Stamping is not a panacea, but rectangular metal expansion joints can't do without it
When many people hear the expansion joint, what pops out of their minds is the circles of ripples on the round tube, thinking that the thermal compensation can be done by the deformation of the bellows. What about something with a rectangular cross section? When you talk to it about hydraulic bulging and mechanical bulging, you can only say that the idea is half right-the four corners of rectangular ripples are not the same at all.
The rectangular corrugation is directly pressed out on the flat plate or roll plate, and the four corners are not cracked, shrunk and wrinkled at the same time. Circular bellows can be supported outward by internal pressure, but rectangular ones can't-the material flow path at the corner is short and the resistance is large, so it will be thinned when it is pressed, and it will leak when it is thin. Therefore, the process of stamping seems to be coarse work, but in fact, it requires extremely high mold, pressure and sequence.
Stress concentration in rectangular corners is the number one enemy of stamping
Unlike the circle, at the corner R of the rectangular corrugation, the flow speed of the material in the stamping process is less than 1/3 of that of the straight edge section. The thinning rate is often above 20%, and the stamping parameters of stainless steel such as 304 and 316L vary greatly. 304 elongation is better, and the holding force can be reduced a little, but 316L has low carbon content and fast work hardening, so the holding force is not adjusted properly, and the R angle is directly cracked for you.
I met a customer two days ago, saying that the rectangular pieces they punched themselves leaked after several months of use. I asked him to take a picture of the fracture and at a glance-there was an obvious micro-crack at the R corner, and the typical excessive holding force caused a localized pull crack. So how to set the holding force? It is usually divided into two steps: first, the material is initially formed with low pressure (about 5-8MPa), and then gradually pressurized to 12-15MPa to complete the shaping. The die gap should also be 0.1-0.2mm larger than the plate thickness, leaving some space for the material to flow.
Not all is well after the stamping, there is still a bunch of work waiting
The stamped corrugated section of the rectangular expansion joint must be welded with the straight edge section and flange before it can be installed on the smoke and air duct-just like the metal rectangular expansion joint products of this station, the end is welded with flange, and some are equipped with a guide tube (the function of the guide tube is clearly explained in Q&A: protect the corrugation from high-speed medium erosion). The biggest problem during welding is deformation: the thin-walled corrugated section will shrink once welded, and the spacing of the whole corrugation will be wrong if you are not careful.
Corrective methods? After welding, stress relief heat treatment is done, and the temperature is controlled at about 600-650℃. The time depends on the thickness of the board, and it starts in 20 minutes. In terms of inspection, penetration detection (PT) is necessary. Brush imaging agent directly on the R angle and weld seam, and cracks will appear immediately. Don't save this step. We have seen too many examples of saving PT and repairing it half a year after leaving the factory.
Special working conditions: wear-resistant layer lining, external insulation layer
Metal rectangular expansion joints used in power stations or cement industries can't be carried by just a layer of metal. For example, the metal corrugated expansion joint in the cement industry is filled with high-temperature dusty flue gas, which can't be installed without a guide tube, and the back of the guide tube has to be lined with wear-resistant ceramics or tortoise shell net. When stamping, it is necessary to leave enough margin for the subsequent process-the plate thickness should be increased by 0.5-1mm, because the heat-affected zone will partially soften the corrugation when welding the guide tube; If the external insulation layer is installed, the corrugation depth should be shallower when stamping, otherwise the insulation tampon will be easy to flatten if it doesn't go in.
And guess what? Some customers find that the ripples are obviously shorter after welding the guide tube after flushing themselves, and the compensation amount is not enough-this is to leave no margin. Therefore, when making the process plan, we must confirm the follow-up process synchronously, one step is wrong, one step is wrong.
The life and cost of stamping dies: the gap between small factories and large factories
Rectangular expansion joints have a wide range of sizes-from a few hundred millimeters to two or three meters, so the mold can't be universal. Small factory drawings are cheap, and simple molds are used to knock by hand, which is not only low in efficiency, but also poor in consistency. Make 50 pieces in a batch, and the corrugation depth of the first and the last can be more than 1mm different. However, if the batch size is larger (for example, hundreds of pieces at a time), it is necessary to use a hydraulic press with a special mold. When the large-diameter thick-walled expansion joint (wall thickness more than 6mm) of this station is stamped, the die tonnage is directly up to 500 tons, and the die core should be plated with hard chromium, so that the service life can exceed 5000 times.
Rectangular stamping pays more attention to the thinning of R angle, while thick walls pay more attention to the rebound compensation-once the thick plate is released after pressing, the corrugation angle can bounce back by 2-3 degrees, so the compensation value should be preset for the mold angle. There is a big difference in process, but the core logic is the same: the mold input is exchanged for quality, which can't be saved.
Stamping is only the first step. Material selection and structural design are the key to determining life
The stamping parameters of rectangular metal expansion joints should be determined according to the actual operating temperature, pressure and compensation amount. For example, the circular high-temperature axial expansion joint can be estimated by empirical formula, while the rectangle has to be analyzed by finite element-the stress distribution at the corner is too complicated, and no one dares to pat their chest and say that it can be used for several years without ANSYS or Abaqus.
In addition, don't forget, the metal rectangular expansion joint of this station is also divided into single layer and multi-layer corrugation. Single-layer stamping is formed at one time, and multiple layers (such as two layers and three layers) need to be stamped in separate lanes. The drawing depth of each layer decreases, and the intermediate layer has to be lubricated, otherwise the adhesion between layers will wrinkle once it is pressed. The number of layers is different, so is the number of stamping passes-at least three layers of corrugations are required, and one is annealed between each pass. You think it's done when you're done stamping? The material selection is not good, the structural design is not in place, and it is useless to rush no matter how good it is.
At the end of the day, stamping is the means, not the end. It is the joint force of material selection, structure and technology that really makes the expansion joint last for ten years in smoke and air ducts and power station desulfurization systems.