Variable wall thickness steel pipe, method of processing and oil and gas pipeline connection system
By designing variable wall thickness steel pipes and adopting specific processing methods, the problem of stress concentration in the connection of steel pipes with unequal wall thicknesses was solved, achieving a high-quality connection without stress concentration or secondary processing, thus improving the safety and reliability of oil and gas pipelines.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CHINA NAT PETROLEUM CORP
- Filing Date
- 2022-06-02
- Publication Date
- 2026-05-29
AI Technical Summary
Existing connections between steel pipes with unequal wall thicknesses result in stress concentration, which affects the connection quality of oil and gas pipelines, and the existing connection methods are unable to effectively eliminate stress concentration.
Design a variable wall thickness steel pipe with a constant outer diameter and an inner diameter that gradually increases from one end to the other. It is processed through specific methods such as variable reduction rate rolling, JCOE variable reduction forming, and mechanical dynamic sizing to ensure that the inner and outer diameters are equal to those of the connecting steel pipe and to eliminate wall thickness differences.
By eliminating wall thickness differences, stress concentration is avoided, improving the connection quality and reliability of circumferential welds in oil and gas pipelines, and ensuring the safety and stability of the pipelines.
Smart Images
Figure CN117212564B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of oil and gas pipeline technology, and in particular to a variable wall thickness steel pipe, a processing method, and an oil and gas pipeline connection system. Background Technology
[0002] Bends are crucial components in pressure pipelines, typically located at pipeline turning points. They often become high-stress areas within the system, bearing complex loads and exhibiting lower stiffness than connected straight pipes. They readily absorb the forces and moments of thermal expansion and contraction through deformation. Furthermore, the pipeline's own weight, assembly deviations, and uneven foundation settlement can all subject it to external loads such as bending moments, torques, and axial forces. Connections between bends and straight pipes are mostly achieved through welding, a process prone to defects such as cracks, incomplete penetration, lack of fusion, porosity, and slag inclusions, posing safety hazards. Hot-bent bends, due to their inherent structural thinning, have a thicker wall than straight pipes in oil and gas pipelines, resulting in unequal wall thickness connections. This leads to stress concentration and is a critical area for pressure pipeline supervision and periodic inspections. Because different geological conditions require different pipe wall thicknesses, unequal wall thickness connections also exist between straight pipes at the boundaries of Class I, II, and III areas.
[0003] The existing methods for connecting unequal wall thicknesses can be mainly divided into the following types:
[0004] First, there is the connection of equal outer diameters, which means that the outer diameters of the pipes are the same but the inner diameters are different. After welding, it is not convenient to enter the system to repair the weld and keep the weld smooth. This will cause a large stress concentration in the inner wall of the pipe due to the abrupt transition of wall thickness. This seriously affects the force and torque generated by the pipeline system to absorb thermal expansion and contraction deformation, settlement deformation, etc., and causes safety hazards.
[0005] The second type is the equal inner diameter connection, which means that the pipes have the same inner diameter but different outer diameters. After welding, the operator can perform external repair on the formed weld to keep the weld smooth and smooth. The stress concentration generated by this method is reduced compared with the equal outer diameter connection, but it still cannot eliminate the structural stress caused by welding of unequal wall thickness.
[0006] Third, after boring the inner circle of the thick-walled pipe end, the welded joint is connected with the same outer diameter and wall thickness. The uneven wall thickness that was originally located at the circumferential weld is welded, so that the wall thickness change is moved to the thick-walled pipe. However, the structural stress concentration is not completely eliminated. At the same time, corresponding boring equipment is required for processing.
[0007] With the rapid development of oil and gas pipeline construction, the safe and stable operation of these pipelines has become increasingly important. In recent years, there have been numerous accidents caused by circumferential weld failures. Statistics on circumferential weld failures in recent years show that more than half of these failures occurred at the joints of steel pipes with unequal wall thicknesses. This indicates that the connection of steel pipes with unequal wall thicknesses presents a problem of high stress concentration, affecting the quality of oil and gas pipeline connections. Summary of the Invention
[0008] The purpose of this invention is to provide a variable wall thickness steel pipe, a processing method, and an oil and gas pipeline connection system to solve the problem of large stress concentration at the connection of steel pipes with unequal wall thickness, which affects the connection quality of oil and gas pipelines.
[0009] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows:
[0010] A variable wall thickness steel pipe is used to connect a first steel pipe and a second steel pipe with different wall thicknesses. The outer diameter of the variable wall thickness steel pipe is constant and equal to the outer diameter of the first steel pipe and the second steel pipe. The inner diameter of the variable wall thickness steel pipe gradually increases from one end to the other. The inner diameters at both ends of the variable wall thickness steel pipe are equal to the inner diameters of the first steel pipe and the second steel pipe, respectively.
[0011] Furthermore, the variable wall thickness steel pipe is formed by processing variable wall thickness steel plate, and the thickness of the variable wall thickness steel plate gradually increases from one end to the other.
[0012] In another aspect, the present invention provides a method for processing a variable wall thickness steel pipe, which is used to process the aforementioned variable wall thickness steel pipe, comprising the following steps: rolling a variable wall thickness steel plate; processing a welding bevel on the variable wall thickness steel plate; pre-bending the variable wall thickness steel plate; JCOE variable pressure forming; welding the inside and outside of the steel pipe; mechanical dynamic sizing of the steel pipe; and non-destructive testing of the steel pipe.
[0013] Furthermore, in the rolling process of variable wall thickness steel plates, a variable reduction rate rolling method is adopted: that is, the steel plate is divided into n equal parts along the rolling direction, and each part is regarded as conventional rolling. The forward slip value f and the reduction rate are different when rolling each part of the steel plate.
[0014] The formula for forward sliding is
[0015] Where h i r is the thickness at the rolling exit during each rolling pass. i Let i be the compression rate, i = 1, 2, ..., n.
[0016] Furthermore, during uphill rolling, During downhill rolling,
[0017] in The wedge angle refers to the central angle formed by the line connecting the point where the workpiece and the roll first come into contact when the workpiece begins to roll, and the center of the roll, and the center line of the roll; h is the thickness at the front entrance; x i This is the distance from the rolling exit to the front inlet during each rolling process.
[0018] Furthermore, r i =(H i -h i ) / H i ;
[0019] H i H represents the thickness of the steel plate before each rolling process begins. i The value is constant during the first pass of variable wall thickness rolling, and the measured value is taken at the end of the previous pass starting from the second pass of variable wall thickness rolling.
[0020] Furthermore, in the JCOE variable compression forming process, a dynamic forming method is used to bend the variable wall thickness steel plate into a steel pipe. The compression amount is adjusted according to the difference in deformation and elongation of different thicknesses to control the circumference of the steel pipe after forming.
[0021] Furthermore, in the mechanical dynamic sizing step of the steel pipe, a dynamic expansion method is adopted to obtain a variable wall thickness steel pipe through segmented processing: that is, the steel pipe is divided into multiple segments of equal length, and the diameter of each segment is expanded separately, with the diameter change of each segment gradually decreasing.
[0022] Furthermore, in the non-destructive testing steps for steel pipes, DAC curves are generated using primary and secondary waves from a vertical through-hole at the maximum wall thickness of φ1.6mm and at the minimum wall thickness of φ1.6mm. The test results are then evaluated using the DAC curves.
[0023] A third aspect of the present invention provides an oil and gas pipeline connection system, comprising the aforementioned variable wall thickness steel pipe, and further comprising a first steel pipe and a second steel pipe connected to both ends of the variable wall thickness steel pipe; the first steel pipe, the second steel pipe, and the variable wall thickness steel pipe have the same outer diameter; the first steel pipe has the same inner diameter as the smaller inner diameter end of the variable wall thickness steel pipe; the second steel pipe has the same inner diameter as the larger inner diameter end of the variable wall thickness steel pipe; and the steel pipes are welded to the first steel pipe and the second steel pipe.
[0024] In summary, the technical effects achieved by this invention are as follows:
[0025] The variable wall thickness steel pipe provided by the present invention is used to connect a first steel pipe and a second steel pipe with different wall thicknesses. The outer diameter of the variable wall thickness steel pipe is constant and equal to the outer diameter of the first steel pipe and the second steel pipe. The inner diameter of the variable wall thickness steel pipe gradually increases from one end to the other end. The inner diameters at both ends of the variable wall thickness steel pipe are equal to the inner diameters of the first steel pipe and the second steel pipe, respectively.
[0026] Since the outer diameter of the variable wall thickness steel pipe provided by the present invention is equal to the outer diameter of the first steel pipe and the second steel pipe, and the inner diameters at both ends are equal to the inner diameters of the first steel pipe and the second steel pipe to be connected, and the inner diameter of the variable wall thickness steel pipe gradually increases from one end to the other, the wall thickness difference is eliminated, thereby avoiding stress concentration of the pipe itself and welding stress concentration at the connection, and improving the connection quality of the circumferential weld of the oil and gas pipeline. Attached Figure Description
[0027] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.
[0028] Figure 1 This is a schematic diagram of an oil and gas pipeline connection system provided in an embodiment of the present invention;
[0029] Figure 2 This is a schematic diagram of an oil and gas pipeline connection system provided in an embodiment of the present invention;
[0030] Figure 3 This is the front view of a steel plate with variable wall thickness.
[0031] Figure 4 A schematic diagram showing the connection of steel pipes with unequal wall thicknesses but equal outer diameters;
[0032] Figure 5 A schematic diagram for connecting steel pipes with varying wall thicknesses but the same inner diameter;
[0033] Figure 6 A schematic diagram before boring the connection of steel pipes with unequal wall thicknesses;
[0034] Figure 7 This is a schematic diagram showing the connection of steel pipes with unequal wall thicknesses after boring.
[0035] Figure 8 This is a schematic diagram of the starting position for conventional rolling.
[0036] Figure 9 This is a schematic diagram of conventional rolling.
[0037] Figure 10 A schematic diagram of variable wall thickness rolling on an uphill slope;
[0038] Figure 11 A schematic diagram of rolling with variable wall thickness on a downhill slope;
[0039] Figure 12 This is a schematic diagram of uphill rolling.
[0040] Figure 13This is a schematic diagram of downhill rolling.
[0041] Figure 14 This is a schematic diagram of the diameter expansion.
[0042] Icons: 100 - First steel pipe; 200 - Second steel pipe. Detailed Implementation
[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.
[0044] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.
[0045] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.
[0046] Example 1
[0047] Existing oil and gas pipelines suffer from problems such as high stress concentration at the connection points due to varying pipe wall thicknesses required for different geological conditions and the fact that the wall thickness of bends is greater than that of straight pipes, which affects the quality of oil and gas pipeline connections.
[0048] In view of this, the present invention provides a variable wall thickness steel pipe for connecting a first steel pipe 100 and a second steel pipe 200 with different wall thicknesses. The outer diameter of the variable wall thickness steel pipe is constant and equal to the outer diameter of the first steel pipe 100 and the second steel pipe 200; the inner diameter of the variable wall thickness steel pipe gradually increases from one end to the other; the inner diameters at both ends of the variable wall thickness steel pipe are equal to the inner diameters of the first steel pipe 100 and the second steel pipe 200, respectively.
[0049] Since the outer diameter of the variable wall thickness steel pipe provided by the present invention is equal to the outer diameter of the first steel pipe 100 and the second steel pipe 200, and the inner diameters at both ends are equal to the inner diameters of the connected first steel pipe 100 and the second steel pipe 200 respectively, and the inner diameter of the variable wall thickness steel pipe gradually increases from one end to the other, the wall thickness difference is eliminated, thereby avoiding stress concentration of the pipe itself and welding stress concentration at the connection, and improving the connection quality of the circumferential weld of the oil and gas pipeline.
[0050] The following combination Figures 1-7 The structure and shape of the variable wall thickness steel pipe provided in this embodiment are described in detail below:
[0051] like Figure 4 , Figure 5 As shown, when connecting steel pipes with unequal wall thicknesses, stress concentration easily occurs at the weld joint, affecting the pipeline system's ability to absorb forces and moments generated by thermal expansion and contraction, settlement deformation, etc., which is detrimental to the quality of the circumferential weld. For example... Figure 6 The image shows the state after welding when steel pipes of unequal wall thickness are connected with the same outer diameter. After boring the inner hole, the result is as shown. Figure 7 In the structure shown, the unequal wall thickness is located on the side of the steel pipe with the larger wall thickness, and the weld is a connection with equal wall thickness, which eliminates stress concentration at the weld. However, stress concentration still exists at the steel pipe with the larger wall thickness.
[0052] like Figure 3 As shown, variable wall thickness steel pipes are obtained by processing variable wall thickness steel plates, and are used for, for example... Figure 1 , Figure 2 The connection shown connects the first steel pipe 100 and the second steel pipe 200 into a pipeline, which makes the pipeline wall thickness transition uniformly, avoids stress concentration, effectively protects the circumferential weld, and improves the connection quality.
[0053] Example 2
[0054] The following combination Figures 8-13 The processing method for steel pipes with variable wall thickness is described in detail:
[0055] The processing of variable wall thickness steel pipes includes the following steps:
[0056] S100 - Rolled Variable Wall Thickness Steel Plate:
[0057] Rolling with variable reduction rate: The steel plate is divided into n equal parts along the rolling direction. Each part is treated as conventional rolling. The forward slip value f and reduction rate are different when rolling each part of the steel plate.
[0058] The formula for forward sliding is
[0059] Where h i r is the thickness at the rolling exit during each rolling pass. i Let i be the compression rate, i = 1, 2, ..., n.
[0060] When rolling uphill, During downhill rolling,
[0061] in The wedge angle refers to the central angle formed by the line connecting the point where the workpiece and the roll first come into contact when the workpiece begins to roll, and the center of the roll, and the center line of the roll; h is the thickness at the front entrance; x iThis is the distance from the rolling exit to the front inlet during each rolling process.
[0062] reduction ratio r i =(H i -h i ) / H i ;
[0063] H i H represents the thickness of the steel plate before each rolling process begins. i The value is constant during the first pass of variable wall thickness rolling, and the measured value is taken at the end of the previous pass starting from the second pass of variable wall thickness rolling.
[0064] Specifically, combined Figure 8 , Figure 9 , Figure 10 , Figure 11 The derivation process of the forward sliding formula for variable wall thickness rolling, as shown in Table 1, is as follows:
[0065] Table 1. Bite Angle and Bite Conditions for Conventional Rolling and Variable Thickness Rolling
[0066]
[0067] Horizontal rolling force T x (Horizontal component of frictional force T) and horizontal pushing force P x (The horizontal component of the roll pressure P) is:
[0068]
[0069]
[0070] wedge angle It refers to the central angle formed by the line connecting the point where the workpiece and the roll first come into contact when the workpiece begins to roll into the roll and the center line of the roll.
[0071] The bite angle α refers to the central angle formed by the line connecting the point where the workpiece and the roll first come into contact with each other and the center line of the roll, when the workpiece begins to enter the roll. At this time, the friction angle β = α. The friction angle β is the angle between the resultant force of the frictional force T and the rolling force P and the rolling force P. After the rolling process is completed, the bite condition is β ≥ α / 2.
[0072] The forward slip value in a conventional rolling process is:
[0073] f=[h+D(1-cosγ)]cosγ / h-1 (3)
[0074] Where D is the roll diameter; h is the exit thickness of the rolled piece; γ is the neutral angle (the central angle formed by the line connecting the 1 / 2 wall thickness contact point and the roll center and the vertical center line of the roll);
[0075] Formula for forward slip during uphill rolling:
[0076]
[0077] The formula for forward sliding during downhill rolling:
[0078]
[0079] In the actual rolling process of variable wall thickness steel plates, the horizontal distance x from the steel plate exit to the starting point of the wedge section changes continuously, and the forward slip value f is no longer a constant value, but changes continuously as the rolling progresses. Since the neutral angle γ is difficult to determine, equations (4) and (5) are difficult to apply in practice in engineering.
[0080] Substituting the neutral angle calculation formula obtained during the derivation of the Sims formula into equation (3), we obtain the forward sliding formula (4) considering the full adhesion condition. This formula simplifies the calculation process to obtain the following engineering application formula while ensuring calculation accuracy:
[0081]
[0082] r is the relative reduction rate during the rolling process; R is the roll radius.
[0083] Variable wall thickness steel plates are generally medium-thick steel plates with a thickness of 10mm or more. The temperature of the rolled piece is high, and the friction coefficient between the rolled piece and the roll is very large, which is rolling under full adhesion conditions. The variable wall thickness steel plate is divided into n equal parts, and each part satisfies the forward sliding formula (7) after discretization:
[0084]
[0085] During uphill rolling, the exit thickness of variable wall thickness steel plates during uphill rolling. During downhill rolling, the thickness of the downhill rolling exit of variable wall thickness steel plates
[0086] h is the thickness at the front entrance; x i This is the distance from the rolling exit to the front inlet during each rolling process.
[0087] reduction ratio r i =(H i -h i ) / H i ;
[0088] H i H represents the thickness of the steel plate before each rolling process begins. i The value is constant during the first pass of variable wall thickness rolling. From the second pass onwards, the value is the measured value at the end of the previous pass. In actual operation, H is detected in real time by laser. i .
[0089] The segmentation diagrams for uphill and downhill rolling are as follows: Figure 12 , Figure 13 As shown.
[0090] S200 - Processing welding bevels on steel plates with varying wall thickness: The edges of the steel plate are milled to obtain the welding bevels.
[0091] S300 - Variable wall thickness steel plate pre-bending: The two sides of the variable wall thickness steel plate are bent respectively so that the curvature of the steel plate edge is the same as the curvature of the formed steel pipe, ensuring the roundness of the steel pipe.
[0092] S400-JCOE variable compression forming: It adopts a dynamic forming method to bend steel plates with varying wall thicknesses into steel pipes. The compression amount is adjusted according to the difference in deformation and elongation of different thicknesses, so that the stress on steel plates with different wall thicknesses tends to be consistent, thereby controlling the circumference of the steel pipe after forming and keeping the circumference difference within 5mm.
[0093] S500 steel pipe internal and external welding: Based on the thickness variation, the welding line energy of 1-3 wires is fixed to ensure full filling. The cladding rate of four wires is adjusted according to the wall thickness variation, i.e., the current is dynamically changed within 10%, and the weld reinforcement height variation is controlled within 2mm.
[0094] S600-Mechanical Dynamic Sizing of Steel Pipes: This method employs dynamic diameter expansion, using segmented processing to obtain variable wall thickness steel pipes. The steel pipe is divided into multiple equal segments, and each segment is expanded separately, from the larger inner diameter end to the smaller inner diameter end. The diameter change in each segment gradually decreases to achieve a gradual change in the inner diameter, thus ensuring that the diameter deviation is controlled within 1mm. Figure 14 The diagram shows a schematic of steel pipe expansion. By controlling the diameter of the expansion mold, the inner and outer diameters of the variable wall thickness steel pipe can be controlled, especially the inner diameter.
[0095] S700 - Non-destructive testing of steel pipes:
[0096] Ultrasonic testing of steel pipes with varying wall thickness uses DAC curves generated by primary and secondary waves through a vertical through-hole at the maximum wall thickness (φ1.6mm) and the minimum wall thickness (φ1.6mm). The test results are then evaluated using the DAC curves. Radiographic testing employs a dual-film imaging technique, where two films with different sensitivities are installed in a dark bag. The film with higher sensitivity is suitable for observing and evaluating areas with greater thickness, while the film with lower sensitivity is suitable for observing and evaluating areas with less thickness.
[0097] Example 3
[0098] This embodiment provides an oil and gas pipeline connection system, including the aforementioned variable wall thickness steel pipe, and further including a first steel pipe 100 and a second steel pipe 200 connected to both ends of the variable wall thickness steel pipe; the first steel pipe 100, the second steel pipe 200 and the variable wall thickness steel pipe have the same outer diameter; the first steel pipe 100 has the same inner diameter as the smaller inner diameter end of the variable wall thickness steel pipe; the second steel pipe 200 has the same inner diameter as the larger inner diameter end of the variable wall thickness steel pipe; the steel pipes are welded to the first steel pipe 100 and the second steel pipe 200.
[0099] By using variable wall thickness steel pipes, the uneven wall thickness connections between different regions and between bends and straight pipes are eliminated, achieving connections without secondary processing, structural stress, or wall thickness differences. This improves the reliability of circumferential welds and greatly enhances the inherent safety of the pipeline.
[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.
Claims
1. A method for processing a variable wall thickness steel pipe, used to process a variable wall thickness steel pipe, wherein the variable wall thickness steel pipe is used to connect a first steel pipe (100) and a second steel pipe (200) with different wall thicknesses, the outer diameter of the variable wall thickness steel pipe is constant and equal to the outer diameters of the first steel pipe (100) and the second steel pipe (200); the inner diameter of the variable wall thickness steel pipe gradually increases from one end to the other end; the inner diameters at both ends of the variable wall thickness steel pipe are equal to the inner diameters of the first steel pipe (100) and the second steel pipe (200), respectively, the variable wall thickness steel pipe is formed by processing a variable wall thickness steel plate, the thickness of the variable wall thickness steel plate gradually increasing from one end to the other end, characterized in that... It includes the following steps: Rolling the variable wall thickness steel plate; A welding bevel is machined on the variable wall thickness steel plate; The variable wall thickness steel plate is pre-bent at the edge; JCOE variable pressure forming; Welding of steel pipes inside and out; Mechanical dynamic sizing of steel pipes; Non-destructive testing of steel pipes; In the step of rolling the variable wall thickness steel plate, the rolling method of variable reduction rate is adopted: that is, the steel plate is divided into n equal parts along the rolling direction, and each part is regarded as conventional rolling. The forward slip value f and the reduction rate are different when rolling each part of the steel plate. The formula for forward sliding is , Where h i The thickness at the exit of the rolling mill during each rolling process. Let i be the reduction ratio, i = 1, 2, ..., n; During uphill rolling, h i =h+2x i tan ; During downhill rolling, h i =h-2x i tan ; in The wedge angle refers to the central angle formed by the line connecting the point where the workpiece and the roll first come into contact when the workpiece begins to roll into the roll and the center of the roll, and the center line of the roll. h represents the thickness at the front inlet; x i This is the distance from the rolling exit to the front inlet during each rolling process.
2. The processing method for variable wall thickness steel pipes according to claim 1, characterized in that, r i =(H i -h i ) / H i ; H i H represents the thickness of the steel plate before each rolling process begins. i The value is constant during the first pass of variable wall thickness rolling, and the measured value is taken at the end of the previous pass starting from the second pass of variable wall thickness rolling.
3. The processing method for variable wall thickness steel pipes according to claim 2, characterized in that, In the JCOE variable compression forming step, the variable wall thickness steel plate is bent into a steel pipe using a dynamic forming method. The compression amount is adjusted according to the difference in deformation and elongation of different thicknesses to control the circumference of the steel pipe after forming.
4. The processing method for variable wall thickness steel pipes according to claim 3, characterized in that, In the mechanical dynamic sizing step of the steel pipe, a dynamic diameter expansion method is adopted to obtain the variable wall thickness steel pipe through segmented processing: that is, the steel pipe is divided into multiple segments of equal length, and the diameter of each segment is expanded separately, with the diameter change of each segment gradually decreasing.
5. The processing method for variable wall thickness steel pipes according to claim 4, characterized in that, In the non-destructive testing step of the steel pipe, the variable wall thickness steel pipe uses a vertical through-hole primary and secondary wave at the maximum wall thickness of φ1.6mm and a vertical through-hole primary and secondary wave at the minimum wall thickness to generate a DAC curve, and the test results are evaluated by the DAC curve.