Method and device for stray flux testing of ferromagnetic test material with signal normalisation
Patent Information
- Application Number
- EP2023734941
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-06-30
- Filing Date
- 2023-06-21
- Publication Date
- 2025-05-07
Smart Images
Figure 1.1
Abstract
Description
[0001] METHOD AND DEVICE FOR FLUX LEAKAGE TESTING OF FERROMAGNETIC TEST MATERIAL WITH SIGNAL NORMALIZATION
[0002] FIELD OF APPLICATION AND STATE OF THE ART
[0003] The invention relates to a method for flux leakage testing of ferromagnetic test material for detecting defects according to the preamble of claim 1 and to a device suitable for carrying out the method according to the preamble of claim 11.
[0004] Magnetic flux leakage methods are an important component in the non-destructive testing of semi-finished and finished parts for defects, both during the manufacturing process and during the cyclical inspection of finished parts. Magnetic flux leakage methods are less sensitive to some interfering material properties, such as surface roughness or scale in hot-rolled products, than, for example, eddy current or ultrasonic testing. This results in a better ratio between the useful signal and the interference signal (N / S ratio), enabling more reliable defect detection.
[0005] In a device for detecting defects using flux leakage measurement, a test volume of the test piece is magnetized by a magnetization device and scanned using at least one magnetic-field-sensitive flux leakage probe to detect magnetic stray fields caused by the defects. A relative movement occurs between the flux leakage probe and the surface of the test piece in a scanning direction. During scanning, the flux leakage probe is held at a relatively small but finite distance from the surface of the test piece.
[0006] The magnetic flux or magnetic field generated by the magnetization device in the test material is spatially distributed essentially homogeneously in the defect-free material. In this case, no significant magnetic field gradients occur in the areas close to the surface. Cracks and other defects, such as blowholes, inclusions, or other inhomogeneities such as weld seams, etc., act as areas of increased magnetic resistance, so that field components near a defect are diverted around the defect and forced out of the metal into the area close to the surface. The forced-out field components are detected in the flux leakage method to detect defects. In a flux leakage measurement, a defect is detectable if the field components displaced from the test object reach the area of the flux leakage probe and cause a change in the field there sufficient for detection.
[0007] The electrical probe signals, i.e. the electrical signals of the flux leakage probe, or signals derived therefrom, are evaluated by an evaluation device to qualify the defects.
[0008] When testing pipes, the goal is to detect both external defects, i.e., defects on the outside of the pipe, and internal defects, i.e., defects on the inside of the pipe and defects in the pipe wall. For this purpose, methods using direct-field magnetization (DC flux leakage testing) are typically used. This utilizes a key advantage of direct-field magnetization, namely the great penetration depth, allowing even internal defects and defects in the pipe wall to be detected.
[0009] Test material in the form of bars can also be tested. Bar testing is usually carried out using alternating field magnetization (AC flux leakage testing).
[0010] DE 10 2014 212 499 A1 discloses generic methods and devices for flux leakage testing of ferromagnetic pipes, which allow reliable detection of defects regardless of length and angle and precise differentiation between external and internal defects. The probe arrangement comprises a probe array with a plurality of magnetic field-sensitive probes arranged next to one another in a width direction. By using a probe array, the test width covered in one scanning process can be significantly larger than that covered by a single probe. The spatial resolution in the width direction is determined by the probe width of the individual flux leakage probes. The use of probe arrays enables efficient continuous testing of test specimens.
[0011] Tubes and bars should be inspected as completely as possible. However, when testing the entire length of the test piece, more or less long sections at the ends are typically left uninspected. These sections, known as "uninspected ends," must be inspected manually or automatically using additional equipment, or cut off and discarded. Each of these options causes additional processing time and losses for the manufacturer. TASK AND SOLUTION
[0012] Against this background, it is an object of the invention to provide a method and device for flux leakage testing that enable reliable qualification of defects even in cases where the magnetization of the test object is difficult to control. In particular, the aim is to achieve the greatest possible reduction in the number of untested ends when testing ferromagnetic tubes or rods.
[0013] To achieve this object, the invention provides a method having the features of claim 1 and a device having the features of claim 11. Advantageous further developments are specified in the dependent claims. The wording of all claims is incorporated into the description by reference.
[0014] In the method according to the claimed invention, a test volume of the test material is magnetized using an external magnetic field to achieve a magnetization state of the test volume, which can be characterized by its magnetization. Magnetization is a physical quantity used to characterize the magnetic state of a material. It is a vector field that describes the density of permanent or induced magnetic dipoles in a magnetic material and is calculated as the magnetic moment per volume.
[0015] In this method, a surface of the test material is scanned using a probe assembly that includes at least one magnetic-field-sensitive flux leakage probe for detecting magnetic stray fields caused by defects. The flux leakage probe is held at a finite distance from the surface of the test material during scanning and generates electrical probe signals that are a measure of the stray field strength at the respective scanned location.
[0016] According to the claimed invention, the magnetization state of the test volume in the region of the flux leakage probe is additionally determined or ascertained. For this purpose, at least one magnetic field probe is used, which generates magnetization signals that represent a measure of the magnetization state of the test material in the region of the flux leakage probe. The probe signals are normalized using the associated magnetization signals to determine normalized probe signals. The normalized probe signals are then evaluated to qualify the defects. A device according to the claimed invention is characterized in that the probe arrangement has at least one magnetic field probe for generating magnetization signals that represent a measure of the magnetization state of the test material in the region of the flux leakage probe.The evaluation device is configured to perform a normalization of the probe signals using the associated magnetization signals in order to generate normalized probe signals, which can then be evaluated to qualify the defects.
[0017] The invention is based, among other things, on the following findings and considerations of the inventors. In an ideal test, the signal amplitude of the probe signals of a flux leakage probe when detecting a fault (hereinafter also referred to as the fault signal amplitude) should depend only on the geometry and location of the fault or defect, so that the type and extent of the fault, for example, the fault depth, etc., can be reliably determined based on the fault signal amplitude. In any case, fault signals should be comparable with one another, so that one can speak of a relatively uniform test sensitivity regardless of the fault location.
[0018] However, it has been determined that the error signal amplitude also depends significantly on the level of magnetization in the material in the test volume. However, this magnetization can only be controlled to a limited extent so that the test specimen is evenly magnetized along its entire length. This prevents or impairs reliable interpretation of error signals with conventional devices and methods. When testing ferromagnetic tubes or rods, for example, this leads to error signals not being assessed with sufficient certainty, particularly in the areas near the ends of the test specimen. This can result in untested ends remaining for a relatively long time.
[0019] According to the inventors' proposal, this problem is reduced or eliminated by measuring the magnetization state of the test volume in the area of the flux leakage probe using at least one magnetic field probe capable of generating magnetization signals that represent a measure of the magnetization state of the test material in the area of the flux leakage probe. The probe signals are then normalized using the associated magnetization signals to determine standardized probe signals. These are then evaluated to qualify the defects.
[0020] By normalizing the magnetization signals, the defect signals or the probe signals of the flux leakage probe can be made comparable, even if different defects are located in areas of different magnetization strengths. Thus, a sufficiently uniform test sensitivity can be achieved by continuously recording the magnetization state and normalizing or compensating the defect signal amplitude with this magnetization state. Thus, the variation in test sensitivity depending on the magnetization effective in the test volume can be significantly reduced compared to the state of the art and, if necessary, suppressed to such an extent that a uniform test sensitivity sufficient for the test purposes can be assumed during a test.
[0021] In many embodiments, a magnetic field probe is a separate magnetic field-sensitive probe that is provided in addition to a flux leakage probe, i.e., a separate functional element that is arranged in a suitable spatial relationship to at least one associated flux leakage probe. The flux leakage probe and the at least one magnetic field probe can then each be arranged in a position optimal for their measuring task, possibly at a distance from each other. Furthermore, the signal transmission and evaluation can be optimized separately for both probe types. The same probe principle can be used (e.g., Hall probe), but the probes can also operate according to different principles (e.g., induction probe and Hall probe).
[0022] However, it is also possible for a flux leakage probe to simultaneously function as a magnetic field probe. A magnetic field probe therefore does not have to be provided in addition to a flux leakage probe. Rather, a flux leakage probe can also be used as a magnetic field probe. This integration leverages the knowledge that one and the same magnetic field-sensitive probe can fulfill both tasks, because the probe signal contains both signal components resulting from the detection of a defect and signal components representing the magnetization to be measured. These signal components (error signal component and magnetization signal component) can be separated from one another for evaluation. The signal component separation can be achieved using electronic filter components or filter algorithms.This is possible because the error signal components in a continuous test are in a range of relatively high frequencies, while the magnetization signal components are at low frequencies.
[0023] According to a further development, the device comprises at least one test head in which a probe arrangement comprising at least one flux leakage probe and at least one magnetic field probe are arranged or mounted in a fixed spatial relationship to one another. This allows for a compact design, and the relationship between the flux leakage probe and the associated magnetic field probe (at least one, often several) remains virtually unchanged during operation without any additional measures, thus enabling consistently reliable results to be achieved.
[0024] It is generally possible to place a magnetic field probe outside the probe. However, the detected magnetization state should be representative of the location of the flux leakage probe to be compensated. This is best achieved by spatial proximity and the smallest possible magnetization gradient between the magnetic field sensor and the flux leakage sensor. Therefore, locating magnetic field probes inside the probe is preferred.
[0025] In many cases, the flux leakage probe is arranged with its main sensitivity direction in such a way that a normal component of the stray field oriented perpendicular to the surface of the test object can be detected with high sensitivity.
[0026] In contrast, it is preferably provided that, to determine the magnetization state, a parallel component of the magnetic field is measured in a close range around the flux leakage probe. The parallel component is the component that is essentially parallel to the surface of the test object and essentially parallel to the main magnetization direction or to the field lines of the magnetization field. The magnetic field probe can thus be oriented with its main sensitivity direction more or less orthogonal to the surface normal oriented perpendicular to the surface of the test object and / or to the main sensitivity direction of the flux leakage probe.
[0027] Alternatively, or in addition, a flux leakage probe can also detect changes in the parallel component of the stray field. In this case, the main sensitivity directions of the flux leakage probe and the magnetic field probe would lie in the same plane, possibly parallel to each other.
[0028] Preferably, to detect the magnetization state, a magnetic field component (the parallel component) directed substantially parallel to the surface of the test material and to the main magnetization direction is measured.
[0029] The parallel field corresponds to the component of the magnetic field strength at the test material's surface that runs parallel to the test material's surface. In longitudinal flaw testing, where the main magnetization direction of the magnetization field runs essentially in the circumferential direction of the test material, the parallel component runs in a plane perpendicular to the test material's longitudinal axis. In this case, the parallel component is also referred to as the tangential component in this application. Measuring the tangential field is particularly advantageous when the test material is a ferromagnetic tube.
[0030] In transverse flaw testing, where the main magnetization direction of the magnetizing field runs essentially in the longitudinal or axial direction of the test specimen, the parallel component runs essentially parallel to the longitudinal axis of the test specimen. In this case, the parallel component can also be referred to as the axial component.
[0031] The terms “substantially parallel” or “substantially tangential” mean that small deviations from the mathematically exact directions are possible, e.g. by a maximum of 20° or a maximum of 15° or a maximum of 10°.
[0032] Magnetic field measurement using magnetic field components running parallel to the test specimen's surface outside the test specimen takes into account that the magnetization within a test specimen cannot be measured directly. In the case of pipe testing, it has been shown that the magnetization in the pipe wall can be derived particularly well from the parallel component, in particular the so-called tangential field or T-field. The proportionality factor between the magnetization of the test specimen and the parallel field or the tangential field directly at the pipe surface corresponds to the ratio of the magnetic conductivities of air and the pipe material. This allows the magnetization state in the test volume detected by the flux leakage probe to be determined with a good approximation by measuring the magnetic field component in the immediate vicinity of the flux leakage probe.
[0033] In addition to the parallel component oriented parallel to the main magnetization direction, it may be advantageous to also measure a parallel component that is orthogonal or oblique to it. This allows for a two-dimensional magnetic field measurement. This can be useful, for example, for normalizing error signals under non-ideal magnetization conditions and / or for characterizing oblique errors.
[0034] If the (at least one) magnetic field probe is provided in addition to the (at least one) flux leakage probe, it can be offset both radially and axially from the location of the flux leakage probe; this offset can be taken into account when interpreting the defect signals. Alternatively, both the flux leakage signals and the magnetization signals can be recorded with the same probe. In this case, a magnetic field-sensitive probe positioned at a finite test distance from the surface of the test material records both the DC and AC field components of the magnetic field in the direction of the main magnetization direction. The downstream signal processing device separates the recorded signal into a slowly changing DC field component and the AC field component superimposed on it.In the subsequent processing, the DC field component represents the magnetization state, the AC field component represents the probe signals, which is a measure of the strength of the stray field caused by defects at the scanned location.
[0035] In many cases, it is particularly advantageous if the flux leakage probe and the magnetic field probe are based on the same measuring principle and are simply installed with a different orientation of their sensitivity. For example, the flux leakage probe and the magnetic field probe can each be a Hall element.
[0036] Preferably, a (possibly slowly varying) DC field component of the magnetization signal is determined and used to normalize the probe signal. It has been found that this component correlates particularly reliably with the current magnetization strength in the detected area of the test object.
[0037] To achieve efficient testing with, if necessary, high spatial resolution tailored to the testing task, preferred embodiments provide for the probe arrangement to comprise a probe array with a plurality of flux leakage probes arranged side by side in a straight row in a first direction. Preferably, two or more magnetic field probes are then provided to detect the magnetization state, arranged parallel to this first direction in a straight row at a distance from one another. It may also be sufficient to use only one magnetic field probe.
[0038] The number of magnetic field probes can be significantly smaller than the number of flux leakage probes, so that not every flux leakage probe needs to be assigned its own magnetic field probe. Rather, it can be the case that the magnetization acting at the location of a specific flux leakage probe can be derived by interpolation from the magnetization signals recorded by multiple magnetic field probes. In some embodiments, there are at least ten times as many flux leakage probes as magnetic field probes, which on the one hand achieves sufficient spatial resolution for the flux leakage test and on the other hand can limit the equipment required for magnetic field measurement. Preferably, the flux leakage probes are arranged on a side of the probe arrangement facing the test object, and the magnetic field probe(s) are arranged at a distance behind the flux leakage probes, i.e. at a somewhat greater distance from the test object.This allows a high spatial resolution of the defect detection by leakage flux measurement to be combined with sufficiently accurate recording of the magnetization state of the individual leakage flux probes.
[0039] In some embodiments, the flux leakage probes are arranged at equal intervals from one another, and the magnetic field probes are arranged at uneven intervals from one another, with the density of magnetic field probes preferably being higher in the end regions of the probe arrangement than in a central region of the probe arrangement. This can be advantageous for measuring values in the area of the test object ends.
[0040] In preferred embodiments, the probe signal of a flux leakage probe has a signal amplitude, and to normalize the probe signal, the signal amplitude is multiplied by a compensation factor that at least partially compensates for a magnetization dependence of the test sensitivity. Such a multiplication operation can be performed relatively quickly for many flux leakage probes simultaneously during the evaluation. The compensation factor can, for example, tend to be inversely proportional to the strength of the magnetization of the test volume scanned by the flux leakage probe.
[0041] In preferred methods and devices, suitable compensation factors are not estimated based on theoretical relationships, but rather determined very precisely based on measurements and verified extrapolations and / or interpolations. In some methods, calibration measurements are performed on a calibration section of the test object that has at least one calibration error in order to determine a compensation curve that describes a functional relationship between a magnetization state of the test object under different external magnetic fields, corresponding magnetization signals from a magnetic field probe, and a signal amplitude of the probe signal generated by the calibration error. During the evaluation of the probe signals, compensation factors for normalizing the probe signals are then derived from the compensation curve.The term "calibration error" here describes a standard defect whose width and depth are usually specified by standards to enable comparable test results. In order to ensure that identical defects at different longitudinal positions of a test piece, for example a ferromagnetic tube under test, generate the same probe signal, the magnetization would have to be constant along the length of the test piece. However, it has been found that, particularly at the tube ends or in the end regions of a test piece, the actual magnetization can deviate considerably from the magnetization in the center region of the test piece. Wall thickness variations, such as manufacturing-related polygons or eccentricities, as well as an eccentric position of a test piece and induction effects due to changes in the magnetic field also have a strong influence on the magnetization effectively present in the test volume.
[0042] In some methods, a variation in the magnetization state depending on the axial position of a test section to be tested is taken into account when determining the correction factor to be applied to the test section. This is achieved by determining an axial offset between the calibration section and the test section when determining the correction factor and modifying the correction factor depending on this offset. This makes it possible to achieve appropriate compensation of the error signal amplitude with relatively low computational effort, even if the calibration measurement or the adjustment with a standard defect was not performed in the axial position of the defect to be evaluated later.
[0043] Extensive investigations by the inventors have shown that in many cases, it is possible to determine the correction factor for an axial position in a test section using a shifted compensation curve. The shifted compensation curve has the same shape as the compensation curve determined in the calibration section, and this compensation curve is only shifted by a value corresponding to the axial offset compared to the compensation curve determined in the calibration section. This permissible simplification enables particularly fast calibration measurements to determine a locally correct compensation factor for each axial position on the test specimen.
[0044] BRIEF DESCRIPTION OF THE DRAWINGS
[0045] Further advantages and aspects of the invention emerge from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures. Fig. 1 shows subsystems of an exemplary embodiment of a device for flux leakage testing of ferromagnetic test material, with a rotating subsystem (Fig. 1A) for testing for defects with a predominant component along the test material axis and a stationary subsystem (Fig. 1B) for testing for defects with a predominant component transverse to the test material axis;
[0046] Fig. 2 shows defect type-specific leakage flux fields at a cross-section through a pipe;
[0047] Fig. 3 shows details of the design of the probe arrangement as well as the main magnetization direction directed transversely to the longitudinal axis of the tube for a rotating subsystem according to an embodiment;
[0048] Fig. 4 shows details of the design of the probe arrangement as well as the main magnetization direction directed along the longitudinal axis of the tube for a stationary subsystem according to an embodiment;
[0049] Fig. 5A to 5C show schematic courses of magnetic field lines in different phases of a continuous test to illustrate differences in magnetization between pipe ends and pipe center;
[0050] Fig. 6 shows schematically the course of the strength of magnetization as a pipe passes through a testing device;
[0051] Fig. 7 shows a schematic side view of a test head according to an embodiment with approximately 100 leakage flux probes and five associated magnetic field probes;
[0052] Fig. 8 shows a schematic view of the arrangement shown in Fig. 7 in the longitudinal direction of a pipe during testing;
[0053] Fig. 9 shows a test head between two pole pieces, with the magnetic field strength varying in the axial direction of the test head;
[0054] Fig. 10 shows a diagram illustrating the dependence of the measured and interpolated T-field on the location along the longitudinal direction of the probe;
[0055] Fig. 11 shows compensation curves for internal and external errors; Fig. 12 shows the shift of a compensation curve;
[0056] Fig. 13 shows the determination of correction factors;
[0057] Fig. 14 shows schematically the effect of the compensation on the error signal amplitudes,
[0058] DETAILED DESCRIPTION OF THE EMBODIMENTS
[0059] Embodiments of the claimed invention are explained below using a device for flux leakage testing of ferromagnetic test material in the form of hot-rolled ferromagnetic tubes using a continuous process. The device is designed to detect defects, imperfections, or discontinuities of various types and can, for example, reliably detect rolling defects on both the inside of the tube (internal defects) and the outside of the tube (external defects). Both longitudinal defects (defects with a main extension direction parallel to the longitudinal axis of the tube), transverse defects (defects with a main extension direction in the circumferential direction or perpendicular to the longitudinal axis of the tube), and oblique defects (transverse to the longitudinal direction and the circumferential direction) can be reliably detected and characterized.
[0060] In one embodiment, two subsystems are integrated into a multi-test block. A rotating subsystem is provided for longitudinal defect detection, the basic principle of which is explained with reference to Fig. 1A. A stationary subsystem with an annular arrangement containing several sensor arrays distributed around the circumference of the arrangement, for example, corresponding to the arrangement in Fig. 1B, is provided for transverse defect detection. The subsystems are arranged one behind the other in the direction of travel of the pipe, whereby the sequence can be arbitrary. In other embodiments not shown in detail, a single system may be sufficient, e.g., a single rotating system.
[0061] The rotating subsystem comprises a rotating head with a ring yoke RJ rotating around the test specimen PR. This ring yoke has pole pieces PS oriented radially to the test specimen surface at diametrically opposite locations, to which magnetizing windings MW are attached. This generates a magnetic flux or a magnetic field MF (DC field) in the pipe wall, whose field lines run in the circumferential direction of the test specimen, i.e., perpendicular to the longitudinal direction of the pipe. Probe heads PK are arranged circumferentially between the pole pieces on the rotor, each containing one or more probe arrays SA, with each probe array comprising a plurality of individual flux leakage probes SO. During the test, the ring yoke, together with the pole pieces PS and the probe heads PK, rotates at speeds between approximately 30 and approximately 1200 rpm, depending on the type of rotating subsystem. 1. The pipe to be tested is simultaneously transported forward in the direction of travel at a test speed (e.g. up to 3 m / s or more). The test heads rub against the pipe surface and scan it seamlessly on a helical path. The probes SO of the probe array are arranged within the test heads at a small test distance AB to the surface OB of the test material, which can be in the order of magnitude of 0.2 mm to 2 mm, for example (see Fig. 3). Because the magnetic field lines run in the circumferential direction, this test is particularly sensitive to longitudinal defects LF-A on the outside of the pipe and longitudinal defects LF-I on the inside of the pipe, which maximally disrupt the magnetic flux in the circumferential direction and thereby generate strong stray flux fields (Fig. 2).
[0062] The stationary system (Fig. 1B) for transverse flaw detection uses a DC magnetization device (not shown in detail) that generates a magnetic field MF in the longitudinal direction of the pipe passing through. Two rings of probe arrays with circumferentially spaced probe arrays SA are arranged in a ring around the test specimen and scan the specimen in its longitudinal direction during the continuous test. Since the magnetic flux flows in the longitudinal direction, it is particularly strongly disturbed by circumferentially running transverse flaws on the outside (QF-A) and transverse flaws on the inside (QF-I), so this arrangement for transverse flaw detection exhibits high test sensitivity.
[0063] The electrical signals SIG-SO of the flux leakage probes of the probe arrays, i.e., the probe signals, are fed to a common evaluation unit AW, where the defects are qualified. Since the probe signals are caused by a fault or defect during testing and are characteristic of it, the probe signals are also referred to here as "fault signals" or "test signals."
[0064] Each type of flaw causes certain, flaw-type-specific leakage flux fields, the properties of which can be identified by the signal shape and the frequencies contained in the signal. Fig. 2, for example, shows a section through a pipe perpendicular to the longitudinal direction and the magnetic field lines of the magnetization field MF running in the circumferential direction. An external flaw LF-A running in the longitudinal direction generates a leakage flux field SF-A that is relatively tightly concentrated in the vicinity of the external flaw. An internal flaw LF-I running in the longitudinal direction of the same dimensions, in contrast, generates a less sharp, more locally smeared, or expanded or broadened leakage flux field SF-I with a lower amplitude on the outside of the pipe. Typical signal shapes of the probe signals when a probe overflows in the circumferential direction are shown above the leakage flux fields. The y-axis corresponds to the signal amplitude A and the x-axis to the time t or the location during the probe's rotation.
[0065] 3 and 4, details of the design of the probe arrangements for the rotating system (Fig. 3) and the stationary system (Fig. 4) are now explained. The probe arrangement SA-R for the rotating system has a plurality of nominally identical individual probes SO1, SO2 etc., which form a probe array SA and are arranged in a straight row along a first direction R1 that runs parallel to the longitudinal axis of the pipe. The probe array SA is installed in a test head PK (see, for example, Fig. 7). When the system is rotating, the probe arrangement as a whole moves in the circumferential direction of the test specimen in a second direction R2 that is perpendicular to the first direction R1. Due to the simultaneous longitudinal movement of the test specimen PR, each of the individual probes SO1, SO2 scans a relatively narrow test track PS that runs spirally around the test specimen, with the test track running obliquely to the first and second directions.All probes of the probe array together scan a relatively high test width with a large number of test tracks parallel to each other.
[0066] A corresponding arrangement is obtained with the probe arrangement SA-T for transverse flaw testing (see Fig. 4). The probe arrangement SA-T has a large number of individual probes SO1, SO2 etc., which are arranged next to one another in a row in the first direction R1, whereby the first direction here corresponds to the circumferential direction of the test material PR. The probe arrangement is stationary, while the test material moves parallel to its longitudinal direction, so that the probe array scans the test piece surface in a scanning direction that corresponds to the second direction R2 perpendicular to the first direction R1. Here, too, each individual probe covers a relatively narrow test track PS, whereby the totality of the test tracks in the circumferential direction results in a much larger test width of the probe arrangement. The magnetic field MF running in the longitudinal direction of the pipe is forced out of the test piece material at a transverse flaw QF-A and detected by the probes of the probe array SA.
[0067] The height or amplitude of a fault signal (leakage flux signal) depends not only on the nature of the fault, but also on the strength of the magnetic field in the test object, for example in the pipe wall, at the location of the fault. In order for the same fault to generate the same fault signal at different longitudinal positions in a pipe test, for example, the magnetization would have to be constant along the length of the pipe. Experience has shown, however, that this is not the case. Especially at the pipe ends, the local magnetization differs from the magnetization in the middle of the pipe (seen in the longitudinal direction). Variations in wall thickness can also lead to fluctuations in the magnetization. In addition, dynamic effects can arise when the magnetic fields build up and decay, particularly when a pipe enters and exits a testing apparatus.
[0068] As schematically shown in Fig. 5, as the tube PR or the test piece PR passes between the pole pieces, the magnetic field lines from the air are drawn into the more conductive, ferromagnetic tube (Fig. 5A). This leads to increased magnetization at the tube ends. Only when a certain length of the tube protrudes from the pole piece again (Fig. 5C) does the magnetization reach its nominal value at a considerable distance from the tube ends. Depending on the tube's advance speed, reaching the nominal magnetization can be delayed or influenced in other ways due to induction effects or by controlling the coil current.
[0069] Fig. 6 shows a schematic example of the curve of the magnetization strength as a pipe passes through a testing device. The abscissa indicates the position POS in the longitudinal direction (first direction R1), and the ordinate shows a measure of the magnetization strength MAG, which will be explained later. At the inlet IN position, a higher magnetization initially results due to the field line concentration, which then drops sharply due to transient processes. The current control REG of the magnetic field coils then counteracts this, so that the desired nominal magnetization MAG-N is present with only minor fluctuations over most of the passage or pipe length. At the outlet OUT position, the effects of the control and the field line concentration occur again (see Fig. 5).
[0070] Fluctuations in magnetization, including during entry and exit, lead to fluctuations and undefined states regarding test sensitivity, partly because it is unclear whether a strong defect signal is due to a particularly large defect or strong magnetization. Since the test results at the pipe ends are therefore not sufficiently reliable, they are referred to as "untested ends."
[0071] The following explains how, according to one embodiment of the invention, a uniform test sensitivity can be achieved essentially over the entire pipe length. The uniform test sensitivity is essentially achieved by continuously recording the magnetization state of the test object and normalizing or compensating the error signal amplitudes with this magnetization state.
[0072] Using Figs. 7 and 8, some design measures implemented in the exemplary embodiment to contribute to the uniformity of the test sensitivity are explained. Fig. 7 shows a schematic side view of a test head PK arranged in a test configuration at a distance AB from the surface of the test object PR. Attached to the side facing the test object is the probe array SA, which has a straight row with a large number of flux leakage probes SO or test probes SO, e.g., 40 or more or 70 or more, in the example case between 90 and 100 identical flux leakage probes.
[0073] A smaller number of magnetic field probes SM1 to SM5 are arranged a short distance behind the probe array, also in a straight row. The magnetic field probes are evenly spaced from one another here, but the spacing can also be uneven, particularly smaller at the ends than in the middle. The arrangement is selected such that each of the flux leakage probes is assigned at least one magnetic field probe, which can generate magnetization signals that represent a measure of the magnetization state of the test material in the area of the flux leakage probe. For example, the magnetization at the location of the flux leakage probe SO30 can be determined by interpolation using the magnetization signals from the two nearest magnetic field probes SM2 and SM3, as explained later.
[0074] Fig. 8 shows a schematic view of the arrangement shown in Fig. 7 in the longitudinal direction of the pipe. The probe PK is shown here directly above a longitudinal defect LF-A on the outside of the pipe.
[0075] The PK probe shown is designed to determine a measure of the test object's magnetization by measuring a field component parallel to the test object's surface and parallel to the main magnetization direction, which can be referred to here as the parallel component. More precisely, the so-called tangential field or T-field is measured. The measured value of the magnetization signal is therefore also referred to as the T-field value. The tangential field TAN corresponds to the component of the magnetic field strength at the test object's surface that corresponds to the parallel component that runs tangential to the pipe, i.e., in a plane perpendicular to the pipe's longitudinal axis, parallel to the surface and parallel to the field lines of the circumferential magnetic field MF. The radial component RAD of the magnetic field measurable in the area of the surface runs orthogonally to this, i.e., in the normal direction of the pipe.
[0076] According to the inventors' findings, the measurement of the tangential field is particularly suitable for determining the magnetization when testing for longitudinal defects, e.g. on pipes, since the proportionality factor between the magnetization in the test material, in this case in the interior of the pipe material, and the T-field near the pipe surface corresponds to the ratio of the magnetic conductivities of air (|JL) and the pipe material (JJR).
[0077] Ideally, the main magnetization direction in a rotating system runs exactly perpendicular to the longitudinal axis of the pipe along the pipe circumference. Deviations from the ideal alignment of the magnetic field lines cause a variation in the leakage flux signal, especially when testing for defects that are not exactly aligned with the longitudinal axis of the pipe, which can impair the accuracy of the test result. This variation can be normalized by additionally measuring the component of the magnetic field that is essentially perpendicular to the main magnetization direction and parallel to the longitudinal axis of the pipe. This component can be referred to as the orthogonal component because it is oriented perpendicular to the main magnetization direction. In this case, it can also be referred to as the axial component because, in this measurement configuration, it is parallel to the axial direction of the test material.This component can be detected by one or more additional magneto-sensitive probes. In another embodiment, the same magnetic field probe that detects the parallel field can also detect the components of the magnetization state perpendicular to the main magnetization direction, or even the magnitude and angle of the magnetization state. Thus, if the magnetization is detected in two mutually orthogonal or oblique directions that lie within a tangent plane (parallel to the test material surface), such effects can also be detected and taken into account in the error signal normalization.
[0078] Stray field probes (test probes, flaw probes) SO, on the other hand, are designed to measure the radial component RAD of the magnetic field strength at the surface. This is primarily caused by the stray flux at defects, namely where the magnetic field lines are forced out of the pipe material by a defect. In Fig. 8, the different sensitivity directions of the stray flux probe (measurement of the radial component) and the magnetic field probes (measurement of the T-field) are indicated by arrows.
[0079] The flux leakage probes SO and the magnetic field probes SM are of the same probe type, namely Hall probes. They are structurally identical but differ in the orientation of their main sensitivity direction (arrows in Fig. 8), i.e., the direction of maximum sensitivity. Another difference is that the signals for the T-field are recorded with DC coupling (direct field coupling), while the flux leakage probes (probes for fault detection) operate with AC coupling (alternating field coupling), thus only recording the change in the stray fields.
[0080] The magnetic field probes (or T-field probes) can be positioned in the probe head, for example, as follows: a central magnetic field probe SM3 in the center, one (SM1, SM5) at each of the axial ends of a probe array, and one (SM2, SM4) between the magnetic field probes at the ends and the central magnetic field probe. Thus, five magnetic field probes may be sufficient. Before determining the positions of these T-field probes, the course of the tangential field in the axial direction should be known for the pole pieces, air gaps, and pipe dimensions used. Depending on this, it may be useful, for example, to provide uneven spacing between the magnetic field probes instead of uniform axial spacing (cf. Fig. 7), by inserting The magnetic field probes are located closer together at the edges.
[0081] In contrast to the flux leakage probes SO, which should be arranged as close as possible to the test object surface for fault detection in order to detect high-frequency field changes, the magnetic field probes can have a greater distance to the test object because they tend to detect low-frequency field changes.
[0082] Design aids for the arrangement of magnetic field probes are now explained using Figs. 9 and 10. For the desired T-field compensation (to equalize the test sensitivity), a corresponding T-field value should be available for each flux leakage probe SO. For five magnetic field probes S1 to SM5, the T-field values for those flux leakage probes that are not located directly beneath a magnetic field probe are interpolated.
[0083] Fig. 9 shows a probe PK between two pole pieces PS. The magnetic field generated in the area of the probe varies in the axial direction, with the magnetic field strength (illustrated by the length of the arrows) being greater in the central region than near the axial ends.
[0084] Fig. 10 shows a corresponding diagram illustrating the dependence of the measured T-field TF on the location along the probe. The crosses represent the magnetic field signals (T-field values) of the magnetic field probes. The dashed lines represent linearly interpolated T-field values, and the solid line represents the actual T-field curve. While in the central region (magnetic field probes SM2, SM3, and SM4) the interpolated values are close to the actual values, larger deviations arise in the region of stronger axial gradients of the magnetic field strengths near the pipe ends. In the example case, these deviations can be reduced by positioning the second and fourth magnetic field probes closer to the ends (dashed position), which can result in uneven spacing between magnetic field probes in the axial direction.
[0085] Using Figs. 11 to 13, the measures for compensating for axially uneven magnetization in the exemplary embodiment are now explained. This method takes into account that the varying magnetization strengths of the pipe wall in the axial direction lead to different error signals for the same errors. Using the T-field measurements by the SM magnetic field probes or a factor calculated from them, the magnitude of the error signal (amplitude of the error signal) is corrected for different magnetizations.
[0086] The method determines (at least) one compensation curve (see Fig. 11). For this purpose, calibration measurements are performed. During T-field calibration, the test probes cyclically record two calibration errors of known dimensions, namely an internal error and an external error, while the current for the field coils (or measuring windings MW) at the pole pieces is increased from a minimum value to a maximum value. For each set current, the test head overruns the calibration errors at least once, and the stray field signals (error signals) and the associated T-field values are recorded. Fig. 11 shows a schematic diagram in which the current for the field coils or the associated T-field TF is plotted on the abscissa, and a standardized signal strength SIGN for the external error (solid line AF) and the internal error (dashed line IF) is plotted on the ordinate.
[0087] The T-field calibration thus results in two pairs of values for each T-field: the error signal (external error) plotted against the T-field value and the error signal (internal error) plotted against the T-field value. The compensation curve AF or IF shown can then be interpolated from the measured value pairs, which assigns a compensation value for the error signal to each T-field value.
[0088] It turns out that the measured T-field values for identical current strengths or identical magnetization states vary in the axial direction. This variation can be influenced by different pole piece geometries. For example, with many conventional pole pieces, the T-field values can be smaller at the axial edges than in the center. The inventors' investigations show that, regardless of this axial variation, the course of the compensation curve, i.e., its shape, appears to be essentially independent of the axial position, i.e., independent of the probe position (with which magnetic field probe) at which the T-field was measured. The method also determines a so-called displacement value. Fig.Figure 12 schematically illustrates a shift VS by a shift value between two compensation curves, where the solid line corresponds to an external error in the region of higher magnetic field strength and the solid line corresponds to the same external error in a region of lower magnetic field strength.
[0089] Since the shape of the compensation curve, which indicates the functional relationship between the T-field strength and the resulting signal amplitude plotted on the y-axis, does not change at different feed positions, the T-field compensation in the exemplary embodiment requires only two compensation curves: one for external defects and one for internal defects. In addition, the displacement value is determined for each of the flux leakage probes. The difference between the T-field value of the flux leakage probe and the T-field value of the center T-field probe can be selected as the displacement value. The displacement value VS should be determined separately for each probe.
[0090] The method uses the T-field value of the respective leakage flux probe (corrected by the displacement value) and a reference value REF for T-field compensation. The reference value is selected, for example, to correspond to the T-field value for which the factor for correcting the signal amplitude is equal to one. The T-field value of the average magnetic field probe from the calibration error at nominal current strength is preferably selected as the reference value. For T-field values that lie above the reference value, the error signals of the corresponding leakage flux probes are given a factor less than one (< 1); in the other case (T-field value below the reference value) they are given a factor greater than 1 (> 1). Fig. 13 illustrates this for the curve of the external error; the abscissa represents the strength of the T-field, the ordinate the factor FAK, which is equal to one for the reference value TREF.
[0091] The effect of the compensation strategy is now explained using a schematic example in Fig. 14. The upper part shows the test specimen PR in which a longitudinal external defect LF-A has been introduced as an adjustment error. The arrows in the test specimen represent the magnetization, and the thickness of the arrows represents the strength of the magnetization, which varies axially. The left part EIN represents the running-in phase, the middle part DYN illustrates the dynamic effects associated with the control shortly before running-in, and the right part NOR shows the conditions at a greater distance from the pipe ends, where a stable, normal state of magnetization occurs. The diagram below shows the amplitude of the measured T-field with a solid line T, i.e. the strength of the magnetization signal from the magnetic field probes. The dashed line SIG-SO schematically represents the error signal, i.e. the leakage flux signal from the leakage flux probes SO.The size of the error in the test specimen is the same in all three cases, so that ideally (with axially uniform magnetization) the same error signal amplitudes should occur in all three situations.
[0092] In fact, the SIG-SO curve shows that during the ON run-in phase, where the magnetization is relatively high and, accordingly, the measured T-field is relatively high, a relatively large error signal amplitude is present. In the area of dynamic effects or transient processes, where relatively low magnetic field strengths can occur, the error signal is significantly weaker than during the run-in phase. Only at a greater distance from the pipe end does the error signal reach its "true" amplitude, which corresponds to the geometry of the defect.
[0093] In tests in which two longitudinal external defects of identical size were introduced at different distances from the pipe end (100 mm and 250 mm) in the area of the pipe ends, the defect closer to the pipe end generated a defect signal up to 6 dB higher than the position further away from the pipe end. Similar results were observed for internal defects.
[0094] The lower diagram in Fig. 14 shows the effect of the compensation. The solid line FAK represents the factor explained above, which indicates the value by which the measured error signal amplitude must be multiplied to arrive at the true amplitude of the error according to the compensation. During the run-in period, this factor is below the value that results in the normal state (on the right in the diagram). This reduces the amplitude of the error signal. In the DYN range of dynamic effects, the error signal is slightly amplified; in the normal range, the factor is approximately one, which means that the "correct" signal amplitude is measured directly.
Claims
Patent claims 1. Method for flux leakage testing of ferromagnetic test material, in particular ferromagnetic pipes, for the detection of defects, comprising: Magnetizing a test volume of the test material by means of an external magnetic field to generate a magnetization state of the test volume that can be characterized by a magnetization; Scanning a surface of the test material by means of a probe arrangement with at least one magnetic field-sensitive flux leakage probe for detecting magnetic stray fields caused by defects, wherein the flux leakage probe is held at a finite test distance from the surface of the test material during scanning and generates electrical probe signals which are a measure of the strength of the stray field; characterized by: Determining the magnetization state of the test volume in the region of the flux leakage probe using at least one magnetic field probe to generate magnetization signals that represent a measure of the magnetization state of the test material in the region of the flux leakage probe; Normalizing the probe signals using the associated magnetization signals to determine normalized probe signals; Evaluation of the standardized probe signals to qualify the defects.
2. Method according to claim 1, characterized in that a magnetic field sensitive probe which is separate from the leakage flux probe and is provided in addition to the leakage flux probe is used as the magnetic field probe.
3. Method according to claim 1 or 2, characterized in that, in order to determine the magnetization state, a parallel component of the magnetic field directed substantially parallel to the surface of the test material and parallel to a main magnetization direction is measured in a close range around the leakage flux probe.
4. Method according to one of the preceding claims, characterized in that the test material is a ferromagnetic tube, wherein a magnetic field component directed substantially tangentially to the surface of the test material is preferably measured to detect the magnetization state.
5. Method according to one of the preceding claims, characterized in that a DC field component of the magnetization signal is determined and used to normalize the probe signal.
6. Method according to one of the preceding claims, characterized in that the probe signal of a flux leakage probe has a signal amplitude and that, in order to normalize the probe signal, the signal amplitude is multiplied by a compensation factor which at least partially compensates for a magnetization dependence of the test sensitivity, wherein preferably the compensation factor is substantially inversely proportional to the strength of the magnetization of the test volume scanned by the flux leakage probe.
7. Method according to one of the preceding claims, characterized by: Carrying out calibration measurements on a calibration section of the test material equipped with at least one calibration error to determine a compensation curve which describes a functional relationship between a magnetization state of the test material in the case of external magnetic fields of different strengths, corresponding magnetization signals of a magnetic field probe and a signal amplitude of the probe signal generated by a standard defect, and Deriving compensation factors for normalizing probe signals from the compensation curve when evaluating the probe signals.
8. Method according to claim 7, characterized by: Taking into account a variation in the magnetization state as a function of an axial position of a test section to be tested when determining the correction factor to be applied to the test section by determining an axial offset between the calibration section and the test section when determining the correction factor and modifying the correction factor as a function of the offset.
9. Method according to claim 7 or 8, characterized in that the correction factor for an axial position in a test section is determined on the basis of a shifted compensation curve, wherein the shifted compensation curve has the curve shape of the compensation curve determined in the calibration section, wherein this compensation curve is shifted by a shift value corresponding to the axial offset compared to the compensation curve determined in the calibration section.
10. Method according to one of the preceding claims, characterized in that the probe arrangement has a probe array with a plurality of leakage flux probes which are arranged next to one another in a first direction, wherein two or more magnetic field probes are preferably provided for detecting the magnetization state, which are arranged at a distance from one another in the first direction, wherein the number of magnetic field probes is preferably smaller than the number of leakage flux probes.
11. A device for flux leakage testing of ferromagnetic test material, in particular ferromagnetic pipes, for detecting defects, comprising: a magnetization device for magnetizing a test volume of the test material (PR); a probe arrangement (SA) with at least one flux leakage probe (SO) for detecting magnetic stray fields caused by defects, wherein the flux leakage probe (SO) is configured to be held at a finite test distance (AB) from the surface of the test material during scanning and to generate electrical probe signals with a fault signal amplitude dependent on the flux leakage, which are a measure of the strength of the stray field;an evaluation device (AW) for evaluating the probe signals to qualify the defects, characterized in that the device has at least one magnetic field probe (SM) for generating magnetization signals that represent a measure of the magnetization state of the test material (PR) in the region of the flux leakage probe (SO); and the evaluation device (AW) is configured to perform a standardization of the probe signals using the associated magnetization signals to determine standardized probe signals and to evaluate the standardized probe signals to qualify the defects.
12. Device according to claim 11, characterized in that the magnetic field probe (SM) is a magnetic field sensitive probe separate from the leakage flux probe (SO) and provided in addition to the leakage flux probe (SO).
13. Device according to claim 11 or 12, characterized in that the device has at least one test head (PK) in which a probe arrangement (SA) with at least one leakage flux probe (SO) and at least one magnetic field probe (SM) are arranged in a fixed spatial relationship to one another.
14. Device according to one of claims 11 to 13, characterized in that the flux leakage probe (SO) is arranged to have a substantially perpendicular to the surface of the to detect a normal component of the stray field oriented towards the test object and / or that the magnetic field probe (SM) is arranged to detect a parallel component of the magnetic field directed essentially parallel to the surface of the test object and parallel to the main magnetization direction.
15. Device according to one of claims 11 to 14, characterized in that the probe arrangement has a probe array with a plurality of leakage flux probes (SO) which are arranged next to one another in a first direction in a straight row, wherein preferably two or more magnetic field probes (SM) are provided for detecting the magnetization state, which are arranged at a distance from one another in a straight row in the first direction.
16. Device according to one of claims 11 to 15, characterized in that a number of magnetic field probes (SM) is smaller than a number of stray flux probes (SO), wherein preferably the number of stray flux probes (SO) is at least five times as high, preferably at least one order of magnitude larger than the number of magnetic field probes (SM) and / or that the stray flux probes are arranged at equal distances from one another and that the magnetic field probes are arranged at uneven distances from one another, wherein preferably in end regions of the probe arrangement a density of magnetic field probes is greater than in a central region of the probe arrangement.
17. Device according to one of claims 11 to 16, characterized in that the leakage flux probes (SO) are arranged on a side of the test head (PK) facing the test object (PR) and the magnetic field probes (SM) are arranged at a distance behind the leakage flux probes.