Analysis of the Variable Slope Section of the Forged Weld Neck Flange Neck Made of the High Strength Steel

Based on the research results of forged weld neck flanges, we analyze the forged weld neck flange neck slope section from both theoretical analysis and finite element simulation, focusing on the influence of slope slope and flange neck height on the force characteristics of the flange, and propose the suggested values of forged weld neck flange neck slope section based on the analysis results.

1. Overview

Steel pipe towers are increasingly used in practical engineering. Flange connection is the main connection form of steel pipe structure. Forging weld neck flange connection without welding stiffening ribs, butt welds can be mechanically welded, which can solve the heavy work of manual welding of stiffening ribs with stiffening flanges, improve the production efficiency and mechanization efficiency. At the same time, to a certain extent to make up for the low stiffness of the connection without stiffening flange, it should be said that forging weld neck flange is a better form of flange – steel pipe connection.
Forged weld neck flange in the transmission tower structure are used, especially in Japan in the large span of steel pipe tower ultra-high voltage and conventional voltage level of steel pipe tower are used in the weld neck flange. But forging weld neck flange research, China is still in the initial stage, and forging weld neck flange design theory is not perfect. High neck flange has better connection stiffness and structural performance than the non-stiffened flange, because the flange plate bending moment at the maximum with a variable cross-section of the variable slope section (i.e., the neck axle). Therefore, the weld neck flange has better structural performance and more coordinated force compared with the unstiffened flange. Therefore, in this paper, we analyze the proposed flange dimensions with the help of finite element analysis method and explore the influence of the variable slope section of the flange neck on the performance of the forged weld neck flange.

2. High neck flange force characteristics analysis

Based on the general principle of structural design – “strong nodes, weak members”, i.e., the damage of the main pipe occurs before the flange, and the design provisions of China’s Steel Design Code, and taking into account the economy, the weld neck flange is designed to ensure the structural The size of the weld neck flange is as reasonable as possible on the basis of the performance.
The section corresponding to each bolt is taken as the isolated body, as the object of analysis, as shown in Figure 1.
When the steel pipe is subjected to axial tension Tc, the flange is subjected to bending and shearing due to the bolt force Tb, and the force of the bolt is transmitted to the flange neck. Due to the weld neck forged flange plate thickness t is larger, with sufficient stiffness, so that the flange edge prying force Rf is small or no prying force, the edge off. The flange deformation and disengagement gap is small, mainly caused by the high strength bolt strain. So it is suitable for the node connection of the main material joint of steel pipe tower of transmission line.

Neck height: 20230108112353 30216 - Analysis of the Variable Slope Section of the Forged Weld Neck Flange Neck Made of the High Strength Steel

Slope: θ1=5°, internal slope: θ2=15°.
Where, D is equal to the diameter of the pipe diameter of the main pipe butt; D1, D2 are the size of the butt pipe diameter;
Calculation method for forged weld neck flange:

Neck height: H = Ed – t + H1.

Among them, Ed for butt-welding flange height coefficient, take 2.2; H1 for the top of the flange neck (straight neck section) height.
Slope: not given a clear value.
And Japan’s “transmission line steel pipe tower production standards” clearly specify the vertical angle of the variable section of 8 ° -22 °.
Forging weld neck flange in the transmission tower structure, China is still in its infancy, and the design theory is not perfect. In view of the force characteristics of weld neck forged flanges, it is important to study the selection of weld neck butt weld flanges and optimize the detailed structure of flanges for engineering applications.

3. Finite element simulation

In order to explore the influence of the forged weld neck flange neck slope section on the force performance of the flange, this paper analyzes the forged weld neck flange by changing the slope angle and neck height and using the finite element software ANSYS to explore the value of the forged weld neck flange neck slope section.

3.1 Element selection

Steel pipe and flange are simulated by Solid185 unit; flange and flange contact surface and bolt and flange contact surface are simulated by target unit Targe170 and contact unit Conta174. The material of the main pipe is Q460, the material of the flange is Q420, and the grade of high-strength bolts is 8.8. The yield criterion of the material obeys the VonMises yield criterion and its related flow law. The effect of the weld seam as well as the residual stress of the weld is not considered.
According to the characteristics of the object model, 1/4 simplified modeling is used, all the nodal degrees of freedom at the bottom of the lower steel tube are constrained, and the section is constrained along the vertical plane in the direction of displacement. The lateral constraints of the flange model are shown in Figure 2.
20230108112558 13191 - Analysis of the Variable Slope Section of the Forged Weld Neck Flange Neck Made of the High Strength Steel
Figure.1 Flange calculation section
20230108113022 53432 - Analysis of the Variable Slope Section of the Forged Weld Neck Flange Neck Made of the High Strength Steel
Figure.2 Flange model lateral constraint

3.2 Influence of neck height and variable slope angle

The pipe diameter of main 406×8, the slope type is external slope and internal straight wall. The neck height of the flange is taken as 1.75d, 2d, 2.5d and 3d, and the stress characteristics are analyzed when the slope is 10°, 12.5°, 15°, 17.5°, 20°, 22.5° and 25° respectively, as shown in Table 1. The stress extraction location of the flange neck is shown in Figure 3, the stress at each point is shown in Figure 4, and the overall mass of the flange is shown in Table 2, d is the bolt diameter.
Table.1 Dimensions of flanges

Steel pipe specification Connection size Flange neck Neck height hc
Flange thickness Bolt
Specification Hole diameter Quantity Taper angle Fillet Top thickness
φ406×8 41 mm 30 mm 32 20 10° – 25° 6 10 mm 1.75d – 3d

Table.2 Flange masses with different neck heights and variable slope angles

Neck height Flange neck slope change angle
10° 12.5° 15° 17.5° 20° 22.5° 25°
1.75d 45 47 49 51.1 53.3 55.2 58
2d 47.3 49.7 52.1 54.7 57.2 60.1 63
2.5d 52.2 55.5 58.9 62.4 66 69.8 73.8
3d 57.6 61.9 66.3 70.9 75.7 80.7 86

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Fig.3 Schematic diagram of stress extraction position of flange neck

4. Conclusion

1) With the increase of variable slope angle, the flange bolt circle diameter increases, the flange cantilever increases, so the stress at the top of the flange neck increases, when the angle exceeds 22.5°, the stress decreases slightly because the top cross-sectional area increases.
2) With the increase of variable slope angle, the cross-sectional area of the neck increases, which makes the stress in the middle of the flange neck decrease.
3) The maximum value of the flange stress is at the bottom of the flange neck, and increasing the slope can effectively reduce the stress concentration at the bottom.
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Fig.4 Stress in each part of flange neck and slope angle curve
4) When the slope of the neck is unchanged, increasing the neck height of the flange will improve the stress performance of the flange neck less, and may even increase the stress concentration at the bottom of the flange neck.
Increasing the slope angle can effectively reduce the stress in the neck of the flange and reduce the stress concentration in the neck of the flange, but it will increase the outer diameter of the flange and make the overall mass of the flange increase faster. In a comprehensive comparison, the stress level in the flange neck is relatively small when the slope angle is 17.5° and 20°, so it is recommended that the slope angle of the flange neck be 20° and the flange neck height be 2d.
Source: China Weld Neck Flange Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)

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