- Ultrasonic inspection during the production process of ERW welded pipes
Ultrasonic testing is currently the main non-destructive testing method in the production process of ERW welded pipes. Its main application areas include: 1) Ultrasonic online testing of steel plates. 2) Ultrasonic online inspection of weld seams after ERW welding and internal and external burr removal. 3) Offline inspection of ERW welds. 4) Ultrasonic testing of ERW welded pipe ends.
1. Ultrasonic online inspection of steel plates
Ultrasonic wave online detection of steel plates generally uses dual crystal or polycrystalline probes, coupled with water film or local water immersion methods. Its main purpose is to detect layered defects in the steel plate that are parallel to the surface of the steel plate. There are two main scanning methods: first, scanning along parallel lines in the rolling direction; second, the steel plate moves linearly along the rolling direction, and the probe reciprocates perpendicular to the direction of steel pipe movement, forming a "z" shaped scan. Since the edge of the steel plate forms a weld in the subsequent ERW welding, the detection of defects here is particularly important in the ultrasonic testing of the steel plate. Relevant standards and specifications require 100% scanning of the edge of the steel plate. In actual work, Generally, this is ensured by increasing the number of probes at the edge of the steel plate.
2. Ultrasonic online inspection of welds
ERW weld ultrasonic online inspection is performed after welding and internal and external burr removal. It mainly includes two parts: First, 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 inner wall morphology of the weld after removing the inner burrs in real time, and the graphic display is more intuitive; second, the longitudinal wave is obliquely incident, and the transverse wave generated by its refraction in the welded pipe is used to detect welding defects. Since the weld temperature is relatively high at this time, online inspection generally uses high-temperature probes and uses the local water immersion method.
3. Weld ultrasonic offline inspection and pipe end inspection
Offline ultrasonic testing of ERW welds is generally performed after hydrostatic testing and chamfering, and is mainly used to detect longitudinal defects in the weld and heat-affected zone. In order to improve detection efficiency, automatic detection is generally used. Due to the influence of the blind area at the pipe end in automatic detection. Afterwards, manual ultrasonic scanning of the welds is usually added. The content of pipe end inspection mainly includes the detection of pipe end welds, pipe end base material layered defects, and axial and circumferential defects. The detection of layered defects generally uses split probes, and the axial defects in the weld and base material are and circumferential defects are often scanned with an oblique probe.
- Selection of process parameters for ERW weld ultrasonic testing
Ultrasonic testing of ERW welds mainly includes two methods: automatic testing and manual testing. At present, the automatic detection of ERW welds mainly adopts two forms: wheel probe detection and local water immersion detection. Automatic inspection has the advantages of high detection efficiency and fast speed, but it is not conducive to the precise positioning of defects and qualitative and quantitative analysis; in comparison, manual inspection is more flexible, not only can the defects be accurately located, but also Conduct qualitative and quantitative analysis of defects through echo characteristics and dynamic waveforms. Defects detected by automatic ultrasonic waves are generally further confirmed using manual methods. Process parameters that need to be considered for ultrasonic testing of ERW welds include refraction angle, sound beam width, and detection frequency.
1. Selection of refraction angle
Ultrasonic testing of ERW welds generally adopts longitudinal waves with oblique incidence. The detection is achieved by generating transverse waves through wave mode conversion in the workpiece and weld. The two basic conditions are: 1) Pure transverse waves are excited in the base metal and weld. 2) The transverse wave sound beam scans the inner wall of the steel pipe.
2. Selection of detection frequency
The frequency range of ultrasonic testing is wide, generally 0.5-10MHz. The selection of frequency should mainly consider the following factors:
(1) The sensitivity of ultrasonic detection is about half of the wavelength. Increasing the frequency is beneficial to finding 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 are beneficial to discovering and locating defects. But for the same chip size, the higher the frequency, the larger the near-field area, which is more detrimental to detection.
(3) As the frequency increases, the scattering and absorption attenuation of ultrasonic waves increase sharply, which is detrimental to detection.
During testing, various factors must be considered comprehensively and the testing 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 narrow, and its grain size is basically the same as that of the base material. In actual work, in order to obtain higher resolution, a higher frequency should be selected as much as possible while ensuring detection sensitivity, generally between 2.5-5MHz.
- Reference test block
The reference test block is the basis for determining the sensitivity of ultrasonic testing of ERW welds. API 5L and GB/T9711 standards require the use of comparison test blocks with N10 grooves or 3.2mm vertical holes to artificially reflect the ultrasonic testing of steel pipe welds. 100% of the body echo is used as the criterion for defect determination.
- Conclusion
(1) In the ultrasonic testing of ERW welded pipes, in order to ensure that pure transverse waves are excited in the welded pipes and scan the inner wall of the welded pipes, the lower limit of the shear wave refraction angle range of the welded pipes is 33.2 degrees, and its upper limit depends on the internal and external diameter ratio r/R of the welded pipes. It changes with the change of r/R. The larger the value of r/R, the wider its range.
(2) When the sound beam refraction angle of the inner wall of the welded pipe is 45 degrees, it has high detection sensitivity for surface opening defects in the weld and heat-affected zone. However, in order to take into account the detection of radial area defects inside the weld, transverse wave acoustic beams with large refraction angles should also be used for scanning.
(3) The width of the sound beam of the firing team must be comprehensively considered based on the transverse wave refraction angle and the diameter of the steel pipe. It is necessary to ensure that the upper edge of the sound beam does not excite surface waves in the welded pipe, and to avoid the occurrence of refracted longitudinal waves in the welded pipe.
