FBE internal coating steel pipe fitting with composite corrosion-resistant alloy connector

Abstract

The utility model provides an FBE inner coating steel pipe fitting with a composite corrosion-resistant alloy joint, which comprises a steel base pipe, a base pipe inner protection layer is arranged on the inner wall of the steel base pipe, the base pipe inner protection layer comprises an FBE coating section, a lap joint section of the FBE coating section and a corrosion-resistant alloy lining pipe section, and the corrosion-resistant alloy lining pipe section, the corrosion-resistant alloy lining pipe section is attached to the inner wall of the pipe end of the steel base pipe; the corrosion-resistant alloy lining pipe section comprises a corrosion-resistant alloy lining pipe, a plurality of welding groove holes which are evenly distributed in the circumferential direction are formed in the corrosion-resistant alloy lining pipe, and the corrosion-resistant alloy lining pipe and the steel base pipe are connected through linear composite welding seams in the welding groove holes. According to the utility model, the problem of firm combination between layers of the double-layer composite pipe in the prior art is solved.

Description

Technical Field

[0001] This utility model relates to the technical field of offshore oil and gas extraction, and in particular to an FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint. Background Technology

[0002] Pipeline systems are indispensable for offshore oil and gas extraction facilities, and pipe fittings are an important component of these systems. FPSO and other offshore oil and gas extraction equipment are very expensive, have high operating costs, and require extremely stringent safety and reliability standards for pipelines.

[0003] Since the media transported by offshore oil and gas pipeline systems are mostly highly corrosive, the pipeline systems used must be protected against internal corrosion. FBE coating is the most effective, reliable, and cost-effective form of pipeline corrosion protection, and it is also the most widely used. However, FBE coating also has drawbacks. During pipeline welding, the high temperatures generated can damage the coating at the pipe ends. Therefore, FBE-coated pipelines all require protective integrity treatment.

[0004] The main methods to ensure the integrity of FBE coated pipeline protection include: flange connection, welding connection with robot internal patching, inner lining sleeve connection, corrosion-resistant alloy short section connection, and corrosion-resistant alloy overlay welding connection.

[0005] Although flange connections are simple, they significantly increase weight. FPSOs and other offshore oil and gas exploration equipment are expensive and have strict weight control requirements, so flange connections that add extra weight are usually excluded.

[0006] Connecting and welding with robotic internal jointing not only has requirements on pipe diameter, but is also mainly designed for planar straight pipelines, which is obviously not suitable for offshore pipeline systems with a spatial three-dimensional layout containing a large number of bends.

[0007] For inner lining and sliding sleeve connection, see Figure 7 This is a new technology that entered China in the last two decades. Currently, its main application is in gathering and transportation pipelines in onshore oil fields. For offshore oil and gas pipelines, the inner lining of the sliding sleeve has the following disadvantages and uncertainties: it is difficult to meet the tolerance requirements of the fastening dimensions, which poses a risk of leakage; the high temperature of the post-weld heat treatment may damage the surface of the sleeve, making it impossible to carry out the corresponding inspection; the welding slag produced by welding may enter the gap between the sealing ring and the weld, which may lead to the failure of RT inspection; the design of the sleeve and insulation layer leads to an excessive reduction in the inner diameter of the pipeline.

[0008] The corrosion-resistant alloy short section connection method involves welding a corrosion-resistant alloy pipe short section with the same wall thickness and inner / outer diameter as the base pipe to both ends of the base pipe. Then, an overall FBE (Frost-Resistant Epoxy) inner coating is applied. During on-site installation, the connection welding is performed directly. The corrosion-resistant alloy section serves to prevent corrosion and block welding heat. This method is relatively simple, but its disadvantages include the large amount of corrosion-resistant alloy used and high cost.

[0009] Corrosion-resistant alloy surfacing connection involves welding a corrosion-resistant alloy layer onto the inner wall of a section at one end of the base pipe. Compared to corrosion-resistant alloy short section connections, this method uses less corrosion-resistant alloy and is less expensive, but its disadvantage is low production efficiency. For example, it takes approximately 10 hours (excluding stress-relief heating process) to surface a 100mm long, 3-4mm thick corrosion-resistant alloy layer at both ends of a base pipe with an outer diameter of 406mm.

[0010] Corrosion-resistant alloy-lined composite pipe technology, or simply composite pipe, involves lining a base pipe with a thin-walled corrosion-resistant alloy pipe, sealing both ends to form a double-layer composite pipe with a corrosion-resistant inner pipe and a pressure-bearing outer pipe. This technology has been developed and applied in China for over twenty years, primarily in harsh corrosive environments on the ground, and in surface oil and gas gathering and transportation, and refining projects. Disadvantages include low bonding strength between the lining pipe and the base pipe, making delamination of the inner and outer layers prone to occur when transporting high-pressure media. Patent CN96250817.9 discloses a "double-layer metal corrosion-resistant pipe joint," which proposes a double-layer design of a base pipe + corrosion-resistant alloy-lined pipe, but does not propose a composite method for the inner and outer layers, thus failing to solve the problem of a reliable interlayer bond in double-layer composite pipes.

[0011] Therefore, how to solve the problem of reliable interlayer bonding in double-layer composite pipes has become a technical problem that needs to be addressed. Utility Model Content

[0012] The purpose of this utility model is to provide an FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint, mainly solving the problem of how to reliably bond the layers of a double-layer composite pipe in the prior art. To achieve the above objective, the technical solution adopted by this utility model is: an FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint, characterized in that: the FBE-coated steel pipe fitting includes a steel base pipe, an inner protective layer on the inner wall of the steel base pipe, the inner protective layer including an FBE coating section, an overlap section between the FBE coating section and the corrosion-resistant alloy liner section, and a corrosion-resistant alloy liner section, the corrosion-resistant alloy liner section being fitted to the inner wall of the end of the steel base pipe;

[0013] The corrosion-resistant alloy liner section includes a corrosion-resistant alloy liner, which has multiple circumferentially evenly distributed welding slots. The corrosion-resistant alloy liner and the steel base pipe are connected by a straight composite weld in the welding slots.

[0014] Furthermore, a second outer bevel is provided along the inner edge of the welding slot;

[0015] Furthermore, the distances from both ends of the welding slot to both ends of the corrosion-resistant alloy liner are equal, and the ends of the welding slot are arc-shaped; the second outer bevel is a 45-degree outer bevel.

[0016] Furthermore, the first end of the corrosion-resistant alloy liner extends out of the end of the steel base pipe, and the second end of the corrosion-resistant alloy liner that extends into the steel base pipe is provided with a first bevel. The first bevel, the FBE coated section and the inner wall of the steel base pipe are connected by an inner sealing circumferential weld.

[0017] Furthermore, the longitudinal section of the inner sealing ring weld is inclined in the axial direction, with an inclination angle of 7 to 15 degrees.

[0018] Furthermore, the steel base pipe end is provided with a first outer bevel, which includes a butt welding bevel and a sealing welding bevel, with the butt welding bevel located at the outer edge of the sealing welding bevel;

[0019] The sealing bevel is connected to the first end of the corrosion-resistant alloy liner through an external sealing circumferential weld.

[0020] Furthermore, after grinding the outer sealing ring weld, the angle of the sealing weld bevel superimposed on the outer sealing ring weld coincides with the angle of the butt weld bevel.

[0021] Furthermore, the length of the corrosion-resistant alloy liner is less than the length of the steel base pipe, and the wall thickness of the corrosion-resistant alloy liner is less than the wall thickness of the steel base pipe.

[0022] Furthermore, the angle of the butt weld bevel is 30 degrees, the angle of the sealing weld bevel is 15 degrees, the conical surface of the butt weld bevel and the conical surface of the sealing weld bevel form an intersecting circle, and the distance between the intersecting circle and the inner circle of the steel base pipe is 4mm.

[0023] After grinding the outer sealing circumferential weld, the thickness of the blunt edge left at the end of the corrosion-resistant alloy liner is 2mm.

[0024] Furthermore, the number of welding layers inside the welding slot is 3, and the width of the third layer weld exceeds the width of the welding slot and completely covers the welding slot.

[0025] In view of the above technical features, the present invention has the following beneficial effects:

[0026] 1. This utility model provides an FBE inner-coated steel pipe fitting with a composite corrosion-resistant alloy joint. The corrosion-resistant alloy liner has multiple circumferentially evenly distributed welding slots. The corrosion-resistant alloy liner and the steel base pipe are connected by straight composite welds in the welding slots. The multiple straight composite welds play a major role in connecting and fixing the corrosion-resistant alloy liner and the steel base pipe, solving the problem of reliable interlayer bonding of the double-layer composite pipe.

[0027] 2. The present invention provides an FBE inner-coated steel pipe fitting with a composite corrosion-resistant alloy joint. The inner sealing circumferential weld and the outer sealing circumferential weld together seal the gap between the corrosion-resistant alloy liner and the steel base pipe, and play an auxiliary role in the connection and fixation of the corrosion-resistant alloy liner and the steel base pipe, thereby further improving the reliability of the connection and fixation of the corrosion-resistant alloy liner and the steel base pipe. Attached Figure Description

[0028] Figure 1 This is a schematic diagram (longitudinal section) of an FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint in specific embodiment 1.

[0029] Figure 2 This is a schematic diagram (longitudinal section) of the welded slot structure of the corrosion-resistant alloy liner in specific embodiment 1;

[0030] Figure 3 This is a schematic diagram (cross-section) of the welded slot structure of the corrosion-resistant alloy liner in specific embodiment 1;

[0031] Figure 4 This is a schematic diagram (top view) of the second outer bevel structure of the welding slot in specific embodiment 1;

[0032] Figure 5 This is a schematic diagram (longitudinal section) of the inner sealing weld and bevel overlay weld structure at the first bevel in specific embodiment 1;

[0033] Figure 6 This is a schematic diagram (longitudinal section) of the first outer bevel and outer sealing weld connection structure of the steel base pipe end in specific embodiment 1;

[0034] Figure 7 This is a schematic diagram of the inner lining sliding sleeve structure in the existing technology.

[0035] In the diagram: 1. Steel base pipe; 11. First outer bevel; 111. Butt weld bevel; 112. Sealing weld bevel;

[0036] 2. Inner protective layer of the base pipe;

[0037] 21. FBE coating section;

[0038] 22. The overlap between the FBE coated section and the corrosion-resistant alloy liner section;

[0039] 23. Corrosion-resistant alloy liner section; 231. Inner sealing circumferential weld; 232. Corrosion-resistant alloy liner; 2321. First bevel; 2322. Blunt edge; 233. Outer sealing circumferential weld; 234. Straight composite weld; 235. Welding slot; 2351. Second outer bevel. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0041] See Figures 1 to 6 In specific embodiment 1, this embodiment 1 provides an FBE inner-coated steel pipe fitting with a composite corrosion-resistant alloy joint. The FBE inner-coated steel pipe fitting includes a steel base pipe 1, and an inner protective layer 2 is provided on the inner wall of the steel base pipe 1. The inner protective layer 2 includes an FBE coating section 21, an overlap section 22 between the FBE coating section 21 and the corrosion-resistant alloy liner section 23, and a corrosion-resistant alloy liner section 23. The corrosion-resistant alloy liner section 23 is attached to the inner wall of the pipe end of the steel base pipe 1, that is, the corrosion-resistant alloy liner section 23 is located at the end of the steel base pipe 1.

[0042] The corrosion-resistant alloy liner section 23 includes a corrosion-resistant alloy liner 232, which has a plurality of circumferentially evenly distributed welding slots 235. The corrosion-resistant alloy liner 232 and the steel base pipe 1 are connected by a straight composite weld 234 in the welding slots 235.

[0043] The first end of the corrosion-resistant alloy liner 232 extends out of the end of the steel base pipe 1, and the second end of the corrosion-resistant alloy liner 232 extending into the steel base pipe 1 is provided with a first bevel 2321. The first bevel 2321, the FBE coated section 21, and the inner wall of the steel base pipe 1 are connected by an inner sealing circumferential weld 231. In this embodiment 1, the first bevel 2321 is located between the FBE coated section 21 and the steel base pipe 1, and together with the two, forms a triangular space for the inner sealing circumferential weld 231 to exist.

[0044] The steel base pipe 1 has a first outer bevel 11 at its end. The first outer bevel 11 includes a butt welding bevel 111 and a sealing welding bevel 112. That is, before the outer sealing circumferential weld 233 is welded, the butt welding bevel 111 and the sealing welding bevel 112 form a double bevel. The butt welding bevel 111 is located at the outer edge of the sealing welding bevel 112. The sealing welding bevel 112 is connected to the first end of the corrosion-resistant alloy liner 232 through the outer sealing circumferential weld 233. After the outer sealing circumferential weld 233 is welded, after grinding the outer sealing circumferential weld 233, the angle of the sealing welding bevel 112 superimposed on the outer sealing circumferential weld 233 coincides with the angle of the butt welding bevel 111.

[0045] In this embodiment 1, the corrosion-resistant alloy liner 232 and the steel base pipe 1 are connected as a whole by a straight composite weld 234, an inner sealing circumferential weld 231, and an outer sealing circumferential weld 233. The multiple straight composite welds 234 play a major role in connecting and fixing the corrosion-resistant alloy liner 232 and the steel base pipe 1. The inner sealing circumferential weld 231 and the outer sealing circumferential weld 233 together seal the gap between the corrosion-resistant alloy liner 232 and the steel base pipe 1, and play an auxiliary role in connecting and fixing them.

[0046] The inner edge of the welding slot 235 is provided with a second outer bevel 2351;

[0047] The distances from both ends of the welding slot 235 to both ends of the corrosion-resistant alloy liner 232 are equal, and the ends of the welding slot 235 are arc-shaped; the second outer bevel 2351 is a 45-degree outer bevel (i.e., a 45° outer bevel).

[0048] The longitudinal section of the inner sealing ring weld 231 is inclined in the axial direction, with an inclination angle of 7 to 15 degrees (i.e., the inclination angle of the inclination is 7° to 15°).

[0049] The length of the corrosion-resistant alloy liner 232 is less than the length of the steel base pipe 1, and the wall thickness of the corrosion-resistant alloy liner 232 is less than the wall thickness of the steel base pipe 1.

[0050] This embodiment 1 describes an FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint. The specific processing steps are as follows: Taking the API 5L X60 seamless steel straight pipe fitting commonly used in offshore oil engineering as an example, this pipe fitting (i.e., the steel base pipe 1) has an outer diameter of 406.4 mm, a wall thickness of 12.7 mm, and a length of 500 mm. The corrosion-resistant alloy section designed on the inner wall of the pipe end has a length of 100 mm on each side. The corrosion-resistant alloy liner 232 is made of Inconel 625 nickel-based corrosion-resistant alloy. The weld overlay thickness of the corrosion-resistant alloy liner 232 is 4 mm ± 0.5 mm.

[0051] Step 1: Prepare the steel base pipe 1 and the corrosion-resistant alloy liner 232 that matches the inner diameter of the steel base pipe 1.

[0052] Specifically, Inconel 625 nickel-based corrosion-resistant alloy tubing was prepared as corrosion-resistant alloy liner 232, with the following specifications: outer diameter 380.6mm, wall thickness 4mm, and single length 100mm.

[0053] Step 2: Machining of composite weld slots (i.e., weld slots 235) and bevels (i.e., second outer bevels 2351): Machining several circumferentially evenly distributed weld slots 235 on the corrosion-resistant alloy liner 232, and machining the second outer bevel 2351 along the inner edge of the weld slots. Machining the first bevel 2321 at one end of the corrosion-resistant alloy liner 232 (i.e., the second end of the corrosion-resistant alloy liner 232), and the other end (i.e., the first end of the corrosion-resistant alloy liner 232) is a flat end.

[0054] Specifically, eight circumferentially evenly distributed welding slots 235 are machined on the corrosion-resistant alloy liner 232. Each welding slot 235 is 12mm wide and 60mm long. The distances from both ends of the welding slot 235 to both ends of the corrosion-resistant alloy liner 232 are equal, and the ends of the welding slot 235 are arc-shaped. A 45-degree outer bevel (i.e., the second outer bevel 2351) is machined along the inner edge of the welding slot. A 45-degree outer bevel (i.e., the first bevel 2321) is machined at one end of the corrosion-resistant alloy liner 232 (i.e., the second end of the corrosion-resistant alloy liner 232), without leaving a blunt edge 2322; the other end (i.e., the first end of the corrosion-resistant alloy liner 232) is a flat end. See also... Figure 2 , Figure 3 , Figure 4 .

[0055] The third step is to use a roundness calibration machine to calibrate the roundness of the corrosion-resistant alloy liner 232, ensuring that the ellipticity does not exceed 0.2%.

[0056] Step 4: Process the beveling of the steel base pipe 1 (i.e., the first outer beveling 11). The first outer beveling 11 consists of two parts: 1) a butt-welding beveling 111 located on the outer edge of the end face of the steel base pipe 1, with an angle of 30 degrees; and 2) a sealing weld beveling 112 located on the inner edge of the end face of the steel base pipe 1, with an angle of 15 degrees. The angle of the butt-welding beveling 111 is greater than the angle of the sealing weld beveling 112. The conical surfaces of the butt-welding beveling 111 and the sealing weld beveling 112 form an intersecting circle, and the distance between the intersecting circle and the inner circular surface of the steel base pipe 1 is 4 mm.

[0057] Step 5: Use an internal cylindrical grinder to bore and grind the corrosion-resistant alloy liner sections 23 at both ends of the steel base pipe 1, so that their roundness, surface roughness, and surface cleanliness meet the requirements for the installation of the corrosion-resistant alloy liner. For example, the ellipticity of the corrosion-resistant alloy liner section 23 should not exceed 0.01%, and the surface roughness Ra should be 1.0 μm.

[0058] Step 6: Heat the entire steel base pipe 1 to 100-200℃.

[0059] Step 7: Apply a high-temperature resistant lubricant to the outer wall of the corrosion-resistant alloy liner 232, and then insert it into one end of the steel base pipe 1, so that the end of the corrosion-resistant alloy liner 232 protrudes from the end of the steel base pipe 1 by about 3 to 15 mm, for example, the end of the liner protrudes from the end of the base pipe by about 8 mm.

[0060] Step 8: After the base pipe cools to room temperature, perform external sealing welding using welding wire compatible with the corrosion-resistant alloy liner 232, such as using N06625 welding wire. The welding type is argon arc welding. The welding machine model is FIT-VCS1400-1200-12000 argon arc welding machine.

[0061] Step 9: Perform inner sealing welding and inner sealing bevel surfacing welding. The welding wire and welding machine are the same as in step 8. For details, please refer to [link / document / reference]. Figure 6 Then, the inner sealing bevel is welded and polished to make the bevel smooth and flat. The angle of the polished inner sealing bevel is 8 to 15 degrees.

[0062] Step 10: Perform composite welding of the corrosion-resistant alloy liner 232 and the steel base pipe 1, i.e., weld in the composite weld slot (i.e., welding slot 235) to connect the steel base pipe 1 and the corrosion-resistant alloy liner 232 into one unit. At this stage, the welding wire and welding machine are the same as in step 8. The number of welding layers in the welding slot 235 is 3. The width of the third weld layer exceeds the width of the welding slot 235 and completely covers it.

[0063] Step 11: Perform external sealing weld at the position corresponding to the sealing weld bevel 112 at the end of the steel base pipe 1. The welding wire and welding machine are the same as in Step 8. After completing the external sealing weld, re-grind the bevel on the weld body. The ground bevel (the angle here is the angle after superimposing the sealing weld bevel 112 onto the external sealing circumferential weld 233) coincides with the butt weld bevel 111 of the steel base pipe 1. After grinding the bevel, the thickness of the blunt edge 2322 left at the end of the corrosion-resistant alloy liner is 2mm.

[0064] Step 12: Repeat steps 1 through 11 to process the other end of the steel base pipe 1.

[0065] Step 13: Grind and inspect all weld surfaces.

[0066] Step 14: Perform stress-relieving heat treatment on the weld to form a pipe fitting with a corrosion-resistant alloy liner (i.e., with a corrosion-resistant alloy liner 232). Generally, the two ends of the steel base pipe 1 that need to be welded together are called joints. In this step, "joint with a corrosion-resistant alloy liner" means that the end of the steel base pipe 1 that serves as the joint has a corrosion-resistant alloy liner 232.

[0067] Step 15: Apply FBE coating to the inner wall of the pipe fitting with the corrosion-resistant alloy liner 232 joint.

[0068] After the above steps, the processing of the composite corrosion-resistant alloy joint and the FBE inner coating of the Φ406mm steel base pipe 1 were completed, forming a steel pipe fitting with an outer diameter of Φ406mm and a composite corrosion-resistant alloy joint. Here, "composite" refers to the two-layer pipe wall, as opposed to a single-layer pipe. Therefore, the industry commonly refers to it as a composite pipe. The entire processing took 6 hours. Compared to the processing of a fully butt-welded corrosion-resistant alloy joint, the cost is comparable, but the time is saved by 4 hours, increasing processing efficiency by 40%.

[0069] In this embodiment 1, an FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint is based on a double-layer composite pipe design. Through a composite weld design (i.e., a straight composite weld, an inner sealing circumferential weld, and an outer sealing circumferential weld), the problem of poor bonding strength between the corrosion-resistant alloy liner 232 and the steel base pipe 1 is first solved. Then, the composite weld corrosion-resistant alloy liner 232 is applied to the end of the FBE-coated pipe, forming an FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint. Compared to corrosion-resistant alloy short sections and corrosion-resistant alloy overlay welding technology, this FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint in embodiment 1 reduces costs and improves production efficiency while ensuring the integrity of pipeline protection.

[0070] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0071] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0072] The above description is only a preferred embodiment of the present utility model and does not limit the patent scope of the present utility model. Any equivalent structural or procedural transformations made based on the content of the present utility model specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present utility model.

Claims

1. A steel pipe fitting with an FBE inner coating and a composite corrosion-resistant alloy joint, characterized in that: The FBE inner-coated steel pipe fitting includes a steel base pipe (1), and the inner wall of the steel base pipe (1) is provided with an inner protective layer (2). The inner protective layer (2) includes an FBE coating section (21), an overlap section (22) between the FBE coating section and the corrosion-resistant alloy liner section (23), and a corrosion-resistant alloy liner section (23). The corrosion-resistant alloy liner section (23) is attached to the inner wall of the pipe end of the steel base pipe (1). The corrosion-resistant alloy liner section (23) includes a corrosion-resistant alloy liner (232), which has multiple circumferentially evenly distributed welding slots (235). The corrosion-resistant alloy liner (232) and the steel base pipe (1) are connected by a straight composite weld (234) in the welding slots (235).

2. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 1, characterized in that: The inner edge of the welding slot (235) is provided with a second outer bevel (2351).

3. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 2, characterized in that: The distances from both ends of the welding slot (235) to both ends of the corrosion-resistant alloy liner (232) are equal, and the end of the welding slot (235) is arc-shaped; the second outer bevel (2351) is a 45-degree outer bevel.

4. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 2 or 3, characterized in that: The first end of the corrosion-resistant alloy liner (232) extends out of the end of the steel base pipe (1), and the second end of the corrosion-resistant alloy liner (232) extending into the steel base pipe (1) is provided with a first bevel (2321). The first bevel (2321), the FBE coated section (21) and the inner wall of the steel base pipe (1) are connected by an inner sealing circumferential weld (231).

5. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 4, characterized in that: The longitudinal section of the inner sealing ring weld (231) is inclined in the axial direction, with an inclination angle of 7 to 15 degrees.

6. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 5, characterized in that: The steel base pipe (1) has a first outer bevel (11) at its end. The first outer bevel (11) includes a butt welding bevel (111) and a sealing welding bevel (112). The butt welding bevel (111) is located at the outer edge of the sealing welding bevel (112). The sealing bevel (112) is connected to the first end of the corrosion-resistant alloy liner (232) by an outer sealing circumferential weld (233).

7. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 6, characterized in that: After grinding the outer sealing ring weld (233), the angle of the sealing weld bevel (112) superimposed on the outer sealing ring weld (233) coincides with the angle of the butt weld bevel (111).

8. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 7, characterized in that: The length of the corrosion-resistant alloy liner (232) is less than the length of the steel base pipe (1), and the wall thickness of the corrosion-resistant alloy liner (232) is less than the wall thickness of the steel base pipe (1).

9. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 8, characterized in that: The angle of the butt weld bevel (111) is 30 degrees, the angle of the sealing weld bevel (112) is 15 degrees, the conical surface of the butt weld bevel (111) and the conical surface of the sealing weld bevel (112) form an intersecting circle, and the distance between the intersecting circle and the inner circle of the steel base pipe (1) is 4 mm. After grinding the outer sealing ring weld (233), the thickness of the blunt edge (2322) left at the end of the corrosion-resistant alloy liner (232) is 2mm.

10. The FBE-coated steel pipe fitting with a composite corrosion-resistant alloy joint according to claim 9, characterized in that: The number of welding layers inside the welding slot (235) is 3. The width of the third layer weld exceeds the width of the welding slot (235) and completely covers the welding slot (235).

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