Marine stainless steel retained austenite contrast display erosion method

Abstract

The invention provides a marine martensitic stainless steel retained austenite contrast display erosion method, which comprises an erosion agent formula and an erosion process, and is characterized in that the erosion agent formula comprises 50 ml of water, 4-5 ml of phosphoric acid, 2-3 ml of nitric acid and 2-2.5 g of potassium permanganate. The method is easy to operate, good in effect and capable of achieving contrast display of the retained austenite.

Description

Technical Field

[0001] The present invention relates to the technical field of metallographic inspection of metal materials, in particular to a method for contrast-enhanced display of corrosion of retained austenite in marine martensitic stainless steel. Background Art

[0002] 17-4PH steel, also known as 05Cr17Ni4Cu4Nb steel, combines the good corrosion resistance of Cr-Ni stainless steel and the high strength of martensitic steel. It is mostly used in high-strength components under harsh working conditions such as marine ship shafting, last-stage moving blades of gas turbines, and valve stems of nuclear power plants.

[0003] The heat treatment process for 17-4PH steel typically involves solution treatment followed by aging. Strengthening elements in this steel, such as copper and niobium, have high solubility in austenite and low solubility in martensite. Therefore, solution treatment creates a supersaturated martensite structure containing copper and niobium, achieving initial strengthening.

[0004] During the subsequent aging treatment, elements such as copper and niobium in the supersaturated solid solution gradually precipitate as fine, dispersed intermetallic compounds or enriched phases, further increasing the material's strength and hardness. Therefore, the final microstructure of 17-4PH steel is primarily composed of tempered martensite.

[0005] However, during the solution cooling process, not all austenite can be completely transformed into martensite, and some austenite is retained in the form of retained austenite. In addition, during the high-temperature aging stage, reverse transformation from martensite to austenite may occur. The resulting reversed austenite is highly stable and difficult to transform into martensite during the subsequent cooling process, and ultimately remains in the microstructure as retained austenite.

[0006] In 17-4PH steel, an appropriate amount of retained austenite improves the toughness of martensitic precipitation-hardened stainless steel. However, excessive retained austenite reduces the hardness and strength of the matrix, thereby affecting the overall mechanical properties of the material. Therefore, its content must be strictly controlled. This also highlights the need for accurate quantification and clear characterization of retained austenite in martensitic precipitation-hardened stainless steel.

[0007] Chinese patent CN110068492A discloses a metallographic etchant and method for staining retained austenite in advanced high-strength steel. The etchant is composed of an alcohol solution and a surfactant. The weight percentages of the alcohol solution are: 3.5-15% sulfuric acid; 12-15% hydrochloric acid; 7-10% FeCl3; and the balance is anhydrous alcohol. This metallographic etchant produces sufficient contrast and effectively distinguishes the various structures in the steel. When metallographically etching advanced high-strength steel with this etchant, the bainite, martensite, ferrite, and retained austenite structures within the metallographic structure are all clearly visible, exhibiting distinct colors. However, this method is not suitable for martensitic stainless steel. Summary of the Invention

[0008] The present invention aims to provide a method for contrast display of retained austenite corrosion in marine martensitic stainless steel. The present invention is easy to operate, has good effect, and can realize contrast display of retained austenite.

[0009] To achieve the above-mentioned object, the present invention provides a method for contrast-enhancing corrosion of retained austenite in marine martensitic stainless steel. The technical solution of the present invention is achieved as follows:

[0010] A method for contrast-enhancing retained austenite in marine martensitic stainless steel includes an etchant formulation and an etching process. The etchant formulation comprises 50ml of water, 4-5ml of phosphoric acid, 2-3ml of nitric acid, and 2-2.5g of potassium permanganate. The etchant selectively oxidizes the metal surface: different phases or components respond differently to the oxidant, resulting in a potential difference. The areas surrounding the precipitated phase are preferentially oxidized, creating contrast. The reduction products of the potassium permanganate adhere to the surface, deepening the color.

[0011] Furthermore, the etching process includes the following steps:

[0012] S1, etchant preparation and sample preparation;

[0013] S2, etchant heating;

[0014] S3, specimen erosion;

[0015] S4, terminate the sample reaction.

[0016] The above steps are used to realize the contrast display of retained austenite in marine martensitic stainless steel.

[0017] Furthermore, in S1, the etchant preparation includes: weighing water into a beaker, sequentially adding phosphoric acid, nitric acid, and potassium permanganate to the water, and stirring evenly. The order of adding the ingredients complies with safety operating requirements. The etchant is stirred evenly to ensure that the components are fully mixed to form a uniform oxidizing etchant.

[0018] Furthermore, in said S1, the sample preparation steps include: coarse grinding, fine grinding, polishing, cleaning and drying to obtain a bright and scratch-free polished surface.

[0019] Furthermore, in S2, the etchant is heated in a water bath to 85-90°C. This improves the selectivity and uniformity of the etchant. The differences in electrochemical activity between the different phases are amplified, making the etchant more selective and clearly distinguishing the matrix from the precipitated phase. Potassium permanganate reacts more readily with the metal surface at higher temperatures, forming a uniform oxide film (such as MnO2 and Fe2O3) that appears gray-brown and facilitates tissue identification.

[0020] Furthermore, the S3 sample erosion step includes:

[0021] S31, immersing the sample in the etchant under the conditions of S2;

[0022] S32, after soaking for a set time, take out the sample and remove the surface corrosion products;

[0023] S33, repeat S31 and S32 until the sample surface turns gray-brown.

[0024] By etching and rinsing the sample multiple times in a short time, the degree of etching can be better controlled, ultimately obtaining a clear metallographic image. The appearance of a gray-brown color indicates that the structure has fully emerged and is an important basis for determining the end point of etching.

[0025] Furthermore, in S32, the set time is 1 to 3 minutes to better control the degree of erosion.

[0026] Furthermore, in S32, surface corrosion products are washed away with running water to facilitate determination of the corrosion endpoint.

[0027] Furthermore, in S33, S31 and S32 are repeated 3 to 5 times to better control the degree of erosion.

[0028] Furthermore, in S4, the sample is first rinsed with running water, then rinsed with anhydrous ethanol, and finally dried with a hair dryer to terminate the sample reaction.

[0029] Compared with the prior art, the method for contrast-based corrosion analysis of retained austenite in marine martensitic stainless steel disclosed in the present invention has the following advantages:

[0030] 1. Excellent color contrast: can clearly distinguish retained austenite and matrix structure, accurately displaying their content and distribution characteristics.

[0031] 2. Reagents are common and easily available: The chemicals used are conventional analytical reagents, which are easy to obtain and simple to prepare.

[0032] 3. Efficient and convenient operation: The erosion process is fast and the method is stable, suitable for batch sample testing and industrial field application. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a metallographic image of 17-4PH steel etched at 500 times the magnification using the etching method described in Example 1 of the present invention;

[0034] Figure 2 This is a metallographic image of 17-4PH steel etched at 500 times the magnification using the etching method described in Example 2 of the present invention;

[0035] Figure 3 This is a metallographic diagram of 17-4PH steel etched with ferric chloride and hydrochloric acid aqueous solution at 200 times magnification as described in Comparative Example 1;

[0036] Figure 4 This is a metallographic image of 17-4PH steel etched with ferric chloride and hydrochloric acid aqueous solution at 500 times magnification as described in Comparative Example 1;

[0037] Figure 5 This is a metallographic diagram of 17-4PH steel corroded by potassium permanganate and sulfuric acid aqueous solution at 200 times the magnification described in Comparative Example 1;

[0038] Figure 6 This is a metallographic diagram of 17-4PH steel corroded by potassium permanganate and sulfuric acid aqueous solution at 500 times the magnification described in Comparative Example 1;

[0039] Figure 7 This is a metallographic diagram of 17-4PH steel etched 100 times by the etching method of the present invention described in Comparative Example 1;

[0040] Figure 8 This is a metallographic image of 17-4PH steel etched 500 times by the etching method of the present invention as described in Comparative Example 1. DETAILED DESCRIPTION

[0041] To make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the embodiments described are part of the embodiments of the present invention, rather than all of the embodiments. The specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. It should be noted that, unless there is a conflict, the features in the embodiments and embodiments of the present invention may be combined with each other.

[0042] A method for contrast-displaying etching of retained austenite in marine martensitic stainless steel includes an etching agent formula and an etching process.

[0043] The etchant formulation comprises 50 ml of water, 4-5 ml of phosphoric acid, 2-3 ml of nitric acid, and 2-2.5 g of potassium permanganate. The etchant is an oxidizing etchant suitable for revealing precipitated phases, grain boundaries, and structural changes after aging treatment.

[0044] From the perspective of corrosion mechanism, the corrosive agent causes selective oxidation of the metal surface: different phases or components respond differently to the oxidant, resulting in a potential difference; the area around the precipitated phase is preferentially oxidized, forming a contrast; the reduction products of potassium permanganate adhere to the surface, deepening the color.

[0045] Adding phosphoric acid to the formula can, on the one hand, provide an acidic environment; on the other hand, it can inhibit excessive corrosion, play a corrosion inhibition role, and help form a uniform oxide film.

[0046] The etching process comprises the following steps:

[0047] S1, Etching agent preparation and sample preparation:

[0048] Weigh water into a beaker. Add phosphoric acid, nitric acid, and potassium permanganate to the water in sequence and stir thoroughly. Add the ingredients in the order that meets safety requirements. Stir thoroughly to ensure that all components are fully mixed to form a uniform oxidizing solution.

[0049] The martensitic precipitation hardened stainless steel specimens were subjected to coarse grinding, fine grinding, polishing, cleaning and drying to obtain a bright and scratch-free polished surface.

[0050] S2, heating the etchant: in a water bath to 85~90℃.

[0051] This improves the selectivity and uniformity of etching. The differences in electrochemical activity between the different phases are amplified, making the etching more selective and clearly distinguishing the matrix from the precipitated phase. Potassium permanganate readily oxidizes the metal surface at higher temperatures, forming a uniform oxide film (such as MnO2 and Fe2O3) that appears gray-brown and facilitates tissue identification.

[0052] S3, the sample etching step includes:

[0053] S31, immersing the sample in the etchant under the conditions of S2;

[0054] S32, after soaking for a set time, take out the sample and remove the surface corrosion products;

[0055] S33, repeat S31 and S32 until the sample surface turns gray-brown.

[0056] Preferably, the sample is removed every 1-3 minutes, rinsed with running water to remove surface corrosion products, and then placed back into the etchant. This is repeated 3-5 times. At this point, the sample surface turns gray-brown. More preferably, the sample is removed every 2 minutes, and after the fourth removal, the sample surface turns gray-brown.

[0057] By etching and rinsing the sample multiple times in a short time, the degree of etching can be better controlled, ultimately obtaining a clear metallographic image. The appearance of a gray-brown color indicates that the structure has fully emerged and is an important basis for determining the end point of etching.

[0058] S4, terminate the sample reaction: first rinse the sample with running water, then rinse with anhydrous ethanol, and finally blow dry with a hair dryer to terminate the sample reaction.

[0059] The present invention is particularly suitable for materials such as 17-4PH stainless steel, which require clear display of precipitated phases and microstructures. By properly controlling the heating conditions, high-quality metallographic images can be obtained.

[0060] Example 1

[0061] S1. Etching agent preparation and sample preparation: Pour 50ml of water into a beaker. Add 4ml of phosphoric acid, 2ml of nitric acid, and 2g of potassium permanganate to the solution in that order. Stir thoroughly and set aside. Martensitic precipitation-hardened stainless steel samples are subjected to coarse grinding, fine grinding, polishing, cleaning, and drying to obtain a bright, scratch-free polished surface.

[0062] S2, heat the etchant to 85°C in a water bath.

[0063] S3: Immerse the polished sample with the polished surface facing up in the heated etchant. Take it out every 2 minutes, rinse the surface corrosion products with running water, and then put it into the etchant again. After taking it out for the fourth time, the sample surface turns gray-brown.

[0064] S4, rinse the sample with running water, then rinse the sample with anhydrous ethanol, and finally dry it with a hair dryer before observation.

[0065] The retained austenite morphology is observed under an optical metallographic microscope, with good color contrast and excellent corrosion effect. Figure 1 .

[0066] Example 2

[0067] S1. Etching agent preparation and sample preparation: Pour 50ml of water into a beaker. Add 5ml of phosphoric acid, 3ml of nitric acid, and 2.5g of potassium permanganate to the solution in that order. Stir thoroughly and set aside. Martensitic precipitation-hardened stainless steel samples are subjected to coarse grinding, fine grinding, polishing, cleaning, and drying to obtain a bright, scratch-free polished surface.

[0068] S2, heat the etchant to 90°C in a water bath.

[0069] S3: Immerse the polished sample with the polished surface facing up in the heated etchant. Take it out every 2 minutes, rinse the surface corrosion products with running water, and then put it into the etchant again. After taking it out for the fourth time, the sample surface turns gray-brown.

[0070] S4, rinse the sample with running water, then rinse the sample with anhydrous ethanol, and finally dry it with a hair dryer before observation.

[0071] The retained austenite morphology is observed under an optical metallographic microscope, with good color contrast and excellent corrosion effect. Figure 2 .

[0072] Comparative Example 1

[0073] A comparative experiment was conducted using a conventional erosion method and the erosion method of the present invention:

[0074] A 17-4PH steel sample was etched with cotton wool at room temperature using a mixture of 5g ferric chloride, 10ml hydrochloric acid, and 50ml water for 10 seconds. Under this etch condition, the orientation characteristics of the tempered martensite are clearly visible, showing alternating black and white contrast; however, the white orientation is similar to the retained austenite morphology and is difficult to distinguish effectively using this method. Figure 3 and Figure 4 shown.

[0075] 17-4PH steel was etched using a mixture of 1g potassium permanganate, 10ml sulfuric acid, and 90ml water. The solution was heated to approximately 85°C and the sample was etched for 4 minutes. Under this etch condition, the grain boundary outlines were mainly displayed, such as Figure 5 and Figure 6 shown.

[0076] According to the etching method of the present invention, a mixed solution of 4.5 ml phosphoric acid + 2.5 ml nitric acid + 2.5 g potassium permanganate + 50 ml water is used to etch 17-4PH steel. The tempered martensite matrix is ​​gray-brown, and the retained austenite is bright white. The color contrast between the two is clear, which can effectively show the distribution characteristics and morphological characteristics of the retained austenite. Figure 7 and Figure 8 shown. Figure 7 The low-magnification image shows that the retained austenite is mainly distributed in the dendrite segregation area; Figure 8 The high-magnification images further reveal that its morphology is mainly blocky and lamellar, among which the lamellar retained austenite is mostly distributed along the martensite laths.

[0077] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A method for contrast-displaying erosion of retained austenite in marine martensitic stainless steel, comprising an etchant formulation and an erosion process, characterized in that: The etchant formula comprises: 50 ml of water, 4 to 5 ml of phosphoric acid, 2 to 3 ml of nitric acid and 2 to 2.5 g of potassium permanganate.

2. The etching method according to claim 1, wherein: The etching process comprises the following steps: S1, etchant preparation and sample preparation; S2, etchant heating; S3, specimen erosion; S4, terminate the sample reaction.

3. The etching method according to claim 2, wherein: In S1, the preparation of the etchant includes: weighing water into a beaker, adding phosphoric acid, nitric acid and potassium permanganate to the water in sequence, and stirring evenly.

4. The etching method according to claim 2, characterized in that In S1, the sample preparation steps include: coarse grinding, fine grinding, polishing, cleaning and drying.

5. The etching method according to claim 2, wherein: In S2, the etchant is heated to 85-90° C. in a water bath.

6. The etching method according to claim 2, characterized in that: The S3 sample erosion step includes: S31, immersing the sample in the etchant under the conditions of S2; S32, after soaking for a set time, take out the sample and remove the surface corrosion products; S33, repeat S31 and S32 until the sample surface turns gray-brown.

7. The etching method according to claim 6, characterized in that In the step S32 , the set time is 1 to 3 minutes.

8. The etching method according to claim 6, wherein: In the step S32 , surface corrosion products are washed away with running water.

9. The etching method according to claim 6, wherein: In the S33, the S31 and the S32 are repeated 3 to 5 times.

10. The etching method according to claim 2, wherein: In S4, the sample is first rinsed with running water, then rinsed with anhydrous ethanol, and finally dried with a hair dryer.

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