US2442219A - Magnetic alloy - Google Patents

Description

J. K. STANLEY May 25, 1948.

MAGNETIC ALLOY 2 sheets-sheet i Filed oct. 3o, 194e o/ a//oy guenc/veafrakn 9/0 "6.

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Patented May 25, 1948 MAGNETIC ALLOY James K. Stanley,

Turtle Creek, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 30, 1946, Serial No. 706,583

(Cl. Z- 123) 2 Claims.

This invention relates to alloys and in particular to magnetic alloys of the iron-cobalt type.

Heretofore, alloys of iron and cobalt have been avaliable for use as magnetic material in the manufacture of electrical apparatus. In practice, itvhas been quite diflicult to process the known iron-cobalt alloys as they are quite brittle. Where it has been found possible to process the alloys, such workable alloys did not have as high an electrical resistivity as is required of the metal where the metal is to be employed in alternating current applications with the result that losses due to eddy currents were not as low as desired.

One object of this invention is to provide a magnetic alloy of iron, cobalt and chromium.

Another object of this invention is to provide an iron-cobalt alloy containing a small but effective amount of chromium as an essential element thereof and which will have a high saturation.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which:

Figure 1 is a graph, the curves of which illustrate the quenching response of the alloys to the carbon content on the elongation characteristics of the alloy;

Fig. 21's a graph, the curve of which illustrates the effect of thickness of the quenched alloy on the elongation, and

Figs. 3, 4, 5 and 6 are graphs the curves of which illustrate the effect of the chromium content on the magnetic and electrical characteristics of the alloy.

The alloy of this invention comprises from 34.5% to 35.5% cobalt, from 0.30% to 0.55% chromium, less than 0.005% carbon and the balance substantially all iron. The cobalt content is selected at that value for which maximum saturation occurs in the iron-cobalt system together with low magnetic anistrophy so that the resulting alloy will have the same magnetic quality in all directions.

As the binary iron-cobalt alloys are quite brittle, such compositions are modified in accordance with this invention by the inclusion of from 0.30% to 0.55% chromium to improve the strength of the alloy and so change its structure that the resulting alloy can be hot rolled without fracture. While chromium within the range indicated makes it possible to hot roll the alloy it is also found that such chromium contents also improve the electrical resistance of the alloy rendering such alloy better suited for use in electrical apparatus.

Although the iinal carbon content is given as less than 0.005%, in making the alloy the carbon content is initially maintained at between 0.02% and 0.10% to improve the forgeability characteristics of the alloy and to render it responsive to a quenching treatment as will be explained more fully hereinafter.

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 lprotective atmosphere of hydrogen being employed to pre-` vent oxidation of the iron during the melting. After the charge is molten, the slag is removed and about 0.1% titanium and 0.1% silicon are added as deoxidizers. Sufficient deoxidizing elements are added to complete the deoxidation of the melt after which from 0.4% to 0.45% chromium and about 0.05% carbon are added to the melt. Suincient carbon is added to insure the retention of at least 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 34.5% to 35.5% cobalt, .30% to .55% 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 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. v

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 ductlle. Referring to Fig. 1 the effect of the carbon content of the alloy on the quenching response is illustrated by curves I0 and |12. Curve lll is illustrative of the iron-cobalt-chromium alloys of this invention which contain less than .02% carbon and demonstrates the effect of the quenching temperature on the elongation of such alloys. As illustrated the elongation is improved by quenching from a relatively narrow range of 700 to 760 C. and even then the elongation obtained is only about 10 to 12%. Such range is too critical where the alloy is to be quenched directly asv it leaves the hot reducing rolls and the ductility thus imparted is unsatisfactory. Y

On the other hand, if the alloy contains over .02% carbon, curve l2 illustrates that good elongation, which is a measure of ductility, is obtainable over avi/'ideV range of quenching temperatures of from'750" C. to 950 C. Such temperatures can readily be maintained in the strip as it v leaves the hot rolls and it'is therefore evident that the ,quenching response of the alloys containing over .02% carbon is Vof great importance from commercial processing consideration as the alloys can be quenched directly from commercial hot mills. Y

The curves l and l2 are composite curves based on a number of alloys havingY the ironcobalt-chromium contents given hereinbefore and with carbon contents above and below .02%V

and which were hot'rolle'd'to athickness of .05 inch and quenched from the temperatures indicated.v In all cases the Yquenching response'at'lthe different quenching temperatures formed the `general patternillustrate'd by curves lll `and t2 depending upon whether or not the carbon content was above or' below .02%.

Referring to Fig. 2 curve'lvl4 is la composite curve based on tests of a'representativevalloy within thev range of elements given hereinbeiore ing a thickness of not more than .025 inch and formed to any predetermined shape in which it is to be employed in industry.. InV practice the alloy strip can be readily reduced to aV thin sheet having a thickness of .002 inch. Y

When the strip is worked to size and shape, itV

is then subjected to an annealing treatment'consisting of heating'the sheetffor afperiod'of time at a temperature between875" and 925 C. ina non-carburizing and non-oxidizing" atmosphere such as hydrogen or cracked ammonia. Preferably,V ifthe alloy sheet has a low carboncontent of 'from .02% to .05%, the sheet is annealed-in rcomnfiercialy-dryr hydrogen having a dew point of 30, it being found that With the incidental oxide and' moisture on the alloy sheets and in the K, annealngfurnace that the gaseous atmosphere and illustrates the effect of the thickneesroithe strip on the elongation `when quenched from a temperature of 910 C. Pis illustrated, the elongation decreases as therthi'cknessdecreases it be'- ing undesirable to quench strips of Aless than .05

Yinch as the required ductility. cannot be obtained,

As stated hereinbefore, in accordance with this invention the strip is hot reduced to a thickness of .(15 to .10 inch it' being found. that suchsizedstrips containing more 'than .02% carbonjvill have an elongation of 15% or mor e. Preferably the strip is reduced to .l inch or. less in order to facilitate the cold rolling of the'quenched strip to size. Also it is preferred thatthe thickness be not less than L08 inch as thepthinner strips tend to buckle Vwhen quenched, being sothin that theyV areV hard to handle although strips having a thickness. of .05 inchV can be quenched Without attending buckling if care is exercised inhandling such thin strips. Y y While the quenching .of the hot reduced strips containing over .02% carbon fromk between'7-50 and 950 C. renders thestrips ducti'le, the reason for imparting ductility thereto is not apparent y and noreason for such'a characteristic can be advanced. Apparently the ductility of suchV a quenched strip is .notidependent upon a crystal structure change for observations have proven that there are Y'no crystal. structure changesunless the strip is heated above 950 C. Againfreferring toFig. 2, lthe improvementin ductil-ity imparted by quenching theV hot reduced strip is illustratedby reference to the point lvwhich indicates the elongation-of oneof the/alloys subjected tothe hot reducing ,but Vwhich was. no t 'quenched as compared with theupper righthand end of curve. I4 lwhich represents the elongation.

of the same hot reduced `alloy striplasV quenched from 910 C. The improvement in ductility imparted to the alloy strip by the quenching treat- Y ment is outstanding. K

Afterthe alloy strip isfiquenchedto render it ductile :as Vdescribed 'hereirrbefora it 'canbe readilyzgcoldiwcrked asfbycoldirollingito va sheethavwill effectively decarburize the alloy. If, however, the carbon content of the alloyV sheet is high, for example between .05% and .1 then it is preferred toanneal the sheets in wet hydrogenhaving up to 3% moisture` by volume to effect" the decarburization of the alloy. In lall casesit-is desired to so decarburize the alloy during "the anneal that not more than '005% carbon "re-.j

mains in the alloy. -Byreducing thecarbonin this'mannerga low loss material is obtained'and grain growth in the alloyzis hastened. i

In annealing the alloy strip,`theV stripv is subjected to the annealing temperatureY for av period of time of from 10'to 50 hours, thelattertime 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. Inannealing the sheets, it ispreferred to separate the strips by means of a suitable refractory material such V as magnesia, talc or alumina, in order to prevent sticlingrorwelding of-theadjacent strips. Y Y

Referring Yto Figs.Y 3, 4, 5 and 6 the curves thereof demonstrate the advantages of having a chromium content between 30%' and 255%. In Fig. 3 curve i6 illustrates the effect o i' the chromium content on the coercive force: ofthe alloys, the curve being a composite curve ofdif- Y ferent alloys containing chromium as indicated.

As 'is' evident the alloys containingbetween 130% and .55% and in particular between 30% `and .40% chromium have quite lowjjcoerciveiforce which isa highly desirable characteristic in Amagnetic alloys of this type. Y

CurvesA I8 and 270 oi Figures l and 5,..respee tively, represent the magnetizing.. forces fori-1:10

and IIL-10G oersteds forV alloys havingdifierentchromium. contents. The eiiect of the chromium content is not so critical at the higher lux'densities as represented by curve 2-0'- although-there is a deiinite'decrease in the magnetizing forcek as the chromium content is increased. However; iatsevereA handling sometimes' being 1droppedlbythe".

operator in passing from one step of the processing to another. In order to further strengthen the alloy Without detrminetally affecting its magnetic characteristics, it may be desirable to include from .05% to .20% of metal selected from the group consisting of manganese, molybdenum, tungsten, titanium or silicon in the alloy as it has been found that such elements within the range given will improve the strength of the alloy somewhat. These elements, however, do not aid in enhancing, nor do they detract from, the magnetic properties of the basic iron, cobalt and chromium alloy and satisfactory magnetic alloy sheet can be readily produced either` with or without such minor quantities of strengthening elements.

As examples of some of the thin alloy sheets and their magnetic characteristics produced in accordance with this invention, reference may be had to the following table, it being understood that in addition to the cobalt and chromium contents listed the alloys have less than .005% carbon and the balance iron.

chromium less than .005 carbon and the balance iron with .06% molybdenum to strengthen the alloy when reduced to size as described and annealed at 925 C. for 10 hours had a coercive force of 1.35 oersteds but when the annealing period was extended to hours, the coercive force decreased to .61 oersted. When thus annealed, the alloy sheet had a tensile strength of 59,000 pounds per square inch and an elongation of 3.6%.

In another alloy containing 34.5% cobalt, .41% chromium, less than .005% carbon and the balance iron with less than .20 silicon to strengthen the alloy when reduced to size as described and annealed at 900 C. for 10 hours had a coercive force of 0.68 oersted and a permeability for 11:10 of 1685.

The alloys of this invention can be readily duplicated, the method 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.

I claim as my invention:

1. A magnetic alloy composed of 34.5% cobalt, 0.30% to 0.55% chromium, less than 0.005% carbon, and the balance substantially all iron.

2. A magnetic alloy composed of about 35% cobalt, about 0.37% chromium, less than 0.005% carbon, and the balance substantially all iron.

JAMES K. STANLEY.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS

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