Detection and analysis method for evaluating intergranular corrosion degree of nuclear-grade austenitic stainless steel

Abstract

The invention discloses a method for improving intergranular corrosion resistance of austenitic stainless steel. According to the method, the 316 austenitic stainless steel is extruded through room-temperature ECAP, a large number of small-angle grain boundaries can be introduced into the material, and the intergranular corrosion resistance of the 316 austenitic stainless steel is remarkably improved. According to the method, the 316 austenitic stainless steel is treated through room-temperature ECAP, the austenitic stainless steel with refined grains and a large number of small-angle grain boundaries is obtained, marking is conducted through a hardness tester pressure head, the intergranular corrosion condition of the nuclear-grade austenitic stainless steel is observed in the in-situ state, it is verified that the small-angle grain boundaries have higher corrosion resistance, and the corrosion resistance of the nuclear-grade austenitic stainless steel is improved. The enrichment of the Cr element on a small-angle grain boundary group and the loss near a large-angle grain boundary are observed, different corrosion resistance of the two grain boundaries is explained, and a basis can be provided for preparing a high-corrosion-resistance material.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal stainless steel material analysis and detection, and particularly relates to a method for improving the intergranular corrosion resistance of nuclear-grade austenitic stainless steel and a detection and analysis method for evaluating the intergranular corrosion degree of nuclear-grade austenitic stainless steel. Background Art

[0002] Austenitic stainless steel is widely used in light water reactors for its excellent mechanical properties and corrosion resistance over a wide temperature range. However, service in harsh environments can lead to serious consequences such as intergranular corrosion (IGC) and stress corrosion cracking (IGSCC) of stainless steel. The energy at the grain boundary is higher than that of the surrounding crystals due to the higher disorder of the atomic arrangement, which often serves as the starting point for corrosion. [1,2] Therefore, the study of the corrosion resistance of grain boundaries is of great significance in the application of light water reactor structural components.

[0003] Since the concept of grain boundary engineering was proposed, some research has been carried out at home and abroad to enhance the corrosion resistance of materials by changing the grain boundary structure. [3-5] Grain boundary engineering (GBE) was introduced into face-centered cubic (FCC) metals with medium and low stacking fault energy, and it was believed that all low ΣCSL grain boundaries were corrosion-resistant grain boundaries, and this type of grain boundary was called a special grain boundary. [6,7] It is believed that only by increasing the twin boundary (Σ3) in the special grain boundary can the resistance to IGC be increased, and Hu [8] It is also proposed that only coherent twin boundaries (Σ3c) have the ability to resist IGC. In essence, it is the well-distributed low-energy segments in the grain boundary network that form discontinuous chromium depletion chains that prevent intergranular corrosion from penetrating from the surface. [9] So, Bi

[10] The concentration distribution of Cr elements on different grain boundary types of 304 stainless steel was studied by transmission electron microscopy (TEM), and it was found that the low Cr area of ​​low Σ grain boundary was less than that of random grain boundary.

[11] No changes in the Cr content at the corrosion front of the nickel-based 625 alloy were detected by STEM-EDS. Most of the research has focused on grain boundary engineering to regulate the corrosion resistance of special grain boundaries in large-angle grain boundaries, and there is limited research on the means of obtaining small-angle grain boundaries (2-15°) and their corrosion resistance. Existing studies on the influence of the grain boundary structure of austenitic stainless steel on corrosion resistance are mostly focused on special grain boundaries (low ΣCSL) at large-angle grain boundaries, and there are different conclusions on the distribution of Cr elements at grain boundaries before and after corrosion. Moreover, the existing technology is more difficult to characterize the grain boundary dimensions, especially the grain boundaries after corrosion, and the cost is also higher. In addition, the means of obtaining small-angle grain boundaries are also relatively limited. ECAP through heating often causes a high degree of recovery inside the material, resulting in the disappearance of small-angle grain boundaries. For example, the literature

[12] It is pointed out that the content of small-angle grain boundaries in the Al-Zn-Mg-Cu alloy extruded by ECAP at room temperature is as high as more than 80%, while after ECAP extrusion at 180℃, under the more obvious dynamic recovery effect, the proportion of small-angle grain boundaries is less than 60%. Summary of the Invention

[0004] Based on the above-mentioned prior art, the present invention aims to provide a method for improving the intergranular corrosion resistance of austenitic stainless steel. This method, which extrudes 316 austenitic stainless steel through room-temperature ECAP (equal channel angular pressing), introduces a large number of low-angle grain boundaries within the material, significantly improving the intergranular corrosion resistance of the 316 austenitic stainless steel. Specifically, the present invention utilizes the following technical solutions:

[0005] In one aspect, the present invention provides a method for improving the intergranular corrosion resistance of austenitic stainless steel, comprising the following steps:

[0006] Step 1: Wipe the sample surface with anhydrous ethanol and apply lubricant on the sample surface to reduce friction and lower the mold temperature.

[0007] Step 2: Place the prepared sample into the mold channel, ensuring a tight fit between the sample and the mold.

[0008] Step 3: Extrude the specimen at room temperature at a speed of 1-3 mm / s to ensure smooth extrusion and avoid excessive recovery or even recrystallization. Use the extrusion path bc, with the die's inner and outer angles set at 30° and 90°, respectively, to ensure approximately 100% equivalent strain is introduced into the material with each extrusion. Repeat Step 3 1-4 times.

[0009] Step 4: After extrusion is complete and the heat generated by extrusion dissipates, the sample is removed, resulting in austenitic stainless steel with refined grains and a large number of low-angle grain boundaries. The extrusion process route during this process, namely the bc mode of rotating ECAP extrusion, promotes the proliferation and entanglement of dislocations, significantly increasing the dislocation density within the sample grains and promoting the formation of new grain boundaries.

[0010] On the other hand, the present invention also provides a method for detecting and analyzing the degree of intergranular corrosion of nuclear-grade austenitic stainless steel. The steps for verifying the corrosion resistance of small-angle grain boundaries are as follows:

[0011] (1) Argon is introduced and the original austenitic stainless steel is placed in a tube furnace and sensitized at 680-710℃ for one hour, followed by air cooling to obtain a sample that is prone to intergranular corrosion. Sensitization is only to allow the Cr on the grain boundaries to precipitate in the form of chromium carbide, making it more susceptible to corrosion. Sensitization can improve the integrity of high-angle grain boundaries, making them more susceptible to intergranular corrosion.

[0012] (2) The sample surface was polished with SiC sandpaper and then electropolished in a mixture of perchloric acid and glacial acetic acid in a volume ratio of 1:9. The polishing voltage was controlled at 30 V and the polishing time was 30 s. This process is to remove the stress layer generated on the sample surface due to grinding, and then it is helpful to identify and distinguish the ratio of large and small angle grain boundaries, and prepare EBSD (electron backscatter diffraction) samples for characterization.

[0013] (3) Four hardness indentation points were selected on the surface of the electropolished sample. In order to eliminate the influence of the strain introduced by the indentation, the observation site was selected in the area away from the indenter. Before corrosion, EBSD analysis was performed on the square area surrounded by the four indentation points to determine the distribution of the grain boundary structure, and 6 line scans were performed in this area to obtain the distribution of elements such as Cr and Ni. Then, the sample was immersed in a nitric acid and hydrofluoric acid solution to produce intergranular corrosion, and the in-situ corrosion was observed under SEM. The four hardness indentation points are used to fix the observation range of the sample to locate the observation area, so as to determine, analyze and compare the corrosion resistance of different grain boundary types within a specific range. It should be noted that the distribution of large and small angle grain boundaries must also be determined before corrosion. The grain boundary distribution cannot be seen under SEM before corrosion, so EBSD should be used to confirm it at this time. The grain boundary morphology after corrosion can already be seen under SEM. At this time, it is only necessary to locate the area to find the corresponding grain boundary distribution and the corrosion condition on it.

[0014] (4) Soak in nitric acid and hydrofluoric acid solution for 10-20 minutes to produce intergranular corrosion, and observe the in-situ corrosion under SEM. After corrosion, the boundaries between large and small grains are obvious and can be seen through SEM.

[0015] Preferably, the nitric acid / hydrofluoric acid solution is prepared by mixing 65-70% by mass nitric acid, 38-42% by mass hydrofluoric acid, and water in a volume ratio of 5-6:1-2:20-25. More preferably, the nitric acid / hydrofluoric acid solution is prepared by mixing 68% by mass nitric acid, 40% by mass hydrofluoric acid, and water in a volume ratio of 6:2:22.

[0016] As a preferred solution, the austenitic stainless steel described in step (1) above is obtained by processing the austenitic stainless steel by the following method:

[0017] Step 1: Wipe the sample surface with anhydrous ethanol and apply lubricant on the sample surface to reduce friction and lower the mold temperature;

[0018] Step 2: Place the prepared sample into the mold channel to ensure a tight fit between the sample and the mold;

[0019] Step 3: At room temperature, extrude the sample at an extrusion speed of 1-3 mm / s to ensure smooth extrusion and avoid excessive recovery or even recrystallization behavior; the extrusion path is bc, where the inner and outer angles of the die are 30° and 90° respectively;

[0020] Step 4: After the extrusion is completed and the heat generated by the extrusion dissipates, the sample is taken out to obtain austenitic stainless steel with refined grains and a large number of small-angle grain boundaries.

[0021] The present invention determines a method for detecting and evaluating the degree of intergranular corrosion of nuclear-grade austenitic stainless steel by quantitatively comparing the degree of corrosion within a specified range of a sample containing a large number of high-angle grain boundaries (3) before ECAP treatment and the degree of corrosion on a specified range of the same size containing a large number of low-angle grain boundaries after ECAP treatment.

[0022] ECAP, a large plastic deformation method, can extrude large rod-shaped samples and produce a large number of low-angle grain boundaries within the material. This patented method uses room-temperature ECAP to extrude 316 austenitic stainless steel, achieving up to 80% low-angle grain boundaries in a single extrusion. During the extrusion process, numerous dislocation entanglements and dislocation walls form within the original large grains. As the extrusion proceeds, the interfaces (interfaces) of these dislocation entanglements and dislocation walls gradually transform into subgrain boundaries / low-angle grain boundaries with very small orientation differences. At elevated temperatures, these subgrain boundaries / low-angle grain boundaries gradually transform into high-angle grain boundaries due to enhanced dynamic recovery. This invention is expected to significantly improve the material's resistance to intergranular corrosion.

[0023] The present invention performs equal channel angular extrusion (ECAP) on 316 austenitic stainless steel at room temperature, and after one ECAP pass, the grain size can be refined from the original 45 μm to 20 μm (e.g.Figure 2 ) and introduces up to 80% low-angle grain boundaries within the material, which can inhibit the consumption of chromium after intergranular corrosion, significantly improving the intergranular corrosion resistance of 316 austenitic stainless steel. The inventors found that a single ECAP pass resulted in the most severe grain refinement and introduction of low-angle grain boundaries. Subsequent passes further aggravated the problem, but not as severely as the first pass, and achieved a more uniform refinement.

[0024] The present invention treats 316 austenitic stainless steel through room-temperature ECAP to obtain austenitic stainless steel with refined grains and a large number of low-angle grain boundaries. The indenter of a hardness tester is used for marking, and the intergranular corrosion of the nuclear-grade austenitic stainless steel is observed in situ, verifying that the low-angle grain boundaries have higher corrosion resistance. The enrichment of Cr on the low-angle grain boundary group and the loss near the high-angle grain boundaries are observed, which explains the different corrosion resistance of the two grain boundaries and provides a basis for the preparation of highly corrosion-resistant materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the ECAP extrusion process; the upper figure is a schematic diagram of ECAP extrusion, and the lower figure is a schematic diagram of the extrusion bc path.

[0026] Figure 2 Grain refinement and low-angle grain boundary distribution: (a) original austenitic stainless steel sample; (b) ECAP one-pass austenitic stainless steel sample; Figure 2 The distribution of large and small angle grain boundaries is given.

[0027] Figure 3 It is an in-situ corrosion process;

[0028] Figure 4 It is the corrosion front line scan: the change of Cr element;

[0029] Figure 5 Figures 1 and 2 show the grain boundary distribution and corrosion morphology: a) grain boundary network; b) corrosion morphology; c) magnified image of area ①; d) magnified image of area ⑥.

[0030] Figure 6 The left image shows the distribution of elements near the low-angle grain boundaries and high-angle grain boundaries. The left image shows the low-angle grain boundaries (corresponding to area ③), and the right image shows the high-angle grain boundaries (corresponding to area ①). DETAILED DESCRIPTION

[0031] The following embodiments are further descriptions of the present invention to illustrate the technical content of the present invention, but the essential content of the present invention is not limited to the following embodiments. Ordinary technicians in this field can and should know that any simple changes or replacements based on the essential spirit of the present invention should fall within the scope of protection required by the present invention.

[0032] Example 1

[0033] A method for improving the intergranular corrosion resistance of austenitic stainless steel comprises the following steps:

[0034] Step 1: Wipe the sample surface with anhydrous ethanol and apply lubricant on the sample surface to reduce friction and lower the mold temperature.

[0035] Step 2: Place the prepared sample into the mold channel to ensure a tight fit between the sample and the mold.

[0036] Step 3: At room temperature, extrude the sample at an appropriate extrusion speed (2 mm / s) to ensure smooth extrusion and avoid excessive recovery or even recrystallization. The extrusion path is bc, where the inner and outer angles of the die are 30° and 90° respectively (e.g. Figure 1 ) to ensure that extrusion can introduce approximately 100% equivalent strain into the material.

[0037] Step 4: After the extrusion is completed and the heat generated by the extrusion dissipates, the sample is taken out to obtain austenitic stainless steel with refined grains and a large number of small-angle grain boundaries, such as Figure 2 shown.

[0038] Example 2

[0039] A detection and analysis method for evaluating the degree of intergranular corrosion of nuclear-grade austenitic stainless steel. The steps for verifying the corrosion resistance of small-angle grain boundaries are as follows:

[0040] (1) Argon gas was introduced and the austenitic stainless steel treated in Example 1 was placed in a tube furnace for sensitization at 700°C for one hour, followed by air cooling to obtain a sample that was susceptible to intergranular corrosion.

[0041] (2) The sample surface was polished with #400-2000Sic sandpaper and then electropolished in a 10 vol% perchloric acid + 90 vol% glacial acetic acid solution with a polishing voltage of 30 V and a polishing time of 30 s.

[0042] (3) Four hardness indentation points were selected on the surface of the electropolished sample. In order to eliminate the influence of the strain introduced by the indentation, the observation site was selected in the area away from the indenter. Before corrosion, the designated area was selected for EBSD analysis to determine the grain boundary structure distribution, and 6 line scans were performed in this area to obtain the distribution of Cr elements. Then, the sample was immersed in a nitric acid and hydrofluoric acid solution (68wt% nitric acid, 40wt% hydrofluoric acid and water in a volume ratio of 6:2:22) to produce intergranular corrosion. The in-situ corrosion was observed under SEM. The whole process is as follows: Figure 3 shown.

[0043] Before corrosion, six line scans were performed on the selected area of ​​the sample. Although there were a large number of small-angle grain boundaries distributed in the selected area, namely "small-angle grain boundary groups", the scanning results did not show obvious changes in the Cr element, such as Figure 4 shown.

[0044] Figure 5 (a) shows the grain boundary distribution corresponding to the corrosion area, calculated using Channel 5 software. Green represents low-angle grain boundaries, and black represents high-angle grain boundaries. The entire grain boundary network can be roughly divided into three regions: regions ① and ② containing only high-angle grain boundaries, regions ③ and ④ containing only low-angle grain boundaries (referred to as "low-angle grain boundary clusters"), and regions ⑤ and ⑥ with a few small-angle grain boundaries surrounding the high-angle grain boundaries. Figure 5 (b) is the morphology after intergranular corrosion. It is easy to see that under the same corrosion conditions, the corrosion degree of each boundary is significantly different. In places where only large-angle grain boundaries exist, the corrosion degree is more serious, with obvious gullies appearing. The measured corrosion depth can reach about 1 μm. Figure 5 (c) As shown in the enlarged view of area ①, on the small-angle grain boundary group, like the matrix, there is almost no trace of corrosion, showing excellent corrosion resistance, as shown in Figures ③ and ④; there are also some areas near the large-angle grain boundaries where small-angle grain boundaries are concentrated, corresponding to the areas in Figures ⑤ and ⑥. It can be seen that the corrosion degree of the large-angle grain boundaries that should have been severely corroded is significantly reduced, and the corrosion depth is less than 0.5 μm, as shown in Figures 5 and 6. Figure 5 (d) Enlarged area shown.

[0045] Figure 6 The distribution of elements in the small-angle grain boundary group and the large-angle grain boundary after intergranular corrosion is shown. There seems to be a trend of a slight decrease in the Cr element near the large-angle grain boundary. The maximum loss is calculated to be around 4%. However, a more intuitive enrichment of Cr element appears on the small-angle grain boundary group (the maximum loss is 8%).

[0046] The present invention treats 316 austenitic stainless steel through room-temperature ECAP to obtain austenitic stainless steel with refined grains and a large number of low-angle grain boundaries. The indenter of a hardness tester is used for marking, and the intergranular corrosion of the nuclear-grade austenitic stainless steel is observed in situ, verifying that the low-angle grain boundaries have higher corrosion resistance. The enrichment of Cr on the low-angle grain boundary group and the loss near the high-angle grain boundaries are observed, which explains the different corrosion resistance of the two grain boundaries and provides a basis for the preparation of highly corrosion-resistant materials.

[0047] It should be noted that the technical contents of the present invention described above are only for the purpose of explaining and illustrating the technical essence of the present invention to enable those skilled in the art to understand the technical essence of the present invention. Therefore, the technical contents described above are not intended to limit the substantial protection scope of the present invention. The substantial protection scope of the present invention shall be based on the claims. Those skilled in the art should be aware that any modifications, equivalent substitutions, and improvements based on the substantial spirit of the present invention shall fall within the substantial protection scope of the present invention.

[0048] References

[0049] [1] H.Shi, J.Li, J.Mao, W.Lu, Corrosion behaviors of Zr-Sn-Nb-Fe-Moalloys in pure water at 360℃ / 18.6MPa: Influence of annealing temperature on the anti-corrosion properties, Corrosion Science, 163(2020)108225.

[0050] [2] G. Reiss, J. Vancea, H. Hoffmann, Grain-Boundary Resistance in Polycrystalline Metals, Physical Review Letters, 56 (1986) 2100-2103.

[0051] [3] T. Watanabe, An approach to grain-boundary design for strong and ductile polycrystals, Res Mechanica. 11 (1984) 47–84.

[0052] [4] G. Palumbo, EM Lehockey, P. Lin, Applications for grainboundary engineeredmaterials, Jom 50 (1998) 40–43.

[0053] [5] S. Kobayashi, S. Tsurekawa, T. Watanabe, G. Palumbo, Grainboundary engineering for control of sulfur segregation-induced embrittlementin ultrafine-grained nickel,Scr. Mater. 62 (2010) 294–297.

[0054] [6] M. Shimada, H. Kokawa, Optimization of grain boundary characterdistribution forintergranular corrosion resistant 304 stainless steel bytwin-induced grain boundaryengineering, Acta Mater. 50 (2002) 2331–2341.

[0055] [7] H.Y. Bi, H. Kokawa, Z.J. Wang, M. Shimada, Suppression ofchromium depletion bygrain boundary structural change during twin-inducedgrain boundary engineeringof 304 stainless steel, Scr. Mater. 49 (2003) 219–223.

[0056] [8] C. Hu, S. Xia, H. Li, Improving the intergranular corrosionresistance of 304stainless steel by grain boundary network control, Corros.Sci. 53 (2011)1880–1886.

[0057] [9] Shimada M, Kokawa H, Wang ZJ, Sato YS, Karibe I. Optimization ofgrain boundary character distribution for intergranular corrosion resistant304 stainless steel by twininduced grain boundary engineerin.Acta Mater 2002;50:2331.

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[10] H. Yun Bi, H. Kokawa, Z. Jie Wang, M. Shimada, Y.S. Sato,Suppression of chromium depletion by grain boundary structural change duringtwin-induced grain boundary engineering of 304 stainless steel, ScriptaMaterialia, 49 (2003) 219-223.

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[11] J. Zhang, L. Xu, Y. Han, L. Zhao, B. Xiao, New perspectives onthe grain boundary misorientation angle dependent intergranular corrosion ofpolycrystalline nickel-based 625 alloy, Corrosion Science, 172 (2020) 108718.

[0060]

[12] M.H. Shaeri, M. Shaeri, M. Ebrahimi, M.T. Salehi d, S. HSeyyedein,Effect of ECAP temperature on microstructure and mechanicalproperties of Al–Zn–Mg–Cu alloy, Progress in Natural Science: MaterialsInternational, 26 (2016) 182-191.

Claims

1. A method for improving the intergranular corrosion resistance of nuclear grade austenitic stainless steel, comprising the following steps: Step 1: Wipe the sample surface with anhydrous ethanol and apply lubricant on the sample surface to reduce friction and lower the mold temperature; Step 2: Place the prepared sample into the mold channel to ensure a tight fit between the sample and the mold; Step 3: At room temperature, the sample is extruded at an extrusion speed of 1-3 mm / s, and the extrusion path is bc, where the inner angle and outer angle of the die are 30° and 90° respectively; Step 4: After extrusion is completed and the heat generated by extrusion dissipates, the sample is taken out to obtain austenitic stainless steel with refined grains and a large number of small-angle grain boundaries.

2. A detection and analysis method for evaluating the degree of intergranular corrosion of nuclear-grade austenitic stainless steel, characterized in that: The steps include: (1) Argon gas is introduced and austenitic stainless steel is placed in a tube furnace and sensitized at 680-710°C for one hour, followed by air cooling to obtain a sample that is prone to intergranular corrosion; (2) The sample surface was polished with SiC sandpaper and then electropolished in a mixed solution of perchloric acid and glacial acetic acid with a volume ratio of 1:

9. The polishing voltage was controlled at 30 V and the time was 30 s. (3) Select four hardness indentation points on the surface of the electropolished sample; perform EBSD analysis on the area within the four hardness indentation points to determine the grain boundary structure distribution, and perform 4-8 line scans in this area to obtain the distribution of Cr and Ni elements; (4) Soak in nitric acid and hydrofluoric acid solution for 10-20 minutes to produce intergranular corrosion, and observe the in-situ corrosion under SEM.

3. The method according to claim 2, characterized in that The austenitic stainless steel described in step (1) is obtained by the following treatment method: Step 1: Wipe the sample surface with anhydrous ethanol and apply lubricant on the sample surface to reduce friction and lower the mold temperature; Step 2: Place the prepared sample into the mold channel to ensure a tight fit between the sample and the mold; Step 3: At room temperature, extrude the sample at an extrusion speed of 1-3 mm / s to ensure smooth extrusion and avoid excessive recovery or even recrystallization behavior; the extrusion path is bc, where the inner and outer angles of the die are 30° and 90° respectively; Step 4: After extrusion is completed and the heat generated by extrusion dissipates, the sample is taken out to obtain austenitic stainless steel with refined grains and a large number of small-angle grain boundaries.

4. The method according to claim 3, characterized in that Repeat step 3 1-4 times.

5. The method according to claim 2, characterized in that The Sic sandpaper described in step (2) is 400#-2000#.

6. The method according to claim 2, characterized in that The area within the four hardness indentation points in step (3) is a square area surrounded by the four hardness indentation points.

7. The method according to claim 2, characterized in that The nitric acid-hydrofluoric acid solution in step (4) is prepared by mixing nitric acid with a mass fraction of 65-70%, hydrofluoric acid with a mass fraction of 38-42% and water in a volume ratio of 5-6:1-2:20-25.

8. The method according to claim 7, characterized in that The nitric acid-hydrofluoric acid solution is prepared by mixing nitric acid with a mass fraction of 68%, hydrofluoric acid with a mass fraction of 40% and water in a volume ratio of 6:2:22.

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