Manufacturing Quality Control of U-shaped Multilayer Metal Bellows Expansion Joint

According to the production and manufacturing process of hydraulic forming U-shaped multi-layer metal bellows expansion joint, the quality control methods of tube blank blanking, longitudinal seam tungsten argon arc welding, tube blank cleaning and fitting, hydraulic forming and other processes are given.

Corrugated expansion joint is a very effective compensation element installed on the transport pipeline system, but it is also the weakest link in the entire pipeline system, its manufacturing quality determines the quality of operation of the entire pipeline system. u-shaped multi-layer metal bellows expansion joint manufacturing process is relatively crude, the implementation of process discipline and management is loose, the quality assurance system is not perfect, resulting in product quality instability, Quality accidents often occur. Especially in the chemical industry, oil refining and other key equipment pipelines, if the quality of the expansion joint accidents, the damage will be incalculable.

Bellows expansion joint parts manufacturing quality control

Pipe billet feeding

U-shaped multi-layer metal bellows with a thickness of ≤ 2mm stainless steel plate rolled. Tube billet does not allow the ring weld, longitudinal weld to the principle of minimum, and the spacing between adjacent longitudinal seams should be > 25Omm. the maximum number of welded seams in accordance with Table 1.
When cutting the material should pay attention to the burr of the butt plate to turn to the same side and the front side of the welding on one side, burr height generally <0.3mm. strict control of the shape of the plate error, diagonal error should be controlled not more than GB/1800.3-1998 JS10 level. Strictly control the perimeter size of the prepared layer to ensure the set gap <0.5mm, and strictly protect the surface of the plate during operation, so that there shall be no scratches or dents larger than the deviation under the thickness of the steel plate.

Pipe billet longitudinal seam tungsten arc welding

Sheet corrugated pipe billet longitudinal seam mostly welded with tungsten arc automatic welding. Welding should make the welding heat-affected zone as small as possible, argon gas protection is adequate, does not allow overburning, end weld through, not fused, biting, tungsten slag and other defects. 304, 316, 321 and other commonly used plates have good weldability, but in the production of the following two points must be noted.
Table.1 Maximum number of weld seams

Outer diameter of tube blank/mm Tube blank length/mm (≤1000) Tube blank length/mm (1000)
≤250 ≤1 ≤1
250-600 ≤2 ≤2
250-600 ≤3 ≤4
1 200-1 800 ≤4 ≤6
1 800-2400 ≤5 ≤8
2400-3000 ≤6 ≤10
3000-4000 ≤8 ≤13

(1) Plate cleaning before welding. Argon arc welding on the surface quality of the material requirements are very high, must be strictly cleaned before welding. Clear the workpiece weld near both sides of the surface at least 20mm range of oil, water, dust and oxide film, etc.. Otherwise, the welding process will affect the stability of the arc, deteriorate the weld seam, and may produce porosity, inclusions, non-fusion and other defects. Remove the oil dust can be used organic solvents, such as gasoline, acetone, trichloroethylene, carbon tetrachloride scrubbing, you can also prepare a special chemical solution cleaning. Removal of oxide film can take mechanical cleaning or chemical cleaning methods. After cleaning to the storage time before welding generally does not exceed 24h.
(2) Welding process parameters. For thickness < 0.5mm stainless steel plate, welding current ≤ 30A, welding speed ≥ 22m / h (Table 2), while also considering the impact of welding speed on the effect of gas shielding. The welding speed is too large, the shielding gas is severely biased after the tungsten extremity, arc column, molten pool exposed to the air. Therefore, corresponding measures must be taken, such as increasing the flow of shielding gas or the torch will be tilted forward at an angle to play a good protective role. In order to strengthen the gas protection, the torch can be attached to the back of the towing cover with argon gas, so that the temperature > 400 ℃ weld and heat-affected zone in the protection, but also in the back of the argon gas pad.
Table.2 welding process parameters

Plate thickness/mm Welding current/A Welding speed/(mm/min) Arc length/mm Argon flow/(L/min)
zero point three 20 700 0.8-1.2 10
zero point four 30 600 0.8-1.2 10
zero point five 60 500 1-2 10
zero point eight 40 600 1-2 12

For the thickness < 2mm steel plate welding quality assessment using the method of filming. Because the existence of such a thin plate > Φ 0.2mm porosity or inclusions is very unlikely, even if there is, the method of filming is difficult to find. Appearance inspection of the weld surface should be free of cracks, porosity, bitten edges and misaligned edges. Craters, collapse and residual height should not be greater than 10% of the wall thickness. The width of the weld should be uniform, the reverse side of the weld width of 1 ~ 2mm. from the weld color can be judged by the protection effect, such as weld color silver white, gold, the best protection effect; blue, good; red gray, better; gray, bad; black worst.

Pipe billet cleaning and set

There should be no water, oil, dirt, etc. between the layers of multi-layer bellows. Clean up carefully before the set. If there is hard debris between the layers, the set will cause scratches on the plate surface and convex marks on the surface when forming. If there is water and oil between the layers, it will affect the quality of the bellows and flange welding. In high temperature operation will cause the layer between the water vaporization expansion affect the bellows performance, will also reduce the life of the bellows, and even cause quality accidents. The position of the longitudinal seam of each layer of the multi-layer bellows should be evenly staggered.

Hydraulic forming

Hydraulic forming is currently the most commonly used manufacturing metal bellows a forming method, forming two ways, single-wave continuous forming and multi-wave forming. The method adopted is to use the mold multi-wave forming at one time.

  • (1) Hydraulic forming process principle. Hydraulic forming bellows can be divided into two stages, the first stage of the tube in the tube under the action of liquid pressure to constrain the expansion, the degree of expansion is relatively small, the purpose is to make each forming intermediate template positioning. In the second stage, under the condition that the liquid pressure inside the tube is stable and equal to the expansion pressure, axial force is applied and the tube is expanded by axial compression under the condition of axial force and internal pressure, and then the metal bellows is made.
  • (2) Hydraulic forming characteristics. As a result of axial compression expansion, the stress and strain state in the expansion area is improved, which is conducive to plastic deformation and improves the expansion coefficient. In the second stage of forming, the expansion pressure P must be stable, and its pressure is equal to the pressure value of the constrained expansion. If the expansion pressure P is too large, the billet will break. If the expansion pressure P is too small, a deflated wave is formed. Hydroformed bellows can be formed in a variety of waveforms in which the size and shape of the bellows are guaranteed by the forming die. Compared with other forming methods, the advantage of hydraulic forming is that it is easy to get a higher pressure, and the pressure is uniform, relatively easy to control, and the surface of the workpiece is smooth and good quality; the disadvantage is that the forming device needs to be strictly sealed to prevent the expansion pressure and liquid leakage when the expansion does not reach.
  • (3) Hydraulic forming process points. For the characteristics of hydraulic forming, first of all, when installing the mold to ensure the verticality of the mold end face and the axis of the billet cylinder. Secondly, the wave height, wave distance and corrugation length are controlled according to the manufacturing process, and the tolerance is controlled according to the JS18 level stipulated in GB/1800.3-1998. Again, when the corrugated pipe wave number is more, the corrugated pipe pressing prone to instability, then should be slowly pressed, the use of cushion block to find the right, to overcome instability. The principle is that the unqualified bellows can not be used to manufacture expansion joints.

Manufacture of other components

Single layer thickness <1mm multi-layer bellows is often used flange connection structure. This structure of the expansion joint flange amount is large, the manufacturing quality of the flange is very critical. Manufacturing flange steel plate should be ultrasonically inspected, no delamination defects. Welded flanges must be fully welded through the structure, and heat treatment before mechanical processing after welding. Manufacturing flange materials must meet the design requirements, production in strict accordance with the design requirements of construction. The receiver is a pressurized component, must be made of steel pipe or rolled steel pipe in accordance with the standard requirements.
Ear piece, bolt, hinge seat and other parts although there is no airtight requirements but have strength requirements, must be manufactured in strict accordance with the design requirements.

Bellows expansion joint assembly welding quality control

Assembly welding includes flange welding, combination welding and other structural parts of the welding.

  • (1) Flange welding refers to the bellows and pipe flange, intermediate flange for argon arc welding connection. The point is that the flange and the corrugated end surface paste tight, in order to ensure the verticality of the expansion joint end surface and axis. Welding shall not use the bellows inside and outside the surface grounding arc, welding arc direction should be aligned with the thick pieces, to prevent the welding speed is too slow or welding current is too large around the mouth of the bellows, resulting in the bellows outer layer or inner layer due to excessive current and arc ablation. This situation in use not only low strength, but also cause the bellows layer into the medium.
  • (2) Combination welding and other structural parts of the welding including intermediate flange butt welding, receiver and flange welding, standard flange and receiver welding, guide block welding, lug tendon plate welding, etc.. Welding should be strictly protected to prevent welding spatter burns the surface of the bellows and become a source of pitting corrosion. For gas tightness requirements of the weld, such as the middle flange butt welding and receiver and flange welding, multi-layer welding between the slag must be strictly cleared to prevent the production of cracks, porosity, slag and other defects. Welded parts must be cleared of oxidation layer before welding.

Bellows expansion joint product quality system

Expansion joint is the weakest part of the pipeline system, the normal operation of the pipeline plays a key role, in order to ensure the quality of the product should establish an effective quality assurance system, prevention and control of the whole process of production. First of all, the raw materials burst into the factory to have a complete set of quality certification, to ensure that the quality of the materials used to meet the design requirements, into the factory after the material are re-inspected (including stainless steel plate, steel plate and other materials such as chemical composition analysis and mechanical properties of the test), unqualified resolutely prohibited. Second, the production process to strictly control the quality of special processes, special processes (such as welding, molding, non-destructive testing, testing) must be strictly inspected and have detailed inspection records and strict process supervision. The production site should have obvious inspection and test state markings to ensure that production activities are carried out in a reasonable and orderly manner. Each process with the inspector’s inspection logo for handover, no logo, logo error, unqualified parts are not allowed to transfer the sequence, the next process has the right to reject the product without a qualified logo. Finally, the finished product should be fully inspected before storage and have detailed records and finished product identification and traceability.
Author: Chunfeng Lin, Maochen Fan

Source: China Metal Bellows Expansion Joint Manufacturer – Yaang Pipe Industry Co., Limited (

(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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  • [1]GB/T12777-1999 General technical conditions for metal bellows expansion joints [S].
  • [2]GB16749-1997 Pressure vessel waveform expansion joint [S].
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  • [4] JB/T4730.1-4730.6-2005 nondestructive testing of pressure-bearing equipment [S].

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