The stress corrosion cracking of stainless steel pressure-bearing equipment usually occurs on the surface of the material in contact with the corrosive medium. This type of cracking often has no obvious signs of deformation, which is extremely harmful and seriously threatens the safe operation of the equipment. The GB/T 30579-2014 standard provides corresponding detection and monitoring methods for this type of cracking:
Visual inspection of the material surface and penetration inspection of suspicious parts;
Perform eddy current testing on pipes, heat exchanger tube bundles and equipment surfaces.
The array eddy current detection technology uses electronic means to drive multiple adjacent detection coils in the same probe, and uses the powerful analysis, calculation and processing functions of the eddy current instrument to achieve rapid and effective detection of materials. The GB/T 34362-2017 standard points out that the array eddy current technology can be used to detect cracks on the surface of the material. Compared with the traditional penetration testing, the array eddy current testing has a large single scan coverage area, high detection efficiency, and defects in different directions. It has the same sensitivity, no need for polishing, good coupling of flexible probes, real-time data storage, and crack depth measurement.
For on-line detection of stress corrosion cracks in pressure-bearing equipment, Phased Array Ultrasonic Total Focus Imaging (TFM) technology is an effective method. It has high resolution for small cracks and can basically restore the morphology of defects, which is beneficial The qualitative nature of the defect can greatly improve the measurement accuracy.
Based on this, the inspectors from Ningbo Labor Safety Technology Service Co., Ltd. and Ningbo Special Equipment Inspection and Research Institute used a vertical reactor made of S31603 in a chemical company as the research object, and adopted array eddy current technology on its inner surface. The phased array ultrasonic technology based on TFM is used for detection and compared with the penetration test results. It is hoped that it can provide a new idea for the chloride stress corrosion cracking detection of austenitic stainless steel pressure equipment.
The design pressure of the vertical reactor is 0.6MPa, the design temperature is 85℃, the working pressure is 0.32MPa, the working temperature is 85℃, the working medium is hydrochloric acid and oxygen, the volume is 24.7m3, the inner diameter is 2800mm, and the cylinder design wall thickness It is 10mm, and the design thickness of the head is 14mm.
When the container was inspected for the first time in December 2017, radiographic and phased array ultrasonic testing revealed a strip defect on the girth weld between the lower head and the cylinder, with a length of 10mm and a height of 1.0mm. According to the "Fixed Section 8.5.10 of the "Regulations for Safety Technical Supervision of Type Pressure Vessels" stipulates that its safety status is rated as level 3. The company has not carried out repairs and plans to conduct an open tank inspection in December 2020.
The damage mode of the stainless steel vertical reactor is chloride stress corrosion cracking. The main reason is that the medium contains chloride and dissolved oxygen, and there is a certain welding residual stress in the weld.
Array eddy current detection adopts OmniScan MX type equipment, as shown in Figure 1, it supports up to 64 coil detection, the frequency is 20Hz~6MHz, which can realize multi-frequency scanning. The detection probe is an orthogonal array eddy current probe based on flexible printed circuit board technology, which can be attached to the weld and heat-affected zone for detection. The special MagnaFORM type scanner is adopted, and the manual or semi-automatic mode is adopted to realize the coverage inspection of the entire weld surface including the heat-affected area through a single scan, thereby improving the inspection efficiency.
Figure Array eddy current detection system
The equipment used in the full-focus phased array inspection is the GEKKO portable phased array flaw detector, as shown in Figure 2, which has 128 channels, high-resolution imaging functions and powerful matrix data processing capabilities. The test uses a 64-chip linear array probe with a frequency of 10 MHz.
Array eddy current testing
Probe calibration is required before testing, which mainly includes sensitivity compensation and defect quantitative calibration. The main operating process of sensitivity compensation is: first balance the probe on the standard test block; then scan the entire standard test block for data collection, and perform automatic compensation after selecting defects. The defect quantitative calibration process is: first scan the standard test plate; then calibrate the defects with depths of 0.5, 1, 2, 3, and 4mm. The array eddy current detection frequency is 300kHz, the probe's driving voltage is 1.0V, the gain is 35dB, the scanning speed is 295mm/s, and the scanning width is 70mm. Under this parameter, C-scan imaging is performed on the circumferential weld of the lower head of the vertical reactor and the suspicious part of the heat-affected zone.Full-focus phased array detection
Full focus phased array detection only needs to set the full focus range, resolution and wave mode. The experimental setup model is a flat model. The main parameters are set as follows: the width of the full focus area is 40mm, the height is 16mm; the resolution is 8.5 sampling points/ mm; The wave mode is LL (longitudinal wave-longitudinal wave).
Array eddy current testing
Figure 3 Array eddy current C-scan imaging result of weld
It can be seen from Figure 3 that the array eddy current technology has a higher detection rate for stress corrosion cracks in welds and heat-affected zones. Many stress corrosion cracks span the weld and heat-affected zone. Array eddy current testing can only show the location, size and depth of the cracks, and cannot show the growth morphology of stress corrosion cracks. This is because of the resolution and detection of array eddy current testing. The diameter of the coil is related to the micron-level accuracy of penetration detection, and it is difficult to show local details. In the later stage, you can also use software to set up to further increase the display contrast of the defect, and evaluate the depth of stress corrosion cracking through the display color, amplitude and phase of the defect.
Table 1 Array eddy current test results of welds
| Number of cracks detected/piece | | |
| | | |
| | | |
| | | |
Figure 4 Full-focus phased array inspection imaging results of welds
It can be seen from Figure 4 that the use of fully-focused phased array method to detect stress corrosion cracks from the outside of the vertical reactor has a higher detection rate and resolution, because the fully-focused phased array uses a single wafer to generate incident waves, and multiple wafers Signals from different angles are collected, so the inclined bottom wave signals are clearly visible, which can more truly reflect the morphology and characteristics of the cracks, and the signals related to the cracks can be found, which are less affected by the orientation of the defects, and are more quantitative in the location of cracks. High accuracy. At the same time, it can be noted that the fully-focused phased array has a small surface blind area, only about 0.5mm, which is conducive to the detection of near-surface defects.
Table 2 Full-focus phased array inspection crack parameters of welds
The solvent removal penetrant inspection method is used to detect the weld, the penetrating time is 10min, and the imaging time is 10min.
Figure 5 Penetration inspection results of welds
It can be seen from Figure 5 that Defect 2 is a crack, which straddles the weld and the heat-affected zone, originates in the heat-affected zone, has obvious stress corrosion cracking characteristics, and presents a dendritic bifurcation morphology. It can be seen that penetration testing can qualitatively analyze defects to a certain extent, and the detection results are in good agreement with the results of array eddy current testing and all-focus phased array testing.
Array eddy current technology is used to detect stress corrosion cracks from the inner surface of pressure-bearing equipment, which can detect shallow buried defects near the surface, and can save and analyze data.Using all-focus phased array technology to detect stress corrosion cracks from the outer surface of pressure-bearing equipment, it has a high detection rate and resolution, and has high accuracy in both quantitative and qualitative stress corrosion cracks.
Authors: Hu Jian1, Zhang Zijian1, Shen Jianmin2, Chai Junhui1,2, Xu Bo2, Wu Jiaxi1, Zhang Xiaolong1
Work unit: 1. Ningbo Labor Safety Technology Service Co., Ltd.
2. Ningbo Special Equipment Inspection and Research Institute
The first author: Hu Jian, engineer, is mainly engaged in the research of non-destructive testing of pressure-bearing special equipment.
Corresponding author: Zhang Zijian, master, engineer, mainly engaged in the research and application of new technology of non-destructive testing.
Source: "Non-destructive Testing" Issue 5, 2021
