How to cut metal expansion joint? 5 Key Points and Precautions of On-site Practice
Two days ago, a customer called, and his tone was very urgent: they cut a double hinge transverse expansion joint by 20mm, and after installing it, the tie rod nut was directly suffocated to death, and the compensation amount was completely insufficient. Do you think this sucks or not? Metal expansion joints, especially general-purpose corrugated expansion joints, high-temperature axial joints, large-diameter thick-walled expansion joints, are calculated with good design, material and corrugation parameters. On-site cutting is not a job that can be done by an angle grinder. Today, let's talk about these five key points, so as to save you from stepping on pits later.
1. Find out which expansion joint you have in your hand before cutting
Different structures, cutting strategies are very different. You have to confirm the material first-is it stainless steel, carbon steel or alloy steel? Look at the wall thickness, corrugation shape (U-shaped, ω-shaped or S-shaped), and end connection mode (flange, welding or thread). For example, universal corrugated expansion joint, the corrugation is generally thin, so carefully avoid the root of the corrugation when cutting; The high-temperature axial expansion joint usually has a guide tube and a heat insulation layer. Before cutting, the outer sheath must be opened to see where the wave peak is; Large-diameter thick-walled expansion joints, the wall thickness is always more than ten millimeters, and the cutting tool is not selected correctly.
Measure the total length and compensation requirements, and leave 3~5mm margin when marking the cutting line on the pipe body. Why? Because there is no way to make up for it when you cut too much at once. You cut it short and either scrap it or add a short section, but that again involves weld distortion and stress concentration. We have aMetal expansion joint weight tableChecking can roughly estimate the size margin, but the most reliable thing is to find the manufacturer to confirm the minimum length requirement-especially the models with tie rods such as double hinge transverse type and straight pipe pressure balance type. The length of the tie rod determines the maximum compensation amount, and cutting it short is equal to wasting half of the function.
2. How to choose cutting tools? Don't burn bellows with oxyacetylene
This is the most common mistake. Some people tried to save trouble and directly cut the bellows with oxyacetylene flame. As a result, the stainless steel was partially overheated to above 450℃, resulting in the precipitation of chromium carbide and direct scrapping of intergranular corrosion. Tell me, did you do that before?
- Plasma cutting: Suitable for thick-walled carbon steel parts, such as straight pipe sections with large diameter thick-walled expansion joints, with fast cutting speed and small heat affected zone.
- Laser or waterjet: Most friendly to stainless steel bellows. Especially for the ultra-thin wall (0.5~1.5mm) universal corrugated expansion joint, the edge of laser cutting is smooth, and the waterjet can not only cut multi-layer composite structure without heat influence. However, the cost is high, and it is not realistic to operate in small batches.
- grinding wheel cutting blade: Small-size thin-walled parts (such as DN100 or below) are enough, but they will produce burrs. They must be deburred after cutting, otherwise the burrs will scratch the sealing surface after installing it.
- OxyacetyleneIt is only suitable for cutting straight carbon steel pipe sections, and leave at least 200mm distance from the corrugation. Don't touch the bellows, don't touch the deflector, don't touch the stainless steel.
3. How to cut the corrugated part? Avoid stress concentration zones
If you are cutting a straight section near the corrugation, be careful not to cut to the root of the corrugation-there is a stress concentration zone, and the risk of crack propagation is extremely high if you cut it. It is recommended to cut the outer sheath or insulation first, touch your finger to confirm the crest position (or sweep it with a flaw detector), and then cut the knife on your desired cutting line.
In addition, the expansion joint typically has a flow guide tube (a structure that prevents the medium from washing the corrugations) inside. You have to mark the position of this thing separately, because the guide tube is usually longer than the bellows. If you cut it directly according to the external dimensions, it is likely that the guide tube will be cut off. As a result, the medium directly washes the corrugated wall and wears out in three months. Before, a customer of a cement factory cut a metal corrugated expansion joint in the cement industry, didn't pay attention to the length of the outcrop of the guide tube, and changed it one year after installing it. Go to our website and searchSpecific Function of Expansion Joint Guide TubeYou can see the structure diagram.
4. Treatment after cutting: deburring, pickling and beveling are indispensable
Cut is not finished. First use a file or angle grinder to clean the burrs (the edge R angle is at least above R1.5), and then use a magnifying glass to check for any micro-cracks or ripples near the incision. Stainless steel incisions must be passivated by pickling-otherwise fresh metal will be exposed at the incisions, and there will be no passivation film protection, which will pitting in a few days in the chloride ion environment. The pickling solution can be a citric acid-nitric acid mixture, applied and waited for 15 minutes before rinsing.
In the case of expansion joints for high-temperature steam pipelines (such as corrugated expansion joints used in the power station industry), welding grooves are also involved after cutting. The groove angle (common V-shaped groove and U-shaped groove) of the original design drawing must be referred to, and the roughness of the groove surface should reach within Ra3.2, otherwise unfusion will easily occur during welding. Don't guess for yourself, on our websiteWhat category do steam pipes fall intoAs mentioned in that article, the welding requirements of high-temperature and high-pressure pipelines are particularly strict, and the groove angle is 2° different, so the post-welding heat treatment parameters will be completely changed.
5. Real Lessons: The Consequences of Cutting 20mm Shorter
I asked carefully about the case of the double hinge transverse expansion joint mentioned at the beginning. They found the wrong size in the warehouse and wanted to cut it shorter themselves. Results not checkedMetal expansion joint weight tableI didn't find the manufacturer to confirm the minimum length, but cut 20mm directly according to the installation space. After installation, the tie rod nut is pushed to the limit block, and the compensation amount is reduced by 15%. As soon as the thermal expansion of the pipeline comes up, the nut is directly deformed.
Therefore, directly contact the supplier to customize the length in non-emergency situations, and the risk of on-site cutting is really not low. If you have to cut on site, at least do: check the weight table, measure three times, leave a margin, choose the right tool, and handle the cut. If you are really unsure, take a photo and send it, and we will help you see the model number. It's better than cutting it away.
How to cut metal expansion joint? The answer is actually-don't cut if you can. If you need to cut, remember these 5 points above and avoid detours.