Test method and test equipment for magnetic flux leakage test

Abstract

In a test method for magnetic flux leakage testing of ferromagnetic test objects (110), in particular ferromagnetic pipes, in a continuous process for detecting defects, the test objects are first conveyed in a travel direction (102) through a first subsystem (200) of a test device (100) and then through a second subsystem (300) of the test device, and demagnetizing the test object before leaving the test device. The first subsystem (200) is designed as a lateral defect testing system and has a first magnetization device (220) which generates a magnetization in the test object, the magnetized magnetic field lines extending substantially in the longitudinal direction of the test object. The second subsystem (300) is designed as a longitudinal defect testing system and has a second magnetization device (320) which generates a magnetization in the test object, the magnetized magnetic field lines extending substantially in the circumferential direction of the test object. In each of these subsystems, the surface of the test object is scanned by means of at least one magnetic field-sensitive probe (250, 350) in the area of the test region in order to detect a leakage magnetic field due to the defect, and the electrical probe signal of the probe is evaluated in order to identify the defect. After passing through a test region (230) of the first subsystem (200), while leaving the first subsystem and before entering the second subsystem (300), the test object is mostly or completely demagnetized by means of a direct-current magnetic field of a demagnetizing coil (400) assigned to the first subsystem in the opposite direction of the magnetic field of the first magnetizing device.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for magnetic flux leakage testing of a ferromagnetic test object for detecting defects according to the preamble of claim 1 and to a testing device according to the preamble of claim 7 suitable for carrying out the method. BACKGROUND

[0002] Magnetic flux leakage methods are an important component in non-destructive defect testing of semi-finished and finished parts and are used for quality monitoring during the manufacturing process and in periodic repeated testing of finished parts. In comparison with, for example, eddy current methods or ultrasonic testing, magnetic flux leakage methods are less sensitive to some disturbing properties of the material, such as surface roughness or scale layers in hot-rolled products. As a result, a better ratio between the effective signal and the disturbing signal (N / S ratio) is obtained, which enables more reliable defect recognition.

[0003] In magnetic flux leakage testing, the test volume of the 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 probe (magnetic flux leakage probe) in order to detect the magnetic flux leakage field caused by the defects. Here, a relative movement takes place between the probe and the surface of the test object.

[0004] The magnetic flux or magnetic field generated by the magnetizing device in the test object is distributed substantially uniformly in space in the defect-free material. In this case, no significant magnetic field gradients occur in the region close to the surface. Cracks and other defects, such as cavities, inclusions or other discontinuities, such as weld seams, etc., act as regions of increased magnetic resistance, so that the magnetic field components near the defects are directed around the defects and are squeezed out from the metal to the region close to the surface. In magnetic flux leakage methods, the magnetic field components squeezed out are detected in order to detect the defects. In a magnetic flux leakage measurement, a defect can be detected if the magnetic field components squeezed out from the test object reach the region of the probe and cause a magnetic field change there that is sufficient for detection.

[0005] If the magnetizing device is oriented perpendicular to the main extension of the defect, the squeezing effect of the magnetic field components is particularly strong and the test sensitivity is correspondingly higher. In other words, the more a defect deviates from the (ideal for testing) orthogonal position to the main magnetization direction, the smaller the level of the leakage flux signal of the defect. If the main extension of the defect is oriented parallel to the longitudinal axis of the test object, this is referred to as a longitudinal defect. In order to be able to easily find these longitudinal defects, the magnetizing device should be oriented as perpendicular as possible to the main extension, in particular thus in the circumferential direction of the test object. On the other hand, if the magnetizing device is oriented parallel to the longitudinal axis of the test object, transverse defects, i.e. defects whose main extension is oriented in the circumferential direction of the test object and thereby perpendicular to the longitudinal axis of the test object, can be particularly easily detected. The more steeply the magnetic flux lines act on the defect, the more easily defects whose main extension is oriented obliquely with respect to the longitudinal direction and obliquely with respect to the circumferential direction (oblique defects) can be detected.

[0006] In order to be able to find longitudinal defects and transverse defects as well as oblique defects, a test device of the type discussed in this application has two subsystems, which are arranged one behind the other with respect to the direction of travel of the test object. Here, in a test line, first a first subsystem is passed through, which is designed as a transverse defect test system and, in correspondence therewith, causes a magnetization of the test object in its longitudinal direction. Then, a second subsystem is passed through, which is designed as a longitudinal defect test system and, in correspondence therewith, causes a magnetization of the test object in its circumferential direction. Oblique defects can produce error signals in both subsystems, wherein the signal amplitudes of these error signals depend not only on the size of the defect, but also on the angular position of the defect with respect to the longitudinal direction and the circumferential direction.

[0007] One important criterion for the quality of a leakage flux test device is the repeatability. This means that similar defects must be detected with similar signal amplitudes. Usually, a test device must prove at customer acceptance that it can find similar defects with a repeatability of better than 3 dB. The experience of the inventors shows that these strict criteria, in particular for oblique defects, cannot always be adhered to or can only be adhered to with great technical effort.

[0008] After passing through these subsystems, residual magnetization may remain in the test object, which can cause interference in subsequent processing steps or applications. Therefore, conventional test equipment or leakage wires typically include demagnetizing coils to demagnetize the test object after passing through the first and second subsystems. Demagnetizing coils are usually implemented as simple hollow coils, i.e., they consist only of one or more copper coils without a magnetic flux conductor or concentrator. These copper coils are operated by alternating current, the frequency and amplitude of which are adjusted according to the diameter of the test object, and if necessary, the wall thickness and transport speed. For example, demagnetization by means of an alternating magnetic field is described in US 6,037,767. Summary of the Invention

[0009] Against this background, the purpose of the present invention is to provide a test method and test equipment for performing leakage magnetic field testing on ferromagnetic test objects. The test method and test equipment can reliably test different defect types with good repeatability, that is, including not only longitudinal defects and transverse defects, but also oblique defects.

[0010] To achieve this objective, the present invention provides a testing method having the features of claim 1 and a testing apparatus having the features of claim 7. Advantageous extensions are described in the dependent claims. The wording of all claims becomes part of the specification by reference.

[0011] The test method of the present invention, as claimed, is designed to perform magnetic flux leakage testing on a ferromagnetic test object in a continuous process and to detect defects in the test object. The test object can be, in particular, a ferromagnetic pipe, but may also be a solid rod made of ferromagnetic material if necessary.

[0012] Within the scope of this testing method, the test object is first conveyed along the travel direction through the first subsystem of the testing equipment and then through the second subsystem. Before leaving the testing equipment, the test object is demagnetized using a demagnetizing device until a tolerable residual magnetization is achieved. Here, the term "demagnetization" means that the test object, after passing through the testing equipment, either has no residual magnetization at all, or, according to specifications, the residual magnetization is so low that it will not interfere with further processing of the test object.

[0013] The first subsystem is designed as a lateral defect testing system and includes a first magnetization device that magnetizes the test object, with the magnetized magnetic field lines extending substantially along the longitudinal direction of the test object. A second subsystem downstream of the first subsystem is designed as a longitudinal defect testing system and includes a second magnetization device that magnetizes the test object, with the magnetized magnetic field lines extending substantially along the circumferential direction of the test object. In each of these subsystems, within a test area, the surface of the test object is scanned using at least one magnetic field-sensitive probe (leakage magnetic field probe) to detect leakage magnetic fields caused by defects. This test area is located between the inlet and outlet ends of the subsystem. The electrical probe signals of these probes are evaluated for defect identification.

[0014] The unique feature of this testing method compared to conventional methods is that after the test object passes through the test area of ​​the first subsystem, upon leaving the first subsystem and before entering the second subsystem in both time and space, it is largely or completely demagnetized by a DC magnetic field distributed to the demagnetizing coil of the first subsystem, which is opposite in direction to the magnetic field of the first magnetizing device. Preferably, a single demagnetizing coil is used. Multiple demagnetizing coils can be provided, especially when it is necessary to reduce other components besides the magnetizing component acting in the longitudinal direction.

[0015] To this end, the testing equipment is equipped with a demagnetizing device having a DC power supply and a demagnetizing coil connected thereto. The demagnetizing coil is arranged in the outlet area of ​​the first subsystem and can be operated with DC power, so that the test object magnetized by the first magnetizing device can be completely demagnetized or demagnetized to a tolerable residual magnetization by reducing the magnetization component extending along the longitudinal direction of the test object.

[0016] Therefore, the demagnetizing coil located in the exit region of the first subsystem generates a DC magnetic field, the polarity of which is opposite to that of the DC magnetic field of the first magnetizing device. Thus, this DC magnetic field demagnetizing coil in this application is also called a reverse coil. It can also be called a compensation coil because: thereby, the magnetization generated in the test object by the first magnetizing device can be partially or completely compensated by the DC reverse magnetic field, so that the test object has no residual magnetization or only a tolerable degree of residual magnetization when entering the second subsystem.

[0017] The claimed invention is based on several insights of the inventors. Here, the first aspect relates to the order of the two subsystems. Previously, it was considered that when the two subsystems (lateral defect testing and longitudinal defect testing) are performed consecutively, it was essentially irrelevant whether the lateral defect testing was performed first and then the longitudinal defect testing, or whether the order was reversed. However, according to the claimed invention, this is chosen such that the lateral defect testing is forced to occur first, in which magnetization aligned along the longitudinal direction of the test object is generated in the test object by means of a first magnetization device. Then, as the test object leaves the first subsystem, the magnetization is immediately and specifically eliminated by means of a DC magnetic field, or the magnetization is reduced to the extent that the test object is completely demagnetized or enters the second subsystem (longitudinal defect testing) with only a tolerable residual magnetization.

[0018] Therefore, the residual magnetization along the longitudinal direction, which is particularly critical for residual magnetization, is immediately reduced to a non-critical level upon leaving the first subsystem, allowing downstream longitudinal defect testing to be performed essentially as if it were in a completely unmagnetized fresh test object. Since longitudinal defect testing requires magnetization with magnetic field lines in the circumferential direction of the test object generated by the second magnetization device, the level of residual magnetization along the longitudinal direction remains small upon passing through the second subsystem, and can even be further reduced. This generally eliminates the need for a demagnetizing coil connected downstream of the second subsystem to eliminate the magnetization component aligned along the longitudinal direction of the test object. Therefore, since there is no need to consider the space requirements for demagnetizing coils before and after the second subsystem, the structural length of the second subsystem, which is critical for material guidance, can be kept short.

[0019] The demagnetizing coil, which operates using direct current, is positioned in the outlet region of the first subsystem, i.e., immediately adjacent to the first subsystem, resulting in further advantages. This spatial proximity allows for the absence of any air gap or at most a very narrow air gap between the demagnetizing coil and the magnetically conductive components of the first subsystem, necessitating the overcoming of only small magnetic reluctance in this region. Consequently, the magnetic flux conductor present in the first subsystem can also be used to conduct the DC magnetic field generated by the demagnetizing coil. Therefore, the demagnetizing coil does not require its own magnetic flux conductor, enabling the use of demagnetizing coils with minimal space requirements. Furthermore, the spatial proximity of the demagnetizing coil to the first subsystem allows for high efficiency, as it can operate with relatively low power while still generating a sufficiently strong reverse magnetic field.

[0020] According to an extended design, the demagnetizing coil of the demagnetizing device is fixed to the housing of the first subsystem. This allows it to be positioned as close as possible to the first subsystem, and also allows the magnetically conductive components of the first subsystem to be used to conduct the demagnetizing field. Furthermore, the demagnetizing coil can be installed and adjusted together with the first subsystem.

[0021] In some embodiments, the first magnetizing device (magnetizing device of the first subsystem) has a magnetizing coil on the inlet side and a test object channel nozzle made of ferromagnetic material on the outlet side, which are coupled by a magnetic return conductor. Here, a demagnetizing coil is arranged on the first subsystem such that the magnetic field lines of the demagnetizing field generated by the demagnetizing coil extend through the magnetic flux conductor on the outlet side of the first subsystem.

[0022] According to an extended scheme, the DC power supply is designed to be adjustable, so that the strength of the demagnetizing field can be adapted to the corresponding requirements.

[0023] In a preferred embodiment, a control loop is provided to automatically adjust the strength of the DC magnetic field generated by the demagnetizing coil based on the residual magnetization value, which can be determined during the residual magnetization determination operation. Therefore, the strength of the compensating reverse magnetic field can be optimally adapted to the requirements without operator intervention.

[0024] According to another formulation, a residual magnetization determination operation is provided, wherein a residual magnetization value is determined on the test object after passing through a first subsystem and a demagnetizing coil, the residual magnetization value representing the intensity of the magnetization component aligned along the longitudinal direction of the test object, wherein the intensity of the DC magnetic field generated by the demagnetizing coil is then automatically adjusted based on the determined residual magnetization value.

[0025] To determine the residual magnetization value, a separate measuring device may be provided. Alternatively, in a preferred embodiment, a second subsystem, which is always present, is used as the measuring system for determining the residual magnetization value during this residual magnetization determination operation. Therefore, the second subsystem can serve as a functional part of the control loop for setting the correct demagnetizing field strength.

[0026] According to an extended embodiment, the second subsystem has a rotating head with pole pieces of a second magnetizing device radially oriented relative to the surface of the test object on opposite diametrically opposed sides. This allows the generation of a DC magnetic field (rotating about the longitudinal axis of the test object) within the test object, with the magnetic field lines extending circumferentially along the test object. At least one probe (leakage probe) is arranged between these pole pieces circumferentially. This second subsystem enables the execution of a skew defect correction procedure, in which the intensity of the DC magnetic field generated by the demagnetizing coil is adjusted so that, regardless of the skew defect's tilt posture (angle and direction), each probe scanning the skew defect produces a substantially identical error signal amplitude.

[0027] This unique method is based on the understanding that residual magnetization in the test object, with a magnetization component along the longitudinal direction of the test object, causes oblique defects of a certain size to produce error signal amplitudes with varying intensities depending on the direction. This direction dependence exists as long as a significant magnetization component exists along the longitudinal direction of the test object. On the other hand, if the magnetization component acting along the longitudinal direction is eliminated through appropriate demagnetization, oblique defects of a certain size, whether left-handed or right-handed, will exhibit the same error signal amplitude.

[0028] This effect can be used to adjust or regulate the strength of the demagnetizing DC magnetic field generated by the demagnetizing coil. If the demagnetizing field strength is correctly adjusted so that the magnetization component acting in the longitudinal direction almost disappears, optimal repeatability of oblique defects is also achieved. Attached Figure Description

[0029] Other advantages and aspects of the invention will become apparent from the claims and from the description of embodiments of the invention, which are subsequently set forth with reference to the accompanying drawings.

[0030] Figure 1 A schematic cross-sectional view of the components of an embodiment of the test equipment is shown; Figure 2 The diagram illustrates a test object using a cylindrical coordinate system. Figure 3 The different defect types are illustrated schematically; Figure 4A and Figure 4B exist Figure 4A The different superpositions of the magnetization field components are shown in the figure, and in Figure 4B The figure shows the effect of different defect types on the error signal amplitude in a test object without residual magnetization along the longitudinal direction; Figure 5A and Figure 5B exist Figure 5A The different superpositions of the magnetization field components are shown in the figure, and in Figure 5B The figure shows the effect of different defect types on the error signal amplitude in a test object with residual magnetization along the longitudinal direction. Detailed Implementation

[0031] In the following description, embodiments of the claimed invention are illustrated with respect to a test apparatus 100 configured to perform magnetic flux leakage testing on a ferromagnetic test object in the form of a ferromagnetic pipe in a continuous process. The pipe may be, in particular, a hot-rolled pipe, and if necessary, a welded pipe or a cold-drawn pipe. Figure 1 A schematic cross-sectional view of the key components of this embodiment is shown.

[0032] To describe the pipe and its defects, a cylindrical coordinate system with three coordinates r, r', r'', r'', r''''''''''''''''''''''''''''''' ',"y'",""y ... and z (see z) Figure 2 and Figure 3 Here, r is the radial coordinate. These are angular coordinates, and z is determined by r and The height on the extended plane. The length of the pipe along the axial direction is represented by the z-coordinate, the diameter and wall thickness are described by the r-coordinate, and the circumference of the pipe can be represented by... Use coordinates to describe it.

[0033] The testing equipment 100 is designed to detect different types of defects, incompleteness, or discontinuities, and can reliably detect, for example, rolling defects (internal defects) on the inside of the pipe and rolling defects (external defects) on the outside of the pipe. Here, longitudinal defects LF (defects with a main extension direction parallel to the longitudinal axis of the pipe), transverse defects QF (defects with a main extension direction along the circumferential direction or perpendicular to the longitudinal axis of the pipe), and oblique defects SF (transverse to both the longitudinal and circumferential directions) can be reliably found and characterized.

[0034] In cylindrical coordinates, the longitudinal defect (LF) is oriented along the z-direction, and the transverse defect (QF) is oriented along... Directional orientation. Not oriented along the z-direction, but additionally along either the positive or negative direction. Defects with directional orientation are called oblique defects. Among them, according to the definition used here, right-hand oblique defects (RH) have a positive... The component, and the left-handed oblique defect (LH) has a negative component. The z-component increases, and the z-component also increases.

[0035] The testing equipment 100 operates continuously. To this end, the testing equipment includes: a first subsystem 200 for conveying the test object 110 along a travel direction 102 through the testing equipment 100, and a second subsystem 300 located downstream of the first subsystem along the travel direction. The first subsystem 200 is designed as a transverse defect testing system and has a first magnetizing device 220 configured to generate magnetization in the test object, the magnetized magnetic field lines extending substantially along the longitudinal direction of the test object (z-magnetization, longitudinal magnetization). The second subsystem 300 is designed as a longitudinal defect testing system and has a second magnetizing device 320 configured to generate magnetization in the test object, the magnetized magnetic field lines extending substantially along the circumferential direction of the test object (z-magnetization, longitudinal magnetization). magnetization).

[0036] Each of these subsystems has test areas 230 and 330, each equipped with at least one magnetic field-sensitive probe 250 or 350. This probe scans the surface of the test object to detect leakage magnetic fields caused by defects. The electrical probe signal is evaluated using an evaluation device 500 for defect identification.

[0037] Magnetic flux leakage testing of transverse defects requires magnetic flux extending as parallel as possible to the pipe axis (z-direction), i.e., perpendicular to the defect direction. In this example, such magnetic flux extending longitudinally relative to the pipe axis is generated using two coils 222-1 and 222-2 surrounding the pipe, spaced approximately according to their diameter. The coils of the first magnetizing device 220 are connected to a direct current power supply 224 and are circulated in the same direction with a current of several amperes, sufficient to magnetize the ferromagnetic pipe to achieve an operating point suitable for magnetic flux leakage testing. Magnetic flux conductors, such as nozzles (inlet nozzle 225-1 and outlet nozzle 225-2) made of ferromagnetic material and matched to the diameter of the pipe under test, and a return conductor 227 located outside the coils, minimize magnetic flux resistance, which increases the magnetic flux density in the material at the same current intensity.

[0038] As a portion of the conduit enters the first subsystem 220, magnetic flux is conducted into the conduit through inlet nozzle 225-1. Between these nozzles, a ferromagnetic conduit (test object 110) conducts the magnetic flux to outlet nozzle 225-2. There, the magnetic flux flows out of the material and is conducted back through the outlet nozzle and return conductor 227, thus closing the magnetic field lines.

[0039] The test area 230 of the first subsystem is located in the middle of these nozzles. Test shoes are present there, and within these test shoes are one or more (e.g., up to dozens) sensors or probes 250. The test shoes and probes are arranged such that the pipe can be scanned along its entire circumference. Here, the test shoes slide on the pipe, which is conveyed through the subsystem by drive rollers. At the location of the test shoes, the pipe has the most uniformly high magnetization possible along the z-direction.

[0040] When transverse defects are present, the cross-section of the pipe decreases locally. This results in a redistribution of magnetic flux within the pipe, with a small portion of the flux being expelled from the pipe. This change in magnetic flux on the pipe surface is detected by sensor (probe) 250 and converted into a high-resolution image of the magnetic flux leakage defect using a suitable algorithm.

[0041] After testing a portion of the pipe and transferring the magnetic flux from the pipe to the outlet nozzle 225-2, the pipe remains magnetized. The level of residual magnetization or remanence depends on the material and the level of magnetization. The direction of residual magnetization corresponds to the magnetization direction of the first subsystem, i.e., the longitudinal direction or z-direction of the test object.

[0042] In terms of timing, after the transverse defect test, a longitudinal defect test is performed by means of a second subsystem 300 located a certain distance downstream of the first subsystem 200 along the travel direction. The second subsystem 300 has a rotating head that can rotate about the travel axis. The second magnetizing device includes two coils 322-1, 322-2, each wound around a magnetic core and arranged around the pipe in opposite radial directions (r direction). On the side facing the pipe, each magnetic core has pole shoes 325-1, 325-2 (see, for example, EP 4099007 A1) that match the pipe diameter, which uniformly introduce magnetic flux into the pipe. On the side of the coil radially away from the pipe, the magnetic core is mounted on an annular yoke, which acts as a return conductor for the magnetic flux. The magnetic core, annular yoke, and pole shoes of the second magnetizing device 320 are made of ferromagnetic material to minimize the magnetic reluctance resisted by the magnetic flux.

[0043] When along When the magnetic flux in the z-direction is disturbed by longitudinal defects, i.e., defects extending along the z-direction, these longitudinal defects will generate leakage magnetic signals. The magnetic flux in the direction of the magnetic flux is generated by two pole shoes with opposite polarities. In the pole shoes... A test head (at ~90° and ~270°) is located at positions (0° and 180°), each containing a probe array with up to 100 or more magnetic field-sensitive probes. The pole shoes and test heads are located in test area 330 of the second subsystem. To test the entire surface of the pipe, the entire second magnetizing device—the coil, annular yoke, and pole shoes—along with the test heads and associated electronics, rotates around the pipe as it is conveyed along the z-direction. Thus, the sensors embedded in the test heads scan the pipe surface in a helical pattern.

[0044] It is important to note that although the lengths of the magnetization vectors at the test head locations are the same, the directions of these vectors are opposite for each of the two test heads. After the pipe leaves the test area of ​​the second subsystem 300, the pipe is remagnetized by a rotating magnetic field. This demagnetization proceeds almost entirely along... The magnetic field proceeds in the z-direction, and at least partially in the z-direction, because at the end of the pole piece, the magnetic field also has a z-component.

[0045] To ensure smooth further processing or use of the pipe after magnetic flux leakage testing, the maximum residual magnetization allowed to remain in the pipe must be defined by standards or by the pipe buyer. Residual magnetization along the z-direction is particularly critical. A typical method for determining residual magnetization in a pipe is to use a moving magnetometer to measure the magnetic flux flowing out at the pipe end. According to standards, the average magnetic flux along the pipe circumference at the pipe end is not allowed to exceed, for example, 3.0 mT. In some cases, the allowable magnetic flux density required by the pipe buyer is much lower, for example, less than 1 mT. To systematically achieve these low values, partial demagnetization via a rotating second subsystem is often insufficient.

[0046] Therefore, in conventional testing equipment (sometimes also called leakage wires), a demagnetizing coil is placed behind the second subsystem, on the side opposite to the first subsystem. The demagnetizing coil is typically implemented as a simple hollow coil, i.e., it consists only of one or more copper coils without a magnetic flux conductor or concentrator. These copper coils are powered by alternating current, the frequency and amplitude of which must be adjusted according to the pipe diameter, wall thickness, and transport speed (see, for example, DE 3819033 C2).

[0047] The inventors have recognized that this traditional design has certain drawbacks. They have proposed designs that deviate from this traditional approach, avoiding these drawbacks and improving the performance of the testing equipment.

[0048] The first drawback of the traditional arrangement is that, depending on the design and operating parameters of the AC voltage demagnetizing coil, the effects of the alternating magnetic field under high current can be perceived not only in the adjacent rotating subsystem but also in the first subsystem, which is significantly farther away, and may cause damage to the test signal.

[0049] Another drawback involves the previously unrecognized adverse effects of demagnetization on testing accuracy in the presence of oblique defects. To understand this, we will combine... Figure 4A and Figure 4B as well as Figure 5A and Figure 5B This will be explained in the following text. (Partial appendix) Figure 4A and some appendices Figure 5A The magnetization (field F) between the pole shoes is shown separately, with relevant parts attached. Figure 4B and some appendices Figure 5B The corresponding error signal amplitudes SIG at test heads PK1 and PK2 are shown respectively. The pole shoes (S and N represent the magnetic south and magnetic north poles on the pole shoes) generate magnetic flux in the unmagnetized conduit, which almost exclusively travels along the direction of the magnetic flux in the regions of test heads PK1 and PK2. Direction orientation. The maximum leakage flux signal is generated by LF, which is related to... The orientation is strictly orthogonal. Compared to LF, for oblique defects LH and RH, the oblique orientation of the defect causes a decrease in the leakage magnetic field signal. (The last sentence appears to be incomplete and possibly refers to a technical detail about the direction of the defect.) In directionally magnetized pipes, the reduction in magnetic flux leakage signal is independent of whether the defect is LH or RH. Since the direction difference between the magnetization vectors M1 and M2 between the first test head PK1 and the second test head PK2 is 180°, this difference has no effect on the magnetic flux leakage signal because the angle between the magnetization vectors M1 or M2 and the defect is the same.

[0050] This is the condition when the pipe is not magnetized before entering the second subsystem.

[0051] However, in the magnetic flux leakage testing equipment discussed here, the presence of lateral defects in the pipe is first tested. Therefore, the pipe is magnetized with residual magnetization (M) along the z-direction. z The residual magnetization M is aligned along the longitudinal direction of the pipe in the second rotating subsystem. z Magnetization aligned circumferentially with the second subsystem The two are added together to form a composite magnetization vector M1 or M2. Then, vector M is added... z After that, the direction difference of the magnetization vector M between PK1 and PK2 is no longer 180°, that is, the two magnetization vectors act on the oblique defect at different angles.

[0052] If a right-handed oblique defect RH is detected using the first test head PK1, then the magnetization vector M1 is almost perpendicular to the defect, which produces almost the largest leakage magnetic signal. Conversely, for the second test head PK2, the magnetization vector M2 is almost parallel to the defect, thus producing only a very small leakage magnetic signal. For the defect LH with the opposite direction, the signals from the test heads behave completely oppositely.

[0053] The z-component of magnetization in the region of the second subsystem that causes the difference in oblique defect signals is affected not only by the residual magnetization component after passing through the first subsystem, but may also be affected by the demagnetizing coil located behind the second subsystem in the line.

[0054] This effect, first recognized by the inventors, has a significant impact on the repeatability of oblique defects. Repeatability is a crucial quality criterion for magnetic flux leakage testing equipment. Here, repeatability means that regardless of which of the two test heads is used to detect the defect, similar defects should be detected with similar signal amplitudes. Typically, the testing equipment must demonstrate its ability to detect similar defects with repeatability better than 3 dB during customer acceptance. In pipes pre-magnetized by the first subsystem, the signal intensity difference of LH or RH oblique defects detected by the rotating subsystem can be as high as 2 dB or more. Since, in a typical setup, the defect is fully detected by only one of the two test heads, this 2 dB repeatability in oblique defect detection must be added to the normal repeatability due to other influences, which is also approximately 2 dB. Therefore, it may be impossible to systematically meet the acceptance criteria.

[0055] This invention solves the above-mentioned problems and provides a novel structure for a universal testing device. The main features of this testing device are: significantly improved repeatability of oblique defects; and the test object only has residual magnetization of non-critical intensity after passing through the testing device.

[0056] One aspect of the proposed solution to this problem stipulates that the magnetization component M is determined before the corresponding piping section of the second subsystem is tested. z It should be eliminated as completely as possible.

[0057] To this end, a special arrangement of the magnetizing coil 400 is chosen. This arrangement allows for the reduction of residual magnetization in the pipe after passing through the test area of ​​the first subsystem 200 by magnetization with a DC magnetic field of opposite polarity, and to minimize this reduction as much as possible before the pipe enters the second subsystem (see...). Figure 1 The outlet side of the first subsystem 200 has been proven to be an ideal location for the demagnetizing coil 400, which is used as a compensation coil.

[0058] The demagnetizing coil 400, also known as a compensation coil, can have the same structure as the two magnetizing coils 222-1 and 222-2 of the first magnetizing device 220. In this example, the demagnetizing coil is directly mounted on the outlet side of the first subsystem 200, i.e., without any distance. The distance to the second subsystem is many times the possible distance to the first subsystem.

[0059] By using the spatial proximity of the demagnetizing coil 400 to the magnetic flux conductor present in the first subsystem (a very small air gap, i.e., low magnetic reluctance), a smaller coil (smaller coil space) and / or less electrical power can be used to operate. The local effect of the longitudinal magnetic field demagnetizing coil on the pipe to be demagnetized can be precisely controlled, so that the residual magnetic field left in the pipe can be minimized. The demagnetizing coil 400 is operated independently using a current-regulated DC power supply (adjustable DC power supply 424).

[0060] After the part under test leaves the first subsystem, the demagnetizing coil 400 (compensation coil, reverse coil) immediately takes effect in the following way: the demagnetizing coil establishes a DC magnetic field along the -z direction, thereby ideally reducing the residual magnetism in the pipe to zero when the coercive field strength is achieved. If the reverse coil does not fully achieve the coercive field strength, the second subsystem 300 also uses its rotating magnetic field to affect component M. z The reason for performing partial demagnetization is that, at the edge of the pole piece, the magnetic field does not only extend along... Directional orientation (so-called magnetic field expansion).

[0061] If the residual magnetization M along the z direction z If eliminated, then in the region of the second subsystem, only the magnetization component remains. It works, and in longitudinal defect testing, i.e., when tested with the aid of the second subsystem, the pipe behaves like a new pipe that has not been pre-magnetized.

[0062] Similar to the first subsystem, where the current intensity of the magnetizing coil depends on the pipe diameter, wall thickness, and the protective nozzle used to achieve a suitable magnetic operating point, the current I of the reverse coil... g The adjustments are also affected by these dependencies.

[0063] Due to the soft magnetic properties of ferromagnetic pipes, i.e., achieving a coercive field strength in a smaller magnetic field than the initial magnetization, the reverse coil typically requires a smaller field strength than the magnetizing coils 222-1 and 222-2.

[0064] Besides improved repeatability of oblique defects, a properly adjusted demagnetizing coil offers another advantage: the pipe has virtually no residual magnetization behind the second subsystem. In most cases, when measuring residual magnetization, as mentioned above, the value should be in the range of around 0.5 mT. Therefore, only manufacturers or pipe buyers with the highest requirements need an additional demagnetizing coil. However, this demagnetizing coil should not be placed directly adjacent to the two subsystems, but rather at a greater distance from the second subsystem, such as several meters, for example, at least 5 meters, to eliminate any impact on the repeatability of oblique defects. Pure DC demagnetization can be used particularly effectively, especially when at least a qualitative measurement of the material's magnetization state and precise control of the demagnetizing coil are achieved.

[0065] During DC demagnetization, magnetic domains are oriented such that the magnetic field cannot be measured outside the material. However, the domains formed during DC demagnetization appear to be larger than those formed during AC demagnetization, and therefore, small residual magnetism can reform after a certain period of time or after mechanical influence. However, this effect cannot be observed in the leakage magnetic field test proposed here, which connects the longitudinal defect test, the DC magnetic field demagnetizing coil (reverse coil), and the transverse defect test. It is currently believed that the reason for this advantage is that the large magnetic domains after passing through the DC magnetic field reverse coil 400 are sufficiently reduced due to the demagnetizing characteristics of the downstream rotating subsystem (the z-component at the edge of the pole shoe and, for example, rotation at 1-20 Hz).

[0066] As described above, by using the oblique defect correction procedure, it can be determined what level of demagnetizing field strength is needed to sufficiently reduce the residual magnetization acting in the longitudinal direction of the pipe before testing for the presence of a longitudinal defect. Here, the average value of the correction results of the two test heads can be compared. If the error signal amplitudes are different, the magnetization field strength aligned in the longitudinal direction will not be zero, and the DC current intensity in the demagnetizing coil must be adjusted accordingly.

[0067] In the existing calibration process, the longitudinal defect is moved through the measurement range of the test head of the second subsystem at a low feed rate, so that each probe of both test heads detects the longitudinal defect multiple times. With the help of a homogenized magnetic shoe and an ideal probe, the longitudinal defect produces the same maximum signal in each probe, regardless of the test head and the residual magnetization Mz (Fig. 4a, Fig. 4b).

[0068] During oblique defect correction, the above procedure is performed on the oblique defect. Therefore, in cases where residual magnetization Mz exists, there will be a difference in the maximum probe signal between the two test heads. Figure 5A , Figure 5BInstead of individual probe signals, the average signal value can also be used for each test head. The signal difference between the two test heads can be reduced by adjusting the DC current intensity in the demagnetizing coil, with a slant defect correction procedure performed for each adjustment. The procedure ends successfully if the slant defect signals at both test heads have almost the same amplitude, because at this point, the residual magnetization Mz is also approximately zero.

[0069] In some implementations, fully automatic adjustment of the demagnetizing field strength is provided. For this purpose, a test conduit is used in which oblique defects of defined size and orientation are introduced at sufficiently large distances relative to each other. The distance between the oblique defects should be large enough that the DC current intensity of the demagnetizing coil between these defects can be varied during oblique defect correction. For example, it can be set such that the demagnetizing current is high at the first oblique defect and lower at the second oblique defect. Then, based on the demagnetizing current, two error signal values ​​are generated for each test head, for which a fitted straight line can be calculated from the demagnetizing current. These two lines intersect at a point where the error signals of the two test heads are aligned and the longitudinally aligned magnetization component Mz disappears. The coil current corresponding to this value corresponds to the coil current of the demagnetizing coil that can be used to reduce the magnetization acting in the z-direction to zero. By using more than two pairs of oblique defect demagnetizing current values, the accuracy of automatic adjustment can be improved, allowing higher-order functions to be used instead of fitted straight lines.

Claims

1. A test method for detecting defects by performing magnetic flux leakage testing on ferromagnetic test objects, especially ferromagnetic pipes, in a continuous process. Among them, The test object is first conveyed along the travel direction through the first subsystem of the test equipment and then through the second subsystem of the test equipment, and is demagnetized before leaving the test equipment. The first subsystem is designed as a transverse defect testing system and has a first magnetizing device that magnetizes the test object, the magnetic field lines of which extend substantially along the longitudinal direction of the test object. The second subsystem is designed as a longitudinal defect testing system and has a second magnetizing device that magnetizes the test object, the magnetic field lines of which extend substantially along the circumferential direction of the test object. In each of these subsystems, within the test area, the surface of the test object is scanned using at least one magnetic field-sensitive probe to detect leakage magnetic fields caused by defects; and The electrical probe signal of the probe is evaluated for identification of the defect. Its features are, After passing through the test area of ​​the first subsystem, the test object is demagnetized, either partially or completely, by means of a DC magnetic field distributed to the demagnetizing coil of the first subsystem, which is opposite in direction to the magnetic field of the first magnetizing device, before entering the second subsystem.

2. The test method according to claim 1, characterized in that, The magnetic components of the first subsystem are also used to conduct the demagnetizing field.

3. The test method according to claim 1 or 2, characterized in that, By adjusting the connected DC power supply, the strength of the DC magnetic field of the demagnetizing coil can be adapted to the corresponding requirements.

4. The test method according to any one of the preceding claims, characterized in that... The residual magnetization determination operation, wherein a residual magnetization value is determined on the test object after passing through the first subsystem and the demagnetizing coil, the residual magnetization value representing the intensity of the magnetization component aligned along the longitudinal direction of the test object; and characterized in that: the intensity of the DC magnetic field generated by the demagnetizing coil is automatically adjusted according to the residual magnetization value.

5. The test method according to claim 4, characterized in that, In the residual magnetization determination operation, the second subsystem is used as a measurement system for determining the residual magnetization value.

6. The test method according to any one of the preceding claims, Its features are, The second subsystem has a rotating head with pole pieces of the second magnetizing device radially oriented relative to the surface of the test object on opposite sides of each other, such that a DC magnetic field can be generated inside the test object, the magnetic field lines of which extend along the circumferential direction of the test object; at least one probe is arranged between the pole pieces respectively along the circumferential direction; and a skew defect correction procedure is performed, wherein the intensity of the DC magnetic field generated by the demagnetizing coil is adjusted such that the skew defect produces substantially the same error signal amplitude in each probe scanning the skew defect.

7. A testing apparatus (100) for performing magnetic flux leakage testing on a ferromagnetic test object (110), particularly a ferromagnetic pipe, in a continuous process to detect defects, said testing apparatus comprising: A conveying device is used to convey the test object along the travel direction (102) through a first subsystem (200) of the test equipment and a second subsystem (300) located downstream of the first subsystem along the travel direction, wherein... The first subsystem (200) is designed as a transverse defect testing system and has a first magnetizing device (220) configured to generate magnetization in the test object, the magnetizing magnetic field lines extending substantially along the longitudinal direction of the test object. The second subsystem (300) is designed as a longitudinal defect testing system and has a second magnetizing device (330) configured to generate magnetization in the test object, the magnetizing magnetic field lines extending substantially along the circumferential direction of the test object, and Each of these subsystems has a test area (230, 330) with at least one magnetic field sensitive probe (250, 350) that can scan the surface of the test object to detect leakage magnetic fields caused by defects. An evaluation device (500) is used to evaluate the electrical probe signal of the probe for the purpose of identifying the defect; A demagnetizing device, designed to allow the test object to be demagnetized to a tolerable residual magnetization before leaving the test equipment. Its features are, The demagnetizing device has a DC power supply (424) and a demagnetizing coil (400) connected thereto. The demagnetizing coil is arranged in the outlet region of the first subsystem (200) and can be operated with DC power so that the test object magnetized by the first magnetizing device (220) can be demagnetized by reducing the magnetization component extending along the longitudinal direction of the test object.

8. The testing equipment according to claim 7, characterized in that, The demagnetizing coil (400) is fixed to the housing of the first subsystem (200).

9. The testing equipment according to claim 7 or 8, characterized in that, The first magnetizing device (220) has magnetizing coils (222-1, 222-2) at the inlet side and test object channel nozzles (225-1, 225-2) made of ferromagnetic material at the outlet side, respectively. The test object channel nozzles are coupled through a magnetic return conductor (227). Moreover, the demagnetizing coil (400) is arranged on the first subsystem (200) such that the magnetic field lines of the demagnetizing field generated by the demagnetizing coil extend through the magnetic flux conductor at the outlet side of the first subsystem (200).

10. The testing equipment according to claim 7, 8, or 9, characterized in that, The DC power supply (424) of the demagnetizing device is designed as an adjustable DC power supply, so that the intensity of the demagnetizing field is adjustable.

11. The testing equipment according to any one of claims 7 to 10, characterized in that... A control loop is provided for automatically adjusting the strength of the DC magnetic field generated by the demagnetizing coil (400) based on the residual magnetization value, which can be determined in a residual magnetization determination operation.

12. The testing equipment according to claim 11, characterized in that, The second subsystem (300) is configured as a measurement system for determining the residual magnetization value, such that the second subsystem can be used as a functional part of a control loop for setting the correct demagnetization field strength.

13. The testing equipment according to any one of claims 7 to 12, characterized in that, The second subsystem (300) has a rotating head with pole shoes (325-1, 325-2) of the second magnetizing device (320) radially oriented relative to the surface of the test object on opposite sides of each other in diameter, such that a DC magnetic field can be generated inside the test object, the magnetic field lines of the DC magnetic field extending along the circumferential direction of the test object, wherein at least one probe (350) is arranged between the pole shoes respectively along the circumferential direction, wherein the device is configured to perform a skew defect correction procedure, wherein the intensity of the DC magnetic field generated by the demagnetizing coil (400) is adjusted such that the skew defect (SF) produces substantially the same error signal amplitude in each probe scanning the skew defect, regardless of its tilt orientation.

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