A metallographic etching method for distinguishing Bainite-Martensite-Austenite complex phase structure in steel

Abstract

The present invention discloses a metallographic etching method for distinguishing Bermaault duplex structures in steel. The etchant is a mixed solution of copper sulfate, picric acid, phosphoric acid, nitric acid, anhydrous ethanol, and water. The method for using the metallographic etching solution includes: providing a Bermaault duplex steel sample; sanding and mechanically polishing the sample; immersing the polished sample in a water bath of a metallographic etching solution at 50-80°C for heat-insulating etching, and removing the sample after etching for 1-5 minutes; washing the etched sample with clean water and anhydrous ethanol, respectively, followed by drying, and observing it using a metallographic microscope or a scanning electron microscope. The metallographic etching solution is simple to prepare and easy to use. The metallographic display method can cause bainite, martensite, and retained austenite in the sample to appear in different colors, with martensite appearing brown, bainite appearing off-white, and retained austenite appearing blue. This method can quantitatively analyze the martensite, bainite, and retained austenite in the sample.

Description

Technical Field

[0001] The invention relates to a metallographic corrosion method for distinguishing Bermaiot complex phase structures in steel, belonging to the technical field of metallographic corrosion. Background Art

[0002] Through alloying and appropriate heat treatment of steel, a multiphase structure composed of bainite, martensite, and retained austenite can be obtained. Martensite provides high hardness and strength, while bainite offers a good balance between plasticity and toughness. Retained austenite in steel can trigger a trip effect during deformation, slowing cracking. The proportions of these three structures largely determine material properties, making accurate quantification of martensite, bainite, and retained austenite crucial.

[0003] Metallographic observation provides the most intuitive means of qualitative and quantitative analysis of material structure. Generally speaking, metallographic sample preparation for steel consists of three steps: preparation of the etching solution, sample grinding and polishing, and surface etching. Preparing different etching solutions can achieve varying topographic and color contrast for different sample structures. Grinding and polishing remove surface defects and impurities, allowing for better visualization of the sample's internal structure. Therefore, the preparation of the etching solution and the choice of etching method significantly impact the final metallographic analysis.

[0004] Traditional 3%-4% nital etchants can produce high-contrast microstructures and are effective at corroding grain boundaries. However, for the complex Bermaio structure, nital's contrast is too pronounced, making it difficult to precisely control the extent of corrosion and unable to distinguish retained austenite. Lepra reagent, a mixture of saturated picric acid and hydrochloric acid, is commonly used to target Bermaio structure. This reagent clearly distinguishes bainite from martensite without causing over-corrosion. However, for retained austenite, Lepra reagent fails to provide clear contrast. Therefore, currently used etchants are ineffective for quantitative analysis of Bermaio structure.

[0005] In view of the above background and the technical difficulties existing at this stage, the present invention provides a metallographic corrosion method for distinguishing the Bermaio duplex structure in steel. Summary of the Invention

[0006] In response to the above problems, the purpose of the present invention is to provide a metallographic corrosion method for distinguishing the Bermaiot complex phase structure in steel. The preparation method of the corrosive agent is simple and easy to use. It can make the Bermaiot structure in steel appear in different colors, among which the martensite structure appears brown, the bainite structure appears grayish white, and the residual austenite structure appears blue; thereby achieving the purpose of distinguishing the Bermaiot structure.

[0007] The technical solution of the present invention is:

[0008] A metallographic etching method for distinguishing Bemaao complex phase structures in steel comprises the following steps: grinding and polishing a sample, then metallographically etching the sample with a metallographic etching agent under conditions of water bath heating, and then cleaning and drying the sample. The metallographic etching agent is a mixed solution formed by copper sulfate, picric acid, nitric acid, phosphoric acid, anhydrous ethanol, and water; the volume ratio of the nitric acid, anhydrous ethanol, and water is (3-4):(50-60):(36-47); the copper sulfate is added in an amount of 1-2 g / 100 ml, the picric acid is added in an amount of 3-4 g / 100 ml, and the phosphoric acid is added in an amount of 2-3 g / 100 ml. The etching solution can effectively make the Bemaao complex phase structures in the steel present different colors, thereby distinguishing the Bemaao complex phase structures.

[0009] Preferably, the water bath holding time of the present invention is 50-80° C., and the water bath holding time is 1-5 min.

[0010] Preferably, the nitric acid described in the present invention is commercially available concentrated nitric acid with a concentration of 68%, and the concentration of the anhydrous ethanol is above 99.5%.

[0011] The grinding method of the present invention is a conventional grinding method, preferably: dry grinding is carried out in sequence with 240 mesh, 400 mesh, 800 mesh, 1500 mesh, 2000 mesh, and 3000 mesh sandpaper, and then water grinding is carried out with 5000 mesh sandpaper.

[0012] Preferably, the mechanical polishing of the present invention is a conventional method, preferably: mechanical polishing is performed using 2 μm, 1 μm, and 0.5 μm diamond polishing pastes in sequence, and finally a 0.05 μm silica fine polishing solution is used for final polishing.

[0013] Preferably, during the mechanical polishing process of the present invention, the polishing disc rotates at a speed of 300 r / min.

[0014] Unless otherwise specified, the raw materials used in the present invention are commercially available analytically pure, and the concentrations are all expressed in mass percentage.

[0015] The effects of the present invention are as follows:

[0016] (1) The preparation method of the etchant provided by the present invention is simple and easy to use. It can clearly corrode the structure and make the Bemao structure in the steel appear in different colors, among which martensite is brown, bainite is grayish white, and retained austenite is blue.

[0017] (2) The method of the present invention has a short corrosion time and a low corrosion temperature, and does not require pre-heat treatment of the sample, which is beneficial to preserving the original structure of the sample and the corrosion effect is controllable.

[0018] (3) Among the etchants provided by the present invention, the corrosion effect of nitric acid can better make different tissues of the sample present different corrosion contrasts; the combined effect of phosphoric acid and picric acid can make the grain boundaries appear more clearly; the Cu ions in copper sulfate have a high affinity with austenite, which can make the retained austenite appear blue under a metallographic microscope; compared with traditional 4% nitric acid alcohol and Lepera reagent, the etchant provided by the present invention can better present and distinguish the Bermaault structure in steel.

[0019] (4) In the metallographic corrosion method for distinguishing the Bermaio complex phase structure in steel provided by the present invention, water bath heating can conveniently change the corrosion environment of the sample. By adjusting parameters such as the holding temperature and the holding time, the corrosion rate and corrosion degree of the sample can be controlled to obtain different corrosion contrast effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a 1000x metallographic photograph of Sample 1 in the embodiment of the present invention;

[0021] Figure 2 This is a 1000x metallographic photograph of Sample 2 in Example 1 of the present invention;

[0022] Figure 3 This is a 1000x metallographic photograph of Sample 3 in Example 1 of the present invention;

[0023] Figure 4 This is a 1000x metallographic photograph of Sample 4 in Example 1 of the present invention;

[0024] Figure 5 This is a 1000x metallographic photograph of Sample 5 in Example of the present invention;

[0025] Figure 6 This is a 1000x metallographic photograph of Sample 6 in Example 1 of the present invention;

[0026] Figure 7 This is a 1000x metallographic photograph of Sample 7 in Example 1 of the present invention;

[0027] Figure 8 This is a 1000x metallographic photograph of Sample 8 in Example of the present invention;

[0028] Figure 9 This is a 1000-fold metallographic photograph of Comparative Example 1 of the present invention;

[0029] Figure 10 This is a 1000-fold metallographic photograph of Comparative Example 2 of the present invention;

[0030] Figure 11 This is a 1000-fold metallographic photograph of Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0031] In order to better illustrate the technical solution of the present invention and more intuitively present the effects of the present invention, the implementation scheme of the present invention will be described in detail below with reference to examples.

[0032] The method described in the embodiment of the present invention specifically includes the following steps:

[0033] (1) Samples provided: Samples were taken from Fe-Mn-Si Bemao composite forging liner.

[0034] (2) Preparation of etching solution: First, measure deionized water and anhydrous ethanol, then measure nitric acid and add it to the container, weigh copper sulfate, picric acid, and phosphoric acid and add them to the beaker; after mixing, stir with a glass rod until completely dissolved. The amount added is shown in Table 1, and finally 8 etching solutions are obtained.

[0035] (3) Grinding and polishing of the sample: Dry grind the sample with 240 mesh, 400 mesh, 800 mesh, 1500 mesh, 2000 mesh, and 3000 mesh sandpaper in turn, and then use 5000 mesh sandpaper for water grinding; mechanical polishing is carried out with 2 μm, 1 μm, and 0.5 μm diamond polishing paste in turn, and finally 0.05 μm silica fine polishing liquid is used for final polishing. During the mechanical polishing process, the polishing disk speed is 300 r / min.

[0036] Corrosion method: Heat the corrosive solution to 50°C in a water bath, immerse the sample in the corrosive solution and keep it warm in a water bath at 50°C; when a uniform gray-yellow color appears on the surface of the sample, take out the sample, wash it with clean water and anhydrous ethanol in turn, blow it dry, and observe it under a metallographic microscope.

[0037] Table 1 Preparation ratio of different corrosive solutions

[0038]

[0039] Table 2 Characteristics of different samples

[0040]

[0041] As shown in Table 2, combined with the accompanying figures, the etching solution, prepared using a rational combination of nitric acid, phosphoric acid, and picric acid, demonstrates significant microstructural differentiation in Bermaord-type steel. Martensite appears gray, bainite appears white, and retained austenite appears blue. Nitric acid provides significant corrosion contrast, clearly highlighting the structure and grain boundaries; it also renders martensite brown and bainite white. Copper sulfate provides copper ions that bind better to the retained austenite in the sample, resulting in a blue appearance under an optical microscope. Phosphoric acid, picric acid, and nitric acid contribute to corrosion contrast.

[0042] Comparative Example 1

[0043] (1) Providing samples: Sample collection is the same as in Example 1

[0044] (2) Preparation of etching solution: Measure 4 ml of nitric acid and 96 ml of anhydrous ethanol and add them into a beaker and mix well.

[0045] (3) Sample grinding and polishing: The sample grinding and polishing methods are the same as in Example 1.

[0046] Corrosion method: Use a medical cotton ball to dip a small amount of corrosive agent and evenly apply it to the surface of the sample. After the surface of the sample changes color evenly, wash it with clean water and anhydrous ethanol in turn, blow it dry, and observe it under a metallographic microscope; the metallographic structure diagram of this comparative example is as follows Figure 9 shown.

[0047] It can be seen from the metallographic organization diagrams of Example 1 and Comparative Example 1 that the metallographic corrosion method for distinguishing the Bermaord complex phase structure in steel provided by the present invention can effectively distinguish bainite, martensite and retained austenite in steel through different color contrasts; while the nitric acid alcohol corrosion method provided in Comparative Example 1 cannot effectively distinguish the retained austenite structure.

[0048] Comparative Example 2

[0049] Provide samples: Sample collection is the same as in Example 1.

[0050] Preparation of etching solution: Measure 2 ml of concentrated hydrochloric acid and mix with 98 ml of anhydrous ethanol, then add to a beaker; weigh 4 g of picric acid, add to a beaker, and stir evenly with a glass rod.

[0051] Corrosion method: Heat the corrosive solution to 60℃ in a water bath, immerse the sample in the corrosive solution and keep it in a water bath at 60℃; when the surface of the sample appears a uniform dark yellow, take out the sample, wash it with water and anhydrous ethanol in turn, blow it dry, and observe it with a metallographic microscope; the metallographic structure diagram of this comparative example is shown in Figure 10 shown.

[0052] It can be seen from the metallographic organization diagrams of Example 1 and Comparative Example 2 that, compared with the Lepera reagent corrosion method provided in Comparative Example 2, the metallographic corrosion method for distinguishing the Bemao complex phase structure in steel provided by the present invention can more effectively distinguish bainite, martensite, and retained austenite structures, while the Lepera reagent corrosion method cannot effectively present the retained austenite.

[0053] Comparative Example 3

[0054] Provide samples: Sample collection is the same as in Example 1.

[0055] Preparation of the etching solution: same as in Example 1.

[0056] All conditions are the same as those in Example 1, except that no water bath heating is performed and the corrosion is carried out at room temperature. Figure 11 shown.

[0057] By comparing the metallographic organization diagrams of Example 1 and Comparative Example 3, it can be seen that the water bath heating in the present invention can make the staining of the retained austenite in the sample more obvious, and can effectively make the retained austenite appear blue under a metallographic microscope.

Claims

1. A metallographic etching method for distinguishing the Bermaault complex structure in steel, comprising grinding and polishing a sample, then etching the sample with a metallographic etchant in a water bath, and then cleaning and drying the sample, characterized in that: The metallographic etchant is a mixed solution of copper sulfate, picric acid, nitric acid, phosphoric acid, anhydrous ethanol and water; The volume ratio of the nitric acid, anhydrous ethanol and water is (3-4): (50-60): (36-47), the amount of copper sulfate added is 1-2 g / 100 ml, the amount of picric acid added is 3-4 g / 100 ml, and the amount of phosphoric acid added is 2-3 g / 100 ml; The water bath insulation time is 50-80° C., and the water bath insulation time is 1-5 minutes.

2. The metallographic corrosion method for distinguishing the Bermaiot duplex structure in steel according to claim 1, characterized in that: The concentration of the anhydrous ethanol is above 99.5%.

3. The metallographic corrosion method for distinguishing the Bermaiot duplex structure in steel according to claim 2, characterized in that: The sandpaper grinding method is to use 240 mesh, 400 mesh, 800 mesh, 1500 mesh, 2000 mesh, and 3000 mesh sandpaper for dry grinding in sequence, and then use 5000 mesh sandpaper for water grinding.

4. The metallographic corrosion method for distinguishing the Bermaio duplex structure in steel according to claim 3, characterized in that: The mechanical polishing method is to use 2μm, 1μm, and 0.5μm diamond polishing pastes for mechanical polishing in sequence, and finally use 0.05μm silica fine polishing liquid for final polishing.

5. The metallographic corrosion method for distinguishing the Bermaio duplex structure in steel according to claim 4, characterized in that: During the mechanical polishing process, the polishing disc speed is 300 r / min.

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