ERW welded pipe is made by using the skin effect and proximity effect of high-frequency current to quickly heat the edge of the pipe blank to the welding temperature and then squeeze and weld it. Compared with seamless steel pipes, ERW welded pipe has the advantages of high dimensional accuracy, low price and high production efficiency, and its grain size and compactness of the structure are better than seamless steel pipes; compared with submerged arc welded pipes of the same specification, ERW welded pipes The production speed is fast, and there is no local thinning of the anti-corrosion layer at the welding seam. Its application areas involve casing used in oil drilling, subsea oil and gas pipelines in the offshore oil industry, oil and gas pipelines and gas distribution pipes for trunk lines and urban pipeline networks, etc.
Although China introduced
ERW steel pipe production technology relatively early, there are still serious shortcomings in digestion, absorption and innovation. Compared with developed countries, there is still a certain gap in the quality of ERW welded pipe in my country, and its use in domestic long-distance pipelines is also subject to certain restrictions. The welding quality of ERW welded pipe is the main factor that affects its performance. The pressure test of ERW welded pipe and the cracking and pipe burst during use are mostly caused by welding defects. Therefore, the inspection level of ERW welded pipe is improved, and the defect of welded pipe is improved. The detection rate is of great significance for ensuring the quality of ERW welded pipe and ensuring the safe operation of ERW welded pipe and pipeline.
There may be a variety of welding defects in ERW welded pipes, some of these defects come from the base metal, and some arise from the welding process. Different defects have different effects on welding quality. Area defects such as cracks and unfusion are prone to stress concentration under stress conditions, which are the main cause of low-stress brittle fracture of welds; while volume defects such as pores and slag inclusions, although their cracking sensitivity is less than area defects However, the effective cross-sectional area of the weld is reduced, and the strength of the weld is reduced. Under the action of external force, these defects will often become the source of cracks and eventually lead to the crack of the weld.
1. Ultrasonic inspection in the production process of ERW welded pipe
Ultrasonic inspection is currently the main non-destructive testing method in the production process of ERW welded pipes. Its main application areas include: 1) On-line ultrasonic testing of steel plates. 2) ERW welding, ultrasonic on-line inspection of welds after removal of internal and external burrs. 3) Offline inspection of ERW welds. 4) Ultrasonic inspection of ERW welded pipe ends.
1.1 Ultrasonic online inspection of steel plate
Steel plate ultra-raising online detection generally uses dual crystal or polycrystalline probes, coupled with water film or partial water immersion, and its main purpose is to detect delamination defects in the steel plate parallel to the surface of the steel plate. There are two main scanning methods: one is scanning along parallel lines in the rolling direction; the other is the linear movement of the steel plate along the rolling direction and the reciprocating movement of the probe perpendicular to the direction of movement of the steel pipe, forming a "z"-shaped scan. Since the edge of the steel plate forms a weld in the subsequent ERW welding, the detection of defects at this place is particularly important in the ultrasonic inspection of the steel plate. Related standards and specifications require 100% scanning of the edge of the steel plate. In actual work It is generally ensured by increasing the number of probes on the edge of the steel plate.
1.2 Ultrasonic online inspection of welds
ERW weld ultrasonic on-line inspection is carried out after welding and internal and external burrs are removed. It mainly includes two parts: one is to use A-scan or B-scan to detect the scraping effect of internal and external burrs. Compared with A-scan, B-scan can display the morphology of the inner wall of the weld after removing the internal burrs in real time, and the graphic display is more intuitive; the second is the oblique incidence of longitudinal waves, and the transverse waves generated by the refraction in the welded pipe are used to detect welding defects. Due to the high weld temperature at this time, high temperature probes are generally used for on-line inspection, using local water immersion methods.
1.3 Weld ultrasonic offline inspection and pipe end inspection
Offline ultrasonic testing of ERW welds is generally carried out after hydraulic test and chamfering, and is mainly used to detect longitudinal defects in welds and heat-affected zones. In order to improve detection efficiency, automatic detection is generally adopted. Due to the automatic detection of the influence of the blind zone of the tube end. Afterwards, manual ultrasonic scanning of welds is generally added. The content of pipe end inspection mainly includes the detection of pipe end welds, pipe end base material delamination defects, and axial and circumferential defects. The detection of delamination defects generally uses a split probe, and the axial direction in the weld and base metal And circumferential defects are mostly scanned with oblique probes.
2. Selection of ERW welding seam ultrasonic testing process parameters
Ultrasonic testing of ERW welds mainly includes two methods: automatic testing and manual testing. At present, the automatic inspection of ERW welds mainly adopts two forms: wheel probe inspection and local water immersion inspection. Automatic detection has the advantages of high detection efficiency and fast speed, but it is not conducive to precise positioning and qualitative and quantitative analysis of defects. In comparison, manual detection is more flexible, not only can accurately locate defects, but also Perform qualitative and quantitative analysis of defects through echo characteristics and dynamic waveforms. For defects detected by automatic ultrasonic, manual methods are generally used for further confirmation. The process parameters that need to be considered for ultrasonic testing of ERW welds include refraction angle, sound beam width, and testing frequency.
2.1 Choice of refraction angle
Ultrasonic testing of ERW welds generally adopts oblique incidence of longitudinal waves, and the detection is achieved by generating transverse waves in the workpiece and welds through wave form conversion. The two basic conditions are: 1) pure transverse waves are excited in the base metal and welds. 2) The inner wall of the steel pipe is scanned by the transverse sound beam.
2.2 Selection of detection frequency
The frequency range of ultrasonic testing is relatively wide, generally 0.5-10MHz. The following factors should be considered in the selection of frequency:
(1) The sensitivity of ultrasonic testing is about one-half of the wavelength. Increasing the frequency will help to find smaller defects. In addition, the higher the frequency, the smaller the pulse width and the higher the resolution.
(2) High frequency, short wavelength, small half-diffusion angle, good sound beam directivity, and concentrated energy, which are conducive to finding and locating defects. But for the same wafer size, the higher the frequency, the larger the near-field area, which is more unfavorable for detection.
(3) As the frequency increases, the scattering and absorption attenuation of ultrasonic waves increase sharply, which is unfavorable for detection.
In the detection, various factors should be considered comprehensively, and the detection frequency should be selected reasonably. For ERW welded pipes, the base material is generally hot-rolled coils with relatively fine grains. The width of the weld fusion zone and heat-affected zone after induction heating and extrusion molding is relatively narrow, and its grain size is basically the same as that of the base material. In actual work, in order to obtain a higher resolution, a higher frequency should be selected as much as possible under the premise of ensuring the detection sensitivity, generally between 2.5-5MHz.
3. Reference test block
The basis for determining the sensitivity of ultrasonic testing of ERW welds, API 5L and GB/T9711 standards for ultrasonic testing of steel pipe welds require the use of contrast test blocks with N10 grooves or 3.2mm vertical through holes, which are echoed by artificial reflectors. 100% as the criterion of defect judgment.
4.Conclusion
(1) In the ultrasonic testing of ERW welded pipe, in order to ensure that pure transverse waves are excited in the welded pipe and the inner wall of the welded pipe is scanned, the lower limit of the refraction angle of the welded pipe's transverse wave is 33.2 degrees, and the upper limit is proportional to the inner and outer diameter ratio r/R of the welded pipe. The larger the r/R value, the larger the range.
(2) When the refraction angle of the sound beam on the inner wall of the welded pipe is 45 degrees, it has a higher detection sensitivity for the surface opening defects of the weld and the heat-affected zone. However, in order to take into account the detection of radial area defects inside the weld, a transverse acoustic beam with a large refraction angle should also be used for scanning.
(3) According to the transverse wave refraction angle and the steel pipe diameter, the sound beam width of the shooting team should be considered comprehensively. It is necessary to ensure that the upper edge of the sound beam does not excite surface waves in the welded pipe, but also to avoid the appearance of refracted longitudinal waves in the welded pipe.
