Characteristics of large diameter electric resistance welded pipe

Date:2024-02-19Tags:electric resistance welded pipe
Characteristics of large diameter electric resistance welded pipe

The performance of Electric resistance welded pipe can well meet the requirements of long-distance pipeline transportation of oil and natural gas, so it is widely used in the field of transportation. At present, the maximum diameter of electric resistance welded pipe designed according to API standards can reach 630mm-660mm in the world, the wall thickness is 8-20mm, and the hot coil used can reach 2000mm.

Electric resistance welded pipe is different from other types of welded pipes. No metal is added to the weld seam. The edge of the steel strip is rapidly heated through the skin effect and proximity effect characteristics of high-frequency current. Then through an effective extrusion process, the edges of the steel strip are forged and welded together to form a weld, and the dissolved bright line is only 0.15~0.2mm wide. At the same time, the welding speed is fast (generally greater than 15m/min) and the heat-affected zone is narrow. The subsequent annealing treatment eliminates the welding residual stress, and then the normalizing heat treatment improves the structure of the welding zone.


electric resistance welded pipe


Therefore, the structure, composition and hardness of the weld area are close to those of the base metal, and the structure of the weld area is in the hot-rolled state rather than the cast state. Due to the process characteristics of ERW, the steel pipe is relatively fully deformed during the forming process, and cold sizing is required after extrusion and welding. By alternating and shrinking the steel pipe in the longitudinal and transverse directions, the residual stress during cold forming of the steel pipe is reduced. The axial notch test shows that the opening displacement of the electric resistance welded pipe is indeed very small.

Electric resistance welded pipe has no high weld reinforcement, and the internal and external burrs are basically flush with the outline of the steel pipe, so there is basically no stress concentration during use. The existence of residual stress causes the stress of the steel pipe to be superimposed during the working process. In transport media containing hydrogen sulfide, hydrogen sulfide stress corrosion is very sensitive to stress. However, erw steel pipes do not have the weakest zone at the root of the weld.
At the same time, the surface of electric resistance welded pipe is smooth and flat, the wall thickness is uniform, and the roundness is good, which can make the anti-corrosion coating uniform. During on-site construction, docking is easy and construction speed is fast.

Characteristics of large diameter electric resistance welded pipe

ERW production equipment requires large investment and many processes, so the processing cost of steel pipes is high. Since the large-diameter electric resistance welded pipe has no repair welding and is not allowed to face the head pipe, the relative yield is low. Therefore, the direct price of large-diameter electric resistance welded pipe is higher, but from the overall perspective of pipeline construction, the cost has not increased.
(1) Large diameter electric resistance welded pipe has good quality and long service life.
(2) Large-diameter electric resistance welded pipes have good surface quality and uniform coating, saving a lot of coating materials and low anti-corrosion costs for steel pipes.
(3) Large-diameter electric resistance welded pipe has high geometric dimensional accuracy, good roundness, fast butt welding speed, and high on-site construction efficiency, which greatly reduces construction costs.
(4) The welding speed of large-diameter electric resistance welded pipe is much higher than that of SSAW steel pipe, and the production efficiency is high. Timely and necessary construction can be guaranteed.

Ultrasonic testing of Electric resistance welded pipe

Electric resistance welded pipe uses the skin effect and proximity effect of high-frequency current to quickly heat the edge of the pipe to the welding temperature and then extrusion welding. Compared with seamless steel pipes, electric resistance welded pipes have the advantages of high dimensional accuracy, low price, and high production efficiency. Their grain size and structure are superior to seamless steel pipes. Compared with straight seam submerged arc welded pipes (LSAW steel pipes) of the same specifications, the production speed of electric resistance welded pipes is fast, and there is no local thinning of the anti-corrosion layer at the weld. Its application areas include casings for oil drilling, submarine oil and gas pipelines in the offshore oil industry, oil and gas pipelines and gas transmission pipelines in trunk lines and urban pipeline networks, etc.

Electric resistance welded pipe may have a variety of welding defects, some of which come from the base material, 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 are the main causes of low-stress brittle fracture of welds. For volume defects such as pores and slag inclusions, although their cracking sensitivity is lower than area defects, the effective cross-sectional area of the weld is reduced and the welding strength is reduced. Under the action of external forces, these defects often become sources of cracks, eventually leading to weld cracking.

Ultrasonic testing in the production process of Electric resistance welded pipe

Ultrasonic testing is currently the main non-destructive testing method in the production process of electric resistance welded pipe materials. Its main application areas include:

1) Ultrasonic online testing of steel plates.
Ultrasonic online testing of steel plates generally uses dual crystal or polycrystalline probes, plus water film or partial water immersion methods. Its main purpose is to detect layered defects in the steel plate parallel to the surface of the steel plate. steel plate. There are two main scanning methods: one is scanning with parallel lines along the rolling direction; the other is scanning with parallel lines along the rolling direction. The other is that the steel plate moves linearly along the rolling direction, and the probe reciprocates perpendicular to the moving direction of the steel pipe, forming a "z" shaped scan. Since the edge of the steel plate forms a weld seam in the subsequent ERW welding, the detection of defects there is particularly important in the ultrasonic inspection of the steel plate. Relevant standards and specifications require 100% scanning of steel plate edges. In actual work, the method of increasing the number of steel plate edge probes is generally used to ensure this.

2) ERW welding, remove internal and external burrs and then conduct ultrasonic online inspection of the weld.
ERW weld ultrasonic online inspection is performed 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 internal burrs in real time, which is more intuitive; second, oblique incidence of longitudinal waves, using the transverse waves generated by refraction in the welded pipe to detect welding defects. Since the temperature of the weld is relatively high at this time, high-temperature probes are generally used for online inspection, and partial water immersion is used.

3) Offline inspection and pipe end inspection of ERW welds.
Offline 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 used. Due to the influence of the dead zone at the pipe end in automatic detection. Manual ultrasonic scanning of welded tanks is typically added at a later stage. The content of pipe end inspection mainly includes the detection of pipe end welds, pipe end base material layer defects, axial and circumferential defects. The detection of delamination defects generally uses split probes, and the axial and circumferential defects in the weld and base metal are mostly scanned by oblique probes.

Selection of process parameters for ultrasonic testing of ERW welds.


Ultrasonic testing of ERW welds mainly includes automatic testing and manual testing.

1) Automatic detection
At present, the automatic inspection of ERW welds mainly adopts two forms: wheel inspection and partial immersion inspection. Automatic inspection 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.
2) Manual detection
In contrast, manual inspection is more flexible. It can not only accurately locate defects, but also conduct qualitative and quantitative analysis of defects through echo characteristics and dynamic waveforms. For defects detected by automatic ultrasonic inspection, manual methods are generally used for further confirmation.

Summarize


1) In the ultrasonic testing of ERW welded pipes, in order to ensure that pure shear waves are excited in the welded pipes, the inner wall of the welded pipes is scanned. The lower limit of the shear wave refraction angle range of the welded pipe is 33.2 degrees, and the upper limit changes with the change of the internal and external 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 surface opening defects in the weld and heat-affected zone. However, in order to take into account the detection of defects in the radial area inside the weld, a transverse wave acoustic beam with a large refraction angle should also be used for scanning.

3) The sound beam width of the shooting team should 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.


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