Study on the Strengthening Effect of Excitation Magnetic Field on Stress-Induced Magnetic Signal of X80 Steels

Weak magnetic excitation stress detection has good engineering application prospects in the pipeline non-destructive testing field. To obtain the quantitative variation law between the excited stress-induced magnetic signal and the axial stress value of X80 steel, an axial tensile experimental test of X80 steel under weak magnetic excitation is conducted in this paper. The enhancing effect of the excitation magnetic field on the sensitivity of stress-induced magnetic signals has been quantitatively analyzed. The results indicate that the value of normal magnetic flux density B z increases by 24 times when the excitation magnetic field reaches 600 A/m. Within the scope of 400 A/m to 700 A/m, the excitation magnetic field of 400 A/m has the best excitation effect on X80 steel.


Introduction
The pipe material of the China-Myanmar (CM) natural gas pipeline is X80 steel, where 70% of the CM pipeline passes through mountainous areas.The pipeline is susceptible to geological hazards.The pipe axial tensile stress due to external loads increases the pipeline fracture failure risk [1][2][3][4].Therefore, it is necessary to detect and evaluate the axial stress status of the X80 steel.
At present, the traditional methods for pipeline axial stress inspection include the small hole method [5], the X-ray diffraction method [6], and the ultrasonic stress detection method [7].However, the small hole method can cause damage to pipelines and is not suitable for stress detection of in-service pipelines.Furthermore, the X-ray diffraction method has problems with the shallow measurement depth and complexity of use [8].Besides, ultrasonic stress detection has high accuracy, fast operation, and no radiation to the human body, but it cannot effectively detect discontinuous structures such as circumferential welds.Magnetic testing technology measures the self-leakage magnetic field of pipes to evaluate its stress state.Therefore, comprehensive inspection of the pipeline body, circumferential welds, and elbows can be carried out through force magnetic inspection technology.
The core foundation of magnetic testing technology is the accusation of the force magnetic coupling effect.Zhang introduced an anisotropic nonlinear force magnetic coupling model aimed at predicting stress magnetization curves in ferromagnetic materials subjected to the simultaneous influence of various loading force directions and environmental magnetic fields [9].Zhao et al. simplified the expression of magneto-strictive strain, taking the effects of demagnetization and linear stress dependence on magneto-strictive effect into account, and proposed a more concise form of force magnetic coupling model [10].Kim proposed a novel force-magnetic coupling model that takes into account the influence of external stress on magnetization strength, magnetic permeability, and magnetostrictive strain, accurately reflecting the intrinsic nonlinear coupling characteristics of ferromagnetic materials.[11].He considered the distinction between the stress of the girth weld and the surrounding environment's magnetic field interference and developed a three-dimensional magnetic signal calculation model specific to the weld.This model allowed for a quantitative analysis of the three components of magnetic flux density within the weld under varying stress conditions and at different detection heights.[12].He established a calculation model for the attenuation of spatial magnetic signals in steel pipelines with distance, and formed a magnetic signal correction method at different heights [13].He eliminates the influence of jitter on the magnetic stress detection results by establishing a magnetic dipole model [14].Zhao developed positioning methods for different pipeline constructions by establishing a magnetic dipole calculation model [15].
In summary, abundant research has been carried out on the stress-magnetic coupling response mechanism and experimental testing of ferromagnetic materials.However, it is challenging to quantitatively study the relationship between stress and magnetic signals under the Earth's magnetic field due to the influence of the initial magnetization state of ferromagnetic materials and external noise interference.This paper conducted axial tension tests on X80 steel under weak magnetic excitation.It analyzed the quantitative variations in the normal magnetic signal Bz and axial stress σa of X80 steel under different excitation magnetic fields.The study investigated the enhancing effect of the excitation magnetic field on stress-induced magnetic signals in X80 steel.The main parameters of the material are shown in table 1 and table 2

Experimental Equipment and Method
The experimental testing system is shown in figure 2. The system consists of a three-phase electric furnace, demagnetizer, microcomputer-controlled electronic universal testing machine, Helmholtz coil, strain gauge, and TMR magnetometer.The MDT USB27053 magnetometer includes a magnetoresistive sensor and data acquisition software.The maximum working magnetic field of the MDT USB27053 magnetometer is 1000 Oe, with an accuracy of 0.35 mOe and a sampling frequency of 40 Hz to 250 Hz. 2) Experimental testing process: figure 3 shows the testing method for measuring the Z-axis component of the X80 steel stress-induced magnetic signal by using an AMR magnetometer.With the Helmholtz coil as the excitation source, excitation magnetic intensity is controlled by controlling the coil current, and the range is 400 A/m to 700 A/m.The stress-induced magnetic signal of X80 steel samples is collected under different axial elastic stress.

Analysis of the Experimental Result
Experiments under the same operating conditions will be repeated three times to ensure the reproducibility of the experiment, with one set chosen for analysis.

Strengthening Effect of Excitation Magnetic Field on Normal Magnetic Flux Density
Figure 4 shows the difference in the Stress-induced magnetic signal of X80 steel under different excitation conditions.Under geomagnetic conditions, due to the influence of metal fixtures used for fixing at both ends of the specimen the magnetic signal exhibits a "U" shaped distribution along the detection direction, and the axial stress has little effect on the magnetic signal.Under the axial excitation magnetic field of 600 A/m, the magnetic signal intensity exhibits a linear relationship along the pipeline axis.The magnetic signal strength gradually decays along the detection direction, with a slope K within the range of -1.8 to -0.82.The absolute value of the slope is negatively correlated with the stress value within the range of 250 MPa to 550 MPa.Magnetic field strength is positively correlated with axial stress.Therefore, it can be concluded that the excitation magnetic field has the dual effect of enhancing the sensitivity of magnetic signals to stress responses and reducing the interference from external magnetic fields.Points 3#, 9#, and 6# represent the positions of both ends and middle of the X80 samples.Figure 5 shows the stress-induced magnetic signal of X80 steel at different positions.At the same conditions, the magnetic signals at different points of the sample exhibit consistent changes with stress.Under no excitation condition, the variation value of the magnetic signal at point 6# is about 0.1 Oe.However, the excitation magnetic field magnifies the magnetic signal as the load increases, with a variation value of approximately 2.5 Oe.The average excitation effect of the excitation magnetic field on the normal magnetic flux density is 2100%.When the axial stress is within the range of 250 MPa to 550 MPa, the axial stress in the middle of the specimen exhibits a linear relationship with the magnetic signal, with a slope of K = -0.594.In the axial stress range of 0 MPa to 250 MPa, the correlation between tangential magnetic flux density and axial stress is relatively poor..

Comparison of Strengthening Effects of Different Excitation Magnetic Fields on Normal Magnetic Flux Density
The relationship curve between X80 steel magnetic signal and axial stress under different excitation magnetic field conditions is shown in figure 6.The strength of the magnetic signal is positively correlated with the excitation magnetic field strength.Under different excitation conditions, the magnetic signal strength decays along the detection direction.The attenuation value is positively correlated with the intensity of the excitation magnetic field.

Quantitative Analysis of the Strengthening Effect of Different Excitation Magnetic Fields on Magnetic Signals
The magnetic detection signals of the sample under axial stress conditions of 187.5 MPa, 312.5 MPa, and 437.5 MPa are analyzed.To quantitatively analyze the effect of excitation magnetic field on the normal magnetic flux density of the specimen, subtract the corresponding initial values under the excitation magnetic field conditions from the magnetic signals to obtain the changes in the magnetic signals after excitation ΔBz, as shown in figure 8.Under the axial stress condition of 312.5 MPa, the ∆BZ of the magnetic signal decreases from 0.57 to 0.14.In the range of 400 A/m to 700 A/m, the strengthening effect of the excitation magnetic field on the magnetic signal decreases with the increase of magnetic field strength.The change value of normal magnetic flux density ΔBz is given by equation ( 1): where ΔBz is the change value of normal magnetic flux density, Oe; Bz is the measured value under excitation conditions, Oe; Bg is the measured value under Earth's magnetic field, Oe.The relationship between the intensity of the excitation magnetic field H, the axial stress σa, and is the change value of normal magnetic flux density ΔBz is given by equation ( 2 where H is the intensity of the excitation magnetic field, A/m; σa is the axial stress, MPa.The calculation results and test results are shown in figure 9.The maximum error between the calculated values and the experimental test results is 0.06, and the average error is 0.004.

Conclusions
This article investigates the strengthening effect of the excitation magnetic field on the force magnetic relationship by conducting tensile experiments.The influence of different excitation magnetic field intensities on the force magnetic relationship has also been studied.The conclusion is as follows: 1) Compared with the condition of Earth's magnetic field, the distribution characteristics of axial magnetic signals of the specimen are significantly changed by the exciting magnetic field.The excitation magnetic field could avoid the interference of noise signals.
2) Compared with the Earth's magnetic field condition, the magnetic signal under the excitation magnetic field condition has a better response to different loads, and the magnetic signal changes relatively evenly with the increase of load, improving the sensitivity of magnetic detection.
3) The excitation magnetic field amplifies the effect of axial stress causing changes in magnetic signals.Compared with the Earth's magnetic field condition, the average excitation effect of the excitation magnetic field on the normal magnetic flux density is 2100%.
4) When the axial stress is within the range of 250 MPa to 550 MPa, the axial stress in the middle of the specimen exhibits a linear relationship with the magnetic signal, with a slope of K = -0.594.In the axial stress range of 0 MPa to 250 MPa, the relationship between axial stress and normal magnetic flux density is not obvious.
5) Different levels of excitation magnetic fields will only change the magnitude of the magnetic signal value and will not affect the distribution pattern of the magnetic signal curve.Within the range of 400 A/m to 700 A/m, the excitation magnetic field of 400 A/m has the best excitation effect on X80 steel.

Figure 1
shows the X80 full-size pipeline.The flat sample is taken from the X80 pipeline.To reduce the influence of ferromagnetic chucks of the tensile loading instrument on the magnetic testing signal, the size of the test sample is set to 200 mm × 40 mm × 4 mm.

Figure 2 .
Figure 2. X80 steel weak magnetic excitation axial tensile testing system.The specific experimental steps include: 1) Experimental pretreatment: the TC-4 demagnetizer is used to demagnetize the sample and weaken the influence of initial magnetization strength on experimental results.The test point positions are marked on the sample surface.2) Experimental testing process: figure3shows the testing method for measuring the Z-axis component of the X80 steel stress-induced magnetic signal by using an AMR magnetometer.With the Helmholtz coil as the excitation source, excitation magnetic intensity is controlled by controlling the coil current, and the range is 400 A/m to 700 A/m.The stress-induced magnetic signal of X80 steel samples is collected under different axial elastic stress.

Figure 3 .
Figure 3. Test method for Z-axis component Bz of X80 steel spatial magnetic field.

Figure 4 .
Comparison of the normal magnetic flux density under different excitation conditions.(a) 0 A/m, (b) 600 A/m.

Figure 5 .
Figure 5.The relation between the stress and the magnetic signal of X80 steel at different positions.

Figure 6 .Figure 7 .
Figure 6.Comparison of stress-induced magnetic signals under different excitation conditions.(a) 400 A/m, (b) 500 A/m, (c) 600 A/m, (d) 700 A/m.As shown in figure7, the force-magnetic relationship curves under axial stress conditions of 187.5 MPa, 312.5 MPa, and 437.5 MPa are analyzed.When the excitation magnetic field increases from 400 A/m to 700 A/m, the average value of the change in normal magnetic flux density is about 2 Oe.

Figure 8 .
Figure 8. Enhancement effect of excitation magnetic field and axial stress on magnetic signals.(a) The excitation magnetic field (b) The axial stress

Figure 9 .
Figure 9.The calculation results and test results.

Table 2 .
Mechanical properties of X80 steel specimens.