How to weld metal expansion joint? This question looks simple, but in fact there are many pits. I met a customer two days ago. Within two months of installing the expansion joint I took back, the root of the weld leaked. When disassembled, the typical group pair edge misalignment exceeds the standard + the heat input is too large, which burns through the thin-walled bellows.
Everyone in this business knows that metal expansion joint welding is completely different from ordinary pipe welding. The bellows has thin walls, low stiffness and is sensitive to heat. If you don't pay attention to it, you will crack or burn through the weld. Today, we will explain these five key points thoroughly according to the actual working experience.
1. Look at the material before welding: the wrong selection of welding material will directly waste parts
Don't weld the pipe when you get it. Find out if the material is 304, 316L, Inconel or carbon steel first. The welding processes of these types of materials vary a hundred and eighty thousand miles.
Most of the general corrugated expansion joints and bellows in our station are made of 304 or 316L stainless steel. This kind of austenitic stainless steel is easy to weld, but attention should be paid to controlling the interlayer temperature, which is best if it does not exceed 150℃. Some high-temperature axial expansion joints are made of Inconel alloy, such as Inconel 625 or 800H. This material must be made of nickel-based welding wire, such as ERNiCrMo-3, and the heat input must be strictly controlled, otherwise the corrosion resistance of the weld at high temperature will be directly scrapped. Carbon steel is relatively solid, but preheating and post-weld insulation cannot be saved.
Before selecting welding materials, check the materials of bellows and connectors according to the product information. What materials are used for the product, the welding materials will follow the match, don't guess by experience.
2. Groove and assembly pair: Thin-walled parts are most afraid of edge misalignment
When the expansion joint port is paired with the connector, the bevel angle, blunt edge and clearance must be according to the process card. The wall of the bellows is thin, usually only 0.5mm ~3mm. When the groove is opened wide, it will directly melt through, and when the groove is opened small, the root will not melt through.
When actually working, many people are planted in the wrong side. The specification requires that the amount of misalignment should be controlled within 10% of the wall thickness. What do you mean? For example, if the wall thickness is 2mm, the amount of misalignment cannot exceed 0.2mm. Beyond this number, the root of the weld seam is prone to unfusion, and the surface looks like it is welded. In fact, there is delamination inside, and it will leak when it is pressed.
When assembling, use special fixtures, do not hold your hands and fix them by spot welding. The bellows itself cannot bear the concentrated load, and if it is hard broken when assembling, the corrugation deformation will be wasted.
3. How to choose the welding method: argon arc welding is standard
Thin-walled bellows are all argon arc welded, which is the iron law. The heat input of argon arc welding is controllable and the arc is stable, which can ensure the back shaping. Thick-walled parts such as large-diameter thick-walled expansion joints can be filled with argon arc bottoming + manual arc welding, which is efficient, but the bottoming layer must be argon arc welded.
Note that when filling and cover welding, the arc should not directly face the bellows, and try to be as close to the connecting side, so as to reduce the damage of high temperature to the bellows. The welding current should be adjusted according to the material and thickness. 304 stainless steel, about 1mm bellows, current control at 50~70A. Inconel material is lower, and you have to use the back to protect the gas.
There is no cookie cutter routine for welding methods, but there is one principle: the smaller the heat input, the better, the faster the better, and the thinner the wire, the better.
4. Welding sequence and deformation control: longitudinal seam first and then annular seam
What are bellows most afraid of? Welding splashes and arc scratches. Splash on the ripples is a corrosive starting point. Therefore, before welding, the bellows surface must be blocked with protective cloth or white iron sheet.
In the welding sequence, the longitudinal seam is welded first, and then the circumferential seam is welded. The longitudinal seam is the weld of the bellows itself. If there are drawings, it must be argon arc welded, and it should be smoothed after welding. In circumferential welding, symmetrical welding or segmented desolding is done to reduce local heat buildup. Sectional desoldering, each section is 200~300mm, and the next section is welded after one section is cooled down.
During on-site welding, protective gas should be passed through the inside of the expansion joint. Some people say that it doesn't make sense to save money, but as a result, the back shaping is a mess, and the oxide scale falls off one by one. Remember, the back side is not well formed, and the weld life is directly folded in half. The back of the stainless steel bellows is protected with argon gas, and the purity is not less than 99.99%.
5. Post-welding inspection and pressure test: Don't skip that step
A weld appearance inspection is only the first step. Biting edges, surface pores, crater cracks, these can be seen by the naked eye is definitely unqualified. But what is more critical is internal defects, which must be nondestructive tested according to standards.
For corrugated expansion joints used in power station industry and expansion joints used in pressure pipelines, the welds generally require 100% RT or UT. RT is filming, UT is ultrasound. Conditionally, use phased array, which is more efficient. We provide a complete report on the welded expansion joints in our station, as long as the customer requests.
The same goes for pressure testing. The pressure test of metal expansion joint should be carried with a restraining device to prevent excessive extension of bellows. When the pressure is tested, it is low first and then high, and the pressure is increased gradually. If the pressure gauge has been calibrated, the measuring range should be 1.5~2 times the test pressure. During the hydraulic test, the internal air should be discharged, otherwise the pressure will be unstable.
How to weld metal expansion joint? In fact, the core is one sentence: treat the bellows as a precision part, and every parameter is true. There is no shortcut to welding, but according to the above five steps, the scrap rate can be reduced by more than half. Don't bother, the cost of rework is much higher than the meticulous effort when welding.