A method for measuring residual stress in welded joints based on hardness-stress coefficient

Abstract

A method for measuring residual stress in welded joints based on hardness-stress coefficients comprises the following steps: S1, taking a welded test plate; S2, cutting a tensile specimen on the welded test plate; S3, determining characteristic points on the tensile specimen of the welded test plate, wherein the characteristic points include at least one point each at the weld center, weld edge area, heat-affected zone, and base material area; S4, calibrating the stress coefficient of the tensile specimen of the welded test plate, calculating the stress coefficient and various anisotropic parameters at the position of each characteristic point, and establishing a relationship between the two; S5, measuring the hardness value of each characteristic point, and establishing a hardness value H-stress coefficient K relationship; S6, detecting the hardness value of a test area of ​​the welded part to be tested, and calculating the stress coefficient and various anisotropic parameters of the test area; S7, using an electromagnetic ultrasonic probe to collect polarized shear wave data of the test area, and substituting the stress coefficient and various anisotropic parameters of the test area into a residual stress calculation formula to obtain the residual stress value of the test area.

Description

Technical Field

[0001] The invention relates to a method for measuring residual stress of a weld joint based on hardness-stress coefficient, and belongs to the field of ultrasonic non-destructive testing. Background Art

[0002] As energy demand continues to rise, energy transportation infrastructure continues to expand. Pipelines, as one of the safest modes of energy transportation, are commonly used for long-distance transport of oil, natural gas, and other resources. Welding is an essential step in the pipeline construction process. During pipeline welding, uneven heat generation in the heat-affected zone (HAZ) of the steel produces significant residual stresses. The combination of external loads and residual stresses can negatively impact pipeline operation, leading to stress corrosion cracking, reduced component stiffness, and decreased fatigue strength. Therefore, monitoring residual stress distribution in pipeline joints is essential to ensuring the safety and reliability of pipeline systems.

[0003] Currently, there are many non-destructive testing methods for measuring residual stress, including X-ray diffraction, neutron diffraction, and electromagnetic ultrasonic methods. X-ray diffraction has a low propagation depth and high requirements for the workpiece surface, so it is only suitable for stress detection on the workpiece surface. Compared with X-ray diffraction, neutron diffraction, which uses a high-energy neutron flux as the incident energy beam, has stronger penetration and a propagation depth of decimeters. However, it cannot measure large components and the equipment cannot be moved to the site. Electromagnetic ultrasonic method, as an emerging ultrasonic testing method, combines the advantages of electromagnetic and ultrasonic technologies. It can perform non-destructive and non-contact measurement of residual stress in various materials and geometric shapes. It has the advantages of no coupling, a wide range of measurement methods, and no influence from surface quality. Therefore, it is gradually being widely used for stress detection of components.

[0004] However, electromagnetic ultrasonic testing still faces challenges in residual stress detection in pipeline joints. Localized, uneven heating in welded joints can lead to significant microstructural differences in the material at the joint, which in turn causes variations in the stress coefficient. Because the EMAT probe is large, and the weld and heat-affected zones of the actual test specimens are narrow, multiple areas may exist beneath the probe during testing. Therefore, using the stress coefficient of a single area for calculations inevitably results in errors, making it impossible to obtain accurate stress values. Summary of the Invention

[0005] The object of the present invention is to provide a method for measuring residual stress of a weld joint based on hardness-stress coefficient, which can quickly and accurately detect the residual stress of the weld joint.

[0006] The technical solution adopted by the present invention to achieve its invention object is: a method for measuring residual stress of a weld joint based on hardness-stress coefficient, the steps of which are as follows:

[0007] S1. Take a welding test plate with the same material, thickness and welding process as the weldment to be tested, and determine the weld center, weld area, heat-affected zone and parent material area of ​​the welding test plate;

[0008] S2. Cut a tensile specimen from the welded test plate, wherein the tensile specimen includes the weld center, weld area, heat-affected zone, and base metal area;

[0009] S3. Determine a characteristic point i on the tensile specimen of the welded test plate, wherein the characteristic point includes at least one point each at the weld center, the weld edge, the heat-affected zone, and the base metal zone;

[0010] S4. Use a tensile machine to calibrate the stress coefficient of the tensile specimen of the welded test plate. Use an electromagnetic ultrasonic probe to obtain the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction at each characteristic point under different stresses. The shear wave propagation time parallel to the stress loading direction at the characteristic point i is recorded as t i1 , the propagation time of the shear wave perpendicular to the stress loading direction is t i2 ; According to the residual stress calculation formula Get the stress coefficient K at the characteristic point i i and the anisotropic parameter α i , and according to the stress coefficient K at each characteristic point i i and the anisotropic parameter α i , establish the relationship between stress coefficient K and anisotropic parameter α; in the above formula, σ i is the stress at the characteristic point i, is the anisotropy coefficient at the feature point i;

[0011] S5. Measure and determine the hardness value of each feature point, and record the hardness value at the feature point i as H i , and then the stress coefficient K at each characteristic point is i And hardness value H i Perform fitting to obtain the hardness value H-stress coefficient K relationship;

[0012] S6. Obtain a hardness value of the test area of ​​the weldment using a hardness tester, substitute the hardness value into the hardness value H-stress coefficient K relationship equation obtained in S5, calculate the stress coefficient of the test area, and obtain the anisotropic parameters of the test area based on the relationship between the stress coefficient K and the anisotropic parameter α established in step S4;

[0013] S7. Use an electromagnetic ultrasonic probe to collect the shear wave propagation time of the test area parallel to the weld direction and perpendicular to the weld direction. Substitute the stress coefficient and various anisotropic parameters of the test area obtained in step S6 into the residual stress calculation formula to obtain the residual stress value of the test area.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] Currently, existing electromagnetic ultrasonic residual stress measurement technology only considers three areas: the weld, the heat-affected zone, and the parent material. However, due to the large size of the EMAT probe and the narrowness of the weld and heat-affected zones in the actual test samples, there may be multiple areas under the probe during the test process. Therefore, using the stress coefficient of a single area for calculation will inevitably cause certain errors. The present invention has found through a large number of experiments that the stress coefficient of each area of ​​the weld joint has a certain relationship with various anisotropic parameters and hardness values. Therefore, the present invention first establishes a relationship between the stress coefficient, various anisotropic parameters, and hardness values ​​through calibration tests and hardness tests. When measuring the test area, the hardness value of the test area is detected, and the stress coefficient and various anisotropic parameters of the test area are determined based on the hardness value. Then, the residual stress of the test area is calculated, which greatly improves the measurement accuracy of the residual stress of the joint.

[0016] Furthermore, in the step S1 of the present invention, the weld center, weld area, heat-affected zone and base material area of ​​the weld test plate are determined by hardness value or metallographic method.

[0017] Furthermore, there are four characteristic points i determined in step S3 of the present invention, which are located at the center of the weld, the edge of the weld, the heat-affected zone, and the base material zone.

[0018] Furthermore, the weld edge zone described in the present invention is an area within the weld zone that is within 1 / 4 of the weld width at the junction of the weld zone and the heat-affected zone.

[0019] Furthermore, the specific method of measuring and determining the hardness value of each characteristic point in step S5 of the present invention is: when measuring the hardness value of one of the characteristic points, measure the hardness value of the characteristic point and the point 0.5 mm before and after the characteristic point parallel to the weld direction, and take the average hardness value of the three points as the hardness value at the characteristic point.

[0020] Furthermore, in step S4 of the present invention, a tensile machine is used to calibrate the stress coefficient of the tensile specimen of the welded test plate, and the specific method for obtaining the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction of each characteristic point under different stresses through an electromagnetic ultrasonic probe is as follows: a tensile machine is used to apply force to the tensile specimen of the welded test plate, starting from 0 MPa to 400 MPa, and an electromagnetic ultrasonic probe is used to collect shear wave waveform data parallel to the stress loading direction and shear wave waveform data perpendicular to the stress loading direction of each characteristic point at every 50 MPa, then the shear wave waveform data is denoised, and a time delay algorithm is used to calculate the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction under each stress.

[0021] Furthermore, the specific method of establishing the relationship between the stress coefficient K and the anisotropic parameter α in step S4 of the present invention is:

[0022] The anisotropy coefficient under different stress at each feature point i is As an independent variable, stress σ i As the dependent variable, linear fitting is performed to obtain the linear fitting equation According to the linear fitting equation, the stress coefficient K at each characteristic point i can be determined i and the anisotropic parameter α i ; Then the stress coefficient K at each characteristic point i is i As an independent variable, the anisotropy parameter α i As the dependent variable, linear fitting is performed to obtain the relationship between the stress coefficient K and the anisotropic parameter α.

[0023] Furthermore, in step S5 of the present invention, the stress coefficient K at each characteristic point position is i And hardness value H i The specific method for fitting and obtaining the hardness value H-stress coefficient K relationship is: the hardness value H at each characteristic point i is i As an independent variable, the stress coefficient K i As the dependent variable, linear fitting is performed to obtain the relationship between hardness value H and stress coefficient K.

[0024] The present invention will be further described in detail below through specific implementation methods and drawings, but this does not mean to limit the scope of protection of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the locations of characteristic points and hardness measurement points in an embodiment of the present invention. DETAILED DESCRIPTION

[0026] Example

[0027] A method for measuring residual stress in welded joints based on the hardness-stress coefficient. In this example, the weldment to be measured is an X80 pipeline steel pipe welded joint. The measurement method steps are as follows:

[0028] S1. Take a welding test plate with the same material, thickness and welding process as the weldment to be tested, and determine the weld center, weld area, heat-affected zone and parent material area of ​​the welding test plate. In this example, the weld center, weld area, heat-affected zone and parent material area of ​​the welding test plate are determined by hardness value.

[0029] S2. Cut a tensile specimen from the welded test plate, wherein the tensile specimen includes the weld center, weld area, heat-affected zone, and base metal area;

[0030] S3. Determine a characteristic point i on the tensile test specimen of the welded test plate. The characteristic points include at least one point each at the weld center, weld edge, heat-affected zone, and parent metal zone. In this example, there are four characteristic points, located at the weld center, 1 mm from the boundary between the weld and heat-affected zones within the weld zone, and 0 mm, 8 mm, 10 mm, and 20 mm from the weld center within the heat-affected zone and parent metal zone. Figure 1 ;

[0031] S4. Use a tensile machine to calibrate the stress coefficient of the tensile specimen of the welded test plate. Use an electromagnetic ultrasonic probe to obtain the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction at each characteristic point under different stresses. The shear wave propagation time parallel to the stress loading direction at the characteristic point i is recorded as t i1 , the propagation time of the shear wave perpendicular to the stress loading direction is t i2 ; According to the residual stress calculation formula Get the stress coefficient K at the characteristic point i i and the anisotropic parameter α i , and according to the stress coefficient K at each characteristic point i i and the anisotropic parameter α i , establish the relationship between stress coefficient K and anisotropic parameter α; in the above formula, σ i is the stress at the characteristic point i, is the anisotropy coefficient at the feature point i;

[0032] S5. Measure and determine the hardness value of each feature point, and record the hardness value at the feature point i as H i , and then the stress coefficient K at each characteristic point is i And hardness value H i Perform fitting to obtain the hardness value H-stress coefficient K relationship;

[0033] S6. Obtain a hardness value of the test area of ​​the weldment using a hardness tester, substitute the hardness value into the hardness value H-stress coefficient K relationship equation obtained in S5, calculate the stress coefficient of the test area, and obtain the anisotropic parameters of the test area based on the relationship between the stress coefficient K and the anisotropic parameter α established in step S4;

[0034] S7. Use an electromagnetic ultrasonic probe to collect the shear wave propagation time of the test area parallel to the weld direction and perpendicular to the weld direction. Substitute the stress coefficient and various anisotropic parameters of the test area obtained in step S6 into the residual stress calculation formula to obtain the residual stress value of the test area.

[0035] The specific method of measuring and determining the hardness value of each characteristic point in step S5 of this example is: when measuring the hardness value of one characteristic point, measure the hardness value of the characteristic point and the point 0.5mm before and after the characteristic point parallel to the weld direction, and take the average hardness value of the three points as the hardness value of the characteristic point, that is, the measurement points of the hardness value include the characteristic point and the point 0.5mm before and after the characteristic point parallel to the weld direction, and the point 0.5mm before and after the characteristic point parallel to the weld direction is as follows: Figure 1 The middle hardness value is shown.

[0036] In this example, step S4 uses a tensile machine to calibrate the stress coefficient of the tensile specimen of the welded test plate, and the specific method for obtaining the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction at each characteristic point under different stresses through an electromagnetic ultrasonic probe is as follows: a tensile machine is used to apply force to the tensile specimen of the welded test plate, starting from 0 MPa to 400 MPa, and an electromagnetic ultrasonic probe is used to collect shear wave waveform data parallel to the stress loading direction and shear wave waveform data perpendicular to the stress loading direction at each characteristic point at intervals of 50 MPa, then the shear wave waveform data is subjected to noise reduction processing, and a time delay algorithm is used to calculate the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction under each stress.

[0037] The specific method of establishing the relationship between the stress coefficient K and the anisotropic parameter α in step S4 of this example is:

[0038] The anisotropy coefficient under different stress at each feature point i is As an independent variable, stress σ i As the dependent variable, linear fitting is performed to obtain the linear fitting equation According to the linear fitting equation, the stress coefficient K at each characteristic point i can be determined i and the anisotropic parameter α i ; Then the stress coefficient K at each characteristic point i is i As an independent variable, the anisotropy parameter αi As the dependent variable, linear fitting is performed to obtain the relationship between the stress coefficient K and the anisotropic parameter α.

[0039] The stress coefficients and anisotropic parameters of each characteristic point obtained in step S4 of this example are shown in the following table. The relationship between the obtained stress coefficient K and the anisotropic parameter α is α=0.0316K+1032.9.

[0040] Distance from weld center 0mm 8mm 10mm 20mm <![CDATA[Stress coefficient K i > 61544 58146 68679 55319 <![CDATA[Anisotropy parameter α i > 2980.8 2887.7 3201.3 2769.6

[0041] In this example, in step S5, the stress coefficient K at each characteristic point is i And hardness value H i The specific method for fitting and obtaining the hardness value H-stress coefficient K relationship is: the hardness value H at each characteristic point i is i As an independent variable, the stress coefficient K i As the dependent variable, linear fitting is performed to obtain the relationship between hardness value H and stress coefficient K.

[0042] The hardness values ​​and stress coefficients of the characteristic points obtained in step S5 of this example are shown in the following table. The obtained relationship between the hardness value H and the stress coefficient K is K=617.61H-79532.

[0043] Distance from weld center 0mm 8mm 10mm 20mm <![CDATA[Hardness value H (HV1)]]> 225.58 225.7 239.98 218.4 <![CDATA[Stress coefficient K i > 61544 58146 68679 55319

[0044] The following table shows the measurement results and accuracy comparison of the X80 pipeline steel pipe weld joint measured by this measurement method and the existing method.

[0045] Small hole method measurement value (MPa) 327.4 334.9 178.3 67.5 -23.6 Average error (%) Current electromagnetic ultrasonic measurement value (MPa) 271.3 272.4 226.4 50.1 -27.3 21％ Measurement method in this example Measurement value (MPa) 281.3 292.5 152.8 57.1 -26.8 14％

[0046] As can be seen from the above table, compared with the existing electromagnetic ultrasonic residual stress measurement technology, the average error of the residual stress obtained in this example is smaller, indicating that the method of the present invention improves the measurement accuracy of the residual stress of the weld joint.

Claims

1. A method for measuring residual stress in welded joints based on hardness-stress coefficient, comprising the following steps: S1. Take a welding test plate with the same material, thickness and welding process as the weldment to be tested, and determine the weld center, weld area, heat-affected zone and parent material area of ​​the welding test plate; S2. Cut a tensile specimen from the welded test plate, wherein the tensile specimen includes the weld center, weld area, heat-affected zone, and base metal area; S3. Determine a characteristic point i on the tensile specimen of the welded test plate, wherein the characteristic point includes at least one point each at the weld center, the weld edge, the heat-affected zone, and the base metal zone; S4. Use a tensile machine to calibrate the stress coefficient of the tensile specimen of the welded test plate. Use an electromagnetic ultrasonic probe to obtain the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction at each characteristic point under different stresses. The shear wave propagation time parallel to the stress loading direction at the characteristic point i is recorded as t i1 , the propagation time of the shear wave perpendicular to the stress loading direction is t i2 ; According to the residual stress calculation formula Get the stress coefficient K at the characteristic point i i and the anisotropic parameter α i , and according to the stress coefficient K at each characteristic point i i and the anisotropic parameter α i , establish the relationship between stress coefficient K and anisotropic parameter α; in the above formula, σ i is the stress at the characteristic point i, is the anisotropy coefficient at the feature point i; S5. Measure and determine the hardness value of each feature point, and record the hardness value at the feature point i as H i , and then the stress coefficient K at each characteristic point is i And hardness value H i Perform fitting to obtain the hardness H-stress coefficient K relationship; S6. Obtain a hardness value of the test area of ​​the weldment using a hardness tester, substitute the hardness value into the hardness value H-stress coefficient K relationship equation obtained in S5, calculate the stress coefficient of the test area, and obtain the anisotropic parameters of the test area based on the relationship between the stress coefficient K and the anisotropic parameter α established in step S4; S7. Use an electromagnetic ultrasonic probe to collect the shear wave propagation time of the test area parallel to the weld direction and perpendicular to the weld direction. Substitute the stress coefficient and various anisotropic parameters of the test area obtained in step S6 into the residual stress calculation formula to obtain the residual stress value of the test area.

2. The method for measuring residual stress in welded joints based on hardness-stress coefficient according to claim 1, characterized in that: In step S1, the weld center, weld area, heat-affected zone and base material area of ​​the weld test plate are determined by hardness value or metallographic method.

3. The method for measuring residual stress in welded joints based on hardness-stress coefficient according to claim 1, characterized in that: There are four characteristic points i determined in step S3, which are located at the center of the weld, the edge of the weld, the heat-affected zone, and the base material zone.

4. The method for measuring residual stress in welded joints based on hardness-stress coefficient according to claim 1, characterized in that: The weld edge area is an area within the weld area that is within 1 / 4 of the weld width at the junction of the weld area and the heat-affected zone.

5. The method for measuring residual stress in welded joints based on hardness-stress coefficient according to claim 1, characterized in that: The specific method of measuring and determining the hardness value of each characteristic point in step S5 is: when measuring the hardness value of one of the characteristic points, measure the hardness value of the characteristic point and the point 0.5 mm before and after the characteristic point parallel to the weld direction, and take the average hardness value of the three points as the hardness value at the characteristic point.

6. The method for measuring residual stress in welded joints based on hardness-stress coefficient according to claim 1, characterized in that: In step S4, a tensile machine is used to calibrate the stress coefficient of the tensile specimen of the welded test plate, and a specific method for obtaining the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction at each characteristic point under different stresses by using an electromagnetic ultrasonic probe is as follows: a tensile machine is used to apply a force to the tensile specimen of the welded test plate, starting from 0 MPa to 400 MPa, and an electromagnetic ultrasonic probe is used to collect shear wave waveform data parallel to the stress loading direction and shear wave waveform data perpendicular to the stress loading direction at each characteristic point at intervals of 50 MPa, then the shear wave waveform data is subjected to noise reduction processing, and a time delay algorithm is used to calculate the shear wave propagation time parallel to the stress loading direction and the shear wave propagation time perpendicular to the stress loading direction under each stress.

7. The method for measuring residual stress in welded joints based on hardness-stress coefficient according to claim 1, characterized in that: The specific method of establishing the relationship between the stress coefficient K and the anisotropic parameter α in step S4 is: The anisotropy coefficient under different stress at each feature point i is As an independent variable, stress σ i As the dependent variable, linear fitting is performed to obtain the linear fitting equation According to the linear fitting equation, the stress coefficient K at each characteristic point i can be determined i and the anisotropic parameter α i ; Then the stress coefficient K at each characteristic point i is i As an independent variable, the anisotropy parameter α i As the dependent variable, linear fitting is performed to obtain the relationship between the stress coefficient K and the anisotropic parameter α.

8. The method for measuring residual stress in welded joints based on hardness-stress coefficient according to claim 1, characterized in that: In step S5, the stress coefficient K at each characteristic point is i And hardness value H i The specific method for fitting and obtaining the hardness value H-stress coefficient K relationship is: the hardness value H at each characteristic point i is i As an independent variable, the stress coefficient K i As the dependent variable, linear fitting is performed to obtain the relationship between hardness value H and stress coefficient K.

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