Corrosion method of high-manganese austenite low-temperature steel metallographic specimen for LNG (Liquefied Natural Gas) storage tank

Abstract

The invention relates to the technical field of metallographic sample preparation, in particular to a corrosion method of a high-manganese austenite low-temperature steel metallographic sample for an LNG (Liquefied Natural Gas) storage tank, which comprises the following steps: 1) grinding and polishing the high-manganese austenite low-temperature steel metallographic sample for the LNG storage tank to enable the surface of the sample to be in a scratch-free mirror surface state; 2) preparing a nitric acid alcohol solution with a volume fraction of 10-18%, and marking the nitric acid alcohol solution as a corrosive agent A; preparing a hydrochloric acid alcohol solution with the volume fraction of 10%-15%, and marking the hydrochloric acid alcohol solution as a corrosive agent B; (3) the polished surface of the metallographic sample is soaked in the corrosive liquid A for 60-120 s and then taken out, the surface of the sample is corroded, and after a mirror surface on the polished surface disappears, the polished surface of the sample is soaked in corrosive liquid B for 30-60 s; the ratio of the erosion time of the metallographic specimen in the corrosive liquid A to the erosion time of the metallographic specimen in the corrosive liquid B is (2-2.5): 1; and (4) taking out the metallographic sample, washing the metallographic sample with distilled water and absolute ethyl alcohol in sequence, and drying the metallographic sample with an air blower. The metallographic structure is clear when the metallographic sample is subjected to corrosion test at normal temperature.

Description

Technical Field

[0001] The invention belongs to the technical field of metallographic sample preparation, and in particular relates to a corrosion method for a high-manganese austenitic low-temperature steel metallographic sample for an LNG storage tank. Background Art

[0002] With the rapid development of the global economy and rising environmental awareness worldwide, the demand for clean energy is growing. Natural gas, as one of the cleanest energy sources, is gaining increasing attention worldwide for its development and utilization. Under normal pressure, natural gas can be liquefied at temperatures as low as -162°C. Liquefied natural gas (LNG) has a volume approximately 1 / 625 of its gaseous state, making it extremely easy to store and transport and offering excellent safety. Currently, the main materials used for LNG storage tanks include aluminum alloys, austenitic stainless steel, 9% nickel steel, and Invar alloy. However, these traditional low-temperature materials present numerous challenges, including high cost, complex processes, manufacturing difficulties, and poor weldability.

[0003] Since LNG storage tanks primarily operate in ultra-low temperature environments, the steel used in these tanks must maintain ideal performance and good structural stability in these conditions. Against this backdrop, high-manganese austenitic steel, characterized by low cost, high ductility, excellent fatigue performance, and low-temperature resistance, has great application potential in LNG tank construction and is attracting increasing attention. High-manganese austenitic low-temperature steel improves its low-temperature performance by adding a high content of Mn to the steel, resulting in a significantly lower cost than 9Ni steel and stainless steel. High-manganese steel incorporates more than 20% Mn to replace Ni, resulting in a stable austenitic structure at room temperature. Furthermore, the addition of a certain amount of C stabilizes the austenite and inhibits phase transformations. The addition of a certain amount of Cr significantly increases its hardness and toughness while significantly improving its corrosion resistance.

[0004] Metallographic analysis is a method for analyzing the structure and composition of metallic materials. It obtains information about the material's structure and composition through microscopic observation and chemical analysis. As one of the key research methods in materials science, metallography reveals structural characteristics of metallic materials, such as grain structure, grain boundary distribution, porosity, and inclusions, through microscopic observation. This can help researchers gain a deeper understanding of the material's microstructure, thereby guiding the rational preparation and processing of materials and improving their performance and quality. Furthermore, metallographic analysis can help understand the causes of material failures such as fracture, deformation, and corrosion, identify improvement measures, and increase the reliability and service life of the material.

[0005] Metallographic specimens are typically etched using a 4% by volume nital solution. When a metal surface comes into contact with nital, nitrate ions form complexes with oxides on the metal surface, causing corrosion. The corrosive action of nital creates a black precipitate on the specimen surface, creating contrast and allowing the microstructure to be clearly visualized. However, high-manganese austenitic low-temperature steel used in LNG tanks contains 22.5% to 25.5% manganese and 3.0% to 4.0% chromium, resulting in excellent corrosion resistance. Using a conventional 4% by volume nital solution, it is difficult to clearly etch the microstructure of high-manganese austenitic steel used in LNG tanks, especially when the content of austenite grain boundary precipitates is low, making it difficult to clearly visualize the austenite grain boundaries. Increasing the concentration of nital easily forms a yellow-black oxide film on the specimen surface, obscuring the observation of twins within the austenite grains. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for corroding a metallographic sample of high manganese austenitic low-temperature steel for LNG storage tanks, which has a clear metallographic structure and can clearly observe the microscopic features of austenite grain boundaries and grains.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions:

[0008] A method for corroding a metallographic specimen of high-manganese austenitic low-temperature steel for an LNG storage tank comprises the following steps:

[0009] (1) Grind and polish the metallographic specimens of high manganese austenitic low-temperature steel used for LNG storage tanks to make the surface of the specimens mirror-like without scratches;

[0010] (2) Prepare a 10% to 18% nitric acid alcohol solution by volume, labeled as etchant A; prepare a 10% to 15% hydrochloric acid alcohol solution by volume, labeled as etchant B;

[0011] (3) Immerse the polished surface of the metallographic specimen in the etching solution A for 60 to 120 seconds and then take it out. The surface of the specimen is corroded. After the mirror surface on the polished surface disappears, immerse the polished surface of the specimen in the etching solution B for 30 to 60 seconds. The ratio of the etching time of the metallographic specimen in the etching solution A to the etching time in the etching solution B is 2 to 2.5:1.

[0012] (4) After taking out the metallographic sample, rinse it with distilled water and anhydrous ethanol in turn, and blow dry the metallographic sample with a hair dryer for metallographic structure detection.

[0013] The chemical composition of the high manganese austenitic low-temperature steel for LNG storage tanks is as follows by weight: C: 0.35% to 0.55%, Si: 0.10% to 0.50%, Mn: 22.50% to 25.50%, P≤0.030%, S≤0.010%, Cr: 3.00% to 4.00%, Cu: 0.30% to 0.70%, B≤0.005%, N≤0.050%, and the balance is Fe and other inevitable impurities.

[0014] Step (1) The sample was polished in sequence with 120#, 240#, 400#, 800# and 1200# sandpaper, and then polished with 2.5μm metallographic polishing agent to a scratch-free mirror state, washed with clean water and anhydrous ethanol, and blown dry for use.

[0015] The preparation method of the etchant A in step (2) is as follows: 14 ml of concentrated nitric acid with a mass fraction of 68% is poured into 63.8-126 ml of anhydrous ethanol, stirred evenly, and a nitric acid alcohol solution with a volume fraction of 10%-18% is prepared;

[0016] The preparation method of the corrosive agent B is as follows: pour 12.5 ml of concentrated hydrochloric acid with a mass fraction of 38% into 70.8-112.5 ml of anhydrous ethanol, stir evenly, and prepare a hydrochloric acid alcohol solution with a volume fraction of 10%-15%.

[0017] Compared with the existing technology, the beneficial effects of the present invention are:

[0018] The present invention performs a corrosion test on a metallographic sample at room temperature, does not require heating of the corrosive agent, and the corrosive agent is easily available and has a high safety factor.

[0019] High-manganese austenitic low-temperature steel used in LNG storage tanks has a high alloy content and good corrosion resistance. The present invention uses a 10% to 18% volume fraction nital solution to erode metallographic specimens. When the concentration of the nital solution is below 10%, the corrosion effect on the high-manganese steel metallographic specimens is weakened, potentially resulting in uneven corrosion and difficulty in clearly identifying the internal structure of the specimens. When the concentration of the nital solution is above 18%, the high-manganese steel metallographic specimens are prone to over-passivation corrosion, leading to intergranular corrosion and grain loss. Compared to a 4% volume fraction nital solution, a 10% to 18% volume fraction nital solution is relatively high. During the erosion process, a yellow-black oxide film forms on the surface of the metallographic specimen, making it difficult to etch out grains when austenite grain boundaries are not clearly visible. Alternatively, after etching out austenite grain boundaries, it becomes difficult to observe the twin morphology within the austenite grains. Hydrochloric acid alcohol solution is very suitable for high-alloy steel containing chromium and nickel. The acidity of hydrochloric acid can dissolve the oxide film generated on the surface of the sample during the nitric acid alcohol etching process. When the volume fraction of the hydrochloric acid alcohol solution is lower than 10%, it cannot effectively remove impurities and defects on the metal surface, making it impossible to clearly display the microstructure of the sample; but when the volume fraction of the hydrochloric acid alcohol solution is higher than 15%, it will aggravate electrochemical corrosion, resulting in excessive corrosion of the sample surface and unclear microstructure display. Therefore, the present invention can clearly etch the metallographic structure and micromorphology of high-manganese austenitic low-temperature steel for LNG storage tanks. The metallographic structure is clear, and the microscopic features of the austenite grain boundaries and the grains can be clearly observed.

[0020] The corrosion method of the present invention is highly applicable and can be used on hot-rolled, heat-treated, heat-affected zones after welding thermal cycles, and even cold-rolled specimens. It exhibits high stability and good reproducibility. This method facilitates the development of rational controlled rolling, controlled cooling, and heat treatment processes, thereby achieving better grain refinement and meeting the microstructural and mechanical property requirements of high-manganese steel used in LNG storage tank construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the metallographic structure diagram of the high manganese austenitic low-temperature steel for LNG storage tanks corroded in Example 1.

[0022] Figure 2 This is the metallographic structure diagram of the high manganese austenitic low-temperature steel for LNG storage tanks corroded in Example 2.

[0023] Figure 3 This is the metallographic structure diagram of the high manganese austenitic low-temperature steel for LNG storage tanks corroded in Comparative Example 1.

[0024] Figure 4 This is the metallographic structure diagram of the high manganese austenitic low-temperature steel for LNG storage tanks corroded in Comparative Example 2. DETAILED DESCRIPTION

[0025] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means more than two.

[0026] Unless otherwise specified or limited, the terms "mounted," "connected," and "connected" should be interpreted broadly. For example, they can refer to fixed, removable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediary; and internal communication between two components. Those skilled in the art will understand the specific meanings of these terms in the present invention based on specific circumstances.

[0027] A method for corroding a metallographic specimen of high-manganese austenitic low-temperature steel for an LNG storage tank comprises the following steps:

[0028] (1) Grind and polish the metallographic specimens of high manganese austenitic low-temperature steel used for LNG storage tanks to make the surface of the specimens mirror-like without scratches;

[0029] (2) Prepare a 10% to 18% nitric acid alcohol solution by volume, labeled as etchant A; prepare a 10% to 15% hydrochloric acid alcohol solution by volume, labeled as etchant B;

[0030] (3) Immerse the polished surface of the metallographic specimen in the etching solution A for 60 to 120 seconds and then take it out. The surface of the specimen is corroded. After the mirror surface on the polished surface disappears, immerse the polished surface of the specimen in the etching solution B for 30 to 60 seconds. The ratio of the etching time of the metallographic specimen in the etching solution A to the etching time in the etching solution B is 2 to 2.5:1.

[0031] (4) After taking out the metallographic sample, rinse it with distilled water and anhydrous ethanol in turn, and blow dry the metallographic sample with a hair dryer for metallographic structure detection.

[0032] The chemical composition of the high manganese austenitic low-temperature steel for LNG storage tanks is as follows by weight: C: 0.35% to 0.55%, Si: 0.10% to 0.50%, Mn: 22.50% to 25.50%, P≤0.030%, S≤0.010%, Cr: 3.00% to 4.00%, Cu: 0.30% to 0.70%, B≤0.005%, N≤0.050%, and the balance is Fe and other inevitable impurities.

[0033] Step (1) The sample was polished in sequence with 120#, 240#, 400#, 800# and 1200# sandpaper, and then polished with 2.5μm metallographic polishing agent to a scratch-free mirror state, washed with clean water and anhydrous ethanol, and blown dry for use.

[0034] The preparation method of the etchant A in step (2) is as follows: 14 ml of concentrated nitric acid with a mass fraction of 68% is poured into 63.8-126 ml of anhydrous ethanol, stirred evenly, and a nitric acid alcohol solution with a volume fraction of 10%-18% is prepared;

[0035] The preparation method of the corrosive agent B is as follows: pour 12.5 ml of concentrated hydrochloric acid with a mass fraction of 38% into 70.8-112.5 ml of anhydrous ethanol, stir evenly, and prepare a hydrochloric acid alcohol solution with a volume fraction of 10%-15%.

[0036] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, and these simple modifications all fall within the scope of protection of the present invention. It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner unless there is any contradiction. In order to avoid unnecessary repetition, the present invention will no longer describe various possible combinations separately. In addition, the various different embodiments of the present invention can also be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

[0037] To make the objectives, technical solutions, and technical effects of the present invention more clear, the technical solutions in the embodiments of the present invention are now clearly and completely described. However, the embodiments described below are only some of the embodiments of the present invention, not all of them. In combination with the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0038] Example 1:

[0039] A method for corroding a metallographic specimen of high-manganese austenitic low-temperature steel for LNG storage tanks. The metallographic specimen is a hot-rolled high-manganese steel plate having the following chemical compositions by weight: C: 0.45%, Si: 0.20%, Mn: 24.50%, P: 0.015%, S: 0.005%, Cr: 3.85%, Cu: 0.50%, B ≤ 0.005%, N ≤ 0.050%, with the remainder being Fe and other unavoidable impurities. The thickness is 20 mm.

[0040] The method comprises the following steps:

[0041] (1) The metallographic specimens of high manganese austenitic low-temperature steel for LNG storage tanks were polished in sequence with 120#, 240#, 400#, 800# and 1200# sandpapers, and then polished to a scratch-free mirror state with a 2.5μm metallographic polishing agent. After cleaning with clean water and anhydrous ethanol, the metallographic specimens were blown dry for later use;

[0042] (2) Pour 14 ml of 68% concentrated nitric acid into 86 ml of anhydrous ethanol and stir to prepare a 14% nitric acid alcohol solution; Pour 12.5 ml of 38% concentrated hydrochloric acid into 87.5 ml of anhydrous ethanol and stir to prepare a 12.5% hydrochloric acid alcohol solution;

[0043] (3) Immerse the polished surface of the metallographic specimen in a 14% nitric acid alcohol solution for 80 seconds and then remove it. The surface of the specimen is corroded. After the mirror surface on the polished surface disappears, immerse the polished surface of the metallographic specimen in a 12.5% hydrochloric acid alcohol solution for 40 seconds.

[0044] (4) After taking out the metallographic sample, rinse it with distilled water and anhydrous ethanol in turn, blow dry the metallographic sample with a hair dryer, and place the metallographic sample under an optical microscope for observation.

[0045] It has been observed that the hot-rolled high manganese austenitic low-temperature steel for LNG storage tanks is a single-phase austenite structure at room temperature, with elongated grains and obvious austenite grain boundaries. 23 C6 carbides are evenly dispersed at the austenite grain boundaries, and the larger twins inside the austenite grains are clearly visible, such as Figure 1 shown.

[0046] Example 2:

[0047] A method for corroding a metallographic specimen of high-manganese austenitic low-temperature steel for LNG storage tanks. The metallographic specimen is high-manganese steel that has undergone a coarse-grained heat-affected zone welding heat cycle treatment, with a welding heat cycle peak temperature of 1300°C. The chemical composition of the specimen is the same as that of Example 1.

[0048] The method comprises the following steps:

[0049] (1) The metallographic grinding and polishing steps of high manganese austenitic low-temperature steel for LNG storage tanks are the same as those in Example 1;

[0050] (2) The method of preparing the etching solution is the same as that in Example 1;

[0051] (3) Immerse the polished surface of the metallographic specimen in a 14% nitric acid alcohol solution for 100 seconds and then remove it. After the mirror surface on the polished surface disappears, immerse the polished surface of the metallographic specimen in a 12.5% hydrochloric acid alcohol solution for 50 seconds.

[0052] (4) The washing, drying and microstructure observation of the metallographic specimens were the same as those in Example 1.

[0053] After observation, the room temperature structure of the high manganese steel metallographic specimen treated by coarse grain heat affected zone welding heat cycle is single phase austenite, the austenite grains grow into equiaxed shape, the grain size is between 50 and 100 μm, and the (Cr, Mn) at the austenite grain boundary is 23 C6 carbides have basically disappeared, with only a small amount of precipitates distributed at the austenite grain boundaries. Due to the high peak temperature of the welding heat cycle of 1300°C and the short holding time, a certain degree of grain boundary liquefaction occurs, such as Figure 2 shown.

[0054] Comparative Example 1:

[0055] A method for corroding a metallographic sample of a high-manganese austenitic low-temperature steel for an LNG storage tank. The metallographic sample is a hot-rolled high-manganese steel plate, and its chemical composition is the same as that of Example 1.

[0056] The method comprises the following steps:

[0057] (1) The metallographic grinding and polishing steps are the same as those in Example 1;

[0058] (2) Pour 4 ml of concentrated nitric acid into 96 ml of anhydrous ethanol and stir to prepare a 4% nitric acid alcohol solution;

[0059] (3) Immerse the polished surface of the metallographic specimen in a 4% nitric acid alcohol solution for 60 to 120 seconds and then remove it;

[0060] (4) The washing, drying and microstructure observation of the metallographic specimens were the same as those in Example 1.

[0061] It was observed that the grain boundary corrosion of the metallographic specimen was incomplete and the microstructure inside the austenite grains could not be observed, such as Figure 3 shown.

[0062] Comparative Example 2:

[0063] A method for corroding a metallographic sample of a high-manganese austenitic low-temperature steel for an LNG storage tank. The metallographic sample is a hot-rolled high-manganese steel plate, and its chemical composition is the same as that of Example 1.

[0064] The method comprises the following steps:

[0065] (1) The metallographic grinding and polishing steps are the same as those in Example 1;

[0066] (2) Pour 14 ml of concentrated nitric acid into 86 ml of anhydrous ethanol and stir to prepare a 14% nitric acid alcohol solution.

[0067] (3) Immerse the polished surface of the metallographic specimen in a 14% nitric acid alcohol solution for 60 seconds and then remove it;

[0068] (4) The washing, drying and microstructure observation of the metallographic specimens were the same as those in Example 1.

[0069] After observation, there is a yellow-black oxide film on the surface of the metallographic sample. The surface of the metallographic sample is dirty. The austenite grain boundary can be observed, but the microstructure inside the austenite grain cannot be observed. Figure 4 shown.

[0070] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations may be made to these embodiments without departing from the principles and basic spirit of the invention, and the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for corroding a metallographic specimen of high manganese austenitic low-temperature steel for LNG storage tanks, characterized in that: The following steps are involved: (1) Grind and polish the metallographic specimens of high manganese austenitic low-temperature steel used for LNG storage tanks to make the surface of the specimens mirror-like without scratches; (2) Prepare a 10% to 18% nitric acid alcohol solution by volume, labeled as etchant A; prepare a 10% to 15% hydrochloric acid alcohol solution by volume, labeled as etchant B; (3) Immerse the polished surface of the metallographic specimen in the etching solution A for 60 to 120 seconds and then take it out. The surface of the specimen is corroded. After the mirror surface on the polished surface disappears, immerse the polished surface of the specimen in the etching solution B for 30 to 60 seconds. The ratio of the etching time of the metallographic specimen in the etching solution A to the etching time in the etching solution B is 2 to 2.5:

1. (4) After taking out the metallographic sample, rinse it with distilled water and anhydrous ethanol in turn, and blow dry the metallographic sample with a hair dryer for metallographic structure detection.

2. The corrosion method of a high manganese austenitic low temperature steel metallographic specimen for LNG storage tanks according to claim 1, characterized in that: The chemical composition of the high manganese austenitic low-temperature steel for LNG storage tanks is as follows by weight: C: 0.35% to 0.55%, Si: 0.10% to 0.50%, Mn: 22.50% to 25.50%, P≤0.030%, S≤0.010%, Cr: 3.00% to 4.00%, Cu: 0.30% to 0.70%, B≤0.005%, N≤0.050%, and the balance is Fe and other inevitable impurities.

3. The method for corroding a metallographic specimen of high manganese austenitic low-temperature steel for LNG storage tanks according to claim 1, characterized in that: Step (1) The sample was polished in sequence with 120#, 240#, 400#, 800# and 1200# sandpaper, and then polished with 2.5μm metallographic polishing agent to a scratch-free mirror state, washed with clean water and anhydrous ethanol, and blown dry for use.

4. The method for corroding a metallographic specimen of high manganese austenitic low-temperature steel for LNG storage tanks according to claim 1, characterized in that: The preparation method of the etchant A in step (2) is as follows: 14 ml of concentrated nitric acid with a mass fraction of 68% is poured into 63.8-126 ml of anhydrous ethanol, stirred evenly, and a nitric acid alcohol solution with a volume fraction of 10%-18% is prepared; The preparation method of the corrosive agent B is as follows: pour 12.5 ml of concentrated hydrochloric acid with a mass fraction of 38% into 70.8-112.5 ml of anhydrous ethanol, stir evenly, and prepare a hydrochloric acid alcohol solution with a volume fraction of 10%-15%.

Images