CN102759567B - The EDDY CURRENT identification of steel pipe inside and outside wall defect and evaluation method under DC magnetization - Google Patents

Abstract

The EDDY CURRENT identification of steel pipe inside and outside wall defect and an evaluation method under DC magnetization, the present invention discloses EDDY CURRENT identification and the evaluation method of steel pipe inside and outside wall defect under a kind of DC magnetization.The method adopts through mode DC magnetization coil to carry out unsaturation magnetization to steel pipe, test coil of eddy current is utilized to pick up the electromagnetic distu information of fault location, rational DC magnetization scope is selected according to phase correlation, thus detect inside and outside wall defect and identified by phase difference, and by setting the alarm threshold value of inside and outside wall defect respectively, realize the consistency checking of the inside and outside wall defect to identical damage equivalent.Pick-up unit must comprise phase sensitive detection module, to eliminate the interference in fault leakage magnetic field; The inventive method detection speed is fast, easily is automated, goes for the defects detection of various ferromagnetic pipeline, have more wide application prospect.

Description

Identification and evaluation method of eddy current testing for inner and outer wall defects of steel pipe under DC magnetization

technical field

The invention relates to an eddy current detection identification and evaluation method for defects on the inner and outer walls of steel pipes under direct current magnetization.

Background technique

Eddy current testing is suitable for non-destructive testing of conductor materials and is widely used in testing practice. Initially, it was mainly aimed at the defect detection of non-ferromagnetic materials such as aluminum and copper. There is a linear change between the defect depth and phase, but the method is extended When it is applied to flaw detection of steel pipe, the signal noise of eddy current detection is caused by the fluctuation of magnetic permeability on the surface of steel pipe, so that the defect information is submerged and it cannot be detected. Generally, the fluctuation of material permeability is suppressed by applying a saturated magnetization field to improve the signal-to-noise ratio of defects. The Chinese national standard "Test method for eddy current testing of steel pipes" (GB/T 7735-2004) and the machinery industry standard "Non-destructive testing of pressure equipment Part VI: Eddy current testing" (GB/T 4730.6-2005) both involve this method, but The signal phases of defects at different depths on the outer wall of the steel pipe are basically consistent, resulting in phase failure, so only the amplitude index can be used to represent the defect equivalent. However, it is now found that when the magnetic saturation method is used, the inner wall defects of the steel pipe are also detected by the eddy current probe, and its phase is still consistent with the outer wall defects, resulting in signal aliasing, which makes the evaluation of inner and outer wall defects of steel pipes a technical problem.

The Chinese patent literature discloses a "method for identification of inner and outer wall defects based on AC-DC composite magnetization", which involves a magnetic flux leakage inspection technology for distinguishing inner and outer wall defects of steel pipes, mainly applying DC and AC excitation The magnetic field magnetization steel pipe uses the outer wall defect to only respond to the DC signal component, and the inner wall defect responds to both the AC and DC signal components, so as to realize the identification of defects, but this method is inconvenient to quickly detect flaws by placing the sensor inside the pipe wall. In addition, the sensor needs electromagnetic shielding, and if the shielding effect is not good, misjudgment may occur. The patent document "A Device for Inner and Outer Wall Defect Detection Based on Far-field Eddy Current" discloses another detection technology for inner and outer wall defects of steel pipes, which involves a far-field eddy current detection method that uses multiple magnetoresistive devices to completely cover the circumference of the pipe. To realize the detection of defects on the inner and outer walls of the pipe, but the detection speed of this technology is slow, and it cannot distinguish and identify the defects on the inner and outer walls of the steel pipe.

Contents of the invention

The object of the present invention is to provide an eddy current detection, identification and evaluation method for inner and outer wall defects of steel pipes under DC magnetization, which has the advantages of simple operation, fast detection speed and applicable to various ferromagnetic materials.

The present invention is achieved like this, and this method comprises the steps:

1. Use the through-type magnetization coil to excite the uniform static magnetization field, implement axial magnetization on the steel pipe inside the effective magnetization zone, and suppress the fluctuation of the permeability of the steel pipe;

2. Use close-packed coils as eddy current detection probes. The coils are passed with AC excitation currents, and eddy current fields are generated inside the steel pipes in the detection area. Generally, there are two eddy current probes that are connected in reverse differentially, and have a certain distance, and are coaxial with the steel pipes. The inner diameter is slightly larger than the outer diameter of the steel pipe to ensure smooth running of the steel pipe;

3. The eddy current detection probe picks up the defect information of the steel pipe to be inspected, converts the magnetic field information caused by the disturbance of the eddy current field at the defect of the steel pipe under DC magnetization into an eddy current impedance electrical signal, and sends it to the computer after amplification, phase-sensitive detection and A/D conversion ;

4. The computer uses impedance analysis software to analyze the eddy current impedance signal. Due to the different DC excitation of the magnetizing coil and the AC excitation of the eddy current probe, the amplitude and phase of the eddy current detection signal on the inner and outer walls of the steel pipe will change. Therefore In order to realize the identification of positional defects at different levels, the detection implementation parameters are processed as follows:

⑴ AC excitation signal frequency of eddy current probe :

Determine the excitation signal frequency of the eddy current probe according to the walking speed of the steel pipe to be inspected, which can be calculated by formula (1):

(1)

In the formula, is the walking speed of the steel pipe, in m/s;

(2) Exciting current of DC magnetizing coil :

Take the standard rectangular groove with the same parameters on the inner and outer walls of the steel pipe as the defect of the comparison sample, and adjust the excitation current of the DC magnetizing coil , and implement eddy current detection, when the signal phase difference when the current recorded as ; when the signal amplitude when the current recorded as ; current It can be determined by formula (2):

(2)

(3) The initial phase angle of the impedance signal of the eddy current detection system :

When the DC magnetizing coil excitation current for When , adjust the initial phase angle of the system impedance signal , can be determined by formula (3):

(3)

In the formula, for when The initial phase angle of the inner wall defect when is zero, at this time, the phase angle of the inner wall rectangular groove signal Adjust to 90o.

5. Use the phase and amplitude comparison method to distinguish and evaluate the inner and outer wall defect signals:

⑴ Signal distinction between inner and outer wall defects

When the steel pipe to be inspected passes through the DC magnetizing coil and the eddy current probe at a constant speed, the computer obtains the impedance signal of the defect, and the inner and outer wall defects can be judged by the formulas (4) to (5): when the formula (4) is established, the signal is the inner wall defect The generation signal of , when the formula (5) is established, the signal is the generation signal of the outer wall defect;

(4)

(5)

In the formula, according to the difference of ferromagnetic material, 5～10o; is 30o;

⑵ Set alarm thresholds for inner and outer wall defect signals respectively and , which can realize the determination of inner and outer wall defects with the same damage equivalent, and the method for determining the defect equivalent of eddy current testing, wherein .

The present invention is also characterized in that the eddy current detection system must include the signal conditioning part of the phase sensitive detection, the detection system includes a differential through-type eddy current probe, a balance bridge, an amplification filter conditioning, a phase sensitive detection module and a data Acquisition transmission card, computer and signal processing software part. The main reason for using the eddy current detection system is that the leakage magnetic field will be generated at the defect when the DC magnetization is implemented. After the eddy current probe quickly scans through the defect, not only the detection signal contains defect information, but the leakage magnetic field also generates an induced electromotive force in the probe. Interference is caused to the test results, resulting in misjudgment. The phase-sensitive detection module can extract defect information and eliminate the disturbance effect of defect leakage magnetic field.

The method proposed by the present invention based on the eddy current detection of unsaturated DC magnetization to distinguish and identify the inner and outer wall defects of steel pipes is essentially to use the joint action of the defect electrical conductivity and magnetic permeability characteristic changes (outer wall defects) of steel pipes or only the magnetic permeability characteristic changes ( Inner wall defect) acts on the eddy current detection signal, so as to implement the distinction and identification of inner and outer wall defects. Even under magnetic saturation DC magnetization, regardless of the outer wall defect or the inner wall defect, the magnetic permeability is still in a non-saturated state in the edge area, resulting in local distortion of the magnetic permeability. This factor makes the inner wall defect detected, and the eddy current detection The imaginary component of the signal is much larger than the real component, so the impedance diagram shows that the phases are basically the same, and cannot be distinguished. Weaken the DC magnetization field to the unsaturated magnetization region, the outer wall defect is affected by the change of conductivity at the defect, the permeability distortion of the defect edge and the uniform permeability of the adjacent defect area. Under different magnetic field strengths, the contribution of the three There will be different degrees of rise or fall, so that the phase of the eddy current detection impedance signal changes accordingly; while the inner wall defect is still only detected by the diffusion of the uneven magnetic permeability caused by the magnetization to the outer wall, thus ensuring that its phase is basically Keep it still, so that the inner and outer wall defects can be detected in a suitable non-saturation magnetization state, and the discrimination and identification can be realized.

The method proposed by the present invention based on the eddy current detection of unsaturated magnetized DC magnetization to distinguish and identify the defects of the inner and outer walls of steel pipes adopts the DC magnetization coil to excite the static magnetic field. Reasonable DC magnetization range, and then the steel pipe to be inspected is subjected to eddy current inspection with the same inspection process parameters, so as to detect inner and outer wall defects and identify them by phase difference. Finally, by setting the thresholds of inner and outer wall defects separately, the consistency judgment of inner and outer wall defects with the same damage equivalent is realized.

The steel pipe eddy current detection method proposed by the present invention breaks through the limitation that the "magnetic saturation method" described in the existing national standards cannot identify inner and outer wall defects, and avoids the same damage equivalent damage caused by the original evaluation method only through the signal amplitude Non-uniform evaluation; at the same time, the method of the present invention is low in detection cost, fast in detection speed, easy to realize automation, and due to the consideration of the difference in magnetic properties of different ferromagnetic materials, it can be applied to the defect detection of various ferromagnetic pipelines, and has a wide range of advantages. application prospects.

Description of drawings

Figure 1 is a schematic diagram A of the conventional uniform permeability-influenced point of view detection principle of eddy current detection of steel pipe outer wall defects under magnetic saturation.

Figure 2 is a schematic diagram B of the conventional uniform magnetic permeability effect point of view detection principle of eddy current detection of steel pipe outer wall defects under magnetic saturation.

Figure 3 is the simulation calculation distribution map A of the magnetic permeability of the outer wall defect of the steel pipe under magnetic saturation.

Fig. 4 is the simulation calculation distribution map B of the magnetic permeability of the inner wall defect of the steel pipe under magnetic saturation.

Fig. 5 is a schematic diagram A of the magnetic characteristic influence detection principle of the eddy current detection of the outer wall defect of the steel pipe under magnetic saturation.

Fig. 6 is a schematic diagram B of the magnetic characteristic influence detection principle of the eddy current detection of the inner wall defect of the steel pipe under magnetic saturation.

Fig. 7 is a schematic diagram of the failure principle of the eddy current detection of steel pipe under magnetic saturation to distinguish the inner and outer wall defects.

Fig. 8 is a schematic diagram of the detection method of the present invention.

Fig. 9 is a schematic diagram of the detection method probe and signal conditioning structure of the present invention.

Fig. 10 is a schematic diagram of the detection probe structure and bridge connection of the present invention.

Figure 11 shows the output waveforms of the eddy current detection probes 3a and 3b when the DC excitation ampere-turns is 11000 when the present invention detects inner and outer circumferential grooves with a width of 1mm, a depth of 0.5mm and 1mm (the wall thickness of the steel pipe is 6mm). picture.

Figure 12 shows the detection of the inner and outer circumferential grooves with a width of 1 mm, a depth of 0.5 mm and a depth of 1 mm (the wall thickness of the steel pipe is 6 mm), when the eddy current detection probes 3a and 3b have a DC excitation ampere-turn of 10,000 When the output waveform diagram.

Detailed ways

The present invention will be described in further detail below in conjunction with accompanying drawings and examples.

As shown in Figures 1 and 2, in the eddy current testing of steel pipe magnetic saturation method, the conventional view is that the magnetic permeability of the outer wall defect 2 or inner wall defect 2′ area after the steel pipe 1 is magnetized is uniformly distributed, so according to the skin depth calculation formula of eddy current testing (where, is the excitation signal frequency; is the magnetic permeability of the steel pipe; is the electrical conductivity of the steel pipe), the eddy current field 4 generated by the eddy current probe 3 can penetrate into the outer wall defect 2, so it can be detected, but the inner wall defect 2′ of the thick-walled steel pipe cannot be detected because the eddy current field 4 cannot penetrate to this position. However, the simulation calculation results of the magnetic permeability of the steel pipe 1 are shown in Figures 3 and 4, which show that even if the DC magnetization field H1 makes the steel pipe 1 reach saturation magnetization, the magnetic permeability of the edge regions of the outer wall defect 2 and the inner wall defect 2′ is still in a non-saturated state , this distorted permeability distribution is inconsistent with the conventional view.

As shown in Figures 5 and 6, when using the magnetic saturation method to perform eddy current testing on the steel pipe 1, for the outer wall defect 2, the eddy current detection signal is affected by the change of the electrical conductivity and magnetic permeability at the gap of the defect and the local variation of the magnetic permeability in the vicinity of the defect. The common influence of the non-uniform area 5, so as to realize the detection of defects; for the inner wall defect 2', although the eddy current field 4 generated by the detection probe cannot penetrate into the defect, the inhomogeneous area of magnetic permeability caused by the magnetization of the inner wall defect 2' 5' has diffused to the outer wall of the steel pipe 1, affected by this, an eddy current detection signal is generated.

Due to the sufficient magnetization, for the outer wall defect 2, the dominant factor is the local unsaturated region 5 in the vicinity of the defect, while the inner wall defect 2' is only the magnetic permeability inhomogeneity region 5', which leads to eddy current testing. The imaginary part of the signal impedance is much larger than the real part, so that the signal phases tend to be consistent and cannot be distinguished, as shown in the actual detection results in Figure 7. Therefore, by reducing the intensity of the DC magnetization field H1, the contribution equivalent of the three factors of the eddy current signal caused by the outer wall defect 2 is changed, so as to obtain a signal that is different from the inner wall defect 2′, which can be detected under a non-saturated DC magnetization field And distinguish the inner and outer wall defects of the components to form an eddy current testing method for the unsaturated magnetization of the steel pipe 1.

As shown in Figures 8, 9 and 10, the main implementation steps of the eddy current detection method for unsaturated magnetized steel pipes proposed by the present invention are as follows:

1) Excite the uniform static magnetization field by using the through-type magnetization coil 6, the ampere-turn number of the coil is selected between 2000 and 40000, and implement axial magnetization on the steel pipe 1 inside the effective magnetization zone;

2) Use close-packed coils as the eddy current detection probe 3. Generally, there are two eddy current probes and reverse differential connections. The coils 3a and 3b have AC excitation current. The number of coil turns is selected within 30-100 turns, with a certain distance. Arranged concentrically with the steel pipe 1, its inner diameter is slightly larger than the outer diameter of the steel pipe 1;

3) The eddy current detection coils 3a and 3b pick up the defect 2 or 2′ information of the steel pipe 1 to be inspected, and convert the magnetic field information caused by the disturbance of the eddy current field 4 at the defect of the steel pipe 1 under DC magnetization into an eddy current impedance electrical signal, which can be recorded as V ₁ , V ₂ is sent to the computer through the phase-sensitive detection 10 and the A/D acquisition system 13 to form an impedance diagram with the real part of the impedance as the abscissa and the imaginary part of the impedance as the ordinate;

4) The computer uses impedance analysis software to analyze the eddy current impedance signal. Due to the different DC excitation of the magnetizing coil 6 and the AC excitation of the eddy current probe 3, the amplitude and phase of the eddy current detection signal of the steel pipe 1 defect will change. , so in order to realize the identification of positional defects at different levels, the detection implementation parameters are processed as follows:

⑴ AC excitation signal frequency of eddy current probe :

Determine the excitation signal frequency of the eddy current probe according to the walking speed of the steel pipe to be inspected, which can be calculated by formula (1):

(1)

In the formula, is the walking speed of the steel pipe, in m/s;

(2) Exciting current of DC magnetizing coil 6 :

Take the standard rectangular groove with the same parameters on the inner and outer walls of the steel pipe 1 as the defect of the comparative sample, and adjust the excitation current of the DC magnetizing coil 6 , and implement eddy current detection, when the signal phase difference when the current recorded as ; when the signal amplitude when the current recorded as ; current It can be determined by formula (2):

(2)

(3) The initial phase angle of the impedance signal of the eddy current detection system :

When the DC magnetizing coil 6 excitation current for , rotate the phase adjustment module 11 to adjust the initial phase angle of the system impedance signal , can be determined by formula (3):

(3)

In the formula, for when The initial phase angle of the inner wall defect when is zero, at this time, the phase angle of the inner wall rectangular groove signal Adjust to 90o.

5) Use the phase and amplitude comparison method to distinguish and evaluate the inner and outer wall defect signals:

⑴ Signal distinction between inner and outer wall defects

When the steel pipe 1 to be inspected passes through the DC magnetizing coil 6 and the eddy current probe 3 at a constant speed, the outer and inner wall defects 2 and 2′ can be judged by the formulas (4) to (5): when the formula (4) holds true, the signal is an inner wall defect 2′, when the formula (5) is established, this signal is the generation signal of outer wall defect 2;

(4)

(5)

In the formula, according to the difference of ferromagnetic material, 5～10o; for 30°;

⑵ Alarm thresholds are set for external and internal wall defect 2 and 2′ signals respectively and , can realize the determination of the outer and inner wall defects 2 and 2' with the same damage equivalent, and the above-mentioned eddy current detection defect equivalent determination method .

The probe and signal conditioning part of the unsaturated magnetized steel pipe eddy current detection system provided by the present invention are shown in Figure 10, mainly including a through-type eddy current detection probe 3, a bridge 8, a first-stage signal amplification and filtering module 9, and a phase-sensitive detection module 10. A phase adjustment module 11, a second-stage signal amplification and filtering module 12, and an A/D data acquisition system 13, wherein the bridge 8 includes coils 3a, 3b, and bridge arm resistors 8a and 8b. The eddy current detection probe 3 is fixed on the penetrating coil 6 through the sensor fixing mounting plate to ensure that the coil is concentric with the steel pipe. The phase-sensitive detection module 10 is mainly used to convert the electrical information of the probe containing defects into signals of the real part and imaginary part of the impedance, and can also filter out the interference caused by the leakage magnetic field signal of the defect.

Detection example: when carrying out eddy current detection on the steel pipe 1, a magnetization field of a certain intensity is applied to the through-type DC magnetization coil 6, and the steel pipe 1 can pass through the magnetization coil 6 and the eddy current detection probes 3a and 3b at a constant speed by using the pair of rollers 7 and 7', and the computer obtains After the defect impedance information is obtained, the frequency of the AC excitation signal of the eddy current probe is determined according to formulas (1)-(3) , The excitation current of the DC magnetizing coil , The initial phase angle of the impedance signal of the eddy current detection system , to realize the detection of the steel pipe 1. The detection eddy current probes 3a and 3b are shown in FIG. 9 .

When the detection is carried out, the moving speed of the steel pipe is 0.5m/s, and the excitation signal frequency of the eddy current probe is calculated according to formula (1) kHz, so 10kHz can be chosen. For the circumferential notch defects with inner and outer wall widths of 1 mm and depths of 0.5 mm and 1 mm respectively in a steel pipe with a wall thickness of 6 mm, when the signal phase difference hour, is 13A; when the signal amplitude hour, 9A; current It can be determined by formula (2):

 

The number of turns of the DC magnetizing coil is 1000, so its number of ampere-turns is 11000 . According to the formula (3) to adjust the inner wall groove defect is 90o, at this time It is 118o.

The detection signal waveforms picked up by the eddy current probes 3a and 3b under this detection condition are shown in Figure 11, and the signals are denoted as , , and , and the corresponding phase of its signal is denoted as , , and .

for and , the phase relationship of the signal is as follows:

                      

In the formula, is 10o.

Therefore, the formula (4) is satisfied, indicating that the signal is an inner wall defect signal, which is consistent with the actual defect position.

for and , the phase relationship of the signal is as follows:

                      

In the formula, for 30o.

Therefore, the formula (5) is satisfied, indicating that the signal is an outer wall defect signal, which is consistent with the actual defect position.

Further, the judgment of the outer wall defect or inner wall defect signal at different depths is carried out according to the signal amplitude, as shown in Figure 11, so the alarm area can be set in the impedance signal area of the eddy current test, and the inner wall defect alarm threshold of the alarm area is Greater than the inner wall defect alarm threshold , which can realize the identification and evaluation of the inner and outer wall defects of the same parameter.

In addition, when the current of DC magnetizing coil 6 When it is 10A, its ampere-turns is 10000 , the corresponding detection signal waveform is shown in Figure 12, and the signal is denoted as , , and , and the corresponding phase of its signal is denoted as , , and , judged according to formulas (4) and (5), and for inner wall defects, and It is an outer wall defect, indicating that when the excitation current fluctuates slightly, the signal discrimination is still valid and will not affect the characteristics of the signal.

Claims (2)

1. An eddy current detection identification and evaluation method for defects on the inner and outer walls of steel pipes under DC magnetization. The eddy current detection system must include the signal conditioning part of the phase-sensitive detection. The detection system includes a differential external pass-through eddy current probe, a balance bridge, In the filter conditioning, phase-sensitive detection module and data acquisition transmission card part, the phase-sensitive detection module extracts defect information and eliminates the disturbance effect of defect leakage magnetic field; the following steps are also included: 1) Use the through-type magnetizing coil to excite a uniform static magnetization field, and implement axial magnetization on the steel pipe inside the effective magnetization zone to suppress the fluctuation of the steel pipe permeability; 2) Use close-packed coils as eddy current detection probes, the coils are passed with AC excitation current, and eddy current fields are generated inside the steel pipe in the detection area. Generally, there are two eddy current probes and reverse differential connections, with a certain distance, coaxial with the steel pipe The inner diameter is slightly larger than the outer diameter of the steel pipe to ensure smooth running of the steel pipe; 3) The eddy current detection probe picks up the defect information of the steel pipe to be inspected, converts the magnetic field information caused by the disturbance of the eddy current field at the defect of the steel pipe under DC magnetization into an eddy current impedance electrical signal, and sends it to the computer; 4) The computer uses impedance analysis software to analyze the eddy current impedance signal; It is characterized by: ⑴ AC excitation signal frequency of eddy current probe : Determine the excitation signal frequency of the eddy current probe according to the walking speed of the steel pipe to be inspected, which can be calculated by formula (1): (1) In the formula, is the walking speed of the steel pipe, in m/s; (2) Exciting current of DC magnetizing coil : Take the standard rectangular groove with the same parameters on the inner and outer walls of the steel pipe as the defect of the comparison sample, and adjust the excitation current of the DC magnetizing coil , and implement eddy current detection, when the signal phase difference , the current is recorded as ; when the signal amplitude , the current is recorded as ; current It can be determined by formula (2): (2) (3) The initial phase angle of the impedance signal of the eddy current detection system : When the DC magnetizing coil excitation current for When , adjust the initial phase angle of the system impedance signal , can be determined by formula (3): (3) In the formula, for when The initial phase angle of the inner wall defect when is zero; At this time, the phase angle of the signal of the rectangular slot on the inner wall Adjust to 90o. 2. A method for eddy current detection, identification and evaluation of inner and outer wall defects of a steel pipe under direct current magnetization according to claim 1, characterized in that: ⑴ Signal distinction between inner and outer wall defects When the steel pipe to be inspected passes through the DC magnetizing coil and the eddy current probe at a constant speed, the computer obtains the impedance signal of the defect, and the inner and outer wall defects can be judged by the formulas (4) to (5): when the formula (4) is established, the signal is the inner wall defect The generation signal of , when the formula (5) is established, the signal is the generation signal of the outer wall defect; (4) (5) In the formula, according to the difference of ferromagnetic material, 5～10o; is 30o; ⑵ Set alarm thresholds for inner and outer wall defect signals respectively and , which can realize the determination of inner and outer wall defects with the same damage equivalent, and the method for determining the defect equivalent of eddy current testing, wherein .