nominal composition:

Percent Cobalt 85.9 Tungsten 12.0 Titanium 1.0 Zirconium 0.5 Carbon 0.6

The subject alloys were prepared by vacuum induction melting. However, it is contemplated that these alloys can also be prepared by induction melting under a protective atmosphere of argon.

The subject alloys were prepared in a vacuum induction melting furnace capable of reaching pressures as low as 10* torr. and holding pressures as low as 5 X10 torr. during active boiling of the melt. Raw materials were charged both into a zirconia crucible as well as into an additions-maker. After the crucible charge was melted, the melt was refined by permitting the oxygen in the charge to react with carbon to produce a so-called carbon boil. After refining, the portion of the charge containing the reactive elements, titanium and zirconium, was added to the melt which was then heated to an appropriate casting temperature. The melt was then poured into ceramic shell moldsheated to 1600" F.

Although vacuum induction melting was used in the development of this alloy series, similar alloys have been successfully induction melted under an argon atmosphere. These alloys are likewise amenable to melting by the electro-slag process where protection from the atmosphere is partially provided by a layer of slag rather than solely by vacuum or an inert gas.

The above listed alloys derive their high elevated temperature strength from the solid solution strengthening of cobalt by tungsten, by the precipitation of an intermediate phase of WCo and by the presence of dispersed tungsten, titanium and zirconium carbides. The good magnetic and strength properties are the result of a compromise between the tungsten and iron content in the alloy. The iron contributes to the high induction and structural stability while the tungsten contributes to the high strength.

Various magnetic and mechanical properties of the preferred composition compared with those of one of the applied fields at elevated temperatures consisting essentially of from 5% to 14% tungsten, from .3% to 1.8%

TABLE l.MAGNETIC AND MECHANICAL PROPERTIES OF SELECTED ALLOYS Test tempera- Alloy Property ture, F

7.5W-2.5Fe W-5I e 12 W Nivco Ultimate tensile strength, p.s.i 1, 300 57, 200 58, 000 59, 000 3 95, 000 1, 400 46, 350 48, 600 52, 600 49, 500 l, 500 41, 000 49, 000 50, 000 3 29, 000 1, 600 22, 000 1, 700 33, 600 32, 200 35, 500 Stress rupture life at stress 0145,000 p.s.i. in hours 1,200 2 50 90 1 1, 268 750 300 25 433 1, 290 30 1, 400 1. 2 96 392 1 Magnetic induction in kilogausses for field of 100 oersteds a 1, 400 10. 3 9. 2 8. 2 9. 4 Curie temperature, F 2 1,820 1, 710 1, 600 1, 600

1 Stress 50,000 p.s.i. Extrapolated. 3 Interpolated.

There are other alloys suitable for magnetic applications at elevated temperatures. Among these are H-11 and 18% nickel-maraging steel. However, their use is limited to temperatures below 1000 F. because of microstructural instabilities. The lowest test temperature shown in Table I is 1200" F.

It has also been found that the stress rupture properties of the alloy series can be improved by heat treating. Two such heat treatments have been found to improve the stress rupture properties of the alloy Co-7.5W-2.5Fe-1Ti- O.5Zr-O.6C in the range from l200 to 1400 F. According to the invention, the alloy was heated for 72 hours at either 1700 F. or 1500 F. Table II illustrates the effect of such heat treatments on mechanical properties of the alloy as compared to Nivco 10.

titanium, from 0.1% to 1% zirconium, from 0.3% to 0.7% carbon, up to 10% iron, and the rest essentially cobalt.

2. A cobalt base alloy having a high Curie temperature, high strength and high magnetic induction for low applied fields at elevated temperatures consisting essentially of 1.0% titanium, 0.5% zirconium, 0.6% carbon, from 7.5% to 10% tungsten, 2.5% to 5% iron, and the rest cobalt.

3. A cobalt base alloy as claimed in claim 2 containing 7.5% tungsten, 2.5% iron and about 87.9% cobalt.

4. A cobalt base alloy as claimed in claim 2 including 10% tungsten, 5% iron, and 82.9% cobalt.

5. A- cobalt base alloy as claimed in claim 2 including 10% tungsten, 2.5% iron, and about 85.4% cobalt.

TABLE II.EFFECT OF HEAT TREATMENT ON MECHANICAL PROPERTIES OF SELECTED ALLOY Alloy Property Test temperature F. Heat-treated-aging N ivco As-east temperature 1,700 F. 1,500 E. Average ultimate tensile strength (p.s.i.) 1,200 55, 800 53, 400 53, 300

1, 300 57, 200 51, 500 51, 100 95, 000 1, 400 46, 350 46, 000 46, 400 49, 500 1, 500 41, 100 42, 300 29, 000 Average stress repture life at 45,000 p.s.i. in hours. 1, 200 1 2, 500 750 1, 300 400 l, 700 Average stress rupture life at 40,000 p.s.i. in hours. 1, 400 25 30 300 0. 4 Magnetic induction in kilogausses 1, 400 10. 3 10. 3 9. 4

1 Extrapolated.

The beneficial technical effect of heat treatment on stress rupture properties is associated with the formation of a finely dispersed precipitate in the material. The aging treatments do not significantly affect the tensile strength. Depending on the test conditions, both slight increases and slight decreases in tensile strength have been observed. However, the greatest room temperature ductility was obtained after aging at 1700 F. for 72 hours. Both heat treatments resulted in improved properties over the as cast condition and over the commercial alloy Nivco 10.

The examples set forth above describe the properties of castings made in accordance with the invention. It is contemplated that the cast alloys will be worked into sheet material by forging or rolling for use in making laminated structures, such as rotors, when it is desired to minimize eddy current losses. Conventional hot rolling techniques have been used to obtain sheet material for these alloys. A satisfactory annealing treatment for the sheet has been found to be one-half hour at 2350 F.

It is understood that equivalents or modifications of or substitutions for parts of the above-described embodiments of the invention may be made without departing from the spirit of the invention or the scope of the subjoined claims.

What is claimed is:

1. A cobalt base alloy having a high Curie temperature, high strength and high magnetic induction for low 6. A cobalt base alloy consisting essentially of 12% tungsten, 1.0% titanium, 0.5% zirconium, 0.6% carbon, and about 85.9% cobalt.

7. A method of making a high strength ferromagnetic material having improved electrical properties at temperatures upto 1400 F. comprising the steps of casting an alloy having a composition essentially in the range of about 5% to 14% tungsten, .3% to 1.8% titanium, 0.1% to 1% zirconium, 0.3% to 0.7% carbon, up to 10% iron, and the rest cobalt, and

heat treating the alloy to improve the strength thereof by aging for about 72 hours at a temperature in the range from about 1300 F. to about 1700 F.

8. A method of making a high strength ferromagnetic material as claimed in claim 7 including the step of working the cast alloy into sheet.

9. A method of making a high strength ferromagnetic material as claimed in claim 8 including the step of annealing the. sheet at 2350 F. for one-half hour.

References Cited UNITED STATES PATENTS 9/1966 Freche et al. 75l70 10/1966 Freche et al. 75-17O

Claims (1)

1. A COBALT BASE ALLOY HAVING A HIGH CURIE TEMPERATURE, HIGH STRENGTH AND HIGH MAGNETIC INDUCTION FOR LOW APPLIED FIELDS AT ELEVATED TEMPERATURES CONSISTING ESSENTIALLY OF FROM 5% TO 14% TUNGSTEN, FROM .3% TO 1.8% TITANIUM, FROM 0.1% TO 1% ZIRCONIUM, FROM 0.3% TO 0.7% CARBON, UP TO 10% IRON, AND THE REST ESSENTIALLY COBALT.