1 Overview

    When stainless steel pipes need to change their direction or use point path design requirements during the layout of the project, they need to be connected with stainless steel elbows, the angles of which are 45°, 90°, etc. The elbow forming production can adopt different methods: orbiting stainless steel elbow forming, push-bending forming and press-bending forming, etc.

 2. Production process of stainless steel elbow

     The production process of stainless steel elbow is as follows:

     Incoming tube blank: inspection, forming, cleaning, shaping, chamfering, lettering, polishing, cleaning, drying, deburring, finished product inspection, packaging and warehousing

 3. Orbiting elbow forming

 (1) A core elbow

    The cored bend tube is a forming method in which a mandrel is used on a tube bending machine to make the tube material orbit and bend along the bending tire mold. The working principle of the cored elbow is shown in Figure 5-1. The bending tire mold 4 is fixed on the main shaft of the pipe bending machine and rotates with the main shaft. One end of the stainless steel tube blank 6 is pressed on the bending tire mold by the clamping block 3 . In the vicinity of the tangent point between the stainless steel tube blank and the bending tire mold, a pressure block 1 is installed on the outer side of the bending, an anti-wrinkle block 5 is installed on the inner side of the bending, and a mandrel 2 is inserted inside the tube blank. When the bending tire mold rotates, the stainless steel tube is gradually bent and formed around the bending tire mold. The bending angle of the stainless steel elbow is controlled by a stopper (not shown in the figure). When the bending tire mold turns to the required bending angle of the stainless steel pipe, it will hit the stopper and stop the bending tire mold from rotating. The design and manufacture of the main parts of the tool and die are described below.

A Mandrel

     The mandrel is an important part of the cored pipe bending device. Its function is to support the pipe wall from the inside of the stainless steel tube blank and prevent the cross-section deformation and wrinkle of the stainless steel pipe. The structure of common round mandrel and multi-ball mandrel is shown in Figure 5-2.

     The round-head mandrel is simple in shape and convenient to manufacture. However, due to the small contact area between the core rod and the pipe wall, the effect of preventing cross-sectional deformation is poor, and it is usually used for bending of stainless steel pipe fittings that are not demanding. The core rod is usually made of 3Cr2W8V material, and the heat treatment hardness is 52-56HRC.

     The flexible shaft type multi-ball mandrel is formed by connecting multiple bowl-shaped spheres in series with a flexible shaft, which can swing in any direction in space, and is suitable for single, multiple and spatially curved stainless steel pipe fittings of thin-walled tubes.

    The multi-ball mandrel elbow is used, and the pipe can be rounded when the mandrel is drawn out by the cylinder after bending. The main disadvantage of the multi-ball mandrel is that it is troublesome to manufacture. The material and heat treatment requirements of the mandrel are the same as the single-ball mandrel.

    In the elbow with mandrel, the shape, size and working position of the mandrel have a greater impact on the quality of the stainless steel elbow. The size of the round head mandrel and the working position into the tube are shown in Figure 5-3.

   In order to insert the mandrel into the stainless steel tube smoothly, the diameter of the mandrel d (mm)-generally should be about 0.5 -1.5mm smaller than the inner diameter of the tube, that is

    d=D1-(0.5 -1.5)

    The length L of the cylinder of the mandrel is:

    L= (3-5)d

    When the diameter d of the mandrel is large, the coefficient takes a small value, and vice versa.

    The working position of the mandrel should be a certain distance e ahead of the bending centerline. Generally speaking, the size of e should be determined according to the diameter of the stainless steel tube blank, the bending radius and the size of the gap between the inner diameter of the tube blank and the mandrel, and then adjust appropriately according to the actual production situation. The e value can be calculated as follows:

     e=√2（R+D1/2）z一z2 (5-1)

   Where D1——inner diameter of tube blank, nim;

    R——The bending radius of the center layer, mm;

    z——The gap between the inner wall of the stainless steel tube blank and the mandrel, mm, z=D1-d0

    There is also a ball head bat commonly used on site, its use and manufacturing difficulty are just between the above two core rods. The parts of the ball mandrel are shown in Figure 5-4.

B 防crease block

    The stainless steel tube billet material that is close to the tangent point and does not enter the bending deformation zone, its appearance is not supported by the curved tire model groove, even if the mandrel is used, the stainless steel tube may still wrinkle here when the tube is bent, so it should be installed as The anti-wrinkle block shown in Figure 5-5. The front end of the anti-wrinkle block is in the shape of a circular arc edge, inserted between the bending tire mold and the tube blank, and the front end should be close to the tangent point between the tube wall and the bending tire mold, effectively filling the bending of the bending tire mold and the stainless steel tube blank. The gap between the inner sides plays a role in supporting the tube wall from the outside and preventing wrinkles.

    Since the shallow circular groove on the anti-wrinkle block is in contact with the outer surface of the tube blank, sliding friction occurs during work, so the working surface of the anti-wrinkle block should have a certain degree of hardness and low surface roughness. The anti-wrinkle block is generally made of tool steel or chromium molybdenum steel, and the hardness after quenching reaches 50-55HRC.

C Bending tire mold

    The bending tire mold is another important part of the core bending device, and its radius depends on the bending radius of the stainless steel bend. However, considering that the bend pipe will produce a certain amount of springback during cold bending, the radius of the designed bending tire mold should be slightly smaller than the required bending radius of the stainless steel bend pipe. Generally, it can be determined according to the following empirical data, namely

    When R/D =3 -4: R1=(0.96 -0.98)R

    In the formula, R1——the radius of the bending tire mold, mm;

    R——the bending radius of the central layer of the elbow, mm;

    D——Outer diameter of tube blank, mm.

    When the R/D is larger, the smaller value is used; when the R/D is smaller, the larger value is used, and finally the correction depends on the trial mode.

    In addition to the wheel type, the bending tire mold also has a mold column type. The parts of the mold column are shown in Figure 5-6.

  Coreless elbows are basically not used in the production of pipe joints.

(2) Push bend

    Push bending is a more common method of bending pipes in the bending of pipe materials, and is mainly used for bending elbows. According to the characteristics of the push-bending process, it can be divided into two types: cold push-bending and mandrel hot-pushing.

    Cold push tube bending is a process of pushing and bending the tube blank with the aid of a tube bending device on an ordinary hydraulic press or a crank press. The stainless steel straight tube blank is pressed into a mold with a bending cavity at room temperature to form stainless steel. elbow.

    The cold push pipe bending device is shown in Figure 5-8, which is mainly composed of a pressure column 1, a guide sleeve 2 and a bending die 4. The bending mold is made up of two centered pieces to facilitate its cavity processing. When the tube is bent, the blank tube 3 is placed in the guide sleeve for positioning, and the pressing column goes down to apply an axial thrust to the tube blank port to force the tube blank into the bending cavity, thereby causing bending deformation.

    Cold push elbows are suitable for bending stainless steel elbows with smaller bending radius. The minimum bending radius that can be bent is R/D≈1.3; the elbow section has a small ellipticity (≤3%-5%); the outer tube wall The amount of thinning is small (≤9%); the pipe bending device has a simple structure, does not require special equipment, and has high productivity. However, it is generally required that the relative thickness of the tube is t/D≥0.06, otherwise, the tube blank will often lose its stability due to poor rigidity, causing the inner side of the elbow to wrinkle or twist.

    The stainless steel pipe fitting factory should also pay attention to the following points for the cold push bending process:

    (a) In order to reduce the frictional resistance, extend the service life of the bending die, and improve the surface quality of the stainless steel elbow, the tube blank must be lubricated. Practice has proved that painting the surface of stainless steel tube billet with No. 40-50 engine oil and then applying a layer of graphite powder can ensure good lubrication during the bending process.

    (b) During the bending process, the end of the tube blank easily collapses under the action of axial thrust. For this reason, a core as shown in Figure 5.9 can be placed in the tube blank, which will accompany the tube elbow during the bending process. Together they were pushed out by the pressure column.

    (c) For stainless steel thin-walled elbows, in order to prevent instability and wrinkles during the push-bending process, fillers should be filled in the tube blank, which is not only convenient for operation, but also has a good anti-wrinkle effect.

(3) Press bending

    Using molds to press stainless steel pipe fittings with straight sections has the advantages of high production efficiency and easy mold adjustment. In order to prevent the pipe wall from wrinkling and flattening, the stainless steel pipe fittings factory needs to fill in fillers or insert cores from both ends of the thin-walled pipe with an outer diameter of 10mm or more before bending. The die bending method also has some shortcomings. For example, at the beginning of the contact between the tube blank and the concave and convex molds, there will always be some distortions in the tube section, which makes the quality of the bent tube unsatisfactory. In addition, the bending radius, angle and bending shape are also subject to certain restrictions, so there are fewer applications in production. Figure 5-10 is the V-shaped pipe bending die, and Figure 5-l1 is the U-shaped pipe bending die. The working surface of the convex mold matches the shape of the tube blank and is placed in the middle of the left and right swinging concave mold.

    Pressed stainless steel elbow is a process method for bending tube blanks on a hydraulic press. It is divided into two types: hot pressing and cold pressing. It can press carbon steel elbows, alloy steel elbows, stainless steel elbows and copper. , Aluminum elbow, etc. The diameter of the pressed elbow is 25-406mm, and the wall thickness is 2.5-40mm. The bending radius is R≥D, generally R=1.5D.