Thin-walled metal expansion joint welding, what is the difficulty? How to weld without accident?
Thin-walled metal expansion joint welding is a headache for many welders. The difficulty is not "welding", but "no cracking or deformation after welding". The wall thickness of bellows is usually only 0.5mm to 2mm, which is as thin as paper. When the hand shakes, it will be a hole. If the current is large, it will burn through directly, and if the current is small, it will not melt through. This degree is difficult to grasp.
Difficulties in welding of thin-walled metal expansion joints: It is not to be welded firmly, but to not crack or deform after welding
Let's talk about why it's difficult first. The corrugated part of the thin-walled expansion joint is formed by hydraulic pressure or rolling, and there is residual stress inside the corrugation. When the local heating reaches thousands of degrees during welding, the stress of thermal expansion and contraction superimposes the existing residual stress, and the weld and heat-affected zone are prone to problems. Crack, is both cold crack and hot crack; Deformation means that the corrugation pitch has changed, the wave height is short, it can't be installed or the compensation amount is insufficient.Welding of thin-walled metal expansion jointIt's not as simple as welding a water pipe-it can pass water without leakage after welding, which is a basic skill; After welding corrugation or corrugation, size or size, and performance are not discounted, that is the ability.
Selection of welding process: Why is argon arc welding the mainstream, and how to determine current, speed and number of layers
The mainstream process is argon arc welding (GTAW/TIG), which is basically uncontroversial. Why don't you use electrode arc welding? The penetration is too great to control; Why not use CO₂? The splash is large, the bead is poorly formed, and the sheet is easy to collapse. Argon arc welding has concentrated heat, stable arc and good protection effect, especially suitable for thin-walled stainless steel.
How to set the parameters? Take 304 stainless steel and 1.0mm wall thickness bellows butt welding as an example: the current is about 35~55A, the tungsten electrode diameter is 1.6mm or 2.0mm, the welding speed is controlled at 8~15cm/min, and the argon flow rate is 8~12L/min. This range is not a dead number, and it has to be fine-tuned according to the actual operation-the vertical welding position should be down a little, the thin-walled overlap should be down a little, and if there is no protective gas on the back, the heat should be lowered on the front.
What about the number of layers? Remember one principle: if you can weld single-sided and double-sided, don't weld two layers, and if you can weld one layer, don't weld two layers.Thin-walled parts are welded in two layers, and the risk of burning through the first layer is high, and the accumulated deformation of heat input in the second layer comes. If the wall thickness exceeds 2mm, it can be divided into two layers, the first layer is coated without filament or filament, and the second layer is coated with filament. Don't pursue how beautiful the welding bead is. The thicker the welding bead of thin-walled parts, the more dangerous it is.
Post-weld Deformation and Residual Stress Control: Different Treatment Approaches from Universal Corrugated Expansion Joints to Large Diameter Thick Wall Expansion Joints
To control deformation, the key is to "hold down the heat input" and "create conditions for it to deform".
General-purpose corrugated expansion joint has many waves and thin walls. If conditions permit during welding, it is best to fix the two ends of the corrugation first with clamps to restrict it from running in the direction of contraction. The welding sequence is also particular-welding the middle wave first, and then welding to both sides, so that the deformation is dispersed and not concentrated in a certain section. Don't rush to loosen the fixture after welding, wait until the temperature drops below 100℃ before loosening it, and the deformation rebound will be much smaller.
The same cannot be said for large-diameter thick-walled expansion joints. The wall thickness is 4mm or more than 6mm, the stiffness is large, the deformation problem is not the most important, Residual stress concentration and weld cracking are the key points。 This kind of thick-walled parts often need stress relief heat treatment (such as integral annealing or local heating) after welding, otherwise the weld root is prone to fatigue cracking after the pipeline runs for a long time. In addition, the groove form of thick-walled parts is also particular. The V-shaped groove angle is controlled at 60 ° ~70 °, and the blunt edge is left at 1~2mm, all of which are to ensure penetration without overburning.
Common welding defects: burn-through, unfused, intergranular corrosion – the easiest pits to step on site
Thin ones burn through easily, which is the first pit. The solution is two words: heat control. The current is smaller and the welding speed is faster. If it doesn't work, add a copper pad on the back for heat dissipation.
The second pit is unfused-the surface looks welded, but in fact the roots of the grooves don't melt together at all, and they leak when pressed. Unfused mostly occurs when the current is small and the welding wire is fed too far ahead. The inspector took the color to detect the flaw and swept it, and the strip-shaped bright line was displayed under the fluorescence that was not fused. One of them was the job of judging the waste.
The third pit is intergranular corrosion. This one is the most hidden. When austenitic stainless steel stays for too long in the temperature range of 450~850℃, carbon and chromium will combine to form chromium carbide, resulting in chromium depletion at the grain boundary, and the weld will crack from the grain boundary after encountering corrosive medium. How to avoid it? Select stabilized materials containing titanium or niobium, or use ultra-low carbon stainless steel (304L, 316L). Another is to control the heat input during welding, so that the high temperature residence time is as short as possible. The temperature of welding thin-walled parts is high, and the thinner the part, the easier it is to burn over, so the rhythm should be fast, and a seam should be welded in one go.
Welding quality inspection: appearance, coloring, airtightness, which links cannot be saved
Appearance inspection is the first link, and it is also the easiest perfunctory level. The welding bead should be smooth, without cracks, biting edges, pores and welding nodules. The uneven welding bead is not an unattractive problem, but a source of stress concentration. As soon as the expansion joint expands and contracts, the broken place starts here. If the appearance is unqualified, it will be repaired directly. Don't hesitate.
Then there is colored flaw detection (PT). The inner surface of the weld (the side in contact with the medium) must be done. Colored flaw detection can detect fine cracks, unfused and pores in surface openings. This step can't be saved, and saving it is equivalent to betting on the service life of the expansion joint.
Finally, there is the airtightness or pressure resistance test. Before leaving the factory, the general corrugated expansion joint usually does a pressure test or air tightness test of 1.5 times the nominal pressure, and the pressure holding time is at least 5 minutes to observe whether the weld seam has leakage. If the pressure gauge reading drops fast, there must be a leak. Don't fool it with "micro-osmosis"-there is no concept of "micro-osmosis" in welding. On-site welding (such as welding expansion joints during pipeline installation), after welding, the pressure test of the same grade as that of the pipeline should be done, and it will be counted only if it is qualified.
From welding to type selection: metal hoses, corrugated expansion joints, PTFE-lined hoses, welding concerns have their own emphasis
Welding problems not only exist in the production and manufacturing process, but also may be involved in on-site pipe modification and maintenance. Different products, the focus of welding is different.
Metal Hoses: The circumferential weld at the connection of the mesh sleeve and bellows is important. The steel wire of the mesh sleeve and the thin plate of the corrugated pipe are welded together, and the difference between the two thicknesses is large. When welding, the current should take care of the thin wall side, and the corrugated pipe will be perforated if you don't pay attention to it.
Corrugated expansion joint (especially general-purpose corrugated expansion joint): There are many welds, such as the welds between the end pipe and the corrugated pipe, the welds of the guide tube and the welds of the tie rod ear plate. Each weld has its own force characteristics. The ear plate weld is a force weld. If the size of the fillet weld is not enough, it will be a big accident if it is broken during hoisting.
PTFE-lined metal hose: The welding difficulty of this product is not in the metal part, but in how to protect the PTFE liner from being scalded by the high temperature of welding. When welding, it is best to circulate cooling water on the back of the weld, or finish welding first and then press lining with PTFE layer. If you need to start a fire next to the installed PTFE-lined hose on the spot, you should also do a good job of thermal insulation protection. The temperature resistance of PTFE is only about 200℃, and the welding slag will be finished if it splashes on it.
To put it bluntly, welding is an experience job and a conscience job. Welding of thin-walled metal expansion jointWhether it is done well or not is not only the technical problem of the welder, but also the embodiment of the whole quality management system-from the material confirmation of the material in storage, to the qualification examination of the welder, to the pre-welding preheating and post-welding inspection, every step is stared at, so as to ensure that the expansion joint can run safely on the pipeline for ten years.