Differences between High Frequency Resistance Welded Pipe and SAW Pipe

1. Comparison of process characteristics and quality performance of several steel pipes:

ERW Pipe is divided into induction welding and contact welding according to different welding methods. Using hot-rolled wide strip steel coils as raw materials, through pre-bending, continuous forming, welding, heat treatment, sizing, straightening, cutting and other processes, compared with spiral welded pipes, it has shorter weld seams, high dimensional accuracy, uniform wall thickness and surface quality good, with the advantages of high pressure, but the disadvantage is that only small and medium-caliber thin-walled pipes can be produced, and the welds are prone to gray spots, unfused, and groove-like corrosion defects. At present, the most widely used areas are city gas, crude oil and refined oil transportation.

Spiral submerged arc welded pipe (SSAW) is a strip steel coiled pipe whose advancing direction has a forming angle (adjustable) with the center line of the forming pipe. It is welded while forming, and its weld is spiral. The advantage is that the same specification of strip steel can be produced steel pipes of various diameters have a large range of raw materials. The welding seam can avoid the principal stress and the stress is better. The disadvantage is that the geometrical size is poor. The length of the weld welding defects such as slag inclusion and welding deviation, the welding stress is in the state of tensile stress. The general design code for long-distance oil and gas pipelines stipulates that spiral submerged arc welded pipes can only be used in Category 3 and Category 4 areas. In foreign countries, this process is improved and the raw material is changed to steel plate to separate the forming and welding. After pre-welding and precision, and cold expansion after welding, the welding quality is close to U0E pipe. At present, there is no such process in China. It is a spiral pipe in my country. The direction of plant improvement. The spiral steel pipe used in the "West-East Gas Pipeline" is still produced according to the traditional technology, but the pipe end has been expanded in diameter. The United States, Japan and Germany generally reject submerged arc welding and believe that submerged arc welding is not suitable for the main line; Canada and Italy use submerged arc welding in some parts, and Russia uses submerged arc welding in a small amount, and they have formulated very strict supplementary conditions. Due to historical reasons , Most domestic trunk lines still use submerged arc welding.

Longitudinal submerged arc welded pipe (LSAW) is produced by using a single medium and thick plate as the raw material, pressing (rolling) the steel plate in a mold or forming machine into a tube blank, using double-sided submerged arc welding and expanding the diameter. The modern JCO molding machine is a stepping compression molding machine controlled automatically by a computer. During the entire forming process, the movement of the upper and lower molds with the feeding and discharging mechanism is controlled by a computer. The reduction amount, reduction force and the steel plate feed amount can be automatically adjusted according to different steel grades, wall thicknesses, and plate widths. At the same time, the upper and lower molds have The deformation compensation function effectively avoids the adverse effects of mold deformation on the forming and ensures the flatness of the full length of the steel plate during the pressing process. The feeding step is uniform during forming, ensuring the roundness of the tube blank and the flatness of the welding edge. The finished product has a wide range of specifications, and the weld has good toughness, plasticity, uniformity and compactness. It has the advantages of large pipe diameter, pipe wall thickness, high pressure resistance, low temperature resistance and strong corrosion resistance. In the construction of high-strength, high-toughness, high-quality long-distance oil and gas pipelines, most of the steel pipes required are large-diameter thick-walled LSAW steel pipes. According to API standards, in large-scale oil and gas pipelines, when passing through Class 1 and Class 2 areas such as alpine regions, seabeds, and densely populated urban areas, LSAW pipe is the only designated applicable pipe type.

2. Comparison of ERW and Submerged Arc Welding Process

A) ERW Pipe has fast heating speed, highly concentrated heat and no filler metal. It is not possible to use wire flux to add alloying elements to compensate for the burning of alloy elements in the welding process like submerged arc welding, and it cannot effectively improve the structure and performance of welded joints like submerged arc welding (microalloying is to refine the grain The main way, grain refinement is the only way to simultaneously improve strength and toughness).

B) ERW does not form a welding pool, so the welding dross ability is poor. If there is slag inclusion on the edge of the raw material, it can only remain in the weld and become a quality risk. Since submerged arc welding forms a welding pool, it is beneficial for slag inclusions to float in the slag.

C) There is no slag/gas protection during ERW welding (submerged arc welding has slag protection formed by flux), and the welded joint structure at high temperature is easily oxidized, which makes the welded joint have a tendency to embrittlement.

D) ERW has the characteristics of high local heating temperature and short cooling time. The welding temperature field has a large gradient, which is prone to hardening phase and large welding stress. The plasticity and toughness of the welded joint are not ideal. Therefore, all ERW steel pipes in the country require post-weld heat treatment (Q+T or Q+N), but domestic the post-weld heat treatment process is not yet mature.

E) Compared with submerged arc welding, the ERW welding speed is too fast, the edge quality of the raw material cannot be tested by NDT, and the NDT test of the weld after welding is also difficult to ensure the quality.

F) Since ERW is welded by pressing molten metal, it is difficult to use NDT for effective inspection. Therefore, incomplete fusion and gray spots are the problems of ERW steel pipes, which have been difficult to effectively solve for decades. The ditch corrosion in ERW pipeline failure is mainly caused by unfusion. (Refer to Welded Pipe "Oil and Gas Transmission Pipeline Failure Accidents and Typical Cases")

G) ERW will inevitably form more T-joints during pipeline construction, while submerged arc welding will not. As we all know, the T-joint is the place where the stress is the most concentrated, which reduces the fatigue life of the welded joint.

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