Demagnetisation of materials

Abstract

 A method of at least partially demagnetising a workpiece comprises applying a magnetic field with a first polarity to the workpiece for a first duration. Thereafter, magnetic fields are applied with successively reversed polarities and shorter durations.

Description

[0001] 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.

[0002] In the past the most common way of demagnetising a magnetic material has been to apply an alternating magneto-magnetic force (MMF) of decreasing amplitude. Typically a coil in which alternating current (commonly directly derived from the main line supply, 50 or 60Hz) is flowing is used, and either the current steadily reduced to substantially zero, or the object and coil separated from each other by an increasing distance until the flux induced from the coil has reduced sufficiently in intensity. This prior method is satisfactory for small objects or thin materials, but on large objects or thick materials only results in a partial demagnetisation. In particular, this approach can result in an apparent demagnetisation where there is a low or virtually no external field from the object. However, it has not been demagnetised throughout but only a limited depth has been affected. This often results in a surrounding zone near the surface of the object which shunts the internal field, giving the seeming lack of magnetisation as measured externally. However, melting the edges as in welding or cutting the object, generally exposes such trapped internal fields.

[0003] 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. However, it is not readily applied where 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.

[0004] In a further development of the alternating method first mentioned above, and in order to obtain lower frequencies than can be conveniently derived from the line supply, or from electronic oscillators operating at low frequencies such as 10Hz or less, 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. However, 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.

[0005] In the context of welding by an electric arc or by an electron beam, 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.

[0006] In accordance with one aspect of the present invention, 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.

[0007] Thus, a method of demagnetisation is employed 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.

[0008] Thus, initially 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. 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. For example, with two circular flat coils of mean diameter D set approximately 0.5D apart, 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. Thus an object may be placed conveniently between the two coils or, if less than the internal diameter, within an equivalent solenoid coil.

[0009] After the initial magnetisation, 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. The degree of time reduction depends on the magnetic properties of the material, but for most ferromagnetic materials can be arbitrarily set at p = 0.9. The field is again reversed and applied preferably for times p²T, p³T and so forth until pⁿT is less than, say, 1 second.

[0010] At this stage, the demagnetising cycle must be terminated satisfactorily during a second phase. Three strategies are possible which may be used either singly or in combination. In the past, 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. In the second, the time period may be reduced still further by the factor p as before. As the initial rate of change of current I_(t) in the demagnetising coils, which have an inductance L, is related to the supply voltage V by the relationship


at some value of pⁿT, the current I_(t) will not reach its usual value I_M. As pⁿT becomes successively smaller, the peak current reaches a negligible value, which is


where C might be 100. At this point, the demagnetising cycle is complete. This may be expressed as:


The inductance L will vary depending on the component being demagnetised; however, it may conveniently be assumed to be its minimum value (i.e. when there is no component to be demagnetised) without significantly affecting the result.

[0011] In the third strategy, 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 qⁿ is less than, say, 0.01.

[0012] Some examples of methods and apparatus according to the present invention will be apparent from the following description and illustrations in which:

Figure 1 shows the axial field strength distribution for a simple, approximately square, solenoid coil (also illustrated);

Figure 2 shows the axial field strength distribution between two flat coils at different separations;

Figure 3 shows the penetration of a field in a ferromagnetic material at different time durations;

Figure 4 shows a typical sequence of applied fields (coil currents) according to the invention;

Figure 5 tabulates a typical sequence of durations;

Figure 6 illustrates a suitable computer flow control;

Figure 7 illustrates an overall system according to an example of the invention;

Figure 8 illustrates an arrangement for demagnetising part of an extended structure;

Figure 9 illustrates an arrangement for demagnetising a structure of complex shape;

Figure 10 illustrates an arrangement for demagnetising a ring or box type structure;

Figure 11 illustrates the use of a fabricated flexible laminated yoke in soft magnetic material;

Figure 12 illustrates the use of magnetic bridging pieces to distribute the flux more evenly; and,

Figure 13 illustrates the origin of apparent B field increase when an AC field is applied.

[0013] Figure 1 shows the field strength distribution along the centre axis 1 of a simple linear solenoid 2 of approximately square format. In this example, the diameter (D) is 5m and the length (L) is 4.5m or 0.9D. For a uniform distribution of 222 turns and a current 800A 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. In this case, 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.

[0014] It should be noted that in general, however strong a field has been applied a simple block of material cannot remain fully saturated since the presence of the surrounding air space leads to a self-demagnetisation so that the remnant field is of the order of half or less of the saturation field strength.

[0015] 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, 2½ and 2m respectively (curves i, ii, iii, and iv). In this example, 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. For objects of more complex shape than can be conveniently placed either within a solenoid or between two short coils than 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. According to the invention, 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. On subsequent reversals of the applied field of the same constant value B₀ 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. As shown, at a known depth X this results in reversing fields of decreasing strength until, with a sufficient short time, the reversing field strength at the distance X has been reduced to a low value.

[0016] During a first phase 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. At the end of the first phase, the polarity reversals continue at a fixed frequency but with reducing amplitude during a second phase as shown. For large coils it is convenient to set a maximum time of the order of 1 sec at the start of the second phase and thereafter to reduce the current in the coil, either by a controlled increment or by a controlled function until it is nominally say only 1% of the initial current. This sequence of controlled reducing time and controlled reducing amplitude is shown in Figure 4.

[0017] During the first phase, 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. Typically the ratio ranges from 1.1 to nearly 2 for values of K of nominally 0.95 to 0.7. Preferably in application the range should fall within ratios of 1.2 to 1.3.

[0018] 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. The demagnetising cycle pulse times (t_n) are given by:


Total time for main cycle, T


Time for finishing cycle ≈5 seconds.
Total time for demagnetising cycle (excluding switching time) 4.6 minutes.

[0019] It should be noted that this results in a considerable saving in time compared with using a constant low-frequency or long duration cycle. In this example, the overall time for any 12 steps is of the order of 5 times the longest duration concerned, neglecting any switching times required for reversal of the field current. This saving in overall duration is more apparent the greater the initial duration. Thus for very thick materials requiring an initial magnetisation time of say 1000 sec the total demagnetisation time is only a few hours instead of one or more days.

[0020] 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. As shown in Figure 7, 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. It should be noted that 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.

[0021] The control computer 5 carries out the steps set out in Figure 6. Initially, the computer 5 is supplied with a maximum current value (I_M), an initial skin depth (δ), a constant (k) relating to the material which is to be demagnetised, and an initial value (K), as previously defined, (steps 30-33). The computer 5 then calculates a value for the initial duration (t₀) in accordance with the formula:


in a step 34.

[0022] The operator then indicates in a step 35 that the process can commence following which the computer 5 causes the unit 4 to pass the current from the current source 6 at its maximum value I_M for the computed time t₀ (step 36). After the time t₀ has expired, the computer 5 causes the unit 4 to reverse the polarity of the current supplied to the coils 3 (step 37) and computes a new duration t_n+1 in accordance with the formula:


where n = 0 initially.

[0023] In 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.

[0024] If the duration has reduced to less than t_min (which may be set for example at 0.1s) then processing continues to step 41 which commences the second phase (as seen in Figure 4). In step 41, an initial current I₀ is supplied to the coils 3 for the duration t_min and then in a step 42 the polarity of the current is reversed by the unit 4. The computer 5 then computes a new value for current I_M+1 in accordance with the formula:


where M = 0 initially.

[0025] 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.

[0026] In some cases, particularly with a large structure, it may not be necessary or practical to demagnetise the whole structure concerned. For example, as shown in Figure 8, if it is designed to demagnetise the boss or protruding part 10 of a large structure 11 the demagnetisation can be carried out as previously described with respect to the boss alone by placing a coil 12 around the boss. There will be, of course, a small degree of self magnetisation due to the fields present in the remaining structure, but in general this will not amount to an unacceptable degree of local magnetisation, particularly if the object such as the boss has been thoroughly demagnetised in depth and through its length. Alternatively, to obtain even lower degrees of demagnetisation 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.

[0027] However, as indicated earlier 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. In some cases 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.

[0028] For structures which present a closed path to the magnetic fields it is often sufficient to apply 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. Here, of course, 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.

[0029] In an alternative arrangement to that shown for a closed loop, Figure 10, 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.

[0030] With objects with re-entrant or irregular shape it is preferable to improve the match between the magnetic properties of the component and the surrounding or integral air spaces. For this, finely divided ferromagnetic material of the same general characteristics as the parent object may conveniently be suspended in a flexible container or bag 20 to fill such discontinuities and provide a more uniform magnetic path as shown in Figure 12a. Alternatively, machined components 21 of the requisite dimension can be inserted in such discontinuities to minimise the air gap and associated spaces of low or unit permittivity (Figure 12b).

[0031] A further variation in the demagnetising technique (not shown) may be realised when using a variation of the fabricated laminated yoke in Figure 11.

[0032] In this variation, 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, α, represents the magnetic state of the region between the ends of the yoke. When a small AC current is applied, this causes a small AC field of ±ΔH in the workpiece. This quickly moves the locus of the magnetic state of the material to a minor hysteresis loop centred on A' (Figure 13B), owing to the shape of the initial magnetisation curve (Figure 13A). The initial magnetisation curve depicted in Figure 13A is typical in shape for many ferritic steels. It may be seen that there is only one point on the initial magnetisation curve where there is not a net increase in the mean magnetic field when a small ±ΔH field is applied. This is when the initial magnetic state A is at the point B=H=0. It will also be seen that the amount of increase in B field measured by the field sensing element gives an indication of the degree of residual magnetism originally present.

[0033] In a second mode of operation, 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. When the current is reduced to zero, further tests in mode 1 may be carried out to assess the remaining residual magnetic field, and the process repeated as required.

[0034] In a third mode of operation, 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.

[0035] Thus it is apparent that 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.

Claims

1. A method of at least partially demagnetising a workpiece comprising 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.

2. A method according to claim 1, wherein each duration has a length p_nT, where n is the duration number, p is a factor less than 1, and T is the length of the first duration.

3. A method according to claim 2, wherein p is in the range 0.5-0.99.

4. A method according to any of the preceding claims, further comprising a second phase, following the first phase, of applying magnetic fields with successively reversed polarities at a constant frequency but with successively smaller amplitudes.

5. A method according to claim 4, wherein the current amplitude for the m+1th cycle in the second phase is given by:

where K is a constant.

6. A method according to claim 4 or claim 5, wherein the minimum current during the second phase is set at 2% of the maximum current in the second phase.

7. A method according to any of the preceding claims, wherein the magnetic field is applied by an electrical coil.

8. A method according to any of the preceding claims, wherein the first duration is at least 100 seconds.

9. A method according to claim 8, wherein the first duration is at least 1000 seconds.

10. A method according to any of the preceding claims, wherein each duration (t_n+1) during the first phase is given by:

11. A method according to any of the preceding claims, wherein the minimum duration during the first phase is substantially 0.1 seconds.

Drawing