The ERW pipe is made by using the skin effect and proximity effect of high-frequency current to rapidly heat the edge of the pipe to the welding temperature, and then extrude and weld it. Compared with seamless steel pipes, ERW pipes have the advantages of high dimensional accuracy, low price, and high production efficiency, and their grain size and structure are superior to seamless steel pipes. Compared with the straight seam submerged arc welded pipe (LSAW steel pipe) of the same specification, the production speed of the straight seam submerged arc welded pipe is fast, and there is no local thinning of the anti-corrosion layer at the weld. Its application fields involve casing for 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.
ERW pipes may have a variety of welding defects, some of which come from the base metal, and some are generated during the welding process. Different defects have different effects on welding quality. Regional defects such as cracks and lack of fusion are prone to stress concentration under stress conditions, which is the main reason for low-stress brittle fracture of welds. For volume defects such as pores and slag inclusions, although the cracking sensitivity is lower than that of surface defects, it will reduce the effective cross-sectional area of the weld and reduce the welding strength. Under the action of external force, these defects often become the source of cracks, which eventually lead to cracking of the weld.
Ultrasonic testing in the production process of ERW pipe
Ultrasonic testing is currently the main non-destructive testing method in the production process of ERW pipes. Its main application areas include:
1) Ultrasonic online detection of steel plates.
Ultrasonic on-line detection of steel plates generally uses twin-crystal or poly-crystal probes, combined with water film or partial water immersion methods, and its main purpose is to detect layered defects parallel to the surface of steel plates. There are two main scanning methods: one is parallel line scanning along the rolling direction; the other is that the steel plate moves linearly along the rolling direction, and the probe reciprocates perpendicular to the steel pipe moving direction to form a “Z” shape scanning. Since the edge of the steel plate forms the weld seam in the subsequent ERW welding, it is particularly important to detect defects there in the ultrasonic flaw detection of the steel plate. Relevant standards and specifications require 100% scanning of the edge of the steel plate. In actual work, the method of increasing the number of probes at the edge of the steel plate is generally adopted to ensure this.
2) ERW welding, ultrasonic on-line detection of weld seam after internal and external burrs are removed.
Ultrasonic online inspection of ERW weld seam is carried out after welding is completed 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 appearance of the inner wall of the weld after removing the internal burr in real time, which is more intuitive; the second is oblique incidence of longitudinal waves, and the transverse waves generated by refraction in the welded pipe are used to detect welding defects. Due to the high temperature of the weld at this time, high-temperature probes are generally used for online detection, and local water immersion is used for detection.
3) Off-line detection and pipe end detection of ERW welds.
Off-line ultrasonic testing of ERW welds is generally performed after hydrostatic testing 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 influence of the dead zone at the pipe end in the automatic detection. Manual ultrasonic scans of welded tanks are generally post-added. The content of pipe end inspection mainly includes the detection of pipe end welds, layered defects of pipe end base metal, axial and circumferential defects. The detection of delamination defects generally adopts split probes, and the axial and circumferential defects of welds and base metals are mostly scanned by oblique probes.
