A method for improving corrosion resistance of 825 nickel-based alloy seamless steel pipe

Abstract

The present application belongs to the technical field of alloy pipe processing, and particularly relates to a method for improving the corrosion resistance of 825 nickel-based alloy seamless pipes. First, the hot-rolled blank pipe is subjected to first cold rolling, the cold-rolled seamless pipe is subjected to high-temperature annealing treatment to obtain a uniform structure in which titanium elements are fully dissolved, and then the second cold rolling is performed to the finished product specification, the uniform structure after annealing is inherited to the finished product, and finally the 825 nickel-based alloy seamless pipe is subjected to solid solution treatment, the solid solution temperature is reduced and the holding time is controlled, the deformed grains are fully and uniformly refined, TiC is formed to fix C, and a large amount of Cr 23 C6 is prevented from being generated, and the corrosion resistance of the pipe is reduced. The method of the present application not only meets the requirements of the structure and mechanical properties of the 825 nickel-based alloy seamless pipe, but also improves the structure and the corrosion resistance of the seamless pipe, especially the intergranular corrosion resistance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of alloy pipe processing, and particularly relates to a method for improving the corrosion resistance of 825 nickel-based alloy seamless steel pipes. BACKGROUND

[0002] 825 nickel-based alloy is a titanium-stabilized solid solution strengthening type full austenitic Ni-Fe-Cr-based alloy, which belongs to both high-temperature alloy and corrosion-resistant alloy. Since the 825 nickel-based alloy seamless pipe is long-term served in the harsh working conditions of high temperature and high pressure, high requirements are put forward for the chemical composition, microstructure, mechanical properties, corrosion resistance and surface quality of the pipe material, so the pipe processing and heat treatment process of the alloy is an important link in the production and application of the alloy.

[0003] The 825 nickel-based alloy pipe is usually produced by hot extrusion to form a rough pipe, and then is produced into a finished pipe through multi-pass cold rolling, intermediate high-temperature annealing and solid solution treatment. In the production process, unreasonable cold working and heat treatment process will cause chromium-rich phase to precipitate at the intergranular ， which greatly increases the intergranular corrosion sensitivity of the 825 nickel-based alloy, thereby affecting the service performance of the 825 nickel-based alloy, especially the intergranular corrosion resistance. SUMMARY

[0004] The application solves the technical problem of providing a method for improving the corrosion resistance of 825 nickel-based alloy seamless steel pipes.

[0005] The method for improving the corrosion resistance of 825 nickel-based alloy seamless pipes provided by the application comprises the following steps:

[0006] (1) selecting the 825 nickel-based alloy seamless pipe after hot rolling piercing as the research object, and the specification of the steel pipe is Φ79mmx10mm.

[0007] (2) performing two-pass cold rolling on the LG-60 cold rolling mill. The first cold rolling deformation is more than 70%, and the rolling speed is 2.5m / min; the high-temperature annealing treatment is performed on the cold-rolled steel pipe, the temperature is 1050℃-1120℃, the holding time is 20min-40min, and the water cooling is performed.

[0008] (3) performing the second cold rolling on the water-cooled and pickled steel pipe, the deformation is 39%, the rolling speed is 3m / min, and the solid solution treatment is performed on the cold-rolled steel pipe, that is, the temperature is 940℃-980℃ in the solid solution heating furnace, the holding time is 10min-20min, and the water cooling is performed.

[0009] (4) cutting a circular ring sample from the finished pipe, and performing hardness test analysis by using a Rockwell hardness tester.

[0010] (5) Take mechanical samples from the finished pipe and use a multi-functional tensile testing machine to test and analyze their mechanical properties.

[0011] (6) Cut a section of the finished tube into a circular sample and sensitize it. Keep it at 750℃ for 2 hours. Then, perform an intergranular corrosion test on the sensitized circular sample using 10% oxalic acid solution.

[0012] (7) The specific methods for testing and analyzing the final product are as follows:

[0013] 1) The cross-sectional microstructure was observed using an optical metallographic microscope, and the grain size grade was calculated;

[0014] 2) Hardness testing was performed using a Rockwell hardness tester;

[0015] 3) Use a multi-functional tensile testing machine to conduct relevant mechanical property tests and analyses;

[0016] 4) A 10% oxalic acid solution was used at a current of 1 A / cm. 2 After 90 seconds of corrosion, observe the intergranular corrosion.

[0017] The beneficial effects of this invention are:

[0018] This invention optimizes the production process of 825 nickel-based alloy by synergistically optimizing three aspects: cold rolling deformation, annealing process, and solution treatment. By optimizing the cold rolling deformation and annealing process parameters, a uniform microstructure with fully dissolved titanium is obtained in the intermediate tube, providing excellent microstructure preparation for the finished tube. Finally, by lowering the solution treatment temperature and controlling the holding time, the grains of the cold-rolled deformation are made more uniform and refined. Simultaneously, TiC is formed during the solution treatment, fixing the carbon elements in the alloy and preventing the formation of large amounts of Cr. 23 C6 slows down the formation of intergranular corrosion and improves the corrosion resistance of products. Attached Figure Description

[0019] Figure 1 The image shows the microstructure of 825 nickel-based alloy after 71% deformation and high-temperature annealing at 1050℃ for 20 min (Example 1).

[0020] Figure 2 The image shows the microstructure of 825 nickel-based alloy after 71% deformation and high-temperature annealing at 1100℃ for 40 min (Example 2).

[0021] Figure 3 The image shows the microstructure of 825 nickel-based alloy after 71% deformation and high-temperature annealing at 1100℃ for 20 minutes (Example 3).

[0022] Figure 4The image shows the microstructure of 825 nickel-based alloy after 71% deformation and high-temperature annealing at 1120°C for 20 minutes (Example 6).

[0023] Figure 5 The image shows the microstructure of 825 nickel-based alloy after 50% deformation, high-temperature annealing at 1100℃ for 20 min, cold rolling, and solution treatment at 940℃ for 10 min (Comparative Example 1).

[0024] Figure 6 The image shows the microstructure of 825 nickel-based alloy after 71% deformation, high-temperature annealing at 1100℃ for 20 min, cold rolling, and solution treatment at 940℃ for 10 min (Example 3).

[0025] Figure 7 The image shows the microstructure of 825 nickel-based alloy after 71% deformation, high-temperature annealing at 1100℃ for 20 min, cold rolling, and solution treatment at 940℃ for 20 min (Example 4).

[0026] Figure 8 The image shows the microstructure of 825 nickel-based alloy after 71% deformation, high-temperature annealing at 1100℃ for 20 min, cold rolling, and solution treatment at 980℃ for 10 min (Example 5).

[0027] Figure 9 The product is an 825 nickel-based alloy that has undergone 50% deformation, high-temperature annealing at 1050℃ for 20 min, cold rolling, and solution treatment at 940℃ for 10 min. After sensitization treatment at 750℃ for 2 h, the microstructure is obtained by electrolytic intergranular corrosion using 10% oxalic acid solution (Comparative Example 1).

[0028] Figure 10 The microstructure of the 825 nickel-based alloy after 71% deformation and solution treatment at 940℃ for 10 min, followed by sensitization treatment at 750℃ for 2 h and intergranular corrosion by electrolysis with 10% oxalic acid solution (Comparative Example 2).

[0029] Figure 11 The product is an 825 nickel-based alloy that has undergone 71% deformation, high-temperature annealing at 1100℃ for 20 min, cold rolling, and solution treatment at 980℃ for 20 min. After sensitization treatment at 750℃ for 2 h, the microstructure is obtained by electrolytic intergranular corrosion using 10% oxalic acid solution (Example 4).

[0030] Figure 12The product is an 825 nickel-based alloy that has undergone 71% deformation, high-temperature annealing at 1100℃ for 20 min, and solution treatment at 940℃ for 10 min. After sensitization treatment at 750℃ for 2 h, the microstructure is obtained by intergranular corrosion electrolysis with 10% oxalic acid solution (Example 3). Detailed Implementation

[0031] The present invention will be further described below with reference to embodiments, but is not limited thereto.

[0032] Example 1

[0033] (1) Hot-rolled 825 nickel-based alloy seamless tubes were selected as the research object, with steel tube specifications of Φ79mm×10mm;

[0034] (2) The steel pipe is subjected to a first cold rolling at a rolling speed of 2.5 m / min. The rolled steel pipe has a specification of Φ45 mm × 5 mm and a cold rolling deformation of 71%. The cold rolled steel pipe is then subjected to high-temperature annealing. The annealing process is to hold at 1050℃ for 20 min and then water-cool.

[0035] (3) The steel pipe is subjected to a second cold rolling at a rolling speed of 3m / min. The rolled steel pipe has a specification of Φ38mm×3.5mm and a cold rolling deformation of 39%. The cold rolled steel pipe is then subjected to solution treatment and kept at 940℃ for 10min.

[0036] (4) Cut a section of circular sample from the finished tube and perform hardness testing and analysis using a Rockwell hardness tester.

[0037] (5) Take mechanical samples from the finished pipe and use a multi-functional tensile testing machine to test and analyze their mechanical properties.

[0038] (6) Grind the cut sample into a mirror finish using a 10% oxalic acid solution at a current of 1 A / cm. 2 After 90 seconds of corrosion, the intergranular corrosion was observed using a metallographic microscope.

[0039] Example 2

[0040] The annealing process in step (2) of Example 1 was changed to holding at 1100℃ for 40 minutes and then water cooling. The rest is the same as in Example 1.

[0041] Example 3

[0042] The annealing process in step (2) of Example 1 was changed to holding at 1100℃ for 20 minutes and then water cooling. Everything else was the same as in Example 1.

[0043] Example 4

[0044] The annealing process in step (2) of Example 1 was changed to holding at 1100℃ for 20 minutes and then water-cooled; the solution treatment process in step (3) was changed to holding at 940℃ for 20 minutes and then water-cooled. The rest was the same as in Example 1.

[0045] Example 5

[0046] The annealing process in step (2) of Example 1 was changed to holding at 1100℃ for 20 minutes and then water-cooled; the solution treatment process in step (3) was changed to holding at 980℃ for 10 minutes and then water-cooled. The rest was the same as in Example 1.

[0047] Example 6

[0048] The annealing process in step (2) of Example 1 was changed to holding at 1120℃ for 20 minutes and then water cooling. Everything else was the same as in Example 1.

[0049] Comparative Example 1

[0050] The cold rolling deformation amount in step (2) of Experimental Example 1 was changed to 50%, and the annealing process was changed to holding at 1100℃ for 20 minutes. The rest was the same as in Example 1.

[0051] Comparative Example 2

[0052] (1) Hot-rolled 825 nickel-based alloy seamless tubes were selected as the research object, with steel tube specifications of Φ79mm×10mm;

[0053] (2) The steel pipe is cold rolled with a deformation of 71%. The cold-rolled steel pipe is then subjected to solution treatment, which involves holding at 940℃ for 10 minutes and then water cooling.

[0054] (3) Cut a section of circular sample from the finished tube and perform hardness testing and analysis using a Rockwell hardness tester;

[0055] (4) Take mechanical specimens from the finished pipe and use a multi-functional tensile testing machine to test and analyze their mechanical properties.

[0056] (5) Grind the cut sample into a mirror finish using a 10% oxalic acid solution at a current of 1 A / cm. 2 After 90 seconds of corrosion, the intergranular corrosion was observed using a metallographic microscope.

[0057] Table 1 Mechanical properties of finished 825 nickel-based alloy seamless tubes under different processes

[0058] Production process Tensile strength Mpa Yield strength Mpa Elongation % Hardness HRB Example 1 268 630 42 73 Example 2 273 634 42 75 Example 3 286 659 40 79 Example 4 259 627 46 69 Example 5 281 647 41 77 Example 6 276 643 41 76 Comparative Example 1 255 621 49 68 Comparative Example 2 248 615 50 66

[0059] Table 2 Grain size and characteristics of finished 825 nickel-based alloy seamless tubes under different processes.

[0060] Production process Grain size / grade Grain characteristics Example 1 6 Grain uniform Example 2 5 Grain non-uniform Example 3 6.5 Grain uniform Example 4 6 Grain non-uniform, some grains abnormally large Example 5 7 Grain non-uniform, presence of fine grains Example 6 4.5 Grains too large Comparative Example 1 6 Grain non-uniform, some grains abnormally large Comparative Example 2 5 Grain relatively uniform

[0061] Table 3. Degree of intergranular corrosion in finished 825 nickel-based alloy seamless tubes under different processes.

[0062]

[0063] Obviously, from Figures 1-2 It can be seen that as the high-temperature annealing time is extended, the grains gradually grow, but fine grains still exist, which aggravates this non-uniformity in the next cold rolling pass. Under the same softening time, increasing the high-temperature annealing temperature makes the fine grains disappear and the grains relatively uniform, providing good microstructure preparation for the next cold rolling pass and the finished product. When the temperature rises to 1120℃, the grains become coarse, affecting the uniformity of the microstructure. Therefore, the annealing temperature of 1100℃ is the most suitable.

[0064] The comparison revealed that, under the same heat treatment process, the finished tube sample with a cold rolling deformation of more than 70% had a more uniform grain size than the sample with a deformation of 50%. The larger deformation amount indicated that the grains were broken up more thoroughly during the cold rolling process. The finished tube sample with a deformation of 50% had a large grain size difference and contained coarse grains, which affected the corrosion resistance of the product.

[0065] from Figures 4-7 It can be seen that by lowering the solution treatment temperature and controlling the holding time, the degree of grain growth can be reduced and the uniformity of the microstructure of the finished tube can be improved. The microstructure uniformity and the shallowest degree of intergranular corrosion were obtained by using a cold rolling deformation of more than 70% followed by high-temperature annealing at 1100℃ for 20 minutes, and holding at 940℃ for 10 minutes. Therefore, this process improves the corrosion resistance of 825 nickel-based alloy to a certain extent.

[0066] The mechanical properties of the finished pipes prepared under different process conditions all meet the standards.

[0067] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for improving the corrosion resistance of 825 nickel-based alloy seamless steel pipes, characterized in that: The process steps of the method are as follows: (1) The hot-rolled 825 nickel-based alloy seamless steel pipe is subjected to the first cold rolling deformation; The steel pipe has a specification of Φ79mm x 10mm; the deformation during the first cold rolling is over 70%, and the rolling speed is 2.5m / min. (2) The seamless steel pipe after cold rolling is subjected to high-temperature annealing treatment; Among them, the high-temperature annealing treatment is held at 1050℃~1120℃ for 20min~40min; (3) After annealing, the steel pipe is water-cooled, pickled, and then subjected to secondary cold rolling deformation to the finished product specifications; The deformation during the second cold rolling was 39%, and the rolling speed was 3 m / min; (4) Solution treatment is performed on the steel pipe after secondary cold rolling; The solution treatment temperature is 940℃~980℃, and the holding time is 10min~20min.

2. A seamless 825 nickel-based alloy steel pipe obtained by the method according to claim 1.

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