Bending forming of pipes in pressure vessels

Thread Content

Bending methods There are various ways to bend pipes, generally divided into manual bending and mechanical bending. There are also various methods for mechanical tube bending, such as press bending, roll bending, bend-back bending, and extrusion bending. Regardless of the bending method used, the main challenge throughout the bending process is how to overcome local deformation. The most widely used processes in engineering are manual bending and loop bending. The bending method involves bending pipes on a rotary pipe bender, and it can be further divided into die-based bending and die-free bending. Manual pipe bending does not require specialized equipment or complex processing tools; it can bend pipes with various radii, angles, and spatial orientations. However, this bending method requires high labor intensity, has low productivity, and results in unstable quality. Hot bending is commonly used for manually bending steel pipes, while cold bending is more suitable for stainless steel and non-ferrous metals. Before bending the pipe, filler is first inserted into its interior; the filler for steel pipes is usually pure, dry fine sand. For stainless steel and non-ferrous metal pipes, low-melting-point substances such as rosin and lead are suitable as fillers, in order to prevent wrinkles and reduce ellipticity. There are die-bending machines; all types of such bending machines use dies for bending. The main molds are of two types: disc-shaped sheaves and horn-shaped core heads. When bending pipes using a disc-shaped cam, the bending die is located on the outside of the pipe; half of the pipe lies within the groove, while the other half of the pipe in the bending area is pressed by a grooved small roller (also known as a compression roller). The end of the pipe is fixed to a disc-shaped bending die using a chuck; if the pressing roller does not move, the disc-shaped bending die rotates on its own to complete the bending of the pipe, which is known as pull-bending ； If the tube is pushed, causing the disc-shaped bending die to rotate accordingly in order to bend the tube, this is called push-bending ； If the disc-shaped bending die remains stationary and the compression roller presses the tube to rotate around it in order to achieve bending, this method is known as press-bending. When bending tubes using a ram’s horn-shaped mandrel, the bending die is located on the inside of the tube; the ram’s horn-shaped mandrel resembles a ram’s horn, with an axis length equal to 1/4 of the circumference, and its bending radius is the same as that of the tube being bent. The maximum bending angle is 180°. The thin end of the horn core is slightly smaller in diameter than the inner diameter of the tube blank; the tube blank is inserted from the thin end, while the thick end serves as the outlet and is slightly larger in diameter than the inner diameter of the tube blank. When bending pipes, the pipe billet is first heated and then inserted into the mold; under thrust, the pipe undergoes both bending and expansion processes, and it takes on its bent shape once it slides out from the end of the mandrel. Dieless bending: This bending method does not require the use of a bending die designed for a specific bending radius. It can be divided into pull-bending and push-bending types. The pipe bender has a rotating arm whose length can be adjusted. During bending, the pipe is fixed to this arm, and the distance from the pipe’s central axis to the rotation center of the arm constitutes the bending radius. During processing, a copper induction coil is wrapped around the tube; an alternating current of intermediate frequency (used for thicker tubes) or high frequency (used for thinner tubes) is passed through this coil to locally heat the tube to 900–950°C, after which the tube is bent. If the rotating arm is used to actively bend the tube back into shape, it is called stretch bending ； When the end of the tube is subjected to a thrust force, the tube pushes the rotating arm, thereby causing the tube to bend – this process is known as push-bending. Immediately thereafter, water is sprayed from a ring of small holes on the inner side of the induction coil onto the already bent pipe section to cool it down rapidly. The purpose of spraying water is to keep the heating section and the deformation zone within a very small area, thereby preventing the tube from wrinkling or being flattened.