Find out before welding: What is the material and structure of your expansion joint?
Two days ago, I met a customer, who took a drawing and asked if he could weld it, saying that it was a general-purpose corrugated expansion joint of 304 stainless steel. As a result, the weld cracked as soon as the primer welding was put on. Why? It was marked 316L on the drawing, and he welded it as 304. If the material is mixed up, the primer welding will crack. This is the first iron law.
304 stainless steel、316L stainless steel,Carbon steel、Heat-resistant steelThe requirements of these materials for welding process are very different. Just use ER308 wire for 304, ER316L for 316L, J422 or J507 for carbon steel, and heat-resistant steel (such as 15CrMo) has to be preheated to above 150℃. If you take carbon steel wire to weld stainless steel bellows, hehe, intergranular corrosion is waiting for you.
Let's talk about structure types. Also stainless steel,Universal corrugated expansion joint bellows is thin (0.3-2mm), which is most afraid of burning through;High temperature axial expansion jointOften with multi-layer corrugations, problems will arise if the interlayer temperature is not well controlled;directly buried expansion jointWith outer sleeve, the space is limited during welding, and the heat dissipation is poor;Large diameter thick wall expansion jointThe wall thickness may be more than 10mm, and there are many welding beads and great stress. You take the parameters of welding thin walls to weld thick walls, and the fusion is poor; Take the parameters of welding thick walls to weld thin walls and burn a hole directly. Therefore, ask yourself before starting: What is the material of my expansion joint? What structure?
How to determine the welding process parameters? Current, welding wire, shielding gas, one step wrong, one step wrong
Thin-walled bellows-between 0.3 and 2mm, the first reaction of many novice welders is to "burn with a big current", and the result is instantaneous perforation. Correct practice: Primer welding must be usedArgon Tungsten Arc Welding (TIG)The current is controlled at 40-80A and the wire diameter is 1.0-1.6mm. Interlayer temperature exceeds 60°C? Stop! Wait until it is cooled below 60℃ before welding, otherwise the intergranular corrosion will come quietly once the sensitization temperature range (450-850℃) passes. The corrosion will not appear immediately, but after half a year leaks, who do you want to cry?
The diameter and groove angle of the filling wire are not determined by patting the head. According to GB/T 12777 "General Technical Specifications for Expansion Joints of Metal Corrugated Pipe", the groove angle is 60°-70°, the blunt edge is 0.5-1mm, and the gap is 2-3mm. Pure argon (99.99%) was used as shielding gas, and the flow rate was 8-12L/min. Someone using a mixture to save money? The back of the bellows is oxidized, the welds are black, and the PT is full of cracks when sprayed. Is it enough to save that little gas money for ten expansion joints?
The welding wire grade must match the base metal. For example, the power station industryCorrugated expansion joint for power plant industryIf it is a high temperature section, change the Inconel 625 welding wire; Used in cement industryMetal corrugated expansion joints in cement industry should be stabilized after welding (850℃ insulation + air cooling). You were lazy and didn't do it. After half a year, you missed it. The customer came to you to claim compensation, which was a list of hundreds of thousands.
Welding deformation and stress control-reverse order, compensation amount scrapped
The bellows of the expansion joint are flexible pieces that are themselves designed to absorb displacement. Your welding sequence is reversed, which directly leads to corrugation distortion and compensation scrapping. What about that? Remember the three routines:
- Sectional jump welding: Do not weld from one end to the other, weld 80-100mm per section, jump welding to allow the heat to spread evenly.
- Anti-deformation of fixture: Pre-pull or pre-press the bellows with a rigid fixture before welding to offset the shrinkage deformation of welding. Many old masters will use wedge blocks against the corrugated roots.
- Hydrogen removal treatment after welding: For carbon steel and low alloy steel, heat to 200-300℃ immediately after welding, keep warm for 2 hours, and cool slowly. Hydrogen-induced cracks (cold cracks) are most afraid of this.
Is hydrogen removal treatment necessary after welding? To tell you the truth, carbon steel thick wall expansion joints (such asLarge diameter thick wall expansion joint) If hydrogen elimination is not done, the weld cools quickly, the hydrogen atoms cannot diffuse out, and micro-cracks will occur inside. After installing the pipe, it cracked after a few vibrations. That scene, tsk, no one wants to see.
Welding inspection standards-porosity, crack and unfused are the three killers
After welding, don't rush to fit. If you can't pass the test, it's all for nothing. There are three most afraid problems in metal expansion joint welding: pores (inadequate argon protection or moisture of welding wire), cracks (hot cracks or cold cracks), and unfused (small current or deviation of welding torch angle).
How to check surface cracks? usePermeation test (PT), spray with imaging agent and wait 10 minutes to see if there are any red lines. What about internal flaws?Radiographic inspection (RT), film to see pores, slag inclusions, unfused. Small factories may be too expensive to do RT, but if you want the expansion joint to run at high temperature and pressure for more than a year, this money can't be saved.
Don't miss inspection of fillet welds at the guide tube. The function of the guide tube is to guide the flow direction of the medium and protect the bellows. If its fillet weld is unfused, the medium erosion will cause the weld to fall off, and the guide tube will fall and smash the bellows. There was a cement factoryThe metal corrugated expansion joint in cement industry was due to the missed inspection of the fillet weld of the guide tube. After three months of operation, the guide tube fell off, the bellows was pierced, and the whole production line stopped for 48 hours.
After welding, don't forget to install and fit the guide tube and the tie rod
When the welding is completed and the inspection is qualified, do you think it will be fine? Don't forgetDeflector andInstallation of tie rods. When the guide tube is positioned and welded, if the welding nodule bumps into the bellows, the expansion joint will be installed in vain. Spot welding for positioning welding, the diameter of the solder joint shall not exceed 2mm, and the solder joint shall be polished smoothly after welding. It is recommended to fix it by spot welding first, and then sectional welding. After welding, check the surface of the bellows for scratches and welding nodules.
AgainAdjustment of tie rod nuts and screws. The function of the tie rod is to limit the displacement range of the expansion joint under normal working conditions and prevent over-stretching or over-compression. When installing, nuts and screws must be adjusted with factory pre-tension reserved. PurchaseUniversal corrugated expansion joint orFor high-temperature axial expansion joints, manufacturers will give a pre-stretching or pre-compression amount (generally about half of the compensation amount). If you screw the nut to death directly after welding, without reserving the amount of pre-tension, the bellows will be crushed to death as soon as the upper bolts are tightened, and the compensation function will be directly lost. The consequence is that the thermal expansion of the pipe has nowhere to release, the bracket is deformed, and even the pipe is cracked.
How to tune it? Look at the installation length mark in the product manual, or measure the free length of the bellows with a tape measure, and adjust the bolt position according to the design value. For specific operation, please refer to the FAQ of this site "How to adjust the tie rod nut of expansion joint".
Practical Case: Lessons from Welding and Renovation of Corrugated Expansion Joints in Power Station Industry
Tell two real cases, both of which are lessons bought with real money.
Case 1: Cracking of ring weld of high-temperature axial expansion joint in a power plant
For power plantsHigh temperature axial type expansion joint, material is 304 stainless steel and Q345B carbon steel dissimilar steel joint. There was no preheating during welding, so austenitic stainless steel welding electrode (A102) was selected for welding electrode. Results After three months of operation, the whole circle of circumferential weld cracked. Check out the reasons: when different steels are welded, the heat-affected zone on the carbon steel side cools quickly, resulting in martensitic structure and great brittleness; In addition, the electrode has high carbon content and high hot crack sensitivity. Solution: Switch to Inconel 625 nickel-based welding wire, preheat the carbon steel side at 150℃ before welding, and slowly cool after welding. After replacing it and running for two years, the welds are intact.
Case 2: Metal corrugated expansion joint in a cement plant leaked half a year after welding
For cement plantsMetal corrugated expansion joint for cement industry, medium temperature 450 ℃, containing dust particles. After the welding is completed, the pipeline is installed directly without stabilization treatment. Half a year later, intergranular corrosion cracks appeared near the weld, and media leaked. This is because the weld stays for too long in the sensitization temperature range, and chromium precipitates to form chromium-depleted zone, which accelerates corrosion in the high-temperature dust environment. Later, we returned to our factory, re-welded and stabilized at 850℃, and the problem was solved.
You see that? Metal expansion joint welding is not as simple as "just melt the molten iron together". Material, parameters, sequence, inspection, installation and cooperation, one link is wrong, which can be reworked, scrapped or even caused by safety accidents. Next time you encounter metal expansion joint welding, spend 10 minutes comparing this article, which can save you a lot of trouble.