1

Select steel pipes similar in shape and material to the natural gas pipeline, and process flat-bottomed holes of different depths as simulated test blocks, as shown in Figure 1, and the set flat-bottomed hole parameters are shown in Table 1.

Table 1 Flat-bottomed hole parameters set on the simulated test block

X-rays are used to transilluminate the flat-bottomed holes of different depths in the simulated test block. The same transillumination layout and exposure parameters are used for the transillumination. After the images are collected, the gray levels of the flat-bottomed holes and adjacent base materials are measured. The numbering sequence of the flat-bottomed holes is as follows As shown in Figure 2, the gray measurement results are shown in Table 2.

Table 2 Gray measurement results

2

Theoretically, under the same exposure conditions, the gray scale of the base material should be the same, but in actual inspection, the difference in focal length and the thickness of the irradiation direction will cause the gray level difference of the base material and the gray level difference of the corrosion pits. The deviation is reduced by the normalization calculation method, and the normalization calculation results are shown in Table 3.

From the calculation formula of the curve slope k k=ΔG/Δh (ΔG is the gray level difference, Δh is the depth of the flat-bottom hole), the average slope=1262, the relative offset of the data is -1.20%~15.82%, and the offset is- 0.12~0.32mm, the statistical results are shown in Table 4.

The calculation formula of the corrosion pit depth h is：h=ΔG×

Through data analysis, the gray difference has a linear relationship with the depth of the flat-bottomed hole, and the relationship curve is shown in Figure 3.

3

Acquire DR images according to the operating instructions. When selecting the gray scale measurement points, the first thing to consider should be the influence of scattered rays. Two points with close scattering ratios should be selected as reference points as much as possible to reduce the gray difference caused by different scattering ratios. The scattering ratio is related to the focal length, the irradiation field, the ray energy, the penetration thickness, the weld reinforcement, the curvature of the steel pipe, etc.; secondly, the change in the thickness of the transillumination caused by the deviation of the ray beam from the center should be considered. The eccentricity of the ray beam also affects the transmittance Certain influence.

Affected by the geometrical layout characteristics of radiographic testing, the penetrating thickness in the circumferential direction of the steel pipe varies greatly. The more the measuring point deviates from the central axis of the pipe, the greater the thickness change, while the axial penetrating thickness changes less. Therefore, when selecting the base material corrosion pit measurement point, the first point is the gray value of the measured corrosion pit, and the second point is the good part of the base material that is offset by a certain distance in the axial direction. The upper and lower symmetrical parts of the corrosion pit take the average gray scale of two points (point 2 and point 3) as the measurement value of the base metal gray scale. The location of the measurement point of the base metal corrosion pit is shown in Figure 4.

When ray transilluminating the weld, the rays of the thinner part will be scattered to the thicker part, and the scattering ratio of different parts is complicated and difficult to quantify. Therefore, it is necessary to reduce the influence of the scattering ratio by selecting an appropriate measuring point. The selection of the corrosion pit measurement point near the fusion line mainly considers the change of the transilluminated thickness caused by the superposition of the internal and external residual heights. Therefore, when selecting the measurement point, the first point is selected at the gray level of the corrosion pit, and the second point is selected near this point along the weld to a non-corrosive location. The location of the corrosion pit measurement point near the fusion line is shown in Figure 5. Shown.

When selecting weld corrosion pit measurement points, the main consideration should be the change of weld reinforcement, the superposition of inner and outer reinforcement, and the change of transillumination thickness and scattering ratio caused by the superposition of two welds during double-wall transillumination. The first point is to choose the gray-scale part of the corrosion pit, and the second point is to choose the part that is offset along the weld to the non-corrosive part near this point; when choosing the second point, the transillumination caused by the decenter of the ray beam should also be considered The thickness changes, as the measuring point is far away from the exposure center point, the thickness deviation becomes larger. Therefore, setting the exposure center in the middle of the first point and the second point can reduce the influence of this factor. The location of the weld corrosion pit measurement point is shown in the figure. 6 shown.

According to the measured grayscale data, the corrosion pit depth is obtained according to the above calculation method. The corrosion pit grayscale and depth data are shown in Table 5.

In the DR inspection of the corrosion pit depth of the in-service pipeline, the image gray level is related to the thickness of the workpiece, the ray energy, the material absorption coefficient, the scattered rays, the irradiation field, the medium in the tube, the content of the corrosion pit, etc.; The degree will also affect the thickness of the transillumination and the ray transmittance; the scattering ratio of each part of the image is also different due to the curvature of the steel pipe. In practical applications, the measurement accuracy of the corrosion pit depth of the pipeline can be improved only by comprehensively considering the above factors.