Method and device for a floating magnetic flux test of a ferromagnetic test material using signal normalization

Abstract

The present invention relates to a method for detecting defects in ferromagnetic test materials, in particular for the flux leakage test of ferromagnetic pipes, comprising magnetizing a test volume of the test material by means of an external magnetic field in order to generate a magnetization state of the test volume that can be characterized by magnetization, and scanning the surface of the test material by means of a probe arrangement having at least one magnetic field-sensitive flux leakage probe for detecting the flux leakage field caused by the defects, the flux leakage probe being held at a finite test distance from the surface of the test material during scanning and generating an electrical probe signal which is a measure of the strength of the flux leakage field. The method is characterized by determining the magnetization state of the test volume in the region of the flux leakage probe and generating a magnetization signal representing that measure, determining a probe signal normalized by the assigned magnetization signal, and evaluating the normalized probe signal to identify defects.

Description

Technical Field

[0001] The present invention relates to a method for a leakage flux test of a ferromagnetic test material for detecting defects according to the preamble of claim 1, and further to an apparatus suitable for carrying out this method according to the preamble of claim 11.

Background Art

[0002] In the context of non-destructive inspection of defects in semi-finished and finished parts, the leakage flux method is an important component for monitoring quality both during the manufacturing process and during periodic repeated tests of finished parts. The leakage flux method is less affected, for example, by some interfering characteristics of the material such as surface roughness or scale coating in the case of hot-rolled products than, for example, eddy current methods or ultrasonic testing. This results in a better ratio (S / N ratio) between the signal used and the noise signal, and as a result, more reliable flaw discrimination becomes possible.

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

[0004] The magnetic flux or magnetic field generated by the magnetization device of the test material is spatially substantially uniformly distributed in a defect-free material. In this case, no significant magnetic field gradient occurs even in the region close to the surface. Cracks and other defects, such as voids, inclusions, etc., or other inhomogeneities, such as weld lines, etc., act as regions of increased magnetic resistance. Therefore, the magnetic field components near the defect are induced around the defect and pushed out from the metal to the region near the surface. The pushed-out magnetic field components are detected by the magnetic flux leakage method for detecting defects. In magnetic flux leakage measurement, a defect can be detected when the magnetic field components displaced from the test piece reach the region of the magnetic flux leakage probe and cause a magnetic field change sufficient for detection there.

[0005] The electrical probe signal, that is, the electrical signal of the magnetic flux leakage probe or the signal derived therefrom, is evaluated by an evaluation device to identify defects.

[0006] Testing of pipes involves attempting to detect both external flaws, that is, flaws or defects on the outside of the pipe, and internal flaws, that is, flaws on the inside of the pipe and further flaws in the pipe wall. A method using DC magnetic field magnetization (DC magnetic flux leakage test) is usually used for this purpose. The significant advantage of DC magnetic field magnetization, that is, a large penetration depth, is utilized here, and as a result, internal flaws and flaws in the pipe wall can also be detected.

[0007] A rod-shaped test material can be tested in a similar manner. In rod testing, generally, AC magnetic field magnetization is used (AC magnetic flux leakage test).

[0008] DE 10 2014 212 499 discloses a general method and apparatus for the magnetic flux leakage testing of ferromagnetic pipes, which method and apparatus enable reliable identification of defects regardless of length and angle and accurate discrimination between external and internal defects. The probe configuration has a probe array including a number of magnetic field sensitive probes arranged adjacent to each other in the width direction. By using the probe array, it is possible to make the test width covered during the scanning process significantly larger than the test width covered by a single probe. In this case, the spatial resolution in the width direction is determined by the probe width of the individual magnetic flux leakage probes. By using the probe array, it becomes possible to efficiently test the test object in a continuous manner.

[0009] Pipes and rods should be tested as completely as possible. However, usually, for the entire length of the test section in the test object, there remains an untested length of a certain part at the end, albeit with a difference in degree. These parts, the so-called "untested ends", have to be tested or cut and discarded either manually or by an automated method using additional equipment. Each of these options results in additional processing time and losses for the manufacturer. SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[0010] In view of this background, the problem addressed by the present invention is to provide a method and an apparatus for magnetic flux leakage testing that enable reliable identification of defects even when it is difficult to control the magnetization of the test object. In particular, when testing ferromagnetic pipes or rods, it is intended to achieve a significant reduction in the untested ends as much as possible.

[0011] To solve this problem, the present invention provides a method having the features described in claim 1 and an apparatus having the features described in claim 11. Advantageous developments are specified in the dependent claims. The expressions of all claims are incorporated herein by reference into the content of this specification.

Means for Solving the Problem

[0012] In the method according to the claimed invention, the test volume of the test material is magnetized using an external magnetic field in order to reach the magnetization state of the test volume that can be characterized by magnetization. Magnetization is a physical variable for characterizing the magnetic state of a material. It is a vector field that describes the density of permanent magnetic dipoles or induced magnetic dipoles in a magnetic material and is calculated as the magnetic moment per unit volume.

[0013] In this method, the surface of the test material is scanned by a probe configuration having at least one magnetic field sensitive leakage magnetic flux probe in order to detect the leakage magnetic field caused by defects. During the scanning, the leakage magnetic flux probe is held at a finite test distance from the surface of the test material and generates an electrical probe signal that is a measure of the intensity of the leakage magnetic field at each scanned location.

[0014] According to the claimed invention, the magnetization state of the test volume in the region of the leakage magnetic flux probe is further determined or ascertained. At least one magnetic field probe that generates a magnetization signal representing a measure of the magnetization state of the test material in the region of the leakage magnetic flux probe is utilized for this purpose. The probe signal is normalized by the assigned magnetization signal in order to determine a normalized probe signal. The normalized probe signal is then evaluated to identify defects.

[0015] The device according to the claimed invention is distinguished by the fact that the probe configuration has at least one magnetic field probe in order to generate a magnetization signal representing a measure of the magnetization state of the test material in the region of the leakage magnetic flux probe. The evaluation device is configured to normalize the probe signal by the assigned magnetization signal in order to generate a normalized probe signal, and then the normalized probe signal can be evaluated to identify defects.

[0016] The present invention is based, inter alia, on the following insights and considerations of the inventors. In the case of an ideal test, the signal amplitude of the probe signal of the leakage flux probe during flaw detection (hereinafter also referred to as flaw signal amplitude) should depend only on the geometric shape and position of the flaw or defect, so that as a result, the type and extent of the flaw, such as flaw depth, etc., can be reliably established based on the flaw signal amplitude. In any case, the flaw signals should be considered equivalent to each other so that it can be said that they have a relatively uniform test sensitivity independent of the location of the flaw.

[0017] However, it has been confirmed that the flaw signal amplitude essentially also depends on the magnitude of the magnetization of the material in the region of the test volume. However, this magnetization can only be controlled within a limited range so that the test object is magnetized uniformly over its entire length. In conventional devices and methods, this hinders or adversely affects a reliable interpretation of the flaw signal, for example, during the testing of ferromagnetic pipes or rods, there is an effect that the flaw signal cannot be evaluated with sufficient certainty, especially in the region of the ends of the test material. A relatively long untested end may consequently remain.

[0018] According to the proposal of the inventors, this problem is reduced or eliminated by the magnetization state of the test volume in the region of the leakage flux probe being determined metrologically using at least one magnetic field probe capable of generating a magnetization signal representing a measure of the magnetization state of the test material in the region of the leakage flux probe. The probe signal is then normalized by the assigned magnetization signal in order to determine the normalized probe signal. The normalized probe signal is then evaluated to identify the defect.

[0019] By normalizing using the magnetization signal, even when different defects exist in regions having different magnetization intensities, the flaw signals or probe signals of the leakage magnetic flux probe can be made equivalent to each other. In this way, by continuously detecting the magnetization state and using this magnetization state to normalize or compensate the flaw signal amplitude, a sufficiently uniform test sensitivity can be generated. As a result, it is possible to significantly reduce the variation in test sensitivity that depends on effective magnetization within the test volume as compared with the prior art, and, where appropriate, suppress the variation to such an extent that a sufficiently uniform test sensitivity for the test purpose can be assumed during the test.

[0020] In many embodiments, the magnetic field probe is a separate magnetic field sensitive probe provided in addition to the leakage magnetic flux probe, i.e., a separate functional element arranged in an appropriate spatial relationship with respect to at least one assigned leakage magnetic flux probe. Then, the leakage magnetic flux probe and the at least one magnetic field probe can optionally, in each case, be arranged at a distance from each other at positions that are optimal for the measurement task. Moreover, signal transmission and evaluation can be optimized separately for both types of probes. The same probe principle may be utilized (e.g., Hall probe), but the probes may also operate according to different principles (e.g., induction probe and Hall probe).

[0021] However, it is also possible for a leakage flux probe to simultaneously perform the function of a magnetic field probe. As a result, there is no need to provide a magnetic field probe in addition to the leakage flux probe. Rather, the leakage flux probe can also be used as a magnetic field probe. This integration utilizes the insight that the same magnetic field sensitive probe can perform both tasks because the probe signal contains both a signal component due to the detection of a flaw and a signal component representing the magnetization to be measured. These signal components (the flaw signal component and the magnetization signal component) can be separated from each other for evaluation. The separation of the signal components can be achieved by an electronic filter component or a filter algorithm. This is possible because during continuous testing, the flaw signal component is in a relatively high frequency range, while the magnetization signal component is at a low frequency.

[0022] According to one development, the device has at least one test head, and within the test head, a probe configuration including at least one leakage flux probe and further including at least one magnetic field probe is arranged or mounted in a fixed spatial relationship to each other. As a result, a compact setup can be achieved, and the assignment between the leakage flux probe and the assigned magnetic field probe(s) (at least one, often further multiple) remains virtually unchanged during operation without further measures, and as a result, a permanently reliable result can be achieved.

[0023] In principle, it is possible to arrange the magnetic field probe outside the test head. However, the detected magnetization state should represent the location of the leakage flux probe to be compensated. The best way to do this is by the spatial proximity between the magnetic field probe and the leakage flux probe and the smallest possible gradient of the magnetization state. Therefore, it is preferred to house the magnetic field probe in the test head.

[0024] Often, the leakage flux probe is in a situation where it is arranged in the main sensitivity direction so that it can detect the normal component of the leakage magnetic field directed perpendicular to the surface of the test object with high sensitivity.

[0025] In contrast thereto, preferably, in order to determine the magnetization state, it is achieved that the parallel component of the magnetic field is measured at a short distance around the leakage flux probe. The parallel component is the component directed substantially parallel to the surface of the test material and substantially parallel to the magnetic field lines of the main magnetization direction or magnetization field. Thus, the magnetic field probe can be aligned such that its main sensitivity direction is approximately orthogonal to the surface normal directed perpendicular to the surface of the test body and / or to the main sensitivity direction of the leakage flux probe.

[0026] As an alternative to or in addition thereto, the leakage flux probe can also detect changes in the parallel component of the leakage magnetic field. In this case, the main sensitivity directions of the leakage flux probe and the magnetic field probe will be present parallel to each other within one plane, optionally.

[0027] Preferably, in order to detect the magnetization state, the magnetic field component (parallel component) directed substantially parallel to the surface of the test material and to the main magnetization direction is measured.

[0028] The parallel magnetic field corresponds to that component of the magnetic field strength at the surface of the test material that extends parallel to the surface of the test material. During longitudinal flaw testing, the main magnetization direction of the magnetization field extends substantially in the circumferential direction of the test material, and the parallel component extends within a plane perpendicular to the longitudinal axis of the test material. In this case, in the present application, the parallel component is also referred to as the tangential component. Measuring the tangential magnetic field is particularly advantageous when the test material is a ferromagnetic pipe.

[0029] During transverse flaw testing, the main magnetization direction of the magnetization field extends substantially in the longitudinal or axial direction of the test material, and the parallel component extends substantially parallel to the longitudinal axis of the test material. In this case, the parallel component may also be referred to as the axial component.

[0030] The indication “substantially parallel” or “substantially tangential” means that small deviations from the mathematically exact direction are possible, for example, deviations of up to 20°, up to 15°, or up to 10°.

[0031] Magnetic field measurements by magnetic field components that extend parallel to the surface of the test specimen outside the test specimen take into account that the magnetization of the test specimen cannot be directly measured. In the case of pipe testing, it has been found that the magnetization of the pipe wall can be particularly appropriately derived by the parallel component, in particular by the so-called tangential magnetic field or T magnetic field. The proportionality coefficient between the magnetization of the test material and the parallel magnetic field or tangential magnetic field exactly at the pipe surface corresponds to the ratio of the magnetic permeability of air to the magnetic permeability of the pipe material. Therefore, the magnetization state of the test volume detected by the leakage flux probe can be determined with good approximation by measuring the magnetic field components at a short distance around the leakage flux probe.

[0032] In addition to the parallel component oriented parallel to the main magnetization direction, it may also be advantageous to measure a parallel component that extends orthogonally or obliquely to the main magnetization direction. As a result, two-dimensional magnetic field measurement is realized. The latter may be advantageous, for example, for normalizing flaw signals under non-ideal magnetization conditions and / or for determining the characteristics of oblique flaws.

[0033] (At least one) magnetic field probe is provided in addition to (at least one) leakage flux probe, and the magnetic field probe can be offset with respect to the leakage flux probe both radially and axially with respect to the location of the leakage flux probe, and this offset can be taken into account in the interpretation of flaw signals.

[0034] Alternatively, both the leakage flux signal and the magnetization signal may be recorded by the same probe. In this case, a magnetic field sensitive probe positioned at a finite test distance from the surface of the test material detects both the DC magnetic field component and the AC magnetic field component of the magnetic field in the direction of the main magnetization direction. A downstream signal processing device separates the signals thus detected into a DC magnetic field component that varies only slowly and an AC magnetic field component superimposed on the DC magnetic field component. In downstream processing, the DC magnetic field component represents the magnetization state, and the AC magnetic field component is the probe signal, which is a measure of the strength of the leakage magnetic field caused by defects at the scanned location.

[0035] It is often particularly advantageous if the leakage flux probe and the magnetic field probe are based on the same measurement principle and are simply mounted in different directions of the sensitivity direction. As an example, the leakage flux probe and the magnetic field probe can each be a Hall element.

[0036] Preferably, the DC magnetic field component of the magnetization signal (optionally varying slowly) is determined and used for normalization of the probe signal. This component has been found to correlate particularly reliably with the current magnetization intensity in the detected region of the test body.

[0037] To achieve an efficient test with an optionally high spatial resolution adapted to the test task, in a preferred embodiment, the probe configuration is realized to have a probe array including a number of leakage flux probes arranged linearly adjacent to each other in a first direction. Preferably, then, two or more magnetic field probes arranged linearly at a distance from each other parallel to the first direction are provided to detect the magnetization state. It may be sufficient to use only one magnetic field probe.

[0038] In this case, the number of magnetic field probes may be clearly less than the number of leakage flux probes. As a result, it is not necessary for all leakage flux probes to be assigned dedicated magnetic field probes. Rather, it may be a situation where the magnetization acting at the location of a particular leakage flux probe can be derived in all cases by interpolation from magnetization signals detected by a plurality of magnetic field probes. In some embodiments, there are at least 10 times as many leakage flux probes as magnetic field probes. As a result, firstly, sufficient spatial resolution of the leakage flux test can be achieved, and secondly, the equipment cost for magnetic field measurement can be limited.

[0039] The leakage flux probes are arranged on the side of the probe configuration that will be directed towards the test object, and the magnetic field probes are preferably arranged at a distance behind the leakage flux probes, that is, at a somewhat greater distance from the test material. As a result, the high spatial resolution of flaw detection by leakage flux measurement can be combined with a sufficiently accurate detection of the magnetization state at each individual leakage flux probe.

[0040] In some embodiments, the leakage flux probes are arranged at equal distances from each other, the magnetic field probes are arranged at unequal distances from each other, and the density of the magnetic field probes is preferably greater in the end region of the probe configuration than in the central region of the probe configuration. This can be advantageous for detecting measured values in the region of the test object end in suitable cases.

[0041] In a preferred embodiment, the probe signal of the leakage flux probe has a signal amplitude, and in order to normalize the probe signal, it is realized that the signal amplitude is multiplied by a compensation coefficient that at least partially compensates for the magnetization dependence of the test sensitivity. Such a multiplication operation can be performed relatively quickly simultaneously for a large number of leakage flux probes in the context of evaluation. The compensation coefficient may, for example, tend to be substantially inversely proportional to the intensity of the magnetization of the test volume scanned by the leakage flux probe.

[0042] In preferred methods and apparatuses, the appropriate compensation factor is not estimated based on theoretical relationships, but rather is determined very precisely based on measurements, tested extrapolations, and / or interpolations. In some methods, calibration measurements are made on a relevant portion of the test object, which has at least one correlation flaw, in order to determine a compensation curve that describes the functional relationship between the magnetization state of the test object for different intensities of an external magnetic field, the corresponding magnetization signal of a magnetic field probe, and the signal amplitude of the probe signal generated by a correlation flaw. Then, during the evaluation of the probe signal, the compensation factor is derived from the compensation curve for the normalization of the probe signal. Here, the term "correlation flaw" describes a standard defect, the width and depth of which are generally predefined by a standard specification in order to enable comparable test results.

[0043] In order to achieve the result that the same flaw at different longitudinal positions of a test material, for example a ferromagnetic pipe to be tested, generates the same probe signal, the magnetization has to be constant over the length of the test material. However, it has been ascertained that mainly at the pipe ends or in the end regions of the test object, the actual magnetization can deviate considerably from the magnetization in the central region of the test material. Furthermore, wall thickness variations such as polygonal shapes or eccentricities determined, for example, by manufacturing, and, for example, the eccentric position of the test material, and furthermore the induction effect in the case of magnetic field variations, have a major influence on the magnetization actually present in the test volume.

[0044] Some methods include taking into account the variation in the magnetization state depending on the axial position of the test portion to be tested when determining the correction factor to be applied to the test portion by a procedure in which the axial offset between the relevant portion and the test portion is determined and the correction factor is corrected according to this offset. What can be achieved in this way is that an appropriate compensation of the flaw signal amplitude is possible with a relatively small computational effort even if the calibration measurement or the correlation with the standard defect was not carried out at the axial position of the defect to be evaluated later.

[0045] Sophisticated research by the inventors can often determine the correction coefficient for the axial position in the test section based on a displaced compensation curve, where the displaced compensation curve has the curve shape of the compensation curve determined in the calibration section, and it has been shown that this compensation curve is simply displaced by a displacement value corresponding to the axial offset with respect to the compensation curve determined in the calibration section. Based on this acceptable simplification, particularly rapid calibration measurements are possible to determine the locally correct compensation coefficient for each axial position of the test material.

[0046] Further advantages and aspects of the present invention will be apparent from the claims and from the description of the exemplary embodiments of the present invention described below with reference to the figures.

Brief Description of the Drawings

[0047]

Figure 1

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Figure 5B

Figure 5C

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Embodiments for Carrying Out the Invention

[0048] Exemplary embodiments of the claimed invention are described below based on an apparatus for the magnetic flux leakage testing of ferromagnetic test materials in the form of hot-rolled ferromagnetic pipes in a continuous manner. The apparatus is designed to detect various types of defects, imperfections or discontinuities, and can, for example, reliably detect rolling imperfections on both the inside (internal flaws) and the outside (external flaws) of the pipe. In this case, longitudinal flaws (flaws having a main extension direction parallel to the pipe longitudinal axis), and transverse flaws (flaws having a main extension direction in the circumferential direction or perpendicular to the pipe longitudinal axis), and inclined flaws (transverse to the longitudinal and circumferential directions) can be reliably found and their characteristics determined.

[0049] In one embodiment, two subsystems are integrated into a multi-test block. A rotating subsystem is provided for longitudinal flaw testing, the basic principle of which is described with reference to FIG. 1A. For transverse flaw testing, a stationary subsystem is provided, for example, having a ring-shaped configuration in which a plurality of sensor arrays are distributed around the circumference, as in the configuration in FIG. 1B. The subsystems are arranged in sequence in the passage direction of the pipe, where the order can be arbitrary. In other embodiments not shown in more specific detail, a single system, for example, a single rotating system, may be sufficient.

[0050] The rotating subsystem has a rotary head with a ring yoke RJ that rotates around the test material PR, the ring yoke having pole pieces PS aligned at diametrically opposite points with respect to the surface of the test body, and magnetization windings MW being attached to the pole pieces. By this means, in the pipe wall, a magnetic flux or magnetic field MF (DC magnetic field) is generated, the magnetic lines of force of which extend in the circumferential direction of the test body, that is, perpendicular to the longitudinal direction of the pipe. On the rotor, test heads PK are arranged in each case circumferentially between the pole pieces, each test head including one or more probe arrays SO, each probe array including a number of individual magnetic flux leakage probes SO.

[0051] The ring yoke, together with the pole piece PS and the test head PK, rotates at a rotational speed between about 30 min -1 and about 1200 min -1 during the test, depending on the type of the rotating subsystem. The pipe to be tested is simultaneously transferred forward in the passing direction at a test speed (e.g., 3 m / s or more at maximum). In this case, the test head slides on the pipe surface and scans the pipe surface without interruption in a spiral path. The probe SO of the probe array is arranged in the test head at a small test distance AB from the surface OB of the test material, and the test distance can be, for example, on the order of 0.2 mm to 2 mm (see Fig. 3). Due to the fact that the magnetic field lines extend circumferentially, this test is particularly sensitive to longitudinal flaws LF-A on the outside of the pipe and longitudinal flaws LF-I on the inside of the pipe, which disrupt the magnetic flux in the circumferential direction to the maximum extent, and as a result, generate a strong leakage magnetic flux field (Fig. 2).

[0052] In the case of the stationary system (Fig. 1B) for transverse flaw testing, a DC magnetic field magnetization device (not shown in more specific detail) that generates a magnetic field MF in the longitudinal direction of the passing pipe is utilized. Two rings of the probe array consisting of probe arrays SA arranged intermittently in the circumferential direction are arranged in a ring around the test object and scan the test object in its longitudinal direction during continuous testing. Since the magnetic flux extends longitudinally, it is particularly greatly disrupted by the transverse flaw (QF-A) extending circumferentially on the outside and the transverse flaw (QF-I) on the inside, and as a result, this configuration of the transverse flaw test has high test sensitivity.

[0053] The electrical signal SIG-SO of the leakage magnetic flux probe of the probe array, that is, the probe signal, is supplied to a common evaluation device AW that identifies defects. Since the probe signal during the test is caused by and characteristic of flaws or defects, the probe signal is also referred to here as the "flaw signal" or the "test signal".

[0054] Each type of flaw causes a leakage magnetic flux field specific to that particular flaw type, and its characteristics can be identified from the signal waveform and the frequencies contained in the signal. Figure 2 shows, for example, a cross-sectional view of a pipe perpendicular to the longitudinal direction and, further, the magnetic field lines of the magnetization field MF extending in the circumferential direction. The outer flaw LF-A extending in the longitudinal direction generates a leakage magnetic flux field SF-A that is relatively narrowly concentrated in the vicinity of the outer flaw. In contrast, the inner flaw LF-I of the same dimensions extending in the longitudinal direction generates a leakage magnetic flux field SF-I that is less steep, more spatially blurred or enlarged or widened, with a smaller amplitude on the outside of the pipe. The typical signal waveforms of the probe signals when the probe passes in the circumferential direction are shown above the leakage magnetic flux fields in both cases. In this case, the y-axis corresponds to the signal amplitude A, and the x-axis corresponds to the time t or the location during the probe's circulation.

[0055] Next, details regarding the design of the probe configurations for the rotating system (Figure 3) and the stationary system (Figure 4) will be described with reference to Figures 3 and 4. The probe configuration SA-R for the rotating system has a number of nominally identical individual probes SO1, SO2, etc., which form a probe array SA and are linearly arranged along a first direction R1 extending parallel to the longitudinal axis of the pipe. The probe array SA is assembled to a test head PK (see, for example, Figure 7). In the rotating system, the probe configuration as a whole moves circumferentially around the test object in a second direction R2 extending perpendicular to the first direction R1. As a result of the simultaneous longitudinal movement of the test object PR, each of the individual probes SO1, SO2 scans a relatively narrow test track PS that spirals around the test object, and the test track extends obliquely with respect to the first and second directions. All the probes of the probe array jointly scan a relatively high test width with a number of test tracks parallel to each other.

[0056] The corresponding configuration occurs in the case of the probe configuration SA-T for the transverse flaw test (see Fig. 4). The probe configuration SA-T has a number of individual probes SO1, SO2, etc., which are arranged adjacent to each other in series in a first direction R1, which here corresponds to the circumferential direction of the test material PR. The probe configuration is stationary, but the test material moves parallel to its longitudinal direction, so that the probe array scans the surface of the test body in a scanning direction corresponding to a second direction R2 perpendicular to the first direction R1. Here too, each individual probe covers a relatively narrow test track PS, and the overall test track in the circumferential direction produces a test width many times larger than the probe configuration. The magnetic field MF extending in the longitudinal direction of the pipe is pushed out from the test body material at the transverse flaw QF-A and detected by the probes of the probe array SA.

[0057] The magnitude or amplitude of the flaw signal (leakage magnetic flux signal) depends not only on the structure of the flaw, but also on the test body at the location of the flaw, i.e., on the strength of the magnetic field in the pipe wall, for example. During pipe testing, for example, for the same flaw at various longitudinal positions of the pipe to generate the same flaw signal, the magnetization must be constant over the length of the pipe. However, experience has shown that this is not the case. Primarily at the pipe ends, the locally present magnetization deviates from the magnetization at the center of the pipe (seen longitudinally). Variations in wall thickness can also cause fluctuations in magnetization. Moreover, dynamic effects can occur when the magnetic field increases and dissipates, especially when the pipe enters and exits the test device.

[0058] As schematically shown in FIG. 5, as a result of the pipe PR or the test object PR entering between the pole pieces, magnetic field lines from the air are drawn into the ferromagnetic pipe with higher permeability (FIG. 5A). This results in a higher magnetization at the pipe ends. The magnetization reaches the nominal value of magnetization at a large distance from the pipe ends only when a specific length of the pipe protrudes from the pole pieces again (FIG. 5C). Depending on the rapidity of the forward speed of the pipe, the achievement of the nominal magnetization may be delayed due to the induction effect or as a result of the control of the coil current, or may be affected in some other way.

[0059] FIG. 6 schematically shows an exemplary profile of the magnetization intensity when the pipe passes through the test device. The position POS in the longitudinal direction (first direction R1) is shown on the abscissa, and the measure of the magnetization MAG intensity (to be further explained later) is shown on the ordinate. At entry EIN, first, a higher magnetization occurs due to magnetic field line concentration and then drops sharply due to the transient process. Then, the current adjustment REG of the magnetic field coil cancels it out, and as a result, the desired nominal magnetization MAG-N exists with only small fluctuations over most of the passing section or the pipe length. At exit AUS, then, the effects of control and magnetic field line concentration appear again (see FIG. 5).

[0060] In particular, the magnetization fluctuations at entry and exit bring about variations and uncertainties regarding the test sensitivity because it is not clear whether the strong flaw signals are due to particularly large flaws or due to strong magnetization. Therefore, the test results at the pipe ends are not fully reliable, and this is called the "untested end".

[0061] Hereinafter, according to one embodiment of the present invention, a method for obtaining substantially uniform test sensitivity over the entire length of the pipe will be described. In this case, the uniform test sensitivity is essentially generated by continuously detecting the magnetization state of the test object and using this magnetization state to normalize or compensate the flaw signal amplitude.

[0062] To contribute to making the test sensitivity more uniform, several design means implemented in exemplary embodiments will be described with reference to FIGS. 7 and 8. 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 a test piece PR. The probe array SA is attached to the side facing the test piece and has a linear array including a large number of leakage flux probes SO or test probes SO, for example, 40 or more, or 70 or more, and in this example, between 90 and 100 identical leakage flux probes arranged in a line with each other.

[0063] A smaller number of magnetic field probes SM1 to SM5 are arranged at a small distance behind the probe array, specifically, also linearly arranged in the same way. The magnetic field probes are arranged at equal distances from each other here, but the distances can also be unequal, and in particular, can be made smaller in the end regions than in the central region. This configuration is selected such that at least one magnetic field probe capable of generating a magnetization signal representing a measure of the magnetization state of the test material in the region of the leakage flux probe is assigned to each of the leakage flux probes. For example, by interpolation, the magnetization at the location of the leakage flux probe SO30 can be determined using the magnetization signals of the two closest magnetic field probes SM2 and SM3, as will be further described later.

[0064] FIG. 8 is a schematic view of the configuration shown in FIG. 7 in the longitudinal direction of the pipe. The test head PK is shown here directly above a longitudinal flaw LF-A on the outside of the pipe.

[0065] The illustrated test head PK is designed to determine a measure of the magnetization of a test body by measuring the magnetic field component that is parallel to the surface of the test material and parallel to the main magnetization direction. This magnetic field component can here be referred to as the parallel component. More precisely, the so-called tangential magnetic field or T magnetic field is measured. Therefore, the measured value of the magnetization signal is also referred to as the T magnetic field value. The tangential magnetic field TAN corresponds to that component of the magnetic field strength at the surface of the test body, which is in a plane (perpendicular to the pipe longitudinal axis) that extends tangentially to the pipe, i.e., parallel to the surface and parallel to the magnetic field lines of the circumferential magnetic field MF. Orthogonal to that, the radial component RAD of the magnetic field, which is measurable in the region of the surface, extends in the normal direction to the pipe.

[0066] According to the inventors' insight, for example, the measurement of the tangential magnetic field during testing for longitudinal defects in a pipe is particularly well-suited for determining the magnetization because the proportionality constant between the magnetization of the test material, i.e., here the magnetization inside the pipe material, and the T magnetic field near the pipe surface corresponds to the ratio of the magnetic permeability of air (μ L ) and the magnetic permeability of the pipe material (μ R ).

[0067] In the case of a rotational system, the main magnetization direction ideally extends along the pipe circumference exactly perpendicular to the longitudinal axis of the pipe. During the testing of defects that are not accurately aligned with the longitudinal axis of the pipe, the deviation of the magnetic field lines from their ideal alignment can cause fluctuations in the leakage flux signal that may reduce the accuracy of the test results. The normalization of this fluctuation can be achieved by the additional measurement of that component of the magnetic field that is substantially perpendicular to the main magnetization direction and parallel to the longitudinal axis of the pipe. This component may sometimes be called the orthogonal component since it is directed perpendicular to the main magnetization direction. In this case, it may also be called the axial component since it is parallel to the axial direction of the test material in the case of this measurement configuration. This component can be detected by one or more additional magnetic susceptibility probes. In a further embodiment, the same magnetic field probe that detects the parallel magnetic field can also detect the component of the magnetization state perpendicular to the main magnetization direction, or else the absolute value and angle of the magnetization state. If the magnetization is thus detected in two directions that are orthogonal to each other or extend at an angle to each other and lie in the tangential plane (parallel to the surface of the test material), such effects can likewise be detected and taken into account in the flaw signal normalization.

[0068] In contrast, the leakage magnetic field probe (test probe, flaw probe) SO is designed to measure the radial component RAD of the magnetic field strength at the surface. This is mainly formed by the leakage flux at the location of the defect, i.e., where the magnetic field lines are pushed out of the pipe material by the defect. In FIG. 8, the different sensitivity directions of the leakage flux probe (measurement of the radial component) and the magnetic field probe (measurement of the T magnetic field) are characterized by arrows.

[0069] The leakage magnetic flux probe SO and the magnetic field probe SM are, here, of the same probe type, i.e., Hall probes. They are structurally identical to each other, but the directions of their main sensitivity directions (arrows in Fig. 8), i.e., the directions of maximum sensitivity, are different from each other. A further difference is that the signal of the T magnetic field is detected by DC coupling (DC magnetic field coupling), while the leakage magnetic flux probe (probe for flaw detection) operates with AC coupling (AC magnetic field coupling), i.e., it detects only changes in the leakage magnetic field.

[0070] For the magnetic field probe (or T magnetic field probe), in the test head, for example, as follows, the central central magnetic field probe SM3, one each at the axial ends of the probe array (SM1, SM5), and one each between the end magnetic field probes and the central magnetic field probe (SM2, SM4) can be positioned. Therefore, five magnetic field probes may be sufficient. Before the positions of these T magnetic field probes are set, the axial tangential magnetic field progression in the magnetic pole pieces, air gaps, and pipe dimensions used should be known. Accordingly, for example, instead of an equal axial distance between the magnetic field probes (see Fig. 7), it may be advisable to provide an unequal distance by placing the magnetic field probes closer to each other in the axial edge regions.

[0071] In contrast to the leakage magnetic flux probe SO, which should be placed as close as possible to the surface of the test object for flaw detection to detect high-frequency magnetic field changes, the magnetic field probe can be at a greater distance from the test object, rather, to detect low-frequency magnetic field changes.

[0072] Next, design assistance regarding the configuration of the magnetic field probe will be described with reference to Figs. 9 and 10. When T magnetic field compensation is required (to make the test sensitivity more uniform), the relevant T magnetic field values are intended to exist for each leakage magnetic flux probe SO. In the case of the five magnetic field probes SM1 to SM5, the T magnetic field values of those leakage magnetic flux probes not arranged directly below the magnetic field probes are interpolated.

[0073] In this regard, FIG. 9 shows a test head PK between two magnetic pole pieces PS. The magnetic field generated in the region of the test head varies axially, and the magnetic field strength (indicated by the length of the arrow) is greater in the central region than in the vicinity of the axial ends.

[0074] FIG. 10 shows a corresponding diagram showing the measured dependence of the T magnetic field T-F on the location along the test head. The x marks represent the magnetization signal (T magnetic field value) of the magnetic field probe. The dashed line represents the linearly interpolated T magnetic field value, and the solid line represents the actual T magnetic field profile. The interpolated values are close to the actual values in the central region (magnetic field probes SM2, SM3, and SM4), but greater deviations occur in the region of the greater axial gradient of the magnetic field strength near the pipe ends. In this example, the deviation can be reduced by positioning the second and fourth magnetic field probes closer to the ends (at the positions of the dashed lines), respectively, resulting in an unequal distance between the magnetic field probes axially.

[0075] Next, means for compensating for the non-uniform magnetization in the axial direction in an exemplary embodiment will be described with reference to FIGS. 11 to 13. This method takes into account that the variation in the magnetization of the pipe wall in the axial direction results in different sizes of flaw signals for the same flaw. The size of the flaw signal (amplitude of the flaw signal) for different magnetizations is corrected using the measured value of the T magnetic field by the magnetic field probe SM or a coefficient calculated therefrom.

[0076] In this method, (at least one) compensation curve is confirmed (see Fig. 11). Correlation measurements are performed for this purpose. During the T magnetic field correlation, the test probe periodically detects two correlation flaws with known dimensions, namely, one inner flaw and one outer flaw, while the current intensity of the electromagnetic coil (or measurement winding MW) of the pole piece increases from the minimum value to the maximum value. For each set current intensity, the test head crosses the correlation flaw at least once, and in this process, the leakage magnetic field signal (flaw signal) and the related T magnetic field value are recorded. Fig. 11 shows a schematic diagram in which the current intensity of the electromagnetic coil or the related T magnetic field T-F is recorded on the abscissa, and the normalized signal intensity SIG N of the outer flaw (solid line AF) and the inner flaw (dashed line IF) is recorded on the ordinate.

[0077] Therefore, the T magnetic field correlation results in a pair of two values for each T magnetic field, namely, the flaw signal (outer flaw) with respect to the T magnetic field value and the flaw signal (inner flaw) with respect to the T magnetic field value. From the measured value pairs, the compensation curves of AF or IF shown here can then be interpolated, and this compensation curve assigns the compensation value of the flaw signal to each T magnetic field value.

[0078] Variations in the T magnetic field values measured for the same current intensity or the same magnetization state in the axial direction are found. This variation may be affected by different pole piece geometries. In the case of many conventional pole pieces, the T magnetic field value may be smaller, for example, at the axial edge than at the center. Investigations by the inventors show that, independent of this axial variation, the profile of the compensation curve, that is, its shape, appears to be substantially independent of the axial position, that is, independent of the test head position where the T magnetic field is measured (by the magnetic probe).

[0079] In this method, a so-called displacement value is further determined. Fig. 12 schematically shows the displacement VS due to the displacement value between two compensation curves, where the solid line corresponds to the outer flaw in the region with a higher magnetic field intensity, and the solid line corresponds to the same outer flaw in the region with a lower magnetic field intensity.

[0080] Since the shape of the compensation curve showing the functional relationship between the T magnetic field strength and the signal amplitude obtained as the result recorded on the y-axis does not change at different forward positions, the T magnetic field compensation in the exemplary embodiment is managed using only two compensation curves, namely, the compensation curve for outer defects and the compensation curve for inner defects. In addition, the displacement value is determined for each of the leakage flux probes. The difference between the T magnetic field value of the leakage flux probe and the T magnetic field value of the central T magnetic field probe can be selected as the displacement value. The displacement value VS should be determined separately for each test head.

[0081] In this method, the T magnetic field value (corrected by the displacement value) of each leakage flux probe and, further, the reference value REF are used for T magnetic field compensation. The reference value is selected, for example, such that it corresponds to the T magnetic field value for which the correction factor of the signal amplitude is equal to 1. The T magnetic field value of the central magnetic field probe from the correlated defect for the nominal current intensity is preferably selected as the reference value. For T magnetic field values above the reference value, the defect signal of the corresponding leakage flux probe is given a factor less than 1 (<1), and in other cases (T magnetic field values below the reference value), a factor greater than 1 (>1) is given. FIG. 13 shows this for the curve of outer defects, where the abscissa represents the strength of the T magnetic field and the ordinate represents the factor FAK, which is equal to 1 at the reference value T REF is equal to 1.

[0082] Next, the effect of the compensation strategy will be explained based on an example schematically illustrated with reference to FIG. 14. The upper part shows the test body PR into which the longitudinal outer defect LF-A has been introduced as a correlated defect. The arrows in the test body represent magnetization, and the thickness of the arrows represents the strength of the magnetization varying in the axial direction. The left part EIN is intended to represent the entry stage, the middle part DYN shown is intended to represent the dynamic effects related to the control near the entry, and the right part NOR is intended to represent the relationship at a greater distance from the pipe end, where a stable normal state of magnetization occurs.

[0083] In the figures shown below, the solid line T indicates the amplitude of the measured T magnetic field, i.e., the intensity of the magnetization signal of the magnetic field probe. The dashed line SIG-SO schematically represents the flaw signal, i.e., the leakage magnetic flux signal of the leakage magnetic flux probe SO. The size of the flaw in the test object is the same in all three cases, and therefore, ideally (assuming uniform magnetization in the axial direction), the same flaw signal amplitude should occur in all three situations.

[0084] However, in reality, at the entry stage EIN where the magnetization is relatively high and thus the measured T magnetic field is relatively high, the curve SIG-SO shows a relatively large flaw signal amplitude. In the region of dynamic effects or transient processes where relatively low magnetic field intensities can occur, the flaw signal is clearly weaker than during the entry stage. The flaw signal is established at its "true" amplitude corresponding to the geometric shape of the flaw only at a greater distance from the pipe end.

[0085] In the region of the pipe end, in an experiment where two longitudinal outer flaws of the same size were introduced at different distances (100 mm and 250 mm) from the pipe end in all cases, the flaw closer to the pipe end generated a flaw signal up to 6 dB higher than the flaw at a greater distance from the pipe end. A similar situation was also observed for inner flaws.

[0086] Next, the lower figure in Figure 14 shows the effect of compensation. Here, the solid line FAK represents the coefficient described above that indicates the value by which the measured flaw signal amplitude must be multiplied to reach the true amplitude of the flaw according to the compensation. During entry, this coefficient is below the value that occurs in the normal state (right side of the figure). As a result, the amplitude of the flaw signal is reduced. In the region DYN of dynamic effects, the flaw signal is slightly amplified, and in the region of normal relationship, the coefficient is approximately 1, which means that here the "correct" signal amplitude is measured directly.

Claims

1. A method for detecting defects in ferromagnetic test materials, particularly for testing the magnetic flux leakage of ferromagnetic pipes, In order to generate a magnetized state of the test volume characterized by magnetization, the test volume of the test material is magnetized by an external magnetic field, A method comprising scanning the surface of a test material with a probe configuration including at least one magnetic field-sensitive leakage flux probe for detecting a leakage magnetic field caused by a defect, wherein the leakage flux probe is held at a finite test distance from the surface of the test material during scanning and generates an electrical probe signal which is a measure of the intensity of the leakage magnetic field, The magnetization state of the test volume in the region of the leakage flux probe is determined by using at least one magnetic field probe to generate a magnetization signal representing a measure of the magnetization state of the test material in the region of the leakage flux probe, In order to determine the normalized probe signal, the probe signal is normalized by the assigned magnetization signal, In order to identify the aforementioned defect, the normalized probe signal is evaluated and A method characterized by including

2. The 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. The method according to claim 1, characterized in that, in order to determine the magnetization state, the parallel component of the magnetic field directed substantially parallel to the surface of the test material and parallel to the principal magnetization direction is measured at a short distance around the leakage flux probe.

4. The method according to claim 1, characterized in that the test material is a ferromagnetic pipe, and a magnetic field component oriented substantially tangentially to the surface of the test material is preferably measured to detect the magnetization state.

5. The method according to claim 1, characterized in that the DC magnetic field component of the magnetization signal is determined and used to normalize the probe signal.

6. The method according to claim 1, wherein the probe signal of the leakage flux probe has a signal amplitude, and in order to normalize the probe signal, the signal amplitude is multiplied by a compensation coefficient that at least partially compensates for the magnetization dependence of the test sensitivity, wherein the compensation coefficient is preferably substantially inversely proportional to the intensity of the magnetization of the test volume scanned by the leakage flux probe.

7. The method involves performing calibration measurements on the correlated portion of the test material to determine a compensation curve that describes the functional relationship between the magnetization state of the test material, the corresponding magnetization signal of the magnetic field probe, and the signal amplitude of the probe signal generated by standard defects, wherein the correlated portion has at least one correlation defect. The method according to claim 1, characterized in that, during the evaluation of the probe signal, a compensation coefficient 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 coefficient, the axial offset between the calibration portion and the test portion is determined, and when determining the correction coefficient to be applied to the test portion by a procedure in which the correction coefficient is modified according to the offset, the variation in the magnetization state according to the axial position of the test portion to be tested is taken into consideration.

9. The method according to claim 7, characterized in that the correction coefficient for the axial position in the test portion is determined based on a displaced compensation curve, the displaced compensation curve has the curve shape of the compensation curve determined in the calibration portion, and the compensation curve is displaced by a displacement value corresponding to the axial offset with respect to the compensation curve determined in the calibration portion.

10. The method according to any one of claims 1 to 9, wherein the probe configuration has a probe array comprising a number of leakage flux probes arranged adjacent to each other in a first direction, and preferably two or more magnetic field probes arranged at a distance from each other in the first direction for detecting the magnetization state, wherein the number of magnetic field probes is preferably less than the number of leakage flux probes.

11. An apparatus for detecting defects in ferromagnetic test materials, particularly for testing the magnetic flux leakage of ferromagnetic pipes, A magnetization device for magnetizing the test volume of the aforementioned test material (PR), A probe configuration (SA) comprising at least one leakage flux probe (SO) for detecting a leakage magnetic field caused by a defect, wherein the leakage flux probe (SO) is held at a finite test distance (AB) from the surface of the test material during scanning and is configured to generate an electrical probe signal having a defect signal amplitude dependent on the leakage flux, the probe signal being a measure of the intensity of the leakage magnetic field, In order to identify the aforementioned defect, an evaluation device (AW) for evaluating the probe signal and Includes, The apparatus has at least one magnetic field probe (SM) for generating a magnetization signal that represents a measure of the magnetization state of the test material (PR) in the region of the leakage flux probe (SO), The apparatus is characterized in that the evaluation device (AW) is configured to normalize the probe signal by an assigned magnetization signal in order to determine the normalized probe signal, and to evaluate the normalized probe signal in order to identify the defect.

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

13. The apparatus according to claim 11, characterized in that the apparatus has at least one test head (PK) comprising a probe configuration (SA) which includes at least one leakage flux probe (SO) and at least one magnetic field probe (SM) arranged in a fixed spatial relationship with respect to each other.

14. The apparatus according to claim 11, characterized in that the leakage flux probe (SO) is arranged to detect the normal component of the leakage magnetic field directed substantially perpendicular to the surface of the test specimen, and / or the magnetic field probe (SM) is arranged to detect the parallel component of the magnetic field directed substantially parallel to the surface of the test material and parallel to the principal magnetization direction.

15. The apparatus according to claim 11, characterized in that the probe configuration has a probe array comprising a number of leakage flux probes (SO) arranged adjacent to each other in a linear manner in a first direction, and preferably two or more magnetic field probes (SM) arranged at a distance from each other in a linear manner in the first direction, which are provided for detecting the magnetization state.

16. The apparatus according to claim 11, characterized in that the number of magnetic field probes (SMs) is less than the number of leakage flux probes (SOs), the number of leakage flux probes (SOs) is preferably at least five times more than the number of magnetic field probes (SMs), preferably at least one order of magnitude more, and / or the leakage flux probes are arranged at equal distances from each other, the magnetic field probes are arranged at uneven distances from each other, and the density of magnetic field probes is preferably greater in the end regions of the probe configuration than in the central region of the probe configuration.

17. The apparatus according to any one of claims 11 to 16, characterized in that the leakage flux probe (SO) is positioned on the side of the test head (PK) which should be directed toward the test specimen (PR), and the magnetic field probe (SM) is positioned at a distance behind the leakage flux probe.

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