US2512358A - Magnetic alloy - Google Patents

Description

June 20, 1950 R. K. MOGEARY 2,512,358

MAGNETIC ALLOY Filed Aug. 6, 1948 Coercive Force Permeability l l l J 0 I I I o soo- 660 Annealing Temperature C WITNESSES: INVENTOR 5 Robert K. McGeory...

ATTOR Patented June 20, 1950 MAGNETIC ALLOY Robert K. McGeary, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Bittsburgh, Pa, a corporation of Pennsylvania Application August 6, 1948, Serial No. 42,845-

2 Claims. .3

This invention relates to alloys, and in partic ular to magnetic alloys of the iron-cobalt type.

A magnetic alloy which is to be used in the form of laminations for cores of alternating current electrical apparatus should meet certain requirements. The alloy must have a high permeability, since the higher the permeability, the greater the amount of magnetic flux a given amount of the alloy can carry, and hence, the less the amount of core material necessary in a given apparatus to carry the required amount of flux. The alloy should also have a low coercive force, to prevent excessive hysteresis loss, and a high resistivity to prevent excessive eddy current loss in the core due to voltage induced by the alternating current.

Alloys of iron and cobalt have been found to have relatively good magnetic properties for such applications. Such iron-cobalt alloys, however, have the disadvantage of being extremely brittle and consequently they are diificult to roll into sheets to be punched to shape to form laminations. Also, the resistivity of pure iron-cobalt alloys is low, which causes laminations made of these alloys to have high eddy current losses.

Although various methods have been proposed to improve iron-cobalt alloys, no alloy has heretofore been found which completely satisfies allthe requirements, since an improvement in one property is usually obtained at the expense oi. another. For example, the addition of a small amount of certain alloying elements to an ironcobalt alloy can increase the resistivity and make the alloy more ductile, but such additional elements will also cause a decrease in permeability and an increase in coercive force. However, it such alloying elements are not present, it is practically impossible to form the alloy into lamina= tions, and the resistivity is so low that a magnetic core of this material will have excessive eddy current losses.

. 2 than 0.01 carbon, and the balance substantially all iron, which alloy develops desirable magnetic properties after being subjected to a treatment consisting of heating the alloy at a temperature of between 725 C. and 775 C. in an non-carburizing and non-oxidizing atmosphere.

In making the alloy, a small amount of carbon in an amount equivalent to 0.1% of the charge to be melted is deposited in the bottom of the melting crucible for deoxidation purposes, after which the proper proportions of unannealed iron and unannealed cobalt is charged, a protective atmosphere of hydrogen being employed to prevent oxidation of the iron during the melting.

After the charge is molten, the slag is removed.

and about 0.1% silicon is added to the melt as a deoxidizer. After suflicient deoxidizer is added to complete the deoxidation of the melt, from 1.9% to 2.1% chromium and about 0.05% carbon are added to the melt. Suflicient carbon is added to insure the retention of about 0.02% carbon in the cast alloy. When all of the additions are completely melted, the molten alloy is poured into a chill mold and an ingot having a composition of 33% to 37% cobalt, 1.9% to 2.1% chromium, 0.2% to 10% carbon and the balance substantially all iron is obtained.

In practice, the ingot is hot rolled on a suitable plate mill to a size of about 3" thick by 6" wide after which it is cogged at a temperature of about 950 C. by-conventional practice to a one inch thick sheet bar. The sheet bar is then heated lid to a temperature between 900 C. and 1100 C. after which it is passed through reducing rolls to reduce it to a strip having a thickness between .05 and .10 inch and preferably between .08 and .10 inch. As the strip leaves the hot rolls it is at red heat and preferably at a temperature between 750 C. and 950 C. The hot strip is passed directly into a quenching bath of water or the like to render it ductile.

After the alloy strip is quenched to render it ductile as described hereinbefore, it can be readily cold worked as by cold rolling to a sheet having a thickness of not more than .025 inch and formed to any predetermined shape in which it is to be employed in industry. In practice the alloy strip can be readily reduced to a thin sheet having a thickness of .002 inch.

When the strip is worked to size and shape, it is then subjected to an annealing treatment consisting of heating the sheetfor aperiod of time at a temperature between 725 C. and 775 C. in a non-carburizing and non-oxidizing atmosphere such as hydrogen or cracked ammonia.

Referring now to the drawing, curve ll represents the variation of permeability at a flux density of oersteds and curve I! the variation in coercive force obtained for an alloy composed of 35% cobalt, 2% chromium, with the remainder iron and incidental impurities as annealed at different annealing temperatures between 500 C. and 900 C. As can be readily seen, the maximum permeability and minimum coercive force is achieved by annealing the alloy at a temperature of about 750 C. Optimum magnetic characteristics are obtained by annealing the alloy within the narrow range of 725 C. and 775 C. An annealing temperature of above about 775 C. or below about 725 C. results in a radical decrease in the magnetic quality of the alloy.

Preferably, if the alloy sheet has a low carbon content of from .0296 to .05%, the sheet is annealed in commercially dry hydrogen having a dew point of 30, it being found that, with the incidental oxide and moisture on the alloy sheets and in the annealing furnace, the gaseous atmosphere will eifectively decarburize the alloy.

If, however, the carbon content of the alloy sheet is high, for example between .05% and .1 then it is preferred to anneal the sheet in wet hydrogen having up to 3% moisture by volume to effect the decarburization of the alloy. In all cases it is desired to so decarburize the alloy during the anneal that not more than 0.01% carbonremains in the alloy. By reducing the carbon in this manner. a low loss material is obtained and grain growth in the alloy is hastened.

In annealing the alloy strip, the strip is subjected to the annealing temperature between 725 C. and 775"v C. for a period of time of from 10 to 50 hours, the latter time being preferred as such period of time gives the grains an opportunity to grow and thereby produce a magnetic material having a low hysteresis value. In annealing the sheets, it is preferred to separate the strips by means of a suitable refractory material such as magnesia, talc or alumina, in order to prevent sticking or welding of the adjacent strips.

While the curves II and ii are based on an 4 alloy of 35% cobalt, 2% chromium, with the remainder iron, the curves are representative of alloys having a composition of between 83% and 37% cobalt, between 1.9% and 2.1% chromium, with the remainder iron, and variations of composition within these limits do not appreciably affect the optimum annealing temperature.

The addition or the chromium to the alloy greatly increases the resistivity, and also improves the workability of the alloy. For instance,

, the alloy consisting of 35% cobalt, 2% chromium,

with the remainder iron, was found to have a resistivity of 35 micro-ohm centimeters. After annealing for 25 hours in hydrogen, the coercive force was found to be .46 oersted and the permeability was 1605 at a flux density of 10 oersteds, and 219 at a flux density of oersteds.

The alloys of this invention can be readily reproduced, the menthod described making it possible to produce thin sheets of the alloy and to fabricate it into the form necessary for utilizing the magnetic alloy as a component in electrical apparatus.

1 claim as my invention:

1. A magnetic alloy composed oi 33% to 37% cobalt, 1.9 to 2.1% chromium, less than 0.01% carbon, and the balance substantially all iron, which has been annealed at a temperature of between 725 C. and 775 C. in an atmosphere of hydrogen.

2. A magneticalloy composed of 35% cobalt, I

ROBERT K. ldcGEARY.

REFERENCES CITED The following references are of record in the file 0! this patent:

UNITED STATES PATENTS Name I Date Stanley May 25, 1948 Number

Claims (1)

1. A MAGNETIC ALLOY COMPOSED OF 33% TO 37% COBALT, 1.9 TO 2.1% CHROMIUM,LESS THAN 0.01% CARBON, AND THE BALANCE SUBSTANTIALLY ALL IRON, WHICH HAS BEEN ANNEALED AT A TEMPERATURE OF BETWEEN 725*C. AND 775* IN AN ATMOSPHERE OF HYDROGEN.

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