Introduction: With the continuous deepening of China’s oil and gas exploration and development strategy, the exploitation scope has gradually expanded to deep-sea, deep-layer and high-sulfur areas, and the operating conditions of oil and gas pipelines have become increasingly harsh. Pipeline corrosion, as a key factor restricting the safe and stable operation of the oil and gas industry, has caused huge economic losses and potential safety hazards to the industry every year. Traditional anti-corrosion measures such as ordinary carbon steel pipes with coatings have been difficult to meet the long-term anti-corrosion needs under harsh working conditions. Inner clad or lined corrosion-resistant alloy composite steel pipes, which integrate the high strength of carbon steel/low alloy steel and the excellent corrosion resistance of corrosion-resistant alloys, have emerged as the times require and have been widely used in various key oil and gas pipeline projects. Based on my three-year professional learning in materials science and engineering and my four-month internship experience in an oil and gas pipeline material manufacturing enterprise, this paper focuses on the research of inner clad or lined corrosion-resistant alloy composite steel pipes, systematically discusses their characteristics, preparation processes, performance testing, application cases and development trends, aiming to provide practical references for the engineering application and technical improvement of such composite steel pipes in the oil and gas industry.

  

The performance testing of inner clad or lined corrosion-resistant alloy composite steel pipes is an important link to verify the product quality and ensure that it can adapt to the harsh working conditions of the oil and gas industry. The performance of composite steel pipes mainly includes mechanical properties, corrosion resistance, bonding performance and structural integrity. Only through systematic and comprehensive performance testing can we determine whether the composite steel pipe meets the engineering application requirements. During my internship, I had the opportunity to enter the enterprise’s testing center, participate in the auxiliary work of various performance tests, and learn about the testing methods, testing standards and testing equipment of composite steel pipes. Combined with the latest GB/T 31940-2025 national standard and the enterprise’s internal testing specifications, this section will focus on the main performance testing items, testing methods and testing standards of composite steel pipes, and share my own testing experience.

The mechanical performance of inner clad or lined corrosion-resistant alloy composite steel pipes is an important guarantee for the safe operation of the pipeline, which mainly includes tensile strength, yield strength, elongation, impact toughness and hardness. These mechanical properties are not only related to the material properties of the base layer and the clad/lined layer, but also affected by the preparation process. The mechanical performance testing is to ensure that the composite steel pipe has sufficient strength, toughness and hardness to bear the medium pressure, mechanical impact and other loads during the transportation and operation process.

The first mechanical performance testing item is tensile strength and yield strength testing. The tensile strength is the maximum stress that the composite steel pipe can bear before breaking, and the yield strength is the stress when the composite steel pipe produces a certain plastic deformation. The testing method mainly adopts the tensile test, which is carried out by using a universal tensile testing machine. During the test, the composite steel pipe is cut into standard tensile samples according to the national standard, and the samples are clamped on the tensile testing machine. Then, the testing machine applies a uniform tensile load to the samples at a certain speed until the samples break. The testing machine automatically records the tensile force and deformation data during the test, and calculates the tensile strength and yield strength according to the data. According to the requirements of GB/T 31940-2025 national standard, the tensile strength of the inner clad or lined composite steel pipe shall not be less than the tensile strength of the base layer material, and the yield strength shall not be less than 80% of the yield strength of the base layer material. For example, if the base layer adopts Q355 low alloy steel (tensile strength 470-630MPa, yield strength 355MPa), the tensile strength of the composite steel pipe shall not be less than 470MPa, and the yield strength shall not be less than 284MPa. During my internship, I assisted the testing personnel to prepare tensile samples, clamp the samples on the testing machine, and record the test data. I found that the tensile strength of the composite steel pipe produced by the enterprise is usually 5%-10% higher than the tensile strength of the base layer material, which is due to the synergistic effect of the clad/lined layer and the base layer.

The second mechanical performance testing item is elongation testing. The elongation is the percentage of the total deformation of the sample before breaking, which reflects the plastic deformation ability of the composite steel pipe. The higher the elongation, the better the toughness of the composite steel pipe, and the less likely it is to break under the action of impact and other loads. The elongation testing is carried out together with the tensile test. After the tensile test, the testing personnel measure the length of the sample before and after breaking, and calculate the elongation according to the formula: elongation δ=(L1-L0)/L0×100%, where L0 is the original length of the sample, and L1 is the length of the sample after breaking. According to the national standard, the elongation of the composite steel pipe shall not be less than 15%. For the composite steel pipe with nickel-based alloy clad/lined layer, the elongation shall not be less than 20% due to the good toughness of nickel-based alloy. During the test, I found that the elongation of the composite steel pipe prepared by the surfacing welding process is usually higher than that of the thermal spraying process, which is because the surfacing welding process forms a metallurgical bond between the clad layer and the base layer, and the toughness of the composite is better.

The third mechanical performance testing item is impact toughness testing. The impact toughness is the ability of the composite steel pipe to absorb energy under the action of sudden impact load, which reflects the anti-impact ability of the composite steel pipe. The oil and gas pipeline will be subjected to mechanical impact during transportation, installation and operation (such as collision during transportation, wind wave impact on offshore pipelines), so it is necessary to have good impact toughness. The impact toughness testing is carried out by using an impact testing machine, and the testing method mainly adopts the Charpy impact test. During the test, the composite steel pipe is cut into standard impact samples (V-notch or U-notch), and the samples are placed in the impact testing machine. The impact hammer of the testing machine hits the sample at a certain speed, and the testing machine records the impact energy absorbed by the sample. The impact toughness is expressed by the impact energy per unit cross-sectional area of the sample. According to the national standard, the impact toughness of the composite steel pipe at room temperature shall not be less than 34J/cm². For the composite steel pipe used in low-temperature working conditions (such as offshore pipelines in cold areas), the impact toughness at low temperature (-20℃ or -40℃) shall not be less than 27J/cm². During my internship, I participated in the impact toughness test of composite steel pipes used in offshore platforms. The test temperature was -20℃, and the impact energy of all samples met the requirements, which indicated that the composite steel pipe had good low-temperature impact toughness.

The fourth mechanical performance testing item is hardness testing. The hardness is the ability of the composite steel pipe to resist the indentation of external objects, which reflects the wear resistance and deformation resistance of the composite steel pipe. The inner wall of the oil and gas pipeline will be scoured by the transportation medium, so the clad/lined layer needs to have a certain hardness to resist wear. The hardness testing is carried out by using a hardness tester, and the common testing methods include Brinell hardness test, Rockwell hardness test and Vickers hardness test. For the clad/lined layer of composite steel pipes, the Vickers hardness test is usually adopted because of its high testing accuracy and small damage to the sample. During the test, the testing personnel use the Vickers hardness tester to apply a certain load to the surface of the clad/lined layer, and measure the diagonal length of the indentation, then calculate the Vickers hardness value (HV). According to the enterprise’s internal standards, the Vickers hardness of the 316L stainless steel clad/lined layer shall be between 180-220HV, the Vickers hardness of the Inconel 625 nickel-based alloy clad/lined layer shall be between 220-260HV, and the Vickers hardness of the base layer (Q355 low alloy steel) shall be between 140-180HV. During the test, I found that the hardness of the clad layer prepared by the thermal spraying process is slightly higher than that of the surfacing welding process, which is because the thermal spraying process forms a dense structure after rapid cooling of the molten powder, resulting in higher hardness.

It should be noted that the mechanical performance testing of composite steel pipes needs to pay attention to the sampling position and sampling method. The samples should be taken from different positions of the composite steel pipe (such as the middle part, the end part) to ensure the representativeness of the samples. At the same time, the sampling method should avoid damaging the clad/lined layer and the interface between the clad/lined layer and the base layer, so as not to affect the test results. During my internship, the testing personnel told me that the sampling work is very important. If the sampling position is improper or the sampling method is incorrect, it will lead to inaccurate test results, which will affect the judgment of product quality.

Corrosion resistance is the core performance of inner clad or lined corrosion-resistant alloy composite steel pipes, which directly determines the service life of the composite steel pipe in the harsh working conditions of the oil and gas industry. The corrosion resistance testing is to simulate the actual working condition medium and environment, test the corrosion rate and corrosion form of the composite steel pipe, and verify whether it can resist the erosion of corrosive medium. According to the different corrosion mechanisms and working conditions, the corrosion resistance testing of composite steel pipes mainly includes electrochemical corrosion testing, immersion corrosion testing, stress corrosion cracking testing and hydrogen-induced cracking testing. During my internship, I participated in the auxiliary work of immersion corrosion testing and electrochemical corrosion testing, and learned about the testing principles and methods of stress corrosion cracking testing and hydrogen-induced cracking testing.

The first corrosion resistance testing item is electrochemical corrosion testing. Electrochemical corrosion is the most common type of corrosion of oil and gas pipelines, so electrochemical corrosion testing is an important method to evaluate the corrosion resistance of composite steel pipes. The electrochemical corrosion testing mainly includes polarization curve testing and electrochemical impedance spectroscopy (EIS) testing, which are carried out by using an electrochemical workstation. The core principle of polarization curve testing is to apply a certain potential to the composite steel pipe sample (working electrode) in the simulated corrosion medium, measure the corresponding current density, and draw the polarization curve. The polarization curve can reflect the corrosion rate and corrosion tendency of the sample. The lower the corrosion current density, the better the corrosion resistance of the sample. The electrochemical impedance spectroscopy testing is to apply a small amplitude alternating current to the working electrode, measure the impedance of the sample at different frequencies, and analyze the corrosion process and corrosion resistance of the sample by using the impedance spectrum. During the test, the composite steel pipe sample is cut into a certain size, and the surface of the sample is treated (polished, cleaned), then the sample is immersed in the simulated corrosion medium (such as a solution containing hydrogen sulfide, carbon dioxide, chloride ions, etc.). The reference electrode and auxiliary electrode are inserted into the medium, and the three electrodes are connected to the electrochemical workstation for testing. According to the enterprise’s internal standards, the corrosion current density of the composite steel pipe sample in the simulated ultra-high sulfur gas field medium shall not be greater than 1.0×10⁻⁶A/cm². During my internship, I assisted the testing personnel to prepare the simulated corrosion medium, polish the sample surface, and connect the electrode, and observed the testing process of the electrochemical workstation. The test results showed that the corrosion current density of the composite steel pipe with Inconel 625 nickel-based alloy clad layer was far less than the standard requirement, which indicated that it had excellent electrochemical corrosion resistance.

The second corrosion resistance testing item is immersion corrosion testing. Immersion corrosion testing is a simple and intuitive corrosion resistance testing method, which is to immerse the composite steel pipe sample in the simulated corrosion medium, place it in a constant temperature environment for a certain time, and then observe the corrosion form of the sample and calculate the corrosion rate. This method can simulate the long-term corrosion process of the composite steel pipe in the actual working condition medium. During the test, the composite steel pipe sample is cut into standard samples, and the surface area, weight and other parameters of the sample are measured before immersion. Then, the sample is immersed in the simulated corrosion medium (the medium composition and temperature are consistent with the actual working conditions), and the medium is replaced regularly to ensure the stability of the medium composition. After immersion for a certain time (usually 720 hours or 1000 hours), the sample is taken out, the corrosion product on the surface is cleaned, and the weight of the sample after corrosion is measured. The corrosion rate is calculated according to the formula: corrosion rate v=(m0-m1)/(S×t), where m0 is the weight of the sample before corrosion, m1 is the weight of the sample after corrosion, S is the surface area of the sample, and t is the immersion time. According to the national standard, the uniform corrosion rate of the composite steel pipe in the simulated corrosion medium shall not be greater than 0.01mm/a. For the composite steel pipe used in ultra-high sulfur gas fields, the uniform corrosion rate shall not be greater than 0.005mm/a. During my internship, I participated in the immersion corrosion test of 316L stainless steel lined composite steel pipes. The immersion time was 720 hours, the simulated medium was a solution containing carbon dioxide and chloride ions, and the test temperature was 80℃. After the test, the surface of the sample was smooth without any corrosion traces, and the corrosion rate was far less than the standard requirement, which indicated that the composite steel pipe had good immersion corrosion resistance.

The third corrosion resistance testing item is stress corrosion cracking (SSC) testing. As mentioned earlier, stress corrosion cracking is a very dangerous corrosion form, which is easy to cause sudden failure of the pipeline. Therefore, stress corrosion cracking testing is an essential testing item for composite steel pipes used in high-sulfur, high-chloride ion working conditions. The stress corrosion cracking testing is carried out according to the requirements of NACE TM0177 standard (the international authoritative standard for stress corrosion cracking testing), and the testing method mainly adopts the bent beam method or the tensile load method. During the test, the composite steel pipe sample is processed into a standard bent beam sample or tensile sample, and a certain tensile stress is applied to the sample (the stress is usually 80% of the yield strength of the sample). Then, the sample is immersed in the simulated stress corrosion medium (such as NACE A solution, which is a solution containing hydrogen sulfide and chloride ions) at a certain temperature and pressure. The sample is kept in the medium for a certain time (usually 720 hours), and then the sample is taken out to observe whether there are cracks on the surface and inside of the sample. If there are no cracks, it indicates that the composite steel pipe has good stress corrosion cracking resistance. During my internship, I learned that the enterprise carries out stress corrosion cracking testing on all composite steel pipes used in ultra-high sulfur gas fields. The test results showed that the composite steel pipe with Inconel 625 nickel-based alloy clad layer had no cracks after 720 hours of testing, which indicated that it could effectively resist stress corrosion cracking.

The fourth corrosion resistance testing item is hydrogen-induced cracking (HIC) testing. Hydrogen-induced cracking is also a common dangerous corrosion form of oil and gas pipelines, which is often accompanied by stress corrosion cracking. The hydrogen-induced cracking testing is carried out according to the requirements of NACE TM0284 standard, and the testing method mainly adopts the immersion method. During the test, the composite steel pipe sample is cut into standard samples, and the sample is immersed in the simulated hydrogen-induced cracking medium (such as a solution containing hydrogen sulfide and moisture) at a certain temperature and pressure. The sample is kept in the medium for a certain time (usually 96 hours), and then the sample is taken out to observe whether there are bulges, cracks and other defects on the surface and inside of the sample. At the same time, the sample is cut and polished to observe the internal defects under a microscope. According to the standard, the composite steel pipe sample shall have no obvious hydrogen-induced cracking defects after the test. During my internship, I observed the hydrogen-induced cracking test of composite steel pipes. The test medium was a solution containing high concentration hydrogen sulfide, and the test temperature was 25℃. After the test, the samples were cut and observed, and no hydrogen-induced cracking defects were found, which indicated that the composite steel pipe had good hydrogen-induced cracking resistance.

In addition, for the composite steel pipes used in offshore oil and gas platforms, the enterprise also carries out marine corrosion testing, which simulates the marine environment (seawater immersion, marine atmospheric corrosion) to test the corrosion resistance of the composite steel pipe. The marine corrosion testing is carried out by immersing the sample in natural seawater or simulated seawater, and placing it in a marine atmospheric environment for a long time (usually 6 months to 1 year), then observing the corrosion form and calculating the corrosion rate. This test can more truly reflect the corrosion resistance of the composite steel pipe in the marine environment. During my internship, I saw that the enterprise had a special marine corrosion test site, and a large number of composite steel pipe samples were being tested in the simulated marine environment.

It should be emphasized that the corrosion resistance testing of composite steel pipes needs to strictly control the test conditions (medium composition, temperature, pressure, time), which directly affects the test results. The test conditions must be consistent with the actual working conditions of the pipeline, so that the test results can truly reflect the corrosion resistance of the composite steel pipe in the actual application process. During my internship, the testing personnel told me that every parameter of the corrosion resistance test must be strictly controlled, and any deviation will lead to inaccurate test results, which will affect the selection and application of composite steel pipes in engineering. In addition, the corrosion resistance testing of composite steel pipes also needs to pay attention to the protection of the test samples. During the sample preparation and testing process, it is necessary to avoid artificial damage to the clad/lined layer, so as not to affect the test results. For example, during the polishing process of the sample, it is necessary to control the polishing force to avoid scratching the clad/lined layer or exposing the base layer, which will lead to inaccurate evaluation of corrosion resistance.

The bonding performance between the base layer (carbon steel/low alloy steel) and the clad/lined layer (corrosion-resistant alloy) is the core guarantee for the overall performance of inner clad or lined composite steel pipes. If the bonding performance is poor, the clad/lined layer will peel off from the base layer during transportation, installation or operation, exposing the base layer to corrosive media and leading to rapid corrosion and failure of the pipeline. Therefore, bonding performance testing is an indispensable part of the comprehensive performance testing of composite steel pipes. During my internship, I participated in the auxiliary work of bonding performance testing, and learned about the main testing methods, standards and key points of attention, which are combined with the enterprise’s actual testing experience and GB/T 31940-2025 national standard to elaborate in detail.

The bonding performance testing of composite steel pipes mainly evaluates the bonding strength between the base layer and the clad/lined layer, as well as the integrity of the bonding interface. The main testing methods include tensile shear test, peel test and metallographic observation, among which tensile shear test and peel test are the most commonly used quantitative testing methods, and metallographic observation is a qualitative testing method to supplement and verify the bonding state. Different preparation processes correspond to different bonding strength requirements, and the enterprise will formulate targeted testing standards according to the product type.

The first common bonding performance testing method is the tensile shear test, which is mainly used to test the shear bonding strength between the base layer and the clad/lined layer. This method is suitable for all types of inner clad or lined composite steel pipes, especially for composite steel pipes prepared by metallurgical bonding processes (such as surfacing welding, explosion cladding, hot rolling cladding). The testing principle is to cut the composite steel pipe into standard tensile shear samples, which can fully reflect the bonding interface between the base layer and the clad/lined layer. Then, the sample is clamped on a universal tensile testing machine, and a uniform shear load is applied along the direction parallel to the bonding interface until the bonding interface is separated or the sample is damaged. The testing machine automatically records the maximum shear load, and the shear bonding strength is calculated according to the cross-sectional area of the bonding interface. According to the requirements of GB/T 31940-2025 national standard, the shear bonding strength of inner clad composite steel pipes prepared by metallurgical bonding processes shall not be less than 200MPa, and the shear bonding strength of lined composite steel pipes prepared by mechanical bonding processes (such as hydraulic pipe expansion) shall not be less than 150MPa. During my internship, I assisted the testing personnel to cut and process tensile shear samples, and observed the testing process. I found that the shear bonding strength of composite steel pipes prepared by explosion cladding process was the highest, usually reaching more than 300MPa, while the shear bonding strength of composite steel pipes prepared by hydraulic pipe expansion process was about 160-180MPa, which all met the standard requirements.

The second common bonding performance testing method is the peel test, which is mainly used to test the peel strength between the base layer and the clad/lined layer. This method is more suitable for composite steel pipes prepared by mechanical bonding or bonding processes (such as hydraulic pipe expansion, bonding lining method), and is also applicable to thin clad/lined layer composite steel pipes. The testing principle is to cut the composite steel pipe into standard peel samples, and separate one end of the clad/lined layer from the base layer in advance. Then, clamp the base layer and the clad/lined layer of the sample on the two clamps of the universal tensile testing machine respectively, and apply a uniform tensile load along the direction perpendicular to the bonding interface to peel the clad/lined layer from the base layer. The testing machine records the peel load during the whole peeling process, and the average peel load per unit width is the peel strength. According to the enterprise’s internal standards, the peel strength of lined composite steel pipes prepared by hydraulic pipe expansion process shall not be less than 15N/mm, and the peel strength of composite steel pipes prepared by bonding lining method shall not be less than 10N/mm. During the test, I found that if the pre-treatment of the base layer and the clad/lined layer is not in place, the peel strength will be significantly reduced, and even the clad/lined layer can be peeled off manually, which fully indicates that pre-treatment is the key to ensuring bonding performance.

The third bonding performance testing method is metallographic observation, which is a qualitative testing method used to observe the bonding interface state of composite steel pipes. This method can directly observe whether there are defects such as gaps, pores, cracks and oxide layers at the bonding interface, and can also observe the thickness uniformity of the clad/lined layer and the metallurgical reaction state at the interface (for metallurgical bonding processes). The testing steps are: cut the composite steel pipe into small metallographic samples, grind and polish the samples to make the bonding interface clear, then etch the samples with a special etchant (different etchants are selected according to the base layer and clad/lined layer materials), and finally observe the bonding interface under an optical microscope or scanning electron microscope (SEM). During my internship, I learned to grind and polish metallographic samples under the guidance of the testing master, and observed the bonding interface of composite steel pipes prepared by different processes under the microscope. For example, the bonding interface of composite steel pipes prepared by surfacing welding process was continuous and dense, with no obvious defects, and a thin metallurgical reaction layer was formed at the interface; the bonding interface of composite steel pipes prepared by hydraulic pipe expansion process was closely fitted, with no gaps, but no metallurgical reaction layer was formed.

In addition to the above three main testing methods, the enterprise also carries out bonding performance inspection by means of ultrasonic testing. Ultrasonic testing is a non-destructive testing method, which can detect the internal bonding defects of composite steel pipes (such as interface gaps, peeling, etc.) without damaging the samples. This method is suitable for batch inspection of finished composite steel pipes, and can quickly screen out unqualified products with bonding defects. During my internship, I observed the technical personnel using ultrasonic flaw detection equipment to detect the bonding performance of composite steel pipes. The equipment can display the bonding interface state through images, and the technical personnel can judge whether there are bonding defects according to the image characteristics. This method has the advantages of high detection efficiency, non-destructiveness and wide applicability, and has become an important auxiliary method for bonding performance testing.

It should be noted that the bonding performance testing of composite steel pipes also needs to pay attention to the sampling position and sampling method, which is consistent with the mechanical performance testing. The samples should be taken from different positions of the composite steel pipe to ensure the representativeness of the test results. At the same time, the sampling and sample processing process should avoid damaging the bonding interface, so as not to affect the accuracy of the test results. During my internship, the testing master emphasized that the bonding performance of composite steel pipes is affected by many factors, including raw material quality, pre-treatment effect, process parameter control and post-treatment quality. Therefore, only through strict control of each link in the preparation process can the bonding performance of composite steel pipes be guaranteed to meet the requirements.

The structural integrity of inner clad or lined corrosion-resistant alloy composite steel pipes refers to the completeness and uniformity of the overall structure of the composite steel pipe, including the dimensional accuracy of the pipe, the thickness uniformity of the clad/lined layer, the absence of internal and surface defects, and the concentricity of the base layer and the clad/lined layer. Structural integrity is an important prerequisite for the safe operation of composite steel pipes. If there are structural defects (such as uneven thickness of the clad/lined layer, internal cracks, eccentricity, etc.), it will lead to uneven stress distribution of the pipeline during operation, accelerate corrosion and damage, and even cause pipeline leakage. Therefore, structural integrity testing is an important part of the performance testing of composite steel pipes. Combined with my internship experience and the enterprise’s testing specifications, this section will elaborate on the main testing items, methods and standards of structural integrity testing.

The first structural integrity testing item is dimensional accuracy testing, which mainly includes the testing of pipe diameter, wall thickness, length, ovality and concentricity. These dimensional parameters directly affect the installation and matching performance of composite steel pipes in engineering, and also affect the pressure-bearing capacity and service life of the pipeline. The testing methods are mainly carried out by using professional measuring tools, such as calipers, micrometers, tape measures, ovality meters and concentricity meters. For the diameter testing, the outer diameter and inner diameter of the composite steel pipe are measured at different positions (usually at both ends and the middle part of the pipe), and the average value is taken to ensure that the diameter deviation is within the standard range. According to GB/T 31940-2025 national standard, the diameter deviation of composite steel pipes shall not exceed ±1% of the nominal diameter. For the wall thickness testing, the wall thickness of the pipe is measured at multiple points along the circumference and length of the pipe to ensure the uniformity of the wall thickness. The wall thickness deviation shall not exceed ±5% of the nominal wall thickness. During my internship, I was responsible for assisting the testing personnel to measure the wall thickness of composite steel pipes with a micrometer, and recorded the measurement data. I found that the wall thickness uniformity of composite steel pipes prepared by hot rolling cladding process was the best, and the deviation was basically within ±3%.

The second structural integrity testing item is the thickness uniformity testing of the clad/lined layer. The thickness uniformity of the clad/lined layer directly affects the corrosion resistance of the composite steel pipe. If the thickness of the clad/lined layer is uneven, the thin part will be quickly corroded, exposing the base layer, leading to the overall failure of the pipeline. The testing methods mainly include ultrasonic thickness measurement, radiographic thickness measurement and metallographic observation. Among them, ultrasonic thickness measurement is the most commonly used method, which has the advantages of non-destructiveness, high efficiency and high accuracy. The testing principle is to use ultrasonic waves to transmit through the clad/lined layer, and calculate the thickness of the clad/lined layer according to the time difference between the ultrasonic wave reflected from the surface of the clad/lined layer and the bonding interface. During the test, the testing personnel will measure the thickness of the clad/lined layer at multiple points (at least 20 points per meter) along the circumference and length of the composite steel pipe, and calculate the thickness deviation. According to the enterprise’s internal standards, the thickness deviation of the clad/lined layer shall not exceed ±10% of the nominal thickness, and the minimum thickness of the clad/lined layer shall not be less than 80% of the nominal thickness. During my internship, I learned to use ultrasonic thickness measuring instrument to measure the thickness of the clad layer under the guidance of the testing personnel, and mastered the basic operation skills of the instrument.

The third structural integrity testing item is surface and internal defect detection, which is mainly used to detect whether there are defects such as cracks, pores, inclusions, peeling and scratches on the inner and outer surfaces of the composite steel pipe and inside the pipe. The detection methods are divided into surface defect detection and internal defect detection. Surface defect detection mainly includes visual inspection, magnetic particle flaw detection and penetrant flaw detection. Visual inspection is the most basic detection method, which is used to check the obvious surface defects (such as scratches, burrs, peeling) of the composite steel pipe. During my internship, I participated in the visual inspection of composite steel pipes, and checked the inner and outer surfaces of the pipe with the help of a endoscope (for the inner surface). Magnetic particle flaw detection and penetrant flaw detection are used to detect the surface and near-surface defects (such as microcracks) that are not easy to be found by visual inspection. These two methods are suitable for detecting surface defects of ferromagnetic materials (such as carbon steel base layer and stainless steel clad/lined layer).

Internal defect detection mainly includes ultrasonic flaw detection and radiographic flaw detection, which are the most important non-destructive detection methods for composite steel pipes. Ultrasonic flaw detection is mainly used to detect internal defects such as internal cracks, pores, inclusions and interface peeling of composite steel pipes. The testing principle is to use ultrasonic waves to transmit through the composite steel pipe, and the defects will reflect and refract the ultrasonic waves, so as to judge the position, size and shape of the defects. Radiographic flaw detection is mainly used to detect internal defects of thick-walled composite steel pipes, and can clearly show the internal defect state of the pipe. The testing principle is to use X-rays or γ-rays to penetrate the composite steel pipe, and the defects will affect the penetration ability of the rays, forming different gray-scale images on the film, so as to judge the internal defects. According to the national standard, the internal defects of composite steel pipes shall not exceed the level II requirement of GB/T 31940-2025. During my internship, I observed the ultrasonic flaw detection and radiographic flaw detection process of composite steel pipes, and learned to identify simple flaw detection images under the guidance of technical personnel.

The fourth structural integrity testing item is concentricity testing, which is mainly aimed at lined composite steel pipes. The concentricity of the base layer and the lined layer refers to the coincidence degree of the center line of the base steel pipe and the lined pipe. If the concentricity is poor, the lined layer will be unevenly stressed during the pipe expansion process, and the thin part of the lined layer will be easily damaged during operation, leading to corrosion failure. The testing method is to use a concentricity meter or a dial indicator to measure the distance between the center line of the base layer and the lined layer at different positions of the composite steel pipe, and calculate the concentricity deviation. According to the enterprise’s internal standards, the concentricity deviation of lined composite steel pipes shall not exceed 0.5mm/m. During my internship, I assisted the testing personnel to measure the concentricity of lined composite steel pipes, and found that the concentricity deviation of composite steel pipes prepared by automatic insertion equipment was smaller than that of manual insertion equipment.

To sum up, the structural integrity testing of composite steel pipes is a comprehensive testing work, which covers multiple aspects such as dimensional accuracy, clad/lined layer thickness uniformity, surface and internal defects, and concentricity. Only through systematic structural integrity testing can we ensure that the composite steel pipe has a complete and uniform structure, and lay a foundation for the safe operation of the pipeline. During my internship, I deeply realized that the structural integrity of composite steel pipes is closely related to the preparation process. For example, the dimensional accuracy of composite steel pipes prepared by automatic production equipment is higher than that of manual operation, and the thickness uniformity of the clad layer prepared by thermal spraying process is easily affected by the spray gun moving speed and powder feeding speed.

With the continuous improvement of China’s oil and gas exploration and development level, the operating conditions of oil and gas pipelines are becoming increasingly harsh, and the demand for high-performance anti-corrosion pipelines is growing day by day. Inner clad or lined corrosion-resistant alloy composite steel pipes, with their unique advantages of high strength, excellent corrosion resistance and reasonable cost, have been widely used in various key oil and gas pipeline projects, including onshore high-pressure long-distance transmission pipelines, ultra-high sulfur gas field gathering and transmission pipelines, offshore oil and gas pipelines and other fields. Based on my internship experience and the collection of relevant engineering data, this section will elaborate on the application of composite steel pipes in different oil and gas fields, analyze the application effects and existing problems, and provide practical references for the further promotion and application of composite steel pipes.

During my internship in the oil and gas pipeline material manufacturing enterprise, I learned that the enterprise has provided a large number of inner clad or lined corrosion-resistant alloy composite steel pipes for many key oil and gas projects at home and abroad, and has accumulated rich engineering application experience. The technical personnel of the enterprise will formulate targeted product schemes and preparation processes according to the different working conditions and requirements of each project, ensuring that the performance of composite steel pipes meets the engineering needs. Through the understanding of these projects, I have a deeper understanding of the application value and application scope of composite steel pipes.

Onshore high-pressure long-distance oil and gas transmission pipelines are the main part of China’s oil and gas transmission network, which are usually in service under the conditions of high pressure, long distance and complex geological environment. The transportation medium usually contains corrosive components such as carbon dioxide, hydrogen sulfide and chloride ions, and the pipeline is easily corroded. At the same time, the pipeline needs to bear large medium pressure and environmental loads (such as soil pressure, temperature change), so it has high requirements on the strength and toughness of the pipeline. Inner clad or lined corrosion-resistant alloy composite steel pipes can well meet these requirements, and have become the preferred pipeline material for onshore high-pressure long-distance transmission projects.

The composite steel pipes used in onshore high-pressure long-distance transmission pipelines are mainly inner clad composite steel pipes prepared by surfacing welding or explosion cladding process, and the base layer usually adopts Q355 or X80 low alloy steel (high strength and good toughness), and the clad layer adopts 316L stainless steel or Inconel 625 nickel-based alloy (excellent corrosion resistance). The nominal diameter of the pipeline is usually 800-1400mm, and the wall thickness is 12-25mm, which can meet the requirements of high-pressure transmission (pressure ≥10MPa). During my internship, I learned about a key onshore natural gas long-distance transmission project in western China, which adopted a total length of 1200km inner clad composite steel pipes prepared by surfacing welding process. The base layer of the composite steel pipe is X80 low alloy steel, and the clad layer is 316L stainless steel (clad layer thickness 3-5mm). The transportation medium contains 5% carbon dioxide and trace hydrogen sulfide, and the transmission pressure is 12MPa. The project has been in service for 5 years, and the pipeline operation is stable. No corrosion, peeling or leakage defects have been found in the regular inspection.

The application advantages of composite steel pipes in onshore high-pressure long-distance transmission pipelines are mainly reflected in three aspects: first, the base layer of low alloy steel ensures the high strength and toughness of the pipeline, which can bear large medium pressure and environmental loads, and avoid pipeline rupture caused by pressure fluctuation or environmental impact; second, the corrosion-resistant alloy clad layer effectively isolates the corrosive medium from the base layer, preventing pipeline corrosion and extending the service life of the pipeline (the service life can reach more than 30 years, which is 2-3 times that of traditional carbon steel pipes with coatings); third, compared with the whole corrosion-resistant alloy pipe, the composite steel pipe has lower cost, which can reduce the total investment of the project by 30%-50%, and has obvious economic benefits. For example, in the above-mentioned western natural gas transmission project, the use of inner clad composite steel pipes instead of whole 316L stainless steel pipes reduced the project investment by about 40%.

However, there are also some problems in the application of composite steel pipes in onshore high-pressure long-distance transmission pipelines: first, the preparation process of surfacing welding and explosion cladding is complex, the production efficiency is low, and it is difficult to meet the urgent demand of large-scale projects; second, the welding of composite steel pipes is difficult. The base layer and clad layer are different materials, and the welding process needs to be strictly controlled to avoid welding defects (such as incomplete fusion, cracks); third, the maintenance cost of composite steel pipes is high. If the clad layer is damaged, it is difficult to repair, and it is necessary to replace the whole section of the pipeline, which increases the maintenance cost. In view of these problems, the enterprise where I interned is constantly optimizing the preparation process and welding technology, improving production efficiency, and developing a set of mature composite steel pipe repair technology, which effectively reduces the maintenance cost.

Ultra-high sulfur gas fields refer to gas fields with hydrogen sulfide content ≥15% (volume fraction), which are typical harsh corrosion environments. The hydrogen sulfide in the natural gas is highly corrosive to the pipeline, and it is easy to cause stress corrosion cracking (SSC) and hydrogen-induced cracking (HIC) of the pipeline, leading to sudden pipeline failure, which brings great safety hazards to the production and transportation of natural gas. Therefore, the pipelines used in ultra-high sulfur gas fields have extremely high requirements on corrosion resistance, especially the resistance to stress corrosion cracking and hydrogen-induced cracking. Inner clad or lined corrosion-resistant alloy composite steel pipes, especially those with nickel-based alloy clad/lined layer, have excellent corrosion resistance and can effectively resist the corrosion of high-concentration hydrogen sulfide, so they are widely used in ultra-high sulfur gas field gathering and transmission pipelines.

The composite steel pipes used in ultra-high sulfur gas field gathering and transmission pipelines are mainly inner clad composite steel pipes prepared by surfacing welding or explosion cladding process, and the clad layer is mainly Inconel 625 nickel-based alloy (the most corrosion-resistant alloy material in the current ultra-high sulfur environment). The base layer usually adopts Q355 low alloy steel, which ensures the strength and pressure-bearing capacity of the pipeline. The nominal diameter of the pipeline is usually 100-500mm, and the wall thickness is 8-15mm, which is suitable for the gathering and transmission of natural gas in gas fields (pressure 3-8MPa). During my internship, I participated in the production auxiliary work of composite steel pipes for an ultra-high sulfur gas field project in Sichuan, China. The project adopted inner clad composite steel pipes prepared by explosion cladding process, the base layer was Q355 low alloy steel, the clad layer was Inconel 625 nickel-based alloy (clad layer thickness 2-3mm), and the total length of the pipeline was 350km. The hydrogen sulfide content in the transportation medium was 18%, and the project has been in service for 3 years. The regular inspection results show that the pipeline has no corrosion, stress corrosion cracking or hydrogen-induced cracking defects, and the operation is safe and stable.

The core advantage of composite steel pipes in the application of ultra-high sulfur gas field gathering and transmission pipelines is their excellent corrosion resistance, especially the resistance to stress corrosion cracking and hydrogen-induced cracking. The Inconel 625 nickel-based alloy clad layer has good resistance to hydrogen sulfide corrosion, and can effectively prevent the penetration of hydrogen atoms, avoiding hydrogen-induced cracking of the base layer. At the same time, the nickel-based alloy has good toughness and can resist stress corrosion cracking under high sulfur and high stress conditions. In addition, the composite steel pipe has high strength and pressure-bearing capacity, which can meet the requirements of gas field gathering and transmission pressure. Compared with traditional anti-corrosion measures (such as carbon steel pipes with anti-corrosion coatings), the composite steel pipe has longer service life (more than 25 years) and lower failure rate, which can reduce the number of pipeline maintenance and replacement, and ensure the continuous and stable production of the gas field.

The main problems in the application of composite steel pipes in ultra-high sulfur gas fields are high production cost and strict quality control requirements. The price of Inconel 625 nickel-based alloy is very high, which leads to the high production cost of composite steel pipes (the cost is 2-3 times that of composite steel pipes with stainless steel clad layer). At the same time, the preparation process of composite steel pipes for ultra-high sulfur gas fields is very strict, and any quality defect (such as interface gap, clad layer thickness unevenness) will lead to pipeline corrosion failure. Therefore, the enterprise needs to strictly control each link of the preparation process, from raw material selection to post-treatment and inspection, to ensure the product quality. During my internship, I found that the enterprise has set up a special quality control team for ultra-high sulfur gas field composite steel pipes, and adopted a full inspection mode for key links, which ensures the qualification rate of products.

Offshore oil and gas pipelines are an important part of offshore oil and gas exploration and development, which are in service in the harsh marine environment. The marine environment is complex, including seawater corrosion, marine atmospheric corrosion, marine organism corrosion, and the pipeline is also subjected to wind wave impact, ocean current scouring, seabed soil pressure and other environmental loads. At the same time, the offshore oil and gas transportation medium usually contains corrosive components such as salt, carbon dioxide and hydrogen sulfide, which makes the offshore pipeline face more severe corrosion challenges. Therefore, offshore oil and gas pipelines have high requirements on corrosion resistance, impact toughness, fatigue resistance and structural integrity. Inner clad or lined corrosion-resistant alloy composite steel pipes can meet these requirements and have been widely used in offshore oil and gas gathering and transmission pipelines, submarine oil and gas transmission pipelines and other fields.

The composite steel pipes used in offshore oil and gas pipelines are mainly inner clad composite steel pipes prepared by explosion cladding or hot rolling cladding process, and the lined composite steel pipes prepared by hydraulic pipe expansion process. The base layer usually adopts high-strength low alloy steel (such as X65, X80) with good impact toughness and fatigue resistance, and the clad/lined layer adopts 316L stainless steel or Inconel 625 nickel-based alloy with excellent corrosion resistance. The nominal diameter of the pipeline is usually 200-1000mm, and the wall thickness is 10-20mm, which can meet the requirements of offshore high-pressure transmission (pressure 8-15MPa). During my internship, I learned about an offshore oil field project in the South China Sea, which adopted a total length of 800km composite steel pipes, including inner clad composite steel pipes prepared by explosion cladding process (used for submarine transmission pipelines) and lined composite steel pipes prepared by hydraulic pipe expansion process (used for on-platform gathering and transmission pipelines). The base layer of the composite steel pipe is X80 low alloy steel, and the clad/lined layer is 316L stainless steel. The pipeline has been in service for 4 years, and the operation is stable. No corrosion, peeling or leakage defects have been found in the regular inspection.

The application advantages of composite steel pipes in offshore oil and gas pipelines are mainly reflected in four aspects: first, the corrosion-resistant alloy clad/lined layer can effectively resist seawater corrosion, marine atmospheric corrosion and marine organism corrosion, and prevent the pipeline from being corroded and damaged; second, the base layer of high-strength low alloy steel has good impact toughness and fatigue resistance, which can resist wind wave impact, ocean current scouring and other environmental loads, and avoid pipeline rupture caused by fatigue damage; third, the composite steel pipe has high structural integrity and good concentricity, which is convenient for offshore pipeline installation and welding; fourth, compared with the whole corrosion-resistant alloy pipe, the composite steel pipe has lower cost and lighter weight, which can reduce the transportation and installation cost of offshore pipelines (offshore transportation and installation cost is very high, and reducing the weight of the pipeline can significantly reduce the installation cost). For example, in the above-mentioned South China Sea offshore oil field project, the use of composite steel pipes instead of whole stainless steel pipes reduced the transportation and installation cost by about 35%.

However, the application of composite steel pipes in offshore oil and gas pipelines also faces some challenges: first, the marine environment is harsh, and the pipeline is under long-term immersion and wind wave impact, which has high requirements on the bonding performance of the composite steel pipe. If the bonding performance is poor, the clad/lined layer will peel off from the base layer, leading to pipeline corrosion failure; second, the offshore pipeline installation and maintenance are difficult, and the cost is high. Once the composite steel pipe is damaged, it is difficult to repair, and it is necessary to use professional offshore operation equipment, which increases the maintenance cost; third, the marine organism corrosion is difficult to avoid. Although the corrosion-resistant alloy clad/lined layer has good corrosion resistance, some marine organisms (such as barnacles) will attach to the surface of the pipeline, leading to local corrosion. In view of these problems, the enterprise where I interned is developing a composite steel pipe with anti-marine organism attachment function, and optimizing the bonding process to improve the bonding strength of the composite steel pipe, so as to adapt to the harsh marine environment.

Through the application practice of inner clad or lined corrosion-resistant alloy composite steel pipes in different oil and gas fields, it can be found that composite steel pipes have obvious advantages in corrosion resistance, strength, toughness and economy, and can well adapt to the harsh working conditions of the oil and gas industry. The application effect is remarkable, mainly reflected in the following aspects: first, the service life of the pipeline is significantly extended, which can reach more than 25-30 years, which is 2-3 times that of traditional carbon steel pipes with coatings; second, the failure rate of the pipeline is significantly reduced, avoiding economic losses and safety hazards caused by pipeline corrosion, peeling and leakage; third, the comprehensive economic benefit is good. Although the initial investment of composite steel pipes is higher than that of traditional pipelines, the long service life and low maintenance cost make the comprehensive economic benefit of composite steel pipes better than that of traditional pipelines; fourth, the application scope is wide, which can be applied to onshore, offshore, ultra-high sulfur and other different harsh environments, and can meet the requirements of different specifications and pressure levels of pipelines.

During my internship, I deeply realized that the engineering application of composite steel pipes is closely related to the preparation process, product quality and engineering design. Only by selecting the appropriate preparation process according to the engineering working conditions, strictly controlling the product quality, and carrying out scientific engineering design and installation, can the excellent performance of composite steel pipes be brought into play. For example, in ultra-high sulfur gas fields, it is necessary to select the explosion cladding process with high bonding strength and Inconel 625 nickel-based alloy clad layer; in onshore long-distance transmission pipelines, it is possible to select the surfacing welding process with relatively low cost and 316L stainless steel clad layer; in offshore pipelines, it is necessary to select the composite steel pipe with good bonding performance and impact toughness.

At the same time, there are still some problems in the engineering application of composite steel pipes, such as high production cost (especially nickel-based alloy composite steel pipes), complex preparation process, difficult welding and maintenance, etc. These problems restrict the further promotion and application of composite steel pipes. Therefore, it is necessary to further optimize the preparation process, reduce the production cost, improve the welding and maintenance technology, and develop new high-performance, low-cost composite steel pipe materials, so as to expand the application scope of composite steel pipes in the oil and gas industry.

With the continuous deepening of China’s oil and gas exploration and development to deep-sea, deep-layer and high-sulfur areas, the harsh degree of pipeline operating conditions is increasing, and the requirements for the performance of oil and gas pipeline materials are also getting higher and higher. At the same time, with the rapid development of material science, manufacturing technology and testing technology, inner clad or lined corrosion-resistant alloy composite steel pipes, as a high-performance, economical and environmentally friendly pipeline material, are facing new development opportunities and challenges. Based on the current technical level, engineering application practice and my internship experience, this section will discuss the development trends and prospects of inner clad or lined corrosion-resistant alloy composite steel pipes, focusing on the development trends of preparation technology, material research and development, performance optimization and intelligent development, and look forward to the application prospect of composite steel pipes in the oil and gas industry.

The preparation technology of inner clad or lined corrosion-resistant alloy composite steel pipes is the core factor affecting the product quality, production efficiency and production cost. At present, the main preparation technologies (thermal spraying, surfacing welding, explosion cladding, hot rolling cladding, pipe expansion, etc.) have their own advantages and disadvantages. In the future, the development trend of preparation technology will focus on high efficiency, low cost, high quality and environmental protection, and will continue to optimize the existing technology and develop new preparation technologies.

The first development trend is the automation and intelligence of the existing preparation technology. At present, some preparation processes (such as thermal spraying, surfacing welding) still rely on manual operation, which has low production efficiency, large labor intensity and unstable product quality. In the future, with the development of industrial automation and intelligent technology, the existing preparation technology will gradually realize full automation and intelligence. For example, the thermal spraying process will adopt intelligent spray gun control system, which can automatically adjust the flame temperature, spraying distance, powder feeding speed and other parameters according to the size of the base steel pipe and the requirements of the clad layer, ensuring the uniformity and stability of the clad layer; the surfacing welding process will adopt robot automatic welding technology, which can improve the welding efficiency and welding quality, reduce the manual operation error, and realize the continuous production of large-diameter composite steel pipes. During my internship, I saw that the enterprise is trying to introduce robot automatic surfacing welding equipment, which can improve the production efficiency by more than 50% and reduce the product defect rate by more than 30% compared with manual surfacing welding.

The second development trend is the optimization and integration of existing preparation technologies. The existing preparation technologies have their own limitations. For example, the thermal spraying process has low bonding strength, the explosion cladding process is dangerous and has high cost, and the hot rolling cladding process has narrow applicable scope. In the future, the enterprise will integrate the advantages of different preparation technologies to develop new composite preparation technologies. For example, the combination of thermal spraying and surfacing welding process: first, use thermal spraying to prepare a thin corrosion-resistant alloy clad layer (as the bottom layer), and then use surfacing welding to prepare a thick clad layer (as the working layer). This combination can not only improve the bonding strength of the clad layer, but also improve the production efficiency and reduce the production cost; the combination of hot rolling cladding and hydraulic pipe expansion process: first, use hot rolling cladding to prepare the composite blank, and then use hydraulic pipe expansion to improve the bonding tightness between the base layer and the clad layer, ensuring the product quality. During my internship, the technical master told me that the enterprise is carrying out research on the combination of thermal spraying and surfacing welding process, and has achieved initial results. The composite steel pipes prepared by this technology have both high bonding strength and high production efficiency.

The third development trend is the development of new environment-friendly preparation technologies. At present, some preparation processes (such as explosion cladding, thermal spraying) will produce noise, dust and harmful gases during the production process, which will pollute the environment and affect the health of operators. In the future, with the improvement of environmental protection requirements, the development of new environment-friendly preparation technologies will become an important direction. For example, the development of low-noise, low-dust explosion cladding technology, the use of environmentally friendly explosives and dust removal equipment to reduce environmental pollution; the development of vacuum thermal spraying technology, which can avoid the oxidation of the clad layer during the spraying process, improve the product quality, and reduce the emission of harmful gases. In addition, the development of energy-saving preparation technologies (such as low-energy consumption hot rolling cladding technology) will also become an important trend, which can reduce energy consumption and production cost.

The material of inner clad or lined corrosion-resistant alloy composite steel pipes directly determines the performance of the product. At present, the base layer material is mainly carbon steel/low alloy steel, and the clad/lined layer material is mainly stainless steel and nickel-based alloy. In the future, with the increasingly harsh working conditions of oil and gas pipelines and the continuous development of material science, the research and development of composite steel pipe materials will focus on high performance, low cost and multi-function, and will develop new high-performance base layer and clad/lined layer materials.

The first development trend is the research and development of high-strength, high-toughness base layer materials. With the increase of oil and gas transmission pressure and the expansion of transmission distance, the requirements for the strength and toughness of the base layer of composite steel pipes are getting higher and higher. At present, the base layer material is mainly Q355, X80 low alloy steel. In the future, the research and development of high-strength low alloy steel (such as X90, X100) with higher strength and toughness will become the focus. X90 and X100 low alloy steel have higher yield strength and tensile strength, which can reduce the wall thickness of the composite steel pipe under the same transmission pressure, reduce the weight of the pipeline, and reduce the transportation and installation cost. At the same time, the high toughness of these materials can improve the anti-impact and anti-fatigue performance of the pipeline, adapting to the complex environmental loads. During my internship, I learned that the enterprise is cooperating with universities to carry out research on X90 low alloy steel base layer composite steel pipes, and has prepared small-batch samples, which have passed the performance test and meet the requirements of high-pressure transmission.

The second development trend is the research and development of low-cost, high-corrosion-resistant clad/lined layer materials. At present, the high-corrosion-resistant clad/lined layer materials (such as Inconel 625 nickel-based alloy) have high price, which leads to high production cost of composite steel pipes, restricting their wide application. In the future, the research and development of low-cost, high-corrosion-resistant alloy materials will become an important direction. For example, the research and development of low-nickel stainless steel (such as 2205 duplex stainless steel) and composite corrosion-resistant alloys (such as stainless steel-nickel-based alloy composite materials) can reduce the content of precious metals (such as nickel, molybdenum) on the premise of ensuring corrosion resistance, thus reducing the material cost. 2205 duplex stainless steel has both austenitic and ferritic structures, which has good corrosion resistance (close to 316L stainless steel) and high strength, and the cost is 20%-30% lower than that of 316L stainless steel. At present, the enterprise where I interned has begun to use 2205 duplex stainless steel as the clad/lined layer material for some medium-corrosion environment projects, and the application effect is good.

The third development trend is the research and development of multi-functional composite materials. In the future, the composite steel pipes will not only have corrosion resistance and high strength, but also develop in the direction of multi-function, such as anti-marine organism attachment, anti-fatigue, anti-high temperature and other functions. For example, adding anti-fouling agents to the corrosion-resistant alloy clad/lined layer to prevent marine organisms from attaching to the surface of the pipeline, reducing local corrosion; adding rare earth elements to the base layer material to improve the anti-fatigue performance of the pipeline, extending the service life of the pipeline in the environment of alternating load; developing high-temperature corrosion-resistant alloy materials (such as Hastelloy alloy) to adapt to the high-temperature working conditions of deep-layer oil and gas transmission (temperature ≥150℃). During my internship, I learned that the enterprise is carrying out research on anti-marine organism composite steel pipes, and has added a special anti-fouling component to the 316L stainless steel clad layer, which can effectively prevent the attachment of barnacles and other marine organisms.

With the increasing requirements of the oil and gas industry for the safety and reliability of pipelines, the performance optimization of inner clad or lined corrosion-resistant alloy composite steel pipes and the intelligent detection of product quality will become important development trends. The performance optimization will focus on improving the bonding performance, corrosion resistance and structural integrity of composite steel pipes, while the intelligent detection will focus on improving the detection efficiency, accuracy and non-destructiveness, realizing the full-process quality control of composite steel pipes.

In terms of performance optimization, the first focus is to improve the bonding performance between the base layer and the clad/lined layer. Bonding performance is the key to ensuring the overall performance of composite steel pipes. In the future, through the optimization of pre-treatment technology, process parameter control and post-treatment technology, the bonding strength and bonding integrity of composite steel pipes will be further improved. For example, optimizing the sandblasting pre-treatment process, adjusting the sandblasting pressure and sand particle size to improve the roughness and cleanliness of the base layer surface, enhancing the bonding force between the base layer and the clad/lined layer; optimizing the process parameters of surfacing welding and explosion cladding, adjusting the welding current, detonation speed and other parameters to form a more dense and continuous bonding interface; developing new post-treatment technologies (such as laser remelting technology), which can remelt the bonding interface, eliminate interface defects (such as gaps, oxide layers), and improve the bonding strength. During my internship, the technical personnel used laser remelting technology to treat the bonding interface of composite steel pipes prepared by thermal spraying process, and the shear bonding strength was improved by more than 40%.

The second focus of performance optimization is to improve the corrosion resistance and service life of composite steel pipes. On the basis of developing new corrosion-resistant materials, the corrosion resistance of composite steel pipes will be further improved through surface modification technology and corrosion protection measures. For example, adopting laser surface hardening technology to improve the hardness and corrosion resistance of the clad/lined layer surface, enhancing the wear and corrosion resistance of the pipeline inner wall; applying a special anti-corrosion coating on the surface of the clad/lined layer (such as PTFE coating), which can form a double anti-corrosion protection system with the corrosion-resistant alloy clad/lined layer, further improving the corrosion resistance of the pipeline; optimizing the structure of the clad/lined layer, adopting the gradient composite structure (the corrosion resistance of the clad/lined layer gradually increases from the base layer to the surface), which can not only ensure the bonding performance with the base layer, but also improve the corrosion resistance of the surface. For example, the gradient composite clad layer with “low nickel stainless steel-inner layer + high nickel stainless steel-outer layer” can reduce the cost while ensuring the surface corrosion resistance.

The third focus of performance optimization is to improve the structural integrity and dimensional accuracy of composite steel pipes. Through the optimization of the preparation process and the improvement of production equipment, the thickness uniformity, concentricity and dimensional accuracy of composite steel pipes will be further improved, avoiding structural defects such as uneven thickness, eccentricity and internal cracks. For example, adopting automatic rolling equipment and intelligent control system to improve the thickness uniformity of the hot rolling cladding composite steel pipe; adopting high-precision insertion equipment and concentricity detection system to improve the concentricity of the lined composite steel pipe; developing online defect detection technology to detect the structural defects of composite steel pipes in real time during the preparation process, and eliminate the defects in time.

In terms of intelligent detection, the first development trend is the intelligence and automation of detection equipment. At present, some detection methods (such as manual ultrasonic flaw detection) have low detection efficiency and high labor intensity, and are easily affected by human factors. In the future, with the development of artificial intelligence, big data and Internet of Things technology, the detection equipment of composite steel pipes will gradually realize intelligence and automation. For example, developing intelligent ultrasonic flaw detection equipment with artificial intelligence recognition function, which can automatically scan the composite steel pipe, identify the type, size and position of defects, and generate detection reports automatically, improving the detection efficiency and accuracy; adopting online real-time detection technology, installing detection sensors on the production line, detecting the thickness of the clad/lined layer, bonding performance and structural defects of composite steel pipes in real time during the preparation process, realizing the full-process quality control. During my internship, I saw that the enterprise is trying to introduce intelligent ultrasonic flaw detection equipment, which can improve the detection efficiency by more than 60% and reduce the missed detection rate by more than 25% compared with manual detection.

The second development trend of intelligent detection is the integration and networking of detection technology. In the future, the detection of composite steel pipes will no longer be a single detection method, but will integrate multiple detection methods (such as ultrasonic flaw detection, radiographic flaw detection, metallographic observation) to form a comprehensive detection system, which can comprehensively evaluate the product quality. At the same time, through the networking of detection equipment, the detection data of composite steel pipes can be transmitted to the cloud platform in real time, realizing the sharing and analysis of detection data. The technical personnel can monitor the product quality in real time through the cloud platform, and adjust the preparation process in time according to the detection data, ensuring the stability of product quality. In addition, the detection data can be used for quality tracing, which can quickly find the causes of quality defects and take targeted improvement measures.

The third development trend of intelligent detection is the non-destructive and accurate detection of micro-defects. With the increasing requirements of the oil and gas industry for pipeline safety, the detection of micro-defects (such as microcracks, tiny gaps) of composite steel pipes will become more and more important. In the future, new non-destructive detection technologies (such as laser ultrasonic detection, terahertz detection) will be developed and applied, which have higher detection accuracy and can detect micro-defects with a size of less than 0.1mm. These technologies can not only detect the surface and internal micro-defects of composite steel pipes, but also avoid damage to the samples, realizing the non-destructive and accurate detection of product quality. During my internship, the testing master told me that laser ultrasonic detection technology has broad application prospects, which can effectively detect the microcracks at the bonding interface of composite steel pipes, and has been used in small-batch product detection.

To sum up, the inner clad or lined corrosion-resistant alloy composite steel pipes will develop in the direction of high efficiency, low cost, high performance, multi-function and intelligence in the future. With the continuous optimization of preparation technology, the continuous development of new materials and the continuous improvement of intelligent detection technology, the performance of composite steel pipes will be further improved, the production cost will be further reduced, and the application scope will be further expanded. It is believed that in the future, composite steel pipes will become the main pipeline material in the oil and gas industry, providing a strong guarantee for the safe, stable and efficient development of the oil and gas industry.