The expansion joint of the metal welding joint sounds quite twisted. To put it bluntly, it is the expansion joint connected between the bellows, the connection pipe and the flange by welding. Turn through the general corrugated expansion joints and high-temperature axial expansion joints in our station. Most of them have this structure. Some people think this thing is simple, isn't it just a round of welding-it really is not. Is it the same welding port, single-sided or double-sided? With or without bushing? Is the flaw detection ratio 100% or random inspection? These details directly determine whether it can last on high-temperature and high-pressure steam lines for ten years without cracking.
The first killer of welding failure: not fatigue, but intergranular corrosion
Many people think that bellows cracking is a fatigue problem, wrong. The most feared of welding joints is the stress concentration of welding toe and intergranular corrosion. Two days ago, I met a customer in the cement industry. It took less than eight months for the metal corrugated expansion joint to leak in the heat-affected zone of the weld. After dismantling, it shows that the line energy is not well controlled during welding. The residence time of austenitic stainless steel in the sensitization temperature range of 450-850℃ is too long, and carbides precipitate along the grain boundary, which directly embrittles and cracks when stressed. Do you think the bellows wall is thick enough? Is the wavenumber much? None of it worked. When the welding spots rotten, everything was lost.
Therefore, choose the expansion joint of the metal welding joint, and don't just stare at the wall thickness and wave number of the bellows. First, see if the manufacturer can take out the welding process evaluation report (PQR), welding material grade, welding method, line energy range and heat treatment parameters, which must be written clearly in black and white. Don't touch it no matter how cheap it is without a PQR quote.
Three checkpoints to keep rotten welds out of the door
When the goods are accepted, check according to these three points, and you will basically not step on a big pit.
- A look at the weld forming.The fish scale pattern should be uniform, without biting edges, pores and slag inclusions. If the surface ripples are rough and different in width, there is a high probability that there will be problems internally. You take a flashlight to the weld and give it a circle, and emphasize the places where there are depressions or bumps.
- Second, look at the flaw detection report.When the design pressure exceeds 1.6MPa or above DN300, 100% ray (RT) or ultrasonic (UT) flaw detection must be performed before leaving the factory. Note that it is every ring seam, not a random inspection.
- Third, look at the post-weld heat treatment record.When welding austenitic stainless steel or different steel with a thickness exceeding a certain grade, whether solid solution or stabilization treatment should be used after welding directly relates to the corrosion resistance of the weld. There is no heat treatment record, which means that the welding stress has not been eliminated, and the later stress corrosion cracking is waiting for you.
When the medium contains chloride ion or sulfide, the material should not be degraded randomly. 316L is the bottom line, and 254SMO can be considered for working conditions with high chloride ion concentration. The welding materials also have to be matched-I have seen 308L welding wire to weld 316L pipe. It was fine at that time after welding, but the weld rusted into a sieve half a year later.
There are more pits in the installation stage than expected
Metal welded joint expansion joints come out of the factory with tie rods or transport supports. This thing is to protect the bellows, not decorations. Some customers tried to save trouble, and the tie rod was disassembled before the pipeline was assembled. As a result, the bellows was displaced ahead of time under its own weight and installation stress, and the steam was not passed yet, so its life was folded by half.
First, spot weld the expansion joint and position it. After all the pipelines are assembled and paired, the fixed bracket and the guide bracket are in place, then disassemble the transportation support. Finally, adjust the tie rod nut according to the design requirements. The function of the tie rod is to restrain the bellows from working within the normal displacement range, not for you to use it to calibrate the coaxiality of the pipeline-it is strictly forbidden to use the expansion joint to forcibly correct the deviation, and the weld joint is deadpulled if the misaligned edge amount exceeds 2mm. This expansion joint is equal to direct scrapping.
The insulation layer does not cover the weld seam. In some projects, in order to look good, the thermal insulation cotton is directly attached to the ring seam package, and it takes half a day for the inspection to see the weld status. Leave room for overhaul, no shame.
For routine maintenance, just keep an eye on the weld
The leakage point of the expansion joint of the weld joint is 80% at the circumferential weld and the connection between the connector and the flange. When inspecting, focus on whether there are yellowing, blackening or rust marks on the weld surface. Normal austenitic stainless steel welds are silvery white or pale yellow, and darkening indicates poor protection during welding or abnormal use temperature.
If you are using external pressure single axial expansion joint or double hinge transverse expansion joint, check the outside of the bellows for foreign matter accumulation. Dust and rain are mixed together and deposited in the trough all the year round, which is a hotbed of electrochemical corrosion. Don't bother with it. Take the hose and flush it. Remember this phrase: The weld is the hardest part of the expansion joint, and it is also the most brittle part.
Exactly how long will it last? Counting your thermal cycles first
The design fatigue life shouts 5000 starts, and how long it can actually last depends on the operating temperature and start-stop frequency. It is normal that the steam pipeline starts and stops once a day and once a week, and the welding life difference is three or five times. The reason is simple: with each hot and cold cycle, the weld zone is subjected to alternating thermal stress, and the grain boundary microcracks continue to propagate under repeated loading until they penetrate the wall thickness.
So instead of directly asking "how many years will it last", first calculate how many times your pipes heat cycle a year. The underlying logic of selection is to make the expansion joint always work within a reasonable displacement range. No matter how high the welding strength is, it can't withstand over-the-range fatigue. This is understood, and many entanglements in selection, installation and maintenance will naturally be solved.