A method for efficiently distinguishing bulk structure and carbon increase on surface of medium carbon steel
By performing specific heat treatment on medium carbon steel and observing the austenite grain morphology after sample preparation, the problem of distinguishing between bulk structures and carburized structures on the surface of medium carbon steel was solved, achieving efficient and rapid structure identification and improving production efficiency and product quality judgment accuracy.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-03-31
- Publication Date
- 2026-06-16
Smart Images
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Abstract
Description
Technical Field
[0001] This invention relates to the field of metallographic structure testing technology for metallic materials, specifically to an efficient testing method for distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel. Background Technology
[0002] In the field of steel material testing, the bulk microstructure and surface-carburized microstructure on the surface of medium carbon steel are highly similar in micromorphology and metallographic characteristics under normal conditions, making it difficult to accurately distinguish them using existing metallographic analysis methods. This morphological confusion leads to difficulties in identifying the material microstructure type, directly affecting the judgment of product quality and the accuracy of process control.
[0003] In the existing technology, there is no technical solution that can simultaneously achieve rapid judgment, batch adaptation, and detection accuracy that meets the requirements for shallow carbonization identification on the surface of medium carbon steel. This has become a technical problem that urgently needs to be solved in this field. Summary of the Invention
[0004] To address the shortcomings of existing technologies, this invention provides an efficient inspection method for distinguishing between large block structures and carbonized structures on the surface of medium carbon steel. This method can differentiate between large block structures and carbonized structures, providing direction for production improvement and increasing production efficiency.
[0005] The technical solution adopted by this invention to solve its technical problem is a test method for efficiently distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel, the specific steps of which are as follows:
[0006] S1. Sampling of steel with carbonized structure or similar blocky structure on the surface: The sample is obtained by cutting and cutting according to the sampling requirements specified in GB / T13298-2015 "Metallic Microstructure Test Method".
[0007] S2. Place the cut sample into a heat treatment furnace for the first heat treatment. The first heat treatment is carried out in an inert protective atmosphere to prevent oxidation and decarburization. The temperature is 850℃~860℃. After the temperature is reached, keep it in the furnace for 60min~70min, and then immediately take it out for oil cooling.
[0008] S3. After cooling, the sample undergoes a second heat treatment at a temperature of 150℃ to 170℃ for 5 hours, followed by air cooling.
[0009] S4. Sample preparation for the sample after the second heat treatment: the cross section is rough ground → fine ground → polished → etched.
[0010] S5. Place the etched sample under an optical microscope for observation. Observe the morphology of the austenite grains in the sample. If the austenite grains are uniform, it is a carbonized structure. Otherwise, if the austenite grains are in a non-uniform mixed crystal state, it is a bulk structure.
[0011] Furthermore, the steel is medium carbon steel.
[0012] Furthermore, the steel is 40Cr.
[0013] Furthermore, in step S1, the sample is prepared using an abrasive wheel cutter, and the cutting fluid is kept constantly cooling the sample during the sample processing.
[0014] Furthermore, in step S2, the inert protective atmosphere is nitrogen.
[0015] Furthermore, in the sample preparation process of step S4, the sample is coarsely ground by a spectral grinding machine, then finely ground by 180-mesh, 600-mesh, and 1000-mesh metallographic sandpaper in sequence, and finally polished. The entire process requires cooling with water.
[0016] Furthermore, in step S4, the etching process involves immersing the polished sample in a saturated sodium picrate solution with the polished surface facing upwards, heating it in a water bath to 75°C for 25 minutes, and then removing it, washing it, and drying it.
[0017] The beneficial effects of this invention are as follows:
[0018] This invention involves sampling steel with abnormal surface structures and then subjecting the samples to anti-oxidation heat treatment (with strict control over oxidation during the heat treatment). Utilizing the different grain morphologies produced by the bulk and carburized structures after heat treatment, the austenite grains in the carburized structure are uniform, while those in the carburized structure are non-uniform. This allows for effective and rapid identification of the microstructure. This invention can efficiently and quickly distinguish between carburized structures and similar bulk structures in steel, providing direction for production improvements and preventing the production of defective products. Attached Figure Description
[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments.
[0020] Figure 1 These are photographs of abnormal metallographic structures in steel. The left image shows a blocky structure, and the right image shows a carburized structure.
[0021] Figure 2 These are metallographic photographs of abnormal tissue areas after anti-oxidation heat treatment observed under a microscope in Embodiment 1 of the present invention; the left image is a photograph of bulk austenite grains, and the right image is a photograph of carbonized austenite grains.
[0022] Figure 3These are metallographic photographs of abnormal tissue areas after anti-oxidation heat treatment observed under a microscope in Embodiment 2 of the present invention; the left image is a photograph of bulk austenite grains, and the right image is a photograph of carbonized austenite grains.
[0023] Figure 4 These are metallographic images of abnormal tissues observed under a microscope after heat treatment in Comparative Example 1 of the present invention; the left image is a photograph of a large blocky tissue after this test method, and the right image is a photograph of a carbonized tissue after this test method.
[0024] Figure 5 These are metallographic photographs of abnormal tissue areas observed under a microscope in Comparative Example 2 of this invention after heat treatment; the left image is a photograph of a large blocky tissue after this test method, and the right image is a photograph of a carbonized tissue after this test method.
[0025] Figure 6 These are metallographic photographs of abnormal tissue areas observed under a microscope in Comparative Example 4 of this invention after heat treatment; the left image is a photograph of a large blocky tissue after this test method, and the right image is a photograph of a carbonized tissue after this test method.
[0026] Figure 7 These are metallographic photographs of abnormal tissue areas observed under a microscope in Comparative Example 5 of this invention after heat treatment; the left image is a photograph of a large blocky tissue after this test method, and the right image is a photograph of a carbonized tissue after this test method.
[0027] Figure 8 These are metallographic photographs of abnormal tissue areas observed under a microscope in Comparative Example 6 of this invention after heat treatment; the left image is a photograph of a large blocky tissue after this test method, and the right image is a photograph of a carbonized tissue after this test method.
[0028] Figure 9 These are metallographic images of abnormal tissues observed under a microscope in Comparative Example 7 of the present invention after heat treatment; the left image is a photograph of a large blocky tissue after this test method, and the right image is a photograph of a carbonized tissue after this test method.
[0029] The scale bars in the figure are all 200μm. Detailed Implementation
[0030] The present invention will now be described in detail with reference to specific embodiments.
[0031] The steel used in the following examples and comparative examples is 40Cr.
[0032] The present invention uses the principle that bulk microstructures and carburized microstructures exhibit significantly different microstructure morphologies after heat treatment in the austenite region as the core basis for distinguishing between the two. Specifically, carburized microstructures, due to their higher carbon content, will have uniform grains after heat treatment in the austenite region, while bulk microstructures will have non-uniform mixed-crystal austenite grains after heat treatment, and the morphological differences between the two are clearly visible.
[0033] Example 1
[0034] S1. Sampling of steel with carbonized structure or similar blocky structure on the surface: The sample is obtained by cutting and cutting according to the sampling requirements specified in GB / T13298-2015 "Metallic Microstructure Test Method".
[0035] S2. Place the cut sample into a heat treatment furnace for specific heat treatment. After the sample is placed in the heat treatment furnace, it is heated to 850°C. At the same time as the temperature rises, nitrogen gas is added to prevent oxidation and decarburization. After the temperature is reached, the sample is kept in the furnace for 60 minutes and then immediately taken out for oil cooling.
[0036] S3. After cooling, the sample is placed in a heat treatment furnace at 150°C and kept at that temperature for 5 hours, then removed and air-cooled.
[0037] S4. Sample preparation after heat treatment: the cross section is rough-ground → fine-ground → polished → etched. The specific steps of etching are as follows: pour an appropriate amount of saturated sodium picrate solution into a beaker, put in the polished sample with the polished surface facing up (the saturated sodium picrate solution should cover the polished sample), heat to 75℃ in a water bath and etch for 25 minutes, then take it out and rinse it clean, and finally use a hair dryer to dry the sample surface.
[0038] S5. Place the etched sample under an optical microscope for observation. Observe the austenite grain morphology of the sample. The observation results are as follows: Figure 2 As shown, the carbon-reinforced austenite grains are relatively uniform, while the bulk austenite grains in the similar carbon-reinforced structure are not uniform.
[0039] Specifically, in step S1, the sample is cut using an abrasive wheel cutter. During the sample processing, the cutting fluid must be kept constantly cooling the sample to prevent it from burning. In step S4, the entire process requires cooling with water to prevent the sample surface from overheating.
[0040] Example 2
[0041] S1. Sampling of steel with abnormal surface structure: The sample is obtained by cutting and cutting according to the sampling requirements specified in GB / T 13298-2015 "Metallic Microstructure Test Method".
[0042] S2. Place the cut sample into a heat treatment furnace for specific heat treatment. After the sample is placed in the heat treatment furnace, it is heated to 860°C. At the same time as the temperature rises, nitrogen gas is added to prevent oxidation and decarburization. After the temperature is reached, the sample is kept in the furnace for 70 minutes and then immediately taken out for oil cooling.
[0043] S3. After cooling, the sample is placed in a heat treatment furnace at 170°C and kept at that temperature for 5 hours, then removed and air-cooled.
[0044] S4. Sample preparation after heat treatment: the cross section is rough-ground → fine-ground → polished → etched. The specific steps of etching are as follows: pour an appropriate amount of saturated sodium picrate solution into a beaker, put in the polished sample with the polished surface facing up (the saturated sodium picrate solution should cover the polished sample), heat to 75℃ in a water bath for 25 minutes, then take it out and rinse it clean, and finally use a hair dryer to dry the sample surface.
[0045] S5. Place the etched sample under an optical microscope for observation. Observe the austenite grain morphology of the sample. The observation results are as follows: Figure 3 As shown, it can efficiently and quickly identify whether the abnormal structure of steel is a carbonized structure or a large blocky structure similar to a carbonized structure.
[0046] Specifically, in step S1, the sample is cut using an abrasive wheel cutter. During the sample processing, the cutting fluid must be kept constantly cooling the sample to prevent it from burning. In step S4, the entire process requires cooling with water to prevent the sample surface from overheating.
[0047] Comparative Example 1
[0048] S1. Sampling of steel with abnormal surface structure: The sample is obtained by cutting and cutting according to the sampling requirements specified in GB / T 13298-2015 "Metallic Microstructure Test Method".
[0049] S2. Place the cut sample into a heat treatment furnace for specific heat treatment. After the sample is placed in the heat treatment furnace, it is heated to 850°C, held in the furnace for 60 minutes, and then cooled in the furnace.
[0050] S3. Sample preparation after heat treatment: the cross section is rough-ground → fine-ground → polished → etched. The specific steps of etching are as follows: pour an appropriate amount of saturated sodium picrate solution into a beaker, put in the polished sample with the polished surface facing up (the saturated sodium picrate solution should cover the polished sample), heat to 75℃ in a water bath for 25 minutes, then take it out and rinse it clean, and finally use a hair dryer to dry the sample surface.
[0051] S4. Place the etched sample under an optical microscope for observation. The observation results are as follows: Figure 4 As shown.
[0052] Comparative Example 2
[0053] S1. Sampling of steel with abnormal surface structure: The sample is obtained by cutting and cutting according to the sampling requirements specified in GB / T 13298-2015 "Metallic Microstructure Test Method".
[0054] S2. Place the cut sample into a heat treatment furnace at 150°C and keep it at that temperature for 5 hours, then remove it and air cool it.
[0055] S3. Sample preparation after heat treatment: the cross section is rough-ground → fine-ground → polished → etched. The specific steps of etching are as follows: pour an appropriate amount of saturated sodium picrate solution into a beaker, put in the polished sample with the polished surface facing up (the saturated sodium picrate solution should cover the polished sample), heat in a water bath to 75℃ for 25 minutes, then take it out and rinse it clean, and finally use a hair dryer to dry the sample surface.
[0056] S4. Place the etched sample under an optical microscope for observation. The observation results are as follows: Figure 5 As shown.
[0057] Comparative Example 3
[0058] S1. Sampling of steel with abnormal surface structure: The sample is obtained by cutting and cutting according to the sampling requirements specified in GB / T 13298-2015 "Metallic Microstructure Test Method".
[0059] S2. Place the cut sample into a heat treatment furnace for specific heat treatment. After the sample is placed in the heat treatment furnace, it is heated to 800°C. At the same time as the temperature rises, nitrogen gas is added to prevent oxidation and decarburization. After the temperature is reached, the sample is kept in the furnace for 30 minutes and then immediately taken out for oil cooling.
[0060] S3. After cooling, the sample is placed in a heat treatment furnace at 170°C and kept at that temperature for 5 hours, then removed and air-cooled.
[0061] S4. Sample preparation after heat treatment: the cross section is rough-ground → fine-ground → polished → etched. The specific steps of etching are as follows: pour an appropriate amount of saturated sodium picrate solution into a beaker, put in the polished sample with the polished surface facing up (the saturated sodium picrate solution should cover the polished sample), heat to 75℃ in a water bath and etch for 25 minutes, then take it out and rinse it clean, and finally use a hair dryer to dry the sample surface.
[0062] S5. Place the etched sample under an optical microscope for observation.
[0063] In Comparative Example 3, due to the excessively low temperature and insufficient time of the first heat treatment, the microstructure failed to fully austenitize, resulting in the inability to distinguish the abnormal microstructure after heat treatment. The metallographic observation results were inconsistent with... Figure 4 The phenomenon shown in (Comparative Example 1) is consistent, and it is impossible to effectively distinguish between large blocky structures and carbonized structures.
[0064] Comparative Example 4
[0065] The difference between Comparative Example 4 and Example 1 is that in Example 1, step S2, "the sample is heated to 840°C in the furnace," while the rest is the same as in Example 1. The observation results are as follows: Figure 6 As shown.
[0066] Comparative Example 5
[0067] The difference between Comparative Example 5 and Example 1 is that in Example 1, step S2, "the sample is heated to 870°C in the furnace," while the rest is the same as in Example 1. The observation results are as follows. Figure 7 As shown.
[0068] Comparative Example 6
[0069] The difference between Comparative Example 6 and Example 1 is that in Example 1, step S2, "keeping the furnace warm for 50 minutes," is the same as in Example 1. The observation results are as follows: Figure 8 As shown.
[0070] Comparative Example 7
[0071] The difference between Comparative Example 7 and Example 1 is that in Example 7, step S2, "keeping the furnace at room temperature for 80 minutes," is the same as in Example 1. The observation results are as follows: Figure 9 As shown.
[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.
Claims
1. A highly efficient inspection method for distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel, characterized in that... Includes the following steps: S1. Take samples of steel with carburized structures or similar large blocky structures on the surface. S2. Place the cut sample into a heat treatment furnace for the first heat treatment. The temperature of the first heat treatment is 850-860℃. The first heat treatment is carried out under an inert protective atmosphere. The time of the first heat treatment is 60-70 minutes. After the first heat treatment is completed, immediately perform oil cooling. S3. After cooling, the sample undergoes a second heat treatment at a temperature of 150–170°C for 5 hours. After the second heat treatment, the sample is then air-cooled. S4. The sample preparation after the second heat treatment includes the following steps: rough grinding, fine grinding, polishing, and etching of the cross section; S5. Place the prepared sample under an optical microscope for observation. Observe the austenite grain morphology of the sample. If the austenite grains are uniform, it is a carbon-reinforced structure. Conversely, if the austenite grains are in a non-uniform mixed crystal state, it is a bulk structure.
2. The inspection method for efficiently distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel as described in claim 1, characterized in that: In step S1, the sample is prepared using a grinding wheel cutting machine, and the cutting fluid is kept constantly cooling the sample during the sample processing.
3. The inspection method for efficiently distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel as described in claim 1, characterized in that: In step S2, the inert protective atmosphere is nitrogen.
4. The inspection method for efficiently distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel as described in claim 1, characterized in that: In the sample preparation process of step S4, the sample is coarsely ground by a spectral grinding machine, then finely ground by 180-mesh, 600-mesh, and 1000-mesh metallographic sandpaper in sequence, and finally polished. The entire process requires cooling with water.
5. The inspection method for efficiently distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel as described in claim 1, characterized in that: The etching process in step S4 involves immersing the polished sample in a saturated sodium picrate solution with the polished surface facing upwards, heating it in a water bath to 75°C for 25 minutes, and then removing it, washing it, and drying it.
6. The inspection method for efficiently distinguishing large blocky structures and carbon enrichment on the surface of medium carbon steel as described in claim 1, characterized in that: The steel is medium carbon steel.