Discussion on Quality Control of U-shaped Multilayer Metal Bellows Expansion Joint

In the actual process of manufacturing U-shaped multilayer metal bellows expansion joints, the relevant processes and standards for the production and manufacturing of hydraulic forming U-shaped multilayer metal bellows expansion joints must be strictly followed. A series of work, such as tube blank blanking, longitudinal seam tungsten argon arc welding of tube blank, tube blank cleaning and fitting, can ensure the quality in this process and effectively improve the efficiency. Note that the quality control of hydraulic forming and other processes is also covered in the above scope.

The transmission pipeline of corrugated expansion joint is essentially a compensation element, and the main installation position is the transmission pipeline system. In the whole pipeline system, corrugated expansion joints also exist as the weakest link. The manufacturing quality of bellow expansion joints will directly affect the operation of the entire pipeline system. Rough process, improper management and imperfect quality assurance system are the main problems and defects in the manufacturing of U-shaped multi-layer metal bellow expansion joints, which are also the main reasons for unstable product quality. In serious cases, quality accidents will occur, which will bring considerable damage and threat.

1. Manufacturing quality control of bellows expansion joint components

1.1 Blanking of tube blank

U-shaped multi-layer metal corrugated pipe is rolled with stainless steel plate with thickness ≤ 2mm. The tube blank shall not have circumferential welds, and the longitudinal welds shall be at least, and the spacing between adjacent longitudinal welds shall be>250mm. During shearing and blanking, it should be noted that the burrs of butt plates should turn to the same side and be on the same side with the front side of welding, and the height of burrs is generally<0.3mm. The shape error of the board surface shall be strictly controlled, and the diagonal error shall not exceed JS10 specified in GB/1800.3-1998. The perimeter dimension of the preparation layer shall be strictly controlled to ensure that the package clearance is less than 0.5mm. The surface of the plate shall be strictly protected during operation, and there shall be no scratches or dents greater than the deviation under the thickness of the steel plate.

1.2 Tungsten argon arc welding of longitudinal seam of tube blank

1) Cleaning of plates before welding. Argon arc welding puts forward strict requirements on material surface quality, which must be strictly cleaned before actual operation. Oil dirt, moisture, dust and oxide film are the main contents of cleaning. The cleaning range is roughly maintained on both sides of the surface near the weld crater of the workpiece, and at least not less than 20mm. This is an effective means to minimize the impact on the stability. If this requirement cannot be met, the weld formation will show a deteriorating state, especially there will be various defects, mainly involving pores, inclusions and non fusion. Various methods can be used to remove oil and dust. At this stage, gasoline, acetone, trichloroethylene, carbon tetrachloride and other means are generally used, and special chemical solutions can also be configured for thorough removal. Mechanical or chemical cleaning methods are generally used in the process of removing oxide film. Storage shall be carried out after cleaning and before welding, and the maximum storage time shall not exceed 24h.
2) Welding process parameters. For stainless steel plates with thickness less than 0.5mm, the welding current is ≤ 30A, and the welding speed is ≥ 22m/h. At the same time, the influence of welding speed on gas shielding effect should be considered. If the welding speed is too high and the shielding gas is seriously backward, the tungsten terminal, arc column and molten pool will be exposed to the air. Therefore, corresponding measures must be taken, such as increasing the flow of shielding gas or tilting the welding torch forward at a certain angle, to play a good role in protection.

2. Manufacturing of other parts

Flange connection structures are generally carried out in multi-layer corrugated pipes with a thickness of less than 1mm. Expansion joints using this structure require a large number of flanges for support. Therefore, the quality research of flange manufacturing is of great practical significance. Ultrasonic testing and work without delamination defects are the conditions that must be met in the manufacturing of flange steel plates. The main methods used in the all welded structural splicing of flanges should pay attention to strict heating treatment before welding.
The materials for manufacturing flanges must be strictly inspected, and can only be put into use after they meet the relevant standards and requirements, and the relevant construction requirements must be strictly observed during production. The pressure element is the main component of the nozzle, which is mainly made of steel plate or rolled steel pipe. Lugs, bolts, hinges and other parts are indispensable components. Although there is no high requirement for air tightness in this part, the strength must be guaranteed, especially the design content.

3. Welding quality control of bellows expansion joint assembly

Argon arc welding is mainly carried out between bellows and tubular flange as well as intermediate flange, which is the essence and goal of flange welding. In order to improve the compactness of flange and bellows end face in a real sense, the perpendicularity of expansion joint end face and axis must be guaranteed. The grounding arc work shall be involved in the welding process. Note that the internal and external surfaces of the bellows cannot be used for welding. During the welding process, the direction of the arc must be toward the position of thick parts to effectively prevent the problem of slow welding speed. If the welding current is too large, the problem of winding through the corrugated pipe orifice will occur, which will eventually lead to arc ablation, and the problem that the outer or inner layer of the corrugated pipe will be affected will also be involved. In this case, the strength will decrease continuously on the original basis, and even some media will enter the corrugated pipe interlayer.
Butt welding of intermediate flange, welding of connecting pipe and flange, and welding of standard flange and connecting pipe are all covered by combination welding and welding of other components. Note that welding of guide block and padeye are also included. Therefore, the above contents must be comprehensively considered in the welding work, and the surface of the corrugated pipe shall be protected from being burned by splashes to avoid the problem of pitting source to the maximum extent. Some welds put forward certain requirements for air tightness. Butt welding of intermediate flange and welding of nozzle and flange can directly reflect them. During welding, strict slag removal must be carried out to effectively avoid cracks, pores and slag inclusion.

4. Bellows expansion joint product quality system

In the whole pipeline system, expansion joints exist as the weakest components, which directly affect the overall operation of the pipeline. The establishment of quality assurance system for the whole production process is mainly to improve the overall quality of products, and strictly prevent and control the production process. Raw materials must have complete formalities and quality certificates before entering the factory to further ensure that the inspection results of materials used meet relevant requirements and standards. Strict re inspection shall be carried out after mobilization, and unqualified inspection results shall not be used.
Author: Xiao Wei

Source: China Expansion Joints Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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