Corrosion method for grain size of novel ultrahigh-strength martensitic stainless steel containing Co, Mo and W

Abstract

The invention relates to the technical field of metallographic analysis of metal materials, and discloses a corrosion method for the grain size of novel ultrahigh-strength martensitic stainless steel containing Co, Mo and W. The corrosion method comprises the steps that martensitic stainless steel is ground and mechanically polished; preparing a mixed solution of potassium permanganate, dilute sulphuric acid and deionized water; heating the mixed solution in a water bath to a target temperature range; placing a to-be-tested surface of a to-be-tested sample in the mixed solution upwards, heating and eroding until the surface of the to-be-tested sample becomes dark or purple red, and taking out the sample; preparing a mixed solution of oxalic acid and deionized water as a film-removing cleaning solution, and rapidly putting the eroded sample into the film-removing cleaning solution for cleaning; and further washing the cleaned sample with absolute ethyl alcohol, and blow-drying. In a specific temperature range, the original austenite grain boundary of the material can be effectively obtained by adopting the erosion solution with a specific proportion, and the grain size of the material can be conveniently graded.

Description

Technical Field

[0001] The present invention relates to the technical field of metallographic analysis of metal materials, and in particular to a corrosion method for the grain size of a new ultra-high strength martensitic stainless steel containing Co, Mo, and W. Background Art

[0002] The grain size is a core parameter in steel materials, and it has an important influence on the mechanical properties of steel materials. Therefore, it is necessary to grade the grain size of steel materials, which helps researchers reasonably regulate the heat treatment system and then reasonably regulate the grain size of steel materials so that their mechanical properties can meet different service scenarios. For martensitic steel, its microstructure substructure is complex and the grain boundaries are not clear enough, and conventional metallographic etching methods are difficult to accurately identify and measure the grain size.

[0003] With the rapid development of China's aerospace field, more stringent requirements are put forward for steel materials, and advanced steel materials with higher strength, elongation and better corrosion resistance are continuously developed. The present invention relates to a new ultra-high strength martensitic stainless steel containing Co, Mo, and W, and the strengthening mechanisms are mainly martensitic transformation strengthening, aging strengthening, etc., and it is mainly used for aircraft landing gears. The heat treatment system of martensitic steel is usually relatively complex, and there are obvious differences in the grain sizes obtained under different heat treatment systems. How to analyze the grain size of the new ultra-high strength martensitic stainless steel involved in the present invention and then obtain its optimal strength and toughness matching is crucial. The most commonly used method for etching the metallographic structure of martensitic steel is to use a mixed solution of ferric chloride and hydrochloric acid, aqua regia or a mixed solution of picric acid and hydrochloric acid for etching. However, through attempts, it is found that the existing technical methods cannot observe the clear and complete prior austenite grain boundaries of a new ultra-high strength martensitic stainless steel containing Co, Mo, and W at low magnification, and it is impossible to evaluate the average grain size range of its grains.

[0004] Therefore, there is a need to improve the corrosion method for the grain size of a new ultra-high strength martensitic stainless steel containing Co, Mo, and W in the prior art. Summary of the Invention

[0005] In view of this, the purpose of the embodiments of the present invention is to provide a corrosion method for the grain size of a new ultra-high strength martensitic stainless steel containing Co, Mo, and W. By using an etching solution with a specific ratio at a specific temperature range, the prior austenite grain boundaries of the above materials can be effectively obtained, which is convenient for grading its grain size.

[0006] The embodiments of the present invention provide a corrosion method for the grain size of a new ultra-high strength martensitic stainless steel containing Co, Mo, and W, including the following steps: S1: Grind and mechanically polish the martensitic stainless steel; S2: Prepare a mixed solution of potassium permanganate, dilute sulfuric acid and deionized water; S3: Heat the mixed solution in a water bath to the target temperature range; S4: Keep the water bath heating within the target temperature range, place the test surface of the test sample face up in the mixed solution for heating and etching, and take out the sample after the surface of the test sample turns dark gray or purplish red; S5: Prepare a mixed solution of oxalic acid and deionized water as a film-removing cleaning solution, and quickly put the etched sample into the film-removing cleaning solution for cleaning; S6: Further rinse and dry the cleaned sample with absolute ethanol.

[0007] In some embodiments, in S1, the grinding process includes: gradually grinding with silicon carbide wet sandpaper with particle sizes of 400#, 800#, 1000#, and 2000# respectively, and the scratches of the next pass must completely cover the scratches of the previous pass.

[0008] In some embodiments, the polishing process includes: mechanical polishing using nano-silica polishing liquid as a lubricant, the rotation speed of the polishing disc is 800 revolutions per minute, and the polishing time is 5 - 10 min.

[0009] In some embodiments, in S2, the specific ratio and preparation method of the mixed solution are: add 50 ml of dilute sulfuric acid to 250 ml of deionized water, and then add 30 g of potassium permanganate, and stir well.

[0010] In some embodiments, in S3, the heating rate during water bath heating is 5 - 6 °C / min, and the temperature is raised to 70 ± 3 °C.

[0011] In some embodiments, in S4, the etching time in the mixed solution is 20 ± 5 min.

[0012] In some embodiments, in S5, the ratio of the film-removing cleaning solution is: the solid-liquid ratio of oxalic acid solid to water is 1:(9 - 10) g / ml.

[0013] In some embodiments, in S5, the cleaning method includes: quickly put the sample completed in step S4 into the prepared cleaning solution for cleaning for 30 ± 10 s to ensure that the mixed solution prepared in step S2 does not remain on the surface of the sample.

[0014] In some embodiments, in S6, the rinsing method includes: rinsing the sample completed in step S5 with absolute ethanol solution for 15 ± 5 s to ensure that the film-removing cleaning solution prepared in step S5 does not remain on the surface of the sample.

[0015] In some embodiments, when drying, use a blower to dry until the test surface of the sample is bright.

[0016] The present invention has at least the following beneficial technical effects: When the corrosion method for the grain size of a new type of ultra-high strength martensitic stainless steel containing Co, Mo, and W according to the present invention is specifically operated, it is usually difficult to accurately evaluate the macroscopic grain size of this new type of steel by existing technical means, and it is impossible to accurately obtain the size range of its average grain. The present invention adopts water bath heating combined with a new etching solution to clearly and completely display the original austenite grain boundaries of a new type of ultra-high strength martensitic stainless steel containing Co, Mo, and W, and then detects the average size range of the grains in the Mo- and Co-containing martensitic steel specimens by the intercept method. The operation is simple and convenient, and is convenient for popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other embodiments can be obtained based on these drawings.

[0018] Figure 1 It is a flowchart of an embodiment of a corrosion method for the grain size of a new type of ultra-high strength martensitic stainless steel containing Co, Mo, and W provided by the present invention; Figure 2 、 3 、4 are the metallographic structures provided by Embodiments 1, 2, and 3 of the present invention, Figure 2 -A, 3-A, 4-A are specifically the metallographic structures of this steel type after solution treatment at 930°C for 30 min, cryogenic treatment at -73°C for 2 h, and aging treatment at 540°C for 4 h, Figure 2 -B, 3-B, 4-B are specifically the metallographic structures of this steel type after solution treatment at 1080°C for 30 min, cryogenic treatment at -73°C for 2 h, and aging treatment at 540°C for 4 h; Figure 5 、 6 、7 are the metallographic structures provided by Comparative Examples 1, 2, and 3 of the present invention, Figure 5 -A, 6-A, 7-A are specifically the metallographic structures of this steel type after solution treatment at 930°C for 30 min, cryogenic treatment at -73°C for 2 h, and aging treatment at 540°C for 4 h, Figure 5 -B, 6-B, 7-B are specifically the metallographic structures of this steel type after solution treatment at 1080°C for 30 min, cryogenic treatment at -73°C for 2 h, and aging treatment at 540°C for 4 h. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the following further elaborates on the embodiments of the present invention in detail with reference to specific embodiments and the accompanying drawings.

[0020] The terms "comprising", "having" and any variations thereof in the description, claims and above-mentioned drawings description of the present invention are intended to cover non-exclusive inclusion; the terms "first", "second", etc. in the description, claims or above-mentioned drawings of the present invention are used to distinguish different objects, rather than to describe a specific order. The meaning of "plurality" is two or more, unless otherwise specifically defined.

[0021] In addition, the mention of "embodiment" herein means that the specific features, structures or characteristics described in connection with the embodiment can be included in at least one embodiment of the present invention. The appearance of this phrase at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0022] As Figure 1 shown is a method for corroding the grain size of a novel ultra-high strength martensitic stainless steel containing Co, Mo, and W provided by the present invention, comprising the following steps: S1: Grinding and mechanical polishing of the martensitic stainless steel; S2: Preparing a mixed solution of potassium permanganate, dilute sulfuric acid and deionized water; S3: Water bath heating the mixed solution to a target temperature range; S4: Maintaining the water bath heating within the target temperature range, placing the test surface of the test sample facing upwards in the mixed solution for heating and erosion, and taking out the sample after the surface of the test sample turns dark gray or purplish red; S5: Preparing a mixed solution of oxalic acid and deionized water as a film-removing cleaning solution, and quickly putting the eroded sample into the film-removing cleaning solution for cleaning; S6: Further rinsing and drying the cleaned sample with absolute ethanol.

[0023] Furthermore, ultra-high strength steel is an advanced steel material with extremely high tensile strength and yield strength. Usually, the tensile strength exceeds 1600 MPa and the yield strength exceeds 1400 MPa. Such steel materials have important applications in high-end fields such as aerospace, national defense, automotive manufacturing, and energy equipment. In addition, due to the further expansion of the service environment, higher requirements are put forward for the corrosion resistance of materials. Therefore, ultra-high strength stainless steel has been developed and designed on the basis of ultra-high strength steel.

[0024] Further, in S1, the grinding process includes: gradually grinding with silicon carbide water sandpaper with particle sizes of 400#, 800#, 1000#, and 2000# respectively, and the scratches of the next pass must completely cover the scratches of the previous pass. Specifically, grinding and mechanical polishing of the specimen can eliminate the work-hardened layer, achieve atomic-level flatness, ensure uniform spreading of the etching solution, and reduce abnormal corrosion caused by local concentration polarization.

[0025] Further, the polishing process includes: mechanical polishing using nano-silica polishing liquid as a lubricant, the rotation speed of the polishing disc is 800 revolutions per minute, and the polishing time is 5 - 10 minutes.

[0026] Further, in S2, the specific ratio and preparation method of the mixed solution are: take 50 ml of dilute sulfuric acid and add it to 250 ml of deionized water, and then add 30 g of potassium permanganate and stir well.

[0027] Further, in S3, the heating rate during water bath heating is 5 - 6 °C / min, and it is heated to 70 ± 3 °C. The advantage of water bath temperature control is that it can precisely control the kinetics, maintain a stable corrosion rate, and reduce the interference of thermal stress.

[0028] Further, in S4, the erosion time in the mixed solution is 20 ± 5 minutes.

[0029] Further, in S5, the ratio of the film removal cleaning solution is: the solid-liquid ratio of oxalic acid solid to water is 1:(9 - 10) g / ml.

[0030] Further, in S5, the cleaning method includes: quickly putting the specimen that has completed step S4 into the prepared cleaning solution and cleaning for 30 ± 10 s to ensure that there is no residue of the mixed solution prepared in step S2 on the specimen surface.

[0031] Further, in S6, the rinsing method includes: rinsing the specimen that has completed step S5 with absolute ethanol solution for 15 ± 5 s to ensure that there is no residue of the film removal cleaning solution prepared in step S5 on the specimen surface.

[0032] Further, when drying, use a blower to dry until the surface to be measured of the specimen is bright.

[0033] The following further explains the present invention in combination with specific embodiments.

[0034] Example 1 Step 1: Grind and mechanically polish a new ultra-high-strength martensitic stainless steel containing Co, Mo, and W. Specifically, the new high-strength martensitic stainless steel is pre-ground with silicon carbide water-grinded sandpaper, and the particle size sequence is 400#, 800#, 1000#, and 2000#. Each scratch must completely cover the previous scratch. After the last grinding, a nano-silicon dioxide suspension with a particle size of 1.5 μm is used for mechanical polishing for 10 minutes. The polishing disk rotates at 800 rpm until the surface of the sample is bright and free of obvious scratches. The sample is then placed in an ultrasonic cleaning device for ultrasonic vibration cleaning for 15 minutes. The cleaning solution is anhydrous ethanol.

[0035] Step 2: Prepare a corresponding metallographic etching solution, specifically a mixed solution of potassium permanganate, dilute sulfuric acid and deionized water, wherein the ratio of potassium permanganate, dilute sulfuric acid and deionized water is 30g:20ml:250ml. After preparing the corresponding solution, stir it thoroughly until the potassium permanganate solid is completely dissolved and the solution turns purple.

[0036] Step 3: Heat the prepared mixed solution in a water bath at a heating rate of 5°C / min to 70°C. After keeping the temperature for 5 minutes, place the sample with the test surface facing up in the solution and etch for 20 minutes until the test surface turns gray.

[0037] Step 4: While the etching solution is being heated, a corresponding film removal cleaning solution is prepared for the test sample, specifically a mixed solution of oxalic acid and deionized water, wherein the ratio of oxalic acid to deionized water is 50 g:450 ml. After the etching of the above-mentioned test sample is completed, it is quickly placed in the film removal cleaning solution and cleaned for 30 seconds to ensure that there is no purple substance remaining on the test surface. Then, the test surface is rinsed with anhydrous ethanol for 15 seconds and blown dry.

[0038] Step 5: Use a metallographic microscope to observe the metallographic structure and evaluate the macro grain size of the above samples. For relevant test results, refer to the accompanying drawings. Figure 2 .

[0039] Example 2 Step 1: Grind and mechanically polish a new ultra-high-strength martensitic stainless steel containing Co, Mo, and W. Specifically, the new high-strength martensitic stainless steel is pre-ground with silicon carbide water-grinded sandpaper, and the particle size sequence is 400#, 800#, 1000#, and 2000#. Each scratch must completely cover the previous scratch. After the last grinding, a nano-silicon dioxide suspension with a particle size of 1.5 μm is used for mechanical polishing for 10 minutes. The polishing disk rotates at 800 rpm until the surface of the sample is bright and free of obvious scratches. The sample is then placed in an ultrasonic cleaning device for ultrasonic vibration cleaning for 15 minutes. The cleaning solution is anhydrous ethanol.

[0040] Step 2: Prepare a corresponding metallographic etching solution, specifically a mixed solution of potassium permanganate, dilute sulfuric acid and deionized water, wherein the ratio of potassium permanganate, dilute sulfuric acid and deionized water is 30g:20ml:250ml. After preparing the corresponding solution, stir it thoroughly until the potassium permanganate solid is completely dissolved and the solution turns purple.

[0041] Step 3: Heat the mixed solution prepared above in a water bath at a heating rate of 5°C / min to 67°C. After keeping warm for 5 minutes, place the sample to be tested with the test surface facing up in the solution and etch for 25 minutes until the test surface turns gray.

[0042] Step 4: While the etch solution is being heated, a corresponding film removal and cleaning solution is prepared for the test sample. Specifically, the solution is a mixture of oxalic acid and deionized water, wherein the ratio of oxalic acid to deionized water is 50 g:450 ml. After the etch is completed, the test sample is quickly placed in the film removal and cleaning solution for 40 seconds to ensure that no purple substance remains on the test surface. The test surface is then rinsed with anhydrous ethanol for 20 seconds and blown dry.

[0043] Step 5: Use a metallographic microscope to observe the metallographic structure and evaluate the macro grain size of the above samples. For relevant test results, refer to the accompanying drawings. Figure 3 .

[0044] Example 3 Step 1: Grind and mechanically polish a new ultra-high-strength martensitic stainless steel containing Co, Mo, and W. Specifically, the new high-strength martensitic stainless steel is pre-ground with silicon carbide water-grinded sandpaper, and the particle size sequence is 400#, 800#, 1000#, and 2000#. Each scratch must completely cover the previous scratch. After the last grinding, a nano-silicon dioxide suspension with a particle size of 1.5 μm is used for mechanical polishing for 10 minutes. The polishing disk rotates at 800 rpm until the surface of the sample is bright and free of obvious scratches. The sample is then placed in an ultrasonic cleaning device for ultrasonic vibration cleaning for 15 minutes. The cleaning solution is anhydrous ethanol.

[0045] Step 2: Prepare a corresponding metallographic etching solution, specifically a mixed solution of potassium permanganate, dilute sulfuric acid and deionized water, wherein the ratio of potassium permanganate, dilute sulfuric acid and deionized water is 30g:20ml:250ml. After preparing the corresponding solution, stir it thoroughly until the potassium permanganate solid is completely dissolved and the solution turns purple.

[0046] Step 3: Heat the mixed solution prepared above in a water bath at a heating rate of 5°C / min to 73°C. After keeping warm for 5 minutes, place the sample to be tested with the test surface facing up in the solution and etch for 15 minutes until the test surface turns gray.

[0047] Step 4: While the etching solution is being heated, a corresponding film removal and cleaning solution is prepared for the test sample. Specifically, the solution is a mixture of oxalic acid and deionized water, wherein the ratio of oxalic acid to deionized water is 50 g:450 ml. After the etching of the test sample is completed, the test sample is quickly placed in the film removal and cleaning solution for 20 seconds to ensure that no purple substance remains on the test surface. The test surface is then rinsed with anhydrous ethanol for 10 seconds and blown dry.

[0048] Step 5: Use a metallographic microscope to observe the metallographic structure and evaluate the macro grain size of the above samples. For relevant test results, refer to the accompanying drawings. Figure 4 .

[0049] Comparative Example 1 Step 1 and step 5 are the same as those in Example 1. Step 2: prepare a conventional metallographic etching solution, specifically a mixed solution of ferric chloride, hydrochloric acid and deionized water, wherein the ratio of ferric chloride, hydrochloric acid and deionized water is 10g:20ml:100ml. After preparing the corresponding solution, stir it thoroughly until the ferric chloride solid is completely dissolved and the solution is yellow.

[0050] Step 3: Place the polished surface of the test sample into the prepared metallographic etching solution and etch for 25 seconds until the surface turns grayish.

[0051] Step 4: Immediately rinse the surface to be tested with anhydrous ethanol for 15 seconds and blow dry. For relevant test results, refer to the accompanying figure. Figure 5 .

[0052] Comparative Example 2 Step 1 and step 5 are the same as step 1 and step 5 in Example 1.

[0053] Step 2: prepare a conventional metallographic etching solution, specifically a mixed solution of ferric chloride, hydrochloric acid and deionized water, wherein the ratio of ferric chloride, hydrochloric acid and deionized water is 10g:20ml:100ml. After preparing the corresponding solution, stir it thoroughly until the ferric chloride solid is completely dissolved and the solution is yellow.

[0054] Step 3: Place the polished surface of the test sample into the prepared metallographic etching solution and etch for 15 seconds until the surface turns grayish.

[0055] Step 4: Immediately rinse the surface to be tested with anhydrous ethanol for 10 seconds and blow dry. For relevant test results, refer to the accompanying drawings. Figure 6 .

[0056] Comparative Example 3 Step 1 and step 5 are the same as those in Example 1. Step 2: prepare a conventional metallographic etching solution, specifically a mixed solution of ferric chloride, hydrochloric acid and deionized water, wherein the ratio of ferric chloride, hydrochloric acid and deionized water is 10g:20ml:100ml. After preparing the corresponding solution, stir it thoroughly until the ferric chloride solid is completely dissolved and the solution is yellow.

[0057] Step 3: Place the polished surface of the test sample into the prepared metallographic etching solution and etch for 40 seconds until the surface turns grayish.

[0058] Step 4: Immediately rinse the surface to be tested with anhydrous ethanol for 25 seconds and blow dry. For relevant test results, refer to the accompanying figure. Figure 7 .

[0059] like Figure 2 、 3 4 is the grain size of the new ultra-high strength martensitic stainless steel containing Co, Mo and W in different solid solution states and the same aging state according to the method of the embodiment of the present invention, Figure 5 、 6 , 7 are the grain sizes of the new ultra-high strength martensitic stainless steel containing Co, Mo and W in different solid solution states and the same aging state under the original method in comparative example.

[0060] Specifically, Figure 2 -A, 3-A, and 4-A are the metallographic structures of the steel after solution treatment at 930°C for 30 minutes, cryogenic treatment at -73°C for 2 hours, and aging treatment at 540°C for 4 hours. Figure 2 -B, 3-B, and 4-B specifically represent the metallographic structures of the steel after solution treatment at 1080°C for 30 min, cryogenic treatment at -73°C for 2 h, and aging treatment at 540°C for 4 h. Figure 5 -A, 6-A, and 7-A are the metallographic structures of the steel after solution treatment at 930°C for 30 minutes, cryogenic treatment at -73°C for 2 hours, and aging treatment at 540°C for 4 hours. Figure 5 -B, 6-B, and 7-B specifically represent the metallographic structures of the steel after solution treatment at 1080°C for 30 min, cryogenic treatment at -73°C for 2 h, and aging treatment at 540°C for 4 h. Figure 2 -A, 3-A, 4-A and Figure 5 -A, 6-A, 7-A have the same heat treatment system. Figure 2 -B, 3-B, 4-B and Figure 5 -B, 6-B, 7-B have the same heat treatment system. In order to better verify the technology of the present invention, two different heat treatment systems were used for verification. Figure 2-7 It can be clearly found that the embodiment of the present invention has more obvious prior austenite grain boundaries and grain size than the comparative example, which is more conducive to grain size grading.

[0061] The above has introduced in detail a method for corroding the grain size of a new type of ultra-high strength martensitic stainless steel containing Co, Mo, and W provided by the embodiments of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application; at the same time, for those of ordinary skill in the art, according to the idea of the present application, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present application.

[0062] The above are exemplary embodiments disclosed by the present invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments disclosed by the present invention defined by the claims. The functions, steps, and / or actions of the method claims according to the disclosed embodiments herein do not need to be performed in any specific order. In addition, although the elements disclosed by the embodiments of the present invention can be described or claimed in individual form, they can also be understood as plural unless explicitly limited to the singular.

[0063] It should be understood that, as used herein, unless the context clearly supports an exception, the singular form "a" is also intended to include the plural form. It should also be understood that the "and / or" used herein refers to any and all possible combinations of one or more of the related listed items.

[0064] The serial numbers of the disclosed embodiments of the present invention above are only for description and do not represent the advantages or disadvantages of the embodiments.

[0065] Those of ordinary skill in the art should understand that: the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the embodiments disclosed by the present invention (including the claims) is limited to these examples; under the idea of the embodiments of the present invention, the technical features between the above embodiments or different embodiments can also be combined, and there are many other variations in different aspects of the embodiments of the present invention as above, which are not provided in detail for the sake of brevity. Therefore, any omission, modification, equivalent replacement, improvement, etc. made within the spirit and principle of the embodiments of the present invention shall be included within the protection scope of the embodiments of the present invention.

Claims

1. A corrosion method for the grain size of a novel ultra-high strength martensitic stainless steel containing Co, Mo, and W, characterized in that, Including: S1: Grinding and mechanical polishing of martensitic stainless steel; S2: Preparing a mixed solution of potassium permanganate, dilute sulfuric acid and deionized water; S3: Heating the mixed solution in a water bath to a target temperature range; S4: Keeping the water bath heating within the target temperature range, placing the test surface of the test sample upward in the mixed solution for etching, and taking out the sample after the surface of the test sample turns dark gray or purplish red; S5: Preparing a mixed solution of oxalic acid and deionized water as a film-removing cleaning solution, and quickly putting the etched sample into the film-removing cleaning solution for cleaning; S6: Further rinsing and drying the cleaned sample with absolute ethanol.

2. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S1, the grinding process includes: Gradually grinding with silicon carbide water sandpaper, with particle sizes of 400#, 800#, 1000#, and 2000# respectively, and the scratches of the next pass must completely cover the scratches of the previous pass.

3. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S1, the polishing process includes: Mechanical polishing using nano-silica polishing liquid as a lubricant, the rotation speed of the polishing disc is 800 revolutions per minute, and the polishing time is 5 - 10 minutes.

4. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S2, the specific ratio and preparation method of the mixed solution are: Take 50 ml of dilute sulfuric acid and add it to 250 ml of deionized water, then add 30 g of potassium permanganate, and stir well.

5. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S3, the heating rate during water bath heating is 5 - 6 °C / min, and the temperature is raised to 70 ± 3 °C.

6. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S4, the etching time of the sample in the mixed solution is 20 ± 5 minutes.

7. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S5, the ratio of the film-removing cleaning solution is: The solid-liquid ratio of oxalic acid solid to water is 1:(9 - 10) g / ml.

8. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S5, the cleaning method includes: Quickly putting the sample completed in step S4 into the prepared cleaning solution for cleaning for 30 ± 10 s to ensure that there is no residue of the mixed solution prepared in step S2 on the surface of the sample.

9. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, In S6, the rinsing method includes: Rinsing the sample completed in step S5 with absolute ethanol solution for 15 ± 5 s to ensure that there is no residue of the film-removing cleaning solution prepared in step S5 on the surface of the sample.

10. The corrosion method for the grain size of the novel ultra-high strength martensitic stainless steel containing Co, Mo, and W according to claim 1, characterized in that, By mass percentage, the chemical composition of martensitic stainless steel includes: C: 0.05% - 0.15%; Cr: 12.20% - 14.50%; Co: 12.00% - 14.00%; Mo: 4.00% - 5.50%; Ni: 2.00% - 3.50%; W: 0.50% - 1.80%; V: 0.18% - 0.45%, and the rest is Fe.

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