US3876932A

US3876932A - Omnidirectional magnetic particle flaw detecting apparatus

Info

Publication number: US3876932A
Application number: US379336A
Authority: US
Inventors: Hitoshi Domon; Takaharu Yasuda; Katsuhiro Kawashima
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1972-07-20
Filing date: 1973-07-16
Publication date: 1975-04-08
Anticipated expiration: 1992-04-08
Also published as: DE2336677B2; DE2336677A1; SE401406B; DE2336677C3

Abstract

An omnidirectional magnetic particle flaw detecting apparatus is disclosed. The apparatus includes two sets of magnetizing coils crossed in such a manner that an elliptical rotating magnetic field may be formed with its major axis running along the width direction of test material at the center of the crossed coils at the time when said coils are air-cored, and a circular rotating magnetic field may be formed on the surface part of the test material at the time when such material is inserted into the coils as the material progresses in the lengthwise direction.

Description

United States Patent 1191 Domon et al. 1 Apr. 8, 1975 OMNIDIRECTIONAL MAGNETIC 2.338.793 1/1944 Zuschlag 324/34 R PARTICLE FLAW DETECTING 3,354,385 ll/l967 Wood et al. APPARATUS 3,495.l66 2/1970 Lorenzi et al 324/37 [75] Inventors: Hitoshi Domon; Takaharu Yasuda, O EIG ATENTS 0R A PLICATIONS both of Kitekyttshu; Katsuhiro 847,661 6/1952 Germany 324/37 Kawashirna, Fukuoka, all of Japan [73] Assignee: Nippon Steel Corporation, Tokyo, Primary Examiner-Robert J. Corcoran Japan Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [22] Filed: July 16, 1973 [21] Appl. No.: 379,336 [57] ABSTRACT An omnidirectional magnetic particle flaw detecting [30] Foreign Application Priority. Data apparatus is disclosed. The apparatus includes two sets July 20. 1972 Ja an 47-72716 of magnetizing coils crossed Such a manner that an Aug. 8, 1972 Ja an 47-92970 elliptical rotating magnetic held y be formed With June 13. 1973 Ja an 48-66680 its major axis running along the Width direetieh of test material at the center of the crossed coils at the time 52 us. c1 324/38; 324/37 when Said eehe ere eh-eeted, end e eheulet rotating 1511 1m. 01. 001p 33/12 megnetie held y be formed on the surfeee P of [58] Field of Search 324/37. 38, 40, 41 the test materiel at the time When Sueh material is serted into the coils as the material progresses in the 5 References Cited lengthwise direction.

UNITED STATES PATENTS 8 Claims, 13 Drawing Figures 1.129.584 2/1915 Murphy 324/37 mEMEsAPR 19. 5 3.878832 SHEET 1 0F 5 FIG. 3

A a A m 5 2' con. TURN RATIO aITEIIITs-IPII 3,876,932

SIIZET 2 [IF 5 MAJOR AXIS- MINOR AXIS RATlO- N 0-! J U1 (D 30 60 90 I2'o I5IO COIL CROSSING ANGLE (gouss) FIG. 5

LEAKAGE FLUX FROM A SLIT (AS {FLAW) LYING PERPENDICULARLY TO THE DIRECTION OF PROGRESS x {LEAKAGE FLUX FROM A SL|T(AS FLAW) LYING AT 45- AGAINST THE DIRECTION OF PROGRESS LEAKAGE FLUX FROM A SLITIAS FLAWILYING PARALLEL WITH THE DIRECTION OF PROGRESS -80 -60 -40 -2o 0 20 4'0 60 sbmm FLAW DETECTING POSITION sum 3 OF 5 FIG. 6

c b Cl FIG. 7

[I a b l9 FIG. 8

(gouss) {LEAKAGE FLUX FROM A SLIT (AS FLAW) u LYING PERPENDICULARLY TO THE 50- DIRECTION OF PROGRESS x LEAKAGE FLUX FROM A SLIT (AS FLAW) LYING AT 45 AGAINST THE DIRECTION OF PROGRESS LEAKAGE FLUX FROM A SLIT(AS FLAW)LYING PARALLEL WITH THE DIRECTION OF PROGRESS 8O -o 40 -20 6 2'0 4'0 6'0 8 0mm FLAW DETECTING POSITION DETECTING SENSITIVITY N PATENTEDAPR 81975 3 876 932 sninsu g FIG. I2

, 1 a OMNIDIRECTIONAL MAGNETIC PARTICLE FLAW DETECTING APPARATUS BACKGROUNDOF THE INvENTIoN 1. Field of the Invention The present invention relates to amagnetic particle testing apparatus for flaw detectionof magnetizied articles and, more particularly, to an omnidirectional magnetic particle flaw detecting apparatus which is able to detect flaws regardless of their directionrelative-to the device so that flaws in a test material lying in a direction perpendicular to the, direction of travel of the test material can be detected with the same level ofdetecting sensitivity as flaws lying along the direction of travel of test material.

2. Description of they Prior Art.

In the case of flaw detection of magnetized steel or :the, like, it is difficult to detect flaws lying along the.di- 1 rection oftravel (lengthwise) of the ,material; under scrutiny while-detecting flaws lying in the direction perpendicular to the direction oftravel since the magnetizing direction must be perpendicular to the direction alongwhich flaws lie.-I n such case, therefore, different magnetizing units for flaw detecting are used in the lengthwise and widthwise directionsv of test material. Thus, according to.the conventionalamethods for magnetic particle flaw detection,.at least two different units are required, making operationscomplicated,and its automation very difficult in view of the necessity of taking into-considerationthe relationships between magnetizing units and between the test material and each respective unit.

As, one of the ,improvements made to the, conventionalmagnetic particle flaw detecting apparatus in order to solve the. above-mentioned problem, so-called .crossed coils, which cross each other at right angles,

are used as magnetizing coils. This system utilizes current with phases'different by' 1205' ina three-phase current source. However, this system, to say nothing of the conventional apparatus, has not been; successful in solving the problem ?of detecting flaws lying in various directions, all in thesame level of detecting sensitivity.

SUMMARY OF THE INVENTION It is an object oftthe present invention to provide a magnetic particle flaw detecting apparatus, which is very handy, and which can detect flaws'jlying in various directions,-all in nearly the same level of detecting sensitivity.

vAnother objectof the present invention is to provide a magnetic particle flawdetecting apparatuswhich can detect flaws at any position on-the surface away from the intersection of coils: that is,,which can perform omnidirectional detection.

A further object of the present invention is to provide a magnetic particle flaw detecting apparatus which is so efficient that it can detect flaws at the, same time all over the entire width of wide, plain materials, such as slab or thick plate, in all directions, without contacting flaws.

These and further objects of the present invention lar rotating magnetic field should be formed on the surface of the test materials. However, in case magnetizing coils, which are to form a circular rotating magnetic field when no test material is positioned therein, are

used for flaw} detection of materials having a length greater than the width, and particularly in case such materials are inserted into the coils along theirlengthwise direction, the magne tic field produced on the test material in the lengthwise direction extends to a much greater extent than that in the widthwise direction, as the material itself has been magnetized. This results in the reduction of the magnetic field produced in the widthwise direction relative t o'the lengthwise direction,

making it impossible to conduct tests with constant detecting sensitivity in every direction over the test material. I

As a result of efforts made by the inventors of the present invention, it was found that omnidirectional constant sensitivity flaw detection is made possible by BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of magnetizing coils of the flaw detecting apparatus according to the present invention.

. FIG. 2 shows a circuit for the connection of the electric source with the coils of FIG. 1.

FIG. 3 shows the relationship between coil turn ratio and the major axis-minor axis ratio of the elliptical rotating-magnetic field formed at'the center of the coils, when these coils are air-cored.

FIG. 4 shows the relationship between the angles formed by crossing magnetizing coils and the major axis-minor axis ratios of the elliptical rotating magnetic field formed at the center of the coils, when these coils are air-cored.

FIG. 5 shows the result of flaw detection achieved by the flaw detecting apparatus of the present invention. FIG. 6 shows the slit flaws in billets used for the test .of FIG. 5.

I FIG. 7 is a front view of a billet as it is inserted into a magnetizing coil.

FIG. 8 shows the result of flaw detection carried out by a prior art flaw detecting apparatus.

' FIG. 9 is a perspective view of the apparatus of the present invention having two sets of crossing magnetizing coils.

' FIG. 10 is a schematic illustration of a device for switching the electric source between coils for use in the apparatus of FIG. 9.

FIG..11 is a perspective view of magnetizing coils .used in the embodiments of the present invention.

. FIG. 12 is a perspective view of a magnetizing com- ,posite coil made of the magnetizing coils of FIG. 11.

FIG. 13 shows an electric power circuit connected with the magnetizing composite coil of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a magnetizing coil 1 is constructed with a first composite coil 2 and a second composite coil 3.

Thefirst composite coil 2 has a rectangular shape, and has an outer coil 4 and an inner coil 5. The outer coil 4 and inner coil 5 are wound on the same reel, but have different numbers of turns.

The second composite coil 3 is constructed in the same manner as the first composite coil 2. The two

composite coils

2 and 3 are arranged to cross each other at the centers respectively of their corresponding horizontal parts, thus forming the magnetizing coil 1.

FIG. 2 shows the connection between the electric sources respectively for the first composite coil 2 and the second composite coil 3.

In FIGS. 1- and-2,

numerals

8 and 9 denote terminals of the outer coil 4 of the first composite coil 2 for the connectionof an electric source; numerals 10 and 11 denote terminals of the inner coil 5 for the connection of the electric source; numerals l2 and 13 denote terminals of the outer coil 6 of the second coil composite 3 for the connection of the electric source; and numerals 1 4 and 15 denote terminals of the inner coil 7 for the connection of the electric source. Letters u, v, and w of three-phase current source 17 denote phases for the output terminals on the secondary side of the transformer which is subjected to V wire connection in the three-phase electric source.

Alternating current is supplied to the outer coil 4 of the first composite coil 2 from the

terminals

8 and 9, connected with the alternating current source 17; and an alternating current with a certain phase difference, in this case 120 from the phase of the current supplied to saidouter coil 4, is supplied to the inner coil 5 in the reverse direction to that to the outer coil 4. The same description applies toalternating current supplied to the second composite coil 3.

The coil turn ratio between the outer and the inner coils of respective composite coils is determined by considering the shape of the test material, the phase difference between alternating current supplied to the coils and other factors, so that a nearly circular rotating magnetic field may be formed on the surface of the test material during the flaw detecting process.

The crossing angle 0 between the

composite coils

2 and 3, defined in the region through which the test materials travel, is so determined that a circular rotating magnetic field may be formed on the surface of the test material.

The shape of the composite coils is not limited to a rectangle, but may vary appropriately according to the sectional form of test materials, and in each composite coil the two coils may be placed side by side instead of the above-mentioned superposition of one coil over the other. 7 i

The following is a detailed explanation of the magnetic particle flaw detecting apparatus according to the present invention. As shown in FIG. 2, alternating current having a phase difference of 120 relative to each other, is supplied to the

coils

4 and 5 of the first composite coil 2. The same thing applies to the

coils

6 and 7 of the second composite coil 3. Also, as shown in FIG. 2, the relation between the first composite coil 2 and the second composite coil 3 is maintained through their connection with the electric sources.

The coil turn ratio between the outer coil 4 and the inner coil 5 of the first composite coil 2 is preferably between 1.5 and 2.5. The same ratio also applies to the second composite coil 3.

Outer coils

4 and 6 and

inner coils

5 and 7 have, respectively, the same number of turns.

If the coil turn ratio is less than 1.5, the major axisminor axis ratio of an elliptical rotating magnetic field will greatly increase, but this is not practical, since this requires a large amount of power to attain the required detecting sensitivity. On the other hand, if the coil turn ratio is more than 2.5, the major axis-minor axis ratio will approach\/ 3 which situation will appear as if the major axis and the minor axis of the elliptical rotating magnetic field were reversed in comparison with the air-cored arrangement when the material is in the flaw detecting test, thereby causing a difference in detecting sensitivity between tests in the width and in the length of the test material, which does not meet the purpose of the present invention.

FIG. 3 shows the relationship between the abovementioned coil turn ratio and the ratio between the major axis and the minor axis of the elliptical rotating magnetic field at the center when the coils are aircored; if the coil turn ratio is less than 1.5, the major axis-minor axis ratio will gradually approach infinity as the ratio approaches 1; and as the coil turn ratio becomes greater, the major axis-minor axis ratio gradually approachesv 3.

As mentioned above, the desired rotating magnetic field can be obtained by determining the appropriate coil turn ratio according to the present invention. Further, for readily adjusting the ratio between the major axis and the minor axis of the elliptical rotating magnetic field to the desired value, the composite coils having the above-mentioned coil turn ratio, are made to cross each other, but it is then necessary to make the crossing angle 0 acute. In case this angle 0 is an obtuse angle, that is, more than it will be impossible to obtain an elliptical rotating magnetic field having its major axis running along the width of the test material when the coils are air-cored.

, According to the knowledge and experience of the inventors of the present invention, an angle 0 of less than 60 is effective to obtain an optimum elliptical rotating magnetic field; if the angle 0 is too small, the effective width of the passage of material will be narrowed, making it necessary to increase the size of the magnetizing coils. FIG. 4 shows the relationship between the crossing angle of coils on one hand and the ratio between the major axis and the minor axis of the elliptical rotating magnetic field at the center of the coils on the other hand. It is apparent that the optimum major axis-minor axis ratio can be obtained from a crossing angle between 60 and 30. Detection signals generated by the magnetic flaw detecting apparatus of the present invention are processed in a known circuit and are output as flaw detecting signals.

In connection with the above explanation of the construction of the apparatus of the present invention, the following is an example of flaw detection, using the apparatus of the present invention.

Magnetic flaw detection (the determination of leakage flux) was conducted with a flaw detecting apparatus constructed with magnetic coils, each having a coil turn ratio of 4 :'2 obtained by making the number of actual turns of the outer coil and of the inner coil, respectively,"4 and 2. The composite coils are then crossed,

four magnetic particle sprayers'19, one being directed against each side of the billet, as is well known in the art. The results of the measurement of leakage flux, for quantitative confirmation of the efficacy of the subject invention, are shown in FIG. 5. Asis obvious therefrom, all the slits a, b, and c were detected as flaws with nearly the same detecting sensitivity even though slit a was perpendicular to the direction of travel A of the billet of rolling billets; slit b was at 45. to this direction and slit was parallel with said; direction. This clearly establishes that all slits can'beldetected with nearly the same detecting sensitivity. FIG. 8 shows the result of a flaw detection test which was made on a material of the same kind and size as mentioned above, that. is, a square steel billet, with a prior art apparatus constructed with two coils, each coil having six turns and crossed at right angles. As seen in the figure, the apparatus does not overcome the prior art difficulties specified in the instant application and achieves poor results compared with the test obtained with the subject invention. More.particularly, the level of detecting sensitivity foraslit lying perpendicular to and 'parallelwith the direction of progress is reversedbetween FIG. 8 and FIG. so far as the detecting position is in the vicinity of the center of the in tersection of the magnetizing coils. This proves that the apparatus ofthe present invention is efficient; it works with, nearly the same level of detecting sensitivity regardless of the directions of slits (flaws) made on the apparatus may work with low detecting sensitivity.

FIG. 9 is a, perspective view of a magnetic particle flaw detecting apparatus designed toobviate this disadvantage. Magnetizing coil 21 consists of

composite coils

23 and 24, constructedas detailed above, and is superposed with similarly constructed magnetizing coil 22 consisting of

composite coils

26 and 27. The device .may be coupled to a power source in a manner similar .to that detailed above. The coils are positioned such that an imaginary lineconnectingthe' points of intersection 25 of the composite coils 23, 24 and an imaginary line connecting the points of intersection 28 of the "composite coils 26, 27 cross each other at right angles .and at the. respective center of the imaginary lines.

If the angle of vintersection formed by the imaginary line s deviates from 90, the rotating magnetic field is deformed, making uniformly effective detection impossible. As forjniaterials'having such sectional form as of shaped steel, the'angle should be changed according to the section form. a

The apparatus of the present invention is constructed as mentioned above, and shall be operated as follows:

A test material, such as a square steel billet, is introv duced into the structure built as shown in FIG. 9 in the direction of the arrow However, current should not be supplied simultaneously to the magnetizing coils 21 and 22; this introduction would create a mixture of rotating magnetic fields formed by the

respective coils

21 and 22 which effects the range of the flaw detection. In order to avoid this complication, the apparatus of the present invention is so operated that the current supply is alternated at a high rate between the

coils

21 and 22, so that they alternately and singly operate for a limited time. This method of current supply will not cause mixing of the elliptical rotating magnetic fields produced by respective coils,- making it possible to carry out flaw detection on amulti-face basis.

FIG. 10 shows an embodiment of a device for switching the power sourcefor the above-mentioned purpose. This device is so constructed that a thyristor 32 and a step-down transformer 33 are connected in series, thereby alternating the current from the source be tween the

coils

21 and 22. If the rate of travel of the test materials is assumedto be 60 m/min. (l in/sec.) and an alternating current of 60 Hz is switched every 2 cycles (1/30 sec.), the material will progress 6.7 cm during ,present invention, to supply a small amount of current,

say, less than 10 percent of the normal supply, since no substantial rotating magnetic field isformed thereby.

Thus, flaw detection of squarebillets over the entire surface area can be made in one pass through the apparatus of the present invention with constancy in every direction and with a high level of detecting sensitivity.

The operation of the magnetic particleflaw detecting apparatus for testing broad articles like plate, is explained, as follows: In tliis case, the composite f'coils made of two sets of magnetizing coils are not suff cient to cover the breadth of such material. Thereforefa magnetizing coil series made of more than two sets of magnetizing coils connected in series at the intersections thereof are used.

However, in such a case, magnetic fields interfere witheach other at the conjunction of the intersections of coils in a series, so that the magnetic fields in the widthwise direction are cancelled, leaving those of lessenedlevel'of sensitivity only in the direction of travel.

The crossed coils shown in FIG. 12 may be used to solve the above-mentioned problem.

FIGQll shows a magnetizing element coil 41,'which isniadefby' crossing two

coils

42 and 43 and forminga certain crossing angle Obetween their respective top parts 44 and 47' and also between their respective bottorn partsj45' and 48.

Numerals

46 and 49 indicate sides, respectively, of the

coils

42 and 43. The crossing angle 0 and the current phase difference between the c'oils42 and 43 are left to choice in view of the above disclosure.

Tabs

73, 74, and 76 indicate terminals for connecting the coils with an electric power source. According to the present invention, more than two of such unit magnetizing element coils, as shown in FIG. 12, may be connected in series into two conjunctive magnetizing coil series. Two

such series

51 and 52 may be superposed so that the intersection 56 of the magnetizing element coils of the second conjunctive magnetizing coil series 52 is placed at the center of the opening 55 between the connecting

parts

53 and 54 of the magnetizing element coils of the first conjunctive magnetizing coil series 51.

Thus, conjunctive magnetizing composite coils are formed having chain-like intersections at the top and thebottom parts. Superposition can be made in any ent invention. Also, according to the present invention,

the number of. magnetizing element coils may be adjustedin accordance with the width of the test materials. The intersection 56 of the second conjunctive magnetizing coil series needs not be strictly the center of the opening 55 between the intersection 57 of the first conjunctive magnetizing coil series, but may be in the vicinity of the center. The magnetizing coils are serially coupled at their top and bottom portions, respectively.

At the center, that is, the boundary of the intersections 57 of the adjoining conjunctive magnetizing coil series, the magnetic fieldsproduced at the intersection 56 interfere with each other to cancel the magnetic field in the direction of connecting magnetizing element coils, that is, in the width direction B, leaving a weakened field in the direction of travel A. I

Therefore, it is impossible to have flaw detecting on an omnidirectional basis carried out at a position far away from the above-mentioned boundary part, that is, the intersections of magnetizing element coils. In order to solve this problem, the first and the second conjunctive magnetizing coil series are superposed, as mentioned above, and the current supply is switched alternately between these series, thus preventing simultaneous supply.

Numerals

61,62, 63, 64, and 65 and 66 indicate terminals of respective conjunctive magnetizing coil series for the connection with their electric power source. Each of these numerals is used for two 7 terminals in order to point out the connection positions 'to ,t he current source output terminals shown in FIG. .13, q

The magnetic particle flaw detecting apparatus of the present invention, which is constructed as mentioned above, is operated in the following manner. A plate-like magnetic article, such as thick steel plate, is charged into the space formed by the

conjunctive magneticcoil series

51 and 52 along the direction line A, and when it is in the air-core of the conjunctive magnetic composite coil for flaw detection, if it is so devised as to have. the magnetic field produced on the surface of the test material form a circular rotating magnetic field, an omnidirectional magnetic field will periodically travel over the full width of the test material, making. it possible to conduct flaw detection with the same level of detecting sensitivity on an omnidirectional basis I In flaw detection of plate-like magnetic articles: by using the apparatus of the present invention, a stronger magnetic field is produced in the direction of travel A i of test materials, as such materials have already been magnetized from outside. However, the strengthening travel)-of an elliptical rotating magnetic field to be between 1 and 2 at the time the conjunctive composite core is empty. In order to determine an appropriate value in the range between 1 and 2, such various factors as current phase difference and the crossing angle of crossed coils are considered; the most useful phase difference being 120.

In this case, the crossing angle 0 of magnetizing element coils is preferably between 120 and FIG. 13 shows a circuit for connecting flaw detection coils 51 and 52 of the apparatus of the, present invention to the current source.

In the figure,

numerals

67, 68 and 69 indicate periodical current switches, such as 32 in FIG. 10. In the up position of the switches, only conjunctive series 51 is energized. In the down position, only conjunctive series 52 is energized.

Numerals

70 and 71 denote respective transformers having V-connection with the three-phase current source. Letters U, V, W, u, v and w denote, respectively: the primary and the secondary phases of the terminals of the transformer. Letters R, S and T denote the phases of lines of the three-phase current source.

The inventors of the present invention were successful in omnidirectional flaw detection over the entire surface of thick steel plate test materials by using the magnetic particle flaw detecting apparatus constructed with the connection of magnetizing element coils inter 'secting at right angles, as shown in FIG. 12, and by using a three-phase alternating current having a phase difference of n This being the achievement of the apparatus of the present invention, it is now possible to subject broad plate-shaped magnetic articles to flaw detection on an omnidirectional basis over the whole surface area without contacting materials at high accuracy and efficiency.

What is claimed is:

1. An omnidirectional magnetic particle flaw detecting apparatus, comprising a first set of magnetizing coils consisting of a first planar composite coil and a second planar composite coil the planes positioned to intersect each other, each said composite coil being composed of two coils sharing one coil reel but having different numbers of coil turns, the coil turn ratio between the two coils of each coil composite being so determined as to produce 'a nearly circular rotating magnetic field on the surface of a test material when the test material is inserted into the magnetizing coils, and an alternating current source means coupled to the two coils of each of said composite coils and supplying alternating current respectively differing in phase. relative to the two coils, whereby surface flaws can be detected with the same sensitivity independent of the flaw direction.

2. The apparatus claimed in claim 1, wherein the angle formed by the intersection of the planes of the first composite coil and the second composite coil in the direction in which the test material is inserted is acute.

5. The device of claim 1, further comprising a second set of magnetizing coils consisting of a third planar composite coil and a fourth planar composite coil, the planes of said third and' fourth composite coils being positioned to intersect each other and being positioned concentrically with said first and second composite coils such that an imaginary line connecting points of intersection of said first and second composite coils and an imaginary line connecting points of intersection of said third and fourth composite coil are at right angles and intersect at their respective centers, said third and fourth composite coils each consisting of two coils sharing one coil reel but having different numbers of coil turns, the coil turn ratio therebetween being such as to form a nearly circular rotating magnetic field on the surface part of test materials when the test material is inserted into the magnetizing coils, and said third and fourth composite coils being coupled to said alternating current source means.

6. The apparatus of claim 5 wherein the angles respectively formed by the intersection of the planes of the first and second composite coils and the third and fourth composite coils in the direction in which the test material is inserted are acute.

7. The apparatus claimed in claim 5, wherein alternating current supplied respectively to said coils has a phase difference of and the said coil turn ratio is in the range between '1.5 and 2.5.

8. The apparatus claimed in claim 5, wherein the angle formed by the intersection respectively of the planes of the first composite coil and the second composite coil and the third and fourth composite coils are between 30 and 60.

Claims

1. An omnidirectional magnetic particle flaw detecting apparatus, comprising a first set of magnetizing coils consisting of a first planar composite coil and a second planar composite coil the planes positioned to intersect each other, each said composite coil being composed of two coils sharing one coil reel but having different numbers of coil turns, the coil turn ratio between the two coils of each coil composite being so determined as to produce a nearly circular rotating magnetic field on the surface of a test material when the test material is inserted into the magnetizing coils, and an alternating current source means coupled to the two coils of each of said composite coils and supplying alternating current respectively differing in phase relative to the two coils, whereby surface flaws can be detected with the same sensitivity independent of the flaw direction.

3. The apparatus claimed in claim 1, wherein the alternating currents supplied to said composite coils have a phase difference of 120.degree., and said coil turn ratio is between 1.5 and 2.5.

4. The apparatus claimed in claim 3, wherein the angle formed by the intersection of the planes of the first composite coil and the second composite coil is between 30.degree. and 60.degree..

5. The device of claim 1, further comprising a second set of magnetizing coils consisting of a third planar composite coil and a fourth planar composite coil, the planes of said third and fourth composite coils being positioned to intersect each other and being positioned concentrically with said first and second composite coils such that an imaginary line connecting points of intersection of said first and second composite coils and an imaginary line connecting points of intersection of said third and fourth composite coil are at right angles and intersect at their respective centers, said third and fourth composite coils each consisting of two coils sharing one coil reel but having different numbers of coil turns, the coil turn ratio therebetween being such as to form a nearly circular rotating magnetic field on the surface part of test materials when the test material is inserted into the magnetizing coils, and said third and fourth composite coils being coupled to said alternating current source means.

7. The apparatus claimed in claim 5, wherein alternating current supplied respectively to said coils has a phase difference of 120.degree.; and the said coil turn ratio is in the range between 1.5 and 2.5.

8. The apparatus claimed in claim 5, wherein the angle formed by the intersection respectively of the planes of the first composite coil and the second composite coil and the third and fourth composite coils are between 30.degree. and 60.degree..