Multi-layer metal expansion joint welding has always been a headache in pipeline engineering. Two days ago, I met a friend who was doing power plant maintenance, saying that they had a high-temperature pipeline expansion joint that cracked in less than a maintenance cycle. After removing it, the weld cracked from the outer layer to the inner layer. This kind of thing is not uncommon, but the root often lies not in the moment of welding, but in several links before and after welding.
Why do you have to use multiple layers? Is a single layer not enough?
Single-layer bellows is not unusable, but it can't bear the working conditions of high temperature, high pressure and frequent displacement. As soon as the pressure is high, the wall thickness of the single layer has to be increased, but as the wall thickness increases, the stiffness goes up, and the compensation amount decreases instead. This is a dead knot, and multilayer structures are here to solve this knot.
Disassemble a thick plate into two, three or even more layers of thin plates, and the thickness of each layer can be about 0.3mm to 0.8mm. It can slide freely between layers, and the overall stiffness is much lower than that of single-layer structures with the same wall thickness, but the fatigue life is doubled up. 2-layer and 3-layer are usually used in general pressure pipelines, and 4-layer or more are mostly used in high temperature and high pressure situations that require large compensation. This point is consistent with the structural design idea of high-temperature axial expansion joint and general-purpose corrugated expansion joint-use a reasonable number of layers to match the working conditions, instead of blindly stacking the thickness.
Material selection: Matching is more important than material selection itself
In a multi-layer structure, each layer does the same work, and the materials can't be all the same. The layer in contact with the corrosive medium, usually with austenitic stainless steel, such as 304, 316L, is responsible for resisting corrosion. The intermediate layer and the inner and outer layers subjected to alternating stress sometimes have to use superalloys, such as Inconel 625 or 316L in the solid solution strengthened state.
This piece of thickness combination is more particular. For the same 4-layer structure, the fatigue life of 0.5×4 and 0.8×2+0.5×2 are completely two concepts. The empirical data is that the thinner the thickness of a single layer, the smaller the strain of each layer at deformation, and the longer the fatigue life. However, there are too many layers, and the friction heat generation and fretting wear between layers will eat up part of the life. Therefore, the thickness combination is not determined by patting the head. The stress should be calculated according to the design pressure, and then the thickness of the single layer should be inferred backwards according to the fatigue life curve.
The hidden danger of material mismatch is the most hidden. Some manufacturers try to save trouble and use the same material inside and out. As a result, after the inner layer is corroded and thinned by the medium, the crack naturally expands to the middle layer. Another is that the potential difference between different layers of materials is too large, which directly causes galvanic corrosion in humid environment. This kind of corrosion will not be exposed immediately, and it will be discovered until the air tightness test can't hold the pressure. It's too late.
Welding process: four stuck neck links
The longitudinal seam welding of multi-layer bellows is the most testing link in the whole manufacturing process. First, the longitudinal seams must be welded before corrugation. This timing can't be chaotic-if molding is first and then welding, the residual stress at the bottom of the corrugation valley will superimpose the welding stress, and the probability of micro-cracks in the weld will be much greater.
Interlayer cleanliness directly determines weld quality. The oil, dust and sweat stains between each layer will become the source of pores and slag inclusions during high-temperature welding. The practice in actual production is to wipe layer by layer with acetone or dust-free cloth before laminating, and the operator is required to wear clean gloves. This link looks simple, but there are not many workshops that really strictly implement it.
The hot input should be controlled within a narrow window. When the current is large, the weld burns through or collapses; The current is small, the melt is impermeable, and the unfused defect appears. For thin plates about 0.5mm, small current, high-speed welding and argon protection are usually recommended. The back shielding gas is particularly easy to overlook-many people only pay attention to the protection of the front of the weld, and they think it's okay if the oxidation phenomenon on the back can't be seen. In fact, the longitudinal joint of bellows needs to be filled with argon both inside and outside, otherwise the oxide scale on the back will crack first due to stress concentration in the subsequent fatigue test.
The amount of misplaced edges between layers in the welding process also has to be kept an eye on. When the misalignment exceeds 0.3mm, the stress concentration zone will appear in the weld under the action of cyclic stress, and the fatigue crack will start from there.
Post-welding inspection: Every level is life and death
Welding is not the end of the matter, inspection is the time to see the true chapter.
First, cosmetic inspection. There are no cracks, biting edges, craters and pores on the weld surface, and the residual height is generally between 0 and 0.5mm. If the longitudinal seam residual height of the bellows is too high, the bellows will generate additional bending stress when compressed.
Radiographic flaw detection does not need to be completely inspected, but the location of sampling inspection is particular-in addition to pumping the longitudinal seam body, it is also necessary to cover the heat-affected zone within 50mm at both ends of the weld, because the microstructure here changes the most and cracks are most prone to appear. The sampling ratio is generally 10% to 20% of the batch, but the first batch of products is recommended to be fully inspected.
The strength is verified by hydraulic pressure test first. The test pressure is usually 1.5 times of the design pressure, and the pressure holding time is not less than 30 minutes; Then air tightness test to verify the sealing performance, the test pressure is generally 1.1 times of the design pressure. There is an easy pit here: it must be thoroughly dried after the hydraulic test, otherwise the residual moisture in the bellows interlayer will cause false leakage during the airtight test, and it is more likely to cause chloride ion stress corrosion after the equipment is put into operation.
In all likelihood, the common weld cracking accident on the site is not unlucky welding operation, but a loose link in front. Either the materials are mismatched, the cleanliness is not up to standard, or the post-welding inspection has stolen the work.
Installation: Don't let the front effort go to waste
After the welding is completed, the bellows must be assembled with the guide tube and the connector. The welding sequence here will also affect the stress state of the bellows body. The core principle is to first weld the fixed weld on the inner side of the guide tube, then weld the sealing weld on the outer side, and finally weld the circumferential weld between the bellows and the joint. In turn, the stress caused by the thermal expansion of the guide tube will directly act on the bellows, which will pull and deform the thin-walled bellows.
Compared with other expansion joint products, the focus of on-site construction is different. The welding of metal hoses (including PTFE-lined metal hoses) focuses on the sealing and vibration resistance of joints, with short welds and small influence range of heat input; Because the transverse expansion joint of compound hinge has hinge structure, attention should be paid to the symmetry of hinge position during construction to avoid additional torque to bellows; For straight pipe pressure balance expansion joints, we should pay attention to the coaxiality between the intermediate pipe and the bellows. The sensitivity of thin-walled multilayer corrugated pipe to assembly stress is higher than these. When installing, the corrugated pipe body cannot be forcibly aligned, but it has to be aligned by adjusting the tie rod or pipe support.
So in the final analysis, when it comes to welding multilayer metal expansion joints, the effort is before welding-materials, cleanliness, welding parameters, everything has to be thought through before starting. More effort is after welding-the inspection is made solid item by item, and the installation sequence is not sloppy. One link drops the chain, and all the previous efforts can turn into that one crack in the weld.