EP0686984A1 - Démagnétisation des matériaux - Google Patents

Démagnétisation des matériaux Download PDF

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Publication number
EP0686984A1
EP0686984A1 EP95303955A EP95303955A EP0686984A1 EP 0686984 A1 EP0686984 A1 EP 0686984A1 EP 95303955 A EP95303955 A EP 95303955A EP 95303955 A EP95303955 A EP 95303955A EP 0686984 A1 EP0686984 A1 EP 0686984A1
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EP
European Patent Office
Prior art keywords
duration
phase
field
current
demagnetising
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP95303955A
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German (de)
English (en)
Inventor
Bruce Guy Irvine Dance
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Welding Institute England
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Welding Institute England
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Filing date
Publication date
Application filed by Welding Institute England filed Critical Welding Institute England
Publication of EP0686984A1 publication Critical patent/EP0686984A1/fr
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F13/00—Apparatus or processes for magnetising or demagnetising
    • H01F13/006—Methods and devices for demagnetising of magnetic bodies, e.g. workpieces, sheet material

Definitions

  • This invention relates to the demagnetisation of materials, particularly ferromagnetic materials of relatively thick section.
  • the method is not limited by the size or shape of the object and is effective for all magnetic materials, irrespective of their magnetic state or orientation.
  • Another prior art method is to deliberately magnetise the object such that an equal but opposing field is developed, so that little or no field is apparent externally.
  • This approach is satisfactory, particularly for objects of simple shape such as a plain tube, and where the existing magnetisation is uniform.
  • the object is irregular in shape and/or of substantial thickness or changing section, or where the magnetisation is non-uniform or irregular as can commonly occur across the face of a thick section. In these circumstances, the most satisfactory approach is complete demagnetisation of the material concerned.
  • the demagnetising field is obtained from a coil carrying unidirectional current which is periodically reversed.
  • This method is more satisfactory in that thicker sections can be demagnetised compared with that obtained at line frequency.
  • the same limitations exist relative to the geometry of the object concerned. Thus, for very thick and/or irregular sections it is possible to obtain an apparent external demagnetisation due to surface or near surface effects masking the internal magnetisation.
  • any internal magnetisation can cause unacceptable deflection. This arises from heating above the Curie point (about 700°C for some ferromagnetic materials), especially as the components to be jointed are melted at their mating faces, which upsets the local surface field distribution and exposes the internal magnetisation from more deeply positioned regions.
  • a method of at least partially demagnetising a workpiece comprises at least a first phase of applying a magnetic field with a first polarity to the workpiece for a first duration; and thereafter applying magnetic fields with successively reversed polarities and shorter durations.
  • a method of demagnetisation which not only demagnetises surface or near surface regions but which extends to any desire degree of depth into the magnetised material, particularly where the latter is of significant thickness such as in excess of 50mm or even in excess of 150mm.
  • the method develops electromagnetic fields capable of fully penetrating all regions of the workpiece, in such a manner as to reduce the time required for demagnetisation over conventional methods.
  • a uni-directional field is applied from a suitable electromagnetic coil surrounding the object for an appreciable time, T, such as for example, 100 seconds. Depending on thickness this time can be as much as 1000 seconds or more.
  • T an appreciable time
  • a single solenoid coil can be utilised or, depending on the shape of the object and its overall size, one or more flat coils can be used.
  • the axial field between the coils in free space does not fall more than 10% below the axial field within one solenoid coil of the same total excitation and of diameter D and overall length L equal to D.
  • an object may be placed conveniently between the two coils or, if less than the internal diameter, within an equivalent solenoid coil.
  • the field is reversed by reversing the current and applying it for a lesser time such as pT, where p can be any suitable factor less than 1, for example between say 0.5 and 0.99.
  • pT can be any suitable factor less than 1, for example between say 0.5 and 0.99.
  • the field is again reversed and applied preferably for times p2T, p3T and so forth until p n T is less than, say, 1 second.
  • the demagnetising cycle must be terminated satisfactorily during a second phase.
  • Three strategies are possible which may be used either singly or in combination.
  • the component and the demagnetising coil or coils are separated gradually, thus reducing the induced magnetisation in the component with each field reversal. This technique is well known.
  • the time period may be reduced still further by the factor p as before.
  • the maximum field current I t is reduced in amplitude at substantially constant duration (frequency) until it is virtually zero or less than, say, 1% of the initial maximum field current I M , as is common practice for demagnetising small or thin components.
  • the reduction factor may be nominally a constant decrement in amplitude, i, or a nominally constant proportion, q, such that on subsequent cycles of nominally constant duration (frequency) the peak current is qI M , q2I M and so forth until q n is less than, say, 0.01.
  • Figure 1 shows the field strength distribution along the centre axis 1 of a simple linear solenoid 2 of approximately square format.
  • the diameter (D) is 5m and the length (L) is 4.5m or 0.9D.
  • the field strength along the centre axis 1 is some 17kA/m at the extremities, rising to about 27kA/m at the centre of the solenoid.
  • the excitation is of the order of 40kA/m length. This excitation and field strength in free space is sufficient to magnetise ferromagnetic material placed within the solenoid space to the order of 50% of its saturation field strength.
  • Figure 2 shows the corresponding field strength on the centre axis for two short coils nominally 5m in diameter and spaced apart some 4, 3, 21 ⁇ 2 and 2m respectively (curves i, ii, iii, and iv).
  • the field strengths are calculated for coils of nominally 111 turns each, carrying 800A. It is seen that with a separation approximating to the coil diameter the field strength is approximately uniform along the centre axis and amounts to about 23kA/m.
  • further similar coils can be utilised on axes normal to, or at an appropriate orientation with respect to, the axis of the above mentioned solenoid or pair of flat coils.
  • the demagnetising field is applied for an extended time period to obtain sufficient penetration into the depth of thick material and thereafter at lesser time durations.
  • the effect of this is illustrated in Figure 3, where for the same field strength B0 the field developed in the depth of the material is a function both of the distance (X) and the duration for which the field is applied (t). With a sufficient duration any degree of depth can be magnetised to the field strength approaching that applied.
  • the time duration is reduced by preferably a nominally constant proportion (p) so that subsequent times t2, t3, t4, t5 correspond to pt, p2t, p3t, p4t and so forth.
  • p nominally constant proportion
  • the applied excitation current is initially constant (apart from the rise and fall which is largely controlled by the self inductance of the coil concerned) and applied for a reducing time as shown in Figure 4.
  • the initial time may be typically in excess of 100 sec or even, for larger structures, in excess of 1000 sec.
  • the minimum time period is determined largely by the self inductance of the coil and the EMF of the applied electrical supply which control the maximum rate of rise of the current in the coil.
  • the polarity reversals continue at a fixed frequency but with reducing amplitude during a second phase as shown.
  • the ratio of durations between one and the next for subsequent demagnetisation currents presuming no change in magnetic properties is given by (1-logK)2 where K is the reduction in field strength desired.
  • K is the reduction in field strength desired.
  • the ratio ranges from 1.1 to nearly 2 for values of K of nominally 0.95 to 0.7.
  • the range should fall within ratios of 1.2 to 1.3.
  • a typical sequence of durations is illustrated by the table in Figure 5, for a K of 0.9 corresponding to a time ratio of nominally 1.25.
  • FIG. 6 A typical control sequence is set out in Figure 6 which, together with a programmable power supply, Figure 7 provides the necessary demagnetisation system for coils as provided.
  • the demagnetising coils 3 are connected to a DC current polarity reversal unit 4 whose operation is controlled by a control computer 5.
  • DC current is supplied to the unit 4 from a source 6.
  • solenoid coil as described in Figure 1 or 2 has an inherent inductance of the order of 0.15H which at a supply of 600V has a rise time of the order of 0.2 sec.
  • a three phase thyristor bridge can be utilised in the unit 4 simply as an on/off switch for long time periods down to a minimum of say 10 seconds. Thereafter a suitable phase sequence of switching is preferable for operating times down to 1 second or less. Thereafter the current can be reduced in amplitude by phase delay on the thyristor controls as is well known in the field, for example, of resistance welding.
  • a step 39 the computer 5 compares the latest duration t n+1 computed in step 38 with a preset minimum time duration t min and if the calculated duration is not less than t min processing returns to step 36.
  • step 41 commences the second phase (as seen in Figure 4).
  • step 44 The latest computed value I M+1 is then compared with a minimum current value I min (typically 2% of I0) in a step 44 and if the computed value is not less than I min the second phase is continued and processing returns to step 41. Otherwise, the demagnetisation cycle is complete and the operator is invited to indicate whether or not the cycle is to be repeated (step 45). If it is, processing returns to step 35 but otherwise to step 46 where the operator indicates whether any new parameters are needed. If they are, processing returns to step 30 but otherwise the process terminates.
  • I min typically 2% of I0
  • the extended structure can be shunted by further magnetic material so that its inherent field is bypassed with respect to the boss or component to be demagnetised, together with, if, necessary, further field coils to counteract any remnant magnetisation in the parts of the structure within the shunt path.
  • an object of complex shape may be satisfactorily demagnetised according to the invention with a group of coils 13 covering each major part of the overall component 14, as illustrated in Figure 9.
  • the coils can be excited all together, but in others it may be preferable to excite the coils in pairs or in a sequence to give the necessary degree of flux and flux reversal in the more remote parts of the structure.
  • a coil 15 to induce fields within the closed loop, as illustrated in Figure 10.
  • Such coils may be conventionally split into mating halves so that they may be readily joined together, rather than be wound individually for each such structure.
  • much higher flux densities are induced since there is no self-demagnetising effect and it is generally preferable to drive the material into saturation at the start of the demagnetising cycle according to the invention and thereafter to reduce the time durations and hence the flux levels as previously described.
  • a flexible or laminated demagnetisation facility can be provided as shown in Figure 11 with the solenoid coil 16 energising an articulated or laminated magnetic yoke 17.
  • This laminated yoke 17 may be terminated in suitable pole pieces 18 to match the dimensions of the object 19 to be demagnetised and hence objects larger than the dimensions of the coil 16 can be treated according to the invention.
  • a further variation in the demagnetising technique may be realised when using a variation of the fabricated laminated yoke in Figure 11.
  • the yoke contains a field sensing element, consisting of a Hall effect device, search coil, or similar, which measures the magnetisation of the yoke.
  • the demagnetising coil itself is connected to both AC and DC supplies, which may be used either singly or in combination.
  • This device may then be used in a number of modes. In the first mode, it can be used to determine the presence of a weak or residual magnetic field in a component or part thereof. Initially the yoke is attached to the workpiece, and a small AC signal is applied to the demagnetising coil. If the net average magnetisation measured in the yoke is increased significantly, then it indicates the presence of a weak or residual field present in the workpiece originally. The origin of this effect is apparent by examination of Figure 13.
  • the first point, ⁇ represents the magnetic state of the region between the ends of the yoke.
  • a small AC current is applied, this causes a small AC field of ⁇ H in the workpiece.
  • a DC current is passed through the magnetising coil, creating a magnetic field in opposition to the residual field.
  • the amount required may be estimated by examining the results of tests carried out in mode 1.
  • the current is applied for sufficient time to allow the magnetic field to penetrate the material to the desired depth.
  • further tests in mode 1 may be carried out to assess the remaining residual magnetic field, and the process repeated as required.
  • the AC and DC current may be passed simultaneously in a controlled and automated sequence, so as to leave the component or a part thereof in a measurably demagnetised state.
  • devices of this type may be used to i) measure the residual magnetic field locally within a component, controlling the effective depth of measurement by controlling the frequency of the applied AC current, ii) demagnetise components wholly or locally, again controlling the depth of the demagnetising field by controlling the time for which it is applied, and iii) combine both measurement and demagnetisation in an automatically controlled cycle.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Magnetic Treatment Devices (AREA)
  • Electromagnets (AREA)
EP95303955A 1994-06-09 1995-06-08 Démagnétisation des matériaux Withdrawn EP0686984A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9411594 1994-06-09
GB9411594A GB9411594D0 (en) 1994-06-09 1994-06-09 Demagnetisation of materials

Publications (1)

Publication Number Publication Date
EP0686984A1 true EP0686984A1 (fr) 1995-12-13

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EP95303955A Withdrawn EP0686984A1 (fr) 1994-06-09 1995-06-08 Démagnétisation des matériaux

Country Status (3)

Country Link
EP (1) EP0686984A1 (fr)
JP (1) JPH08172014A (fr)
GB (1) GB9411594D0 (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001020621A2 (fr) * 1999-09-16 2001-03-22 Redcliffe Magtronics Ltd. Demagnetisation de composants magnetiques
EP1353342A1 (fr) * 2002-04-12 2003-10-15 Albert Maurer Procédé et dispositif pour démagnétiser des objets
DE102010001999A1 (de) * 2010-02-16 2011-09-08 Schenck Rotec Gmbh Auswuchtmaschine mit Entmagnetisiervorrichtung
EP2851911A1 (fr) * 2013-09-06 2015-03-25 Albert Maurer Élimination du magnétisme anhystérétique dans des corps ferromagnétiques

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3895270A (en) * 1974-04-29 1975-07-15 Western Electric Co Method of and apparatus for demagnetizing a magnetic material
JPS6199308A (ja) * 1984-10-19 1986-05-17 Hitachi Metals Ltd 脱磁方法
EP0243564A2 (fr) * 1986-04-21 1987-11-04 MANNESMANN Aktiengesellschaft Méthode et dispositif pour démagnétiser des aciers

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3895270A (en) * 1974-04-29 1975-07-15 Western Electric Co Method of and apparatus for demagnetizing a magnetic material
JPS6199308A (ja) * 1984-10-19 1986-05-17 Hitachi Metals Ltd 脱磁方法
EP0243564A2 (fr) * 1986-04-21 1987-11-04 MANNESMANN Aktiengesellschaft Méthode et dispositif pour démagnétiser des aciers

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 010, no. 279 (E - 439) 20 September 1986 (1986-09-20) *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001020621A2 (fr) * 1999-09-16 2001-03-22 Redcliffe Magtronics Ltd. Demagnetisation de composants magnetiques
WO2001020621A3 (fr) * 1999-09-16 2001-09-27 Redcliffe Magtronics Ltd Demagnetisation de composants magnetiques
EP1353342A1 (fr) * 2002-04-12 2003-10-15 Albert Maurer Procédé et dispositif pour démagnétiser des objets
DE102010001999A1 (de) * 2010-02-16 2011-09-08 Schenck Rotec Gmbh Auswuchtmaschine mit Entmagnetisiervorrichtung
DE102010001999B4 (de) 2010-02-16 2022-05-05 Schenck Rotec Gmbh Auswuchtmaschine mit Entmagnetisiervorrichtung
EP2851911A1 (fr) * 2013-09-06 2015-03-25 Albert Maurer Élimination du magnétisme anhystérétique dans des corps ferromagnétiques

Also Published As

Publication number Publication date
GB9411594D0 (en) 1994-08-03
JPH08172014A (ja) 1996-07-02

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