Wrinkling or cracking of the pipe wall is a common problem during the bending process of stainless steel hot water pipes, mainly due to insufficient material ductility, uneven internal stress distribution, or improper control of process parameters. These defects not only affect the appearance of the pipe fittings but also reduce their pressure-bearing capacity and service life. A comprehensive approach is needed, encompassing material selection, process optimization, equipment adjustment, and operational procedures, to achieve high-quality bending.
Material selection is fundamental to defect prevention. Stainless steel hot water pipes should be made of austenitic stainless steel with a high nickel content, such as 304 or 316L. Nickel significantly improves the material's toughness, reduces work hardening tendency, and makes it easier for the pipe to release stress through plastic deformation during bending, reducing the risk of cracking. If the carbon content of the material is too high, chromium carbide is easily formed at the grain boundaries, leading to decreased corrosion resistance in chromium-depleted areas and increased brittleness. Therefore, carbon content must be strictly controlled, and ultra-low carbon stainless steel should be prioritized.
The rationality of the bending process directly affects the quality of the pipe fittings. Cold bending is suitable for small-diameter, thin-walled pipes, which are formed by multiple roll forming on a bending machine. However, the deformation per pass must be controlled to avoid excessive stretching that would thin the pipe wall. For large-diameter, thick-walled pipes, hot bending is more suitable. Local heating softens the material and reduces its resistance to deformation. During heating, the temperature must be uniformly controlled to prevent localized overheating that could lead to oxidation or grain coarsening. During cooling, water spraying or immersion should be used for rapid cooling to refine the grains and improve strength. If die-free bending technology is used, the pipe blank is locally heated using a medium-frequency induction or flame heating coil, followed by bending and water cooling. This method eliminates the need for a die and offers flexible bending radii, but precise control of heating temperature and cooling rate is crucial to prevent cracking due to uneven thermal stress.
Matching the die and mandrel is key to preventing wrinkling. When using a mandrel, the mandrel diameter should be slightly smaller than the inner diameter of the pipe, the bilateral gap should be controlled within a reasonable range, and an appropriate mandrel extension should be set to ensure that the mandrel consistently supports the pipe wall during bending, preventing wrinkling of the inner material due to pressure instability. For coreless bending, the die can be designed with an anti-deformation groove structure to offset the distortion during bending through a pre-set deformation amount. Alternatively, a polyvinyl fluoride (PVC) film can be used as a lubricant, as its high tear strength and elongation can form a lubricating film between the pipe and the die, reducing friction and wrinkling tendency.
Lubrication and cooling measures effectively reduce deformation resistance. During bending, the friction between the stainless steel hot water pipe and the die generates a large amount of heat, accelerating material hardening and increasing the risk of cracking. Therefore, a special lubricant should be applied to the pipe surface, or a PVC film should be used to wrap it; its porous structure can absorb lubricating oil, forming a durable lubricating layer. For hot bending processes, during the cooling stage, it is necessary to ensure that the cooling medium evenly covers the pipe wall to avoid excessive local temperature differences that could lead to stress concentration.
Proper operational details are equally important. Before bending, the surface quality of the pipe should be checked, removing burrs, scratches, and other defects to prevent stress concentration from causing cracks. During bending, the pipe axis should be kept aligned with the die centerline to avoid eccentric bending that could lead to excessive thinning of one side of the pipe wall. For pipe fittings with multiple bends, the bending sequence must be arranged reasonably to avoid additional stress on the bent sections due to subsequent processing. After bending, the pipe fittings should be visually inspected and dimensionally measured to ensure there are no defects such as cracks or wrinkles. If necessary, penetrant testing or ultrasonic testing can be performed to detect micro-cracks.
Post-processing can further improve the performance of the pipe fittings. Residual stress can be eliminated and the material's toughness restored through solution treatment after bending. The specific method involves heating to a high temperature followed by rapid cooling to fully dissolve carbides and refine the grains. For pipe fittings with oxide scale, pickling and passivation treatment is required to remove the surface oxide layer and regenerate a dense passivation film, improving corrosion resistance. In addition, regular maintenance of the bending equipment to ensure that the die wear is within allowable limits is also an important measure to prevent defects.
