US2489520A - Method of making magnetic impulse record members - Google Patents

Description

Nov. 29, 1949 M. CAMRAS ET AL 2,439,520

METHOD OF MAKING MAGNETIC IMPULSE RECORD MEMBERS Filed Aug. 7, 1947 2 Shets-Sheet 1 RESIDUAL MAGNETI sm GAUSS APPLIED FIELD- H- OERSTEDS COERCIVE FORCE c OERSTEDS APPLIE FlELD-H- OERSTEDS hlET'll-UZ" 5 M44? w C'mmens firm/M E fin/vases Nov. 29, 1949 M. CAMRAS ETAL METHOD OF MAKING MAGNETIC IMPULSE RECORD MEMBERS 2 Sheets-Sheet 2 Filed Aug. 7, 1947 HIGH FREQUENCY OSCILLATOR [rm En [0F 5 Me w (:QMEJ-S f/rewv f: fin/vases AUDIO AMPLIFIER Patented Nov. 29, 1949 METHOD OF MAKINGMAGNETIC IMPULSE RECORD MEMBERS Marvin Camras, Chicago, Ill., and Hyrum E. Flanders, Salt Lake City, Utah, assignors to Armour Research Foundation of Illinois Institute 01' Technology, Chicago, Ill., a corporation of Illinols Application August 7, 1947, Serial No. 767,290 5 Claims. (Cl. 148-12) 1 This invention relates to a magnetic impulse record member and to a method of making the same.

of a wire, tape, ribbon, or the like, formed of normally non-magnetic stainless steel that has been cold worked to impart thereto desirable magnetic properties.

. The present invention is a continuation-inpart of our pending application Serial No. 610,678,

filed August 13, 1945.

According to our present invention, stainless steel, preferably of the type generally referred to as 18-8 stainless steel, is formed into a magnetic impulse record member by successive series of cold working steps, each series being preceded in each instance by a soft annealing step. By properly controlling the composition of the stainless steel alloy and the conditions under which the cold working and annealing is carried out, a magnetic impulse record member can be obtained that possesses very desirable magnetic properties for use in magnetic sound recorders, and the like.

We have found that the composition of the stainless steel alloy may be varied through a considerable range with respect to the chromium and nickel content, and through a smaller range with respect to the carbon content, provided that the elements are balanced to provide definite stability limits of the austenite toward decomposition on cold working. For instance, the chromium content can be varied between 12 and 25%, the nickel content between 5.5 and 12% and the carbon content between 0.06 and 0.20%, but the nickel content should be relatively lower when the chromium content is relatively higher, and the nickel content should also be lower when the carbon content is higher, at all times, however, keeping the percentages within the broad ranges just given.

With respect to the magnetic properties that are desired in a magnetic sound impulse record member for use in a magnetic sound recorder, we have found that the two most important properties to control are the residual magnetism, Br, and the coercive force, He. The low frequency response of the magnetic record member, is, in general, proportional to its Br, while its high frequency response is improved by a relatively high He. Also, in order for the magnetic-impulse rec- More particularly, the invention relates 'to a magnetic impulse record member in the form ord member to be relatively permanent in its retention of magnetically recorded impulses, it should have a relatively high coercive force.

There are other magnetic characteristics that we have found desirable in magnetic impulse record members and that are found-in the magnetic impulse record member of our present invention.

For one thing, it is not desirable that there should be a straight linear relationship between the applied field and the residual magnetism, or between the applied field and the coercive force. If there were such a linear relationship in the case of residual magnetism, a magnetized portion of the record member would tend to magnetize an adjacent unmagnetized portion of the record member, as for instance an adjacent strand of the wire in the same reel. The curve produced by plotting the residual magnetism against the applied field,

' therefore, should rise very slowly for values of acteristics are shown in the drawings.'

We have further found that in order to produce magnetic impulse record members havin these desirable magnetic properties and characteristics, it is necessary to subject the stainless steel alloy from which the record member is to be made to a series oi cold working steps in which the final amount of reduction in cross sectional area is at least within the range of between and 95%, preferably in the neighborhood of and as a result of which the record member is reduced to a diameter or a thickness preferably of not greater than about'0.005 inch. Where the record member in its final form is a wire of circular cross section, the diameter of the wire should preferably be less than 0.005 inch and usually of the order of 0.004 inch, while if the record memberin inch. We have found that even though the composition of the alloy and the percentage reduction effected by cold working are identical in two wires of difierent diameters, the smaller diameter wire, if within the ranges indicated, may have magnetic properties imparted to it that render it eminently suited for use in magnetic sound recorder devices, whereas the wire of a diameter lying substantially outside of the specified diameters, as for instance one having a final diameter of 0.04 inch, will not attain those same desirable magnetic properties. Accordingly, the diameter, or thickness, of the magnetic impulse record member must be kept within the limits herein specifled for maximum development of its desirable magnetic properties.

It is therefore an important object of this invention to provide a magnetic impulse record member having magnetic properties and characteristics peculiarly adapting it for use in magnetic sound recorders and the like.

It is a further important object of this invention to provide an elongated magnetic impulse record member in the form of a wire, tape, ribbon or the like, of a diameter or thickness of the order of 0.004 inch and formed of a normally non-magnetic chrome-nickel steel that has been cold worked to impart thereto particularly de sirable magnetic properties.

It is a further important object of this invention to provide a method of producing an elongated magnetic impulse record member, starting from a non-magnetic, chrome-nickel steel of such composition as to be susceptible of acquiring desirable magnetic properties upon being cold worked, and cold working the alloy to produce such magnetic properties.

It is a further important object of this invention to provide a method of making a magnetic impulse record member from a normally nonmagneticstainless steel, the analysis of whichis controlled within certain limits as to chromium, nickel and carbon so that the alloy is capable of having imparted thereto the desired magnetic properties, and then effecting the reduction of the cross-sectional area of blank of such alloy by-a series of cold working steps each series being preceded by a soft annealing step and including a final reduction oi between 50 and 95% to produce a member having at least one dimension reduced by cold working to not greater than about 0.004 inch and possessing the desired magnetic properties and characteristics.

Other and further important objects of this invention will be apparent from the disclosures in the specification and the accompanying drawings.

On the drawings:

Figure 1 is a chart of curves obtained by plotting residual magnetism against applied fields up to 1000 oersteds showing optimum, minimum and maximum values of residual magnetism and showing a shaded area between such curves representing residual magnetism values and characteristics found suitable in magnetic impulse record members of our invention.

Figure 2 is a chart of curves obtained by plotting coercive force against applied fields up to 1000 oersteds, showing optimum, minimum and maximum values of coercive force, and showing a shaded area between such curves representing coercive force values and characteristics found suitable in the magnetic impulse record member I 4 We may use as the starting material a stainless steel alloy within the following broad ranges of percentages by weight:

Per cent Chromium 12 to 25 Nickel 5.5 to 14 Carbon 0.08 to 0.20 Accessory elements, (maganese,

silicon, nitrogen, titanium, columbium, molybdenum and impurities like sulphur and phosphorus) Less than 8.5

Iron. balance In general, the constituents which we prefer to vary in maintaining the proper balance of the alloy are nickel, carbon, chromium and iron.

Accessory elements, such as manganese, silicon and nitrogen, are ordinarily held substantially constant at the usual commercial levels, including less than 2.0% manganese, less than 0.75% silicon, and less than 0.30% nitrogen. Nitrogen and manganese can be substituted for part of the carbon or nickel in eflecting a balanced alloy. The nitrogen range can extend from 0.003% to 0.30%. Elements commonly used for purposes other than control of the magnetic properties may also be present in the alloy as, for example, titanium and/or columbium, in amounts ordinarily used for stabilization. Thus, titanium may be present in amounts equivalent to four times the carbon content or upto about 0.8%, and columbium may be present in amounts equivalent to eight times the carbon content or up to about 1.6%. Molybdenum up to 3% may be used to improve corrosion resistance. These elements and the small content of deoxidizers and impurities, such as sulphur and phosphorus, normally present in commercial steels have been grouped together for purposes of this specification as "accessory elemen The sulphur and phosphorus contents should be less than about 0.04% each.

The term accessory elements" as used herein and in the claims, therefore designates ingredients such as specified in the preceding paragraph, other than chromium, nickel, carbon, and iron, which may be present in commercial stainless steels.

A very suitable stainless steel alloy falls within the following ranges of percentages by weight:

Per cent Chromium 12 to 25 Nickel 5.5 to 14 Carbon 0.06 to 0.20 Manganese Less than 2.00 Sulphur Less than 0.04 Phosphorus Less than 0.04 Silicon Less than 0.75

Iron, balance and 19.5%, the nickel content should be higher mouse when the carbon content is lower and vice versa within the following limits: I

0.06 to 0.08% carbon-9.0 to 11% nickel 0.08 to 0.10% carbon-8.5 to 10% nickel 0.10 to 0.15% carbon-8.0 to 9.5% nickel 0.15 to 0.20% carbon-7.5 to 9.0% nickel Giving effect to these relationships, we have found the following narrower ranges to be preierred:

Per cent Chromium 17.5 to 19.5 Nickel 7.5 to 11.0 Carbon 0.06 to'0'.20

Iron, balance except for accessory elements In selecting a specific alloy composition within the above preferred range, if the carbon content is on the low side, the nickel content should be on the high side, as indicated by the table showing the relationship between carbon and nickel contents. It is only where the chromium lies outside of the range of 17.5 to 19.5% that nickel outside of the range of 7.5 to 11.0% might be desirable, and in that case a low nickel content will be used with a high chromium content, and vice versa, but still within the broad range first above given.

As a specific example, the following is given:

Example 1 The starting material was a rod of inch in diameter having the following analysis:

Per cent Chromium 18.01 Nickel 8.63 Carbon 0.12 Manganese 0.52 Sulphur 0.016 Phosphorus 0.017 1 Silicon 0.48

Iron, balance A rod of this analysis and of the diameter specified was hydrogen annealed at between 1950 and 2050 F. for a sufiicient length of time, usually a matter of seconds, to give it a dead soft anneal. The rod was then treated in a sequence of steps as follows:

If the last anneal in the foregoing sequence of steps is carried out at a temperature close to 1950 F., the record member so produced will have residual magnetism values lying on the curve AB of Figure 1 and coercive force values lying on the curve DF of Figure 2; whereas if the last anneal is carried out at a temperature close to 2050 F., the record member so produced will have residual magnetism values lying on the curve AC of Figure 1 and coercive force values lying on the curve DE of Figure 2. These curves Draw to efiect a reduction of approximately and the areas defined thereby will be more fully explained'hereina'fter.

Example 2 The starting material was a rod of inch diameter having the following analysis:

Per cent Chromium Nickel Carbon Iron, balance except for accessory elements The rod was subjected to the same sequence of steps as those numbered (1) through (8) of Example 1, but in step (9) was cold drawn to effect a reduction of approximately 84% to produce a wire having a diameter of about 0.004 inch. With an applied field of 1000 gausses. the finished wire showed a coercive force, He, of 300 and a residual magnetism, Br, of 2400. A wire produced inthe same way except for a final reduction of 77%, instead of 84%, showed an Hc of 300 and a Bl' of 1700 with an applied field of 1000 ausses.

Example 3 The starting material was a rod of as, inch diameter having the following analysis:

Per cent Chromium 18.5 Nickel 8.8 Carbon 0.14

Iron, balance except for accessory elements The rod was subjected to the same sequence of steps numbered as'(l) through (6) of Example 1,

-' but in step (7) was cold drawn to effect a reduction of approximately 95% to produce a wire having a diameter of about 0.004 inch. With an applied field of 1000 gausses, the so finished wire showed a coercive force, He, of about 300 and a residual magnetism, Br, of 1250.

Example 4 A inch rod was used "having the following analysis:

' Per cent Chromium 18.73 Nickel 9.06 Carbon 0.12

Iron, balanceexcept for accessory elements Using the same sequence of steps as steps (1) through (9) of Example 1, a wire of 0.004 inch diameter was produced having an He of 265 and a Bl' of 1000 for an applied field of 1000 gausses.

Example 5- The rod started with had the following analysis:

Percent Chromium 18.6 Nickel 9.4 Carbon 0.06

The-rod was-subjected to the same sequence of steps (1) through (9) of Example 1 to produce a wire of 0.004 inch diameter having an He of 250 and a Br of. 1000 with an applied field of 1000 gausses.

Example 6 The starting rod had the following analysis:

Per cent Chromium 17.87 Nickel 9.23 Carbon 0.105

Iron, balance except for accessory elements assasso The subjected to the same sequence of steps (1) through (9) of Example 1 to produce a wire of 0.004 inch diameter havingan m of 250 and a Br of 1500 with an applied field of 1000 gausses. g

' Example 7 a The starting rod had the following analysis:

. Per cent Chromium 18.67 me a 9.31 Carbon 0.085

Iron, balance except for accessory elements The rod was subjected to the same sequence of steps 1) through (9) of Example 1 to produce a wire of 0.004 inch diameter having an He of 250 and a B1- of 1200 with an applied field of 1000 gausses.

In general, the dimensions of the rod or other blank used as the starting material for our methdare unimportant. Originally, the alloy is in the form of an ingot. The ingot may be reduced to some suitable dimensions by any hot or cold forging operation. For wire. drawing, however, the starting point is usually a rod having a diam-' eter of V inch or less.

In each of the foregoing examples the draw was eiiected at ordinary room temperatures, which, in general, would be from 15 to 30 C. and certainly below 50 0. While the upper limit of the temperature of the wire during the preliminary cold drawing steps is not particularly critical, the temperature during the final reduction step should certainly be below its transformation point, which is around 1200 F. in order to have the most favorable magnetic properties imparted to it as a result ofthe final cold working step. The final reduction step should be a reduction in cross-sectional area to about 50 to 95%, and preferably about 65%. There should be no final annealing step after the final reduction step since if a sufiiciently high temperature is used to efiect a soft anneal, the magnetic properties would be destroyed. It is possible to heat treat at a relatively low temperature, such as between 800 and 1200 F., after the final reduction step without harming the magnetic properties of the wire, but we prefer to omit any final annealing step entirely.

It will be understood that similar magnetic properties can be obtained if, instead of cold drawing, the material is subjected to an equiv alent amount of cold forging, rolling, swaging or extruding. Where the final form of the magnetic impulse record member is that of a circular wire, the diameter should be of the order of 0.004 inch and in any event less than 0.010 inch. In the case of ribbons, tapes or sheets, the thickness should be less than 0.010 inch and preferably of the order of from 0.001 to 0.004 inch.

We have found that the'two most important magnetic properties to be controlled in the magnetic impulse record member of our invention, are residual magnetism and coercive force. In general, the record member should be capable of reaching a residual magnetism of between 1000 and 3000 gausses when the applied field is of the order of 1000 oersteds and should be approximately saturated at that value for the applied field. Figure 1 shows limiting curves AB and AC representing maximum and minimum values of residual magnetism, Br, obtained by plotting residual magnetism against the applied field, expressed in oersteds and denoted by H. The

resents values that have been found suitable in the case of magnetic impulse record members of our invention. While it will be understood that the curves AB and AC can be extended out to the right of the points 3 and C, the extended portions of these curves would be substantially fiat and are therefore not significant. Accordingly, the area defined by the curves AB and AC will be considered as the shaded area lying between these curves and to the left of the ordinate joining the points B and C.

The slope of the curves AB and AC is particularly significant. As shown, for a relatively low intensity of applied field, as for instance a field below 100 oersteds. the residual magnetism is correspondingly low. This is very important,

since it means that for low applied field, the record member does not become appreciably magnetized. Consequently, the record member is not easily magnetized by the proximity of magnetic fields of low intensity, as would be the case if there were a more nearly linear relationship such as represented by the dot-dash line AB shown in Figure 1. Other magnetic impulse record members that we have tested more closely approximate the slope of the dot-dash line AB for low intensity of applied field and are objectionable for that reason, since they tend to become magnetized by stray fields.

On the other hand, as the applied field increased in value above oersteds, the slope of the curves AB and AC rises quite steeply, so that at fields of moderate intensity, say between 500 and 1000 oersteds, there is a corresponding substantial residual magnetism of the order of between 1000 and 3000 gausses. These are values that are readily obtained by the ordinary construction of recorder heads in magnetic sound recorders.

Figure 2 shows in solid line two curves, DF and DE that represent the values obtained by plotting coercive force, He, against applied field, H, for magnetic impulse record members having a coercive force at saturation within the desired limits of 200 to 300 oersteds. Since the curves DE and DF intersect, as at K, two areas would, in fact, be defined by these curves, but in order to include points plotted for values obtained in the testing of other satisfactory magnetic impulse record members included in the foregoing examples, smooth composite curves have been formed by joining the solid line portions DG and HE by a dotted line portion GH, and by joining the solid line curves DI and JF by a dotted line portion LI, and the area so included between composite curves DGHE and DIJF has been shaded. This shaded area represents values for coercive force that has been found satisfactory for magnetic impulse record members of our invention.

It is preferable for ease of erasing the recorded magnetic impulses that the coercive force, He, of the record member be not over 300, but except for this reason the coercive force could be higher. In general, with lower annealing temperatures, around 1950 F., higher residual magnetism values, Br, and lower coercive force values, He, are obtained. .Such control of the annealing temperature used, therefore, affords a way of varying these magnetic properties for the same composition of alloy.

As indicated by the curves on the drawings, the materialis practically saturated at fields of around 500. This makes for ease of erasing the shaded area between the curves AB and AC rep- :5 m n i ally recorded impulses. esbecia y the use of a high frequency field, which has been found the most desirabietype of field to use from the standpoint of low noise level.

In use, the magnetic impulse record members of our invention are magnetized in accordance with the magnetic impulses which the members are subjected. Our invention is therefore intended to include such magnetized record members.

As diagrammatically illustrated in Figure 3 of the drawing, the magnetic impulse record member II is arranged to have intelligence recorded thereon by passing the record member over a head l2 which varies the magnetic state of an incremental length of the record member H in accordance with time variations of the intelligence. In reproduction, the record member H is again passed over the head l2 in the same.

direction and the condition or state of the record member along the incremental length thereof is reproduced as a signal, thereby converting the variations in the magnetic state of the record member along its length to a time varying signal corresponding to the recorded intelligence.

A wide variety of apparatus has been developed in the past for effecting such operations, but the details of such apparatus form no part of the present invention One of the common systems includes transferring the magnetic impulse record member H from a storage reel l3 mounted on a shaft It to a -take-up reel l5 mounted on a shaft It. The shaft 18 may be driven by any suitable source of power (not shown), and the shaft It may have a braking force applied in any suitable manner (not shown) to apply a slight tension to the impulse record member H as it passes first over a demagnetizing head I! and then over the recording and play back head l2. The demagnetizing head I] is for the purpose of uniformly demagnetizing the magnetic impulse record member ll before a ma netic record is made thereon by the head l2. When an audible signal is to be magnetically recorded on the traveling record member II, it

-is first converted by a microphone l8 into a fluctuating electric current which is then amplified by an audio amplifier l9, and is then fed through a switch 20 and an input circuit 2| to the head l2. A source of high frequency electric current such, for example, as the high frequency oscillator 22, is connected to the erase head I! through switch 23 and an energizing circuit 2|. This conditions the record member ll immediately prior to recording by demagnetizing it. High frequency current from the oscillator 22 is also fed through switch 25 and a. circuit 26 to the input circuit 2! of the recording head l2 to superimpose a high frequency bias current on the signal and thereby improve the recording characteristics of the apparatus.

After a magnetic record is made on the record member ll by varyingly magnetizing succeeding incremental lengths thereof, the member H is rewound onto the storage reel I! with switches 22 and 25 in their dotted line positions and with switch 20 in its intermediate open circuit position. Thereafter, if it is desired to play back the record which has been made on the record member II, the member II is again transferred from the storage reel H to the take up reel it, but this time the switches 20, 23 and 25 are placed in their respective dotted line positions so that no high frequency energy is fed to either 10 the erase head I! or the recording and play back head i2. The varying magnetic state of the record member II induces an electric current in the signal coil of the head l2, and this current is fed through the circuit 2| and the switch 20 (in its dotted line position) through the circuit 28 to the input side of the audio amplifier IS. The output of the audio amplifier is connected to a loud speaker 29 which converts the fluctuating electric signal current into an audible signal corresponding to the original signal previously recorded.

We claim as our invention:

1. The method of making a magnetic impulse record member, which comprises successively and repeatedly soft annealing at a temperature of between about 1950 and 2050" F. and cold working a normally austenitic chromium-nickel alloy having an analysis within the followin percentages by weight:

Per cent Chromium 12 to 25 Nickel 5.5 to 14 .Carbon 0.06 to 0.20

Iron, substantially the balance,

keeping the percentage of nickel low when the percentage of carbon is high and keeping the percentage of nickel low when the percentage of chromium is high and vice versa within the above specified ranges, and effecting a final cold work reduction of between 50 and in cross-sectional area, with no subsequent annealing, to produce a member having at least one dimension less than 0.005 inch and having a residual ma netism, Br, for a given applied field, H, lying within the area defined by the curves AB and AC and the ordinate joining the points B and C of Fig. l and a coercive force, He, for a given applied fleld, H, lying within the area defined by the curves DGHE and DIJF and the ordinate joining the points E and F of Fig. 2 of the accompanying drawings.

2. The method of making a magnetic impulse record member, which comprises successively having an analysis within the following percentages by weight:

Per cent Chromium 12 to 25 Nickel 5.5 to 14 Carbon 0.06 to 0.20

Iron, substantially the balance,

keeping the percentage of nickel low when the percentage of carbon is high and keeping the percentage of nickel low when the percentage of chromium is high and vice versa within the above specified ranges, and efiecting a final cold work reduction of between 50 and 95% in crosssectional area, with no subsequent annealing, to produce a wire of circular cross-section having a diameter of about 0.004 inch and having a residual magnetism, Br, for a given applied field, H, lying within the area defined by the curves AB and AC and the ordinate joining the points B and C of Fig. l and a coercive force, He, for

a given applied field, H, lying within the area defined by the curves DGHE and DIJF and the ordinate joining the points E and F of Fig. 2 of the accompanying drawings.

3. The method of making a magnetic impulse Per cent Chromium. 17.5 to 19.5 Nickel 7.5 to 11 Carbon 0.06 to 0.20

Iron, substantially the balance,

keeping the percentage of nickel low when the percentage of carbon is high and keeping the percentage of nickel low when the percentageof chromium is high and vice versa within the\ above specified ranges and efiecting a final cold work reduction of between 50 and 95% in crosssectional area, with no subsequent annealing, to

produce a member having at least one dimension less than 0.005 inch and having a residual magnetism, Br, for a given applied field, H, lying within the area defined by the curves AB and AC and the ordinate joining the points E and F of Fig. 1 and a coercive force, He, for a given applied field, H, lying within the area defined by the curves DGHE and DIJF and the ordinate joining the points E and F of Fig. 2 of' the accompanying drawings.

4. The method of making a magnetic impulse record member, which comprises successively and repeatedly annealing and cold drawing a chromium-nickel-iron alloy having an analysis within about the following ranges of percentages by weight:

Per cent Chromium 17.5 to 19.5 Nickel 7.5 to 11.0 Carbon 0.06 to 0.20 Iron, substantially the balance,

keeping the percentage of nickel toward the lower side of its specified range when the percentage of carbon is toward the higher side of its specified range and vice versa, annealing within the temperature range of about 1950 to 2050 F. between successive cold drawing steps, eifecting a reduction in cross-sectional area of between about 50% and 95% in the final cold drawing step without subsequent annealing to produce a wire of circular cross-section having a diameter of the order of 0.004 inch, and for the composition of alloy selected controlling the temperature of the last anneal before the final cold drawing step and the extent of the last cold reduction so as to impart to said wire magnetic properties eminently suiting the same for use as a magnetic impulse record member, said properties including inappreciable residual magnetism for an applied field below 100 oersteds but substantial residual magnetism of the order of between 1000 and 3000 gausses for applied fields of between 500 and 1000 oersteds and a coercive force at saturation of at least 200 oersteds.

weight:

Per cent Chromium 17.5 to 19.5 Nickel 7.5 to 11.0 Carbon 0.06 to 0.20

Iron, substantially the balance,

keeping the percentage of nickel toward the lower side of its specified range when the percentage of carbon is toward the higher side of its specified range and vice versa, annealing within the' temperature range of about 1950 to 2050 F. between successive cold working steps, effecting a reduction in cross-sectional area of between about 50% and in the final cold working step without subsequent annealing to produce a member having at least one dimension less than 0.005 inch, and for the composition of alloy selected controlling the temperature of the last anneal before the final cold working step and the extent of the last cold reduction so as to impart to said member magnetic properties eminently suiting the same for useas a magnetic impulse record member, said properties including inappreciable residual magnetism for an applied field below oersteds but substantial residual magnetism of the order of between 1000 and 3000 gausses for applied fields of between 500 and 1000 oersteds and a coercive force at saturation of at least 200 oersteds.

MARVIN CAMRAS. HYRUM E. FLANDERS.

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

UNITED STATES PATENTS Number Name Date 1,915,766 Smith et al. June 27, 1933 2,085,118 Noll June 29, 1937 2,114,183 Haase et al Apr. 12, 1938 2,167,188 Schaarwachter et al. July 25, 1939 FOREIGN PATENTS Number Country Date 463,901 Great Britain Apr. 8, 1937 482,037 Great Britain Mar. 22, 1938 OTHER REFERENCES Book of Stainless Steels by Thum, published by the American Society for Metals, Cleveland, Ohio, 1935, pages 117-119, 372-373.

"The Alloys of Iron and Chromium," by Kinzel and Franks. volume 2, published by McGraw Hill Book Co., N. Y., 1938, pp. 338-340.

Journal of the Institute of Electrical Communication Engineers of Japan," March 1938, pages 144-148.

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