Repair welding of metal expansion joints, is it saving money or laying mines?
Two days ago, I met an old brother who was doing power plant maintenance, and spat with me: a DN600Corrugated expansion joint for power plant industry, after three years of use, a two-millimeter-wide crack appeared in the bellows. What the leaders of the factory mean-weld it to save money. As a result, he found a random welder outside, and in less than two months, it leaked again, and even took the next twoMetal hoseAlso corroded out of the pit. The last three devices were all replaced, and the cost was quadrupled.
Alas, this kind of thing is quite rare in the industry.Repair welding of metal expansion jointIs it really saving money or laying mines? Let's break it down today.
The Necessity of Repair Welding: What Situations Force You to Repair Welding?
Not all cracks can be mended. Find out the situation first.
Local corrosion perforations or fatigue cracks appear on the bellows surface, and the crack depth does not exceed 10% of the wall thickness, and the length is within 30 mm. Such asUniversal corrugated expansion jointUsed on steam lines, the water hammer causes fine cracks in the bottom of the corrugated valley. This kind of damage has not yet damaged the structural bones, and repair welding is feasible.
Weld cracking caused by detachment or wear of the guide tube。 Such asHigh temperature axial expansion jointThe inner guide tube was flushed and displaced, which cracked the connecting weld between the bellows and the end tube. The cracking location is in the non-confined area (strictly speaking, the non-corrugated area), so the repair welding difficulty is relatively low and the construction period is short.
But note-if the crack has penetrated the entire wall thickness, or appears in the crest and trough stress concentration zone of the corrugation (the so-called "dead center"), then repair welding is to dig a hole for yourself. And guess what? Many manufacturers even dare to take this kind of work. After welding, they use coloring inspection to take a photo. On the surface, it looks fine, but the internal stress has already destroyed the material organization.
Easy to step on pits: Why is it said that repair welding is more of a test of craftsmanship than replacement?
What is the most taboo in repair welding? Inexperienced welders take ordinary welding rods to the bellows. And the result?Stainless steel bellows (such as 316L) are extremely sensitive to heat inputAfter welding, a chromium-depleted zone is formed, and the corrosion resistance drops linearly. It turned out to be just a small pitting corrosion, but it accelerated the corrosion when it was repaired.
What's worse isThe wall thickness of bellows is usually only 1~2 mmThe welding gun can be burned through with a shiver. Moreover, the corrugated structure is complex, and the influence of different wave pitch and wave height on heat conduction is completely different. You look at the same rootLarge diameter thick wall expansion jointIf the wall thickness difference is 0.1 mm, the welding parameters have to be re-adjusted.
Welding material mismatch。 The list of products on our site is very clear- -PTFE-lined hose、PTFE compensatorThis kind of non-metallic lining, repair welding high temperature directly destroys the lining. Some masters don't care, the lining blisters and falls off after welding, and the whole compensator is directly scrapped.
Therefore, repair welding is much harder than replacement. There are drawings and process documents for new replacement, just install them according to the steps. Repair welding is an uncertain damage, which depends on the welder's experience to judge. When you encounter inexperienced ones, you will lay mines.
The correct repair welding process: from material matching to post-weld heat treatment
How should the formal repair welding process go? Three steps, one is indispensable.
Step 1: Material matching and pre-weld preparation.Find out the bellows material first. Such asMetal Corrugated Expansion Joints in Cement Industry304 or 316L is commonly used, so you have to use stainless steel welding wire of the same grade, or choose nickel-based alloy welding wire (such as ERNiCrMo-3) to ensure corrosion resistance. Before welding, the oil stain and oxide scale in the crack area must be removed, and a V-shaped groove must be grinded with a grinding wheel. The groove angle is 60~70 °, and the blunt edge is left 0.5 mm, so as to reduce the heat input.
Step 2: Welding operation.Must useArgon Tungsten Arc Welding (TIG)The current is controlled at 30-80A, the voltage is 10-14V, and the argon gas flow rate is 8-12L/min. Multi-layer and multi-pass welding is adopted, the width of each weld seam does not exceed 3 times the diameter of the welding wire, and the interlayer temperature is controlled below 100℃ (stare at it with an infrared temperature measuring gun). ForDirect buried (fully buried) type expansion jointThis kind of equipment that cannot be disassembled needs to be set up with a local protective cover on the spot, and the back is filled with argon gas for protection to prevent oxidation.
Step 3: Post-weld heat treatment.Stainless steel bellows must be weldedSolid solution treatment (1050~1100℃ rapid cooling)Or at least doLow temperature stress relief heat treatment (300~400℃ for 2 hours)。 Some masters try to save trouble and install it back directly after welding. As a result, the hardness of the weld area is much higher than that of the base metal, and it is only a matter of time before the stress concentration leads to secondary cracking.
After the repair welding is completed, doColor penetration detection(According to JB/T 4730 standard), ensure that there are no cracks, pores and unfused. You have to do it if you have the conditionsAir pressure strength test, the test pressure is 1.5 times of the design pressure, and the pressure is held for 10 minutes without leakage.
Acceptance criteria after repair welding: Light water-tight is not enough
Many users accept the repair welding effect, that is, irrigate it to see if it leaks. This is called acceptance? It's called taking chances.
- Dimensional tolerance: The variation of the original wave pitch and wave height of the bellows does not exceed ±5%. If the deformation is too large after repair welding, the displacement compensation ability of the compensator will decrease. For example: a power plantExternal pressure single axial expansion jointRepair welding was carried out, but it was not reset after welding, and the axial stiffness increased by 15%. As a result, the fixed bracket of the adjacent pipe was crooked.
- Surface quality: The residual height of the weld shall not exceed 0.5mm, and there shall be no biting edges or craters on the surface. The bellows itself is a thin-walled piece, and if the residual height is too large, it will become the stress concentration source.
- Material recovery: withFerrite analyzerThe ferrite content in the weld zone is measured and controlled between 3% and 12% (for austenitic stainless steel). The ferrite content is too high, and the weld brittleness is large; If it is too low, it will have poor intergranular corrosion resistance, so this job is in place.
By the eyes alone? It didn't work. The quality of repair welding needs data to speak for itself.
When is it time to decisively replace it? Calculate an economic account
This is probably the most useful part. When exactly should I change it? Settle the account for you.
Suppose a rootCorrugated expansion joint for power station industry, specification DN400, design pressure 1.6MPa, material 316L. The new purchase cost is about 8,000 yuan. Once welded, the formal quotation (including welder's working hours, materials and testing) is about 1500~2500 yuan. It looks like the repair welding is about 70% saved. However, note that the life of the equipment after repair welding is usually only 30% ~50% of the original equipment. The design life of the original equipment is 10 years, and it can be used for up to 3~5 years after repair welding. Moreover, after this repair welding, if the follow-up breaks down again, each shutdown loss (the loss of one hour of power plant shutdown may be tens of thousands or even hundreds of thousands), plus the labor cost of disassembly and installation-an absolute loss.
- Corroded area exceeding 15% of bellows surface areaOr the number of cracks is more than 3-this can't be repaired, and the metal structure has deteriorated as a whole.
- High temperature condition (> 450℃)underHigh temperature axial expansion jointAfter repair welding, it cannot recover its thermal fatigue resistance, and replacement is the only option.
- Non-metallic lined expansion joint(e.g.PTFE-lined hose、rubber compensator), repair welding high temperature directly destroys the lining, it is better to change it directly.
- Expansion joints of Class 300 and above on pressure pipelines, such as forDesulfurization flue gas baffle doorSystems,Steam pipelineThis type of pressure-bearing special equipment needs to be re-tested after repair welding, and the cost is higher than replacement, so no one wants to do this.
If the equipment has been used for more than half of its design life, or the damage is in the stress concentration zone, replace it directly. Repair welding is not impossible, but to find a qualified professional manufacturer, it is best to set up a record sheet on the spot-the batch number of welding materials, welding parameters and test reports are all indispensable.
Do you want to spend a small amount of money on something big, or do you want to spend a small amount on a ticking time bomb?Repair welding of metal expansion jointThink clearly about this matter before starting.