US1762730A - Heat treatment of magnetic materials - Google Patents

Description

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UNITED `s'mrlas "PATENT OFFICE LOUIS W. MGKEEHAN, F MAPLEWOOD, NEW JERSEY, ASSIGNOR TO BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. YI, A CORPORATION OF NEW YORK HEAT TREATMENT lOIE MAGNETIC MATERIALS Application led October 30, 1926. Serial No. 145,124.

This invention'relates to magnetic materials, and particularly to producing desirable magnetic characteristics in nickel-iron alloys.

A feature of the invention resides in treating certain magnetic compositions, particularly nickel-iron alloys containing fram 60% to 80% of nickel, to give them substantially constant permeability with accompanying lovsT hysteresis loss at low magnetizing forces of the order employed in signaling.

Recent discoveries have taught that magneti materials containing nickel and iron as the principal ingredients can be given very high permeability at low magnetizing forces and a low hysteresis loss as compared with iron. Examples `of such materials and methods of heat-treatingthem' to obtain the desired properties are disclosed in U. S.-

Patent No. 1,586,884 to G. .W. Elmen, issued June 1, 1926.

The usual method of preparing nickel-iron compositions comprises casting the elements into an alloy, alternately 4working and annealing the alloy until the desired shape or form is obtained, and then heat-treating it in its finalform. Many of the alloys con-- taining nickel and iron are sensitive to strain, the permeability changing markedly 3o whenstress is applied. One reason for heattreating the materials after they are in their final form is to .remove unfavorable residual strains.

' The heat treatment to which these materials are subjected depends largely upon `the characteristics whiclrare desired in the linished product. For example,'one treatment which may -be employed eectively to develop high permeability consists in heating the material to a temperature of about 1200 C. for about twenty hours in a vacuum furnace, and then allowing the material to cool downto room temperature with the furnace. Another treatmentl resulting in high permeability consists in, heating the material to about 875 C., by passing an electric current through it for several minutes, then cutting off the current and allowing it to cool. The rate of cooling in 'the second case, of course, is much more rapid than in the first. Other methods of heattreating the materials are also employed.

The present invention is concerned particularly with nickel-iron alloys containing from about to 80% nickel. When alloys within this range are given the heat treatment hereinafter described, the permeability ,becomes substantially constant for ma nevtizing forces up to at least .1 gauss. T ese alloys, when treated according to the invention, exhibit a much higher initial permeability than iron as well as substantially constant permeability through the range of eld strengths ordinarily employed in signaling circuits.

The characteristics of substantially constant permeability and accompanying low hysteresis loss which are exhibited by these alloys when subjected to different heat treati ments, showing the greater constancy of permeability obtainable by means of this invention. In each case the magnetizing forces are plotted in c.` g. s. units. The magnetic alloys hereinafter considered are prepared by meltingthe constituents together in the desired proportions in an induction furnace. Good commercial gradesof these materials are suitable. The molten metal is then cast into rods or bars which are subsequently rolled or drawn into any desired shape. For convenience in treating and testing samples of these alloys they are sometimes worked into 40 mil wire, or formed into a tape about .006 inch thick and .125 inch Wide, the latter also being a form which has been employed for continuously loading signaling conductors.

The variations of permeability exhibited by two magnetic alloys, heat-treated according to one of the ordinary methods referred to above, are illustrated by the curves of Fig. 1 in which the ordinates represent permeability and the abscissae represent magnetizing forces. These' samples were in the form of 40 mil wire. The curve A desi nates an alloy containing approximate y 7813% nickel and 211/% iron, and the curve B an alloy containing'approximately 7 5% nickel and 25% iron. annealed in a vacuum furnace at about 1200 C. for twenty hours, and were then allowed to cool slowly to room temperature with the furnace. The curves show that prolonged heating of the alloys at high temperatures, followed by very slo`w cooling, gives the alloys .high permeability, and hence such treatment is advantageous where this ychar` acteristic is desired. It will be noted, however, that with this heat treatment the permeability rises when the alloys are subjected to increasing Iield strengths ranging from zero to .l-gauss. The rate of change in permeability with increasing field strengths varies-for the different compositions, but the slope is fairly great in each case.y Slopes of this magnitude are characteristic of ordinary heat .treatments heretofore employed.

At a given magnetic induction and for the same permeability the hysteresis losses in alternating `magnetic fields of the magnitude here. considered are substantially proportional to the rate of variation of the permeability with increasing field strength.

' Reduction of the slope of curves of the sort shown in Fig. 1 isI therefore an important advantage, provided that permeability is not too much depressed in attaining the result. As an example of an artifice which causes non-advantageous reduction of slope may be mentioned the introduction of airgaps into the vmagnetic circuit. Airgaps reduce permeability to an extent which leaves 'hysteresis losses at the same induction substantially unaffected by the simultaneous reduction in slope of the permea- 'bility vs. ield strength curve.

The curves of Fig. 2 illustrate the greater constancy of permeability obtainable by means of the present invention. The curve A represents an alloy containing approximately 781/% nickel and 211% iron, the curve B an alloy containing. approximatel 75% nickel and 25% iron, and the curve an alloy containing approximately 65% nickel and 35% iron. These alloys, like those vdescribed in connection with Fig. 1, were in the form of 40 mil wire. The samples were annealed in a vacuum furnace at a temperature of about 700 C., for

, twenty-two hours, and were then allowed to cool slowly to room temperature with the furnace, the rate of cooling being slightly These alloys were less than 10 C.. per minute. The essential difference between Vthis heat treatment and yplotting these curves thatat low field v alues the hysteresis losses in the material with y 781/2%v nickel are less for the state shown in Fig. 2 byjabout 25 per cent and that the hysteresis losses in the material with nickel are less by about Improvements of this order are typical. l

- The samples of magnetic alloys represented by the curves of Fig. 3 are the same in composition as those represented by the corresponding curves of Fig. 2, but are in the formof tape .006 i-nch thick and .125 inch wide. The heat treatment was also identical except that thetemperature em loyed was-600 C., instead of 700 C., as in t e previous case. The curves show that while the permeability of each of these alloys is lower than that obtained by employing higher temperatures, the permeability is remarkably constant` over the range of magnetizing forces up to .1 gauss.

An alloy containing approximately 75% nickel and 25% iron maintained at 900 C. for about twenty-one hours and cooled slow- 1y with hhafumace also exhibited a high constant permeability as shown by the curve of Fig. 4. This vsample was in the'formof 40 mil wire. The best results have been obtained, according to the invention, with temperatures between about 600 C., and 900 C.

While the time of heating the alloys herein describedwas in the neighborhood of twenty hours, excellent results may be obtained with treatmentsas short as two hours the optimum temperature and duration of the heating being determined by trial in each case. Heating the magnetic alloys at comparatively low temperatures for only a very short time, however, does not give them great constancy o f permeability. Fig. 5 il-v lustrates the sloping characteristic obtained.

ioo i while in a vacuum, for two minutes. lWhen the. heating current was cut olf the tape cooled rapidly to room temperature? This heat treatment gives the alloy a high iniin permeability with increasing field strengths.

It may-be dicult in some cases to heat lthe magnetic materials for several hours after they are in their inal form. It has been found, however, that the valuable properties conferred by long-continued heating at a relatively low temperature can be 4restored by a very short' supplementary treatment when these properties are lost by over-strain in placing the heat-treated material where it is finally to be used. This is illustrated by the curves of Fig. 6, repre-- senting an alloy which is the same in form and composition as that represented by the curve of Fig. 5. The curve E represents an alloy containing approximatelyv 78%76 nickel and QPL/2% iron, maintained at a temperature of 800 C., for twenty hours and allowed to cool to room temperature with'the furnace at a rate slightly less than 10 C. per minute. This curve shows the high and substantially constant charactcristic obtained with this heat treatment which is within the scope of this invention. Following this treatment the tape was overstretched about 4%. The result is indicated by the curve F, showing that the permeability was -greatly reduced. The overstretched tape was'then reheated at a temperature of 800 C. by passing an electric current through it, while in a vacuum, for two minutes, this treatment restoring the material to its previous -condition as illustrated by the curve E. When another-sammon practice in nickel-iron metallurgy.

ple of this same material not previously heat-treated in accordance with this invention was heated to 800 C., for two minutes, this being the heat-treatment according to a method referred to above, the permeabilitywas higher but not nearly so constant, as illustrated bythe curve G. Curve G is generally` similar to the curve of Fig. 5.

Various changes'in the method and compositions described above may be made without departing from the scope and spirit of the invention. For'example, small percentages of chromium or'other materials may be added to the alloys to increase their resistivity, or for other purposes, as is com- What is claimed is: 1. The method of treating a magnetic material containing from 60% to 80% nickel and the balance chiefly iron to give it substantially constant permeability for all field strengths up to at least .1 gauss which comprises heating it to a maximum temperature between about 600 C and about 900 C. for at least two hours and cooling to 'approach room temperature at an average ratel of about 10 C. per minute. 4

.2. The method of treating a magnetic material containing from 60% to 80% nickel and the balance chiefly iron to give it substantially constant permeability for all field strengths up to at least .1 gauss which comprises heating it to a maximum temperature between 600 C. and 800 C. for at least two hours and coolingI to approach room temperature at an average rate of about 10 C. per minute.

3. The method of treating a magnetic` material containing from 60% to 80% nickel and the balance chiefly iron to give it substantially constant permeability for all lield l strengths up to at least .1 gauss which com- 4having substantially constant permeability for all magnetic ield strengths up to at least .1 gauss.

5. A.magnetic material havin substantially constant permeability for di erent magnetic field strengths up to at least .1 gauss, comprising from 60% to 80% nickel and the remainderchieliy iron and a small amount of non-magnetic material for increasing the resistivity, said amount being greater than that of an impurity.

i 6. A magnetic material ,comprising approXimately 7 5% nickel and 25% iron, having substantially constant permeability for different magnetic field strengths up to at least .1 gauss. 7. A magnetic material comprising approximately 781/2% nickel and 211/2% iron,

having substantially constant permeability for a comparatively short time and cooling it to about room temperature at an average rate of about 10 C. er minute.

In witness whereo I hereunto subscribe my name this 29th day of October A. D.,

LOUIS W. MGKEEHAN.

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