Why is on-site welding the "life and death" of corrugated expansion joint?
Anyone who has done pipeline installation knows that the metal corrugated expansion joint is obviously good when it leaves the factory, but there will be problems once it is welded at the site. Leaks, deformations, and even cracked bellows — nine times out of ten, they are buried mines by welding. Why? Because the conditions on the site are simply incomparable to those in the factory. In the factory, the welding machine is stable, the temperature and humidity are controllable, and the operators can take their time; What about the scene? Wind and sun exposure, the pipeline is still stressed, the groove may be rusty, and the welding electrode is not completely baked... If any link pulls the crotch, the bellows will become a scapegoat.
Welding stress is the number one killer. If you think about it, the bellows itself is a thin-walled piece, and the thinnest part may only be a few tenths of a millimeter. Once the welding heat input is not well controlled, the local temperature soars, and the weld shrinks after cooling, pulling the bellows into deformation. Seal failure often does not happen right away, but instead the stress is released after several months of operation and the cracks slowly crawl out. Another problem is intergranular corrosion-stainless steel bellows stay in the sensitization temperature zone of 450~850℃ for too long, carbides precipitate along the grain boundaries, and the weld becomes brittle. Therefore, on-site welding is not just welding, but a tough battle with thermal stress, metallographic structure and sealing surface.
Preparation before welding: beveling, interlayer temperature, base metal matching
Let's start with the bevel. The groove angle of the pipe with different wall thickness is different. The conventional V-shaped groove angle is 30 ° ~35 °, and the blunt edge is 1~2mm. But don't stick to standards-you have to see if there is an ellipticity deviation in the pipe on site. If the amount of misalignment of the counter edge exceeds 10% of the wall thickness, the groove must be repaired, otherwise the weld will not penetrate.
Interlayer temperature is more critical. For austenitic stainless steel bellows, the interlayer temperature is best controlled below 150℃. Beyond this number, the weld surface will turn blue or even black, indicating that oxidation is serious and corrosion resistance is directly compromised. Used in the power station industryCorrugated expansion joint for power station industry, the medium temperature is high, the pressure is high, and the interlayer temperature control is particularly strict-some projects require that every welding is finished, it should be measured with an infrared temperature measuring gun, and if it exceeds 120℃, it has to be stopped for cooling. What about the cement industry?Metal Corrugated Expansion Joints in Cement IndustryMost of them work in a dusty environment, and impurities are easy to attach to the surface of the base metal. Before welding, the 20mm range on both sides of the groove must be wiped clean with acetone or special cleaning agent, otherwise there will be scary pores.
The base metal matches this piece, don't think "it's all stainless steel anyway". The field piping may be 304, the expansion joint bellows may be 316L or 321, and the electrode should be A102 or A132? Depends on the working conditions. If it isHigh temperature axial expansion jointThe working temperature exceeds 500℃, the base metal is usually Incoloy 800 or heat-resistant alloy, and the welding electrode must be nickel-based, such as ENiCrFe-3. You take ordinary stainless steel welding electrode to weld, the weld strength is not enough at high temperature, and it will crack in a few months.
How to determine the welding process parameters?
I won't memorize the parameter table with you here, but talk about a few places that are easy to overturn in practice.
Welding rod grades.The small diameter welding electrode of φ 2.5 is used for bottoming welding, which has small current and shallow melting depth, which can prevent burning through the bellows. The filling and cover surface can be φ 3.2, but pay attention to the line energy when the current is adjusted up-the line energy q = IU/v, in kJ/mm. Generally, the online energy of austenitic stainless steel is controlled to be ≤1.5 kJ/mm, and the risk of intergranular corrosion will come up when it is high.
Multi-layer multi-pass welding.Why does it have to be multi-layered and multi-channel? The heat input of single-pass welding is concentrated, the welding bead is wide, the cooling is slow, and the residence time of sensitization zone is long. Multi-layer multi-pass is equivalent to quickly cooling each weld bead, and the former bead can be "tempered" by the latter bead to refine the grains. I have seen a project in which the worker tried to save trouble and welded the whole ring seam in one go. As a result, PT detected a bunch of micro-cracks.
Prevents hot cracking.Solidification cracks and liquefaction cracks are most likely to occur in stainless steel welds. Note in operation: fill the crater when closing the arc, otherwise the crater crack will extend to the base metal; The swing amplitude of the welding electrode is not too large, and narrow pass welding is conducive to the floating of impurities. In addition, the joints are overlapped by 10 to 15 mm to ensure sufficient fusion.
Common field welding defects: porosity, unfused, biting edges
When it comes to defects, many people wait for pressing and testing to find that they leak, and it is more troublesome to repair welding. In fact, the first two steps can avoid most of the problems.
Pre-heating before welding (not all materials are required, but 100~150℃ pre-heating is recommended when welding thick-walled and different steels); Step 2: Slow cooling with asbestos cloth immediately after welding to prevent quick cooling from producing hardened structure.
Stomata.The most common reasons: the welding electrode is not dried, the purity of the shielding gas is not enough (if argon arc welding is used), and the groove has oil stain. The drying temperature of 302 stainless steel welding electrode is 350℃ ×1h. After taking it out, put it in a thermal insulation cylinder and take it at any time. Poor conditions on site? Then buy a welding rod bucket with an insulated cylinder, and don't save those hundreds of dollars.
Unfused.Often out between the primer weld and the groove sidewall. Solution: Increase the welding current by 10% ~15%, or open the groove angle wider. There is another situation-the welder's technique is too quick, the angle of the welding rod is wrong, and the arc doesn't hit the root. This can only rely on practical training and process supervision.
Bite the edges.The walls of the bellows are thin, and a pit will be burned in the base metal if the current is slightly large or the swing of the welding electrode is too large. If the bite depth exceeds 0.5mm, it has to be repaired, because the stress concentration point is there. Tip: Reduce the welding speed appropriately when cover welding, so that the molten pool has enough time to fill the edges.
Post-welding inspection is not a formality
Many people think that PT (penetration detection) is enough. Wrong.Universal corrugated expansion jointIf it is used in normal temperature and low pressure water pipelines, PT + appearance inspection is sure. But likeHigh temperature axial expansion jointIn this case, the working temperature is high and the pressure fluctuates greatly. It is recommended to do MT (magnetic particle detection) or RT (ray detection) directly, because some internal cracks PT can't be seen at all.
What about hardness control? The welding hardness of austenitic stainless steel generally does not exceed 220HB. If it exceeds, it means that martensitic structure is produced by cooling too quickly, and solution treatment is needed. However, it is unrealistic to make solid solution on the spot. In most cases, it is solved by controlling the interlayer temperature and post-welding heat insulation.
As for stress relief heat treatment – not all cases do it. Thin-walled 304 pipes need not be made after welding, but thick-walled (> 20mm) or different steel joints, or media have a tendency to stress corrosion (such as chloride ion-containing environment), it is recommended to do stress relief annealing at 550~650℃ after welding. Note, though:Direct buried (fully buried) type expansion jointHeat treatment is usually not done after welding, because the thermal expansion and contraction are limited after burial, and heat treatment may change the rigidity of the bellows.
The Essential Difference Between Field Welding and Factory Welding
Someone moved the welding process evaluation of the factory directly to the site for use. It is equivalent to asking a marathon runner to lift weights-the way of exerting power is completely different.
Factory welding is in the workshop, the workpiece can be turned over, and the angle of the welding torch can be adjusted at will. What about the scene? The pipe is fixed dead, and the welder has to be welded overhead, vertical or even all-position. The welding positions used in the process evaluation are usually only flat welding or cross welding, and all the welding positions on site are all, so the heat input parameters must be adjusted. For example, the current in vertical welding is 10% ~15% smaller than that in flat welding, otherwise the molten iron will drip down.
In addition, expansion joints of different structures are welded with respective pits on site. For example, beltguide tubeFor the corrugated expansion joint, you have to spot-weld the guide tube and then weld the annular seam on the bellows side-because the guide tube is a thin plate, it is easy to deform when welded first. Andtie rodInstallation: The weld between the tie rod and the ear plate is easy to crack after being stressed. After welding, it is necessary to check whether the tie rod is consistent in tightness, otherwise the tie rod will bear eccentric load after operation and the bellows will be abnormally compressed.
All in all,On-site welding of metal corrugated expansion jointDon't expect the welder to go up with the welding rod and talk about it. Only by clearing the groove, adjusting the parameters, keeping an eye on the interlayer temperature, and testing after welding in place can the pipeline life be worth the money you spend.