Production technology of stainless steel elbow

Production process of stainless steel elbow

1 overview

when the direction of stainless steel pipeline needs to be changed or the design requirements of service point path need to be changed during the layout of the project, stainless steel elbow connection is required, with angles of 45 ° and 90 °. Different methods can be adopted for elbow forming production: winding stainless steel elbow forming, push bending forming and press bending forming

2 production process flow of stainless steel elbow

production process flow of stainless steel elbow is as follows:

incoming pipe blank - Inspection - forming - Cleaning - adjustment - chamfering - Lettering - polishing - Cleaning - drying - deburring - finished product inspection - packaging and warehousing

3 winding elbow forming

(1) Cored bend

cored bend is a forming method in which the pipe material is bent around the bending die by using the mandrel on the pipe bender. The working principle of the cored elbow is shown in Figure 5-1. The bending die 4 is fixed on the main shaft of the pipe bender and rotates with the main shaft. One end of the stainless steel tube blank 6 is pressed on the bending die by the clamping block 3. Near the tangent point between the stainless steel tube blank and the bending die, a pressing block 1 is installed on the outside of the bending, an anti wrinkle block 5 is installed on the inside of the bending, and a mandrel 2 is inserted inside the tube blank. When the bending die rotates, the stainless steel tube blank is gradually bent around the bending die. The bending angle of the stainless steel elbow is controlled by the stop (not shown in the figure). When the bending die turns to the bending angle required by the stainless steel pipe fitting, it will touch the stop to stop the bending die from rotating. The design and manufacture of the main parts of the tool and die are described below

a mandrel

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

round head mandrel is simple in shape and easy to manufacture. However, due to the small contact area between the mandrel and the pipe wall, the effect of preventing section deformation is poor. It is usually used for bending stainless steel pipe fittings with low requirements. The mandrel is usually made of 3Cr2W8V material with heat treatment hardness of 52 - 56hrc

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. It is suitable for single, multiple and space bending stainless steel pipe fittings of thin-walled tubes

multi ball mandrel elbow is adopted. After bending, the pipe fittings can also be rounded during the extraction of mandrel by oil cylinder. The main disadvantage of multi ball mandrel is manufacturing trouble. The material and heat treatment requirements of mandrel are the same as those of single ball mandrel

in the bend with mandrel, the shape, size and working position of mandrel have a great influence on the quality of stainless steel bend. The size of round head mandrel and its working position in the pipe are shown in Figure 5-3

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

d = D1 - (0.5 - 1.5)

the cylinder length L of the mandrel is:

L = (3-5) d

when the diameter D of the mandrel is large, the coefficient takes the small value, and vice versa

the working position of the mandrel shall not be more than a distance E from the bending centerline. Generally speaking, the size of e should be determined according to the diameter of stainless steel pipe blank, bending radius and the gap between the inner diameter of pipe blank and mandrel, and then adjusted appropriately according to the actual production situation. E value can be calculated according to the following formula:

e = √ 2 (R + D1 / 2) z-z2 (5-1)

where D1 - inner diameter of pipe blank, Nim

R - bending radius of central layer, mm

Z - the gap between the inner wall of stainless steel tube blank and mandrel, mm, z = d1-d0

there is another kind of ball joint rod commonly used on site, and its service condition and manufacturing difficulty are just between the above two kinds of mandrel. The parts of ball head mandrel are shown in Figure 5-4

b anti wrinkle block

for the stainless steel pipe blank material close to the tangent point and not entering the bending deformation area, its appearance is not deeply supported by the bending tire model groove. Even if the mandrel is used, the stainless steel elbow may still wrinkle here, so the anti wrinkle block shown in Figure 5-5 should be added. The front end of the anti wrinkle block is in the shape of an arc edge and is inserted between the bending die and the pipe blank. The front end should be close to the tangent between the pipe wall and the bending die, which effectively fills the gap between the bending die and the inner side of the stainless steel pipe blank, so as to support the pipe wall from the outside and prevent wrinkling

since the shallow circular groove on the anti wrinkle block contacts with the outer surface of the pipe blank, sliding friction occurs during operation, so the working surface of the anti wrinkle block is required to have determined hardness and low surface roughness. The anti wrinkle block is generally made of tool steel or chromium molybdenum steel, and the hardness reaches 50 - 55HRC after quenching

C bending die

the bending die is another important part in the cored elbow device, and its radius depends on the bending radius of the stainless steel elbow. However, considering that the bend will produce a certain amount of rebound during cold bending, the designed bending die radius should be slightly smaller than the required bending radius of stainless steel bend. Generally, it can be determined according to the following empirical data, that is,

when R / D = 3 - 4: R1 = (0.96 - 0.98) r

where R1 - bending die radius, mm

R - bending radius of central layer of elbow, mm

D -- outer diameter of tube blank, mm

when R / D is large, take the small value; When the R / D is small, take the large value and correct it at the final mold test

in addition to wheel type, the bending tire mold also has mold column type

coreless elbow is basically not used in pipe joint production

(2) push bending

push bending is a common bending method in pipe bending, which is mainly used to bend elbows. According to the characteristics of push bending process, it can be divided into cold push bend and mandrel hot push bend

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

as shown in Figure 5-8, the cold push elbow device is mainly composed of pressure column 1, guide sleeve 2 and bending die 4. The bending die is composed of two aligned pieces to facilitate the processing of its cavity. When bending, place the tube blank 3 in the guide sleeve for positioning, press the column downward, apply axial thrust to the tube blank port, and force the tube blank into the bending cavity, resulting in bending deformation

cold push elbow is suitable for bending stainless steel elbow with small bending radius, and the small relative bending radius R / D that can be bent is ≈ 1.3; The section ovality of elbow is small (≤ 3% - 5%); The thinning of the outer tube wall is small (≤ 9%); The bending device has the advantages of simple structure, no one-to-one equipment and good productivity. However, it is generally required that the relative thickness of the pipe T / D ≥ 0.06, otherwise, the pipe blank often loses stability due to poor stiffness, resulting in wrinkling or distortion on the inner side of the elbow

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

(a) in order to reduce friction resistance, prolong the service life of bending die and improve the surface quality of stainless steel elbow, it is necessary to lubricate the pipe blank. Practice has proved that applying 40-50 engine oil on the surface of stainless steel tube blank and a layer of graphite powder can protect the bending process and have a good lubrication effect

(b) during the bending process, the end of the pipe blank is easy to collapse under the action of axial thrust. Therefore, a core as shown in Figure 5.9 can be placed in the pipe blank, which will be pushed out by the pressure column together with the pipe elbow during the bending process

(c) for stainless steel thin-walled elbow, in order to prevent instability and wrinkling during push bending, filler should be loaded into the pipe blank, which is not only convenient to operate, but also has good anti wrinkling effect

(3) bending

use the die to press the stainless steel pipe fittings with straight section, which has the advantages of good production efficiency and simple die adjustment. In order to prevent the pipe wall from wrinkling and flattening, the stainless steel pipe fitting factory shall fill the thin-walled pipe with an outer diameter of more than 10mm or put the core head from both ends respectively before bending. The die bending method also has some disadvantages, such as at the beginning of the contact between the pipe blank and the concave and convex die, there will always be some distortion in the pipe section, so that the quality of the elbow is not ideal. In addition, the bending radius, angle and bending shape are also limited, so they are less used in production. Figure 5-10 shows the V-shaped pipe bending die, and figure 5-l1 shows the U-shaped pipe bending die. The working surface of the punch is consistent with the shape of the pipe blank and is placed in the middle of the left and right swing concave dies

pressing stainless steel elbow is a process method of pressing and bending pipe blank with pressing mould on hydraulic press. It is divided into hot pressing and cold pressing. It can press carbon steel elbow, alloy steel elbow, stainless steel elbow, copper and 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