A method for displaying grain boundaries of high-boron alloy steel

Abstract

This invention discloses a method for grain boundary display of high boron alloy steel, comprising the following steps: (1) Sample preparation: the sample to be tested is inlaid, ground, and polished to obtain a scratch-free mirror surface; (2) Initial grain boundary etching: the polished sample is immersed in an etching solution for 5-30 seconds; the etching solution is prepared from perchloric acid, citric acid, benzotriazole, sodium dodecylbenzenesulfonate, and deionized water; (3) the sample is quickly removed, rinsed with anhydrous ethanol, and dried; (4) Anodizing: the sample is used as the anode and placed in a 5%-10% chromic acid electrolyte for constant-voltage anodizing treatment; (5) after rinsing with deionized water, dehydrating with anhydrous ethanol, and drying, it is observed under a microscope. The method of this invention has the advantages of simple operation, good reproducibility, continuous and clear grain boundary display, and minimal damage to the matrix structure, providing a reliable technical means for the grain size assessment, microstructure analysis, and performance research of high boron alloy steel.

Description

Technical Field

[0001] This invention belongs to the field of microstructure analysis of metallic materials, and specifically relates to a method for displaying grain boundaries in high boron alloy steel. Background Technology

[0002] High-boron alloy steels, due to their high hardness, excellent wear resistance, and potential boride volume strengthening effect, have significant application prospects in the field of wear-resistant components. In their microstructure, boron tends to segregate at austenite grain boundaries and form continuous network borides (such as Fe₂B, MnO₂). 23 (B,C)6, etc.); the morphology, distribution, and continuity of these grain boundary precipitates have a decisive influence on the toughness, brittle fracture behavior, and wear resistance of materials. Clearly and accurately displaying the original austenite grain boundaries of high boron alloy steel is a prerequisite for grain size rating, studying the evolution of microstructure, and evaluating material properties.

[0003] Traditional etchants, such as 4% nitric acid alcohol solution (Nital), are mainly suitable for displaying pearlite or cementite networks in carbon steel and low alloy steel. For high boron alloy steel, these conventional etchants have significant shortcomings: (1) they have weak selective corrosion ability on grain boundary borides, and the grain boundary contrast is not obvious; (2) they are prone to over-corrosion of intragranular ferrite or martensite matrix, resulting in blurred or even completely disappeared grain boundaries; (3) they have poor reproducibility and are extremely sensitive to time, temperature and sample condition.

[0004] While strong oxidizing acids or alkaline solutions may attack borides, they often result in excessively wide grain boundaries and severe damage to the intragranular structure, failing to accurately reflect the true morphology of the grain boundaries.

[0005] Therefore, developing a method specifically designed for the characteristics of borides at the grain boundaries of high-boron alloy steels to achieve clear, continuous, and stable grain boundary display is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a method for displaying grain boundaries in high boron alloy steel, which not only has excellent display effects, but is also simple to operate and has good reproducibility.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for grain boundary display in high boron alloy steel includes the following steps: (1) Sample preparation: The high boron alloy steel sample to be tested is inlaid, ground and polished to obtain a scratch-free mirror surface.

[0008] (2) Initial etching of grain boundaries: The polished sample is immersed in the etching solution for 5-30 seconds; the etching solution is prepared from perchloric acid, citric acid, benzotriazole, sodium dodecylbenzenesulfonate and deionized water.

[0009] (3) Quickly remove the sample, rinse it with anhydrous ethanol and blow it dry.

[0010] (4) Anodizing: The sample is used as the anode and placed in a 5%-10% chromic acid electrolyte for constant voltage anodizing treatment.

[0011] (5) Rinse with deionized water, dehydrate with anhydrous ethanol and blow dry, then observe under a microscope.

[0012] Furthermore, in step (2) of the present invention, the citric acid content in the corrosive solution is 1-5% by mass, the perchloric acid content is 2-8% by volume, the benzotriazole content is 1-3% by mass, and the sodium dodecylbenzenesulfonate content is 0.1-0.2% by mass.

[0013] Furthermore, the corrosion time in step (2) of the present invention is adjusted according to the boron content in the steel: when the boron content is ≤0.5wt%, corrosion is 5-15s; when the boron content is 0.5-1.5wt%, corrosion is 15-25s; when the boron content is >1.5wt%, corrosion is 25-30s.

[0014] Furthermore, the chromic acid electrolyte in step (4) of this invention is prepared from chromium trioxide and deionized water.

[0015] Furthermore, in step (4) of the present invention, the anodizing is performed using a DC regulated power supply, with the sample connected to the anode and a stainless steel or platinum sheet connected to the cathode, and the area ratio of the cathode to the anode is not less than 2:1.

[0016] Furthermore, in step (4) of the present invention, the anodizing voltage is 2-8V and the time is 30-90 seconds; preferably, the voltage is 5V and the time is 60 seconds.

[0017] Furthermore, the high-boron alloy steel of the present invention is an iron-based alloy with a boron content between 0.1wt% and 2.5wt%.

[0018] The inventive principle of this invention lies in: The core of this invention lies in the use of stepwise etching and selective passivation, as detailed below: Step 1: Initial etching is performed using a water-based etching solution prepared with perchloric acid, citric acid, benzotriazole, and sodium dodecylbenzenesulfonate. Perchloric acid and citric acid have a preliminary dissolving and etching effect on grain boundary borides; the anionic surfactant sodium dodecylbenzenesulfonate reduces the surface tension of the etching solution, making it easier to penetrate into the fine grain boundary interstices, ensuring a uniform and controllable initial attack on the continuous network of borides; in addition, a trace amount of the corrosion inhibitor benzotriazole can moderately protect the intragranular matrix and avoid excessive etching. This operation lays the foundation for the subsequent oxidation treatment based on the "micro-region electrochemical activity difference".

[0019] Step 2: Chromate anodizing. After initial etching, the boride in the grain boundary regions is partially dissolved, exposing more fresh metal surface, and the composition (boron-rich phase) differs significantly from that of the intragranular matrix (iron-rich phase). During anodizing, the grain boundary regions, due to their higher chemical activity and relatively concentrated current density, preferentially form a thicker oxide film; while the oxide film formed in the intragranular matrix is ​​thinner. This difference in oxide film thickness directly leads to differences in optical interference colors. Under a bright-field optical microscope, the thicker grain boundary oxide film absorbs more light, appearing as continuous, clear black or dark gray lines; the thinner intragranular oxide film is relatively bright. Even when the high-boron alloy steel matrix has a hard phase (such as martensite), this contrast mechanism based on film thickness difference remains stable and effective.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Excellent display effect: It can stably display the original austenite grain boundaries of high boron alloy steel as continuous, clear and straight dark lines, which facilitates accurate grain size measurement and grain boundary morphology analysis.

[0021] (2) Good selectivity: Based on the principle of electrochemical selective passivation, it mainly images the grain boundary region, with little interference to the matrix structure within the grain, and a clean background, which is conducive to observing the interaction between the grain boundary and the second phase.

[0022] (3) Simple operation and good reproducibility: Stepwise chemical and electrochemical treatment is adopted, the steps are standardized, the parameters are easy to control, and the results between different operators and batches are highly reproducible.

[0023] (4) Wide applicability: It is applicable to high boron alloy steels with different boron contents (0.1-2.5 wt%), and good results can be obtained regardless of the matrix structure.

[0024] (5) Minimal damage to samples: Low-voltage anodizing is used, which is a micro-area surface treatment. It will not drastically change the surface morphology of the sample like deep corrosion, which is beneficial for subsequent surface-sensitive analyses such as EBSD. Attached Figure Description

[0025] Figure 1 The image shown is a metallographic photograph (500x optical microscope bright field image) obtained in Example 1.

[0026] Figure 2 The image shown is a metallographic photograph (500x optical microscope bright field image) obtained in Example 2.

[0027] Figure 3 The image shown is a metallographic photograph (500x optical microscope bright field image) obtained in Example 3.

[0028] Figure 4 The image shown is a metallographic photograph (500x optical microscope bright field image) obtained in Comparative Example 1. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. All reagents used in the following embodiments are commercially available analytical grade, and the water used is deionized water. Example 1

[0030] The high-boron alloy steel in this embodiment has the following composition: B 0.1%, C 0.03%, Si 0.3%, Mn 1.2%, Cr 19.5%, Ni 14%, Ti 0.05%, with the remainder being iron and unavoidable impurities; its grain boundary display method includes the following steps: (1) Sample preparation: Cut a high boron alloy steel sample, heat mount it, and wet grind it step by step with 180#, 400#, 800# and 1000# sandpaper. Then polish it to a mirror finish with diamond polishing paste (2.5μm) on a polishing machine. Clean it with anhydrous ethanol and blow it dry.

[0031] (2) Initial etching of grain boundaries: The polished sample is immersed in the etching solution and etched for 10 seconds at room temperature (25°C); the etching solution is prepared from perchloric acid, citric acid, benzotriazole and sodium dodecylbenzenesulfonate; the mass percentage of citric acid in the etching solution is 1%, the volume percentage of perchloric acid is 8%, the mass percentage of sodium dodecylbenzenesulfonate is 0.1%, and the mass percentage of benzotriazole is 1%.

[0032] (3) Quickly remove the sample, rinse it with a large amount of anhydrous ethanol, and dry it with warm air (<50℃).

[0033] (4) Anodizing: Weigh 7g of chromium trioxide (CrO3) and dissolve it in 93g of deionized water to prepare a chromic acid electrolyte with a concentration of about 7%. Use the treated sample as the anode and a stainless steel sheet as the cathode (area ratio of 3:1) and immerse it in the electrolyte. Connect a DC regulated power supply and oxidize for 60 seconds at a voltage of 5V DC. The electrolyte temperature is room temperature.

[0034] (5) Remove the sample, rinse with deionized water, dehydrate with anhydrous ethanol and blow dry, then observe under an optical microscope. The results are as follows: Figure 1 As shown.

[0035] Depend on Figure 1 As can be seen, the original austenite grain boundaries are clearly and continuously displayed as fine black lines, forming a complete grain network. Boride precipitates are present within the grains, but they do not affect the observation of the grain boundaries. The grain boundary width is moderate, and there is no over-corrosion. Example 2

[0036] The high-boron alloy steel in this embodiment has the following composition: B 1%, C 0.03%, Si 0.3%, Mn 1.2%, Cr 19.5%, Ni 14%, Ti 0.05%, with the remainder being iron and unavoidable impurities; its grain boundary display method includes the following steps: (1) Sample preparation: Cut a high boron alloy steel sample, heat mount it, and wet grind it step by step with 180#, 400#, 800# and 1000# sandpaper. Then polish it to a mirror finish with diamond polishing paste (2.5μm) on a polishing machine. Clean it with anhydrous ethanol and blow it dry.

[0037] (2) Initial etching of grain boundaries: The polished sample is immersed in this etching solution and etched for 20 seconds at room temperature (25°C); the etching solution is prepared from perchloric acid, citric acid, benzotriazole and sodium dodecylbenzenesulfonate; the mass percentage of citric acid in the etching solution is 2%, the volume percentage of perchloric acid is 6%, the mass percentage of sodium dodecylbenzenesulfonate is 0.15%, and the mass percentage of benzotriazole is 1.5%.

[0038] (3) Quickly remove the sample, rinse it with a large amount of anhydrous ethanol, and dry it with warm air (<50℃).

[0039] (4) Anodizing: Weigh 10g of chromium trioxide (CrO3) and dissolve it in 90g of deionized water to prepare a chromic acid electrolyte with a concentration of about 10%. Use the treated sample as the anode and a stainless steel sheet as the cathode (area ratio of 3:1) and immerse it in the electrolyte. Connect a DC regulated power supply and oxidize for 60 seconds at a voltage of 4V DC. The electrolyte temperature is room temperature.

[0040] (5) Remove the sample, rinse with deionized water, dehydrate with anhydrous ethanol and blow dry, then observe under an optical microscope. The results are as follows: Figure 2 As shown.

[0041] Depend on Figure 2 As can be seen, the original austenite grain boundaries are clearly and continuously displayed as fine black lines, forming a complete grain network. Boride precipitates are present within the grains, but they do not affect the observation of the grain boundaries. The grain boundary width is moderate, and there is no over-corrosion. Example 3

[0042] The high-boron alloy steel in this embodiment has the following composition: B 2.5%, C 0.03%, Si 0.3%, Mn 1.2%, Cr 19.5%, Ni 14%, Ti 0.05%, with the remainder being iron and unavoidable impurities; its grain boundary display method includes the following steps: (1) Sample preparation: Cut a high boron alloy steel sample, heat mount it, and wet grind it step by step with 180#, 400#, 800# and 1000# sandpaper. Then polish it to a mirror finish with diamond polishing paste (2.5μm) on a polishing machine. Clean it with anhydrous ethanol and blow it dry.

[0043] (2) Initial etching of grain boundaries: The polished sample is immersed in the etching solution and etched at room temperature (25°C) for 30 seconds; the etching solution is prepared from perchloric acid, citric acid, benzotriazole and sodium dodecylbenzenesulfonate; the mass percentage of citric acid in the etching solution is 5%, the volume percentage of perchloric acid is 2%, the mass percentage of sodium dodecylbenzenesulfonate is 0.2%, and the mass percentage of benzotriazole is 3%.

[0044] (3) Quickly remove the sample, rinse it with a large amount of anhydrous ethanol, and dry it with warm air (<50℃).

[0045] (4) Anodizing: Weigh 5g of chromium trioxide (CrO3) and dissolve it in 95g of deionized water to prepare a chromic acid electrolyte with a concentration of about 5%. Use the treated sample as the anode and a platinum sheet as the cathode (area ratio of 3:1) and immerse it in the electrolyte. Connect a DC regulated power supply and oxidize for 60 seconds at a voltage of 3V DC. The electrolyte temperature is room temperature.

[0046] (5) Remove the sample, rinse with deionized water, dehydrate with anhydrous ethanol and blow dry, then observe under an optical microscope. The results are as follows: Figure 3 As shown.

[0047] Depend on Figure 3 As can be seen, the original austenite grain boundaries are clearly and continuously displayed as fine black lines, forming a complete grain network. Boride precipitates are present within the grains, but they do not affect the observation of the grain boundaries. The grain boundary width is moderate, and there is no over-corrosion.

[0048] Comparative Example 1 Take the same high boron alloy steel sample as in Example 1 and process it according to the traditional metallographic method: after polishing, directly etch it with 4% nitric acid alcohol solution for 8 seconds.

[0049] The sample was removed, rinsed with deionized water, dehydrated with anhydrous ethanol, and dried. It was then observed under an optical microscope. The results are as follows: Figure 4 As shown.

[0050] Depend on Figure 4 It is evident that the sample surface is corroded, with only intragranular boride precipitates visible, but the original austenite grain boundaries are almost completely unrecognizable. Locally, the influence of the borides presents uneven dark areas, but these do not form a continuous grain boundary network. Therefore, effective grain size assessment is impossible.

Claims

1. A method for revealing grain boundaries in high-boron alloy steel, characterized by, The method comprises the following steps: (1) sample preparation: embedding, grinding and polishing the sample to be tested to obtain a scratch-free mirror surface; (2) initial etching of grain boundaries: immersing the polished sample in an etching solution for 5-30 seconds; the etching solution is prepared from perchloric acid, citric acid, benzotriazole, sodium dodecylbenzenesulfonate and deionized water; (3) quickly taking out the sample, rinsing with anhydrous ethanol and blowing dry; (4) anodic oxidation: taking the sample as an anode and placing it in a chromium acid electrolyte with a concentration of 5%-10% for constant voltage anodic oxidation treatment; (5) rinsing with deionized water, dehydrating with anhydrous ethanol and blowing dry, and then observing under a microscope.

2. A method of revealing grain boundaries in a high-boron alloy steel according to claim 1, characterized in that, The mass percentage of citric acid in the etching solution of step (2) is 1-5%, the volume percentage of perchloric acid is 2-8%, the mass percentage of benzotriazole is 1-3%, and the mass percentage of sodium dodecylbenzenesulfonate is 0.1-0.2%.

3. A method of revealing grain boundaries in a high-boron alloy steel according to claim 1, characterized in that, The etching time in step (2) is adjusted according to the boron content in the steel: when the boron content is ≤0.5wt%, etching for 5-15 seconds; when the boron content is 0.5-1.5wt%, etching for 15-25 seconds; and when the boron content is >1.5wt%, etching for 25-30 seconds.

4. The method for grain boundary display of high boron alloy steel according to claim 1, characterized in that, The chromium acid electrolyte in step (4) is prepared from chromium trioxide and deionized water.

5. The method for grain boundary display of high boron alloy steel according to claim 1, characterized in that, In step (4), the anodic oxidation is performed using a direct current stabilized power supply, the sample is connected to the anode, a stainless steel sheet or a platinum sheet is connected to the cathode, and the area ratio of the cathode to the anode is not less than 2:

1.

6. A method of revealing grain boundaries in a high-boron alloy steel according to claim 1, wherein In step (4), the anodic oxidation is performed at a voltage of 2-8V for 30-90 seconds.

7. The method for grain boundary display of high boron alloy steel according to claim 1, characterized in that, The high-boron alloy steel is an iron-based alloy with a boron content of 0.1wt%-2.5wt%.

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