US2293240A - Permanent magnet - Google Patents

Description

Patented Aug. 18, 1942 PERMANENT MAGNET Paul Brauburger, Stuttgart, Germany, assignor to `Robert Bosch Gesellschaft mit beschrnkter Haftung, Stuttgart, Germany Application In March 9, 1940, Serial No. 323,198 Germany April 12, 1939 s claims. (ci. 14s-io) This invention relates to a permanent magnet and to a method of making same.

The usefulness, lor maximum energy available per unit of volume, of the known martensitic permanent magnet steels or of those hardenable by precipitation is determined either by the product of remanence (Br) and coercive force (Hc) or by the quality index (BHmaxJ. This index is equivalent to the greatest possible rectangle inscribed in the quadrant of the hysteresis loop which quadrant is formed by the 4demagnetization part of the loop, the corresponding portions of the axes, of the field-strength H (coercive force Hc) and of the induction B respectively (remanence Br). In' such steels high remanence and relatively high coercive force, that is, high magnetic power, can be obtained only by a heat treatment adapted to the nature of the steels and comprising, as a rule, the steps of quenching at relatively high temperature and of subsequent annealing at relatively low temv perature.

The invention aims at providing a permanent magnet the magnetic properties of which are much better in a preferred directionv than transversely thereto by magnetizing the permanent magnet at higher temperatures, preferably in ing only with the o uenching operation, or delayed cooling, etc., or an equivalent step, or with annealing only. Generally, a combination with annealing will. be found advantageous, because in view of the relatively lower temperatures the arrangements required for performing .the method according to the invention may be made simpler andwill be more easy to handle.

The application of the method according to -the invention brings about a considerable increase in the magnetic power; due' to remanence and coercive force, of permanent magnet steels which, within the meaning of the invention, are restricted to those that yield a product o f more than 0.45X even without applying the method according to the invention. The peculiar feature of the new method surprisingly resides in attaining the maximum power possible in permanent magnets treated accordingly not in all directions but only in one preferred direction, as the longitudinal direction, whilst in the direction extending vertically thereto the power decreases. This feature is, however, not in any way disadvantageous but eminently favorable, since in practical application permanent magnets can be utilized only in one direction in most cases.

In the known permanent magnets the maximum magnetic power aimed at is sometimes obtainable only with the aid of an intricately devised heat treatment which has to be carefully followed to attain maximum values, and it is occasionally quite difficult to operate under such conditions. The invention, on the other hand, makes it possible to do without subtilized heat treating methods,` inasmuch as simpler methods in combination with magnetizing according to the invention will yield greater magnetic power than was attainable hitherto in permanent magnets even through the medium of the most thoroughly worked out heat treating methods.

In practical operation, joint operation of.. magnets of different kind is frequently necessary or desirable. In such instances, it has been necessary prior to this invention to heat-treat each permanent magnet in the manner adapted to it to produce maximum magnetic values, whereas the method according to the invention permits to a certain extent normalizing oi.' diil'erent heat treating methods, particularly when only the present values are to be obtained, because the heat treatment according to the invention brings out additional strength which could not be uti-v lized hitherto.

Further details and advantages of the metho according to the invention will be dealt with in the following examples which, however, do not restrict the invention to the permanent magnet alloy series (iron-nickel-aluminum) .on which they are based or to the representatives (ironnickel-aluminum-cobalt) of this series mentioned.

The invention is illustrated by way of example in the accompanying drawing, in which Figure 1 shows an electromagnet acting as holder for the test pieces, and

Fig. 2 is a graph showing demagnetization curves of a magnetically directed permanent magnet steel.

Referring to Fig. 1, the electromagnet has a soft iron core I about which a coil 2 is placed. The core I is provided with a rigid member 3 and a displaceable member 4 so as to permit variation of the air gap between the members 3, 4 each of which is fitted with an asbestos support 5, 6 between which a test piece 1 to be magnetized is fixed in annealed condition.

'I'he alloys used in the tests were melted in a laboratory high-frequency furnace having acid lining, the aluminum being added last in known manner. The molten alloy was cast into slightly heated iron molds measuring 11 x 20 x 100 mm.

A test piece of the casting measuring x 20 x 10 mm. was annealed in known manner in the furnace at 1,300 C. for fifteen minutes, taken out and in annealed condition instantly fixed in the electrcmagnet shown in Fig. 1. Here the steel cooled down to 600 C. within 300 sec. and while cooling from approximately 1300 C. to approximately 400 C. was under the influence of an electromagnetic fleld. The intensity of the electromagnetic field will vary, depending upon the coercive force which it is desired to produce in the permanent magnet, but in a typical instance a field of 3000 oersted may be used. When the test piece had reached room temperature, its remanence and coercive force were measured both in preferred direction and vertically thereto, whereupon the piece was annealed again at S80-590 for one hour without, however, being subjected to magnetic field action. After the test piece had cooled down to roomtemperature, its magnetic properties in preferred direction and vertically thereto were measured again. All the values found are stated in the following table:

As shown by the above values, retarded cooling in a magnetic field according to the invention produces a permanent magnet whose power as represented by the product of Br and Hc considerably exceeds in preferred direction the values obtainable in a direction extending vertically thereto. By resorting in known manner to subsequent annealing without an additional field the magnetic values in both directions can be improved still more, though their relation is not appreciably affected thereby.

The demagnetization curves of the magnetically directed permanent magnet steel of Example 1 are shown in Fig. 2 in which the abscissa represents coercive force in oersted and the ordinate represents remanence in gauss. The demagnetization curve a was measured at room temperature in preferred direction and the curve b transversely thereto after the magnet had been subjected to retarded cooling associated with directional magnetization and then to annealing without directional magnetization. The two curves diier from one another not only as to their relative values and their relative position in the diagram but also, which is essential, with respect to their curvature factor forming the quotient of the product (BXH) max. divided by the product of BTX Hc (B.H)max.)

This factor 'y (gamma) was found to amount to 0.65 for curve a and only to 0.28 for curve b. The relatively high factor for curve a constitutes an advantage, since the quality of a. permanent magnet increases with the factor y. The demagnetization curve showing the highest curvature factor determines also the most favorable working direction of a magnet. In the case of permanent magnets not treated according to the invention it is usually immaterial how a magnet is arranged in a construction, but when permanent magnets treated according to the invention are used it is of decisive importance to ascertain their preferred working direction, for it is only in this direction that a maximum of energy per unit of volume is available.

The demagnetization curves of permanent magnets not treated according to the invention but of equal composition would be located between the curves a, b of Fig. 2. Although such curves are not shown, it may nevertheless be pointed out in connection therewith that in accordance with the invention the demagnetization curves are varied in preferred direction and transversely thereto (the increase of the demagnetization curve in preferred direction corresponds to the decrease of the demagnetization curve extending transversely thereto). The magnetization in a preferred direction furthermore results, according to the invention, in an increase of the eiiiciency of a permanent magnet to a higher absolute magnetic output in the preferred direction which considerably exceeds those obtainable from a magnet not treated according to this invention. In support of this statement the curvature factors of permanent magnets of equal composition treated or not treated according to the invention may be compared. The maximum hitherto attainable curvature factor of the last-mentioned magnets may be considered as being 0.45. The factor of 0.65 for the others has been stated already, though this is probably not the maximum value. An increase of approximately 45% in the factor compared with prevailing maximum values involves, furthermore, an additional saving in material and also extensive structural improvements in the arrangements usually associated with permanent magnets, which presumably cover more than savings in material.

EXAKPLI 2 A second test piece of the casting measuring 20 x 20 x 10 mm. was annealed in known manner in the furnace at 1,300'C. for fifteen minutes, then taken out and in annealed condition quickly fixed in an electromagnet serving as holder. Thistime the steel cooled down to'600 C. already in 200 seconds and, as in Example 1, while cooling to 400 C. was subjected to the influence of a magnetic field. When the piece had reached about 400 C., it was removed from the holder and cooled to room temperature, whereupon remanence and coercive force in preferred direction and transversely thereto were measured. The magnet was then tempered again for half an hour at`870 C. and during this time subjected to the action of a magnetic field. After its removal from the holder and cooling toroom temperature the magnetic properties, remanence tained are shown in tempered at 800 C. for one hour and subjected to the action. of a magnetic field, and the other one was also tempered at 800 C. but for two hours and without being acted upon by a magnetic field. After cooling to room temperature remanence and coercive force of both magnets were measured in preferred direction and transversely thereto. The numerical values ascer- Tables 3 and 4.

' Table s Mnetic Heat treatment without v u Product.

magnetic field BrXHc Rems Hc Br (l) Bteelannealed at l,300 N V en C. for 15min. and quench. g 9% e en: to 600, C. in 25 sec. magn.vnluee measured at room temp.. 8 3,000 0. 24x10H inprei. direc- (2) Then tempered at 800 tio n a n d C. for 2 hrs. and meastransversely ured at room temp 75 9, 900 0. 7x10 thereto.

Table 4 Magnetic VaLtransv. Product BrXHc Heat treatment mmm R" marke Hc Br Hc Br Pref. dir. Trausv.dir.

(1) steel man. t 1,aoo. c. for 15 min. and

quencieex to 000 C. in 25 sec., measured at 3 room p 8 0m 8 3 000 .24 1 l .24 (2) Then tempered et 800 C. for 1 hr. with o X 0+ o XW magn. field action, meas. at room temp m0 l2, 000 140 6, 000 2. 4x10+6 0. 84;)(10M and coercive force, were ascertained again. The numerical values obtained are stated in Table 2,.-

' Table 2 Magnetic val. Preferred vertc. `directhereto Re' Transv.

marks Beat treatment Bc Br (l) Cooling of steel annealed at l,300 C. to 600 C. in 200 sec. with direct magnetiz ation and meured at -room temp- (2) T hen anv n e al i n g a t 670 C. for l hour with rect magnetiz a t i o n and measured at room temp.

345 13, lll) 5, 950 4. X10 l. 2x10 l2, 650 6, 400 5. 6x10 1. 86)(10M The numerical results of test 2 need not be discussed in detail and will be understood in view of the explanations of the valuesof Example 1. The curvature factor of the demagnetization curve corresponding to that of a in Fig. 2

was ascertained to be 0.63.

. EXAMPLE 3 Two test pieces of the same casting measuring 11x 10x20 mm. were annealed at 1,300 C. for

fifteen minutes, removed from the furnace and quenched to 600 C. in about 25 seconds by applying compressed air, no magnetic eld acting on the steel. After the test pieces had been cooled to room temperature their magnetic properties were measured as usual. According to expectations, there was not much difference in the various measuring directions. vOne piece was then The numerical values of Tables 3 and 4 clearly indicate the veffect of the application of the method according to the invention upon the magnetic properties and the product formed of remanence and coercive force. Whilstannealing in the old way brought about an increase from 0.24x10+5 to only 0.7xl0+, annealing subjected to the action of a magnetic field effected an increase from 0.24 10+5 to 2.4 10+ in magnetizing direction and to 0.84)( 10+*l transversely thereto. 'I'hese figures show that the values obtained according to the invention in non-preferred direction are still higher than those hitherto attained.

The effect of directional magnetization upon magnetic properties as well as the magnetic product formed of remanence and coercive force can be intensified still more by combining direcsteel, as recited in the following example.

Exams! 4 A test piece, measuring 1l x l0 x 20 mm., of a magnet steelI cast in a mold was heated up to 680 C. in a salt bath for half an hour and quickly placed in a magnetic eld in which the steel cooled down to 300 C. After room temperature had been reached, coercive force and remanence of the steel were measured, and it wasfound that the values ascertained did not materially differ with respect to both directions (preferred direction and direction transversely thereto). A second test piece was treated like the iirst, with the difference, however, that while it was subjected to the action of a magnetic iield, pressure4 amounting to 25 to 50 kg./cm.2 was exerted upon it in the vdirection of the field. This application of pressure in addition to directional magnetization had a surprising effect as to values relating to a common direction of magnetization of a. solid permanent ferromagnetic alloy which is subjected to a heat treatment to improve its Table 5 Ma etlc vaflin` vlatgltsig Product ms-sum thereto i B'XBG 1r. lient treatment Pressure exerted Magn. nc Br He Br anddilirr'ess. 'l'gxieftg (l) Magnet steel cast in chill mold, heated in salt bath up to 680 C. for M hr., cooled under influence of magnet. field to 300 C., measur. at rcomtemp hone 175 9,900 176 9,000 1.73 10+ 1.74 10+ (2) The same Pressure of 25-50 kg/cm.7 exerted upon magn. steel during cool. to 300 C 195 8,800 250 9,350 1.72X10H 2.34Xl0 The above numerical valus Show herefOIe 20 magnetic erliciency, the heat treatment involving that the application of pressure improves the magnetic product formed of remanence and coer-l cive force compared with that obtainable through directional magnetization alone. It may be assumed that through directional magnetization as well as through the application of pressure combined therewith apparently new elementary magnets are turned into eld direction. In case of ferromagnetic crystals it is generally assumed that they are built up of zones (Weiss zones) in each of which the magnetic axes of the atoms are parallel dilone the magnetic axes of the atoms are parallel directed but oriented in any way in the various zones. So long as no great mechanical force or magnetic field strength acts upon them, the magnetic axes must lie in one of the crystal axes or be disposed parallel to one of them. Through the influence of an extraneous eld the magnetic axes are first turned into the principal crystal 30:

of the axes into balanced position after removal I of the force. The invention, on the other hand, makes it possible to retain the alignment of the magnetic axes even after removal of the eX- traneous eld and of the mechanical forces that may be applied.

It is known that remanence and coercive force of permanently magnetic alloys vary under the influence of a constant magnetic field if such a cooling which is accelerated during a certain period and/or retarded during another period. Applicants process also distinguishes from the prior art because he applies a mechanical load (e. g. pressure) on the work-piece during cooling and magnetic treatment and/ or by applying magnetic treatment during a tempering operation either alone or in addition to either or both of the two cooling treatments previously mentioned.

80 Applicants process also distinguishes from the alloys are cooled from 1,200 C. to room temperature. In measuring the magnetic properties at room temperature it was found, however, that coercive force was only slightly affected whilst remanence increased to an appreciable extent. Measurements carried out in the direction of the magnetic field or vertically thereto disclosed values differing only about 10% from one another, Whereas permanent magnets treated according to the invention show increases in the values of coercive force. remanence and the product formed of them, which those skilled in the art have hitherto considered to be unattainable. Applicants process differs from the prior art in the use of an integral cast mass consisting entirely 75 prior art by applying a magnetic treatment during cooling within the casting mold, either alone or in connection with either or both of the cooling treatments and/or with a tempering operation.

What is claimed is:

1. A method o1' producing permanent magnets whose magnetic properties in a preferred direction considerably exceed those in a direction extending transversely thereto comprising preforming an integral cast mass consisting entirely of a solid permanent magnet alloy of the ironnickel-aluminum type, heating said mass to a temperature above about 600 C., cooling the mass, subjecting said mass to directional magnetization while in heated condition and during cooling, and exerting intense pressure on said mass during said magnetization to improve the magnetic efdciency (BHmax. value) thereof.

2. The method according to claim 1 in which the pressure is exerted in a direction in conformity with the preferred direction of magnetization.

3. The method according to claim 1 in which the pressure exerted amounts to 25 to 50 kg. /cm2.

4. A method of producing permanent magnets whose magnetic properties in a preferred direction considerably exceed those in a direction extending transversely thereto comprising forming an integral mass purely of a solid permanent magnet alloy, heating said mass to a temperature above about 600 C., cooling the mass, subjecting the mass to an annealing treatment after cooling, subjecting said mass to directional magnetization while in heated condition and during cooling and annealing, and exerting intense pressure on said mass during said magnetization to improve the magnetic efliciency (BHmax, value) thereof.

5. A method of producing permanent magnets as set out in claim 1 in which the cooling is done suddenly.

6. A method of producing permanent magnets as set out in claim 1 in which cooling is retarded.

PAUL BRAUBURGER.

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