METHOD AND DEVICE FOR OBSERVING A MAGNETIC FIELD OF A VOLUME OF MATERIAL AND USE OF THE DEVICE

Description

Field of the invention

[0001] The invention relates to a method and a device for observing a magnetic field of a material volume and to the use of the device according to the preamble of claims 1, 10 and 13, respectively. Background of the invention

[0002] To observe the magnetic field of a material volume in order to determine the properties of a workpiece, inductors, Hall sensors, or similar devices are used. The temporal change of the magnetic field in response to a magnetic excitation is recorded. From the temporal change of the resulting magnetic field, conclusions can be drawn about the properties of the workpiece.

[0003] The methods and devices known from the state of the art are based on macromagnetic effects as a function of time.

[0004] For example, it is known from US 4634976 A to detect material defects by magnetically exciting a workpiece and evaluating the Barkhausen noise. Comparable prior art is known from US 4977373 A and US 20130276546 A1.

[0005] In the above-mentioned methods, the macromagnetic effects are only recorded and evaluated in the amplitude-time domain.

[0006] From PIOTROWSKI L ET AL: "An In-Depth Study of the Barkhausen Emission Signal Properties of the Plastically Deformed Fe-2%Si Alloy", IEEE TRANSACTIONS ON MAGNETICS, Vol. 44, No. 11, (2008-11-01), pages 3828-3831, ISSN: 0018-9464, DOI: 10.1109 / TMAG.2008.2002634, it is known to excite a material volume of a workpiece arranged in a shield with an alternating magnetic field. A time-resolved FFT analysis of the Barkhausen signal and the distribution of the pulses is performed.

[0007] LR PADOVESE ET AL: "Time-frequency and time-scale analysis of Barkhausen noise signals", PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS, PART G: JOURNAL OF AEROSPACE ENGINEERING, Vol. 2007, No. 5, (2009-04-30), pages 577 - 588, ISSN: 0954-4100, DOI: 10.1243 / 09544100JAERO436 describes a time-frequency analysis of Barkhausen noise signals under magnetization of a material sample.

[0008] All known methods and devices have the disadvantage that the workpiece can only be inadequately evaluated. Summary of the invention

[0009] Based on this, the invention is based on the object of creating a method and a device for observing a magnetic field of a material volume as well as the use of the device according to the preamble of claims 1, 10 and 13, respectively, which are more precise.

[0010] This object is achieved according to the features of claims 1, 10 and 13 respectively.

[0011] Accordingly, a method is created for observing a magnetic field of a material volume, in particular for determining properties of a workpiece under mechanical and / or thermal excitation or the like of a material volume of the workpiece, in which the magnetic field of the material volume is recorded as a function of time and frequency with a high-frequency resolution. The high-frequency resolution in the time and / or frequency axis enables precise detection of micromagnetic effects, which allow more precise qualitative conclusions about the material properties.

[0012] Preferably, the micromagnetic response to the excitation is recorded.

[0013] Furthermore, the speed of the response to the stimulus can be recorded.

[0014] The frequency and / or time resolution of the excitation and / or detection is preferably selected according to the micromagnetic effects in the material volume.

[0015] The excitation can be carried out with an alternating magnetic and / or electric field and / or static field and / or by means of eddy current induction and / or by material processing such as induction hardening and / or by applying an alternating or direct current and / or applying an alternating and / or direct voltage.

[0016] During excitation and / or detection, the material may be stationary relative to a sensor and / or an excitation device or may be moved relative to a sensor and / or an excitation device.

[0017] The frequency of the exciting source can be varied, in particular a frequency range can be scanned.

[0018] Multiple excitation devices and / or multiple magnetic field sensors can be used.

[0019] The invention further provides a device for observing a magnetic field of a material volume, in particular for determining properties of a workpiece under mechanical and / or thermal excitation of a material volume of the workpiece, wherein a sensor is provided for detecting the magnetic field of the material volume as a function of time and frequency with a high-frequency resolution.

[0020] Preferably, an excitation device is provided for electrically and / or magnetically exciting a material volume of the workpiece.

[0021] The invention further provides, relates to and / or enables: Detection of cracks in ferritic materials Detection of blowholes or foreign material inclusions in ferritic materials Evaluation of crystal formation during cooling of ferritic materials Creation of a magnetization map of a component Comparison of the magnetization map of a sample component with other components for quality assessment Detection of hardness defects or hardness changes in materials Detection of the hardening depth and the hardness gradient in ferritic materials Detection of the grain or crystallite structure in iron and other ferritic materials Generation of the magnetization of ferritic materials by means of an alternating magnetic field, which is optimized in its temporal progression for a uniform reorientation sequence of the individual Weiss domains Generation of the magnetization by moving the material past a static magnetic field so that all crystallites experience the same gradient of an external magnetic field Calibration of aFast measurement of the magnetization of ferritic materials with a slower, high-resolution measurement for recording Description of the magnetization behavior of a material volume after a single magnetization Description of the mechanical properties of a material volume after a single magnetization Description of the magnetization behavior of a material volume without having reached magnetic saturation in the entire volume Determination of alloying components of ferritic materials based on the magnetization properties of individual crystallites Application of a combination of short-term FFTs and their temporal evaluation to analyze the magnetization properties of all Weiss domains located in a material volume Application of a combination of short-term FFTs and their temporal evaluation to analyze the magnetization properties of all Weiss domains located in a material volume, which are arranged in specific groupsaccording to size and position in the external magnetic field, application of a combination of short-term FFTs and their temporal evaluation to generate a magnetization map of a material area or a component Character description

[0022] Fig. 1 illustrates a recording of the temporal magnetic field curve after a magnetic excitation according to the state of the art. Fig. 2 illustrates a multidimensional recording of the magnetic field curve in the frequency-time diagram. Fig. 3 shows the main emissions from Fig. 2 with frequency, time and amplitude curves. Fig. 4 shows the main emissions from Fig. 3 with projection onto the timeline. Fig. 5 shows the main emissions from Fig. 3 with projection onto the frequency axis. Fig. 6 to 9 show further embodiments. Description of the embodiments Analysis of magnetic fields

[0023] Fluctuations in magnetic fields are analyzed.

[0024] The fields are measured using suitable sensors, inductors, Hall sensors, etc. The frequency bandwidth of the sensors should be as wide as possible. A frequency of up to 1 MHz, 3 MHz, 50 MHz, 100 MHz, 1 GHz, or 10 GHz is preferred. The upper frequency limit correlates with the qualitative resolution; that is, the higher the frequency limit, the smaller the magnetic structures that can be detected. In the GHz range, the change in the magnetic field of individual dipoles can be detected. Thus, there is a correlation between the upper frequency limit and the crystallite structure of the material.

[0025] The measurement signals are preferably subjected to many short-time FFTs so that the broadband frequency dependence can be described at any time and also in its temporal sequence.

[0026] Further transformations that describe the temporal and quantitative magnetic field changes in detail are conceivable. A pulse analysis algorithm generates the necessary information.

[0027] For practical purposes, around 64 million measured values ​​are evaluated for one measurement. However, a data volume ten times higher can also be interesting. Analysis of the microstructure of ferrous materials

[0028] As a ferromagnetic material, iron forms magnetic dipoles in each Weiss domain.

[0029] These dipoles can be reoriented by applying an external magnetic field. As the microdipoles pivot toward the direction of the external magnetic field, its strength increases until eventually all microdipoles are aligned with the external field.

[0030] To overcome the magnetic moment present in the microdipole, a certain opposing magnetic field strength is necessary. The dipoles closest to the field source experience the greatest force. The more microdipoles are reoriented, the greater the resulting field strength in the direction of the external field.

[0031] This allows even more distant microdipoles to be reoriented.

[0032] The change in the magnetic field generates an electric current, which in turn generates an opposing magnetic field. The greater the rate of change of the external magnetic field, the greater the induced counterforce, which slows the reorientation of the microdipoles at greater distances.

[0033] With a very sensitive measurement of the magnetic field changes, the effects of the reorientation of the microdipoles can be resolved.

[0034] By limiting the external magnetic field to a specific volume of material, conclusions can be drawn about the structure and properties of this volume of material. The effects caused by more distant structures can be neglected due to their small number. Since the Weiss domains, i.e., the material volumes that exhibit a uniform orientation of the FE electron spins and thus represent magnetic microdipoles, also exhibit mechanically interesting properties, some mechanical properties can be inferred from their magnetic properties.

[0035] The size distributions of microcrystals in the iron material also describe tensile and compressive strengths.

[0036] If the exact size and spatial extent of the iron crystallites are known, precise statements can be made about the mechanical properties of the material.

[0037] Elements that are intentionally or unintentionally introduced into the iron change the mechanical, electrical and magnetic properties of the material.

[0038] Carbon-containing steels are widely used when hardened iron materials are to be used. The specific distribution of carbon in the iron, the size distribution of the crystallites, the complete or incomplete incorporation of all available carbon atoms in iron lattices, the presence of elemental carbon in the material, etc., all influence the material's properties.

[0039] The same applies to other important alloying elements or interfering elements that unintentionally enter the material.

[0040] Many different methods are used for analysis.

[0041] Compression and tensile tests of defined cross-sections or surfaces as well as load tests of finished components.

[0042] Microscopy of material samples to describe the crystal structures and the inclusions of various alloy components.

[0043] X-ray and eddy current measurements as examples of non-destructive examinations.

[0044] It is well known that alternating magnetic fields are applied and individual magnetic parameters are determined by repeatedly traversing magnetic hysteresis curves, i.e., by repeatedly reversing the polarity of an external magnetic field. The parameters are determined as average values ​​of steady-state conditions.

[0045] Also known from the prior art is the evaluation preferably based on the amplitude, time, and frequency distribution of a single magnetization. Even passing through half of a hysteresis curve in frequency-time analysis provides information about the material being examined. If the hysteresis curve is passed through completely, see Fig. 2using the example of a sinusoidal magnetic excitation, or if several runs are recorded, the reliability of the measurement results increases.

[0046] The excitation can be optimized to ensure the hysteresis curve is as flat as possible. This results in a longer magnetization process and a more meaningful result. The excitation curve can be optimized to suit the material.

[0047] The reliability of the measurement results can be increased by repetition, but this is about reducing the measurement error.

[0048] As is known from the state of the art, the frequency distribution of the magnetic field change and the amplitude jumps associated with the frequencies can be analyzed and from this the size distribution of the Weiss domains can be determined.

[0049] If the sampling speed of the magnetic field changes is greater than the average speed of the reorientation of the individual Weiss domains, the individual domains and their size can be assigned to the location of the magnetic field strength, since the domains in the area of ​​higher field strengths are reoriented first.

[0050] The magnetic field strength or the rate of change of the field strength can be varied to successively add information about deeper material layers and thus obtain a depth profile of the average sizes of the Weiss domains.

[0051] This allows us to generate a description of the hardness and strength profiles of the material as well as a description of the distribution of material defects insofar as they affect the characteristics of the microdipoles. Non-inventive example:

[0052] The average crystallite size is 10µm^3. The relevant field size is 4x8x2mm, or 64mm^3. This means that a total of approximately 64*10^6 microdipoles are affected.

[0053] At a sampling rate of 100MHz and a time to saturation magnetization of one second, the magnetic field increases of the individual dipoles can be described.

[0054] The change in the orientation of the electron spin axis of a single electron can only occur in certain quantizations.

[0055] Neighboring electrons within a Weiss domain are magnetically aligned with each other, meaning they all have coordinated spin axis directions. When the sudden rotation of the spin axis of some electrons begins, this leads to an accelerated flipping of all neighboring electrons, which already experience a similar external magnetic field strength. When the speed of this flipping process is at its maximum, this results in a quasi-impulsive increase in the magnetic field on the order of the magnetic field of the aligned Weiss domain relative to the direction of the external magnetic field. It is precisely these rapid changes within a Weiss domain, which are superimposed on the overall magnetic field, that are captured by our analysis.

[0056] With an increasing number of aligned or largely aligned microdipoles, the resulting total magnetic field grows ever faster.

[0057] By cleverly changing the external magnetic field, this avalanche effect, which is then reflected in the hysteresis curve of the overall magnetic field, can be compensated so that we achieve the most linear progression of the field strength increase possible.

[0058] The preferred intention behind this is to achieve the best possible resolution of the flipping behavior of all crystallites contained in the volume under consideration for a given maximum sampling rate.

[0059] Crystallites that have identical solid angle differences between their internal magnetic field orientation and the external magnetic field, and that are located in regions of identical external field strength, cannot be distinguished in their rotational behavior. In addition, they are subject to the effects of the surrounding crystallites, which also change the magnetic field overall, but also at the microscale. Nevertheless, there may be crystallites whose behavior cannot be separated. Therefore, it also seems sensible to use measurement results that describe the cumulative effects of the magnetic field changes rather than the full resolution.

[0060] This approach can be used to perform a measurement in significantly shorter time. It achieves a compromise between fully capturing all crystallite influences and a very fast measurement.

[0061] It is also conceivable that the high-resolution measurement is carried out with new materials and that these results are then used to calibrate significantly faster but lower-resolution measurements.

[0062] The process requires a magnetic field that varies throughout the volume of the material. This can be achieved, for example, by applying a magnetic field whose strength changes over the desired period of time and / or by moving the material through a static magnetic field.

[0063] If the crystallite structure of the material changes (e.g., due to a change in hardness), this is reflected in a changed distribution of the magnetic field increases. This method thus makes it possible to perform measurements on moving materials, for example, within production processes.

[0064] The measuring probes can be installed in a wire drawing or other strand forming machine to analyze the material for deviations along its entire length during forming and to obtain information about the actual strengths and other mechanical properties.

[0065] By applying repeated measurements, different volume elements of a material can be analyzed, as well as temporal changes, e.g. during cooling in a hardening process, can be investigated.

[0066] This allows the drop below the Curie temperature and the progression of crystallization effects in the material to be observed.

[0067] The measuring probes can be used directly in a forming tool for hot forming and hardening of steel sheets and the process of crystallization and thus hardening can be observed in situ.

[0068] In addition to analyzing the crystallite structure of the material, macroscopic defects such as cracks or cavities can also be detected if they are located in the area of ​​the magnetic field analysis.

[0069] Through fast measurements, the invention enables complete scanning of component surfaces or component layers.

[0070] An analysis down to the hardening depth is particularly helpful for surface-hardened steels. These material areas are particularly exposed to stress both during the hardening process and later during component use.

[0071] For serially manufactured components, a magnetic field change map can be created, resulting from scanning the relevant material layers. The behavior of each subsequent component can then be compared with this map, and deviations can be immediately recorded.

[0072] The method according to the invention is suitable, for example, for detecting new hardening zones in ground surfaces such as those occurring in bearing or gear manufacturing. Examples

[0073] Fig. 1 This figure illustrates the temporal progression of the magnetic field H following magnetic excitation by applying a sinusoidally modulated external magnetic field to a ferromagnetic material sample. The AC components of the changes in the resulting magnetic field are shown. (High-pass filter with a cutoff frequency of approximately 1000 Hz, sampling rate of 50 MHz, downsampling to 6 MHz) The sine period is clearly at 50 Hz. A complete sweep through the hysteresis characteristic is shown.

[0074] Fig. 2is a representation in the time-frequency domain with a high-frequency resolution in the frequency domain, allowing micromagnetic effects to be identified. In addition to the two main emission fields, each of which can be assigned to a magnetic excitation pulse, many typical secondary emissions are visible. The frequency-time diagram forms a fingerprint from which conclusions about the micromagnetic processes can be derived.

[0075] Fig. 3 shows the main emissions from Fig. 2 with frequency, time and amplitude curves.

[0076] Fig. 4A shows the main emissions with projection onto the time axis. Fig. 4B to 4D show alternative signal shapes. Thus, for any excitation, even non-magnetic, the signal shape can be used to infer a material property. Whether the signal shape rises more steeply than it falls, Fig. 4B , or rises more gently than it falls, Fig. 4C , or a double ( Fig. 4D, 4E) or multiple oscillations, is meaningful for characterizing material, tool, or process properties. In particular, the signal shape, whether projected as in Fig. 4 or on average the Fig. 2 or evaluated multidimensionally as in Fig. 2 , 3 , information can be obtained regarding the hardness of the material, the residual stress, the microstructure, alloy components, etc.

[0077] Fig. 5 shows the main emissions projected onto the frequency axis.

[0078] For electrical or magnetic excitation not in accordance with the invention, the excitation frequency is varied or selected differently depending on the evaluation objective. At high frequencies, the penetration depth into the material is lower than at low frequencies. Therefore, it may be useful to traverse a frequency range during excitation in order to capture material properties depending on the depth of the material.

[0079] For example, in Fig. 6 In the embodiment shown, the surface of a rotating wheel 2, e.g. a wheel tire of a train, or of a flat material 3, e.g. a plate, is scanned by means of a magnetic field sensor 1 to create a material property profile, in particular a hardness or strength profile, of the respective surface, wherein the material property can advantageously be detected as a function of the distance from the surface.

[0080] The method can also be used to assess weld seams. By scanning the weld seam, a depth-graded assessment of the weld quality, strength, etc. can be obtained, and defects such as cracks can be detected.

[0081] The methods, devices and uses according to the invention can be used particularly advantageously in process monitoring.

[0082] For example, during wire drawing, wire 4, which is drawn from a wire drawing device along an arrow 5, can be moved along a magnetic field sensor 6 or several magnetic field sensors 6', detecting a magnetic field of the material volume detected by the magnetic field sensor. Excitation is expediently provided by a device 7, e.g., in the form of a magnetization 8. A device 7' downstream in the drawing direction 5 can be provided for demagnetization 8'.

[0083] Instead of the wire 4, a flat material, e.g. a sheet 4, can also be guided past the magnetic field sensor 6.

[0084] Bulk material 9, such as screws, can fall through a device 7 along an arrow 10 following the force of gravity and be detected by a magnetic field sensor 6, see Fig. 8 .

[0085] Device(s) 7, 7'... and magnetic field sensor(s) 6, 6'... can generally be arranged along any material path.

[0086] What these process monitoring variants have in common is that, with a stationary magnetic field sensor arrangement, a continuous material flow enables essentially complete testing. Hardness, density fluctuations of the material, surface waviness, wire diameter, and the like can be detected.

[0087] The excitation, which in the above examples is preferably magnetic and / or electrical, is carried out according to the invention mechanically or thermally.

[0088] Thus, a mechanical action on a workpiece, in particular a plastic and / or elastic deformation or machining and the like, causes a detectable magnetic field change, especially in the case of ferro- or paramagnetic or metallic material. Fig. 9Shown is a wire drawing device 11 with plastic and, if necessary, elastic forming 12 and a magnetic field sensor 6. The inventive detection and evaluation of the magnetic field resulting from the forming 12 allows the quality of the forming to be determined. Thickness variations, surface waves or defects, as well as wire breakage, and the like, can also be detected.

[0089] All embodiments of the invention are preferably usable with ferromagnetic material, but also with paramagnetic material or with non-magnetic material. For example, if the plate 4 is Fig. 7Made of plastic, the damping behavior of the plate 4 can be used by the device 7 and the sensor 6 to determine the thickness, defects such as holes or material inclusions, density variations, cracks, or the like, surface waviness, etc. The invention can also be used with modern composite plastics or plastics containing particles. Here, for example, the particle density, the uniformity of the particle distribution, and the quality of the composite can be monitored, and defects such as faulty connections and detachments in flat composite materials, or inclusions or the like, can be detected.

[0090] Another application is process monitoring during hardening, particularly press hardening. The forming and / or heating of the material serves as the excitation. The magnetic field changes during cooling, during forming, and after forming. By detecting the magnetic field, the crystallization processes can be observed and evaluated. Conclusions can be drawn about the degree of crystallization and the microstructure, or the point in time at which the material can be released from the mold can be determined because the hardening process is complete. In addition, the previously mentioned information is available, such as defects (crack formation during cooling, including microcracks, fractures, etc.), and process optimization is possible by recording the material parameters during a variation of the process parameters: temperature, forming pressure, heating rate, cooling rate, temperature profile as a function of time, forming rate or profile, etc.

[0091] In terms of evaluation technology, the invention preferably provides for working with characteristic vectors. A characteristic vector is an n-tuple of individual values ​​such as frequency distribution, single or multiple peaks, peak slope, magnetic field, etc. Such a characteristic vector can be recorded as a sample for known properties and subsequently compared with characteristic vectors acquired during testing to determine material and / or process properties.

Claims

1. A method for monitoring a magnetic field of a material volume for the determination of properties of a workpiece by exciting a material volume of the workpiece, comprising detection of the magnetic field of the material volume as a function of time and frequency with a high-frequency resolution, characterised by a mechanical and / or thermal excitation.

2. The method according to claim 1, characterised in that the frequency and / or time resolution of the detection is selected corresponding to the micromagnetic effects in the material volume.

3. The method according to claim 1 or 2, characterised in that the speed of the reaction to the excitement is detected.

4. The method according to any one of claims 1 to 3, characterised by a magnetic and / or electrical excitation and / or by the fact that the excitation takes place with a magnetic and / or electrical alternating field and / or static field and / or by means of eddy current induction and / or by material processing such as for example induction hardening and / or by applying an alternating or direct current and / or applying an alternating and / or direct voltage and / or by material deformation.

5. The method according to claim 4, characterised in that a ferromagnetic workpiece is used under magnetic excitation and the magnetic field is detected during the passage of one half of the hysteresis curve, a complete hysteresis curve or a plurality of successive hysteresis curves.

6. The method according to any one of claims 1 to 5, characterised in that the material is stationary relative to the sensor and / or excitation device during the excitation and / or the detection.

7. The method according to any one of claims 1 to 5, characterised in that the material is moved relative to the sensor and / or the excitation device during the excitation and / or the detection.

8. The method according to any of claims 1 to 7, characterised in that the frequency of the exciting source varies, in particular a frequency range is passed through.

9. The method according to any one of claims 1 to 8, characterised in that a plurality of excitation devices and / or a plurality of magnetic field sensors are used.

10. An apparatus for observing a magnetic field of a material volume for the determination of properties of a workpiece by exciting a material volume of the workpiece, comprising a sensor for detecting the magnetic field of the material volume as a function of time and frequency with a high-frequency resolution, characterised by an excitation device for the mechanical and / or thermal excitation of the workpiece or a material volume of the workpiece.

11. The apparatus according to claim 10, characterised by an excitation device for the magnetic and / or electrical excitation of the workpiece or a material volume of the workpiece.

12. The apparatus according to claim 10 or 11, characterised by an evaluation device for the three-dimensional evaluation of the magnetic field in the frequency-time spectrum.

13. Use of the apparatus according to any one of claims 10 to 12 for process monitoring, in particular for testing a continuously fed and / or produced wire-shaped or flat material or bulk material, in particular during wire drawing.

14. The use of the apparatus according to any one of claims 10 to 12 for measuring the hardness and / or strength profile of a surface, in particular a surface of a workpiece with a circular diameter or a weld seam.

15. Method, apparatus or use according to any one of the preceding claims, characterised by a correlation of the detected waveform with a material property such as for example hardness, residual stress, microstructure, alloying constituents.