A heat treatment method to improve the corrosion resistance of N36 alloy strip

Abstract

This invention discloses a heat treatment method for improving the corrosion resistance of N36 alloy strip, comprising: vacuum annealing N36 alloy sheet at 580℃~640℃ for 1~3h to obtain sheet 1; cold rolling sheet 1, followed by vacuum annealing at 580℃~640℃ for 1~3h to obtain sheet 2; rolling sheet 2 to obtain strip with a thickness of 0.35~0.90mm; placing the strip under continuous vacuum annealing at 580℃~620℃ for 5~30min, followed by polishing and pickling; the heat treatment method has the advantages of good process stability and high yield, and the obtained N36 alloy strip has a uniform microstructure, fine grains, and fine, uniformly dispersed second phase particles. Under two aqueous chemical conditions, namely 360℃ / 18.6MPa pure water and 360℃ / 18.6MPa LiOH solution, the corrosion rate is significantly lower than that of Zr-4 alloy strip.

Description

Technical Field

[0001] This invention relates to the field of special material preparation technology, specifically to a heat treatment method for improving the corrosion resistance of N36 alloy strip, the resulting N36 alloy strip being used as a strip for positioning grids of fuel assemblies in nuclear power reactors. Background Technology

[0002] Zirconium alloys, with their low neutron absorption cross-section and excellent mechanical and corrosion resistance in high-temperature, high-pressure water, are widely used in reactor fuel assemblies. As a core component of nuclear power reactors, fuel assemblies, whose structural materials are subjected to high-temperature, high-pressure water and high neutron flux conditions, experience water-side corrosion, hydrogen absorption, abrasion, irradiation creep, and growth, all of which affect their performance. With reactors evolving towards higher fuel burnup and longer refueling cycles, fuel assemblies remain within the reactor for longer periods, thus requiring zirconium alloys with better environmental compatibility. The positioning grid is a crucial component of the fuel assembly, playing a role in guiding fuels, maintaining lateral spacing between fuel rods, mixing coolant, and optimizing the thermal, mechanical, and hydraulic properties of the reactor core. It is one of the key factors determining the performance of the fuel assembly.

[0003] Zr-4 alloy (nominal composition Zr-1.5Sn-0.2Fe-0.1Cr), as a representative long-running zirconium alloy internationally, is widely used in core structures such as fuel element cladding tubes, guide tubes, and positioning grid strips in water-cooled power reactors. However, its corrosion rate increases with burnup, and Zr-4 alloy can no longer meet the requirements for high burnup fuel elements. Currently, high-performance Nb-based zirconium alloys with superior overall performance compared to Zr-4 have been developed internationally, such as the French M5 alloy and the American ZIRLO alloy. These alloys are not only used in fuel element cladding but also as guide tube or grid strip materials in fuel assemblies.

[0004] The preparation of zirconium alloy strips and sheets mainly includes ingot smelting, forging, homogenization treatment, hot rolling of slabs, cold rolling, intermediate annealing, and final annealing. The heat treatment regime during alloy processing will affect its microstructure, and thus the corrosion resistance of the finished strip. Previous studies have shown that for Zr-4 alloys, increasing the heat treatment temperature will increase the size of its second phase, which will improve its resistance to uniform corrosion to some extent. For N36 alloys, due to the certain solid solubility of Nb in the alloy, in zirconium alloys with high Nb content, after forging the β phase, the supersaturated Nb dissolved in the zirconium matrix precipitates as β-Nb or Zr-Nb-Fe second phases during subsequent processing and heat treatment. The size distribution, crystal structure, and composition of the second phase are affected by heat treatment. When the annealing temperature is too high, recrystallization and grain growth easily occur, the second phase coarsens, and second phase aggregation may occur, leading to uneven size distribution of the second phase and even the formation of β-Zr, which is detrimental to the corrosion resistance of the zirconium alloy. When the annealing temperature is too low, it will affect the recrystallization of the alloy and the homogenization of the second phase distribution along the rolling flow line during processing, thereby reducing the alloy's corrosion resistance. Cumulative annealing parameters The effects of annealing temperature and time on the corrosion resistance of zirconium alloys are considered together.

[0005] For Zr-Sn alloys, increasing the cumulative annealing parameter improves their resistance to uniform corrosion and is commonly used to evaluate the corrosion resistance of Zr-Sn alloys. For Zr-Sn-Nb alloys (such as N36 alloy), although corrosion resistance is also affected by the Nb content in the alloy, for low-Nb zirconium alloys, it can still be considered that increasing the cumulative annealing parameter within a certain range will increase the size and area ratio of the second phase in the alloy, further reducing the alloy's corrosion resistance. Currently, the mechanism by which heat treatment affects the microstructure and corrosion resistance of Zr-Sn-Nb alloys is unclear, and no systematic reports have been found on the processing heat treatment technology of N36 alloy strip. Summary of the Invention

[0006] The technical problem to be solved by this invention is that there is no systematic report on the processing and heat treatment process of N36 alloy strip in the prior art.

[0007] The purpose of this invention is to provide a heat treatment method for improving the corrosion resistance of N36 alloy strip, the heat treatment method comprising: N36 alloy sheet was vacuum annealed at 580℃~640℃ for 1~3h to obtain sheet 1; Plate 1 is cold rolled and then vacuum annealed at 580℃~640℃ for 1~3 hours to obtain plate 2; The sheet material 2 is rolled to obtain a strip with a thickness of 0.35~0.90mm; The strip is vacuum annealed at 580℃~620℃ for 5~30 minutes, followed by polishing and pickling.

[0008] As one possible design, the thickness of the N36 alloy sheet is 2~4mm.

[0009] As one possible design, the thickness of the plate 2 is 0.8~3mm.

[0010] As one possible design, the thickness of the strip is 0.35~0.90mm.

[0011] As one possible design, the thickness of the plate 2 is 0.8~3mm, and the thickness of the strip is 0.35~0.90mm.

[0012] As one possible design, the thickness of the sheet 2 is 1.5~3mm, and the thickness of the strip is 0.35~0.90mm.

[0013] As one possible design, the thickness of plate 2 is 0.8~1.5mm, and the thickness of strip is 0.35~0.90mm.

[0014] As one possible design, the thickness of plate 2 is 0.8~1.5mm, and the thickness of strip is 0.60~0.90mm.

[0015] As one possible design, the thickness of the plate 2 is 0.8~1.5mm, and the thickness of the strip is 0.35~0.60mm; preferably, the vacuum annealing temperature during the preparation of the plate 2 is 610℃~640℃; preferably, the vacuum annealing temperature during the preparation of the plate 2 is 580℃~610℃.

[0016] As one possible design, the vacuum annealing temperature during the preparation of plate 2 is 580℃~610℃, and the vacuum annealing temperature of strip is 580℃~610℃ or 610℃~620℃.

[0017] The beneficial effects of this invention are as follows: The heat treatment method disclosed in this invention has the advantages of good process stability and high yield. The obtained N36 alloy strip has a uniform microstructure and fine grains, and the second phase particles are fine and uniformly dispersed. Under two water chemical conditions, namely 360℃ / 18.6MPa pure water and 360℃ / 18.6MPa LiOH solution, the corrosion rate is significantly lower than that of Zr-4 alloy strip. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. The illustrative embodiments and descriptions of this invention are only used to explain this invention and are not intended to limit this invention.

[0019] This invention addresses the lack of systematic reports on the heat treatment process for Zr-Sn-Nb alloy strips (such as N36 alloy, nominal composition Zr-1Sn-1Nb-0.3Fe). However, existing technologies provide preparation processes for Zr-4 alloys (nominal composition Zr-1.5Sn-0.2Fe-0.1Cr), and it has been disclosed that increasing the heat treatment temperature during the preparation of Zr-4 alloys is beneficial for increasing the size of its second phase, which to some extent improves its resistance to uniform corrosion. In Zr-Sn-Nb alloys (such as N36 alloy), because Nb has a certain solid solubility in the alloy, the corrosion resistance of N36 alloy is affected by the Nb content.

[0020] This invention provides a heat treatment method for N36 alloy to improve its corrosion resistance.

[0021] A heat treatment method for N36 alloy includes the following steps: S1. The N36 alloy sheet was vacuum annealed at 580℃~640℃ for 1~3h to obtain sheet 1; S2. Cold roll sheet 1, then vacuum anneal at 580℃~640℃ for 1~3h to obtain sheet 2; S3. Roll the sheet 2 to obtain a strip with a thickness of 0.35~0.90mm; S4. Place the strip in a vacuum annealing environment at 580℃~620℃ for 5~30 minutes, and then polish and pickle it.

[0022] This invention improves the corrosion resistance of N36 alloy by processing the original sheet multiple times and controlling the temperature, processing time, and strip thickness of each processing step.

[0023] In step S1, the thickness of the N36 alloy plate is generally 2~4mm, and the thickness of plate 1 is generally 0.8~3mm.

[0024] The temperature of cold rolling in step S2 is generally [temperature value], and the time for one cold rolling cycle is generally [duration value], which is selected based on the temperature of cold rolling and the desired effect.

[0025] The rolling process in step S3 specifically involves: The solutions and specific methods involved in the polishing and acid washing in step S4 are conventional and commonly used in the field, and will not be described in detail here.

[0026] As one feasible method, the thickness of the sheet after the two cold rolling processes (i.e., sheet 2) is 0.8~3mm; the thickness of the finished strip after rolling is 0.35~0.90mm; the vacuum annealing conditions for the sheet after the two cold rolling processes are: holding at a temperature of 580~640℃ for 1~3h; the vacuum annealing conditions for the finished strip after rolling processes are: holding at a temperature of 580~620℃ for 5~30min.

[0027] As one feasible method, the thickness of the sheet material (i.e., sheet 2) after two cold rolling processes is 1.5~3mm; the thickness of the finished strip after rolling is 0.35~0.90mm; the vacuum annealing of the sheet material after two cold rolling processes is: holding at a temperature of 580~640℃ for 1~3h; the vacuum annealing of the finished strip after rolling processes is: holding at a temperature of 580~620℃ for 5~30min.

[0028] As one feasible method, the thickness of the sheet after the two cold rolling processes is 0.8~1.5mm; the thickness of the finished strip after rolling is 0.35~0.90mm; the vacuum annealing of the sheet after the two cold rolling processes is: holding at a temperature of 580~640℃ for 1~3h; the vacuum annealing of the finished strip after rolling processes is: holding at a temperature of 580~620℃ for 5~30min.

[0029] As one feasible method, the thickness of the sheet after the two cold rolling processes is 0.8~1.5mm; the thickness of the finished strip after rolling is 0.35~0.60mm; the vacuum annealing of the sheet after the two cold rolling processes is: holding at a temperature of 610~640℃ for 1~3h; the vacuum annealing of the finished strip after rolling processes is: holding at a temperature of 580~620℃ for 5~30min.

[0030] As one feasible method, the thickness of the sheet after the two cold rolling processes is 0.8~1.5mm; the thickness of the finished strip after rolling is 0.35~0.60mm; the vacuum annealing of the sheet after the two cold rolling processes is: holding at a temperature of 580~610℃ for 1~3h; the vacuum annealing of the finished strip after rolling processes is: holding at a temperature of 580~620℃ for 5~30min.

[0031] As one feasible method, the thickness of the sheet after the two cold rolling processes is 0.8~1.5mm; the thickness of the finished strip after rolling is 0.35~0.60mm; the vacuum annealing of the sheet after the two cold rolling processes is: holding at a temperature of 580~610℃ for 1~3h; the vacuum annealing of the finished strip after rolling processes is: holding at a temperature of 580~610℃ for 5~30min.

[0032] As one feasible method, the thickness of the sheet after the two cold rolling processes is 0.8~1.5mm; the thickness of the finished strip after rolling is 0.35~0.60mm; the vacuum annealing of the sheet after the two cold rolling processes is: holding at a temperature of 580~610℃ for 1~3h; the vacuum annealing of the finished strip after rolling processes is: holding at a temperature of 610~620℃ for 5~30min.

[0033] Example 1: A heat treatment method for N36 alloy, comprising the following steps: S1. The N36 alloy sheet was vacuum annealed at 580℃ for 3 hours to obtain sheet 1; S2. Cold roll sheet 1, then vacuum anneal at 600℃ for 2 hours to obtain sheet 2. During this process, it should be noted that the furnace exit temperature after vacuum annealing should not be lower than 100℃. S3. Roll the sheet 2 to obtain a strip with a thickness of 0.425 mm; S4. Place the strip in a vacuum annealing process at 600℃ for 10 minutes, followed by polishing and pickling. This process involves continuous degreasing before vacuum annealing.

[0034] The zirconium alloy strip prepared by the above heat treatment technology was subjected to a long-term corrosion test for 500 days under the corrosion conditions of 360℃ / 18.6MPa pure water and 360℃ / 18.6MPa LiOH solution. The corrosion resistance test results are shown in Table 1. Its corrosion rate is significantly lower than that of Zr-4 alloy strip.

[0035] Table 1 Corrosion rates of N36 alloy strip and Zr-4 alloy strip

[0036] Example 2: A heat treatment method for N36 alloy, comprising the following steps: S1. The N36 alloy sheet was vacuum annealed at 600℃ for 2 hours to obtain sheet 1; S2. Cold roll sheet 1, then vacuum anneal at 600℃ for 2 hours to obtain sheet 2. During this process, it should be noted that the furnace exit temperature after vacuum annealing should not be lower than 100℃. S3. Roll the sheet 2 to obtain a strip with a thickness of 0.425 mm; S4. Place the strip in a vacuum annealing process at 640℃ for 10 minutes, followed by polishing and pickling. This process involves continuous degreasing before vacuum annealing.

[0037] The zirconium alloy strip prepared by the above heat treatment technology was subjected to a long-term corrosion test for 500 days under the corrosion conditions of 360℃ / 18.6MPa pure water and 360℃ / 18.6MPa LiOH solution. The corrosion resistance test results are shown in Table 2. Its corrosion rate is significantly lower than that of Zr-4 alloy strip.

[0038] Table 2 Corrosion rates of N36 alloy strip and Zr-4 alloy strip

[0039] Example 3: A heat treatment method for N36 alloy, comprising the following steps: S1. The N36 alloy sheet was vacuum annealed at 610℃ for 1.8h to obtain sheet 1; S2. Cold roll sheet 1, then vacuum anneal at 600℃ for 2 hours to obtain sheet 2. During this process, it should be noted that the furnace exit temperature after vacuum annealing should not be lower than 100℃. S3. Roll the sheet 2 to obtain a strip with a thickness of 0.425 mm; S4. Place the strip in a vacuum annealing process at 580℃ for 10 minutes, followed by polishing and pickling. This process involves continuous degreasing before vacuum annealing.

[0040] The zirconium alloy strip prepared by the above heat treatment technology was subjected to a long-term corrosion test for 500 days in pure water at 360℃ / 18.6MPa and LiOH solution at 360℃ / 18.6MPa. The corrosion resistance test results are shown in Table 3. Its corrosion rate is significantly lower than that of Zr-4 alloy strip, especially in terms of corrosion resistance in alkaline environment.

[0041] Table 3 Corrosion rates of N36 alloy strip and Zr-4 alloy strip

[0042] Example 4: A heat treatment method for N36 alloy, comprising the following steps: S1. The N36 alloy sheet was vacuum annealed at 640℃ for 1 hour to obtain sheet 1; S2. Cold roll sheet 1, then vacuum anneal at 580℃ for 2 hours to obtain sheet 2. During this process, it should be noted that the furnace exit temperature after vacuum annealing should not be lower than 100℃. S3. Roll the sheet 2 to obtain a strip with a thickness of 0.60 mm; S4. Place the strip in a vacuum annealing process at 600℃ for 5 minutes, followed by polishing and pickling. This process involves continuous degreasing before vacuum annealing.

[0043] The zirconium alloy strip prepared by the above heat treatment technology was subjected to a long-term corrosion test for 500 days under the corrosion conditions of 360℃ / 18.6MPa pure water and 360℃ / 18.6MPa LiOH solution. The corrosion resistance test results are shown in Table 4. Its corrosion rate is significantly lower than that of Zr-4 alloy strip, especially in terms of corrosion resistance in alkaline environment.

[0044] Table 4 Corrosion rates of N36 alloy strip and Zr-4 alloy strip

[0045] Example 5: A heat treatment method for N36 alloy, comprising the following steps: S1. The N36 alloy sheet was vacuum annealed at 580℃ for 1 hour to obtain sheet 1; S2. Cold roll sheet 1, then vacuum anneal at 600℃ for 2 hours to obtain sheet 2. During this process, it should be noted that the furnace exit temperature after vacuum annealing should not be lower than 100℃. S3. Roll the sheet 2 to obtain a strip with a thickness of 0.60 mm; S4. The strip is sequentially subjected to continuous vacuum annealing at 580℃ for 10 min and vacuum annealing at 640℃ for 10 min, followed by polishing and pickling. This process involves continuous degreasing before vacuum annealing.

[0046] The zirconium alloy strip prepared by the above heat treatment technology was subjected to a long-term corrosion test for 500 days under the corrosion conditions of 360℃ / 18.6MPa pure water and 360℃ / 18.6MPa LiOH solution. The corrosion resistance test results are shown in Table 5. Within a certain annealing temperature range, the relative corrosion rate decreases with increasing temperature. Therefore, increasing the annealing temperature is beneficial to improving the corrosion resistance rate of the N36 alloy finished strip.

[0047] Table 5 Corrosion rates of N36 alloy strips with a diameter of δ 0.60 mm prepared at different annealing temperatures

[0048] Example 6: A heat treatment method for N36 alloy, comprising the following steps: S1. The N36 alloy sheet was vacuum annealed at 580℃ for 1 hour to obtain sheet 1; S2. Cold roll sheet 1, then vacuum anneal at 600℃ for 2 hours to obtain sheet 2 (δ1.4mm). Note that the furnace exit temperature after vacuum annealing should not be lower than 100℃. S3. Roll the sheet 2 to obtain a strip with a thickness of 0.425 mm and a strip with a thickness of 0.90 mm; S4. Place both types of strips obtained in step S3 under continuous vacuum annealing at 640℃ for 10 minutes, followed by polishing and pickling. This process involves continuous degreasing before vacuum annealing.

[0049] The zirconium alloy strips prepared by the above heat treatment technology were subjected to a long-term corrosion test for 500 days under the corrosion conditions of 360℃ / 18.6MPa pure water and 360℃ / 18.6MPa LiOH solution. The corrosion resistance test results are shown in Table 6. The relative corrosion rates of N36 alloy finished strips with different thicknesses are different. Among them, the smaller the thickness, the smaller the relative corrosion rate.

[0050] Table 6 Corrosion rates of N36 alloy finished strips with δ 0.425 mm and δ 0.90 mm

[0051] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A heat treatment method for improving corrosion resistance of N36 alloy strip, characterized in that, The heat treatment method comprises: vacuum annealing N36 alloy plate at 580-640 DEG C for 1-3h to obtain plate 1; cold rolling plate 1 and then vacuum annealing at 580-640 DEG C for 1-3h to obtain plate 2, wherein the thickness of plate 2 is 0.8-3mm; rolling plate 2 to obtain strip with thickness of 0.35-0.60mm; vacuum annealing the strip at 580-620 DEG C for 5-30min and then polishing and pickling; the furnace-out temperature after vacuum annealing in the preparation of plate 2 should be no less than 100 DEG C; continuously removing oil before vacuum annealing the strip.

2. The heat treatment method for improving corrosion resistance of N36 alloy strip according to claim 1, characterized in that, The thickness of plate 2 is 1.5-3mm.

3. The method of claim 1, wherein the heat treatment is performed at a temperature of 450-550°C for 1-3 hours. The vacuum annealing temperature in the preparation of plate 2 is 580-610 DEG C.

4. The heat treatment method for improving corrosion resistance of N36 alloy strip according to claim 3, characterized in that, The vacuum annealing temperature of the strip is 580-610 DEG C.

Images