Method for displaying original austenite grain boundary after spring steel thermal compression experiment

Abstract

The invention discloses a method for displaying an original austenite grain boundary of spring steel after thermal compression, and belongs to the technical field of physical detection. The method aims at solving the problems that when the original austenite grain boundary of the hot compression spring steel is displayed through a traditional method, display is difficult, and potential safety hazards are caused by using corrosive agents such as picric acid. The method comprises the following steps: firstly, splitting a thermal compression sample along a compression direction, and grinding and polishing the section of the thermal compression sample to obtain a scratch-free mirror surface; then, the polished surface faces upwards, heat preservation is conducted for 15-30 min in an air furnace at the temperature of 850-870 DEG C, and then water quenching is conducted; and finally, corroding the surface of the sample for 5-10 seconds by adopting a nitric acid alcohol solution with the volume concentration of 4%, washing, dehydrating and drying, so that the clear original austenite grain boundary can be observed under a metallographic microscope. Through the steps, clear display of the original austenite grain boundary after hot compression of the spring steel is achieved. According to the method, a nitric acid alcohol solution is adopted to replace a traditional picric acid solution, so that the operation safety is improved; and specific heat treatment is combined with a corrosion process, so that the definition and the stability of crystal boundary display are ensured.

Description

Technical Field

[0001] This invention belongs to the field of physical testing technology, specifically relating to a method for displaying the original austenite grain boundaries after a hot compression experiment on spring steel. Background Technology

[0002] Spring steel, as an important structural material, is widely used in the manufacture of key elastic components in the automotive, railway, and machinery industries. The austenite grain size of spring steel has a significant impact on its mechanical properties, particularly fatigue performance, toughness, and strength. In the research of hot working processes for spring steel, accurately determining the original austenite grain size after hot compression experiments is of significant theoretical and practical value for optimizing hot working process parameters, controlling the evolution of material microstructure, and improving product performance.

[0003] Currently, the common method for revealing the original austenite grain boundaries is chemical etching, with picric acid solution being widely used. However, this method has the following significant drawbacks: (1) Poor safety: Picric acid is a flammable and explosive hazardous chemical. There is a risk of explosion during the heating process, which poses a threat to the safety of experimental personnel and equipment; (2) High toxicity: Picric acid has a strong irritant effect on human skin and respiratory system. Long-term exposure may lead to occupational diseases such as contact dermatitis and conjunctivitis. (3) Unstable corrosion effect: Picric acid corrosive agent has poor adaptability to certain steel grades (especially medium and high carbon spring steel that has undergone plastic deformation), and often causes problems such as uneven corrosion and incomplete grain boundary display; (4) Low operational controllability: It requires precise control of water bath temperature and corrosion time, the operation process is cumbersome and has poor reproducibility.

[0004] To address the aforementioned issues, patent CN106596234A discloses a method for revealing the original austenite grain boundaries in low-carbon microalloyed steel, using a composite corrosive agent consisting of picric acid, hydrochloric acid, ethanol, shampoo, and detergent. While this method improves the corrosion effect to some extent, it still uses picric acid and fails to fundamentally solve the safety and environmental protection problems.

[0005] Patent CN110987576A further improves upon this method by using a composite corrosive agent consisting of saturated citric acid solution, saturated FeCl3 solution, and detergent, completely avoiding the use of picric acid and thus improving safety. However, this method is mainly effective for low-carbon microalloyed steels; its corrosion efficacy and adaptability for medium- and high-carbon spring steels still require further verification.

[0006] Spring steel, as a medium-to-high carbon alloy steel, differs significantly from low-carbon microalloyed steel in its chemical composition and microstructure. Spring steel has a higher carbon content (typically 0.45–0.75%) and different alloying element content compared to low-carbon microalloyed steel, which often makes existing grain boundary display methods ineffective when applied to spring steel. This is especially true for spring steel samples that have undergone hot compression testing, as they have experienced plastic deformation and dynamic recrystallization, resulting in a more complex microstructure and making the display of the original austenite grain boundaries even more difficult.

[0007] Therefore, there is an urgent need in the field to develop a method for displaying original austenitic grain boundaries that is particularly suitable for medium and high carbon spring steel, especially for samples after hot compression tests, and that can balance safety, ease of operation, and clarity and stability of grain boundary display. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for displaying the original austenite grain boundaries after a hot compression test of spring steel. This method has the advantages of high safety, simple operation, clear and stable grain boundary display, and good reproducibility, and is particularly suitable for medium and high carbon spring steel materials.

[0009] The technical solution of this invention is based on the following innovative understanding: (1) In view of the high carbon content of spring steel, the heat treatment process parameters are optimized. By precisely controlling the heating temperature and holding time, a suitable microstructure is obtained, which lays the foundation for subsequent grain boundary display. (2) Using a 4% nitric acid alcohol solution as a corrosive agent has the advantages of high safety, simple operation and stable corrosion effect compared with traditional picric acid corrosive agent.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A method for displaying the original austenite grain boundaries after a hot compression test of spring steel, the specific steps of which are as follows: (1) Sample preparation: The hot-compressed spring steel sample is cut along the compression direction using wire cutting or electric spark cutting. The flatness error of the obtained cross section to be tested does not exceed 0.02 mm and there is no obvious cutting deformation layer. (2) Grinding and polishing: The cross-section obtained in step (1) is ground step by step, and then mechanically polished until a mirror surface without visible scratches is obtained; (3) Heat treatment: Place the sample treated in step (2) with its polished surface facing up in an air furnace at 850-870°C, keep it at that temperature for 15-30 minutes, and then quickly place it in cold water at 5-25°C for water quenching. After quenching and cooling to room temperature, take it out. The quenching water temperature is limited to 5-25°C in order to ensure the rapid transformation of austenite to martensite and avoid the effect of tempering on grain boundary display. (4) Corrosion and observation: The surface of the sample after quenching in step (3) is lightly polished to remove the oxide layer. Then, the surface of the sample is corroded with a 4% nitric acid alcohol solution. After rinsing and drying, metallographic observation is performed. The 4% nitric acid alcohol solution is prepared freshly by mixing concentrated nitric acid and anhydrous ethanol at a volume ratio of 1:24, and the preparation environment temperature is 15-25℃.

[0011] Further, in step (2), the grinding is performed in sequence using sandpaper of 200#, 400#, 800# to 1500#; the mechanical polishing uses W2.5 diamond polishing paste and is lightly polished for 10 to 20 seconds under a pressure of 0.1 to 0.3 MPa to completely remove the surface oxide layer without damaging the internal structure of the sample.

[0012] Furthermore, in step (3), the heating temperature of the air furnace is 860°C.

[0013] Furthermore, in step (4), the sample surface is etched by wiping it with degreased cotton soaked in nitric acid alcohol solution for 5 to 10 seconds.

[0014] Furthermore, the spring steel contains, by mass percentage: 0.45%–0.75% carbon, 1.2%–2.2% silicon, and 0.5%–1.0% manganese.

[0015] Compared with the prior art, the present invention has the following outstanding advantages: (1) High safety: It completely abandons high-risk chemicals such as picric acid and uses nitric acid alcohol solution, which is widely used in conventional laboratories, as a corrosive agent, which significantly reduces safety risks and environmental hazards; (2) Strong targeting and applicability: Through the optimized combination of heat treatment and corrosion process, it is particularly suitable for the original austenite grain boundaries of medium and high carbon spring steel with special chemical composition and hot deformation history, which solves the problem that the existing methods are not effective in this type of material; (3) Simple operation and good reproducibility: The process steps are clear and the parameters are easy to control, avoiding complex and harsh operating conditions, which is conducive to obtaining stable and reliable experimental results; (4) Clear display effect: It can obtain continuous and complete original austenite grain boundaries, which facilitates subsequent quantitative analysis of grain size and microstructure characterization. Attached Figure Description

[0017] Figure 1 This is a metallographic photograph of the original austenite grain boundaries of spring steel, as shown by the method of this invention (heat treatment holding time of 15 min).

[0018] Figure 2This is a metallographic photograph of the original austenite grain boundaries of spring steel, as shown by the method of this invention (heat treatment holding time of 30 min). Detailed Implementation

[0019] The present invention will be further described in detail below through specific embodiments. These embodiments are used to more intuitively illustrate the technical solution and effects of the present invention, but the scope of protection of the present invention is not limited thereto.

[0020] Example 1: This embodiment uses a hot-compression sample of widely used 60Si2Mn spring steel as the object to demonstrate the implementation process of the method of the present invention.

[0021] (1) Sample preparation: The hot-compressed 60Si2Mn spring steel sample was precisely cut along the compression direction using wire cutting to obtain a flat and clean test section. (2) Grinding and polishing: The cross-section obtained in step (1) is mechanically ground and pre-polished. Metallographic sandpaper of specifications 200#, 400#, 800# and 1500# is used in sequence to grind in order to completely remove cutting marks and surface deformation. Then, W2.5 diamond polishing paste is used on a polishing machine to precisely polish its surface until a mirror surface without any visible scratches is obtained; (3) Heat treatment: Place the sample treated in step (2) with its polished surface facing up in an air furnace that has been preheated to 860°C. After holding it at this temperature for 15 minutes, quickly take the sample out and place it in cold water at 5-25°C for quenching. (4) Corrosion: A thin oxide film will form on the surface of the sample after quenching. First, the sample surface is polished very lightly on a polishing machine to completely remove this oxide film and expose the fresh metal surface. Then, use tweezers to pick up degreased cotton, dip it in a sufficient amount of 4% nitric acid alcohol solution (the 4% nitric acid alcohol solution is prepared freshly by mixing concentrated nitric acid and anhydrous ethanol at a volume ratio of 1:24, and the preparation temperature is 15-25℃), and gently wipe the sample surface for about 8 seconds; (5) Observation: After corrosion is completed, immediately rinse the sample surface with running water to stop corrosion, then dehydrate with anhydrous ethanol and dry with a hair dryer. Observe the treated sample under a metallographic microscope.

[0022] The results are as follows Figure 1 As shown, continuous, complete, and highly contrasting original austenite grain boundaries can be clearly observed, and the grain morphology is complete, which is very beneficial for subsequent automatic or manual measurement and analysis of grain size.

[0023] Example 2: The main difference between this embodiment and Embodiment 1 lies in the heat treatment stage's heat preservation time, aiming to illustrate the display effect under different heat preservation conditions.

[0024] In this embodiment, except that the holding time in the air furnace is set to 30 min in step (3), the other steps, including materials (60Si2Mn), preparation, grinding and polishing parameters (200# to 1500# sandpaper, W2.5 polishing paste), heat treatment temperature (860℃), etchant (4% nitric acid alcohol solution by volume) and operation, are exactly the same as in Example 1.

[0025] The results are as follows Figure 2 As shown, even with an extended holding time of 30 minutes, the method of this invention still yields very clear, complete, and continuous original austenite grain boundaries. This indicates that the method of this invention has good stability and reproducibility within a certain process window.

Claims

1. A method for displaying the original austenite grain boundaries of spring steel after hot compression, characterized in that, Includes the following steps: (1) Sample preparation: The hot-compressed spring steel sample is cut along the compression direction using wire cutting or electric spark cutting. The flatness error of the obtained cross section to be tested does not exceed 0.02 mm and there is no obvious cutting deformation layer. (2) Grinding and polishing: The cross-section obtained in step (1) is ground step by step, and then mechanically polished until a mirror surface without visible scratches is obtained; (3) Heat treatment: Place the sample treated in step (2) with its polished surface facing up in an air furnace at 850-870°C, keep it at that temperature for 15-30 minutes, and then quickly place it in cold water at 5-25°C for quenching. After quenching and cooling to room temperature, take it out. (4) Corrosion and Observation: The surface of the sample after quenching in step (3) is lightly polished using W2.5 diamond polishing paste at a pressure of 0.1-0.3 MPa for 10-20 seconds to completely remove the surface oxide layer without damaging the internal structure of the sample. Then, the sample surface is etched with a 4% nitric acid alcohol solution. After rinsing and drying, metallographic observation is performed. The 4% nitric acid alcohol solution is prepared freshly by mixing concentrated nitric acid and anhydrous ethanol at a volume ratio of 1:24, and the preparation environment temperature is 15-25℃.

2. The method according to claim 1, characterized in that, In step (2), the progressive grinding is performed in sequence using sandpaper of 200#, 400#, 800# to 1500#; the mechanical polishing uses W2.5 diamond polishing paste.

3. The method according to claim 1, characterized in that, In step (3), the heating temperature of the air furnace is 860°C.

4. The method according to claim 1, characterized in that, In step (4), the sample surface is etched by wiping it with the nitric acid alcohol solution for 5 to 10 seconds using degreased cotton.

5. The method according to any one of claims 1 to 4, characterized in that, The spring steel contains, by mass percentage: 0.45%–0.75% carbon, 1.2%–2.2% silicon, and 0.5%–1% manganese.

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