Method for displaying original austenite grains in fine blanking medium-low carbon hot-rolled steel plate

Abstract

The invention discloses a method for displaying original austenite grains in a fine blanking medium-low carbon hot-rolled steel plate. The method mainly solves the technical problem that the original austenite grains in the fine blanking hot-rolled steel plate with w [C] of 0.16%-0.51% in the existing steel cannot be clearly displayed. According to the technical scheme, the corrosive liquid for displaying the original austenite grains in the fine blanking medium-low carbon hot-rolled steel plate is characterized in that 500ml of the corrosive liquid is prepared from the following raw materials: 2-3g of FeCl3 powder, 30-60ml of concentrated hydrochloric acid, 10-30ml of concentrated nitric acid, 10-15g of hexadecylamine, 10-15g of lignin sulfonate, 5-10g of dodecyl trimethyl ammonium chloride, 200ml of ethanol solution and the balance of deionized water. The corrosion liquid is good in stability, long in shelf life, suitable for structure display of other low-carbon steel and wide in application range.

Description

Technical Field

[0001] This invention relates to a method for displaying the metallographic structure of steel plates, and particularly to a method for displaying the original austenite grains in fine-stamped low-carbon hot-rolled steel plates; specifically, it relates to a method for displaying the original austenite grains in fine-stamped hot-rolled steel plates with w[C] of 0.16% to 0.51%, belonging to the field of metallographic structure inspection technology of steel materials. Background Technology

[0002] Fine blanking is a high-efficiency and economical processing technology developed from conventional blanking technology. It combines precision blanking and sheet metal forming to manufacture precision and complex stamped parts, producing high-quality components. The automotive industry is a key sector for the promotion and application of fine blanking parts. Currently, 95% of the fine blanking materials worldwide are steel materials, namely fine blanking steel, including carbon structural steel, carbon tool steel, alloy structural steel, alloy tool steel, spring steel, etc.

[0003] Currently, the cold-rolled steel products used by fine stamping enterprises are mainly supplied through a combination of hot-rolled raw materials from large steel enterprises and cold rolling and spheroidizing annealing by specialized cold rolling enterprises. The metallographic structure and heat treatment process are relatively complex. The hot-rolled structure is mainly pearlite, while the structure after cold rolling and spheroidizing annealing is spheroidized annealed.

[0004] The initial austenite grain size of steel directly affects the microstructure changes during controlled rolling and cooling, thus significantly influencing the material's mechanical properties. Finer austenite grains result in higher strength, better plasticity, and better impact resistance after heat treatment. Conversely, coarser austenite grains significantly reduce the steel's impact resistance, decrease crack propagation energy, and increase the brittle transition temperature. Furthermore, steel with coarse grains exhibits increased quenching distortion and cracking tendency. Especially when grain size is uneven, it significantly reduces the steel's structural strength, causes stress concentration, and facilitates brittle fracture.

[0005] There are many methods for revealing the original austenite grain boundaries, such as oxidation, network cementite, and grain boundary corrosion. Among them, grain boundary corrosion is the most effective. The etchant used is a mixture of saturated picric acid and other solutions in different proportions.

[0006] Chinese patent application publication number CN107621398A discloses a method for revealing the original austenite grain boundaries of atmospheric corrosion resistant steel Q450NQR1. The corrosive agent disclosed in this method is 200ml supersaturated picric acid solution, 5ml FeCl3 solution, 5ml detergent, and 10ml benzalkonium chloride.

[0007] Chinese patent application publication number CN109001010A discloses an etchant and etching method for revealing the initial austenite grain boundaries of low and medium carbon steel. The etchant disclosed in this method includes water, picric acid and fatty acid polyoxyethylene ether.

[0008] The aforementioned corrosive solutions all contained picric acid, but picric acid is a strong acid, dangerous, and explosive; many research institutions have banned its use. Technicians began exploring corrosive agents that did not contain picric acid.

[0009] Chinese patent application CN105092437A discloses a method for displaying the original austenite grain size of ultra-supercritical martensitic heat-resistant cast steel. The etching solution of the method is a 6-10% nitric acid alcohol solution. However, the steel type targeted is martensitic heat-resistant steel, with the representative steel type being ZG13Cr9Mo2Co1NiVNbNB. However, the sample needs to undergo pre-heat treatment to promote the formation of the original austenite grain size, and the heat treatment time is as long as 100 hours.

[0010] Chinese patent application CN105865882A discloses an etching method for displaying the austenitic grain boundaries of quenched and tempered low-alloy chromium-molybdenum steel using an etchant. The etchant composition is: 8-15g of sulfosalicylic acid, 100ml of water, 0.1-1ml of 4% nitric acid alcohol (by mass percentage), and 0.2-5ml of detergent. This method is suitable for quenched and tempered low-alloy chromium-molybdenum steels, such as 26CrMo4, 27CrMo47Vs, and 27CrMo44s. However, this method is specific to certain steel grades and is not applicable to medium and low carbon precision stamping steels.

[0011] Existing technologies cannot solve the problem of displaying the original austenite grains in fine-stamped hot-rolled steel plates with w[C] of 0.16% to 0.51%. Summary of the Invention

[0012] The purpose of this invention is to provide a method for displaying the original austenite grains in fine-stamped low-carbon hot-rolled steel plates, mainly solving the technical problem that the original austenite grains in fine-stamped hot-rolled steel plates with w[C] of 0.16% to 0.51% in existing steel cannot be clearly displayed; this invention can clearly display the original austenite grain boundaries, is applicable to a wide range of steel grades, has stable corrosion effect, and high reproducibility.

[0013] The technical concept of this invention is to use an etching solution consisting of an austenite grain boundary etchant and a martensite lath inhibitor to perform low-temperature tempering on quenched samples, thereby promoting the movement of impurity elements towards the austenite grain boundaries and making the austenite grain boundaries more easily visible.

[0014] The technical solution adopted in this invention is an etching solution for displaying the original austenite grains in low-carbon hot-rolled steel plates during fine stamping. 500ml of the etching solution is made from the following raw materials: 2-3g FeCl3 powder, 30-60ml concentrated hydrochloric acid, 10-30ml concentrated nitric acid, 10-15g hexadecylamine, 10-15g sulfonated lignin, 5-10g dodecyltrimethylammonium chloride, 200ml ethanol solution, and the balance being deionized water.

[0015] The ethanol solution was of analytical grade.

[0016] The method for preparing the above-mentioned etchant for displaying the original austenite grains in fine-stamped low-carbon hot-rolled steel plates includes the following steps:

[0017] 1) Measure out hexadecylamine, sulfonated lignin, and dodecyltrimethylammonium chloride according to the specified proportions;

[0018] 2) Add hexadecylamine to 200 ml of ethanol solution and stir until homogeneous to obtain intermediate solution A;

[0019] 3) Take 100ml of deionized water and heat it to 50-70℃. Add sulfonated lignin and dodecyltrimethylammonium chloride to the deionized water in sequence and stir until completely dissolved. After cooling to room temperature, intermediate solution B is obtained. Then mix intermediate solution A and intermediate solution B and stir until well mixed to obtain intermediate solution C.

[0020] 4) Add concentrated nitric acid and concentrated hydrochloric acid to the beaker in sequence and stir well; after cooling to room temperature, add 100 ml of deionized water and FeCl3 powder to the beaker containing concentrated nitric acid and concentrated hydrochloric acid in sequence and stir well to obtain intermediate solution D.

[0021] 5) Mix intermediate solution C and intermediate solution D and stir well; dilute the mixture of intermediate solution C and intermediate solution D to 500 ml with deionized water to obtain the finished corrosion solution.

[0022] The basis for the formulation of the etching solution used in this invention to reveal the composition of the original austenite grains in fine-stamped low-carbon hot-rolled steel plates is as follows:

[0023] The etchant of this invention is composed of an austenitic grain boundary etchant and a martensite lath inhibitor (slow-release agent).

[0024] 1. Setting of austenitic grain boundary etchant components

[0025] The austenitic grain boundary etchant is prepared from FeCl3, concentrated hydrochloric acid, and concentrated nitric acid.

[0026] Concentrated hydrochloric acid and concentrated nitric acid are the main components of aqua regia. Aqua regia is often used to corrode austenitic stainless steel, heat-resistant steel and other steels that are difficult to corrode. This invention uses it after dilution.

[0027] FeCl3 reacts chemically with Fe: 2FeCl3 + Fe → 3FeCl2. Due to the abundance of crystal defects at austenite grain boundaries, this reaction occurs first at the grain boundaries. FeCl3 exhibits better corrosion performance in acidic media. Therefore, the combination of these three reagents can achieve a better corrosion effect.

[0028] Austenite is a high-temperature structure that exists in high-temperature regions above 727°C. During the cooling process, it will transform into other structures. However, during the rapid cooling (quenching) process of austenite to form martensite, the martensite will maintain a coherent relationship with the original austenite. Therefore, room temperature martensite structure retains the original austenite grain boundaries.

[0029] To better reveal the austenite grain boundaries, the quenched sample needs to be subjected to low-temperature tempering to promote the aggregation of impurity elements at the austenite grain boundaries, making the austenite grain boundaries easier to reveal.

[0030] 2. Setting of martensitic lath inhibitor components

[0031] Martensite lath inhibitor is formulated with hexadecylamine, sulfonated lignin, and dodecyltrimethylammonium chloride. It mainly plays a sustained-release role, inhibiting the display of martensite tissue.

[0032] Hexadecylamine and sulfonated lignin are adsorption film type organic corrosion inhibitors. The molecules of these compounds are adsorbed onto the metal surface by their hydrophilic groups, forming a dense hydrophobic film that protects the metal surface from corrosion.

[0033] In addition, if there are corrosion products or scale deposits on the metal surface, it is difficult to form an effective corrosion inhibitor film. In this case, a small amount of surfactant dodecyltrimethylammonium chloride can be added to help the corrosion inhibitor form a film.

[0034] The martensitic lath inhibitor of this invention has the best slow-release effect at a temperature of 50-70°C. Therefore, corrosion must be carried out in water at 50-70°C.

[0035] A method for displaying the proto-austenite grains in fine-stamped low-carbon hot-rolled steel sheets includes the following steps:

[0036] 1) Prepare the test sample by performing heat treatment, quenching treatment, and low-temperature tempering treatment in sequence; cut a fine-stamped medium-low carbon hot-rolled steel plate sample with a length of 20 mm and a width of 20 mm; heat the sample to 30-50°C above the Ar3 temperature of the steel and hold it at this temperature for 25-35 min; then perform water quenching on the sample; after quenching, perform low-temperature tempering on the sample, controlling the tempering temperature at 200-300°C and the tempering holding time at 1-3 h; after tempering, air cool the sample to room temperature to obtain the test sample.

[0037] 2) Prepare metallographic specimens by cutting, mounting, grinding and polishing the specimens to be tested;

[0038] 3) Etching of the metallographic specimens to the original austenite grains, including:

[0039] 3.1) Heat the etching solution to 50-70°C, place a beaker with a glass lid containing 50ml of etching solution in a water bath and let it stand for 4-6 minutes, controlling the water bath temperature to 50-70°C.

[0040] 3.2) Immerse the metallographic sample in an etching solution. Add the metallographic sample to the etching solution in a beaker, ensuring the sample is completely submerged with the test surface facing upwards. Let it stand for 10-15 minutes. When the test surface of the metallographic sample turns dark purple, remove the sample from the etching solution, clean and dry it, and gently polish the test surface to remove oxides.

[0041] 4) Observe and photograph the metallographic structure of the sample. Use a metallographic microscope to observe and photograph the surface of the metallographic sample after the original austenite grains have been corroded and analyze the grain size.

[0042] Compared with existing technologies, this invention has the following positive effects: 1. Without using picric acid, the method of this invention achieves clear display of the original austenite grains in medium and low carbon fine-stamping steel through a newly developed etching solution and a matching heat treatment process; this method can clearly display the original austenite grain boundaries, and the grain boundary display is clear and complete, which is beneficial for the observation of the original austenite structure and the evaluation of grain size. 2. The method of this invention has a wide detection range and is applicable to fine-stamping hot-rolled steel plates with a carbon content of 0.16% to 0.51%. The representative steel grade with a carbon content of 0.16% is 16MnCr5; the representative steel grade with a carbon content of 0.51% is 51CrV4; other fine-stamping steels with carbon contents within this range are also applicable, such as 20MnB5, 20CrMo, 30CrMo, etc. 3. The etching solution of this invention has good stability, long shelf life, stable etching effect, and high reproducibility. Attached Figure Description

[0043] Figure 1 The image shows a metallographic photograph of the original austenite grains of 16MnCr5 in Example 1.

[0044] Figure 2 The image shows a metallographic photograph of the original austenite grains of 30CrMo in Example 2.

[0045] Figure 3 Metallographic photograph of the original austenite grains of 51CrV4 in Example 3. Detailed Implementation

[0046] The present invention will be further described below with reference to the embodiments.

[0047] Example 1: The sample was 16MnCr5 fine stamping steel with the following chemical composition by weight percentage: C: 0.14%, Si: 0.058%, Mn: 1.03%, P≤0.010%, S≤0.005%, Alt: 0.034%, Ti: 0.026%, Cr: 0.90%, with the remainder being Fe and unavoidable inclusions.

[0048] A method for preparing an etchant for revealing the original austenite grains in fine-stamped low-carbon hot-rolled steel plates includes the following steps:

[0049] 1) Measure out 10g of hexadecylamine, 10g of sulfonated lignin, and 5g of dodecyltrimethylammonium chloride respectively;

[0050] 2) Add hexadecylamine to 200 ml of ethanol solution and stir until homogeneous to obtain intermediate solution A;

[0051] 3) Take 100ml of deionized water and heat it to 50-70℃. Add sulfonated lignin and dodecyltrimethylammonium chloride to the deionized water in sequence and stir until completely dissolved. After cooling to room temperature, intermediate solution B is obtained. Then mix intermediate solution A and intermediate solution B and stir until well mixed to obtain intermediate solution C.

[0052] 4) Add 10 ml of concentrated nitric acid and 30 ml of concentrated hydrochloric acid to a beaker in sequence and stir well. After cooling to room temperature, add 100 ml of deionized water and 2 g of FeCl3 powder to the beaker containing concentrated nitric acid and concentrated hydrochloric acid in sequence and stir well to obtain intermediate solution D.

[0053] 5) Mix intermediate solution C and intermediate solution D and stir well; dilute the mixture of intermediate solution C and intermediate solution D to 500 ml with deionized water to obtain the finished corrosion solution.

[0054] A method for displaying the proto-austenite grains in fine-stamped low-carbon hot-rolled steel sheets includes the following steps:

[0055] 1) Prepare the test sample by performing heat treatment, quenching treatment and low-temperature tempering treatment in sequence; cut a fine-stamped medium-low carbon hot-rolled steel plate sample with a length of 20 mm and a width of 20 mm; heat the sample to 930℃ and hold it at this temperature for 30 min; then perform water quenching on the sample; after quenching, perform low-temperature tempering on the sample, controlling the tempering temperature at 200℃ and the tempering holding time at 2 h; after tempering, air cool the sample to room temperature to obtain the test sample.

[0056] 2) Prepare metallographic specimens by cutting, mounting, grinding and polishing the specimens to be tested;

[0057] 3) Etching of the metallographic specimens to the original austenite grains, including:

[0058] 3.1) Heat the etching solution to 50°C, place a beaker containing 50ml of etching solution with a glass lid in a water bath and let it stand for 5 minutes, controlling the water bath temperature to 50°C.

[0059] 3.2) Immerse the metallographic sample in an etching solution. Add the metallographic sample to the etching solution in a beaker, ensuring the sample is completely submerged with the test surface facing upwards. Let it stand for 10 minutes. When the test surface of the metallographic sample turns dark purple, remove the sample from the etching solution, clean and dry it, and gently polish the test surface to remove the oxides.

[0060] 4) Observe and photograph the metallographic structure of the sample. Use a metallographic microscope to observe and photograph the surface of the metallographic sample after the original austenite grains have been corroded and analyze the grain size.

[0061] Example 2: The sample was 30CrMo fine stamping steel with the following chemical composition by weight percentage: C: 0.30%, Si: 0.21%, Mn: 0.52%, P: 0.010%, S: 0.05%, Alt: 0.034%, Mo: 0.022%, Cr: 1.05%, with the remainder being Fe and unavoidable inclusions.

[0062] A method for preparing an etchant for revealing the original austenite grains in fine-stamped low-carbon hot-rolled steel plates includes the following steps:

[0063] 1) Measure out 12g of hexadecylamine, 12g of sulfonated lignin, and 8g of dodecyltrimethylammonium chloride respectively;

[0064] 2) Add hexadecylamine to 200 ml of ethanol solution and stir until homogeneous to obtain intermediate solution A;

[0065] 3) Take 100ml of deionized water and heat it to 50-60℃. Add sulfonated lignin and dodecyltrimethylammonium chloride to the deionized water in sequence and stir until completely dissolved. After cooling to room temperature, intermediate solution B is obtained. Then mix intermediate solution A and intermediate solution B and stir until well mixed to obtain intermediate solution C.

[0066] 4) Add 15ml of concentrated nitric acid and 45ml of concentrated hydrochloric acid to a beaker in sequence and stir well. After cooling to room temperature, add 100ml of deionized water and 2g of FeCl3 powder to the beaker containing concentrated nitric acid and concentrated hydrochloric acid in sequence and stir well to obtain intermediate solution D.

[0067] 5) Mix intermediate solution C and intermediate solution D and stir well; dilute the mixture of intermediate solution C and intermediate solution D to 500 ml with deionized water to obtain the finished corrosion solution.

[0068] A method for displaying the proto-austenite grains in fine-stamped low-carbon hot-rolled steel sheets includes the following steps:

[0069] 1) Prepare the test sample by performing heat treatment, quenching treatment and low-temperature tempering treatment in sequence; cut a fine-stamped medium-low carbon hot-rolled steel plate sample with a length of 20 mm and a width of 20 mm; heat the sample to 890℃ and hold it at this temperature for 30 min; then perform water quenching on the sample; after quenching, perform low-temperature tempering on the sample, controlling the tempering temperature at 250℃ and the tempering holding time at 3 h; after tempering, air cool the sample to room temperature to obtain the test sample.

[0070] 2) Prepare metallographic specimens by cutting, mounting, grinding and polishing the specimens to be tested;

[0071] 3) Etching of the metallographic specimens to the original austenite grains, including:

[0072] 3.1) Heat the etching solution to 60°C, place a beaker with a glass lid containing 50ml of etching solution in a water bath and let it stand for 5 minutes, controlling the water bath temperature to 60°C.

[0073] 3.2) Immerse the metallographic sample in an etching solution. Add the metallographic sample to the etching solution in a beaker, ensuring the sample is completely submerged with the test surface facing upwards. Let it stand for 12 minutes. When the test surface of the metallographic sample turns dark purple, remove the sample from the etching solution, clean and dry it, and gently polish the test surface to remove the oxides.

[0074] 4) Observe and photograph the metallographic structure of the sample. Use a metallographic microscope to observe and photograph the surface of the metallographic sample after the original austenite grains have been corroded and analyze the grain size.

[0075] Example 3: The sample was 51CrV4 fine stamping steel, and its chemical composition by weight percentage was: C: 0.48%, Si: 0.020%, Mn: 0.88%, P: 0.012%, S: 0.001%, Alt: 0.012%, Cr: 1.01%, with the remainder being Fe and unavoidable inclusions.

[0076] A method for preparing an etchant for revealing the original austenite grains in fine-stamped low-carbon hot-rolled steel plates includes the following steps:

[0077] 1) Measure out 15g of hexadecylamine, 15g of sulfonated lignin, and 10g of dodecyltrimethylammonium chloride respectively;

[0078] 2) Add hexadecylamine to 200 ml of ethanol solution and stir until homogeneous to obtain intermediate solution A;

[0079] 3) Take 100ml of deionized water and heat it to 50-60℃. Add sulfonated lignin and dodecyltrimethylammonium chloride to the deionized water in sequence and stir until completely dissolved. After cooling to room temperature, intermediate solution B is obtained. Then mix intermediate solution A and intermediate solution B and stir until well mixed to obtain intermediate solution C.

[0080] 4) Add 20 ml of concentrated nitric acid and 60 ml of concentrated hydrochloric acid to a beaker in sequence and stir well. After cooling to room temperature, add 100 ml of deionized water and 3 g of FeCl3 powder to the beaker containing concentrated nitric acid and concentrated hydrochloric acid in sequence and stir well to obtain intermediate solution D.

[0081] 5) Mix intermediate solution C and intermediate solution D and stir well; dilute the mixture of intermediate solution C and intermediate solution D to 500 ml with deionized water to obtain the finished corrosion solution.

[0082] A method for displaying the proto-austenite grains in fine-stamped low-carbon hot-rolled steel sheets includes the following steps:

[0083] 1) Prepare the test sample by performing heat treatment, quenching treatment and low-temperature tempering treatment in sequence; cut a fine-stamped medium-low carbon hot-rolled steel plate sample with a length of 20 mm and a width of 20 mm; heat the sample to 850℃ and hold it at this temperature for 30 min; then perform water quenching on the sample; after quenching, perform low-temperature tempering on the sample, controlling the tempering temperature at 300℃ and the tempering holding time at 3 h; after tempering, air cool the sample to room temperature to obtain the test sample.

[0084] 2) Prepare metallographic specimens by cutting, mounting, grinding and polishing the specimens to be tested;

[0085] 3) Etching of the metallographic specimens to the original austenite grains, including:

[0086] 3.1) Heat the etching solution to 70°C, place a beaker containing 50ml of etching solution with a glass lid in a water bath and let it stand for 5 minutes, controlling the water bath temperature to 70°C.

[0087] 3.2) Immerse the metallographic sample in an etching solution. Add the metallographic sample to the etching solution in a beaker, ensuring the sample is completely submerged with the test surface facing upwards. Let it stand for 15 minutes. When the test surface of the metallographic sample turns dark purple, remove the sample from the etching solution, clean and dry it, and gently polish the test surface to remove the oxides.

[0088] 4) Observe and photograph the metallographic structure of the sample. Use a metallographic microscope to observe and photograph the surface of the metallographic sample after the original austenite grains have been corroded and analyze the grain size.

Claims

1. A etching solution for revealing the original austenite grains in fine-stamped low-carbon hot-rolled steel plates, characterized in that, The 500ml etching solution is made from the following raw materials: 2-3g FeCl3 powder, 30-60ml concentrated hydrochloric acid, 10-30ml concentrated nitric acid, 10-15g hexadecylamine, 10-15g sulfonated lignin, 5-10g dodecyltrimethylammonium chloride, 200ml ethanol solution, and the balance being deionized water; the w[C] in the fine-stamped low-carbon hot-rolled steel plate is 0.16%-0.51%.

2. The method for preparing the corrosive liquid as described in claim 1, characterized in that, The method includes the following steps: 1) Measure out hexadecylamine, sulfonated lignin, and dodecyltrimethylammonium chloride according to the specified proportions; 2) Add hexadecylamine to 200 ml of ethanol solution and stir until homogeneous to obtain intermediate solution A; 3) Take 100ml of deionized water and heat it to 50-70℃. Add sulfonated lignin and dodecyltrimethylammonium chloride to the deionized water in sequence and stir until completely dissolved. After cooling to room temperature, intermediate solution B is obtained. Then mix intermediate solution A and intermediate solution B and stir until well mixed to obtain intermediate solution C. 4) Add concentrated nitric acid and concentrated hydrochloric acid to the beaker in sequence and stir well; after cooling to room temperature, add 100 ml of deionized water and FeCl3 powder to the beaker containing concentrated nitric acid and concentrated hydrochloric acid in sequence and stir well to obtain intermediate solution D. 5) Mix intermediate solution C and intermediate solution D and stir well; dilute the mixture of intermediate solution C and intermediate solution D to 500 ml with deionized water to obtain the finished corrosion solution.

3. A method for displaying the original austenite grains in fine-stamped low-carbon hot-rolled steel plates, characterized in that, The method using the corrosive liquid of claim 1 comprises the following steps: 1) Prepare the test sample by performing heat treatment, quenching treatment, and low-temperature tempering treatment in sequence; cut a fine-stamped medium-low carbon hot-rolled steel plate sample with a length of 20 mm and a width of 20 mm; heat the sample to 30-50°C above the Ar3 temperature of the steel and hold it at this temperature for 25-35 min; then perform water quenching on the sample; after quenching, perform low-temperature tempering on the sample, controlling the tempering temperature at 200-300°C and the tempering holding time at 1-3 h; after tempering, air cool the sample to room temperature to obtain the test sample. 2) Prepare metallographic specimens by cutting, mounting, grinding and polishing the specimens to be tested; 3) Etching of the metallographic specimens to the original austenite grains, including: 3.1) Heat the etching solution to 50-70°C, place a beaker with a glass lid containing 50ml of etching solution in a water bath and let it stand for 4-6 minutes, controlling the water bath temperature to 50-70°C. 3.2) Immerse the metallographic sample in an etching solution. Add the metallographic sample to the etching solution in a beaker, ensuring the sample is completely submerged with the test surface facing upwards. Let it stand for 10-15 minutes. When the test surface of the metallographic sample turns dark purple, remove the sample from the etching solution, clean and dry it, and gently polish the test surface to remove oxides. 4) Observe and photograph the metallographic structure of the sample. Use a metallographic microscope to observe and photograph the surface of the metallographic sample after the original austenite grains have been corroded and analyze the grain size.

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