Metal Expansion Joint Forming: Process Details and Selection Points from Steel Plate to Bellows

Metal Expansion Joint Forming: Process Details and Selection Points from Steel Plate to Bellows

Metal expansion joint forming, to put it bluntly, is to press stainless steel plate or alloy plate into bellows by hydraulic or mechanical means. This craft looks simple, but there are many doors in it. Two days ago, I met a customer and asked me why the expansion joint, which is also made of 304 material, cracked after less than two years of use at home, while the next factory was still fine after five years of use? The problem lies in the forming process – is it hydroforming or roll forming? What is the number of molding times? Did you do a solution treatment? These details directly determine product longevity.

Hydroforming: mainstream process, but don't be fooled by "one-time forming"

Let's talk about hydroforming first. This is the current mainstream process and is especially suitable forUniversal corrugated expansion jointAndHigh temperature axial expansion jointSuch products. The principle is not complicated: the tube blank is placed in the mold, the inside is pressurized, and the outside is limited by the mold, and it is formed once or multiple times. But guess what? In order to save costs, many small factories form in place at one time. As a result, the stress at the bottom of the corrugation is concentrated, and it won't take long for fatigue cracking. The formal practice is step-by-step molding, with a solution annealing in the middle to release the intergranular stress. For example, we doCorrugated expansion joint for power station industryThe requirements are relatively high, usually divided into 3 to 4 times of molding, and the wall thickness thinning rate is detected after each molding, which is controlled within 15%, so as to ensure the pressure resistance and fatigue life.

Mechanical Roll Forming: A Flexible Choice for Large Diameter and Thick Wall

Let's talk about mechanical roll forming. This process is suitable forLarge diameter thick wall expansion jointOr occasions where special waveforms are required. Such asDouble hinge expansion joint for air-cooled island vacuum pipelineThe diameter can reach more than two meters, the cost of hydraulic mold is too high, and rolling is more flexible. There is a common problem with roll forming: the corrugation pitch is uneven. What to do? Adjust the feed speed and roller pressure, and ensure that the wall thickness tolerance of the tube blank is within ±0.3mm, otherwise the corrugation will be deep on one side and shallow on the other. In addition, the bellows after rolling often need stress relief annealing, especially austenitic stainless steel, otherwise the risk of intergranular corrosion is high.

Molding is only the first step, and subsequent inspection is the watershed

Molding is only the first step, and subsequent assembly and inspection are equally critical. If you think about it, if the bellows is qualified for molding, but it burns through during welding or the welding slag splashes onto the corrugations, the front is all dry in vain. Therefore, many manufacturers have to do air pressure test or water pressure test after welding. LikeCompound hinge transverse expansion jointIf this structure is complicated, stiffness test should be done to ensure that the compensation amount reaches the standard. In addition, the installation direction of the guide tube can't be reversed-the direction of the arrow of the expansion joint refers to the flow direction of the medium, and the guide tube can't play a protective role when installed backwards, and vortex erosion is easy to occur. This has also been mentioned in the FAQ, but it is not uncommon to install the wrong thing on the spot.

How to quickly judge the molding process when selecting?

How to judge whether the molding process is good or not when selecting? An easy way: Look at the fillet radius and wall thickness uniformity of the corrugation. Touch it with your hand. If the corrugated root feels particularly thin or has sharp edges and corners, there is probably something wrong with the molding process. Experts will also ask manufacturers: Has the finished product been tested for fatigue? At least 3000 cycles are required by EJMA standards, but many factories only do 1000 cycles. ForMetal Corrugated Expansion Joints in Cement IndustryIt is recommended to require a fatigue life of more than 5000 times for such harsh working conditions. And,Direct buried (fully buried) type expansion jointBecause it is buried underground, the outer layer anti-corrosion and pickling passivation treatment after molding are also very critical. If the tensile mark during molding is too deep, it is easy to remain after pickling, and the later corrosion starts from there.

Installation and maintenance: Don't let good craftsmanship be ruined in the last step

Finally, the molding process determines the intrinsic performance of the expansion joint, but the installation and use maintenance are equally important. Many customers ask how to adjust the tie rod nut of the expansion joint. In fact, the tie rod only plays a protective role during transportation and installation, and it must be loosened after installation, otherwise the expansion joint can't move freely. Do you still need to remove the screw of the expansion joint? This score situation-if it is a non-metal expansion joint or a fabric fiber expansion joint, the screw is fixed and cannot be disassembled; However, the limit screw on the metal bellows should be loosened after debugging. These details, like the molding process, are fundamental to ensuring the safe operation of the system. Only by choosing the right process and using the right method can the expansion joint play its role.