Method for displaying austenite grain size of martensite structure in stainless steel

Abstract

The invention relates to the technical field of metallographic microstructure detection, and discloses a method for displaying the austenite grain size of a martensitic structure in stainless steel, which comprises the following steps: S1, sample preparation: cutting, grinding and polishing a stainless steel sample until no macroscopic scratch exists on the surface; s2, preparing an electrolyte: mixing 50%-70% of nitric acid and 30%-50% of deionized water to form a basic solution, and adding 3ml-7ml of phosphoric acid and 1ml-5ml of sulfuric acid into every 100ml of the basic solution; and S3, assembling an electrolysis device: a sample clamp made of martensitic stainless steel is adopted as an anode, austenitic stainless steel is adopted as a cathode, and the soaking area of the cathode is not less than 1500mm. By optimizing the electrolyte formula and process parameters, the original austenite grain size of the martensitic structure in the stainless steel can be clearly displayed. The method solves the problem of unclear display of the stainless steel grain boundary in a traditional method, is not only suitable for martensitic stainless steel, but also suitable for precipitation hardening type stainless steel, and has high universality and applicability.

Description

Technical Field

[0001] The present invention relates to the technical field of metallographic microstructure detection, and specifically provides a method for revealing the austenite grain size of martensite structure in stainless steel. Background Art

[0002] Martensitic stainless steel and martensitic precipitation hardening stainless steel are two important types of materials widely used in fields such as aviation, mechanical manufacturing, and energy. They have high strength and excellent toughness. The properties of these two types of stainless steel are closely related to their organizational structures, and the original austenite grain size is an important parameter affecting the formation of martensite structure and material properties. Therefore, accurately revealing the austenite grain size in metallographic detection is a key link in optimizing material properties and guiding heat treatment process design.

[0003] In carbon steel and medium-high alloy steel, the technology for revealing austenite grain size is relatively mature. Usually, corrosion reagents such as picric acid plus corrosion inhibitors are used to corrode the specimen, and the grain boundaries can be quickly revealed. However, for martensitic stainless steel and precipitation hardening stainless steel, this traditional method is difficult to work effectively, mainly because picric acid has no corrosive effect on stainless steel. This problem makes the revelation of martensitic stainless steel grain size a technical difficulty, severely restricting the effectiveness of metallographic detection and heat treatment process optimization. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a method for revealing the austenite grain size of martensite structure in stainless steel, which solves the problem that the original austenite grain size of martensite structure in stainless steel cannot be clearly revealed by traditional corrosion methods.

[0005] To achieve the above objectives, the present invention is realized through the following technical solutions: A method for revealing the austenite grain size of martensite structure in stainless steel, comprising the following steps: S1. Specimen preparation: Cut, grind, and polish the stainless steel specimen until there are no visible scratches on the surface with the naked eye; S2. Prepare the electrolyte: The electrolyte is a basic solution mixed with 50%-70% nitric acid and 30%-50% deionized water. Add 3 ml - 7 ml phosphoric acid and 1 ml - 5 ml sulfuric acid to every 100 ml of the basic solution; S3. Assemble the electrolysis device: The anode is a sample clamp made of martensitic stainless steel, and the cathode is made of austenitic stainless steel. The immersed area of the cathode is not less than 1500 mm²; S4. Electrolyze the specimen: Immerse the specimen in the electrolyte, set the electrolysis voltage to 0.5 V - 10 V, and the electrolysis time to 5 s - 60 s, and reveal the austenite grain size through selective corrosion; S5. Optimize and adjust: If the grains are not completely revealed, adjust the immersed area of the specimen, keep the electrolysis voltage and time unchanged, and repeat electrolysis until it is clearly revealed.

[0006] Preferably, the specimen is polished successively with 400# to 1200# sandpaper by mechanical polishing and then finely polished with 0.5μm diamond polishing agent.

[0007] Preferably, the basic solution of the electrolyte is composed of 65%-68% nitric acid and 32%-35% deionized water, and the ratio of nitric acid to deionized water is fixed at 2:1.

[0008] Preferably, the anode in the electrolysis device is 420 series martensitic stainless steel and the cathode is 300 series austenitic stainless steel.

[0009] Preferably, the electrolysis voltage range is set to 0.5V - 10V, and the specific selection of the voltage is optimized according to the display effect of the specimen grain size to ensure uniform corrosion and clear grain boundaries.

[0010] Preferably, the electrolysis time range is set to 5s - 60s, which is adjusted according to the corrosion depth of the specimen to ensure the integrity of the grain boundaries and no over-corrosion.

[0011] Preferably, during the electrolysis process, the grain display effect is optimized by adjusting the immersion area of the specimen. The initial immersion area is 240mm² and is gradually reduced to not less than 150mm² to improve the clarity of the grain boundary display.

[0012] Preferably, the anode fixture is designed as an elastic clamping structure to ensure stable fixation of the specimen and avoid uneven corrosion.

[0013] Preferably, the surface morphology of the specimen is observed by a metallographic microscope, and the clarity of the grain boundaries and the uniformity of the grain distribution are analyzed.

[0014] Preferably, in the basic solution of the electrolyte, the addition ratio of phosphoric acid to sulfuric acid is configured according to the volume ratio of 1.5:1 to 2:1, and the electrolyte temperature is controlled within the range of room temperature to 50°C.

[0015] The present invention provides a method for displaying the austenite grain size of martensite structure in stainless steel. It has the following beneficial effects: 1. By optimizing the electrolyte formula and process parameters, the present invention can clearly display the original austenite grain size of martensite structure in stainless steel. This method solves the problem of unclear grain boundary display of stainless steel in traditional methods, and is applicable not only to martensitic stainless steel but also to precipitation-hardening stainless steel, with strong generality and applicability.

[0016] 2. The present invention adopts the electrolytic corrosion technology, which can precisely control the display effect of grain boundaries by adjusting the electrolytic voltage, time, and the immersion area of the specimen. Without complex equipment and cumbersome operations, the entire experimental process is simple and easy to implement, and the parameter adjustment range is wide, facilitating flexible adjustment according to the characteristics of different specimens.

[0017] 3. By clearly displaying the austenite grain size of the martensite structure in stainless steel, the present invention provides a scientific basis for the design and optimization of heat treatment processes. Precise grain size measurement can help control the material properties and ensure that the stainless steel has excellent mechanical properties, such as high strength and good toughness.

[0018] 4. The electrolyte used in the present invention is prepared from common chemical reagents, with low material costs and easy availability, avoiding the use of expensive or special reagents in traditional corrosion methods. At the same time, the electrolysis process has high environmental friendliness, the reaction process is controllable, reducing the impact on the environment, and better meeting the requirements of modern green manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a three-dimensional schematic diagram of the electrolysis device in the present invention; Figure 2 is a morphology diagram of 0Cr17Ni4Cu4Nb martensite structure after being corroded by aqua regia in the present invention; Figure 3 is a morphology diagram of the original austenite grain size of 0Cr17Ni4Cu4Nb in the present invention; Figure 4 is a morphology diagram of 07Cr16Ni6 martensite structure after being corroded by aqua regia in the present invention; Figure 5 is a morphology diagram of the original austenite grain size of 07Cr16Ni6 in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the specification of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] Embodiment: Please refer to the attached Figure 1 - attached Figure 5 , the embodiment of the present invention provides a method for displaying the austenite grain size of the martensite structure in stainless steel, including the following steps: S1. Specimen preparation: Cut, grind, and polish the stainless steel specimen until there are no visible scratches on the surface with the naked eye; Two types of stainless steel materials were selected for the experiment: Material A: 0Cr17Ni4Cu4Nb precipitation-hardening stainless steel.

[0022] Material B: 07Cr16Ni6 martensitic stainless steel.

[0023] Specimen preparation process: Cutting: Each material was cut into sample blocks of 25mm×25mm×5mm to ensure that the cutting surface was flat.

[0024] Grinding: Gradually polished with 400# to 1200# sandpapers in sequence. During the water grinding process, the sandpapers were frequently changed to avoid cross-contamination.

[0025] Polishing: Pre-polished with 3μm diamond polishing agent to remove deep scratches.

[0026] Finely polished with 0.5μm diamond polishing agent until the specimen surface was smooth and flawless.

[0027] Cleaning and drying: After each polishing, it was cleaned successively with deionized water and absolute ethanol to avoid chemical residues, and then dried.

[0028] S2. Preparation of electrolyte: The electrolyte was prepared by mixing 50%-70% nitric acid and 30%-50% deionized water to form a basic solution, and 3ml-7ml phosphoric acid and 1ml-5ml sulfuric acid were added to every 100ml of the basic solution; The electrolyte consisted of a basic solution and additive components, and their ranges and regulation were as follows: Basic solution: 50% deionized water + 50% nitric acid (concentration 65%-68%).

[0029] Additive components (per 100ml of basic solution): Phosphoric acid: 3-7ml (concentration ≥85%).

[0030] Sulfuric acid: 1-5ml (concentration 95%-98%).

[0031] Adjustable variables: The following variables were mainly regulated in the experiment to analyze their effects on the grain size display effect: Nitric acid concentration (40%-70%).

[0032] Addition ratio of phosphoric acid to sulfuric acid (phosphoric acid:sulfuric acid from 1:1 to 3:1).

[0033] Voltage range (0.5V-10V).

[0034] Time range (5s-60s).

[0035] S3. Electrolytic cell assembly: The anode uses a sample clamp made of martensitic stainless steel, the cathode uses austenitic stainless steel, and the immersed area of the cathode is not less than 1500 mm²; Anode: A sample clamp made of 420 series martensitic stainless steel is used to fix the specimen, and the exposed part is the corrosion surface.

[0036] Cathode: 300 series austenitic stainless steel, specifically selected as follows: Material A: 304 stainless steel is used, and the immersed area is 2000 mm².

[0037] Material B: 316L stainless steel is used, and the immersed area is 1500 mm².

[0038] Container: A 500 ml corrosion-resistant glass container is selected to ensure that the solution evenly covers the specimen.

[0039] S4. Specimen electrolysis: The specimen is immersed in the electrolyte, the electrolysis voltage is set to 0.5V - 10V, and the electrolysis time is 5s - 60s. The austenite grain size is revealed through selective corrosion; S5. Optimization and adjustment: If the grains are not completely revealed, adjust the immersed area of the specimen, keep the electrolysis voltage and time unchanged, and repeat the electrolysis until it is clearly shown.

[0040] To optimize the experiment, multiple groups of comparative experiments are designed. For each group of experiments, other variables are kept unchanged, and only one variable is adjusted to observe its effect on the grain display effect.

[0041] Experimental comparison of Material A (0Cr17Ni4Cu4Nb precipitation hardening stainless steel): Experimental group A1: Low addition amounts of phosphoric acid and sulfuric acid Conditions: Electrolyte ratio: 50% deionized water + 50% nitric acid + 3 ml of phosphoric acid + 1 ml of sulfuric acid; Voltage: 0.5V; Time: 40 seconds; Cathode material: 304 stainless steel, area 2000 mm².

[0042] Experimental steps: Prepare the specimen and polish it until it is bright and free of scratches.

[0043] Prepare the above electrolyte and connect the cathode and anode to the electrolytic cell respectively.

[0044] Start the electrolytic cell, record the time and observe the electrolysis reaction.

[0045] Clean the surface of the specimen and dry it for standby.

[0046] Experimental group A2: Medium addition amounts of phosphoric acid and sulfuric acid Conditions: Electrolyte ratio: 50% deionized water + 50% nitric acid + 5 ml phosphoric acid + 3 ml sulfuric acid; Voltage: 5 V; Time: 30 s; Cathode material: 304 stainless steel, area 2000 mm².

[0047] Experimental procedure: Prepare the specimen and polish it.

[0048] Prepare the electrolyte and install the device.

[0049] Start the electrolysis process and record the reaction time.

[0050] Observe the surface morphology after cleaning.

[0051] Experimental group A3: Optimize the addition amounts of phosphoric acid and sulfuric acid Conditions: Electrolyte ratio: 50% deionized water + 50% nitric acid + 6 ml phosphoric acid + 3 ml sulfuric acid; Voltage: 0.5 V; Time: 40 s; Cathode material: 304 stainless steel, area 2000 mm².

[0052] Experimental procedure: Prepare and clean the specimen.

[0053] Prepare the electrolyte and adjust the electrolysis parameters.

[0054] Control the electrolysis time to 40 s and then stop immediately.

[0055] Rinse the specimen with deionized water and absolute ethanol.

[0056] Experimental group A4: High addition amounts of phosphoric acid and sulfuric acid Conditions: Electrolyte ratio: 50% deionized water + 50% nitric acid + 7 ml phosphoric acid + 5 ml sulfuric acid; Voltage: 10 V; Time: 20 s; Cathode material: 304 stainless steel, area 2000 mm².

[0057] Experimental procedure: Prepare the specimen and polish it.

[0058] Prepare a high-concentration electrolyte and adjust the voltage to 10 V.

[0059] Perform electrolysis for a short time (20 s) and observe the reaction.

[0060] Clean and microscopically observe the specimen.

[0061] The experimental results are shown in the following table: Result analysis: When the nitric acid concentration is controlled at 50%, the grain display effect is the best. Excessive concentration leads to over-corrosion.

[0062] When the ratio of phosphoric acid to sulfuric acid is 2:1, the boundary display effect is the best.

[0063] Optimal conditions: 50% nitric acid + 6 ml phosphoric acid + 3 ml sulfuric acid, voltage 0.5 V, time 40 s.

[0064] Experimental comparison of Material B (07Cr16Ni6 martensitic stainless steel): Experimental group B1: Low addition amounts of phosphoric acid and sulfuric acid Conditions: Electrolyte formulation: 50% deionized water + 50% nitric acid + 5 ml phosphoric acid + 2 ml sulfuric acid; Voltage: 1 V; Time: 20 s; Cathode material: 316L stainless steel, area 1500 mm².

[0065] Experimental procedures: Prepare the specimen and polish it.

[0066] Prepare the electrolyte and load it into a container.

[0067] Electrolyze at a low voltage for 20 s.

[0068] Clean the specimen and observe it microscopically.

[0069] Experimental group B2: Optimized addition amounts of phosphoric acid and sulfuric acid Conditions: Electrolyte formulation: 50% deionized water + 50% nitric acid + 6 ml phosphoric acid + 4 ml sulfuric acid; Voltage: 4.5 V; Time: 15 s; Cathode material: 316L stainless steel, area 1500 mm².

[0070] Experimental procedures: Prepare the specimen until its surface has no scratches.

[0071] Prepare the optimized electrolyte and install the electrolysis device.

[0072] Start the electrolysis device and record the time as 15 s.

[0073] Clean the specimen and observe its surface with a metallurgical microscope.

[0074] Experimental group B3: High addition amounts of phosphoric acid and sulfuric acid Condition: Electrolyte ratio: 50% deionized water + 50% nitric acid + 7 ml phosphoric acid + 5 ml sulfuric acid; Voltage: 8 V; Time: 25 s; Cathode material: 316L stainless steel, area 1500 mm².

[0075] Experimental procedure: Prepare the specimen.

[0076] Prepare a high-concentration electrolyte.

[0077] Electrolyze for 25 s and observe the reaction rate.

[0078] Record the microscopic morphology after cleaning.

[0079] Experimental group B4: High addition amounts of nitric acid, phosphoric acid, and sulfuric acid Condition: Electrolyte ratio: 50% deionized water + 50% nitric acid + 7 ml phosphoric acid + 5 ml sulfuric acid; Voltage: 10 V; Time: 10 s; Cathode material: 316L stainless steel, area 1500 mm².

[0080] Experimental procedure: Prepare the specimen.

[0081] Prepare a high-concentration electrolyte.

[0082] Electrolyze for 10 s and observe the reaction rate.

[0083] Record the microscopic morphology after cleaning.

[0084] The experimental results are shown in the following table: Result analysis: When the voltage is controlled at 4.5 V, the grain display effect is the best, and too high a voltage causes excessive corrosion of the grain boundaries.

[0085] Controlling the time at 15 s can avoid grain boundary blurring and uneven corrosion.

[0086] Optimal conditions: 50% nitric acid + 6 ml phosphoric acid + 4 ml sulfuric acid, voltage 4.5 V, time 15 s.

[0087] S6. Observe the samples under the optimized conditions using a metallurgical microscope and perform data analysis through image processing software: Material A: The grain boundaries are clear, the grain distribution is uniform, and the boundaries are regular.

[0088] Material B: The grain boundaries are continuous, the grain sizes are relatively regular, and there is no over-corrosion phenomenon.

[0089] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for displaying the austenite grain size of a martensite structure in stainless steel, characterized in that: The following steps are involved: S1. Sample preparation: Cut, grind and polish the stainless steel sample until there are no visible scratches on the surface; S2. Prepare electrolyte: The electrolyte is a mixture of 50%-70% nitric acid and 30%-50% deionized water to form a base solution. 3ml-7ml phosphoric acid and 1ml-5ml sulfuric acid are added to every 100ml of the base solution. S3. Assembly of electrolysis device: The anode is made of martensitic stainless steel, the cathode is made of austenitic stainless steel, and the cathode immersion area is not less than 1500mm²; S4. Sample electrolysis: The sample is immersed in an electrolyte, the electrolysis voltage is set to 0.5V-10V, the electrolysis time is 5s-60s, and the austenite grain size is displayed by selective corrosion; S5. Optimization and adjustment: If the grain display is incomplete, adjust the sample immersion area, keep the electrolysis voltage and time unchanged, and repeat the electrolysis until the display is clear.

2. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: The sample was polished by mechanical polishing using 400# to 1200# sandpaper in sequence, and fine polished using 0.5 μm diamond polishing agent.

3. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: The basic solution of the electrolyte consists of 65%-68% nitric acid and 32%-35% deionized water, and the ratio of nitric acid to deionized water is fixed at 2:

1.

4. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: The anode in the electrolysis device is 420 series martensitic stainless steel, and the cathode is 300 series austenitic stainless steel.

5. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: The electrolysis voltage range is set to 0.5V-10V, and the specific selection of voltage is optimized according to the sample grain size display effect to ensure uniform corrosion and clear grain boundaries.

6. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: The electrolysis time range is set to 5s-60s, which is adjusted according to the corrosion depth of the sample to ensure that the grain boundaries are intact and there is no excessive corrosion.

7. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: During the electrolysis process, the grain display effect is optimized by adjusting the sample immersion area, where the initial immersion area is 240 mm² and gradually reduced to no less than 150 mm² to improve the clarity of grain boundary display.

8. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: The anode fixture adopts an elastic clamping structure to ensure that the sample is fixed stably and avoid uneven corrosion.

9. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: The surface morphology of the samples was observed by metallographic microscope, and the grain boundary clarity and grain distribution uniformity were analyzed.

10. The method for displaying the austenite grain size of the martensite structure in stainless steel according to claim 1, characterized in that: In the base solution of the electrolyte, phosphoric acid and sulfuric acid are added in a ratio of 1.5:1 to 2:1 by volume, and the temperature of the electrolyte is controlled in the range of room temperature to 50°C.

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