Method for displaying prior austenite grain boundaries of bainitic steel and use thereof

Abstract

The application belongs to the technical field of physical detection, and particularly relates to a display method of prior austenite grain boundaries of bainite steel, which comprises preparing a metallographic sample, preparing a corrosion solution and electrolytic corrosion. The corrosion solution comprises the following components with mass concentrations: 5-20 g / L of picric acid, 3-10 g / L of sodium tungstate and 0.009-0.03 g / L of zinc chloride. An electrolytic etching device used in the electrolytic corrosion comprises a resistance heating furnace, an iron stand, a metal rod with a tapered head, a glass beaker and a metal base. The resistance heating furnace is arranged on the iron stand, and the metal rod with the tapered head and the metal base are arranged in the glass beaker. The metal rod with the tapered head is used to connect a cathode of an electrolytic device, and the metal base is used to connect an anode of the electrolytic device. Through the corrosion solution with a special proportion and the electrolytic etching device, the corrosion rate can be effectively controlled, the prior austenite grain boundaries can be clearly displayed, and the grain size of the prior austenite grain boundaries in the bainite steel can be accurately obtained.

Description

Technical Field

[0001] This invention belongs to the field of physical testing technology, specifically relating to a method for displaying the original austenitic grain boundaries of bainitic steel and its application. Background Technology

[0002] Austenite grains can be classified into initiating grains, intrinsic grains, and actual grains according to their formation conditions. The sizes of these three types of grains are expressed as initiating grain size, intrinsic grain size, and actual grain size, respectively. It is generally believed that intrinsic grain size has a greater impact on the processing properties of steel, while its effect on service performance is indirect. Actual grain size has a more direct impact on the service performance of steel. Actual grain size refers to the size of austenite grains obtained under specific heating conditions during heat treatment or hot working. When the actual grain size is large, the mechanical properties of the steel, especially impact toughness and fatigue performance, decrease. When the actual grain size is small, the yield strength, tensile strength, and fatigue strength of the steel increase, while also giving the steel higher plasticity and impact toughness. The determination of actual grain size relies on the use of special methods to reveal the original austenite grain boundaries of the steel.

[0003] Currently, the main grain boundary visualization methods used for austenite grain size determination include carburizing, oxidation, network ferrite, network pearlite (troostite), heating and slow cooling, direct corrosion with supersaturated picric acid solution, and electrolytic corrosion. Among these, carburizing and oxidation are suitable for determining the intrinsic grain size of various steels; the network ferrite method is suitable for determining the actual grain size of hypoeutectoid steels; the network pearlite (troostite) method is suitable for carbon steels and low-alloy steels with low hardenability; the heating and slow cooling method is suitable for determining the actual grain size of hypereutectoid steels; the direct corrosion with supersaturated picric acid solution is suitable for determining the actual grain size of quenched martensitic steels and their low-temperature tempered steels; and the electrolytic method is mainly used to determine the actual grain size of some corrosion-resistant steels.

[0004] Different steel compositions require different methods for revealing austenite grain boundaries. Even for steels of the same composition, different hot workings or heat treatments will result in different room temperature microstructures, and the applicable methods for revealing the original austenite grain boundaries will also differ. For example, for medium and low alloy steels, when the room temperature microstructure is ferrite + pearlite (hypoeutectoid composition), the network ferrite method is suitable; when the room temperature microstructure is martensite or martensite tempered at low temperature, the direct etching method with supersaturated picric acid solution is suitable. When the room temperature microstructure of medium and low alloy steel is bainite, the network ferrite method cannot be applied, and the direct etching method with picric acid solution often requires repeated grinding, polishing, and etching, which is cumbersome and the austenite grain boundary revealing effect is unstable.

[0005] Currently, there is a lack of a simple, efficient, and highly clear method for determining the actual grain size of bainitic steel, specifically for displaying its original austenitic grain boundaries. Existing methods for displaying the original austenitic grain boundaries in bainitic steel mainly fall into two categories: direct etching with a picric acid solution heated in a water bath, and secondary electrolytic etching with an electrolyte containing sulfosalicylic acid and nitric acid. The first method shows good results for displaying grain boundaries in martensitic steel, but its reproducibility is poor, and the grain boundary display is unstable, requiring repeated polishing and etching, leading to low experimental efficiency. The second method can complete etching within one minute, but the phase transformation structure within the austenitic grain boundaries is also severely etched, resulting in low relative contrast of the original austenitic grain boundaries, hindering clear observation and significantly affecting the accuracy of grain size assessment results. If methods such as carburizing and oxidation are used to alter the microstructure of bainitic steel to reveal the original austenitic grain boundaries for determining the intrinsic grain size, the results differ significantly from the actual grain size. Summary of the Invention

[0006] The purpose of this invention is to address the aforementioned technical problems by providing a method for displaying the original austenite grain boundaries in bainitic steel, which can clearly display the original austenite grain boundaries and thereby accurately obtain the actual grain size of the original austenite grain boundaries in bainitic steel.

[0007] The method for displaying the original austenitic grain boundaries of bainitic steel in the technical solution of the present invention includes preparing metallographic samples, preparing etching solutions, and electrolytic etching.

[0008] Furthermore, during the preparation of metallographic specimens, the specimens are cut, ground, and polished sequentially.

[0009] Furthermore, the corrosive solution comprises the following components at mass concentrations: picric acid 5–20 g / L, sodium tungstate 3–10 g / L, and zinc chloride 0.009–0.03 g / L.

[0010] This invention employs a specially formulated etching solution that clearly reveals the original austenite grain boundaries while leaving other structures almost uncorroded. This results in high contrast of the austenite grain boundaries, making them clearer under a metallographic microscope and facilitating easier and more accurate measurement of the actual grain size. Picric acid exhibits a different corrosion rate on the original austenite grain boundaries and intragranular structures of bainitic steel, allowing for differentiated corrosion of the grain boundary and intragranular microstructure morphology. Adding a small amount of picric acid prevents excessive concentration, which could cause the sample to blacken due to rapid corrosion. Sodium tungstate deactivates bainitic steel during electrolysis, achieving a corrosion-resistant state and acting as passivation. The addition of a small amount of zinc ions assists in the rapid film formation of sodium tungstate, further increasing the difference in corrosion rate between the grain boundaries and intragranular microstructure by picric acid, thus preferentially corroding and revealing the grain boundaries, resulting in clear grain boundaries.

[0011] Furthermore, after preparing the etching solution, the pH is adjusted to 1.5–1.8. Adjusting the pH of the etching solution ensures a suitable concentration of hydrogen ions as an oxidant, which increases the electrode potential and promotes the anolyte metal ions to leave the metal surface and enter the solution. This results in higher electrolytic activity for the bainitic steel sample acting as the anode, which is beneficial for the electrolytic corrosion reaction at the grain boundaries, as well as the slow-release effect of sodium tungstate and zinc ions on the intragranular structure.

[0012] Furthermore, the pH of the corrosive solution is adjusted using a hydrochloric acid solution with a concentration of 0.01–0.03 mol / L.

[0013] Furthermore, the preparation of the etchant is carried out at a temperature of 55–65°C.

[0014] Furthermore, the electrolytic etching apparatus used in the electrolytic etching process includes a resistance heating furnace, an iron stand, a metal rod with a conical tip, a glass beaker, and a metal base. The resistance heating furnace is placed on the iron stand, and the metal rod with the conical tip and the metal base are placed in the glass beaker. The metal rod with the conical tip is used to connect to the cathode of the electrolytic equipment, and the metal base is used to connect to the anode of the electrolytic equipment.

[0015] In existing electrolytic corrosion methods, the anode of the electrolytic equipment is connected to a metal clamp, which holds the sample, while the cathode is connected to a metal base or plate. During electrolytic corrosion, the current inside the sample is uneven, easily leading to localized instantaneous corrosion and blackening, failing to achieve a uniform passivation effect. Furthermore, the current magnitude cannot be adjusted to control the electrolytic corrosion rate when the voltage is constant. In this invention, the anode of the electrolytic equipment is connected to the metal base where the sample is placed, with the sample's surface facing upwards and the bottom of the sample in complete contact with the metal base. During electrolytic corrosion, the current magnitude inside the sample is uniform. The cathode of the electrolytic equipment is connected to a tapered metal rod with adjustable height. The current magnitude can be adjusted by the depth of the tapered lower part of the metal rod immersed in the corrosive liquid, thereby controlling the electrolytic corrosion rate of the sample, avoiding excessive corrosion that causes blackening, and clearly displaying grain boundaries for observation of grain boundaries and grain size.

[0016] Furthermore, the voltage during electrolytic corrosion is 25–30V, and the current is 0.6–1.2A. Selecting appropriate voltage and current allows for electrolytic polishing of the intragranular structure, making it less susceptible to corrosion, and ensuring stable and uniform electrolytic corrosion of the grain boundaries. If the voltage is too low, the sample undergoes rapid corrosion, resulting in excessive corrosion and blackening of the intragranular structure. If the voltage is too high, the current within the bainitic steel sample continues to increase, leading to uneven polishing and a disordered and unclear microstructure of the crystal structure.

[0017] Furthermore, the electrolytic corrosion time is 15–120 seconds.

[0018] This invention reduces the temperature sensitivity of the corrosion process by limiting the voltage and current during electrolytic corrosion, allowing electrolytic corrosion to be carried out at the target temperature ±4℃, resulting in a more stable austenite grain boundary display effect.

[0019] The present invention also provides the application of the above-mentioned display method in detecting the actual grain size of the original austenite grain boundaries in bainitic steel, which can accurately obtain the actual grain size of the original austenite grain boundaries in bainitic steel.

[0020] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0021] (1) The present invention uses a specially formulated corrosion solution combined with an electrolytic etching device to effectively control the corrosion rate, thereby clearly displaying the original austenite grain boundaries, which makes it easy to accurately obtain the grain size of the original austenite grain boundaries in bainitic steel.

[0022] (2) The specially formulated etching solution makes the original austenite grain boundaries more visible, while other structures are hardly corroded. This results in high contrast of the austenite grain boundaries, making them clearer under a metallographic microscope and making the actual grain size measurement easier and more accurate.

[0023] (3) Adding a small amount of picric acid can prevent the concentration of picric acid from being too high, which would cause the sample to turn black due to rapid corrosion.

[0024] (4) When the electrolytic etching device of the present invention is used for electrolytic etching, the current inside the sample is uniform. The cone-shaped metal rod connected to the cathode of the electrolytic device is adjustable in height. The current can be adjusted by the depth of the lower part of the cone-shaped metal rod immersed in the etching liquid, thereby controlling the electrolytic etching rate of the sample, avoiding excessive corrosion and blackening, and clearly showing the grain boundaries so as to observe the grain boundaries and grain size.

[0025] (5) By limiting the voltage and current during electrolytic corrosion, the sensitivity of the corrosion process to temperature is reduced, allowing electrolytic corrosion to be carried out at the target temperature ±4℃, and the austenite grain boundary display effect is more stable. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the electrolytic etching apparatus used in this invention;

[0027] In the diagram: 1-Resistance heating furnace, 2-Iron stand, 3-Metal rod with cone tip, 4-Glass beaker, 5-Metal base;

[0028] Figure 2 The results of the manifestation of the original austenite grain boundaries in the bainitic steel of Example 1;

[0029] Figure 3 The results show the appearance of the original austenite grain boundaries in the bainitic steel of Comparative Example 1;

[0030] Figure 4 The results show the appearance of the original austenitic grain boundaries in the bainitic steel of Comparative Example 7. Detailed Implementation

[0031] The technical solution of the present invention will be further described and illustrated below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are only for the purpose of helping to understand the present invention and are not intended to limit the specific scope of the present invention. Furthermore, the accompanying drawings used herein are merely for better illustrating the content disclosed in the present invention and do not limit the scope of protection. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commonly used in the art, and the methods used in the embodiments are all conventional methods in the art.

[0032] The electrolytic etching apparatus used in this invention is as follows: Figure 1 As shown, the electrolytic etching apparatus used in electrolytic etching includes a resistance heating furnace 1, an iron stand 2, a metal rod with a conical tip 3, a glass beaker 4, and a metal base 5. The resistance heating furnace 1 is placed on the iron stand 2, and the metal rod with a conical tip 3 and the metal base 5 are placed in the glass beaker 4. The metal base 5 is used to place the sample, the metal rod with a conical tip 3 is used to connect to the cathode of the electrolytic equipment, and the metal base 5 is used to connect to the anode of the electrolytic equipment.

[0033] Example 1

[0034] The method for revealing the original austenite grain boundaries of bainitic steel in this embodiment includes the following steps:

[0035] (1) A cylindrical sample with a diameter of Φ10*10 was cut from a bar of bainitic non-quenched and tempered steel YG1401. The two ends of the sample were cut flat. The metallographic observation surface of the cylindrical sample was ground on 200#, 400# and 600# sandpaper in sequence and then polished to obtain the sample.

[0036] (2) Heat the corrosive solution at 62°C using a resistance heating furnace. Add 1.0g picric acid, 0.55g sodium tungstate and 0.002g zinc chloride to a glass beaker containing 100ml distilled water and stir evenly with a glass rod. Add 0.01mol / L hydrochloric acid solution to adjust the pH to 1.6.

[0037] (3) Move the glass beaker containing the corrosive solution to the resistance heating furnace 1 of the electrolytic etching device. Connect the anode to the metal base 5 and the cathode to the metal rod 3 with a cone tip. Turn on the electrolytic device, adjust the voltage to 29V, adjust the depth of the cone tip of the metal rod 3 into the liquid surface, and control the current to 1.0A. Place the sample on the metal base 5 with the surface to be etched facing up, ensuring that the surface to be etched is completely immersed in the corrosive solution. Turn on the electrolytic device to electrolytically etch for 50s, turn off the electrolytic device, remove the sample, gently remove the dirt from the surface to be observed, then wash and dry it. Observe the corrosion of the original austenite grain boundaries of the sample under a metallographic microscope.

[0038] like Figure 2As shown, the original austenitic intragranular structure of bainitic steel is in an electropolished state and is not corroded, while the grain boundaries are moderately corroded, making the grain boundaries clearly visible.

[0039] Example 2

[0040] The method for revealing the original austenite grain boundaries of bainitic steel in this embodiment includes the following steps:

[0041] (1) A cylindrical sample with a diameter of Φ10*10 was cut from a bar of bainitic non-quenched and tempered steel YG1401. The two ends of the sample were cut flat. The metallographic observation surface of the cylindrical sample was ground on 200#, 400# and 600# sandpaper in sequence and then polished to obtain the sample.

[0042] (2) Heat the corrosive solution at 65°C using a resistance heating furnace. Add 1.0g picric acid, 0.55g sodium tungstate and 0.002g zinc chloride to a glass beaker containing 100ml distilled water and stir evenly with a glass rod. Add 0.01mol / L hydrochloric acid solution to adjust the pH to 1.8.

[0043] (3) Move the glass beaker containing the etching solution to the resistance heating furnace 1 of the electrolytic etching device. Connect the anode to the metal base 5 and the cathode to the metal rod 3 with a cone tip. Turn on the electrolytic device, adjust the voltage to 28V, adjust the depth of the cone tip of the metal rod 3 into the liquid surface, and control the current to 0.8A. Place the sample on the metal base 5 with the surface to be etched facing up, ensuring that the surface to be etched is completely immersed in the etching solution. Turn on the electrolytic device and electrolytically etch for 60s. Turn off the electrolytic device, remove the sample, gently remove the dirt from the surface to be observed, then wash and dry it. Observe the corrosion of the original austenite grain boundaries of the sample under a metallographic microscope.

[0044] Example 3

[0045] The difference between this embodiment and embodiment 1 is only in step (2) heating in a resistance heating furnace to prepare the corrosion solution at 62°C. 1.5g of picric acid, 0.8g of sodium tungstate and 0.0015g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. A 0.01mol / L hydrochloric acid solution is added to adjust the pH to 1.6.

[0046] Example 4

[0047] The only difference between this embodiment and embodiment 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 1.0g of picric acid, 0.55g of sodium tungstate and 0.002g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. A 0.01mol / L hydrochloric acid solution is added to adjust the pH to 1.2.

[0048] Example 5

[0049] The only difference between this embodiment and embodiment 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 1.0g of picric acid, 0.55g of sodium tungstate and 0.002g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod.

[0050] Example 6

[0051] The difference between this embodiment and embodiment 1 is only in step (3): the glass beaker containing the etching solution is moved to the resistance heating furnace 1 of the electrolytic etching device, the anode is connected to the metal base 5, the cathode is connected to the metal rod 3 with a cone tip, the electrolytic device is turned on, the voltage is adjusted to 20V, the depth of the cone end of the metal rod 3 submerged in the liquid is adjusted, the current is controlled to 1.0A, the sample is placed on the metal base 5 with the surface to be etched facing up, ensuring that the surface to be etched is completely immersed in the etching solution, the electrolytic device is turned on for 50s of electrolytic etching, the electrolytic device is turned off, the sample is clamped out, the dirt on the surface to be observed is gently removed, then it is washed and dried, and the corrosion of the original austenite grain boundaries of the sample is observed under a metallographic microscope.

[0052] Example 7

[0053] The difference between this embodiment and embodiment 1 is only in step (3): the glass beaker containing the etching solution is moved to the resistance heating furnace 1 of the electrolytic etching device, the anode is connected to the metal base 5, the cathode is connected to the metal rod 3 with a cone head, the electrolytic device is turned on, the voltage is adjusted to 32V, the depth of the cone end of the metal rod 3 submerged in the liquid is adjusted, the current is controlled to 1.0A, the sample is placed on the metal base 5 with the surface to be etched facing up, ensuring that the surface to be etched is completely immersed in the etching solution, the electrolytic device is turned on for 50s of electrolytic etching, the electrolytic device is turned off, the sample is clamped out, the dirt on the surface to be observed is gently removed, then it is washed and dried, and the corrosion of the original austenite grain boundaries of the sample is observed under a metallographic microscope.

[0054] Comparative Example 1

[0055] The only difference between this comparative example and Example 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 0.5g of picric acid, 0.55g of sodium tungstate and 0.002g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. The pH is then adjusted to 1.6 by adding a 0.01mol / L hydrochloric acid solution.

[0056] Comparative Example 2

[0057] The only difference between this comparative example and Example 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 2.5g of picric acid, 0.55g of sodium tungstate and 0.002g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. A 0.01mol / L hydrochloric acid solution is added to adjust the pH to 1.6.

[0058] Comparative Example 3

[0059] The only difference between this comparative example and Example 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 1.0g of picric acid, 0.2g of sodium tungstate and 0.002g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. A 0.01mol / L hydrochloric acid solution is added to adjust the pH to 1.6.

[0060] Comparative Example 4

[0061] The only difference between this comparative example and Example 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 1.0g of picric acid, 1.2g of sodium tungstate and 0.002g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. A 0.01mol / L hydrochloric acid solution is added to adjust the pH to 1.6.

[0062] Comparative Example 5

[0063] The only difference between this comparative example and Example 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 1.0g of picric acid, 0.55g of sodium tungstate and 0.005g of zinc chloride are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. A 0.01mol / L hydrochloric acid solution is added to adjust the pH to 1.6.

[0064] Comparative Example 6

[0065] The only difference between this comparative example and Example 1 is that in step (2), the corrosion solution is prepared by heating in a resistance heating furnace at 62°C. 1.0g of picric acid and 0.55g of sodium tungstate are added to a glass beaker containing 100ml of distilled water and stirred evenly with a glass rod. A 0.01mol / L hydrochloric acid solution is added to adjust the pH to 1.6.

[0066] Comparative Example 7

[0067] The method for revealing the original austenite grain boundaries of the bainitic steel in this comparative example includes the following steps:

[0068] (1) A cylindrical sample with a diameter of Φ10*10 was cut from a bar of bainitic non-quenched and tempered steel YG1401. The two ends of the sample were cut flat. The metallographic observation surface of the cylindrical sample was ground on 200#, 400# and 600# sandpaper in sequence and then polished to obtain the sample.

[0069] (2) Heat the corrosive solution at 62°C using a resistance heating furnace. Add 1.0g picric acid, 0.55g sodium tungstate and 0.002g zinc chloride to a glass beaker containing 100ml distilled water and stir evenly with a glass rod. Add 0.01mol / L hydrochloric acid solution to adjust the pH to 1.6.

[0070] (3) Place the sample in a glass beaker containing the etching solution with the surface to be etched facing up, ensuring that the surface to be etched is completely immersed in the etching solution. After etching for 50 seconds, remove the sample, gently remove the dirt from the surface to be observed, then wash and dry it. Observe the corrosion of the original austenite grain boundaries of the sample under a metallographic microscope.

[0071] In Example 4, the pH of the etching solution was adjusted to 1.2, resulting in rapid corrosion and blackening of the sample. In Example 5, the pH of the etching solution was not adjusted, and the pH value was 2.0. The degree of zinc ion hydrolysis was slightly greater, weakening the synergistic effect with sodium tungstate, leading to excessively rapid corrosion and blackening of the sample. In Example 6, the electrolytic corrosion voltage was too low, resulting in a significant increase in the current within the bainitic steel sample, causing rapid corrosion and excessive corrosion of the intragranular structure, leading to blackening. In Example 7, the electrolytic corrosion voltage was too high, resulting in uneven polishing of the sample and a disordered and unclear microstructure of the crystal structure. In Comparative Example 1, the amount of picric acid added was too low, resulting in poor electrolytic corrosion and the inability to observe clear grain boundaries and intragranular structures. Figure 3 As shown, in Comparative Example 2, the excessive addition of picric acid caused rapid corrosion and blackening of the sample. In Comparative Example 3, the insufficient addition of sodium tungstate resulted in simultaneous corrosion of the crystalline structure, making clear grain boundaries difficult to observe. In Comparative Example 4, the excessive addition of sodium tungstate led to a significant passivation effect, but the grain boundaries remained indistinct. In Comparative Example 5, the excessive addition of zinc chloride resulted in a distinct film on the sample surface, preventing corrosion and obscuring grain boundaries. In Comparative Example 6, the absence of zinc chloride weakened the passivation effect of sodium tungstate, leading to corrosion of the intracrystalline structure and insufficient grain boundary visibility. In Comparative Example 7, without electrolysis, the corrosion rates of the grain boundaries and intracrystalline structure were nearly identical, resulting in no clear grain boundaries. Figure 4 As shown. Finally, it should be noted that the specific embodiments described herein are merely illustrative of the spirit of the invention and are not intended to limit the implementation of the invention. Those skilled in the art to which this invention pertains can make various modifications or additions to the described embodiments or use similar methods to replace them; it is neither necessary nor possible to exemplify all embodiments here. However, these obvious variations or modifications derived from the essential spirit of the invention still fall within the scope of protection of this invention, and interpreting them as any additional limitation would contradict the spirit of the invention.

Claims

1. A method for displaying the original austenitic grain boundaries of bainitic steel, characterized in that, The display method includes preparing a metallographic sample, preparing an etching solution, and electrolytic etching. When preparing metallographic specimens, the specimens are cut, ground, and polished in sequence. The corrosive solution comprises the following components at mass concentrations: picric acid 5~20 g / L, sodium tungstate 3~10 g / L, and zinc chloride 0.009~0.03 g / L; After preparing the etching solution, adjust the pH to 1.5~1.8; The pH of the corrosive solution is adjusted using a hydrochloric acid solution with a concentration of 0.01~0.03 mol / L; The preparation of the etchant is carried out at a temperature of 55~65℃; During electrolytic corrosion, the voltage is 25~30V and the current is 0.6~1.2A; The electrolytic corrosion time is 15~120s.

2. The display method according to claim 1, characterized in that, The electrolytic etching apparatus used in electrolytic etching includes a resistance heating furnace, an iron stand, a metal rod with a conical tip, a glass beaker, and a metal base. The resistance heating furnace is placed on the iron stand, and the metal rod with the conical tip and the metal base are placed in the glass beaker.

3. The application of the display method as described in claim 1 in the detection of actual grain size of the original austenitic grain boundaries in bainitic steel.

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