Corrosion method for stainless steel austenite grains of composite board

Abstract

The invention discloses a corrosion method for stainless steel austenite grains of a composite board, which comprises the following steps: step 1, preheating a sample: soaking and preheating a metallographic sample in hot water to obtain a preheated sample; and 2, corrosion and hot bath cleaning of the sample: carrying out corrosion treatment on the sample preheated in the step 1 by using a corrosive liquid, cleaning and heating the sample in hot water after the reaction rate is reduced, removing a surface passivation layer, repeating the operation until the surface of the sample is gray and free of light reflection, and blow-drying the sample to obtain the sample subjected to corrosion treatment. According to the corrosion method, the austenite grain corrosion effect can be effectively improved, the corrosion speed is increased, and the problem that the stainless steel austenite grain boundary display of the composite plate is not clear is solved.

Description

Technical Field

[0001] This invention belongs to the field of microstructure analysis and quantitative characterization detection technology, specifically involving a method for corrosion of austenitic grains in composite stainless steel. Background Technology

[0002] Composite plates typically refer to plates made by metallurgically bonding two or more different metal materials through processes such as rolling composite, explosive composite, or welding composite. Composite plates made of ordinary steel (such as Q235B, Q345R carbon steel or low alloy steel) and stainless steel (such as 304, 316L) are the most widely used type. The core design concept is "complementary advantages": utilizing the strength, rigidity, and low cost of ordinary steel as the base material (base layer), and utilizing the excellent corrosion resistance of stainless steel as the cladding material (cladding layer). While ensuring corrosion resistance on the surface in contact with corrosive media, this significantly reduces equipment manufacturing costs. Stainless steel typically accounts for only 10% to 20% of the overall thickness (common cladding layer thickness is 1.5 to 3 mm), yet it provides 100% corrosion resistance, saving a significant amount of precious metals.

[0003] Grain size is an indicator that measures the size of grains within a metal and has a decisive impact on the properties of metallic materials. For austenitic stainless steel cladding, grain size is a key metallographic indicator for evaluating its quality and performance. Fine-grained cladding itself has higher strength and is less prone to cracking or defects when deforming in conjunction with the base layer (such as during equipment pressure or rolling). In low-temperature environments, fine grains can prevent crack propagation and significantly improve the material's low-temperature impact toughness. Coarse grains, on the other hand, make the material brittle. For composite plate equipment used in low-temperature environments (such as liquefied natural gas storage tanks), the fine grain size of the cladding stainless steel is crucial for ensuring the safety of the overall structure.

[0004] Grain boundaries are high-energy regions in materials and are often where corrosion (especially intergranular corrosion) easily begins. Coarse grains reduce the number of grain boundaries per unit area, but the segregation of impurities (such as carbides) at each grain boundary can be more severe, forming continuous corrosion channels and significantly reducing resistance to intergranular corrosion. Fine grains increase the total number of grain boundaries, making the corrosion path more tortuous. The impurity concentration per unit grain boundary is relatively low, disrupting the formation of continuous corrosion channels and improving resistance to intergranular corrosion. A fine-grained structure usually means a more uniform microstructure, which can reduce the initiation points of pitting corrosion and also has a positive effect on pitting corrosion resistance. Corrosion resistance is the core mission of strata. Controlling the strata to have a fine-grained structure is key to ensuring that the composite plate does not experience corrosion failure within its design life.

[0005] Therefore, assessing whether the austenitic grain size of the composite plate meets the quality requirements is a crucial indicator for determining its qualification. Conventional corrosion cannot uniformly and clearly etch the grains on the austenitic steel side of the composite plate, affecting the assessment of the austenitic grain size. Therefore, it is necessary to address the difficulty in revealing the grain boundaries of austenitic grains in composite plates by innovating a new method for corroding the austenitic microstructure of composite plates, starting from the corrosion agent and corrosion method, and the preparation method. Summary of the Invention

[0006] The purpose of this invention is to provide a metallographic sample preparation technique for corroding austenitic grains in stainless steel composite plates. This technique employs a special method of treating the etchant to corrode the sample, repeatedly removing the surface passivation film to accelerate the corrosion rate. This sample preparation method can effectively improve the corrosion effect of austenitic grains, accelerate the corrosion rate, and solve the problem of unclear austenitic grain boundaries in stainless steel composite plates.

[0007] According to one aspect of the present invention, a method for etching austenitic grains of stainless steel in composite plates is provided, comprising the following steps:

[0008] Step 1, Sample preheating: Immerse the metallographic sample in hot water to preheat it, and obtain the preheated sample. Step 2, corrosion and hot water bath sample: The preheated sample described in Step 1 is corroded with a corrosive solution. After the reaction rate decreases, the sample is placed in hot water for cleaning and heating to remove the surface passivation layer. The operation is repeated until the sample surface is gray and non-reflective. The sample is then dried to obtain the corroded sample.

[0009] Based on the above technical solution, the preheating conditions in step 1 are as follows: The preheating temperature is 60~100℃; The preheating time is 1-2 minutes.

[0010] Based on the above technical solution, the corrosion solution in step 2 is a mixture of concentrated nitric acid and concentrated hydrochloric acid with a volume ratio of 1:3~5, wherein the concentration of concentrated nitric acid is 14mol / L~16mol / L and the concentration of concentrated hydrochloric acid is 11.6mol / L~12.4mol / L.

[0011] Based on the above technical solution, the corrosion treatment process in step 2 is as follows: the preheated sample is immersed in the corrosion solution for 30-60 seconds to carry out the corrosion reaction. The volume ratio of hydrochloric acid in the corrosion solution is adjusted according to the severity of the corrosion to ensure that no bubbles appear on the corrosion surface and the reaction rate is fast.

[0012] Based on the above technical solution, the temperature of the hot water in step 2 is 60~100℃, the heating time is 30~60s, and it is repeated 1~3 times.

[0013] Based on the above technical solution, the corrosion-treated sample obtained in step 2 has austenitic grain boundaries with clear grain boundaries.

[0014] Beneficial effects The technical solution disclosed in this invention uses a special corrosive liquid to corrode the sample, repeatedly removing the surface passivation film. This solves the problem that traditional corrosion methods cannot uniformly and clearly corrode the grains on the austenitic steel side of the composite plate, which seriously affects the evaluation of austenitic grain size. At the same time, it is difficult to display the grain boundaries of austenitic grains in the composite plate. The technical solution disclosed in this invention can effectively improve the corrosion effect of austenitic grains and accelerate the corrosion rate. Attached Figure Description

[0015] Figure 1 This is a microstructure image of the tissue after etching with aqua regia in the comparative example of this invention; Figure 2 The images show the microstructure of the sample after corrosion treatment as described in Example 1 of this invention (left image: 50X, austenitic matrix of composite plate; right image: 500X, austenitic matrix of composite plate).

[0016] Figure 3 The image shows the microstructure of the sample after corrosion treatment as described in Example 2 of this invention (left: austenitic matrix of 50X composite plate, right: junction of 50X composite plate).

[0017] Figure 4 This is a microstructure image of the sample after corrosion treatment as described in Example 3 of the present invention. Detailed Implementation

[0018] To make the objectives and technical solutions of this invention clearer, the following embodiments are provided for further explanation. However, the scope of protection of this invention is not limited to these embodiments; the embodiments are merely for illustrative purposes. Those skilled in the art should understand that any changes or equivalent substitutions that do not depart from the concept of this invention are included within the scope of protection of this invention.

[0019] Unless otherwise specified, all reagents and raw materials used in this invention are obtained through purchase.

[0020] The present invention provides a method for etching austenitic grains of stainless steel in composite plates in the specific embodiments section, comprising the following steps: Step 1, Sample preheating: Immerse the metallographic sample in hot water to preheat it, and obtain the preheated sample. Step 2, corrosion and hot water bath sample: The preheated sample described in Step 1 is corroded with a corrosive solution. After the reaction rate decreases, the sample is placed in hot water for cleaning and heating to remove the surface passivation layer. The operation is repeated until the sample surface is gray and non-reflective. The sample is then dried to obtain the corroded sample.

[0021] Based on the above technical solution, the preheating conditions in step 1 are as follows: The preheating temperature is 60~100℃; The preheating time is 1-2 minutes.

[0022] Based on the above technical solution, the corrosion solution in step 2 is a mixture of concentrated nitric acid and concentrated hydrochloric acid with a volume ratio of 1:3~5, wherein the concentration of concentrated nitric acid is 14mol / L~16mol / L and the concentration of concentrated hydrochloric acid is 11.6mol / L~12.4mol / L.

[0023] Based on the above technical solution, the corrosion treatment process in step 2 is as follows: the preheated sample is immersed in the corrosion solution for 30-60 seconds to carry out the corrosion reaction. The volume ratio of hydrochloric acid in the corrosion solution is adjusted according to the severity of the corrosion to ensure that no bubbles appear on the corrosion surface and the reaction speed is fast.

[0024] Based on the above technical solution, the temperature of the hot water in step 2 is 60~100℃, the heating time is 30~60s, and it is repeated 1~3 times.

[0025] Based on the above technical solution, the corrosion-treated sample obtained in step 2 has austenitic grain boundaries with clear grain boundaries.

[0026] Example 1 Step 1, Preheat the sample: Immerse the prepared metallographic sample (Q345R+316L composite plate) in hot water at 100℃ for 60 seconds until the overall temperature of the sample is uniform, then remove the preheated sample.

[0027] Step 2: Etching the sample and removing the passivation film in a hot water bath: A etching solution was prepared using concentrated nitric acid (15.8 mol / L) and concentrated hydrochloric acid (12.1 mol / L) at a volume ratio of 1:4.5. The preheated sample was etched with this solution. No bubbles appeared on the sample surface, and the reaction rate was relatively fast. The reaction effect was observed. After the reaction rate decreased, the sample was placed back into 80°C hot water for cleaning and heated for 30 seconds to remove the surface passivation layer.

[0028] Step 3, Repeat the operation: Repeat step 2, observing the corrosion status of the corroded surface continuously. After repeating this process three times, the sample surface will turn gray and non-reflective. Dry the sample to obtain the corroded sample. Under a microscope, a clean grain boundary and clear microstructure can be observed. Figure 2 .

[0029] Example 2 The difference from Example 1 lies in the composition of the corrosive solution in step 2; specifically, the volume ratio of concentrated nitric acid to concentrated hydrochloric acid is 1:3.5. The rest of the process remains the same as in Example 1. See the corrosion effect diagram below. Figure 3 .

[0030] Example 3 The differences from Example 1 are as follows: the austenitic microstructure morphology is different, the preheating temperature is different, and the number of etching cycles in step 2 is different. Specifically, the preheating temperature is 80°C, and the etching is repeated twice. The remaining processes are the same as in Example 1. See the etching effect diagram below. Figure 4 .

[0031] Comparative Example Metallographic samples were directly treated with conventional aqua regia, specifically a 3:1 volume ratio mixture of concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3). The etching effect is shown in [the figure]. Figure 1 The results indicate that conventional aqua regia etching solution cannot produce uniform and clear austenitic grain boundaries; the grain boundaries in the sample microstructure are unclear, and selective corrosion is observed.

[0032] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for etching austenitic grains of stainless steel in composite plates, characterized in that, Includes the following steps: Step 1, Sample preheating: Immerse the metallographic sample in hot water to preheat it, and obtain the preheated sample. Step 2, corrosion and hot water bath cleaning of the sample: The sample preheated in Step 1 is corroded with a corrosive solution. After the reaction rate decreases, the sample is placed in hot water for cleaning and heating to remove the surface passivation layer. The above operation is repeated until the sample surface is gray and non-reflective. The sample is then dried to obtain the corroded sample.

2. The corrosion method according to claim 1, characterized in that, The preheating conditions described in step 1 are as follows: The preheating temperature is 60~100℃; The preheating time is 1-2 minutes.

3. The corrosion method according to claim 1, characterized in that, The corrosive solution in step 2 is a mixture of concentrated nitric acid and concentrated hydrochloric acid with a volume ratio of 1:3~5, wherein the concentration of concentrated nitric acid is 14mol / L~16mol / L and the concentration of concentrated hydrochloric acid is 11.6mol / L~12.4mol / L.

4. The corrosion method according to claim 1, characterized in that, The corrosion treatment process in step 2 is as follows: the preheated sample is immersed in the corrosion solution for 30-60 seconds to carry out the corrosion reaction. The volume ratio of hydrochloric acid in the corrosion solution is adjusted according to the severity of the corrosion to ensure that no bubbles appear on the corrosion surface and the reaction rate is fast.

5. The corrosion method according to claim 1, characterized in that, In step 2, the temperature of the hot water is 60~100℃, the heating time is 30~60s, and it is repeated 1~3 times.

6. The corrosion method according to claim 1, characterized in that, The etched sample obtained in step 2 has well-defined austenitic grain boundaries.

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