Welding process of metal corrugated expansion joint of oil storage tank: quality control and key points of welding seam operation
1. Structural characteristics and welding difficulties-the weld is the lifeline
It consists of bellows, end tubes, guide tubes (if available), and flanges. Corrugated pipes are usually made of multi-layer thin-walled stainless steel, such as 304 and 316L, with a thickness of 1.5mm to 3mm. Thin-walled structures are extremely sensitive to welding heat input – with a little carelessness, bellows can deform, intergranular corrosion or even melt through. The internal medium of the oil storage tank is transformer oil, which requires ridiculously high sealing and cleanliness of welds. Any small leakage may cause insulation accidents. So welding is not a simple "fusion", but a precision process control. Think about it, once the transformer oil leaks, the whole transformer may be wasted. Who dares to bear this responsibility?
2. Prepare to match the material before welding-80% of defects are planted in this step
Don't underestimate pre-weld preparation, 80% of welding defects originate here. In terms of base metal, austenitic stainless steel is commonly used for bellows, and the matching end pipe and flange materials must be compatible with bellows-it is no joke that different steel welding produces brittle structure. The groove is processed into V-shape or U-shape, the blunt edge is controlled at 0.5mm ~1mm, and the gap is 2~3mm. For wire selection, ER308L (for 304 base metal) or ER316L (for 316L base metal) with a diameter of 1.0mm ~1.6mm is recommended. Cleaning must be done in place: de-oil the surface of the welding wire, wipe with acetone within 20mm of both sides of the base metal, and welding with rust and oil is strictly prohibited. Two days ago, I met a customer, because the oil stain on the end pipe was not wiped, and the porosity after welding directly exceeded the standard by 30%. You say it was wrong or not?
3. Core control of welding process parameters-fast-in and fast-out, small heat input
The key to welding thin-walled stainless steel is "fast in and fast out, small heat input". It is recommended to use tungsten argon arc welding (GTAW) with direct current connection. The welding current is controlled at 40A ~80A (adjusted according to the wall thickness of 1.5mm ~3mm), the arc voltage is 10V ~15V, and the welding speed is 80mm/min ~150mm/min. The interlayer temperature must be controlled below 100℃, otherwise carbides in the heat-affected zone will precipitate and the corrosion resistance will drop by a cliff. When multi-layer multi-pass welding, each weld should be cleaned with oxidized color, especially the blue and purple areas, which must be polished off with a stainless steel wire brush.
When welding, fill the back of the weld with argon gas for protection-when there is a partition inside the bellows, local argon filling can prevent the back surface from oxidating. In addition, current attenuation or crater filling is used during arc closing to avoid crater cracks. Tsk, this job looks simple. If the current difference is 10A, the bellows may be scrapped in layers.
4. Weld inspection and on-site treatment of common defects
After welding, it doesn't mean it's done. Inspection is hard work. Look at the appearance first: visually with a 5x magnifying glass, no cracks, biting edges, unfused; The weld residual height is controlled at 0~1.5mm, and the width is uniform. Then do penetration detection (PT), brush developer on the weld surface and heat affected zone, and observe whether there is a nonlinear or circular display. If you really don't have PT conditions, at least do a kerosene leakage test-apply chalk powder on one side of the weld, brush kerosene on the other side, wait for 30 minutes and check for any kerosene penetration traces on the back.
Vents → Check argon flow and welding ambient wind speed; Unfused → Adjust the groove angle or increase the welding current; Welding nodule → Increase welding speed or reduce wire feeding amount appropriately. If a biting edge with a depth exceeding 0.3mm is found, it must be repaired and re-tested. Remember, transformer oil is no joke, leaking a drop is an accident.
5. Technological differences with similar expansion joint products in this station
Take the corrugated expansion joint and the high-temperature axial expansion joint used in power station industry as examples. Their welding principles are similar, but the corrugated expansion joint of oil storage tank has more stringent requirements for sealing and cleanliness. Power station industry expansion joints are commonly used in flue gas or steam pipelines, allowing some degree of oxidized color and slight deformation, while oil storage tanks must meet corrosion resistance and zero leakage under long-term immersion of transformer oil. For another example, metal corrugated expansion joints in cement industry are mostly used for dusty airflow, but more attention is paid to wear-resistant lining when welding.
Don't use one universal process to cover all products. Materials, working conditions and service environment are different, so the welding parameters have to be adjusted accordingly. The manual plug-in insulation doors and flue gas baffle doors on our website also have similar structures, but the welding process focuses on wear resistance and high temperature strength, and the sealing of oil storage tanks is two different things. If you take the process of the power station to weld the oil storage tank, it will leak in less than three months.
6. "Pit" in actual operation and summary of experience
Finally, let's talk about a few real cases. In order to catch up with the construction deadline, some customers raised the welding current to 120A. As a result, the bellows were separated between layers and directly scrapped. Still others did not calculate the compensation amount of the length of the expansion joint before welding, resulting in the corrugated pipe being straightened after welding and losing the compensation ability-then what is the use of installing this expansion joint?
The welding sequence is also very important-first weld the flange and end pipe ring seam, then weld the bellows and end pipe ring seam, and finally weld the guide tube. After each circumferential seam is welded, check the corrugation height change of the bellows with a caliper. If the change exceeds 0.2mm, adjust the parameters or re-calibrate the pair. Tsk, in the final analysis, welding the corrugated expansion joint of oil storage tank is competing with the heat sensitivity and sealing requirements of stainless steel, and every step has to be picked up in details.