Cutting rectangular metal expansion joints looks simple, but in fact, the rollover rate is not low. Two days ago, I met a customer, and the cutting size was 2mm off. When it was installed, it leaked, and the whole pipeline system suffered. Don't underestimate these few millimeters. Expansion joints, especially rectangular metal expansion joints used in power stations and cement plants, often have a temperature of hundreds of degrees and a pressure of several thousand Pascals. The cutting quality is not up to standard, and the rest is full of trouble.
Preparation before cutting: drawings, materials and allowances should not be careless
Understand the drawings first. Rectangular metal expansion joint is not a standard square tube cut with a knife. Its interface form, groove angle and weld position are clearly marked on the drawings. When you get the drawings, check three things first: one is the overall dimensions, the other is the thickness of the tube, and the third is the material grade. The cutting parameters of 304 and 316L are different, let alone superalloys.
Let's talk about margin. Machining should leave an allowance, and cutting is the same. Plasma cutting generally leaves a trimming margin of 3~5mm, while laser cutting can leave a small point, and 1~2mm is enough. But don't think that everything will be fine if you leave a margin, and you have to consider thermal deformation. When the sheet is cut, the heat comes up, and the board is warped, leaving less allowance and not even having the chance to repair it.
The material confirmed this piece, and many people stumbled. The appearance of the 316L and the 321 is not much different, but the spark, speed and gas ratio are all different when cutting. If the wrong parameters are used, the cutting surface will not only be rough, but also micro-cracks may occur. Take the spectrometer and hit it before cutting. It takes tens of seconds, don't save it.
Common Methods of Rectangular Expansion Joint Cutting: Plasma, Laser or Waterjet?
There are three mainstream ways, each with its own applicable scenarios.
Plasma cutting is the most commonly used method for rectangular metal expansion joints, with low cost and fast speed, and can handle stainless steel plates with a thickness of 2~12mm. However, the section cut out by plasma has large roughness and wide heat-affected zone, so it must be polished or machined and trimmed after cutting.
Laser cutting has high accuracy, smooth cross section and narrow heat affected zone. However, the equipment investment is large, and the cutting speed of boards exceeding 8mm will come down, and the cost will be higher than that of plasma. With thin-walled rectangular expansion joints and high requirements, laser is the first choice.
Waterjet cutting has no heat-affected zone, and its accuracy is high, but the speed is the slowest. Moreover, the section cut by waterjet is prone to water stains, and the stainless steel material has to be treated with rust prevention. Waterjets are not used much on rectangular expansion joint cutting unless the customer has a hard requirement for the heat affected zone.
To be honest, most factories are equipped with plasma, and with a good cutting nozzle, proper current and gas flow, the cut quality is completely enough. The key depends on whether you can adjust the parameters.
How to control the cutting heat affected zone? Deformations and scales are the worst enemies
Heat-affected zones are the vital gates of cutting quality. When the heat-affected zone is too wide, the grains will be coarsened, and the toughness of the material will decrease. When the expansion joint opens and closes, the stress will concentrate in the heat-affected zone, and the crack will grow from here.
To control the heat affected zone, the first is speed. The faster the cutting speed, the smaller the heat input. But too fast will cause hanging slag on the back of the incision. This degree has to be handled well. Generally, listen to the sound when plasma cutting. The sound is crisp and the spark is even, which indicates that the speed is appropriate. The second is gas. Using nitrogen as plasma gas, the cutting surface is less oxidized, the color is white, and the follow-up treatment is troublesome. Cut with air, fast but black cut, thick oxide skin.
When it comes to deformation, the most prone problem of rectangular expansion joint cutting is the flexural deformation of the long side. The longer the board, the more heat builds up. There are two ways: First, in the cutting order, cut the short side first and then cut the long side, so that the heat has room to dissipate; Second, cut in sections. Don't cut the long side from beginning to end with a knife. Stop at intervals to let the board slow down. When the amount of deformation is controlled, the rear group pair welding will not be mistaken.
Groove treatment and dimensional inspection: Don't wait for installation to find problems
The bevel is a prerequisite for welding quality. Rectangular metal expansion joints usually need to be welded through, the groove angle is generally opened by 30~35 °, and the blunt edge is 1~2mm. The groove, heat-affected zone and oxide scale cut by plasma must be polished clean to expose the metallic luster, otherwise the pores and slag inclusions will all come during welding.
Don't wait until the last for the dimensional inspection. After cutting, while the board is still on the floor, quickly pull the diagonal, measure the side length and check the flatness. The interface dimension tolerance of rectangular expansion joint is generally controlled within ±3mm, and the diagonal difference is not more than 5mm. What if it exceeds? While the margin is still being repaired quickly, and when it is transferred to the next process, it will be found again, and the rework cost will be high.
Flatness is an indicator that many people ignore. If the flange surface of the rectangular expansion joint is uneven, the tightening force of the bolts will be different after installation, and the local stress will be concentrated, so the corrugated part will easily become unstable. Therefore, the cut board must be laid flat, not leaning against the wall, and your own weight can bend it.
What if I make a cut mistake? Judgment Criteria for Repair Welding, Correction and Scrap
People are not sages, and cutting mistakes are inevitable. The key is to have clear criteria for judgment.
Cut small, cut off, if the notch is not large, you can repair welding. Before repair welding, polish the defects clean, fill them with argon arc welding, and then re-cut or grind them. However, it should be noted that the repair welding area is the superposition of the heat affected zone. If the deformation can't be controlled, this position is a hidden danger. Therefore, repair welding is limited to non-stressed parts, and cannot be repaired near corrugated areas and welds.
Correction is another way to go. When the sheet is deformed, it can be corrected by flame, and then quickly cooled after local heating to pull the deformation back. However, flame correction has requirements for materials. The temperature of martensitic stainless steel and austenitic stainless steel is strictly controlled, and carbides may precipitate when heated over 400℃, and the corrosion resistance will decrease. So check the material before correction, and give up if you are not sure.
What circumstances are directly scrapped? Three standards: First, cracks, any cracks visible to the naked eye can't be repaired, and they are directly wasted; Second, the dimension is out of tolerance and cannot meet the tolerance requirements through grinding; Third, the material burns out, and the surface oxidation and decarburization caused by cutting overburn can't be reversed.
Alas, after all, rectangular metal expansion joint cutting is a delicate job. For people who want to save trouble, the plasma pulls a knife casually, and the back is all paying off debts to the sloppy in front. In a factory that does well, every cutting process is recorded, and the parameters, operators and inspection results can be traced. This thing is not just a matter of craftsmanship, it's a matter of management.
The quality of expansion joints depends half on design and half on manufacture. Cutting as the first gateway of manufacturing, is worth using more snacks.