Why is stamping process the core link of rectangular metal expansion joint?
If you look at the smoke and air ducts of power plants and cement plants, every place that needs to absorb large displacements and withstand high-temperature smoke is likely to be equipped withMetal rectangular expansion joint。 This thing is not like a circular expansion joint, which can be directly formed by coiling. The rectangular cross section has corners and straight edges, and the structural stress distribution is much more complicated. There is one reason why stamping technology has become the core, to put it bluntly-it can use the die to press the flat sheet out of the corrugation at one time, so as to ensure the geometric accuracy and mechanical consistency of each wave. Without stamping, the fatigue life of the ripples cobbled together by welding alone is directly cut in half, and the risk of air leakage is doubled.
Two days ago, I met a customer, who insisted that spinning could be used instead of stamping to make rectangular parts. As a result, I tried three times, and all four corners were cracked. Why? The material flow direction and stress state in the fillet area of rectangular corrugation are completely different from those of circular shape. Stamping is to force the metal to deform according to a preset path through die constraint, which can't be replaced by other processes.
Key points of material selection and design that must be known before stamping: the game of material, wave height and wall thickness
When it comes to material selection, many engineers stare at stainless steel grades when they come up, and 304 and 316L come with their mouths open. But the rectangular metal expansion joint stamping is more criticalElongation and yield ratio of materials。 Like doingHigh temperature axial expansion jointUsed Incoloy 825, if the elongation is less than 30%, the punching depth is slightly larger than the wave height, and the rounded corners will directly crack. I have seen a case: the wave height was changed from 80mm to 100mm, and the material was not changed. As a result, the scrap rate soared from 2% to 18%. Therefore, the design stage has to be calculated clearly: for every 10mm increase in wave height, the wall thickness has to be increased by 0.3mm to hedge the thinning amount? Or switch to high elongation materials?
The wall thickness game is more headache. When the stamping is thin, it is easy to wrinkle, when the stamping is thick, the life of the die will decrease and the stamping pressure will soar. UsuallyLarge diameter thick wall expansion joint(For example, the wall thickness is more than 6mm) stamping needs heating assistance, and cold stamping can't be pressed at all. We have measured that when the cold punching wave height of 3mm 304 web is 120mm, the fillet thinning rate has reached 22%, which exceeds the limit value. The design specification generally requires that the thinning rate should not exceed 15%. If it exceeds, it must be made up in the process: either step-by-step stamping or changing the material.
Three key steps of stamping: blanking, molding and springback control
The first step is blanking, don't underestimate it. The unfolding material of rectangular expansion joint cannot be directly calculated according to the projected area, so it should be consideredtensile coefficient。 Part of the remaining material needs to be cut off at the corner to avoid wrinkles, and too much cutting will lead to misalignment of the weld. I have seen the old master use the empirical formula: width of unfolding material = (length from peak to trough ×1.03) + straight edge segment. However, as soon as the third-party test report came out, people used finite element simulation to calculate that the correction difference was 0.5mm, and the final product corrugation depth was more than 1mm different.
The second step of molding, the core isDie clearance and holding force。 The gap at the four fillets of the rectangular mold is 0.2~0.4mm larger than that at the straight edge, because the material thickness at the corners will be slightly thickened. The holding force is too large, and the material cannot flow; Too small and wrinkled. In practice, we giveMetal Corrugated Expansion Joints in Cement IndustryWhen making the mold, the flange holder at the fillet corner is controlled by nitrogen spring alone, distinguished from the straight edge, and the effect is immediate-the wrinkling rate is reduced from 12% to 1.5%.
The third step is rebound control, which is the most headache. After the rectangular corrugation is stamped, the rebound direction of the four fillets and the two long sides is inconsistent, resulting in the corrugation being out of round. The solution isCompensating die surface: According to the material thickness and wave height, the rebound angle is calculated backwards, and the mold profile is made into a "bending" shape. For example, when the thickness of 304 material is 3mm and the wave height is 80mm, the rebound compensation angle is about 2.5 ° to 3 °. No compensation? After installing the pipe, the flange surface is warped, and the gasket can't hold it down. It is a matter of time before air leaks.
Quality inspection after stamping: Bellows stiffness, fatigue life, and guide tube fit
Stamped bellows, first thing to teststiffness。 Check our stationStiffness and Calculation Formula of BellowsThat set of formulas is essentially a simplified model, but the stiffness dispersion of actual stamped parts can reach ±8%. Therefore, during our inspection, we require 3 pieces per batch to be sampled. If the elastic stiffness deviation exceeds ±5%, it will be judged as unqualified. Why? Because the stiffness directly affects the reaction force of the expansion joint on the pipeline, the pipeline support can't stand it if it is large, and the compensation amount is not enough if it is small.
Fatigue life testing is a burning job, but it must be done. Especially forCorrugated expansion joint for power station industryFor rectangular products, the design life usually requires more than 1000 cycles. We did according to EJMA standards and found that the fatigue crack of the stamping corrugation always appears first on the inside of the fillet-where the residual tensile stress is greatest. Solution? Make it after stampingStabilizing heat treatmentOr shot peening strengthening, which can increase the life by more than 30%.
It is easy to neglect the cooperation of the guide tube with this piece. The guide tube of the rectangular expansion joint is usually also a stamped part, and it should be left between it and the bellowsThermal expansion gap。 Some of our customers found abnormal noise after installation, and when they disassembled it, they saw that the guide tube was clamped to death by the bellows, because the fillet radius of the guide tube did not match the bellows during stamping.Specific Function of Expansion Joint Guide TubeIt is to guide the flow of the medium and protect the bellows. If the fitting gap is wrong, the guide tube becomes the source of wear.
What is the difference between stamping for rectangular and circular expansion joints?
Many newcomers think that a rectangle is a circle squashed, which is completely wrong. The stamping die of the circular expansion joint is axisymmetric, the material flows evenly, and the rebound is also uniform. What about rectangles? The material is subjected to biaxial tensile stress at the four fillets and unidirectional tensile stress at the straight edges. This stress imbalance leads toThe distribution of wall thickness is very different。 Actual data: With the same wave height of 100mm, the maximum thinning rate of circular pieces is about 12% at the bottom of the wave peak, and the maximum thinning rate of rectangular pieces can reach 18% ~20% in the fillet area.
Therefore, the die design of rectangular metal expansion joint stamping must compensate for this difference. We giveDesulfurization flue gas baffle doorWith the rectangular expansion joint, the radius of the die fillet at the fillet is 3mm larger than the theory, and the straight edge is 1mm smaller, so that the thinning rate can be pulled back to less than 14%. Moreover, the rebound of rectangular parts is more difficult to predict- -attention should be paid in finite element simulationAnisotropy coefficientThe R value of cold rolled sheet and stainless steel sheet can be twice different.
How to avoid stamping defects? Common pits and solutions in actual combat
wrinkling。 Especially the four corners of the rectangle, if the holding force is not enough, it will wrinkle. Solution: Pressing the edges in different regions, pressing the rounded corner area separately with high-strength springs, or addingdraw bar。 We have tried to carve a circle of draw rib grooves on the mold, the depth is 0.5mm, and the wrinkles are completely eliminated.
cracking。 The wave height design is too large or the material elongation is insufficient. The solution is simple and rude: change the material, or stamp in two steps-first pre-punch 40% depth, then fine punch the remaining 60%. Moreover, an annealing is added in the middle to eliminate work hardening.
Dimensional instability。 The rebound amount of pieces punched out of the same mold is different in each batch. Why? The yield strength fluctuates according to the material batch. We require suppliers to provide mechanical reports per batch and punching parameters to fine-tune holders based on measured yield strength. And guess what? The scrap rate dropped from 8% to below 2%.
Die wear。 Rectangular dies wear 5 times faster at fillets than straight edges. Solution: Use cemented carbide for fillet inserts and Cr12MoV for straight edges, which only increases the cost by 15% and extends the mold life by 3 times.
Finally, the optimization of stamping process is always a dynamic process. Don't expect a set of molds to dominate the world. Every time you change a batch of materials and every time you change the wave height, you have to re-check the mold clearance and holding force. doMetal rectangular expansion jointIn this line, the harder the details are picked, the longer the product will live.