A method of welding a bellows to a spool

Abstract

This invention discloses a welding method for corrugated pipes and connecting pipes, characterized by: selecting an austenitic stainless steel test plate and machining its welding end face into a stepped slope; preparing a multi-layer austenitic stainless steel test plate for resistance seam welding, cutting it into a straight surface along the edge of the resistance weld; assembling the two test plates into a socket-lap welded structure to form a butt joint, leaving sufficient gap for weld penetration; ensuring the assembled test plates are in a horizontal welding position; and using tungsten inert gas (TIG) welding, completing at least two or more multi-layer welds. The specimens obtained by this method exhibit satisfactory mechanical properties and macroscopic metallographic characteristics. The beneficial effects of this invention are that the use of horizontal welding, with the arc offset from the corrugated pipe, prevents the arc from directly acting on the corrugated pipe, as the molten iron melts the pipe downwards, thus preventing the arc from burning through the corrugated pipe. Multi-layer welding disperses the heat input to the resistance weld of the corrugated pipe. The adoption of these process measures overcomes the defects of incomplete resistance seam welding and prevents the corrugated pipe from burning through.

Description

Technical Field

[0001] This invention relates to the field of corrugated pipe expansion joint welding technology, specifically a welding method for corrugated pipes and connecting pipes. Background Technology

[0002] Expansion joints are commonly used metal elastic elements in industry, primarily for instrument sensing elements and pipeline connection compensation components. The bellows connects to a pipe or flange to form an expansion joint. In power transmission pipeline (GIS / GIL) systems, expansion joints composed of thin-walled, multi-layered bellows (e.g., 0.5 mm × (three to seven) layers, austenitic stainless steel) are commonly used as expansion compensation units.

[0003] When the wall thickness of a single layer of a multi-layer corrugated pipe is thin (<0.8 mm), direct welding to other workpieces is difficult. The first layer of the corrugated pipe is easily burned through by the electric arc, making it difficult to repair and rendering the corrugated pipe unusable. To solve this welding problem, an additional sealing process is needed. This involves taking technological measures to ensure that the layers of the straight edge section of the corrugated pipe are tightly bonded together before welding it to other workpieces (connectors or flanges). The purpose of this is to allow the lower layer of the corrugated pipe to act as a welding backing for the upper layer, greatly reducing the welding difficulty.

[0004] Resistance seam welding is a commonly used sealing method and is widely used in the manufacture of multi-layer corrugated pipe expansion joints.

[0005] The purpose of resistance seam welding is to seal the straight-edge sections of multi-layered bellows into a single unit, without separate layers, facilitating the next process: circumferential welding of the bellows ends to the connecting pipe. Therefore, the resistance weld at the bellows end is essentially a process weld, an atypical application of resistance seam welding. It is not necessary to require this type of resistance weld to possess the strength and sealing performance typically expected of a closed thin-walled container. Other methods can replace resistance seam welding, such as using a pressure method to compact the bellows ends during assembly with the connecting pipe, which also helps to easily achieve circumferential welding; however, resistance seam welding is a faster and more convenient method. The resistance-sealed bellows also require a cutting process, where excess straight-wall length of the bellows is cut off along the outside of the resistance weld.

[0006] Based on reference materials and experimental observations, a complete resistance weld can exist in two states: a fusion state with a weld nugget and a plastic adhesion state without a weld nugget. This is related to the capability of the resistance welding machine, the total thickness of the workpiece, the cooling method of the workpiece, and the stability of the welding parameters. Resistance welding becomes more difficult when the workpiece thickness exceeds 4 mm.

[0007] Commonly used resistance welding machines (such as the FN1-150-1 model) have a rated capacity of 150kVA and a rated weld thickness of 2 mm + 2 mm. For resistance seam welding of 0.5 mm × (seven / eight) layers, this is already operating at the upper limit of the equipment's capacity.

[0008] The tear test is the most effective method to inspect the condition of resistance welds. It involves welding resistance test pieces and then performing interlayer stretching on a testing machine to separate the test pieces. If there is a tear, the weld is in a fused state; if there is no tear, the weld can be directly torn apart, indicating a plastic bonded state.

[0009] The tearing phenomenon observed in the test pieces reveals inconsistent interlayer bonding along the resistance weld seam. Both fusion welding and plastic adhesion coexist in the length and thickness directions. In the length direction, a small portion is often welded, while the vast majority remains unwelded. In the thickness direction, the inner layers are often fusion welded, while the desired outermost layer is not. Maintaining plastic adhesion between the first and second layers on the inner side of the bellows is crucial in the welding of bellows cuts and inner lap joints with connecting pipes. Forced delamination eliminates the cushioning effect of the lower layers, easily leading to burn-through of the uppermost layer. Even with a high welding current and the weld surface already showing signs of overheating (further current would cause breakdown) for a 0.5 mm × seven-layer weld, the majority of the weld in the length direction remains in a plastic adhesion state, with only a small portion in a fusion weld state. Plastic adhesion also maintains interlayer bonding and will not naturally delaminate without external force.

[0010] Extensive testing has concluded that it is unrealistic to expect the entire multi-layer resistance weld to achieve a completely fusion weld state! Currently, achieving consistent welding conditions in multi-layer resistance welds remains the ultimate goal of resistance seam welding. Inconsistency in resistance welds also hinders the application of automated (or machine-made) welding for circumferential seams. Automated welding cannot observe the state of the resistance weld and cannot make corresponding adjustments. Furthermore, automated welding has strict requirements for the dimensional and positional tolerances of the workpiece assembly, while machine-made welding requires smooth workpiece rotation. Currently, manual tungsten inert gas (TIG) welding remains the most commonly used and effective welding method for multi-layer thin-walled corrugated pipes and connecting pipes. Manual operation is flexible; the wire feed can be adjusted according to the size of the weld, and the arc deflection can be flexibly controlled. The operator can adjust the arc position and control the heat input in a timely manner based on the observed state of the resistance weld and whether there is a residual "white edge" of the base material at the edge of the resistance weld after cutting.

[0011] To avoid burning through the corrugated pipe in the resistance weld delamination, adjusting the arc position could potentially result in incomplete fusion at the weld root, an unavoidable reality. Extensive engineering practice has proven that a very small number of incomplete root fusions do not fatally impact weld quality and do not affect the reliable operation of the expansion joint on the pipeline. This is thanks to the excellent operating environment of expansion joints for power lines: the internal medium is SF6 inert gas, the internal pressure is low (≤0.5MPa), the operating temperature is low (≤80℃), and the external medium is air. Power lines differ from common industrial pressure pipelines.

[0012] While it is important to prevent the corrugated pipe from tearing during the cutting process while maintaining its plastic bond, it is even more important to adjust the tungsten inert gas welding method to weld the corrugated pipe that is already in a delaminated state. This is the problem that this invention patent aims to solve. Summary of the Invention

[0013] To overcome the inconsistencies in the resistance weld seams of corrugated pipes and the welding difficulties caused by delamination, this invention provides a welding method for corrugated pipes and connecting pipes, which can effectively prevent the corrugated pipes from burning through and ensure the yield rate.

[0014] The objective of this invention is achieved through the following technical solution: a welding method for corrugated pipes and connecting pipes, characterized by comprising the following steps:

[0015] ①. Prepare base material and welding materials: Select one austenitic stainless steel test plate with a thickness of 8 mm to 100 mm. Machining the welding end face into a stepped slope is used for lap splicing with the corrugated pipe. Prepare one multi-layer austenitic stainless steel test plate with resistance weld. Cut along the edge of the resistance weld to make a straight face. Select stainless steel welding materials according to the recommendations of NB / T 47015-2011.

[0016] ②. Assembly: Assemble the stainless steel test plate and the five to ten layers of corrugated pipe test plate into a socket lap welded structure, so that the slope and straight surface of the two components form a semi-V-shaped groove, leaving an assembly gap that can be welded through, and spot weld at both ends of the test plate.

[0017] ③. Welding position: The assembled test plate should be in a vertical position perpendicular to the ground plane, and the weld to be welded should be parallel to the ground plane in the length direction, even if the welding is in a horizontal welding position;

[0018] ④. Tungsten inert gas welding: Use at least two or more layers of welding, and control the interpass temperature within the range of 20℃ to 150℃.

[0019] The austenitic stainless steel described in this invention includes steel grades applicable to the Fe-8-1 steel grade classification group in NB / T 47015-2011, including: S30408, S30403, S31603, and S32168.

[0020] The thickness of the steel plate or forged flange described in this invention is in the range of 8 mm to 90 mm, and the specifications of the multi-layer resistance welded thin steel plate are in the range of (0.4 mm to 0.5 mm) × (six to eight layers).

[0021] In the preparation work described in this invention, the socket slope angle of the austenitic stainless steel test plate is 45±2.5°, and the step depth is the thickness of the multi-layer corrugated pipe; the multi-layer stainless steel test plate is kept in a straight state after the resistance weld side cut.

[0022] The welding wire grade selected in the preparation work described in this invention is matched with the base material, including S308, S308L, S316L, and S321, with a diameter of φ2.0 mm or φ2.5 mm.

[0023] In the second assembly operation of this invention, the assembly gap is determined to be 2.5 mm to 3.0 mm, and spot welding is performed within a range of 20 mm from both ends of the test plate, with a weld length of 10 mm to 15 mm.

[0024] In the placement of the welding position ③ in this invention, a horizontal welding position is adopted.

[0025] In the tungsten inert gas welding (TIG) operation described in this invention, the tungsten electrode diameter is φ3.0mm, the nozzle diameter is φ10mm~12mm, the current is 180A~250A, the shielding gas Ar≥99.99%, and the shielding gas flow rate of the welding torch nozzle is 8L / min~10L / min.

[0026] Compared with existing technologies, the beneficial effects of this invention are: 1) It is no longer the traditional flat welding, but a horizontal welding, with the thick-walled nozzle on top and multiple layers of thin-walled corrugated pipes below. During operation, the electric arc is deflected towards the nozzle side and away from the corrugated pipe side. The electric arc does not act directly on the corrugated pipe, but the corrugated pipe is melted by the downward flow of molten iron. This allows for edge fusion and prevents the electric arc from directly burning through the corrugated pipe. 2) During welding, the electric arc is deflected away from the corrugated pipe, and the filler metal is used to minimize pressure on the resistance weld, thus avoiding weld build-up on the resistance weld and reducing heat input to the corrugated pipe. 3) Leaving a certain gap on the slope and during assembly is beneficial for root penetration fusion. The socket welding structure is a butt joint weld, which is the most ideal stress state. 4) Multi-layer welding not only improves the density of the weld but also disperses the heat input to the resistance weld of the corrugated pipe. This overcomes the defect of incomplete resistance welds and prevents the multi-layer corrugated pipe from burning through. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the welding of the bellows and the connecting pipe of the present invention;

[0028] Figure 2 This is a macroscopic metallographic image of the welded joint in an embodiment of the present invention.

[0029] Among them, 1-steel plate test plate; 2-argon arc weld; 3-welding gun; 4-resistance weld; 5-multi-layer thin plate test plate; 6-metallographic structure of steel plate test plate; 7-metallographic structure of argon arc weld; 8-metallographic structure of resistance weld; 9-metallographic structure of multi-layer thin plate test plate.

[0030] The steel plate test plate 1 and steel plate test plate metallographic 6, argon arc weld 2 and argon arc weld metallographic 7, resistance weld 4 and resistance weld metallographic 8, multi-layer thin plate test plate 5 and multi-layer thin plate test plate metallographic 9 mentioned here are corresponding to each other and refer to the same parts.

[0031] Macroscopic metallographic analysis revealed complete fusion of the tungsten inert gas (TIG) weld seam, with no welding defects found in the weld metal; however, the outermost and second-outermost, and innermost and second-innermost, layers of the 0.5 mm × seven-layer resistance weld seam were not fused together. The focus of this investigation is on the TIG weld seam, not the resistance weld seam.

[0032] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Detailed Implementation

[0033] Example 1

[0034] like Figure 1 As shown, the welding process applied to bellows and connecting pipes in Example 1 includes the following steps:

[0035] The first step is to prepare the base material and welding materials.

[0036] Select a 12 mm thick S30408 ​​austenitic stainless steel plate test plate 1 and machine its welding end face into a socket-type stepped slope using mechanical processing methods; prepare a 0.5 mm × seven-layer S30408 ​​austenitic stainless steel multilayer thin plate test plate 5 with resistance seam weld 4, cut the edge of the resistance seam 4 into a straight face, and select S308 welding wire for tungsten inert gas welding.

[0037] The second step is to assemble the test boards.

[0038] Stainless steel test plate 1 and seven-layer stainless steel multi-layer thin plate test plate 5 are arranged according to... Figure 1 The assembly shown is a socket-type lap welding structure, which forms a semi-V-shaped groove between the slope and straight surface of the two components. The tungsten inert gas welding torch 3 is used for tack welding at the two ends of the test plate.

[0039] The third step is to position the welding area.

[0040] according to Figure 1 As shown, the assembled test plate is placed in a vertical position (i.e., perpendicular to the ground plane), and the weld to be welded is parallel to the ground plane in the length direction, so that the welding is in a horizontal welding position.

[0041] Step 4, welding

[0042] The welding method is manual tungsten inert gas (TIG) welding. The first pass is a self-fusion weld without filler wire on the semi-V-groove, paying attention to arc deviation and root fusion. The second and third passes use S308 welding wire to fill and cap the semi-V-groove, ultimately forming TIG weld 2. A far-infrared thermometer is used to monitor and control the interpass temperature between 20℃ and 150℃ to prevent overheating of the weld and heat-affected zone.

[0043] In the above embodiment, in the first step of preparation, the welding end of the 12 mm thick S30408 ​​austenitic stainless steel test plate 1 is machined into a 45±2.5° slope with a step depth of 3.5 mm; the resistance weld 4 side of the 0.5 mm × seven-layer S30408 ​​austenitic stainless steel multilayer thin plate test plate 5 is cut and kept in a straight state. The diameter of the selected S308 welding wire is φ2.0 mm.

[0044] In the second step of assembly, the gap between the test plates is 2.5 mm. Position welding is performed 20 mm from each end of the test plate, and the weld length is 10 mm to 15 mm.

[0045] In the third step of positioning the welding plate, the test plate is placed in a horizontal welding position.

[0046] In the fourth step of the welding operation, the tungsten inert gas (TIG) welding uses a tungsten electrode with a diameter of φ3.0mm, a nozzle diameter of φ10mm~12mm, a current of 150A~220A, a shielding gas of Ar≥99.99%, and a welding torch nozzle flow rate of 8L / min~10L / min (without back shielding gas).

[0047] The welding process parameters for the fourth step are shown in Table 1.

[0048] Table 1 Welding process parameters for Example 1

[0049]

[0050] To evaluate the mechanical properties of the welded joints obtained using the welding method of this invention, tensile specimens were prepared from the welded joints of Example 1 according to the requirements of NB / T 47014-2011 "Welding Procedure Qualification for Pressure Equipment" and tensile tests were conducted. The tensile strength of the welded joints obtained from the tests is shown in Table 2, and the results are satisfactory.

[0051] Table 2 Tensile strength of the welded joint in Example 1

[0052]

[0053] To evaluate and inspect the weld formation, the macroscopic metallographic morphology of the welded joint was fabricated. Figure 2The metallographic image 6 of the steel plate test piece and the metallographic image 9 of the multi-layer thin plate test piece show that the weld connecting them is an argon arc weld (metallographic image 7). The argon arc weld (metallographic image 7) shows that the weld is fully fused, without any welding defects such as incomplete fusion, incomplete penetration, or porosity. The metallographic image 8 of the resistance weld of the multi-layer thin plate test piece shows that the outermost and second-outermost layers are not fused. The resistance weld is only a process weld and is not the object of further investigation.

[0054] Example 2

[0055] like Figure 1 As shown, the welding process applied to the bellows and nozzle in Example 2 includes the following steps:

[0056] The first step is to prepare the base material and welding materials.

[0057] Select a 12 mm thick S30408 ​​stainless steel test plate 1 and machine its welded end face into a socket-type 45±2.5° slope with a step depth of 3.0 mm. Prepare a 0.5 mm × six-layer S30408 ​​austenitic stainless steel multilayer thin plate test plate 5 with resistance weld 4, and cut the edge of the resistance weld 4 into a straight face. Select S308 welding wire with a diameter of φ2.0 mm.

[0058] The second step is to assemble the test boards.

[0059] Stainless steel test plate 1 and six-layer stainless steel multi-layer thin plate test plate 5 are assembled into a socket lap welding structure, so that the slope and straight surface of the two components form a semi-V-shaped groove with an assembly gap of 2.5 mm. Tungsten inert gas welding torch 3 is used to perform tack welding at a distance of 20 mm from each end of the test plate, with a weld length of 10 mm to 15 mm.

[0060] The third step is to position the welding area.

[0061] Position the test plate in the horizontal welding position.

[0062] Step 4, welding

[0063] The semi-V-groove was subjected to one pass of autofusion welding using manual tungsten inert gas (TIG) welding, followed by two passes of filler wire welding, ultimately forming TIG weld 2. Welding process parameters are shown in Table 3, and the tensile strength of the welded joint is shown in Table 4.

[0064] Table 3 Welding process parameters for Example 2

[0065]

[0066] Table 4 Tensile strength of the welded joint in Example 2

[0067]

[0068] Example 3

[0069] like Figure 1 As shown, the welding process applied to the bellows and nozzle in Example 3 includes the following steps:

[0070] The first step is to prepare the base material and welding materials.

[0071] Select a 12 mm thick S30408 ​​austenitic stainless steel test plate 1. Machining its weld end face into a socket-type 45±2.5° slope with a step depth of 4.0 mm is used. Prepare a 0.5 mm × eight-layer S30408 ​​austenitic stainless steel multilayer thin plate test plate 5 with resistance weld 4, and cut the edge of resistance weld 4 into a straight face. Select S308 welding wire with a diameter of φ2.5 mm.

[0072] The second step is to assemble the test boards.

[0073] Stainless steel test plate 1 and eight-layer stainless steel multi-layer thin plate test plate 5 are assembled into a socket-type lap welding structure, so that the slope and straight surface of the two components form a semi-V-shaped groove with an assembly gap of 3.0 mm. Tungsten inert gas welding torch 3 is used to perform tack welding at a distance of 20 mm from each end of the test plate, with a weld length of 10 mm to 15 mm.

[0074] The third step is to position the welding area.

[0075] Position the test plate in the horizontal welding position.

[0076] Step 4, welding

[0077] The semi-V-groove was subjected to one pass of autofusion welding using manual tungsten inert gas (TIG) welding, followed by two passes of filler wire welding, ultimately forming TIG weld 2. Welding process parameters are shown in Table 5, and the tensile strength of the welded joint is shown in Table 6.

[0078] Table 5 Welding process parameters for Example 3

[0079]

[0080] Table 6 Tensile strength of the welded joint in Example 3

[0081]

Claims

1. A method for welding a corrugated pipe and a connecting pipe, characterized in that: It includes the following steps: ①. Prepare base material and welding materials: Select one austenitic stainless steel test plate with a thickness of 8mm-100mm. Machining its welding end face into a stepped slope is used for spigot and lap joint with the austenitic stainless steel multilayer corrugated pipe test plate. Prepare one austenitic stainless steel multilayer corrugated pipe test plate with resistance weld. Cut along the edge of the resistance weld to make a straight face. Select stainless steel welding materials according to the recommendations of NB / T47015-2011. ②. Assembly: Assemble the stainless steel test plate and five to ten layers of corrugated pipe test plates with a thickness of 0.4mm-0.5mm into a socket-lap welded structure, so that the slope and straight surface of the two components form a semi-V-shaped groove, leaving an assembly gap that allows for full weld penetration. Spot weld at both ends of the assembly gap; during assembly, place the stainless steel test plate on top and the corrugated pipe test plate on the bottom; the socket slope angle of the austenitic stainless steel test plate is 45±2.5°, and the step depth is the thickness of the corrugated pipe test plate; after the resistance weld side cut of the corrugated pipe test plate, keep it in a straight state; the assembly gap at the root of the austenitic stainless steel test plate and the corrugated pipe test plate is 2.5mm-3.0mm; ③. Welding position: The assembled test plate should be in a vertical position perpendicular to the ground plane, and the weld to be welded should be parallel to the ground plane in the length direction, even if the welding is in a horizontal welding position; ④. Tungsten inert gas (TIG) welding: Multi-layer welding with two or more layers is adopted, and the interpass temperature is controlled within the range of 20℃ to 150℃. During welding, the arc is biased towards the stainless steel test plate side and away from the corrugated pipe test plate side, so that the arc does not act directly on the corrugated pipe test plate. Instead, the corrugated pipe test plate is melted by the downward flow of molten iron. This can both fuse the edges and prevent the arc from directly burning through the corrugated pipe.

2. The welding method for the bellows and the connecting pipe according to claim 1, characterized in that: The austenitic stainless steel test plate is applicable to steel grades with the steel grade classification group number Fe-8-1 in NB / T47015-2011, including: S30408, S30403, S31603, and S32168.

3. The welding method for the corrugated pipe and the connecting pipe according to claim 1, characterized in that: The thickness of the austenitic stainless steel test plate is in the range of 8mm-90mm, and the number of layers of the austenitic stainless steel multilayer corrugated pipe test plate is six to eight.

4. The welding method for the bellows and the connecting pipe according to claim 1, characterized in that: The tungsten inert gas (TIG) welding used has a tungsten electrode diameter of Φ3.0mm, a nozzle diameter of Φ10mm-12mm, a current of 150A-220A, a shielding gas of Ar≥99.99%, and a flow rate of 8L / min-10L / min.

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