Method and apparatus for magnetic leakage testing of ferromagnetic test objects with signal normalization

Abstract

A method for magnetic flux leakage testing of a ferromagnetic test object, in particular a ferromagnetic pipeline, for detecting defects, comprising the following steps: magnetizing a test volume of the test object by means of an external magnetic field, in order to produce a magnetization state of the test volume, which can be characterized by the magnetization; and scanning the surface of the test object by means of a probe device having at least one magnetic field-sensitive magnetic flux leakage probe in order to detect a magnetic flux leakage field caused by a defect, the magnetic flux leakage probe maintaining a limited test distance from the surface of the test object and generating an electrical probe signal during the scanning, the electrical probe signals are metrics of the intensity of the leakage magnetic field. The method is characterized in that, using at least the provided magnetic field probe, the magnetization state of the test volume in the region of the magnetic flux leakage probe is determined in order to produce magnetization signals representing a measure of the magnetization state of the test object in the region of the magnetic flux leakage probe; normalizing the probe signals by means of the assigned magnetization signals in order to determine normalized probe signals; and evaluating the normalized probe signals to identify the defects.

Description

Technical Field

[0001] The invention relates to a method for conducting a magnetic flux leakage test on a ferromagnetic test object 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 . Background Art

[0002] The magnetic flux leakage method is an important component in the non-destructive defect testing of semi-finished and finished parts for quality monitoring during the manufacturing process and in the regular re-testing of finished parts. Compared to, for example, eddy current or ultrasonic testing, the magnetic flux leakage method is less sensitive to some interfering characteristics of the material, such as surface roughness or scale layers in hot-rolled products. This results in a better ratio between the useful signal and the interfering signal (N / S ratio), which enables more reliable defect identification.

[0003] In an apparatus for detecting defects by means of magnetic flux leakage measurement, a test volume of a test object is magnetized by means of a magnetizing device and the test volume of the test object is scanned by means of at least one magnetic field-sensitive magnetic flux leakage probe in order to detect the magnetic leakage field caused by the defects. In this case, a relative movement occurs between the magnetic flux leakage probe and the surface of the test object in a scanning direction. During the scanning, the magnetic flux leakage probe maintains a relatively small but finite test distance from the surface of the test object.

[0004] The magnetic flux or magnetic field generated by the magnetizing device in the test object is spatially distributed substantially uniformly in the defect-free material. In this case, no significant magnetic field gradients occur in the near-surface region either. Cracks and other defects, such as voids, inclusions or other inhomogeneities, such as welds, etc., act as areas of increased magnetic resistance, so that the magnetic field component near the defect is guided around the defect and is squeezed out of the metal into this near-surface area. In the magnetic flux leakage method, the magnetic field component squeezed out therefrom is detected in order to detect the defect. In magnetic flux leakage measurement, if the magnetic field component squeezed out of the test object reaches the area of ​​the magnetic flux leakage probe and causes a sufficient magnetic field change there to be detected, the defect can be detected.

[0005] The electrical probe signal, ie the electrical signal of the magnetic flux leakage probe or a signal derived therefrom, is evaluated by means of the evaluation device in order to identify the defect.

[0006] When testing pipelines, the aim is to detect external defects (i.e. flaws or defects on the outside of the pipeline) as well as internal defects (i.e. defects on the inside of the pipeline and defects in the pipeline wall). For this purpose, the DC field magnetization (DC magnetic flux leakage testing) method is usually used. Here, the obvious advantage of DC field magnetization is utilized, namely the large penetration depth, so that internal defects and flaws in the pipeline wall can also be detected.

[0007] It is also possible to test test objects in the form of bars. In the case of bar testing, alternating field magnetization is usually used (AC magnetic flux leakage test).

[0008] DE 102014212499 A1 discloses methods and devices of this type for magnetic flux leakage testing of ferromagnetic pipes, which allow reliable identification of defects regardless of length and angle and allow accurate differentiation between external and internal defects. The probe device has a probe array, which has a plurality of magnetic field sensitive probes, which are arranged side by side in the width direction. By using the probe array, the test width covered during the scanning process can be significantly larger than the test width covered by a single probe. Here, the spatial resolution in the width direction is determined by the probe width of each magnetic flux leakage probe. By using the probe array, the test object can be efficiently tested in a traversal process.

[0009] Tubes and bars should be tested as completely as possible. However, when testing over the entire length of the test part, there are often more or less long sections at the ends that are not tested. These sections, the so-called "untested ends", must be tested manually or automatically with additional equipment, or cut off and discarded. Each of these options causes additional processing time and losses for the manufacturer. Summary of the invention

[0010] Against this background, the object of the present invention is to provide a method and a device for magnetic flux leakage testing which allow reliable identification of defects even in cases where the magnetization of the test object is difficult to control. In particular, when testing ferromagnetic pipes or bars, it is necessary to minimize the number of untested ends.

[0011] 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 developments are described in the dependent claims. The wording of all claims is incorporated by reference into the description.

[0012] In the method according to the claimed invention, a test volume of a test object is magnetized by means of an external magnetic field in order to achieve a magnetization state of the test volume which can be characterized by the magnetization of the test volume. The magnetization is a physical quantity used to characterize the magnetic state of a material. The physical quantity is a vector field describing the density of permanent or induced magnetic dipoles in a magnetic material and is calculated as a magnetic moment per unit volume.

[0013] In the method, the surface of the test object is scanned by means of a probe device having at least one magnetic field-sensitive magnetic flux leakage probe for detecting the leakage magnetic field caused by defects. During the scan, the magnetic flux leakage probe maintains a limited test distance from the surface of the test object and generates electrical probe signals, which are a measure of the strength of the leakage magnetic field at the respective scanned position.

[0014] According to the claimed invention, the magnetization state of the test volume in the region of the magnetic flux leakage probe is additionally determined or measured. 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 object in the region of the magnetic flux leakage probe. The probe signals are normalized with the aid of the assigned magnetization signals in order to determine normalized probe signals. These normalized probe signals are then evaluated in order to identify defects.

[0015] The device according to the claimed invention is characterized in that the probe device has at least one magnetic field probe for generating magnetization signals which represent a measure of the magnetization state of the test object in the region of the magnetic flux leakage probe. The evaluation device is configured to perform a normalization of the probe signals by means of the assigned magnetization signals in order to generate normalized probe signals which can then be evaluated to identify defects.

[0016] The present invention is based in particular on the following recognition and considerations of the inventors. In an ideal test, the signal amplitude of the probe signal of the magnetic flux leakage probe when detecting a defect (hereinafter also referred to as the defect signal amplitude) should depend only on the geometry and orientation of the defect or flaw, so that the type and extent of the defect, such as the defect depth, etc., can be reliably ascertained based on the defect signal amplitude. In any case, the defect signals should be comparable to each other, so that the test sensitivity can be said to be relatively uniform regardless of the location of the defect.

[0017] However, it has been found that the fault signal amplitude also depends significantly on the magnetization level in the material in the area of ​​the test volume. However, this magnetization can only be controlled conditionally so that the test object is magnetized uniformly over its entire length. In conventional devices and methods, this prevents or impairs a reliable interpretation of the fault signal and, for example, when testing ferromagnetic pipes or bars, results in that the fault signal cannot be assessed with sufficient certainty, especially in the end area of ​​the test object. As a result, relatively long ends may remain untested.

[0018] According to the inventor's proposal, this problem is reduced or eliminated by measuring the magnetization state of the test volume in the region of the magnetic flux leakage probe using at least one magnetic field probe, which can generate magnetization signals that represent a measure of the magnetization state of the test object in the region of the magnetic flux leakage probe. The probe signals are then normalized with the aid of the assigned magnetization signals in order to determine normalized probe signals. These normalized probe signals are then evaluated to identify defects.

[0019] By normalizing with the aid of the magnetization signal, the defect signals or probe signals of the magnetic flux leakage probe can be made comparable to one another even if different defects are present in areas of different magnetization strengths. Thus, by continuously detecting the magnetization state and using this magnetization state to normalize or compensate the defect signal amplitude, a sufficiently uniform test sensitivity can be established. Thus, compared to the prior art, variations in the test sensitivity that depend on the magnetization effective in the test volume can be significantly reduced and, if necessary, suppressed to such an extent that during testing it can be assumed that the test sensitivity is sufficiently uniform for the test purpose.

[0020] In many embodiments, the magnetic field probe is a separate magnetic field-sensitive probe provided in addition to the magnetic flux leakage probe, i.e. the magnetic field probe is a separate functional element arranged in a suitable spatial relationship with at least one assigned magnetic flux leakage probe. The magnetic flux leakage probe and the at least one magnetic field probe can then be arranged in respective positions that are optimal for their measurement tasks, if necessary at a distance from one another. Furthermore, the signal transmission and evaluation can be optimized separately for the two probe types. The same probe principle can be used (e.g. Hall probe), however, the probes can also work according to different principles (e.g. inductive probe and Hall probe).

[0021] However, the magnetic flux leakage probe may also simultaneously assume the function of a magnetic field probe. Therefore, it is not necessary to provide a magnetic field probe in addition to the magnetic flux leakage probe. Rather, the magnetic flux leakage probe can also be used as a magnetic field probe. This integration makes use of the recognition that the same magnetic field-sensitive probe can fulfill both tasks, since the probe signal contains not only signal components that trace back to the defect detection, but also signal components that represent the magnetization to be measured. These signal components (defect signal components and magnetization signal components) can be separated or separated from each other for evaluation. The separation of the signal components can be achieved by electronic filtering components or by filtering algorithms. This is possible because, in the case of traversal testing, the defect signal components are in the relatively high-frequency range, while the magnetization signal components are at low frequencies.

[0022] According to one embodiment, the device has at least one test head, in which a probe arrangement is arranged or mounted, the probe arrangement having at least one magnetic flux leakage probe and at least one magnetic field probe in a fixed spatial relationship with respect to one another. As a result, a compact design can be achieved, and the assignment of the magnetic flux leakage probe to the assigned magnetic field probe (at least one, usually also a plurality) remains virtually unchanged during operation without further measures, thereby achieving permanently reliable results.

[0023] In principle, the magnetic field probe can be arranged outside the test head. However, the detected magnetization state should represent the position of the magnetic flux leakage probe to be compensated. This is best achieved by a spatial proximity between the magnetic field sensor and the magnetic flux leakage sensor and a gradient of the magnetization state that is as small as possible. Therefore, it is preferred to place the magnetic field probe in the test head.

[0024] In many cases, the magnetic flux leakage probe is arranged with its main sensitivity direction in such a way that a normal component of the magnetic leakage field oriented perpendicularly to the surface of the test object can be detected with high sensitivity.

[0025] Instead, it is preferably provided that a parallel component of the magnetic field in the close range around the magnetic flux leakage probe is measured to determine the magnetization state. The parallel component is that component which is directed essentially parallel to the surface of the test object and essentially parallel to the main magnetization direction or parallel to the field lines of the magnetization field. Thus, the magnetic field probe can be oriented with its main sensitivity direction more or less orthogonal to a surface normal oriented perpendicular to the surface of the test object and / or orthogonal to the main sensitivity direction of the magnetic flux leakage probe.

[0026] Alternatively or in addition thereto, the magnetic flux leakage probe can also detect changes in the parallel component of the magnetic field leakage. In this case, the main sensitivity directions of the magnetic flux leakage probe and the magnetic field probe are located in one plane and are parallel to each other if necessary.

[0027] Preferably, for detecting the magnetization state, a magnetic field component (parallel component) is measured which is directed essentially parallel to the surface of the test object and essentially parallel to the main magnetization direction.

[0028] The parallel field corresponds to that component of the magnetic field intensity at the surface of the test object which is parallel to the surface of the test object. In the longitudinal defect test, the main magnetization direction of the magnetization field runs essentially in the circumferential direction of the test object, and the parallel component runs in a plane perpendicular to the longitudinal axis of the test object. In this case, the parallel component is also referred to as the tangential component in this application. In particular, when the test object is a ferromagnetic pipe, it is advantageous to measure the tangential field.

[0029] In transverse defect testing, the main magnetization direction of the magnetization field runs substantially along the longitudinal direction or axial direction of the test object, and the parallel component runs substantially parallel to the longitudinal axis of the test object. In this case, the parallel component can also be called the axial component.

[0030] The term “substantially parallel” or “substantially tangential” means that slight deviations from the mathematically exact direction are possible, for example deviations of at most 20° or at most 15° or at most 10°.

[0031] The measurement of the magnetic field by means of a magnetic field component running parallel to the surface of the test object outside the test object takes into account the fact that the magnetization in the test object cannot be measured directly. In the case of pipeline testing, it has been found that the magnetization in the pipeline wall can be particularly well determined by means of parallel components, in particular by means of so-called tangential fields or T fields. The proportionality factor between the magnetization of the test object and the parallel or tangential field immediately on the pipeline surface corresponds to the ratio of the magnetic permeabilities of air and pipeline material. Thus, by measuring the magnetic field components in the close range around the magnetic flux leakage probe, the magnetization state in the test volume detected by the magnetic flux leakage probe can be determined very approximately.

[0032] In addition to the parallel component oriented parallel to the main magnetization direction, it can be advantageous to also measure the parallel component running orthogonally or obliquely thereto. This enables a two-dimensional magnetic field measurement. This two-dimensional magnetic field measurement can be advantageous, for example, for normalizing defect signals under non-ideal magnetization conditions and / or for characterizing oblique defects.

[0033] If (at least one) magnetic field probe is provided in addition to (at least one) magnetic flux leakage probe, then this magnetic field probe can be offset both in the radial direction and in the axial direction relative to the position of the magnetic flux leakage probe, which offset can be taken into account when interpreting the fault signal.

[0034] As an alternative to this, both the magnetic flux leakage signal and the magnetization signal can be recorded with the same probe. In this case, a magnetic field-sensitive probe, which is positioned at a limited test distance from the surface of the test object, detects the DC field and the alternating field components of the magnetic field in the direction of the main magnetization direction. Subsequent signal processing means separate the signal thus detected into a DC field component, which changes only slowly, and an alternating field component superimposed thereon. In the subsequent processing, the DC field component represents the magnetization state and the alternating field component represents the probe signal, which is a measure of the strength of the magnetic field leakage at the scanning position due to the defect.

[0035] In many cases, it is particularly advantageous if the magnetic flux leakage probe and the magnetic field probe are based on the same measuring principle and are mounted only with different orientations of their sensitivity directions. For example, the magnetic 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 intensity in the detection region of the test object.

[0037] In order to achieve efficient testing with a spatial resolution that is suitable for the test task and that is as high as possible, it is provided in a preferred embodiment that the probe device has a probe array, which has a plurality of magnetic flux leakage probes that are arranged side by side in a straight line along a first direction. Subsequently, preferably two or more magnetic field probes are provided to detect the magnetization state, which are arranged in a straight line parallel to the first direction at a certain distance from each other. It may also be sufficient to use only one magnetic field probe.

[0038] In this case, the number of magnetic field probes can be significantly smaller than the number of magnetic flux leakage probes, so that it is not necessary to assign a separate magnetic field probe to each magnetic flux leakage probe. Rather, it is possible that the magnetization at the position acting on a specific magnetic flux leakage probe can be derived from the magnetization signals detected by multiple magnetic field probes by interpolation. In some embodiments, the number of magnetic flux leakage probes is at least ten times the number of magnetic field probes, whereby, on the one hand, sufficient spatial resolution of the magnetic flux leakage test can be achieved and, on the other hand, the equipment expenditure for the magnetic field measurement can be limited.

[0039] Preferably, the magnetic flux leakage probes are arranged on the side of the probe arrangement that is to face the test object, and the magnetic field probe(s) are arranged at a distance behind these magnetic flux leakage probes, i.e. at a slightly greater distance from the test object. Thus, a high spatial resolution of defect detection by magnetic flux leakage measurement can be combined with a sufficiently accurate detection of the magnetization state at the individual magnetic flux leakage probes.

[0040] In some embodiments, the magnetic flux leakage probes are arranged at a uniform distance relative to each other, and the magnetic field probes are arranged at a non-uniform distance relative to each other, wherein preferably, in the end region of the probe device, the density of the magnetic field probes is greater than the density of the magnetic field probes in the middle region of the probe device. This may facilitate the detection of measured values ​​in the end region of the test object.

[0041] In a preferred embodiment, it is provided that the probe signal of the magnetic 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 the magnetization dependency of the test sensitivity. Within the framework of simultaneous evaluation of a plurality of magnetic flux leakage probes, this multiplication operation can be performed relatively quickly. The compensation factor may, for example, tend to be inversely proportional to the magnetization intensity of the test volume scanned by the magnetic flux leakage probe.

[0042] In preferred methods and devices, suitable compensation factors are not estimated based on theoretical relationships, but are determined very accurately based on measurements and tested extrapolations and / or interpolations. In some methods, calibration measurements are performed at a calibration portion of the test object provided with at least one calibration defect in order to determine a compensation curve, which describes the functional relationship between the magnetization state of the test object under external magnetic fields of different strengths, the corresponding magnetization signal of the magnetic field probe and the signal amplitude of the probe signal generated due to the calibration defect. Subsequently, when evaluating the probe signal, a compensation factor for normalizing the probe signal is derived from the compensation curve. Here, the term "calibration defect" describes a standard defect, the width and depth of which are usually specified by a standard in order to enable comparable test results.

[0043] In order to achieve the same probe signal for the same defect at different longitudinal positions of a test object (e.g. a ferromagnetic pipe to be tested), the magnetization must remain constant over the length of the test object. However, it has been found that, especially at the ends of the pipe or in the end regions of the test object, the actual magnetization can differ significantly from the magnetization in the middle region of the test object. Wall thickness variations (such as polygonal shapes or eccentricities due to manufacturing) and, for example, eccentric orientations of the test object and inductive effects in the event of magnetic field changes can also have a strong influence on the magnetization effectively present in the test volume.

[0044] In some methods, when determining the correction factor to be applied to the test section to be tested, the change in the magnetization state depending on the axial position of the test section is taken into account, in that when determining the correction factor, the axial offset between the calibration section and the test section is determined and the correction factor is modified as a function of this offset. By this means, it is achieved that even if the calibration measurement or the calibration with the standard defect is not carried out at the axial position of the defect to be evaluated later, an appropriate compensation of the defect signal amplitude can be achieved with a relatively low computational effort.

[0045] Extensive investigations by the inventors have shown that in many cases the correction factor can be determined for the axial position in the test section on the basis of a shifted compensation curve which has the curve shape of the compensation curve determined in the calibration section and which is shifted relative to the compensation curve determined in the calibration section only by a shift value corresponding to the axial offset. Due to this permitted simplification, particularly rapid calibration measurements can be achieved in order to determine a locally correct compensation factor for each axial position at the test object. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] 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 drawings.

[0047] FIG. 1 shows subsystems of an embodiment of an apparatus for magnetic flux leakage testing of a ferromagnetic test object, the subsystems having: a rotating subsystem for defect testing with a main component along the axis of the test object ( Figure 1A ) and a stationary subsystem for defect testing with a main component transverse to the axis of the test object ( Figure 1B );

[0048] Figure 2 shows the leakage magnetic field specific to the defect type on a cross section through the pipeline;

[0049] Figure 3 The design details of the probe device according to the embodiment and the main magnetization direction of the rotating subsystem directed transversely to the longitudinal axis of the pipeline are shown;

[0050] Figure 4 shows the design details of the probe device according to an embodiment and the main magnetization direction of the stationary subsystem directed along the longitudinal axis of the pipeline;

[0051] Figures 5A to 5C The schematic evolution of the magnetic field lines during the different stages of the traversal test is shown to illustrate the difference in magnetization between the end of the pipeline and the middle of the pipeline;

[0052] Figure 6 The schematic diagram shows the change process of the magnetization intensity when the test device traverses the pipeline;

[0053] Figure 7 shows a schematic side view of a test head according to an embodiment, the test head having approximately 100 magnetic flux leakage probes and five assigned magnetic field probes;

[0054] Figure 8 Shown in Figure 7 Schematic diagram of the device shown in the longitudinal direction of the pipeline during testing;

[0055] Fig. 9 shows a test head between two pole shoes, wherein the magnetic field strength varies along the axial direction of the test head;

[0056] Fig.10 shows a diagram presenting the dependence of the measured and interpolated T-field on the position along the longitudinal direction of the test head;

[0057] Fig.11 Compensation curves for internal and external defects are shown;

[0058] Fig.12 The shift of the compensation curve is shown;

[0059] Fig.13 shows the determination of the correction factor;

[0060] Fig.14 The compensation effect on the defect signal amplitude is schematically shown. DETAILED DESCRIPTION

[0061] In the following, an embodiment of the claimed invention is explained on the basis of an apparatus for conducting a magnetic flux leakage test on a ferromagnetic test object in the form of a hot-rolled ferromagnetic pipe in a traversing process. The apparatus is designed to detect different types of defects or incompleteness or discontinuity and can, for example, reliably detect rolling defects on the inside of the pipe (internal defects) as well as rolling defects on the outside of the pipe (external defects). In this case, longitudinal defects (defects with a main extension direction parallel to the longitudinal axis of the pipe) as well as transverse defects (defects with a main extension direction in the circumferential direction or perpendicular to the longitudinal axis of the pipe) and oblique defects (transverse to the longitudinal direction and transverse to the circumferential direction) can be reliably found and characterized.

[0062] In one embodiment, the two subsystems are integrated in a multi-test block. For longitudinal defect testing, a rotation subsystem is provided, based on Figure 1A For transverse defect testing, a stationary subsystem is provided, which has an annular device with a plurality of sensor arrays distributed around the circumference of the device, for example corresponding to Figure 1B These subsystems are arranged in sequence along the traversal direction of the pipeline, wherein the order can be arbitrary. In other embodiments not shown in more detail, a single system may be used, such as a single rotating system.

[0063] The rotating subsystem has a rotating head with an annular yoke RJ rotating around the test object PR, and the annular yoke has pole shoes PS radially oriented relative to the surface of the test object at diametrically opposite positions, and magnetizing windings MW are attached to these pole shoes. As a result, a magnetic flux or magnetic field MF (DC field) is generated in the pipeline wall, and the magnetic field lines of the magnetic flux or magnetic field run in the circumferential direction of the test object, that is, perpendicular to the longitudinal direction of the pipeline. On the rotor, test heads PK are arranged in the circumferential direction between the pole shoes, and these test heads respectively include one or more probe arrays SA, wherein each probe array includes a plurality of individual magnetic flux leakage probes SO.

[0064] During the test, the annular yoke together with the pole piece PS and the test head PK is rotated in about 30 minutes, depending on the type of rotating subsystem. -1 With about 1200min -1At the same time, the pipe to be tested is conveyed forward in the traversing direction at a test speed (e.g. up to 3 m / s or more). Here, the test head glides over the pipe surface and scans it seamlessly along a spiral path. The probes SO of the probe array are arranged within the test head at a small test distance AB from the surface OB of the test object, which test distance can be, for example, in the order of 0.2 mm to 2 mm (see Figure 3 ). Since the magnetic field lines run in the circumferential direction, the test is particularly sensitive to longitudinal defects LF-A on the outside of the pipe and LF-I on the inside of the pipe, which disturb the magnetic flux in the circumferential direction to the greatest extent and thus generate strong leakage magnetic fields ( Figure 2 ).

[0065] In a stationary system for transverse defect testing ( Figure 1B ), a DC field magnetization device, not shown in more detail, is used, which generates a magnetic field MF in the longitudinal direction of the traversed pipe. Two circles of probe arrays SA with spaced-apart probe arrays in the circumferential direction are arranged in a ring around the test object and scan the test object in the longitudinal direction of the test object during the traversal test. Since the magnetic flux runs in the longitudinal direction, transverse defects (QF-A) on the outside running in the circumferential direction and transverse defects (QF-I) on the inside will particularly strongly interfere with the magnetic flux, so that the device has a high test sensitivity for transverse defect testing.

[0066] The electrical signals SIG-SO, i.e. the probe signals, of the magnetic flux leakage probes of the probe array are supplied to a common evaluation device AW, in which the defects are identified. Since the probe signals are caused by defects or flaws during the test and characterize the defects or flaws, these probe signals are also referred to here as "flaw signals" or "test signals".

[0067] Each type of defect causes certain leakage magnetic fields that are specific to the defect type and whose characteristics can be identified from the signal shape and the frequencies contained in the signal. Figure 2 A cross section through the pipe perpendicular to the longitudinal direction and the magnetic field lines of the magnetizing field MF running in the circumferential direction are shown. An external defect LF-A running in the longitudinal direction generates a leakage magnetic field SF-A which is relatively narrowly concentrated near the external defect. On the other hand, an internal defect LF-I of the same size running in the longitudinal direction generates a less clear, more spatially blurred or enlarged or widened leakage magnetic field SF-I on the outside of the pipe, which has a smaller amplitude. Above the leakage magnetic field, the magnetic field lines when the probe exceeds the travel in the circumferential direction are shown respectively. Here, the y-axis corresponds to the signal amplitude A and the x-axis corresponds to the time t or the position at the overtravel of the probe.

[0068] Now, according to Figure 3 and Figure 4 , explained for the rotating system ( Figure 3 ) and stationary systems ( Figure 4 ). The probe arrangement SA-R for a 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 line along a first direction R1, which runs parallel to the longitudinal axis of the pipeline. The probe array SA is installed in a test head PK (see, for example, Figure 7 ). When the system rotates, the probe device as a whole moves in the circumferential direction of the test object around the test object in a second direction R2, which runs perpendicular to the first direction R1. Through the simultaneous longitudinal movement of the test object PR, each of these individual probes SO1, SO2 scans a relatively narrow test track PS, which runs in a spiral around the test object, wherein the test track runs obliquely with respect to the first direction and the second direction. All probes of the probe array jointly scan a relatively large test width, which has a plurality of test tracks parallel to each other.

[0069] A corresponding arrangement is obtained in the probe arrangement SA-T for transverse defect testing (see Figure 4 ). The probe device SA-T has a plurality of individual probes SO1, SO2, etc., which are arranged side by side in a row along a first direction R1, wherein the first direction here corresponds to the circumferential direction of the test object PR. The probe device is stationary, while the test object moves parallel to its longitudinal direction, so that the probe array scans the surface of the test object along a scanning direction, which corresponds to a second direction R2 perpendicular to the first direction R1. Here, each individual probe also covers a relatively narrow test track PS, wherein the entire test track along the circumferential direction produces a much larger test width of the probe device. The magnetic field MF running in the longitudinal direction of the pipe is squeezed out of the test object material at the transverse defect QF-A and is detected with the help of the probes of the probe array SA.

[0070] The height or amplitude of the defect signal (magnetic flux leakage signal) depends not only on the nature of the defect, but also on the strength of the magnetic field in the test object (i.e., for example, in the pipe wall) at the defect location. For example, in order for the same defect at different longitudinal positions of the pipe to produce the same defect signal during pipeline testing, the magnetization must remain constant over the length of the pipe. However, experience has shown that this is not the case. Especially at the end of the pipe, the local magnetization is different from the magnetization in the middle of the pipe (seen in the longitudinal direction). Variations in wall thickness may also cause magnetization fluctuations. In addition, there are dynamic effects in the establishment and disappearance of the magnetic field, especially when the pipe enters the test device and when it leaves.

[0071] As shown schematically in FIG5 , by inserting the pipe PR or the test object PR between the pole shoes, the magnetic field lines are drawn from the air into the ferromagnetic pipe with better conductivity ( Figure 5A This causes a higher degree of magnetization at the end of the pipe. Only when the pipe extends a certain length from the pole shoe again ( Figure 5C ), the magnetization reaches its nominal value at a greater distance from the end of the pipe. Depending on how fast the pipe is fed, the achievement of the nominal magnetization can be delayed or otherwise influenced, either based on inductive effects or by controlling the coil current.

[0072] Figure 6 The schematic diagram shows an exemplary course of the magnetization intensity when the test device traverses the pipeline. The abscissa indicates the position POS in the longitudinal direction (first direction R1), and the ordinate indicates the measure of the magnetization intensity MAG which will be explained later. When entering the EIN, a higher degree of magnetization is first generated due to the concentration of the magnetic field lines, and then the magnetization drops sharply due to transient processes. Then, the current regulation REG of the excitation coil counteracts this effect, so that over most of the traversal path or pipeline length, there is a desired nominal magnetization MAG-N with only small fluctuations. Then, when leaving the AUS, the effect of control and magnetic field line concentration occurs again (see Figure 5).

[0073] Magnetization fluctuations, especially at the entry and exit, can cause fluctuations in the test sensitivity and uncertainty, especially since it is unclear whether a strong defect signal should be attributed to a particularly large defect or to the strong magnetization. Therefore, the test results at the end of the pipe are not reliable enough and are therefore referred to as "untested ends".

[0074] In the following, it is explained how a uniform test sensitivity can be achieved substantially over the entire pipeline length according to embodiments of the invention. In this case, a uniform test sensitivity is established essentially by continuously detecting the magnetization state of the test object and using this magnetization state to normalize or compensate the fault signal amplitude.

[0075] in accordance with Figure 7 and Figure 8 , explaining some design measures that are implemented in an embodiment to promote uniformity of test sensitivity. Figure 7 A schematic side view of a test head PK is shown, which is arranged in a test configuration at a distance AB from the surface of a test object PR. On the side facing the test object, a probe array SA is attached, which has a large number of magnetic flux leakage probes SO or test probes SO arranged in a straight line, for example 40 or more or 70 or more, in the example case between 90 and 100 mutually identical magnetic flux leakage probes.

[0076] A smaller number of magnetic field probes SM1 to SM5 are arranged shortly behind the probe array, more precisely likewise in a straight line. Here, these magnetic field probes are arranged at uniform distances relative to one another, however, these distances can also be non-uniform, in particular the distances in the end regions are smaller than in the middle. The arrangement is selected so that: each of these magnetic flux leakage probes is assigned at least one magnetic field probe, which can generate a magnetization signal, which represents a measure of the magnetization state of the test object in the region of this magnetic flux leakage probe. For example, by interpolation, the magnetization at the location of the magnetic flux leakage probe SO30 can be determined with the help of the magnetization signals of the two closest magnetic field probes SM2 and SM3, as will be explained later.

[0077] Figure 8 Shown in Figure 7 Schematic diagram in the longitudinal direction of the pipe of the device shown in . Here, the test head PK is shown just above the longitudinal defect LF-A on the outside of the pipe.

[0078] The test head PK shown is designed to determine a measure of the magnetization of the test object by measuring a magnetic field component parallel to the test object surface and parallel to the main magnetization direction, which magnetic field component can be referred to as a parallel component here. More precisely, the so-called tangential field or T field is measured. Therefore, the value of the measured magnetization signal is also called the T field value. The tangential field TAN corresponds to the following component of the magnetic field strength on the surface of the test object, which corresponds to the parallel component that runs tangentially to the pipeline, that is, parallel to the surface in a plane perpendicular to the longitudinal axis of the pipeline and parallel to the magnetic field lines of the magnetic field MF running in the circumferential direction. The radial component RAD of the magnetic field that can be measured in the area of ​​the surface runs orthogonally to this, that is, in the normal direction of the pipeline.

[0079] According to the inventors' knowledge, the measurement of the tangential field is particularly suitable for determining the magnetization when testing for longitudinal defects, for example, on pipes, since the proportionality factor between the magnetization in the test object, i.e. here in the interior of the pipe material, and the T field close to the pipe surface corresponds to the magnetic permeability of the air (μ L ) and the magnetic permeability of the pipe material (μ R ) ratio.

[0080] Ideally, the main magnetization direction in the rotating system runs along the circumference of the pipeline completely perpendicular to the longitudinal axis of the pipeline. In particular, when testing defects that are not completely aligned with the longitudinal axis of the pipeline, deviations from the ideal orientation of the magnetic field lines will cause changes in the leakage magnetic signal, which may damage the accuracy of the test results. Normalization of this change can be achieved by additionally measuring a magnetic field component that is substantially perpendicular to the main magnetization direction and parallel to the longitudinal axis of the pipeline. This component can be called an orthogonal component because it is oriented perpendicular to the main magnetization direction. In the current case, this component can also be called an axial component because it is parallel to the axial direction of the test object in this measurement configuration. This component can be detected by one or more additional magnetic sensitive probes. In another embodiment, the same magnetic field probe that detects parallel fields can also detect the component of the magnetization state perpendicular to the main magnetization direction, or can also detect the size and angle of the magnetization state. Therefore, if magnetization is detected in two directions that are orthogonal to each other or inclined to each other and are in the tangent plane (parallel to the surface of the test object), this effect can still be detected and considered when the defect signal is normalized.

[0081] On the other hand, the leakage magnetic field probes (test probes, defect probes) SO are designed so that they measure the radial component RAD of the magnetic field strength on the surface. This radial component is formed in particular due to magnetic leakage at defects, i.e. at locations where magnetic field lines are squeezed out of the pipe material due to defects. Figure 8 In the figure, arrows are used to represent the different sensitivity directions of the magnetic flux leakage probe (measuring the radial component) and the magnetic field probe (measuring the T field).

[0082] Here, the magnetic flux leakage probe SO and the magnetic field probe SM are of the same probe type, namely a Hall probe. These probes are identical to each other in structure, but they differ from each other in their main sensitivity direction ( Figure 8 Another difference is that the signal of the T field is detected by DC coupling (direct current field coupling), while the magnetic flux leakage probe (defect detection probe) works by AC coupling (alternating field coupling), that is, only the change of the leakage magnetic field is detected.

[0083] The magnetic field probes (or T-field probes) can be positioned in the test head, for example, as follows: an intermediate magnetic field probe SM3 in the middle, one magnetic field probe (SM1, SM5) at each of the axial ends of the probe array, and one magnetic field probe (SM2, SM4) between the magnetic field probes at these ends and the intermediate magnetic field probe. Thus, five magnetic field probes may be sufficient. Before specifying the positions of the T-field probes, the direction of the tangential field in the axial direction of the pole piece, the air gap and the pipe size used should be known. Based on this, it may be useful, for example, to replace the uneven axial distances between the magnetic field probes (see Figure 7 ), providing a non-uniform distance in such a way that the magnetic field probes are placed closer together in the axial edge region.

[0084] Unlike magnetic flux leakage probes SO which should be arranged as close as possible to the surface of the test object for defect detection in order to detect high-frequency magnetic field changes, magnetic field probes can have a greater distance from the test object because they tend to detect low-frequency magnetic field changes.

[0085] Now, according to Fig. 9 and Fig.10 , explaining the design aid for arranging the magnetic field probes. For the required T-field compensation (to make the test sensitivity uniform), each magnetic flux leakage probe SO should have an associated T-field value. In the case of five magnetic field probes SM1 to SM5, the T-field values ​​of those magnetic flux leakage probes that are not arranged directly below the magnetic field probe are interpolated.

[0086] to this end, Fig. 9 The test head PK is shown between two pole shoes PS. The magnetic field generated in the region of the test head varies in the axial direction, wherein the magnetic field strength (indicated by the length of the arrows) is greater in the central region than near the axial ends.

[0087] Fig.10 A corresponding graph is shown, which shows the dependence of the measured T field TF on the position along the test head. The cross represents the magnetic field signal (T field value) of the magnetic field probe. The dashed line represents the linearly interpolated T field value, and the solid line represents the actual T field curve. In the middle area (magnetic field probes SM2, SM3 and SM4), the interpolation is close to the actual value, while in the area with a strong axial gradient of the magnetic field strength near the end of the pipeline, a large deviation occurs. In the example case, these deviations can be reduced by positioning the second and fourth magnetic field probes closer to the end (dashed line position), respectively, so that the distance between the magnetic field probes in the axial direction can be uneven.

[0088] Now, according to Figures 11 to 13 , explaining the measures for compensating for axial inhomogeneous magnetization in this embodiment. The method takes into account that different strengths of magnetization of the pipe wall in the axial direction will cause defect signals of different heights for the same defect. By measuring the T field with a magnetic field probe SM or using a factor calculated therefrom, the height of the defect signal (the amplitude of the defect signal) is corrected for different magnetizations.

[0089] In this method, (at least one) compensation curve is determined (see Fig.11). For this purpose, calibration measurements are performed. During the T-field calibration, the test probe cyclically detects two calibration defects of known size, i.e. an internal defect and an external defect, while the current intensity of the excitation coil (or measuring winding MW) at the pole shoe is increased from a minimum value to a maximum value. For each set current intensity, the test head overtravels these calibration defects at least once, during which the leakage field signal (defect signal) and the associated T-field value are recorded. Fig.11 A schematic diagram is shown in which the abscissa plots the current intensity of the excitation coil or the associated T-field TF and the ordinate plots the normalized signal intensity SIG of an external defect (solid line AF) and an internal defect (dashed line IF) N .

[0090] Thus, for each T field, two value pairs are generated using the T field calibration, namely a defect signal above the T field value (external defect) and a defect signal above the T field value (internal defect). Based on the measured value pairs, the compensation curves AF or IF shown can then be interpolated, which assign a compensation value for the defect signal to each T field value.

[0091] It can be seen that the measured T-field values ​​for the same current intensity or the same magnetization state vary in the axial direction. This variation may be affected by different pole shoe geometries. For many conventional pole shoes, for example, the T-field values ​​at the axial edges may be smaller than in the middle. Studies by the inventors have shown that, regardless of this axial variation, the direction of the compensation curve, i.e. the shape of the compensation curve, appears to be essentially independent of the axial position, i.e., independent of at which test head position (with which magnetic field probe) the T-field is measured.

[0092] In this method, a so-called shift value is also determined. Fig.12 The shift VS of the shift value between two compensation curves is schematically illustrated, wherein the solid line corresponds to an external defect in a region of higher magnetic field strength and the solid line corresponds to the same external defect in a region of lower magnetic field strength.

[0093] Since the shape of the compensation curve (which indicates the functional relationship between the T field strength and the resulting signal amplitude recorded on the y-axis) does not change at different feed positions, the T-field compensation in this embodiment requires only two compensation curves, one for external defects and one for internal defects. Additionally, a shift value is determined for each of these magnetic flux leakage probes. The difference between the T field value of the magnetic flux leakage probe and the T field value of the middle T field probe can be selected as the shift value. The shift value VS should be determined individually for each test head.

[0094] In the method, for T-field compensation, the T-field value of the corresponding magnetic flux leakage probe (corrected for the shift value) is used, and a reference value REF is used. The reference value is selected, for example, so that: the reference value corresponds to the following T-field value, in the case of which the factor for correcting the signal amplitude is equal to one. Preferably, the T-field value of the intermediate magnetic field probe from the calibration defect at the nominal current intensity is selected as the reference value. For T-field values ​​above the reference value, the defect signals of the corresponding magnetic flux leakage probe are equipped with a factor less than one (<1), and in other cases (T-field values ​​below the reference value), these defect signals are equipped with a factor greater than 1 (>1). Fig.13 This is illustrated by the curve for an external defect, where the horizontal axis represents the T field strength and the vertical axis represents the reference value T REF Let's say the factor FAK is equal to one.

[0095] Now, according to Fig.14 , the effect of the compensation strategy is explained with a schematically illustrated example. The upper part shows a test object PR, at which a longitudinal external defect LF-A is introduced as a calibration defect. The arrows in the test object represent the magnetization, the thickness of the arrows indicates the magnetization intensity that varies in the axial direction. The left part EIN is intended to represent the entry phase, the middle part DYN illustrates the control-related dynamic effects close in time to this entry phase, and the right part NOR illustrates the situation at a greater distance from the end of the pipeline, where a stable normal magnetization state is generated.

[0096] The diagram shown below shows the amplitude of the measured T field, i.e. the strength of the magnetization signal of the magnetic field probe, with the solid line T. The dashed line SIG-SO schematically represents the defect signal, i.e. the magnetic flux leakage signal of the magnetic flux leakage probe SO. In all three cases, the size of the defect in the test object is the same, so that ideally (in the case of axially uniform magnetization) the same defect signal amplitude should appear in all three cases.

[0097] In practice, however, the curve SIG-SO shows that: during the entry phase EIN, the magnetization is relatively high and, correspondingly, the measured T field is relatively high, with a relatively high defect signal amplitude. In the region of dynamic effects or transient processes, where relatively low magnetic field strengths may occur, the defect signal is significantly weaker than during this entry phase. Only at a greater distance from the end of the pipe does the defect signal appear with its "real" amplitude, which corresponds to the geometry of the defect.

[0098] In tests where two longitudinal external defects of the same size were introduced in the pipe end region at different distances from the pipe end (100 mm and 250 mm), the defect closer to the pipe end produced a defect signal that was up to 6 dB higher than the one farther from the pipe end. A similar situation was observed for internal defects.

[0099] Now, Fig.14 The lower diagram in shows the compensation effect. In this diagram, the solid line FAK represents the factor explained above, which indicates by what value the measured defect signal amplitude must be multiplied in order to obtain the real amplitude of the defect according to the compensation. This factor is lower during entry than the value that occurs in normal conditions (on the right side of the diagram). As a result, the amplitude of the defect signal is reduced. In the DYN area of ​​dynamic effects, the defect signal is slightly amplified, in the area of ​​normal conditions the factor is approximately one, which means that here the "correct" signal amplitude is measured directly.

Claims

1. A method for performing a magnetic flux leakage test on a ferromagnetic test object, in particular a ferromagnetic pipeline, to detect defects, the method comprising: magnetizing a test volume of the test object by means of an external magnetic field to produce a magnetization state of the test volume that can be characterized by the magnetization; The surface of the test object is scanned by means of a probe device having at least one magnetic field-sensitive magnetic leakage probe in order to detect leakage magnetic fields caused by defects, wherein: During the scanning, the magnetic flux leakage probe maintains a limited test distance from the surface of the test object and generates an electric probe signal, which is a measure of the strength of the magnetic leakage field, Features: determining, using at least one magnetic field probe, a magnetization state of the test volume in the region of the magnetic flux leakage probe to generate a magnetization signal representing a measure of the magnetization state of the test object in the region of the magnetic flux leakage probe; normalizing the probe signal by means of the assigned magnetization signal to determine a normalized probe signal; The normalized probe signal is evaluated to identify the defect.

2. The method according to claim 1, characterized in that A magnetic field-sensitive probe separate from the magnetic flux leakage probe is used as the magnetic field probe, which is provided in addition to the magnetic flux leakage probe.

3. The method according to claim 1 or 2, characterized in that: A parallel component of the magnetic field in a close range around the magnetic flux leakage probe, which component is directed substantially parallel to the surface of the test object and parallel to the main magnetization direction, is measured to determine the magnetization state.

4. The method according to any one of the preceding claims, characterized in that The test object is a ferromagnetic pipe, wherein preferably a magnetic field component directed essentially tangentially to the surface of the test object is measured to detect the magnetization state.

5. The method according to any one of the preceding claims, characterized in that A DC field component of the magnetization signal is determined and the probe signal is normalized using the DC field component.

6. The method according to any one of the preceding claims, characterized in that The probe signal of the magnetic flux leakage probe has a signal amplitude; and in order to normalize the probe signal, the signal amplitude is multiplied by a compensation factor, the compensation factor at least partially compensating for the magnetization dependence of the test sensitivity, wherein, preferably, the compensation factor is substantially inversely proportional to the magnetization strength of the test volume scanned by the magnetic flux leakage probe.

7. The method according to any one of the preceding claims, characterized in that: performing calibration measurements at a calibration portion of the test object provided with at least one calibration defect to determine a compensation curve describing a functional relationship between the magnetization state of the test object under external magnetic fields of different strengths, the corresponding magnetization signal of the magnetic field probe and the signal amplitude of the probe signal resulting from the standard defect; and When evaluating the probe signal, a compensation factor for normalizing the probe signal is derived from the compensation curve.

8. The method according to claim 7, characterized in that: When determining the correction factor to be applied to the test section to be tested, the change of the magnetization state depending on the axial position of the test section is taken into account in that, when determining the correction factor, the axial offset between the calibration section and the test section is determined and the correction factor is modified as a function of the offset.

9. The method according to claim 7 or 8, characterized in that: A correction factor is determined for the axial position in the test section based on a shifted compensation curve, wherein the shifted compensation curve has a curve shape of a compensation curve determined in the calibration section, wherein the compensation curve is shifted by a shift value corresponding to the axial offset relative to the compensation curve determined in the calibration section.

10. The method according to any one of the preceding claims, characterized in that The probe device has a probe array, the probe array has a plurality of magnetic flux leakage probes, the magnetic flux leakage probes are arranged side by side along a first direction, wherein, preferably, in order to detect the magnetization state, two or more magnetic field probes are provided, the magnetic field probes are arranged at a certain distance from each other along the first direction, wherein, preferably, the number of the magnetic field probes is less than the number of the magnetic flux leakage probes.

11. A device for performing a magnetic flux leakage test on a ferromagnetic test object, in particular a ferromagnetic pipeline, to detect defects, the device comprising: a magnetizing device for magnetizing a test volume of the test object (PR); A probe device (SA) having at least one magnetic flux leakage probe (SO) for detecting a magnetic flux leakage caused by a defect, wherein: The magnetic flux leakage probe (SO) is configured to: during scanning, maintain a limited test distance (AB) from the surface of the test object and generate an electrical probe signal having a defect signal amplitude depending on the magnetic flux leakage, the electrical probe signal being a measure of the strength of the magnetic leakage field; an evaluation device (AW) for evaluating said probe signal to identify said defect, It is characterized in that The device has at least one magnetic field probe (SM) for generating a magnetization signal representing a measure of the magnetization state of the test object (PR) in the region of the magnetic flux leakage probe (SO); and The evaluation device (AW) is configured to perform a normalization of the probe signal by means of the assigned magnetization signal to determine a normalized probe signal and to evaluate the normalized probe signal to identify the defect.

12. The device according to claim 11, characterized in that The magnetic field probe (SM) is a magnetic field-sensitive probe which is separate from the magnetic flux leakage probe (SO) and is provided in addition to the magnetic flux leakage probe (SO).

13. The 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) is arranged, which has at least one magnetic flux leakage probe (SO) and at least one magnetic field probe (SM) which are arranged in a fixed spatial relationship with respect to one another.

14. The device according to any one of claims 11 to 13, characterized in that The magnetic flux leakage probe (SO) is arranged to detect a normal component of the magnetic leakage field which is oriented substantially perpendicular to the surface of the test object; and / or the magnetic field probe (SM) is arranged to detect a parallel component of the magnetic field which is oriented substantially parallel to the surface of the test object and parallel to the main magnetization direction.

15. The device according to any one of claims 11 to 14, characterized in that The probe device has a probe array, which has a plurality of leakage magnetic probes (SO), which are arranged side by side in a straight line along a first direction, wherein, preferably, two or more magnetic field probes (SM) are provided to detect the magnetization state, and the magnetic field probes are arranged in a straight line along the first direction at a certain distance from each other.

16. The device according to any one of claims 11 to 15, characterized in that The number of magnetic field probes (SM) is less than the number of magnetic flux leakage probes (SO), wherein, preferably, the number of magnetic flux leakage probes (SO) is at least five times the number of magnetic field probes (SM), preferably at least one order of magnitude larger than the number of magnetic field probes; and / or the magnetic flux leakage probes are arranged at a uniform distance relative to each other, and the magnetic field probes are arranged at a non-uniform distance relative to each other, wherein, preferably, in the end area of ​​the probe device, the density of the magnetic field probes is greater than the density of the magnetic field probes in the middle area of ​​the probe device.

17. The device according to any one of claims 11 to 16, characterized in that The magnetic flux leakage probe (SO) is arranged on a side of the test head (PK) to face the test object (PR), and the magnetic field probe (SM) is arranged at a certain distance behind the magnetic flux leakage probe.

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