Welding process is right, expansion energy saving will take more than three to five years-why can't on-site welding be fooled
Anyone who has worked on the scene knows that once the metal corrugated expansion joint is welded, rework is a trivial matter, and the whole pipeline system will suffer. A while ago, a buddy from a power plant complained to me that the corrugated expansion joint on one of their steam pipelines leaked after less than two years of use. When I disassembled it, I found that the root of the weld did not penetrate at all, and the cracks ran out from the inside. Who takes the blame? The welding process is not in place.
A qualified metal corrugated expansion joint has a design life of at least ten years. However, if on-site welding is perfunctory-for example, excessive current burns through wave nodes and intergranular corrosion caused by mismatch of welding electrodes-the actual life can be cut by more than half. The data does not lie: According to the statistics of a domestic petrochemical base, more than 40% of the early failure cases of expansion joints are directly related to on-site welding defects. Therefore, the idea of "just make do with welding it" is destined to lose several times the cost of later maintenance.
So what is the difficulty of on-site welding? The factory is automatic welding, the environment is controllable, and there are strict regulations for beveling, preheating and interlayer temperature. At the scene, the wind was strong, the pipes were unaligned, the operating space was small, and the level of welders was uneven. To put it bluntly, on-site welding is a "dance in shackles"-you have to bring the rigor in the factory and deal with all kinds of accidents.
Preparation before welding: bevel, positioning, welding material, one step less is all pits behind
Preparation before welding is not enough, which directly determines the quality of weld seam. Let's break it apart.
Bevel.When the expansion joint is connected with the pipeline, the groove angle is generally controlled at 30° ~35°, the blunt edge is 1~2mm, and the clearance is 2~3mm. If the angle is too small, the penetration depth is not enough, and if it is too large, the welding material will be easily deformed. Note: For expansion joints made of stainless steel (such as universal corrugated expansion joints), the surface of the groove should be thoroughly cleaned with oil and oxide scale, otherwise pores will be generated during welding, and it will leak after being pressurized. Two days ago, I met a customer who directly put the groove standard of carbon steel on 304 stainless steel. After welding, I found a bunch of pores, and I was so angry that I slapped my thigh.
Positioning welding (spot fixing).The length, quantity and position of spot welding are particular. For example, large-diameter thick-walled expansion joints above DN300 should be fixed at least at 4 to 6 points, each with a length of 10 to 15mm. The welding electrode or wire of spot welding should be consistent with the formal weld, and the current should be slightly smaller by about 10% to prevent cracks. The most taboo thing is that spot welding is directly welded to the bellows-the walls of the bellows are thin (often only 0.5~2mm), and they are worn as soon as they are burned. The correct way to do this is to fix the point on the straight edge segment at the end of the connector.
Welding materials.Selecting the wrong welding material is equal to laying a mine. I have encountered someone who used ordinary carbon steel electrode to weld austenitic stainless steel expansion joints. As a result, brittle martensite structure appeared in the weld, which cracked before it was put into production. Remember: the base metal of corrugated expansion joints is mostly stainless steel (e.g. 304, 316L, 321) or heat resistant alloys (e.g. Incoloy 800), and the welding materials must be homogeneously matched. For example, when welding high-temperature axial expansion joints, since the operating temperature can reach above 800℃, it is recommended to use nickel-based welding materials (such as ERNiCr-3) to ensure high-temperature strength and oxidation resistance. Before use, the welding electrode should be dried according to the instructions. The drying temperature of the low-hydrogen welding electrode is 350℃ ~400℃, kept warm for 1 hour, and taken as you go-this is often ignored.
Key points of actual welding: From arc start to arc close, controlling deformation and penetration is the key
As soon as you take the welding gun, you will see the real skill. Several key points are emphasized repeatedly:
- The parameters should be stable.Take manual arc welding welding stainless steel expansion joint as an example: the current of 4mm welding electrode is controlled at 140~160A, the voltage is 22~26V, and the welding speed should not exceed 15cm/min. The current is too large, the molten pool temperature is too high, and the stainless steel is prone to hot cracks; The current is small, the penetration depth is not enough, and the roots are not fused. If it is a thin-walled bellows (such as 0.8mm wave height), it is recommended to use argon arc welding, the current is 80~100A, and the interlayer temperature is controlled below 100℃ to avoid trough deformation caused by overheating.
- Multi-layer multi-pass welding.For joints with wall thickness exceeding 8mm, they must be welded in layers, each layer thickness not exceeding the electrode diameter. Primer beads should be thin, ensuring that the back is formed. The cover weld bead should be wide, but don't exceed the edge of the groove, otherwise it will bite the edge. In actual operation, you can first use small diameter welding electrode (3.2mm) to base, and then use 4mm filling and cover.
- Controlling deformation.Welding heat input will cause angular deformation or shrinkage of the expansion joint pipe, and even the tie rod cannot be installed in severe cases. How to break? Symmetric welding-two people start the arc from symmetric positions at the same time, and jump welding a section every 200mm. Or use rigid fixation: temporarily fix the connecting pipe on the tooling with a clamp, and then loosen it after cooling after welding. In addition, the reverse deformation method is also very useful: the connection pipe is inclined in the opposite direction of deformation by 1 ° ~2 ° before welding, and it is corrected just after welding.
- Start and close the arc.The arc should be started in the groove to avoid arc craters on the surface of the base metal. Fill the arc crater when the arc is closed, and use the back-welding method or lead-out plate if necessary to prevent crater cracks. Especially on stainless steel, crater cracks are high frequency defects.
And guess what? Some old masters have been welding all their lives, and what they are most afraid of is closing the arc-when their hands shake, the pores and cracks all come. So don't underestimate these last few millimeters. Slow down the arc closure speed, reduce the current a little, and wait for the molten pool to solidify before removing the welding torch.
Different types of expansion joints pay different attention to welding-high-temperature type and general-purpose type should be treated differently
Expansion joints are not cookie cutter, and the requirements for welding are different in different working conditions.
High temperature axial expansion joint(For example, used on boiler exhaust pipes): High operating temperatures, heat-resistant stainless steel (e.g. 310S, Inconel 625) is commonly used for bellows and connectors. Two points should be paid attention to when welding: First, post-welding heat treatment to eliminate residual stress-if the site conditions do not allow, at least heat insulation and slow cooling should be done to avoid cracks caused by quick cooling; The second is to control the energy of the welding line. Excessive heat input will easily cause the carbide precipitation of the heat-resistant steel and reduce the high-temperature strength. There was a project on the cement line before, and the high-temperature expansion joint was not heat treated after welding. As a result, the weld cracked after three months of operation.
Universal corrugated expansion joint(For normal or medium temperature pipes, such as HVAC, gas): The material is mostly 304 or 316L. The core of welding is to prevent intergranular corrosion-keep the interlayer temperature no more than 150℃, make the welding speed appropriately faster, and reduce the residence time of the sensitization zone. Models containing stabilizing elements (such as titanium and niobium) are recommended for welding wires, such as H0Cr20Ni10Ti. In addition, the general-purpose expansion joint usually has a thin wall thickness, and the back should be filled with argon gas protection during welding to prevent oxidation and blackening.
Corrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryEach has its own characteristics: power station pipelines often have large diameter and thick walls, and the groove should be transitioned from U-shaped to V-shaped, preheated at 100~150℃ before welding, and heat treated immediately after welding. The cement industry is dusty, so dust and oil must be thoroughly removed before welding, otherwise pores will inevitably occur.
Don't take the form of post-welding inspection: how to do non-destructive testing, leak test and defect repair
After welding, everything is not fine, but the inspection link is the true acceptance. Many on-site maps save trouble, and they are done with a glance. As a result, the cost is doubled after leakage and repair welding.
Appearance inspection.Inspect the weld surface with naked eye and magnifying glass: no cracks, no biting edges and no pores. The biting depth shall not exceed 0.5mm, and the continuous length shall not exceed 10% of the total length of the weld. Immediate grinding and repair welding for unqualified.
Non-destructive testing.Select methods according to pressure class and hazard. Generally, the surface defects can be screened out by permeation detection (PT) or magnetic particle detection (MT) in pipe welds. However, high-temperature and high-pressure media (such as steam and toxic gases) must be tested by radiography (RT) or ultrasonic (UT). If unfused, unpenetrated or cracked are found on the radiographic film, it will be directly judged to be scrapped. Note: The bellows itself is not allowed to be welded back and repaired, and can only be replaced once the weld leaks. The same position of the welding seam shall not be repaired more than twice, and the whole section of pipe shall be scrapped directly for the third time.
Leak test.Air pressure test (soapy water leak detection) or hydraulic pressure test are commonly used on site. The hydraulic test pressure is 1.5 times of the design pressure, and there is no leakage after holding the pressure for 10 minutes. However, for corrugated expansion joints, they should be thoroughly dried after hydraulic test to avoid residual water corrosion of corrugated pipes-especially stainless steel is prone to stress corrosion cracking in chloride ion environment. Some project drawings save trouble, and it takes a few minutes to blow compressed air. As a result, it will rot through the bottom of the bellows after half a year. Tsk, this money is not wrongful.
On-site welding rollover case review: the tie rod is not loose, the guide tube burns through... How many have you won?
In our business, who hasn't seen a few rollover scenes?
Case 1: The tie rod is not loose.Once the big tie rod expansion joint was installed, the welder screwed the tie rod fastening nut tightly and pressed it directly after welding. And guess what? The bellows can't expand and contract freely at all, and the stress generated by thermal expansion and contraction is all piled on the weld, and the weld joint is directly torn. Remember: The tie rod nut must be withdrawn to the limit position before welding to ensure that the expansion joint is not subject to additional constraints during welding. After welding, adjust the length of the tie rod according to the instructions.
Case 2: The guide tube burns through.The function of the guide tube is to protect the bellows from being washed by high-speed media, and the wall thickness itself is only 2~3mm. During on-site welding, the welder was quick, and the guide tube and the nozzle were welded together directly by high-current arc welding. As a result, several holes were burned out of the guide tube. What's worse is that after the guide tube burns through, the debris falls into the bellows, causing the bellows to wear and leak. The correct method is to fix the guide tube and the connecting tube with intermittent welding, or use argon arc welding and small current welding. After welding, check whether the guide tube is damaged.
Case 3: The welding electrode does not dry.A chemical plant installed a high-temperature expansion joint, and the welder took a pack of low-hydrogen welding electrodes from the warehouse, which was used directly without drying. Poor gas protection during welding and dense air pores appear in the weld. Suppress a try, and the leakage points are dense. Rework disassembled and re-welded, the construction period was delayed by one week, and Party A almost fined. Therefore, drying the welding electrode is not a formality, and the porosity can be increased five times when the humidity exceeds the standard.
How many of these three pits have you fallen into? You are lucky to fail, and you are not ashamed to fail-the key is to turn lessons into operating norms.
On-site welding of metal corrugated expansion joints is never a trivial matter of "just welding it". From groove preparation to post-weld inspection, every process is adding weight to the safety life of the pipeline system. Instead of waiting for something to go wrong and looking for the reason everywhere, it's better to get the details in place at the welding site. Is that the truth?