A method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes

Abstract

This invention discloses a method for evaluating the corrosion resistance of welds in mechanical nickel-based alloy bimetallic composite pipes. The method includes corrosion resistance testing and evaluation of both straight and circumferential welds in the mechanical nickel-based alloy bimetallic composite pipe. The straight weld corrosion resistance testing and evaluation includes: sample preparation; intergranular corrosion testing; SSC resistance testing; and corrosion resistance testing of the straight weld under simulated corrosion conditions. The circumferential weld corrosion resistance testing and evaluation includes: sample preparation; SSC resistance testing; and corrosion resistance testing of the circumferential weld under simulated corrosion conditions. This evaluation method can simultaneously perform tests on both straight and circumferential welds, conducting corrosion resistance tests on both under simulated actual corrosion conditions and performing environmental cracking evaluation. It accurately reflects the corrosion resistance performance under actual working conditions and comprehensively reflects the corrosion resistance of the welds in the mechanical nickel-based alloy bimetallic composite pipe. The evaluation of the welds is targeted and systematic.

Description

Technical Field

[0001] This invention relates to the field of metal material performance testing technology, specifically to a method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes. Background Technology

[0002] During the development of high-sulfur gas fields, perforation of oil casing and pipes and corrosion leaks in surface gathering and transportation pipelines occur frequently. Corrosion-resistant alloys are a reliable method for preventing and controlling corrosion of oil and gas field pipelines, but they are expensive and uneconomical. To increase the reliability of anti-corrosion measures while reducing the cost of corrosion-resistant alloy pipes, a possible approach is to coat ordinary gathering and transportation pipelines with a thin-walled corrosion-resistant alloy layer, forming a bimetallic composite pipe with both ends welded in the field. Based on the bimetallic interface bonding method, these are classified into mechanical composite pipes and metallurgical composite pipes. Mechanical composite pipes refer to pipes where the corrosion-resistant alloy layer is inserted into the carbon steel base pipe through mechanical expansion, extension, or installation methods. The alloy layer and carbon steel are only in mechanical contact. Mechanical composite pipes have a relatively simple process, are cheaper, and are suitable for large-scale use.

[0003] For surface gathering and transportation composite pipelines, welding of the composite pipes is inevitable. The weld seam is typically the weakest point in a corrosive environment. Bimetallic composite pipes used in high-acid gas fields generally have a nickel-based alloy cladding, which is difficult to weld. Furthermore, the welding of composite pipes involves dissimilar steels, further increasing the difficulty. Because the chemical composition and mechanical properties of the corrosion-resistant layer and the base metal differ, inhomogeneity occurs at the weld seam and interface, potentially leading to cracks; poor distribution of welding residual stress can also cause cracks. Due to differences in linear expansion coefficients, residual stress remains even after heat treatment. During welding, the two different metals mix, forming a low-plasticity micro-quenched structure within the weld seam, resulting in decreased corrosion resistance. The use of bimetallic composite pipes in corrosive environments also depends on the corrosion resistance of the welded areas. Once corrosion occurs at the weld seam, it can cause natural gas leaks, affecting normal production and potentially leading to other safety accidents. Therefore, testing the corrosion resistance of weld seams in mechanically nickel-based alloy bimetallic composite pipes is crucial.

[0004] Currently, the evaluation of the corrosion resistance of bimetallic composite pipes is mainly achieved by evaluating the corrosion resistance of the cladding alloy. Existing technologies for evaluating the corrosion resistance of the alloy body and welds suffer from two main problems: First, these evaluations are primarily conducted indoors, making it impossible to accurately assess the weld's corrosion resistance under actual corrosive conditions. Second, they mainly involve taking a single pipe section sample and conducting various tests such as weight loss and stress corrosion cracking. However, this sampling and testing method only represents the overall corrosion resistance of the composite pipe, and the obtained sample section typically only contains circumferential or straight welds, failing to accurately and comprehensively reflect the weld's corrosion resistance. Furthermore, when bimetallic composite pipes are used in sulfur-containing, highly acidic gas fields, the environmental cracking of the welds must also be evaluated.

[0005] In summary, existing methods for evaluating the corrosion resistance of welds in mechanical nickel-based alloy bimetallic composite pipes cannot accurately and comprehensively reflect the corrosion resistance of welds under actual corrosive conditions. Summary of the Invention

[0006] The technical problem to be solved by this invention is that existing methods for evaluating the corrosion resistance of welds in mechanical nickel-based alloy bimetallic composite pipes cannot accurately and comprehensively reflect the corrosion resistance of welds under actual corrosive conditions. The purpose is to provide a method for evaluating the corrosion resistance of welds in mechanical nickel-based alloy bimetallic composite pipes to solve the above problems.

[0007] The purpose of this invention is to provide a method for evaluating the corrosion resistance of welds in mechanical nickel-based alloy bimetallic composite pipes, including corrosion resistance testing and evaluation of straight welds and circumferential welds in mechanical nickel-based alloy bimetallic composite pipes.

[0008] The corrosion resistance test evaluation of the straight weld seam includes: preparation of test specimens for the straight weld seam; intergranular corrosion test of the straight weld seam; SSC resistance test of the straight weld seam; corrosion resistance test of the straight weld seam under simulated corrosion conditions.

[0009] The corrosion resistance test evaluation of the circumferential weld includes: preparation of test specimens for circumferential weld testing; SSC resistance performance test; and corrosion resistance performance test of the circumferential weld under simulated corrosion conditions.

[0010] In an optional embodiment, the corrosion resistance testing and evaluation process for the straight weld seam includes the following steps:

[0011] S11, Intergranular corrosion test of straight weld seams;

[0012] S12, SSC resistance performance test of straight weld seam;

[0013] S13, Corrosion resistance test of straight weld seams under simulated corrosion conditions;

[0014] If any step in the evaluation of the straight weld seam fails, the test is terminated, and the corrosion resistance test of the weld seam of the mechanical nickel-based alloy bimetallic composite pipe fails.

[0015] In an optional embodiment, the corrosion resistance test and evaluation process for the circumferential weld includes the following steps:

[0016] S21, SSC resistance performance test;

[0017] S22, Corrosion resistance test of circumferential weld under simulated corrosion conditions;

[0018] If any step in the evaluation process of the circumferential weld fails, the test is terminated, and the corrosion resistance test of the mechanical nickel-based alloy bimetallic composite pipe weld fails.

[0019] In an optional embodiment, the method for testing intergranular corrosion of the straight weld seam is as follows:

[0020] The prepared and treated straight weld seam test evaluation sample is embedded in copper shot or a copper abrasive rod, completely immersed in an acidified copper sulfate solution, heated to boiling, and maintained at boiling for 20 h ± 5 h; then the sample is removed.

[0021] If there is residual copper on the removed sample, quickly immerse it in a concentrated nitric acid solution at room temperature to remove the copper.

[0022] A bending test was performed on the sample to test whether intergranular corrosion occurred.

[0023] In an optional embodiment, the preparation process of the specimen for straight weld seam testing and evaluation is as follows:

[0024] Sampling was performed using sawing or flame cutting methods; two samples were taken from the corrosion-resistant alloy layer of the mechanical nickel-based alloy bimetallic composite pipe, and one sample including the straight weld was taken from the weld area.

[0025] The specimen is either a transverse specimen perpendicular to the axial direction of the steel pipe or a longitudinal specimen parallel to the axial direction of the steel pipe.

[0026] In an optional embodiment, the transverse specimen has a length ≥ 50 mm and a width of 20 ± 10 mm; the longitudinal specimen has a length ≥ 50 mm.

[0027] In an optional embodiment, the acidified copper sulfate solution is a 16% sulfuric acid-copper sulfate solution;

[0028] The bending test is a reverse bending test. The bending test process is as follows: bend 180° around an axis with a diameter equal to twice the thickness of the sample; the bending axis is perpendicular to the sample.

[0029] In an optional embodiment, the method for testing the SSC resistance performance is as follows: for each test group, take no less than 3 thin-walled weld C-ring specimens, place the C-ring specimens in the C-ring test device, keep the test solution at the test temperature, load the weld stress level to 100% SMYS of the actual strength, maintain for one test cycle, and test its SSC resistance performance.

[0030] In an optional embodiment, the test solution in the anti-SSC performance test is a saturated H2S solution with a mass fraction of 5% NaCl + 0.5% CH3COOH;

[0031] The test temperature was 24±3℃ and the test period was 720 hours;

[0032] The evaluation requirements for passing the SSC resistance test are as follows: the tensile surface of the C-ring sample is examined under a microscope with a magnification of 10x or more, and the tensile surface of the C-ring sample is free from cracks or any surface damage cracks.

[0033] In an optional embodiment, the processing method of the weld C-ring specimen is as follows: cut off the weld joint and flatten the end face of the joint; bore the hole; turn the outer carbon steel layer of the weld; when the outer carbon steel layer is about to be completely removed and the layering of the inner and outer tubes of the mechanical composite pipe is visible, cut off the section of the pipe where the layering can be seen; add a support ring inside the specimen and continue cutting; scribing and cutting.

[0034] In an optional embodiment, the support ring is made of resin material, the outer diameter of the support ring is the same as the inner diameter of the bimetallic composite tube, and the wall thickness of the support ring is greater than 3 mm.

[0035] In an alternative embodiment, the welded joint is cut off using a wire EDM machine, the end face of the joint is flattened on a lathe, and the boring is stopped after the inner wall is smoothed.

[0036] In an optional embodiment, the corrosion resistance test method for straight welds and circumferential welds under simulated corrosion conditions in steps S13 and S22 is as follows:

[0037] For each test group, no fewer than three thin-walled C-ring specimens were taken. The stress level of the weld seam of the specimens was loaded to 100% of the actual strength (SMYS). The C-ring specimens were suspended in an autoclave using an insulated specimen clamp assembly. The test solution in the autoclave was simulated water from the production site, and the partial pressures of H2S and CO2 were consistent with the production partial pressure. The corrosion resistance was tested by maintaining the test temperature for one test cycle.

[0038] In one optional embodiment, the test period is 720 hours, and the test temperature is the highest operating temperature on site.

[0039] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0040] The evaluation method of this invention prepares test samples for straight welds and circumferential welds separately, allowing for simultaneous testing of both. Both straight and circumferential welds are tested for corrosion resistance under simulated actual corrosion conditions, and environmental cracking evaluations are performed. This accurately reflects the corrosion resistance performance under real-world conditions and provides a comprehensive and accurate assessment of the corrosion resistance of the welds in mechanical nickel-based alloy bimetallic composite pipes. The evaluation method of this invention is targeted and systematic, providing a basis for the delivery technical requirements and welding process requirements of mechanical nickel-based alloy bimetallic composite pipes, and ultimately providing reliable and effective data support for the application of mechanical metal composite pipes in acidic gas fields. Attached Figure Description

[0041] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0042] Figure 1 The flowchart illustrates a method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes, as provided in an embodiment of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0044] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that these specific details are not necessary to practice the invention. In other embodiments, well-known structures, circuits, materials, or methods have not been specifically described in order to avoid obscuring the invention.

[0045] Throughout this specification, references to "an embodiment," "an example," or "an example" mean that a particular feature, structure, or characteristic described in connection with that embodiment or example is included in at least one embodiment of the present invention. Therefore, the phrases "an embodiment," "an example," "an example," or "an example" appearing in various places throughout the specification do not necessarily refer to the same embodiment or example. Furthermore, specific features, structures, or characteristics can be combined in one or more embodiments or examples in any suitable combination and / or sub-combination. Moreover, those skilled in the art will understand that the illustrations provided herein are for illustrative purposes and are not necessarily drawn to scale. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0046] Existing methods for evaluating the corrosion resistance of welds in mechanical nickel-based alloy bimetallic composite pipes cannot accurately and comprehensively reflect the corrosion resistance of welds under actual corrosive conditions.

[0047] To address the aforementioned issues, this invention provides a method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes, including corrosion resistance testing and evaluation of straight weld seams and circumferential weld seams.

[0048] The corrosion resistance test evaluation of the straight weld seam includes: preparation of test specimens for the straight weld seam; intergranular corrosion test of the straight weld seam; SSC resistance test of the straight weld seam; corrosion resistance test of the straight weld seam under simulated corrosion conditions.

[0049] The corrosion resistance test evaluation of the circumferential weld includes: preparation of test specimens for circumferential weld testing; SSC resistance performance test; and corrosion resistance performance test of the circumferential weld under simulated corrosion conditions.

[0050] The evaluation method of this invention prepares test samples for straight welds and circumferential welds separately, allowing for simultaneous testing of both. Both straight and circumferential welds are tested for corrosion resistance under simulated actual corrosion conditions, and environmental cracking evaluation is performed. This accurately reflects the corrosion resistance performance under real-world conditions and provides a comprehensive and accurate assessment of the corrosion resistance of welds in mechanical nickel-based alloy bimetallic composite pipes. The evaluation method of this invention offers targeted and systematic evaluation of welds.

[0051] Furthermore, considering that existing methods for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes still follow the evaluation standards for stainless steel, namely, intergranular corrosion testing methods refer to ASTM A262-2015, and SSC resistance and simulated operating conditions testing refer to NACE TM0177-2005, this standard is not applicable to the evaluation of weld seam corrosion resistance of mechanical nickel-based alloy bimetallic composite pipes due to the difference in the structure of the nickel-based alloy corrosion-resistant layer in the pipe compared to stainless steel. It cannot accurately and comprehensively test and evaluate the weld seams. Moreover, when using NACE TM0177-2005 for simulated operating conditions testing, the methods in this standard can only simulate on-site solution media under normal pressure and temperature conditions, and cannot simulate the high-pressure and high-temperature corrosive environment of oil and gas field production sites. Therefore, in this embodiment of the invention, the corrosion resistance testing and evaluation process for straight weld seams includes the following steps:

[0052] S11, Intergranular corrosion test of straight weld seams;

[0053] S12, SSC resistance performance test of straight weld seam;

[0054] S13, Corrosion resistance test of straight weld seams under simulated corrosion conditions;

[0055] In the corrosion resistance evaluation of straight welds, intergranular corrosion, SSC resistance, and simulated actual corrosion conditions are tested sequentially. If any step fails, the test is terminated, and the corrosion resistance test of the weld of the mechanical nickel-based alloy bimetallic composite pipe fails. Compared to using ASTM A262-2015 and NACE TM0177-2005 standards and conducting intergranular corrosion, SSC resistance, and simulated field condition tests according to both standards, this embodiment of the invention first performs the intergranular corrosion test, and stops the test if any step fails. This greatly shortens the testing and evaluation time and efficiency, reduces costs, and makes the testing and evaluation more targeted.

[0056] Furthermore, the corrosion resistance testing and evaluation process for the circumferential weld includes the following steps:

[0057] S21, SSC resistance performance test;

[0058] S22, Corrosion resistance test of circumferential weld under simulated corrosion conditions;

[0059] If any step in the evaluation process of the circumferential weld fails, the test is terminated, and the corrosion resistance test of the mechanical nickel-based alloy bimetallic composite pipe weld fails.

[0060] Similarly, compared to using ASTM A262-2015 and NACE TM0177-2005 standards to conduct intergranular corrosion, SSC resistance, and simulated field conditions tests according to the two standards, the embodiments of the present invention first conduct intergranular corrosion tests, and stop the test if any step fails, which greatly shortens the test evaluation time and efficiency, reduces costs, and makes the test evaluation more targeted.

[0061] Furthermore, considering that ASTM A262-2015 and NACE TM0177-2005 standards are applicable to the corrosion resistance evaluation of stainless steel materials, their test methods are not applicable to mechanical nickel-based alloy bimetallic composite pipes, and the evaluation results obtained will inevitably be inaccurate. Therefore, in this embodiment of the invention, a method suitable for evaluating the corrosion resistance of mechanical nickel-based alloy bimetallic composite pipes has been identified and optimized, specifically as follows:

[0062] The method for testing intergranular corrosion of the straight weld seam is as follows:

[0063] The prepared and treated test and evaluation sample for straight weld seam is embedded in copper shot or copper grinding rod, completely immersed in the prepared acidified copper sulfate solution, heated to boiling, and kept boiling for 20h±5h.

[0064] Remove the sample. If there is any copper residue on the removed sample, quickly immerse it in a concentrated nitric acid solution at room temperature to remove the copper residue.

[0065] A bending test was performed on the sample to test whether intergranular corrosion occurred.

[0066] The evaluation criteria for the absence of intergranular corrosion are as follows: inspect the bent specimen at low magnification to check for cracks. If there is any doubt about the evaluation, the specimen should be inspected under a 100x microscope, and cracks are not allowed.

[0067] The preparation process of the test specimen for the straight weld seam evaluation is as follows:

[0068] Sampling was performed using sawing or flame cutting methods; two samples were taken from the corrosion-resistant alloy layer of the mechanical nickel-based alloy bimetallic composite pipe, and at least one additional sample including a straight weld was taken from the weld area.

[0069] The specimen can be a transverse specimen perpendicular to the axis of the steel pipe or a longitudinal specimen parallel to the axis of the steel pipe. The transverse specimen has a length ≥ 50 mm and a width of 20 ± 10 mm; the longitudinal specimen has a length ≥ 50 mm.

[0070] The pH of the acidified copper sulfate solution is 16% sulfuric acid-copper sulfate solution;

[0071] The bending test is a reverse bending test, and the bending test process is as follows: bend 180° around an axis with a diameter equal to twice the thickness of the sample; the bending axis is perpendicular to the sample.

[0072] The requirements for the treatment of test specimens for straight welds are as follows: when sampling by flame cutting, the cut edges of the specimens shall be machined or ground to remove the sheared edges; the specimens shall be tested in their existing condition, except when flattening is required; the scratches on the specimens shall be removed by grinding with iron-free alumina abrasive with a grit size of 120; before the test, the grease on the specimens shall be removed with acetone, alcohol and gaseous degreasing agent.

[0073] Furthermore, the test method for the SSC resistance performance of straight welds and circumferential welds is as follows: for each test group, take no less than 3 thin-walled weld C-ring specimens, put the C-ring specimens into the C-ring test device, keep the test solution at the test temperature, load the weld stress level of the specimens to 100% SMYS of the actual strength, maintain for one test cycle, test its SSC resistance performance, and evaluate whether it passes the SSC resistance performance test.

[0074] The test solution used in the anti-SSC performance test was a saturated H2S solution with a mass fraction of 5% NaCl + 0.5% CH3COOH.

[0075] The test temperature was 24±3℃ and the test period was 720 hours;

[0076] The evaluation requirements for passing the SSC resistance test are as follows: the tensile surface of the C-ring sample is examined under a microscope with a magnification of 10x or more, and the tensile surface of the C-ring sample is free from cracks or any surface damage cracks.

[0077] Furthermore, considering that the NACE TM0177-2005 standard's SSC test method has a wide size range for the fixed C-ring test, which is not applicable to mechanical nickel-based alloy bimetallic composite pipes, and that the sampling and processing procedure for the fixed C-ring according to the standard involves directly clamping the cut weld joint with a machine tool fixture, turning the carbon steel outer layer of the weld down to the corrosion-resistant alloy layer, and then cutting it off, the corrosion-resistant coating of the bimetallic composite pipe weld is very thin at the root weld zone, and the weld bead is geometrically irregular. The composite pipe also exhibits varying degrees of misalignment after welding, and residual stress exists in both the transverse and longitudinal directions near the weld. This makes sampling of bimetallic composite pipes difficult, resulting in the inability to obtain valid samples or the presence of localized defects, hindering evaluation or leading to biased test results.

[0078] Therefore, in this embodiment of the invention, the processing method of the weld C-ring specimen is improved, specifically as follows:

[0079] The welded joint was cut off using a wire EDM machine, and the end face of the joint was then flattened on a lathe. Due to the irregular weld bead on the inner wall of the composite pipe welded joint, boring was performed to smooth the inner wall before stopping the boring. To prevent galvanic corrosion, the carbon steel outer layer of the welded joint had to be removed. This was done by turning the carbon steel outer layer of the weld seam. When the carbon steel outer layer was almost completely removed and the delamination between the inner and outer tubes of the mechanical nickel-based bimetallic composite pipe was clearly visible, the wall thickness was already very thin. To prevent deformation, breakage, or other damage to the specimen, the visible delamination section had to be cut off. A support ring was then added inside the specimen, and machining continued. Positioning eye wire cutting was performed to complete the C-ring specimen preparation. The C-ring specimen obtained using this method was defect-free and a valid specimen suitable for corrosion resistance evaluation.

[0080] The support ring is made of resin material to ensure its mechanical support strength; the outer diameter of the support ring is the same as the inner diameter of the bimetallic composite tube, and the wall thickness of the support ring is greater than 3mm to ensure sufficient support strength.

[0081] By processing the C-ring specimen of the weld, the problems of difficult sampling of mechanical nickel-based alloy bimetallic composite tubes, difficulty in obtaining effective parts, or local defects can be overcome, greatly reducing the processing difficulty of the specimen and ensuring the accuracy of the test results.

[0082] Furthermore, the test method for the corrosion resistance of straight welds and circumferential welds under simulated corrosion conditions is as follows:

[0083] For each test group, no fewer than three thin-walled C-ring specimens were taken. The stress level of the weld seam of the specimens was loaded to 100% of the actual strength (SMYS). The C-ring specimens were suspended in an autoclave using an insulated specimen clamp assembly. The test solution in the autoclave was simulated water from the production site, and the partial pressures of H2S and CO2 were consistent with the production partial pressure. The corrosion resistance was tested by maintaining the test temperature for one test cycle.

[0084] The test cycle is 720 hours, and the test temperature is the highest operating temperature on site.

[0085] The evaluation method for the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes provided above is targeted and systematic, and can truly and accurately reflect the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes. It clarifies and optimizes the evaluation process for the specific corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes. Furthermore, using this method can reduce the processing difficulty of samples in SSC testing, ensure the accuracy of test results, provide a basis for the delivery technical requirements and welding process requirements of mechanical nickel-based alloy bimetallic composite pipes, and ultimately provide reliable and effective data support for the application of mechanical nickel-based alloy bimetallic composite pipes in acidic gas fields.

[0086] The following will provide a more detailed explanation through specific examples.

[0087] Example 1:

[0088] The gathering and transportation pipeline of Well X in a high-sulfur gas field is planned to use L245 / 825 mechanical nickel-based alloy bimetallic composite pipe. An indoor evaluation of the corrosion resistance of the composite pipe's weld seams is required. The raw natural gas in this well contains 16.7% H2S and 11.1% CO2, with a pipeline pressure ≤9MPa and a pipeline temperature ≤70℃. The weld seam performance of the L245 / 825 mechanical composite pipe is tested and evaluated below, according to the following... Figure 1 Perform as shown.

[0089] I. Evaluation of Corrosion Resistance of Straight Welds

[0090] 1. Intergranular corrosion test of straight weld seams

[0091] A set of samples was taken from the 825 corrosion-resistant alloy layer and the butt-welded pipe using a sawing method. The sample was 76.2 mm long, 25.4 mm wide, and 9.5 mm thick. The scratches on the sample were removed using iron-free alumina abrasive with a particle size of 120. Then, acetone, alcohol and gaseous degreasing agent were used to remove the grease from the sample.

[0092] The treated sample is embedded in copper pellets and completely immersed in the prepared acidified copper sulfate solution. It is heated to boiling and kept boiling for 24 hours. The sample is then removed, and copper residue remains on the sample. The sample is then quickly immersed in nitric acid at room temperature to remove the copper residue.

[0093] A bending test was performed on the sample after the attached copper was removed. After the test, the bent sample was inspected under low magnification and no cracks were found. No intergranular corrosion occurred in the sample.

[0094] 2. SSC resistance test of straight weld seam

[0095] Three C-ring specimens of the straight weld seam of the 825 inner tube of L245 / 825 mechanical composite pipe were prepared. The C-ring specimens were placed in a C-ring test apparatus for SSC resistance performance testing. The opening angle of the C-ring specimen was 150°, and the geometric dimensions are shown in Table 1.

[0096] Table 1. Geometric dimensions of C-ring specimens for SSC resistance test of straight weld seams

[0097] Specimen Number Outer diameter (mm) Wall thickness (mm) Width (mm) Aperture (mm) Z1 101.36 1.96 14 7 Z2 101.28 1.98 14 7 Z3 100.88 1.96 14 7

[0098] The test solution was a saturated H2S solution of 5% NaCl + 0.5% CH3COOH (solvent was distilled water), the test temperature was 24℃, the stress level of the weld was applied to 100% of the actual strength (SMYS), and the test period was 720 hours.

[0099] After the test period, the tensile surface of the specimen was examined under a microscope with magnification of 10x or higher. No cracks were found on the tensile surface of the specimen, and no surface damage cracks were observed. The specimen passed the SSC resistance test.

[0100] 3. Corrosion resistance test of straight welds under simulated corrosion conditions

[0101] Three C-ring specimens with straight weld seams of L245 / 825 mechanical composite pipe with an 825 inner tube were fabricated. The weld stress level of the specimens was applied to 100% of the actual strength (SMYS). The C-ring specimens were placed in an autoclave using an insulating clamp to conduct corrosion resistance tests under simulated production environment corrosion conditions. The inner surfaces of the straight weld seam specimens numbered Z4, Z5, and Z6 were left unprocessed, maintaining the original state of the material after forming. The opening angle of the C-ring specimens was 150°, and the geometric dimensions are shown in Table 2.

[0102] Table 2. Geometric dimensions of C-ring specimens used in corrosion resistance testing under simulated corrosion conditions of straight weld seams.

[0103] Specimen Number Outer diameter (mm) Wall thickness (mm) Width (mm) Aperture (mm) Z4 101.08 2 14 7 Z5 101.44 2 14 7 Z6 101.18 1.98 14 7

[0104] The test solution was prepared according to the on-site solution ion concentration of Well X (on-site simulated water), and H2S and CO2 were introduced to make the partial pressure of H2S in the container reach 1.5MPa and the partial pressure of CO2 reach 1.0MPa. The total test pressure was 9MPa, and the test temperature was the highest on-site operating temperature of 70℃. One test cycle was maintained for 720 hours.

[0105] After the test cycle was completed, the tensile surface of the specimen was examined under a microscope with magnification of 10x or higher. No cracks or surface damage were found on the tensile surface. Except for the area in contact with the fixture, the specimen showed no pitting or other localized corrosion. The maximum corrosion weight loss was 0.0004g < 0.005g. The specimen passed the corrosion resistance test under simulated corrosion conditions.

[0106] II. Evaluation of Corrosion Resistance of Circumferential Welds

[0107] 1. SSC resistance test of circumferential weld;

[0108] Three C-ring specimens were taken from the welded joint of the L245 / 825 mechanical composite pipe. The middle specimen is the weld specimen, and the specimens on both sides were taken from positions adjacent to the weld along the axial direction. Thus, the three specimens include the circumferential weld, heat-affected zone, and 825 body of the welded joint. The opening angle of the C-ring specimen is 150°, and the geometric dimensions are shown in Table 3.

[0109] Table 3. SSC Resistance Test of Ring Weld: Geometric Dimensions of Ring Specimen

[0110] Specimen Number Outer diameter (mm) Wall thickness (mm) Width (mm) Aperture (mm) H1 100.3 1.88 14 7 H2 101.98 1.74 14 7 H3 97.4 1.18 14 7

[0111] The test method and conditions for the SSC resistance performance of the circumferential weld are the same as those for the straight weld. After the test, the tensile surface of the specimen was examined under a microscope with magnification of 10x or higher. No cracks or surface damage cracks were found on the tensile surface of the specimen, indicating that the circumferential weld of the mechanical composite pipe exhibits good SSC resistance. The specimen passed the SSC resistance performance test.

[0112] 2. Corrosion resistance test of circumferential welds under simulated corrosion conditions

[0113] Three C-ring specimens of straight weld seams from L245 / 825 mechanical composite pipes with 825 inner tubes were prepared. The weld stress of the specimens was horizontally loaded to 100% of the actual strength (SMYS). The C-ring specimens were then placed in an autoclave to conduct corrosion resistance tests under simulated production site corrosion conditions. The opening angle of the C-ring specimens was 150°, and the geometric dimensions are shown in Table 4.

[0114] Table 4. Geometric dimensions of C-ring specimens used in corrosion resistance testing under simulated corrosion conditions of straight weld seams.

[0115] Specimen Number Outer diameter (mm) Wall thickness (mm) Width (mm) Aperture (mm) H4 101.48 1.4 14 7 H5 101.38 1.68 14 7 H6 100.70 1.54 14 7

[0116] The corrosion resistance of circumferential welds under simulated corrosion conditions was tested using the same methods and conditions as those for straight welds. After the test, the tensile surfaces of the specimens were examined under a microscope with magnification of 10x or higher. No cracks or surface damage were observed on the tensile surfaces. Except for the areas in contact with the fixture, no pitting or other localized corrosion was observed. The corrosion weight loss was 0.00248g < 0.005g. This was the corrosion resistance test under simulated corrosion conditions.

[0117] Through the above tests, the weld seam of the L245 / 825 mechanical nickel-based alloy bimetallic composite pipe passed the corrosion resistance test, and the corrosion resistance performance met the requirements.

[0118] This embodiment also demonstrates that the method of the present invention can systematically evaluate the corrosion resistance of mechanical nickel-based alloy bimetallic composite pipes.

[0119] The above specific embodiments further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes, characterized in that, This includes corrosion resistance testing and evaluation of straight weld seams and circumferential weld seams of mechanical nickel-based alloy bimetallic composite pipes; The corrosion resistance test evaluation of the straight weld seam includes: preparation of test specimens for the straight weld seam; intergranular corrosion test of the straight weld seam; SSC resistance test of the straight weld seam; corrosion resistance test of the straight weld seam under simulated corrosion conditions. The corrosion resistance test evaluation of the circumferential weld includes: preparation of test specimens for circumferential weld testing; SSC resistance performance test; corrosion resistance performance test of the circumferential weld under simulated corrosion conditions; The corrosion resistance test and evaluation process for the circumferential weld includes the following steps: S21, SSC resistance performance test; S22, Corrosion resistance test of circumferential weld under simulated corrosion conditions; If any step in the evaluation of the circumferential weld fails, the test is terminated, and the corrosion resistance test of the mechanical nickel-based alloy bimetallic composite pipe weld fails. The method for testing intergranular corrosion of the straight weld seam is as follows: The prepared and treated test and evaluation sample for straight weld seam is embedded in copper shot or copper grinding rod, completely immersed in acidified copper sulfate solution, heated to boiling, and kept boiling for 20h±5h. Remove the sample. If there is any copper residue on the removed sample, quickly immerse it in a concentrated nitric acid solution at room temperature to remove the copper residue. A bending test was performed on the sample to test whether intergranular corrosion occurred. The preparation process of the specimens used for testing and evaluating the straight weld seam is as follows: Sampling was performed using sawing or flame cutting methods; two samples were taken from the corrosion-resistant alloy layer of the mechanical nickel-based alloy bimetallic composite pipe, and one sample including the straight weld was taken from the weld area. The specimen is either a transverse specimen perpendicular to the axial direction of the steel pipe or a longitudinal specimen parallel to the axial direction of the steel pipe. The method for testing the SSC resistance performance is as follows: for each test group, take no less than 3 thin-walled weld C-ring specimens, put the C-ring specimens into the C-ring test device, keep the test solution at the test temperature, load the weld stress level to 100% SMYS of the actual strength, maintain for one test cycle, and test its SSC resistance performance.

2. The method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes as described in claim 1, characterized in that, The corrosion resistance testing and evaluation process for straight weld seams includes the following steps: S11, Intergranular corrosion test of straight weld seams; S12, SSC resistance performance test of straight weld seam; S13, Corrosion resistance test of straight weld seams under simulated corrosion conditions; If any step in the evaluation of the straight weld seam fails, the test is terminated, and the corrosion resistance test of the weld seam of the mechanical nickel-based alloy bimetallic composite pipe fails.

3. The method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes as described in claim 1, characterized in that, The transverse specimen has a length ≥ 50 mm and a width of 20 ± 10 mm; the longitudinal specimen has a length ≥ 50 mm.

4. The method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes as described in claim 1, characterized in that, The acidified copper sulfate solution is a 16% sulfuric acid-copper sulfate solution; The bending test is a reverse bending test. The bending test process is as follows: bend 180° around an axis with a diameter equal to twice the thickness of the sample; the bending axis is perpendicular to the sample.

5. The method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes as described in claim 1, characterized in that, The test solution used in the anti-SSC performance test was a saturated H2S solution with a mass fraction of 5% NaCl + 0.5% CH3COOH. The test temperature was 24±3℃ and the test period was 720 hours; The evaluation requirements for passing the SSC resistance test are as follows: the tensile surface of the C-ring sample is examined under a microscope with a magnification of 10x or more, and the tensile surface of the C-ring sample is free from cracks or any surface damage cracks.

6. The method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes as described in claim 1, characterized in that, The processing method for the weld C-ring specimen is as follows: cut off the weld joint and flatten the end face of the joint; bore the hole; turn the outer carbon steel layer of the weld; when the outer carbon steel layer is about to be completely removed and the layering of the inner and outer tubes of the mechanical composite pipe is visible, cut off the section of the pipe where the layering is visible; add a support ring inside the specimen and continue cutting; scribing and cutting.

7. The method for evaluating the corrosion resistance of weld seams in a mechanical nickel-based alloy bimetallic composite pipe as described in claim 6, characterized in that, The support ring is made of resin material, the outer diameter of the support ring is the same as the inner diameter of the bimetallic composite tube, and the wall thickness of the support ring is greater than 3mm.

8. The method for evaluating the corrosion resistance of weld seams in a mechanical nickel-based alloy bimetallic composite pipe as described in claim 6, characterized in that, The welded joint is cut off using a wire EDM machine, and the end face of the joint is flattened on a lathe. After the inner wall is smooth, the boring is stopped.

9. A method for evaluating the corrosion resistance of weld seams in mechanical nickel-based alloy bimetallic composite pipes as described in claim 1 or 2, characterized in that, In steps S13 and S22, the method for testing the corrosion resistance of straight welds and circumferential welds under simulated corrosion conditions is as follows: For each test group, no fewer than three thin-walled C-ring specimens were taken. The stress level of the specimens was loaded to 100% of the actual strength (SMYS). The C-ring specimens were suspended in an autoclave using an insulated specimen clamp assembly. The test solution in the autoclave was simulated water from the production site, and the partial pressures of H2S and CO2 were consistent with the production partial pressure. The corrosion resistance was tested by maintaining the test temperature for one test cycle.

10. The method for evaluating the corrosion resistance of weld seams in a mechanical nickel-based alloy bimetallic composite pipe as described in claim 9, characterized in that, The test cycle is 720 hours, and the test temperature is the highest operating temperature on site.

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