Stainless steel elbow production process

Stainless steel elbow production process

The production process of stainless steel elbow: Incoming pipe blank – inspection – forming – cleaning – shaping – chamfering and engraving – polishing – cleaning – drying – deburring – finished product inspection – packaging and storage.

1 Overview
When stainless steel pipes need to change direction or use point path design requirements during the layout process of the project, stainless steel elbows need to be connected, with angles of 45°, 90°, etc. Elbow forming production can use different methods: orbiting stainless steel elbow forming, push bending and press bending, etc.
2. Stainless steel elbow production process
The production process of stainless steel elbow is as follows:
Incoming tube blanks – inspection – forming – cleaning – shaping – chamfering – polishing – cleaning – drying – deburring – finished product inspection – packaging and storage
3. Forming around the bent pipe
(1) Core bent pipe
Core bending is a forming method that uses a mandrel on a pipe bending machine to bend the pipe material around the bending mold. The working principle of cored pipe bending is shown in Figure 5-1. The bending 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 mold by the clamping block 3 . Near the tangent point between the stainless steel tube blank and the curved tire mold, a pressing block 1 is installed on the outside of the bend, an anti-wrinkle block 5 is installed on the inside of the bend, and a mandrel 2 is plugged inside the tube blank. When the curved tire mold rotates, the stainless steel tube blank is gradually bent around the curved tire mold. The bending angle of the stainless steel elbow is controlled by a stopper (not shown in the figure). When the curved tire mold reaches the bending angle required by the stainless steel pipe fitting, it hits the stopper and stops the rotation of the curved tire mold. The design and manufacturing of the main parts of the tool mold are explained 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 pipe blank and prevent the stainless steel pipe fittings from deforming in section and wrinkling the pipe wall.
The round head mandrel has a simple shape and is easy to manufacture. However, due to the small contact area between the mandrel and the pipe wall, the effect of preventing cross-sectional deformation is poor, and it is usually used for bending stainless steel pipe fittings with low requirements. The mandrel is commonly made of 3Cr2W8V material, with a heat treatment hardness of 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 realize the swing in any direction in space. It is suitable for single, multiple and spatial bending of thin-walled pipes and stainless steel pipe fittings.
The multi-ball mandrel is used to bend the pipe, and the pipe fittings can be rounded during the process of drawing out the mandrel by the oil cylinder after bending. The main disadvantage of multi-ball mandrels is that they are troublesome to manufacture. The mandrel material and heat treatment requirements are the same as those of single-ball mandrels.
In bent pipes with mandrels, the shape, size and working position of the mandrel have a great impact on the quality of stainless steel bent pipes.
In order to insert the mandrel into the stainless steel tube blank smoothly, the diameter of the mandrel 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 core rod is large, the coefficient takes a small value, otherwise it takes a large value.
The working position of the mandrel should be a certain distance e ahead of the bending center line. Generally speaking, the size of e should be determined according to the diameter of the stainless steel tube blank, the bending radius and the gap between the inner diameter of the tube blank and the mandrel, and then be appropriately adjusted according to the actual production conditions. The e value can be calculated as follows:
e=√2（R+D1/2）z－z2 (5-1)
In the formula, D1——inner diameter of tube blank, nim;
R——bending radius of central 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, whose usage and manufacturing ease are right between the above two types of mandrels. The parts of the ball head mandrel are shown in Figure 5-4.
B anti-wrinkle block
The surface of the stainless steel pipe blank material that is close to the tangent point and has not entered the bending deformation zone is not supported by the bending tire model groove. Even if a mandrel is used, the stainless steel pipe may still wrinkle here when bending, so it should be installed such as The anti-wrinkle block shown in Figure 5-5. The front end of the anti-wrinkle block is in the shape of an arc blade and is inserted between the curved tire mold and the pipe blank. The front end should be close to the tangent between the pipe wall and the curved tire mold, effectively filling the gap between the curved tire mold and the curved stainless steel pipe blank. The gap between the inner and outer sides thus supports the pipe wall from the outside and prevents wrinkling.
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 operation, so the working surface of the anti-wrinkle block is required to have a certain hardness and low surface roughness. Anti-wrinkle blocks are generally made of tool steel or chromium-molybdenum steel, with a hardness of 50 – 55HRC after quenching.
C curved tire mold
The bending mold is another important part in the cored pipe bending device, and its radius depends on the bending radius of the stainless steel bending pipe. However, considering that the bent pipe will produce a certain amount of springback during cold bending, the designed bending mold radius should be slightly smaller than the required bending radius of the stainless steel bent 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——radius of curved tire mold, mm;
R——Bending radius of the center layer of the bent pipe, mm;
D——Outer diameter of tube blank, mm.
When R/D is large, take a small value; when R/D is small, take a large value, and finally correct it during mold testing.
In addition to the wheel type, the curved tire mold also has the mold column type. The parts of the mold column are shown in Figure 5-6.
Coreless bent pipes are basically not used in pipe joint production.
(2) Push and bend
Push bending is a common pipe bending method in pipe bending processing, and is mainly used to bend elbows. According to the characteristics of the push bending process, it can be divided into two categories: cold push bend and mandrel hot push bend.
Cold-pushed pipe bending is a process in which the pipe blank is pushed and bent using a pipe bending device on an ordinary hydraulic press or crank press. The stainless steel straight pipe blank is pressed into a mold with a bending cavity at room temperature to form stainless steel. elbow.
The cold-pushed pipe bending device is shown in Figure 5-8, which mainly consists of a pressure column 1, a guide sleeve 2 and a bending die 4. The bending mold is made of two aligned pieces to facilitate its cavity processing. When bending the tube, after placing the tube blank 3 in the guide sleeve and positioning it, the pressure column moves downward, exerting axial thrust on the port of the tube tube, forcing the tube tube into the bending cavity, thereby causing bending deformation.
Cold-pushed pipe elbows are suitable for bending stainless steel elbows with smaller bending radius. The minimum relative bending radius that can be bent is R/D≈1.3; the cross-section ovality of the elbow is small (≤3%-5%); the outer pipe wall The thinning amount is small (≤9%); the 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 pipe t/D ≥ 0.06. Otherwise, the pipe blank often loses stability due to poor stiffness, causing the inside of the elbow to wrinkle or twist.
Stainless steel pipe fittings factories should also pay attention to the following points for the cold-pushed pipe bending process:
(a) In order to reduce 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 proven that applying No. 40-50 engine oil on the surface of the stainless steel tube blank and then coating it with a layer of graphite powder can ensure good lubrication during the bending process.
(b) During the bending process, the end of the tube blank is easy to collapse under the action of axial thrust. For this reason, a core can be placed in the tube blank, which will be pushed out by the pressure column along with the tube elbow during the bending process.
(c) For stainless steel thin-walled elbows, in order to prevent instability and wrinkles during the pushing and bending process, filler should be installed in the tube blank, which is not only easy to operate, but also has a good anti-wrinkle effect.
(3) Bending
The advantage of using a mold to press stainless steel pipe fittings with straight sections is high production efficiency and simple mold adjustment. In order to prevent the pipe wall from wrinkling and flattening, the stainless steel pipe fittings factory requires thin-walled pipes with an outer diameter of 10mm or more to be filled with filler or cores from both ends before bending. The mold bending method also has some shortcomings. For example, where the tube blank starts to contact the concave and convex molds, the tube section will always have some distortion, making the quality of the bent tube less than ideal. In addition, the bending radius, angle and bending shape are also subject to certain restrictions, so they are rarely used in production.
Pressed stainless steel elbow is a process that uses a pressing mold to bend the tube blank 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 elbows. , 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, and generally R=1.5D is recommended.

Author: Maria Yang