A high-corrosion-resistant bimetallic metallurgical composite pipe for oil and gas production was prepared by high-speed laser cladding technology. The interface bonding, mechanical properties and corrosion resistance of the composite pipe were evaluated. The bimetallic metallurgical composite pipe made by cladding the inner wall of the c110 anti-sulfur pipe with nickel-based alloy by high-speed laser cladding technology can not only play the role of rigid support of the base pipe, but also play the role of corrosion protection through the cladding layer. The flattening test proved that the high-speed laser cladding technology can achieve metallurgical bonding of the composite pipe. The mechanical properties of the composite pipe after cladding can still meet the requirements of API Spec 5CT standard. Stress corrosion cracking tests were carried out on the composite pipe samples under NACE standard working conditions and simulated actual working conditions of a domestic oil field. It was found that no stress corrosion cracking occurred in the samples. The composite pipe can be used for oil and gas production containing corrosive media such as H2S and CO2.
Due to the slow research, development and promotion and application of new energy, traditional fossil energy is still the main force of energy supply. As excellent oil and gas fields are gradually exhausted due to continuous exploitation, the exploration and development of oil and gas is developing towards deep wells, ultra-deep wells and corrosive oil and gas wells. Complex and harsh service conditions place high demands on the mechanical properties and corrosion resistance of oil well pipes. An environment rich in corrosive media such as Cl-, H2S and CO: can cause corrosion damage to pipes. Sulfide stress corrosion cracking, hydrogen-induced cracking, pitting and uniform corrosion are all causes of pipe failure and frequent accidents. Traditional single anti-corrosion technology and materials, such as adding corrosion inhibitors, plastic inner coatings, corrosion-resistant alloys, etc., are difficult to balance corrosion resistance reliability and economic indicators, and it is difficult to meet the needs of oil and gas field development with strong corrosive media.
Bimetallic composite pipes retain the advantages of base pipes and cladding metals, and the cost of pipes is only 1/5 to 1/2 of pure corrosion-resistant alloy pipes. Compared with pure alloy pipes, bimetallic composite pipes can also improve safety and reliability in chloride-containing and (or) acidic environments. Compared with mechanical composite pipes, metallurgical composite pipes have a metallurgical bonding surface between the two metals, and the bonding strength is high. The main metallurgical composite forming processes include hot extrusion, hot rolling, centrifugal casting, explosive welding and powder metallurgy. In recent years, domestic steel pipe manufacturers have used rolled composite plates through JCO forming process and non-melting electrode gas shielded arc welding (Tungsten Inert Gas Welding, TIG) + metal active gas shielded arc welding (Metal Active Gas Welding, MAG) and other welding processes to prepare metallurgical composite pipelines for transportation. Sinopec Petroleum Engineering Technology Research Institute Co., Ltd. developed P110+316L bimetallic metallurgical composite oil pipes through hot rolling process. Wang et al. proposed a multi-billage rotary forging (MBRW) multi-metal composite pipe preparation process, which can obtain a good interface transition zone, high interface bonding strength, and good surface quality of the obtained composite pipe. However, due to the various shortcomings of traditional bimetallic metallurgical composite processes, they have gradually failed to meet the growing market demand.
Different from the traditional laser cladding process, the high-speed laser cladding technology uses a synchronous powder feeding method to melt the added powder at a certain distance from the surface of the base material by using a high-energy density beam, and sprays it onto the surface of the base material at high speed. After rapid solidification, it is metallurgically combined with the base material to form an extremely thin cladding layer, which greatly improves the cladding rate. Compared with surfacing, spraying, electroplating and vapor deposition technologies, high-speed laser cladding technology has the advantages of high cladding efficiency, low dilution, dense structure, thin coating and metallurgical combination with the base material, wide selection of cladding materials, large savings of precious elements, and almost no damage to the base material. At present, this technology has been widely used in offshore platforms, coal, metallurgy, automobiles, ships, aerospace and other industries, but there is no precedent for its large-scale application in the thin and long energy pipe industry. Wang Yanfang et al. prepared a martensitic stainless steel cladding layer on the surface of the 35CrMo sucker rod coupling by high-speed laser cladding technology, which greatly improved the wear resistance and corrosion resistance of the substrate. The high-speed laser cladding technology is used to prepare a Φ88.9 mm× (0.4+6.45) mm bimetallic metallurgical composite pipe. The microstructure, bonding, mechanical properties and corrosion resistance of the composite pipe are analyzed and studied to provide a theoretical basis for its application in oil and gas fields containing highly corrosive media.
1 Experimental materials and methods
The experimental base pipe is a Φ88.9 mmx6.45 mm C110 sulfur-resistant oil well pipe produced by Baoshan Iron and Steel Co., Ltd. The morphology of the nickel-based alloy powder for cladding under a scanning electron microscope (SEM) is shown in Figure 1. It can be seen that the nickel-based alloy powder is spherical and the particle size is in the design range of 10 to 55 μm.
After the inner wall of the Φ88.9 mmx6.45 mm C110 sulfur-resistant oil well pipe is machined or sandblasted to remove the oxide scale, a layer of nickel-based alloy with a thickness of about 400 μm is clad by high-speed laser to obtain a bimetallic metallurgical composite pipe. Among them, the laser power is 3 kW, the cladding line speed is 4 000 mm/min, and the overlap rate is 60%.
According to the requirements of standards such as YB/T 4335–2013 (Metallurgical composite bimetallic seamless steel pipe for fluid transportation), GB/T 37701–2019 (Inner-coated or lined corrosion-resistant alloy composite steel pipe for petroleum and natural gas industry), and GB/T35072–2018 (Corrosion-resistant alloy composite pipe fittings for petroleum and natural gas industry), “For steel pipes with a composite layer thickness of less than 1.9 mm, the base layer and the composite layer adhesion shear strength test can be replaced by a flattening test”, the flattening test is used to evaluate the bonding between the base pipe and the cladding layer of the metallurgical composite pipe. The obtained composite pipe section is placed between parallel plates, and the flattening test is carried out in two steps: ① The distance between the two plates is pressed to 44.7 mm (derived from the calculation formula for the distance between the two plates in the above-mentioned relevant standards) , observe whether there are cracks, fractures and cladding peeling on the inner and outer surfaces and edges of the composite pipe; ② Continue to flatten until the inner wall of the composite pipe fits together, and observe whether the cladding layer and the base pipe peel off.
API Spec 5CT–2018 ((Casing and Tubing Specifications) has requirements for the strength and plasticity of C110 sulfur-resistant oil well pipes. Therefore, according to this standard, samples are taken and machined for tensile tests to obtain the mechanical properties of the composite pipe.
According to the C-ring method in NACE TM0177-2016 “Laboratory Standard Test Method for Resistance of Metals to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments”, the corrosion resistance of the composite pipe in two typical corrosion conditions (NACE standard environment and simulated conditions of a certain oil field in Northwest China) is evaluated. The corrosion conditions are shown in Table 1. To avoid interference, the inner and outer walls of the C-ring are formed by high-speed laser cladding to form a cladding layer.
2 Results and discussion
2.1 Composite interface organization and composition analysis
First, the microstructure of the composite interface was characterized. The thickness of the composite tube cladding layer was analyzed by SEM, as shown in Figure 2. It can be seen that the fluctuation of the cladding layer thickness is very small, and the average thickness is about 400
μm. By changing the laser cladding parameters, it is found that the thickness of the cladding layer increases with the increase of powder feeding amount or the decrease of cladding line speed.
The energy dispersive spectrometer (EDS) analysis spectrum of the content of each element in the composite interface is shown in Figure 3. Among them, Figure 3 (a) shows the composition line scan from the cladding layer to the base tube, and it is found that the Ni, Cr, Fe and Mo elements at the composite interface have very obvious content mutations. In addition, the content gradient distribution of these four elements due to cladding dilution can also be clearly observed inside the cladding layer: along the cladding layer to the base tube, the content of Ni, Cr, and Mo gradually decreases, and the content of Fe gradually increases. Figure 3 (b) to (d) are the surface scanning results of Fe, Ni, and Cr elements, respectively. It can be seen that the distribution of each element content can well correspond to the shape of the cladding layer and the base tube.
The structure of the composite tube sample was observed with an optical microscope, and the results are shown in Figure 4. As can be seen from Figure 4, the sample is clearly divided into three areas from top to bottom: the nickel-based alloy layer, the heat-affected zone, and the base tube. The characteristics of fast heating and cooling of high-speed laser cladding technology make the average thickness of the heat-affected zone only about 300μm. There is a certain fluctuation in the thickness of the heat-affected zone. As shown in Figure 4, the thicker area is about 340μm, and the thinner area is about 280μm. This is because the center temperature of the molten pool generated by the cladding layer is high and the surrounding temperature is low, which causes different thermal effects on the substrate. Figure 5 shows the metallographic structure of the heat-affected zone near the cladding layer. It can be seen that there is no stratification between the cladding layer and the base tube, and the two are metallurgically bonded. In addition, affected by laser cladding, part of the structure of the heat-affected zone is transformed from the tempered bainite of the original base tube to upper bainite.
2.2 Composite interface bonding performance
The composite tube was subjected to a flattening test to evaluate the bonding between the base tube and the cladding layer. After the ductility test, the sample was unloaded and observed. It was found that the loaded sample was still intact, with no cracks or fractures on the surface and edge, and the base tube and the cladding layer were not separated, as shown in Figure 6 (a). The pressure was continued until the inner wall was fitted to complete the flattening test, and the sample did not break; after unloading, the sample rebounded to a certain extent, as shown in Figure 6 (b). The sample was macroscopically observed, and no peeling of the cladding layer and the base tube occurred.
A stereo microscope was used to observe the bonding between the cladding layer and the base tube at the maximum deformation. To facilitate the distinction between the two, the sample was simply polished and corroded. The bonding between the cladding layer and the base tube at the maximum deformation is shown in Figure 7. Although the cladding layer inevitably cracked due to the huge deformation force, the crack did not extend to the base tube, and the cladding layer did not peel off from the base tube. It can be seen that the bonding performance of the metallurgical composite tube manufactured by high-speed laser cladding meets the requirements of relevant standards.
Subsequently, a torsion test was performed to verify the interface bonding. After the composite pipe sample was twisted 360° and observed through a stereo microscope, it was found that the cladding layer and the base pipe did not separate at the junction, which once again proved that the composite pipe had excellent bonding performance.
2.3 Mechanical properties of composite pipe
The mechanical properties of the composite pipe sample are shown in Table 2. It can be seen that the yield strength of the composite pipe sample is high, close to the upper limit of the API Spec 5CT standard; the elongation just exceeds the API standard. As mentioned above, the base pipe near the cladding layer forms a heat-affected zone of a certain thickness composed of upper bainite structure, which has a certain impact on the overall mechanical properties of the composite pipe. If the composite pipe is heat treated with the same quenching and tempering process as the base pipe, its mechanical properties will better meet the standard requirements. In addition, the tensile sample fracture was observed using a stereo microscope. After the tensile neck fracture, the cladding layer and the base pipe did not peel off, which further verified the bonding performance of the composite pipe.
2.4 Corrosion resistance of composite pipe
The C-ring of the composite pipe was immersed in the corrosive medium for 720 hours. The pH changes of the solution before and after the test are shown in Table 3. The side and inner wall (tension surface) of the C-ring were observed by visual inspection, macro camera and stereo microscope to see if cracks occurred. The results showed that the carbon steel base pipe had a certain degree of corrosion; the cladding layer was not corroded in both environments and showed a metallic luster; there were no cracks on the side and inner wall of the C-ring.
The side and inner wall of the C-ring were observed at 20 times using a stereo microscope, and the results are shown in Figure 8. As can be seen from Figure 8 (a) to (b), the base pipe was locally corroded in both corrosion environments, but no cracks occurred; the cladding layer was not corroded. This also proves that the laser cladding process did not affect the sulfur resistance of the base pipe. Figure 8 (c) to (d) shows that the cladding layer on the inner wall of the C-ring did not crack in both corrosion environments.
Taking the C-ring after being corroded under simulated working conditions in an oil field as an example, SEM observation and EDS composition analysis were carried out, and the results are shown in Figure 9 and Table 4. As can be seen from Figure 9, the cladding layer of the composite pipe is still smooth and complete, without any cracks, and no peeling occurs from the base pipe. Table 4 shows the average content of each element in the cladding layer before and after corrosion. It is found that the corrosion process has little effect on the type and content of elements in the cladding layer. The C-type ring after corrosion under the NACE environment shows the same phenomenon and law. It can be seen that neither of the two corrosion environments has any effect on the cladding layer. The high-speed laser cladding process has no effect on the corrosion resistance of the anti-sulfur base pipe. The metallurgical composite pipe shows excellent corrosion resistance in both corrosion environments, meeting the requirements of the NACETM 0177 standard and user needs.
3 Conclusion
(1) The high-speed laser cladding technology has little effect on the C110 anti-sulfur base pipe, and only produces a heat-affected zone with a thickness of 300μm, and its structure is upper bainite. The average thickness of the cladding layer of the composite pipe is only about 400μm, and the elements in the cladding layer have a content gradient distribution due to cladding dilution.
(2) It can be seen from the metallographic photos that the cladding layer and the base pipe are metallurgically bonded. After the flattening test, the cladding layer did not peel off from the base pipe, and the bonding of the composite pipe met the standard requirements.
(3) The mechanical properties of the clad composite pipe meet the API Spec 5CT standard, and the performance of the quenched and tempered pipe is better. In addition, the composite pipe has excellent corrosion resistance under the NACE standard environment and the simulated working conditions of a certain oil field in Northwest China.
