Stainless steel grain size corrosive agent and stainless steel grain size display method

Abstract

The invention discloses a stainless steel grain size corrosive agent and a stainless steel grain size display method, and belongs to the technical field of corrosive agents. The corrosive agent is composed of 0.1-5% of ammonium persulfate, 0.5-8% of phosphoric acid, 3-20% of citric acid, 0.1-5% of sodium chloride, 0.05-2% of a surfactant and 50-65% of deionized water, and all the components have a synergistic effect. The display method comprises the steps of sample pretreatment, corrosion treatment, post-treatment and observation and detection, wherein after pretreatment, a sample is etched for 5-20 minutes at room temperature, and after cleaning and drying, the sample is detected through a metallographic microscope. The corrosive agent is green and safe, has no highly toxic and highly corrosive components, can complete corrosion in a single step without heating, and is simple, convenient and efficient to operate. The original austenite grain boundary of the martensitic stainless steel can be clearly displayed, accurate judgment can be achieved without high magnification times, the problems that a traditional corrosive agent is poor in safety, complex in operation and fuzzy in grain boundary display are solved, and the method is suitable for martensitic stainless steel grain size detection and quality control.

Description

Technical Field

[0001] This invention relates to the field of etchant technology, specifically to a stainless steel grain size etchant and a method for displaying stainless steel grain size. Background Technology

[0002] Martensitic stainless steel, with its excellent comprehensive properties, has been widely used in many important fields such as aerospace. Grain size is a key factor affecting the mechanical properties of martensitic stainless steel, and accurate detection of its grain size is of great significance for its application and quality control.

[0003] Existing technologies for etching martensitic stainless steel grain size have many drawbacks. For example, some etchants use picric acid as the main reagent, which is a highly toxic and easily explosive substance, difficult to procure, and explicitly banned in many laboratories, posing extremely low safety risks. Some etchants use potassium permanganate + sulfuric acid as the main system, often requiring a two-step etching process involving heating, prolonged immersion, and repeated wiping, resulting in long operation cycles and harsh conditions. The etchant recommended in GB / T13298-2015, using FeC13 + hydrochloric acid as the main system, can only observe the metal structure and cannot clearly display the proto-austenite grain boundaries of martensitic stainless steel. Furthermore, observation at magnifications of 100x or higher is required for more accurate grain boundary identification, making it inconvenient to use. In addition, traditional etchants often use strong acids, highly toxic components, or organic solvents, posing significant hazards to the environment and operator health.

[0004] Therefore, we propose a stainless steel grain size etchant and a method for displaying stainless steel grain size. Summary of the Invention

[0005] The purpose of this invention is to provide a stainless steel grain size etchant and a method for displaying stainless steel grain size, thereby solving the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a stainless steel grain size etchant, comprising the following components by mass percentage: 0.1-5% ammonium persulfate, 0.5-8% phosphoric acid, 3-20% citric acid, 0.1-5% sodium chloride, 0.05-2% surfactant, and 50%-65% water; wherein the surfactant is White Cat dishwashing liquid, and the components work synergistically to achieve single-step corrosion display of the original austenitic grain boundaries of martensitic stainless steel through acid etching, oxidation, complexation, and cleaning mechanisms.

[0007] In a preferred embodiment of the present invention, the ammonium persulfate is 1-3% by mass, which acts as a strong oxidizing agent to enhance the oxidation reaction on grain boundaries and improve the clarity of grain boundary display.

[0008] In a preferred embodiment of the present invention, the phosphoric acid has a mass percentage of 2-6%, providing an acidic environment to activate the oxidizing properties of persulfate, and the PO4 produced by its dissociation... 3- It forms complexes with metal ions enriched at grain boundaries, reducing corrosion resistance.

[0009] In a preferred embodiment of the present invention, the citric acid has a mass percentage of 8-15%, which stabilizes the pH value of the solution and inhibits localized electrolytic corrosion, while also reacting with Fe³⁺. + Its strong complexing effect removes oxidation products and ensures uniform and continuous corrosion.

[0010] In a preferred embodiment of the present invention, the sodium chloride is 1-4% by mass, which increases the conductivity of the solution and destroys the passivation film on the stainless steel surface, thereby accelerating the corrosion reaction process.

[0011] In a preferred embodiment of the present invention, the surfactant is 0.5-1.5% by mass, which reduces the surface tension of the solution, improves the wettability of the solution on the stainless steel sample, and ensures uniform corrosion.

[0012] In a preferred embodiment of the present invention, the water is deionized water with a mass percentage of 55-62%, which serves as the main solvent to adjust the pH value of the solution and ensure the stable dissolution and synergistic effect of each component.

[0013] This invention also relates to a method for displaying the grain size of stainless steel, comprising the following steps: Step 1: Sample pretreatment: Grind and polish the surface of the martensitic stainless steel sample to remove surface oxide scale, scratches and impurities, and obtain a sample with a smooth surface. Step 2, corrosion treatment: Immerse the pretreated sample in the corrosive agent and etch it at room temperature for 5-20 minutes; Step 3, Post-processing: Remove the sample, rinse it with clean water, and then dry it. Step 4: Observation and testing: Place the dried sample under a metallographic microscope to observe and test the original austenite grain size.

[0014] In a preferred embodiment of the present invention, in step one, grinding is carried out step by step using 400-2000 grit sandpaper, and polishing is performed using diamond polishing paste to ensure that the surface roughness of the sample is ≤0.05μm.

[0015] In a preferred embodiment of the present invention, in step two, the corrosive agent is kept still or stirred slowly during the etching process, and in step three, the drying process is carried out by cold air drying or natural air drying to avoid secondary oxidation of the sample surface.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention uses a mild organic acid as the corrosive agent instead of a traditional strong acid. It does not contain highly toxic components such as picric acid. The amount of strong oxidant added is small and it is mainly dissolved in aqueous solution. Water is used as the main medium and no organic solvents are added. It is green, safe, low-load, and environmentally friendly, while reducing the harm to the health of operators. The single-step etching method eliminates the need for complex operations such as heating and repeated wiping. It can be directly etched at room temperature with controllable time, requiring less experience and technical skills from the operator, thus greatly improving operational efficiency. Through the synergistic effects of acid etching, oxidation, complexation, and cleaning, the original austenitic grain boundaries of martensitic stainless steel can be clearly displayed, facilitating accurate subsequent grain size detection and enabling accurate identification of grain boundaries without relying on high magnification. Attached Figure Description

[0017] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a flowchart of a stainless steel grain size etchant and a stainless steel grain size display method according to the present invention. Detailed Implementation

[0018] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments. Example 1

[0019] Corrosion agent formulation Ammonium persulfate 1%, phosphoric acid 2%, citric acid 8%, sodium chloride 1%, surfactant (White Cat dishwashing liquid) 0.5%, deionized water 87.5%.

[0020] Steps for stainless steel grain size display method: Step 1: Sample Pretreatment: Select a standard sample of 1Cr13 martensitic stainless steel (10mm×10mm×5mm). Grinding is carried out step by step in the order of 400 grit → 800 grit → 1200 grit → 1500 grit → 2000 grit. During each grinding, the sample is fixed on the polishing machine table, the grinding pressure is controlled at 0.2MPa, and the grinding time is 5 minutes per level to ensure that the scratches of the previous sandpaper are completely covered. Polishing is done with W3.5 diamond polishing paste, the polishing speed is 300r / min, and the polishing time is 8 minutes. Finally, the surface roughness of the sample is measured by a roughness meter to be 0.04μm. There is no oxide scale, scratches, or impurities on the surface, and the surface is smooth and flat.

[0021] Step 2, corrosion treatment: Pour 500mL of the prepared corrosive agent into a corrosion-resistant glass container, and immerse the pretreated sample completely in the corrosive agent to ensure that there are no air bubbles adhering to the sample surface. Keep the corrosive agent stationary for 5 minutes at room temperature (25℃). During the corrosion process, observe the surface condition of the sample every 1 minute to ensure that the corrosion reaction proceeds steadily.

[0022] Step 3, Post-treatment: After removing the sample, immediately rinse it in running water for 3 minutes at a flow rate of 0.5 m / s to thoroughly remove any residual corrosive agent from the surface. Then place the sample in a clean fume hood to air dry naturally for 15 minutes. During this time, avoid contact between the sample and oil or dust to prevent secondary oxidation of the surface.

[0023] Step 4: Observation and testing: Fix the dried sample on the stage of the metallographic microscope, adjust the microscope focus, and the original austenite grain boundaries can be clearly observed at 50x magnification. The grain boundary outlines are continuous and have good contrast. The grain size is tested using image analysis software. The test result is grade 8.2. Compared with the standard test method, the deviation is ≤0.3, which meets the requirements for accurate testing.

[0024] Example 2 Ammonium persulfate 2%, phosphoric acid 4%, citric acid 12%, sodium chloride 2.5%, surfactant (White Cat dishwashing liquid) 1%, deionized water 78.5%.

[0025] Steps for stainless steel grain size display method: Step 1: Sample Pretreatment: Select industrially produced 2Cr13 martensitic stainless steel workpiece slices (size 15mm×15mm×3mm), and successively polish them with 400-grit, 1000-grit, 1500-grit, and 2000-grit sandpaper. During the polishing process, a combination of manual and mechanical methods is used to ensure uniform force on the sample surface. After each polishing stage, rinse with clean water and dry. After checking for no obvious scratches, proceed to the next stage. Polishing is performed using W2.5 diamond polishing paste at a polishing speed of 350 r / min for 10 minutes. The surface roughness of the sample is measured by a roughness tester to be 0.03 μm, which meets the test requirements.

[0026] Step 2, corrosion treatment: Prepare 1000mL of etchant in a polytetrafluoroethylene container, immerse the pretreated sample in the etchant, and slowly stir the etchant with a magnetic stirrer at room temperature (23℃) at a stirring rate of 30r / min to ensure that the etchant is in full contact with the sample surface. The immersion time is 12 minutes. During the stirring process, avoid generating violent eddies to prevent uneven corrosion of the sample surface.

[0027] Step 3, Post-treatment: After etching, remove the sample and rinse it with clean water for 5 minutes to ensure that there is no corrosive residue on the surface; then use a cold air dryer to dry it. The drying temperature is set at 35℃, the wind speed is 1m / s, and the drying time is 8 minutes to quickly remove the moisture from the sample surface and avoid secondary oxidation caused by moisture residue.

[0028] Step 4: Observation and testing: Place the dried sample under a metallographic microscope. The original austenite grain boundaries can be clearly distinguished at 40x magnification. The grain boundary details are complete and there are no blurred areas. The grain size is determined by a combination of manual counting and instrument detection. The result is grade 7.8, which is less than ±0.5 grade from the test result of GB / T6394-2017 standard method, meeting the requirements of industrial quality control.

[0029] Example 3 Ammonium persulfate 3%, phosphoric acid 6%, citric acid 15%, sodium chloride 4%, surfactant (White Cat dishwashing liquid) 1.5%, deionized water 70.5%.

[0030] Steps for stainless steel grain size display method: Step 1: Sample Pretreatment: Select 3Cr13 martensitic stainless steel test blanks and process them into standard-sized samples (12mm×12mm×4mm). Grind them step by step in the order of 400 mesh → 800 mesh → 1200 mesh → 1800 mesh → 2000 mesh, with each grinding time controlled at 5 minutes. During the grinding process, continuously cool the sample with clean water to prevent changes in the sample surface structure caused by frictional heat. Polishing is done with W1.5 high-purity diamond polishing paste at a polishing speed of 400 r / min for 12 minutes. The final sample surface roughness reaches 0.02μm, with a smooth and flawless surface, free of oxide scale, scratches, and impurities.

[0031] Step 2, Corrosion Treatment: Prepare 800 mL of corrosive agent in a corrosion-resistant plastic container, immerse the pretreated sample completely in the corrosive agent, keep the corrosive agent still at room temperature (26℃), and immerse for 20 minutes. During the immersion process, observe the color change of the sample surface regularly to ensure that the corrosion reaction is carried out within a controllable range and avoid excessive corrosion.

[0032] Step 3, Post-processing: After removing the sample, rinse it with running water for 5 minutes, ensuring that the water flow is evenly applied to all surfaces of the sample to ensure that any residual corrosive agent is completely removed; use cold air drying method, with the drying temperature controlled below 40℃ and the drying time 10 minutes, to prevent the sample from undergoing structural transformation due to excessive temperature, which would affect the grain size detection results.

[0033] Step 4: Observation and Testing: Place the dried sample under a metallographic microscope. The original austenite grain boundary outline can be clearly observed at 30x magnification. The grain boundary contrast is high and the boundary is clearly distinguishable. Automatic testing is performed by a professional grain size testing instrument. The test result is grade 8.5. The test accuracy is high and the repeatability is good. It is suitable for precise testing in the laboratory and large-scale quality screening in industrial scenarios.

[0034] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0035] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A stainless steel grain-scale etchant, characterized in that: The product comprises the following components by weight percentage: 0.1-5% ammonium persulfate, 0.5-8% phosphoric acid, 3-20% citric acid, 0.1-5% sodium chloride, 0.05-2% surfactant, and 50%-65% water; the surfactant is White Cat dishwashing liquid. The components work synergistically to achieve single-step corrosion display of the original austenitic grain boundaries of martensitic stainless steel through acid etching, oxidation, complexation, and cleaning mechanisms.

2. The stainless steel grain size etchant according to claim 1, characterized in that: The ammonium persulfate is 1-3% by mass, which acts as a strong oxidizing agent to enhance the oxidation reaction on grain boundaries and improve the clarity of grain boundary display.

3. The stainless steel grain size etchant according to claim 1, characterized in that: The phosphoric acid, at a mass percentage of 2-6%, provides an acidic environment to activate the oxidizing properties of persulfate, and its dissociation produces PO4. 3- It forms complexes with metal ions enriched at grain boundaries, reducing corrosion resistance.

4. The stainless steel grain size etchant according to claim 1, characterized in that: The citric acid, at a mass percentage of 8-15%, stabilizes the solution pH and inhibits localized electrolytic corrosion, while also reacting with Fe³⁺. + Its strong complexing effect removes oxidation products and ensures uniform and continuous corrosion.

5. The stainless steel grain size etchant according to claim 1, characterized in that: The sodium chloride is present in a mass percentage of 1-4%, which increases the conductivity of the solution and destroys the passivation film on the stainless steel surface, thus accelerating the corrosion reaction process.

6. The stainless steel grain size etchant according to claim 1, characterized in that: The surfactant is present in a mass percentage of 0.5-1.5%, which reduces the surface tension of the solution, improves the wettability of the solution on the stainless steel sample, and ensures uniform corrosion.

7. The stainless steel grain size etchant according to claim 1, characterized in that: The water is deionized water, with a mass percentage of 55-62%, which serves as the main solvent to adjust the pH value of the solution and ensure the stable dissolution and synergistic effect of each component.

8. A method for displaying the grain size of stainless steel, applicable to the stainless steel grain size etchant according to any one of claims 1-7, characterized in that: The methods and steps include the following: Step 1: Sample pretreatment: Grind and polish the surface of the martensitic stainless steel sample to remove surface oxide scale, scratches and impurities, and obtain a sample with a smooth surface. Step 2, corrosion treatment: Immerse the pretreated sample in the corrosive agent and etch it at room temperature for 5-20 minutes; Step 3, Post-processing: Remove the sample, rinse it with clean water, and then dry it. Step 4: Observation and testing: Place the dried sample under a metallographic microscope to observe and test the original austenite grain size.

9. The stainless steel grain size etchant and stainless steel grain size display method according to claim 1, characterized in that: In step one, grinding is carried out step by step using 400-2000 grit sandpaper, and polishing is done with diamond polishing paste to ensure that the surface roughness of the sample is ≤0.05μm.

10. The stainless steel grain size etchant and stainless steel grain size display method according to claim 1, characterized in that: In step two, the corrosive agent is kept still or stirred slowly during the etching process. In step three, the drying process is carried out by cold air drying or natural air drying to avoid secondary oxidation of the sample surface.

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