Method for measuring grain size of original austenite of medium carbon steel

Abstract

The invention discloses a method for measuring the original austenite grain size of medium carbon steel. The method comprises the following steps: heating a medium carbon steel sample to a temperature above an original austenitizing temperature, and keeping the temperature for a period of time to precipitate carbides from an original austenite grain boundary; cutting and embedding the heat-treated medium carbon steel sample to prepare a medium carbon steel metallographic sample; carrying out grinding and fine polishing treatment on the medium carbon steel metallographic sample, and carrying out ultrasonic cleaning on the medium carbon steel metallographic sample in alcohol; immersing and corroding for a certain time by using nitric acid alcohol, then cleaning the surface of the medium carbon steel metallographic sample by using alcohol, and drying until an austenite grain boundary is clearly displayed; completing microscopic image rating; and calculating the austenite grain grade by adopting an intercept point method. According to the method, the original austenite grain boundary can be clearly displayed, and the effective size of the original austenite grain size of the medium carbon steel can be rapidly and accurately obtained.

Description

Technical Field

[0001] The invention relates to the technical field of detection and analysis, and in particular to a method for measuring the original austenite grain size of medium carbon steel. Background Art

[0002] Medium carbon steel is a carbon steel with a carbon content of 0.25% to 0.6%. It includes most high-quality carbon structural steels and some common carbon structural steels. This type of steel is primarily used to manufacture various mechanical parts, and some is used in engineering structures. High-strength medium carbon structural steel exhibits a certain degree of plasticity, toughness, and strength, along with good machinability. After quenching and tempering, it exhibits excellent overall mechanical properties.

[0003] In actual production, controlling the original austenite grain size of steel is crucial for its production, as it is one of the primary factors influencing the mechanical properties of steel. Grain refinement is the only optimal method for improving both strength and toughness; other methods increase strength while compromising toughness. Research into grain refinement technology requires first visualizing the austenite grain boundaries of the material and understanding the actual state of the production process. This allows for adjustments to product composition and production processes, ultimately achieving the desired material structure and mechanical properties.

[0004] The national standard GB / T6394-2017, "Methods for Determination of Average Grain Size of Metals," specifies methods for determining prior austenite grain boundaries, including the carburizing method, the ferrite network method, the oxidation method, the direct quenching method, the cementite network method, and the fine pearlite network method. All of these methods require etching with chemical reagents to reveal the prior austenite grain size. The cementite network method utilizes nital, picric acid solution, or alkaline picric acid; the ferrite network method utilizes nital and picric acid solution; the grain boundary oxidation method utilizes hydrochloric acid in ethanol; and the fine pearlite network method utilizes nital and picric acid solution.

[0005] Therefore, the most common traditional method for revealing the original austenite grains of medium carbon steel is to use a etchant made by mixing saturated picric acid, various surfactants, and a small amount of other additives in different proportions. The etchant is usually heated in a water bath to a certain temperature, and then the directly quenched or quenched and tempered specimen is either immersed in the etchant or wiped with cotton wool soaked in the etchant to reveal the grain boundaries of the austenite grains. However, the above chemical corrosion measurement methods have the following technical problems: First, the chemical corrosion method involves the use of different corrosion reagents for different steel grades, which has complicated ratios. In addition, different corrosion reagent concentrations and corrosion times have a significant impact on the corrosion effect. For different steel grades, it takes a lot of time and energy to explore experimental corrosion reagents and methods, which makes it difficult to quickly and effectively etch out the austenite grain size. Second, reagents are controlled and difficult to purchase. Picric acid, the main reagent used as a corrosive agent, is a civilian explosive and is strictly controlled and cannot be purchased, resulting in the inability to carry out this method in the industry.

[0006] In addition, other measurement methods, such as high-temperature laser confocal microscopy, are mentioned in relevant literature. A Chinese patent (application number: CN200810227821.3) describes a method for measuring austenite grain size using a high-temperature laser microscope. After preparing a metallographic specimen, it is heated and maintained in a high-temperature laser microscope furnace. The measurement function of the device controller is used to measure the austenite grain size in real time, and the austenite grain size is further assessed according to standards. However, this high-temperature laser confocal microscopy method has the following technical issues: First, the equipment for high-temperature laser microscopy is expensive and its use is limited; Second, the high-temperature laser confocal measurement method is only applicable to the measurement of austenite grain size in the austenite single-phase region of metal materials heated above the Ac3 point, and the austenite grain size measured by this method needs to be verified. Summary of the Invention

[0007] In view of the shortcomings of the existing technology, the present invention proposes a method for measuring the original austenite grain size of medium carbon steel, which can quickly and accurately obtain the effective size of the original austenite grain size of medium carbon steel without using a high-temperature laser microscope and picric acid reagent.

[0008] To achieve the above object, the present invention provides a method for measuring the original austenite grain size of medium carbon steel, which is particularly characterized in that it includes the following steps: S1) austenite grain boundary formation; The medium carbon steel sample is heated to above the original austenitizing temperature and kept at this temperature for a period of time to allow carbides to precipitate from the original austenite grain boundaries, thereby facilitating the subsequent display of the austenite grain boundaries that existed during the original austenitizing process. Different heat treatment processes are used for medium carbon steel samples with different carbon contents, as follows: S11) For medium carbon steel samples with a carbon content of 0.25% to 0.40%, heat to Ac3+50~70℃, hold for 30 to 40 minutes, then quench and quickly cool to room temperature, then heat the medium carbon steel sample to 570~650℃ and hold for 60 minutes; S12) For medium carbon steel samples with a carbon content of 0.40% to 0.60%, heat the sample to Ac3+30~50℃ and hold it for 20 to 30 minutes, then quench and quickly cool it to room temperature. Then heat the medium carbon steel sample to 500~570℃ and hold it for 60 minutes. S2) austenite grain boundary display; The specific steps include: S21) cutting and mounting the heat-treated medium carbon steel sample to prepare a medium carbon steel metallographic sample; S22) Grinding and fine polishing of medium carbon steel metallographic samples; S23) using nitric acid to etch the medium carbon steel metallographic sample, then cleaning the surface of the medium carbon steel metallographic sample with alcohol and drying it until the austenite grain boundaries are revealed; S24) Take photos of austenite grain boundary samples and complete microscopic image rating; S3) is graded according to the metal average grain size determination method, and the intercept method is used to calculate the austenite grain grade and grain size.

[0009] Furthermore, in S22), the grinding of the medium carbon steel metallographic sample is manual grinding; The manual grinding includes grinding from coarse to fine on different sandpapers. Each time the sandpaper is changed, the sample must be rotated 90 degrees. o Grind in a direction perpendicular to the old wear marks until the old wear marks disappear completely and the new wear marks are uniform. Each time, the sample must be cleaned and dried with water or ultrasound before proceeding to the next sample preparation procedure.

[0010] Furthermore, in S22), the grinding of the medium carbon steel metallographic sample is carried out through 4 passes. The grinding sandpapers for the 1st to 4th passes are 180#, 320#, 600#, and 1000#, respectively. The grinding pressures are 240N, 220N, 200N, and 180N, respectively. The grinding time is 180s, 220s, 260s, and 300s, respectively. The grinding speed is increased from 200 rpm to 300 rpm, respectively.

[0011] Furthermore, in S22), the polishing of the medium carbon steel metallographic sample is mechanical polishing, and the mechanical polishing is manual polishing; The manual polishing includes lightly pressing the sample on the polishing disc and polishing back and forth along the diameter of the disc; controlling the humidity of the polishing cloth to avoid affecting the polishing quality.

[0012] Furthermore, in S22), the polishing of the medium carbon steel metallographic sample was carried out in three passes. The particle diameters of the polishing agents in the first to third passes were 5 mm, 1 mm, and 0.05 mm, respectively. The polishing pressures were 120 N, 110 N, and 100 N, respectively. The polishing times were 300 s, 750 s, and 1200 s, respectively. The polishing speeds were 160 rpm, 180 rpm, and 200 rpm, respectively.

[0013] Furthermore, in S23), for the metallographic specimens of medium carbon steel with a carbon content of 0.25% to 0.40%, 3% to 5% nitric acid alcohol immersion corrosion is used for 10 to 20 seconds; for the metallographic specimens of medium carbon steel with a carbon content of 0.40% to 0.60%, 3% to 5% nitric acid alcohol immersion corrosion is used for 5 to 10 seconds.

[0014] Furthermore, in S3), according to GB / T6394-2017 "Method for Determination of Average Grain Size of Metals", the image analyzer analysis software is used to perform the interception method analysis. The specific analysis steps are as follows: S31) Randomly draw multiple circles of equal diameter on the microscopic image. The circle diameter needs to cover enough grains to avoid duplicate counting. S32) Record the number of intersections between each circle and the grain boundary; S33) calculating the average number of intersections of all circles; S34) Calculate the average intercept length by the average number of intersections of all circles, and convert the average intercept length to ASTM grain size grade using the grain size grade formula.

[0015] Furthermore, in S31), the number of circles with equal diameters must be greater than or equal to 5.

[0016] Furthermore, in S32), the number of intersections between each circle and the grain boundary is counted as follows: if the circle intersects the grain boundary at the grain boundary, the count is 1; if the circle cuts through a grain corner, the count is 0.5.

[0017] Furthermore, in S33), the average number of intersections of all circles is calculated by the following formula: Where, is the average number of intersections of all circles, P i is the number of intersection points of a single circle, N is the total number of circles.

[0018] The average intercept length is calculated by the following formula Where, L m is the average intercept length, is the average number of intersections of all circles, D is the diameter of the circle, in mm; The grain size grade formula is: G = -6.6359 log(L m ) + 12.641 Where, G is the grain size grade, L m is the average intercept length.

[0019] The advantages of the present invention are: 1. Because austenite grain size is related to austenitizing temperature and holding time, the present invention uses different special heat treatment processes for medium-carbon steels with different carbon contents to precipitate cementite at the original austenite grain boundaries. Fine polishing combined with conventional nital etching significantly improves the visualization of the original austenite grain boundaries and the accuracy of the final intercept method. The present invention uses fine polishing combined with conventional nital to replace picric acid, solving the austenite grain size measurement problem caused by picric acid control. 2. Compared with the traditional chemical etching method, the present invention is easier to display the original austenite grain boundaries for medium carbon steel with tempered bainite as the structure, and the displayed medium carbon steel austenite grain boundaries are clearer, and the accuracy of measuring austenite grain size is higher; 3. Based on the austenite formation method of medium carbon steel, the present invention uses different and precise heat treatment processes for different carbon contents, which greatly improves the display effect of austenite grain size and also improves the measurement accuracy of the final intercept method; 4. The present invention develops different nitric acid etching processes for different carbon contents based on a large amount of practical experience, accurately displaying the austenite structure; The method for measuring the original austenite grain size of medium carbon steel of the present invention adopts different special heat treatment processes for medium carbon steels with different carbon contents, and then performs fine polishing and conventional nitric alcohol etching methods to clearly show the original austenite grain boundaries and avoid the precipitation of cementite by using picric acid or alkaline picric acid as an etching agent. This not only solves the problem caused by the shortage of picric acid, but also can quickly and accurately obtain the effective size of the original austenite grain size of medium carbon steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Flow chart of the method for determining the original austenite grain size of carbon steel of the present invention; Figure 2 The original austenite grains of the medium carbon steel shown in the embodiment of the present invention; Figure 3 The intercept method is used in the embodiment of the present invention to calculate the austenite grain grade and grain size. DETAILED DESCRIPTION

[0021] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the invention.

[0023] The invention provides a method for measuring the original austenite grain size of medium carbon steel, which is divided into three parts: austenite grain boundary formation, austenite grain boundary display, and austenite grain grade calculation.

[0024] like Figure 1 As shown, the specific steps are as follows: S1) Austenite grain boundary formation.

[0025] The medium carbon steel sample is heated to above the original austenitizing temperature and kept at this temperature for a period of time to allow carbides to precipitate from the original austenite grain boundaries, thereby facilitating the subsequent display of the austenite grain boundaries that existed during the original austenitizing process.

[0026] Since austenite grain size is related to the austenitizing temperature and holding time, it is necessary to accurately select the relevant temperature and holding time. Usually, samples with a diameter or side length of 25 mm or greater are used for testing. This invention adopts different heat treatment processes for medium carbon steel samples with different carbon contents, as follows: S11) For medium carbon steel samples with a carbon content of 0.25% to 0.40%, heat to Ac3+50~70℃, hold for 30 to 40 minutes, then quench and quickly cool to room temperature, then heat the medium carbon steel sample to 570~650℃ and hold for 60 minutes; S12) For medium carbon steel specimens with a carbon content of 0.40% to 0.60%, heat the specimen to Ac3+30~50°C and hold for 20 to 30 minutes. Then quench and rapidly cool to room temperature. Then heat the medium carbon steel specimen to 500~570°C and hold for 60 minutes.

[0027] Among them, Ac3 is the temperature at which austenite transformation is completed during heating. It is one of the core parameters for determining the heating temperature in the steel heat treatment process and directly affects the formation and uniformity of austenite grains.

[0028] In this embodiment, medium carbon steel is cut into squares with a side length of 30 mm for heat treatment. The sample is heated to 830°C and kept at this temperature for 30 minutes. Then, it is quenched and rapidly cooled to room temperature. The sample is then heated to 500-570°C and kept at this temperature for 60 minutes.

[0029] S2) Austenite grain boundaries revealed.

[0030] During the cooling process of steel in the austenitic state, cementite precipitated at the grain boundaries is used to reveal the original austenite grain size. In the prior art, picric acid solution or alkaline picric acid is generally used as an etchant. This method uses fine polishing + conventional nitric acid alcohol to replace the picric acid etchant to reveal the original austenite grain size.

[0031] The austenite grain boundary display in the present invention specifically includes the following steps: S21) cutting and mounting the heat-treated medium carbon steel sample to prepare a medium carbon steel metallographic sample; specifically, cutting the heat-treated medium carbon steel sample into a suitable mounting size, and then mounting the cut sample into a metallographic sample.

[0032] S22) Grinding and fine polishing the medium carbon steel metallographic sample, and then ultrasonically cleaning the medium carbon steel metallographic sample in alcohol.

[0033] First, after the specimen is mounted, it is ground flat, cleaned, and blown dry before being ground manually or automatically.

[0034] Specifically, the manual grinding includes grinding on different sandpapers from coarse to fine. Each time the sandpaper is changed, the sample must be rotated 90 degrees. o Grind in a direction perpendicular to the old wear marks until the old wear marks disappear completely and the new wear marks are uniform. Each time, the sample must be cleaned and dried with water or ultrasound before proceeding to the next sample preparation procedure.

[0035] Specifically, automatic grinding includes placing sandpaper or grinding discs of different grit sizes from coarse to fine on a mechanical grinding machine and grinding them in sequence.

[0036] Manual or automatic grinding requires four passes. Automatic grinding is preferred because grinding directly affects the quality of subsequent inspection. For the first to fourth passes, sandpaper is used, in order: 180#, 320#, 600#, and 1000#. The grinding pressure is 240N, 220N, 200N, and 180N, respectively. The grinding time is 180s, 220s, 260s, and 300s, respectively. The grinding speed increases from 200 rpm to 300 rpm.

[0037] Secondly, the grinding marks on the sample are polished to achieve a mirror finish without grinding defects. Specifically, the polishing of the medium carbon steel metallographic sample is mechanical polishing, which can be manual polishing or automatic polishing.

[0038] The manual polishing includes lightly pressing the sample on the polishing disc and polishing back and forth along the diameter of the disc; controlling the humidity of the polishing cloth to avoid affecting the polishing quality.

[0039] The automatic polishing equipment fixes the sample on the fixture, and the fixture drives the sample to move along a certain trajectory in the polishing disk. The force between the fixture and the polishing disk, the speed direction, etc. can be adjusted as needed.

[0040] Manual or automatic grinding and polishing requires three passes. Automatic polishing is preferred because grinding directly affects the quality of subsequent inspection. The particle diameters of the polishing compound used for the first through third passes are 5mm, 1mm, and 0.05mm, respectively. The polishing pressures are 120N, 110N, and 100N, respectively. The polishing times are 300s, 750s, and 1200s, respectively. The polishing speeds are 160 rpm, 180 rpm, and 200 rpm, respectively.

[0041] After the last polishing with a 0.05 mm diameter polishing agent at a low speed for a long time, the sample was placed in a glass container filled with alcohol and cleaned with ultrasonic waves to remove surface oxides.

[0042] S23) A medium carbon steel metallographic sample was etched with nitric acid, and then the surface of the medium carbon steel metallographic sample was cleaned with alcohol and dried until the austenite grain boundaries were visible.

[0043] Specifically, for medium carbon steel metallographic samples with a carbon content of 0.25% to 0.40%, 3% to 5% nitric acid alcohol immersion corrosion is used for 10 to 20 seconds; for medium carbon steel metallographic samples with a carbon content of 0.40% to 0.60%, 3% to 5% nitric acid alcohol immersion corrosion is used for 5 to 10 seconds; then, the surface of the medium carbon steel metallographic sample is cleaned with alcohol and blown dry until the austenite grain boundaries are clearly displayed; the specific conditions of alcohol cleaning and blowing dry shall be based on the clear display of the austenite grain boundaries.

[0044] S24) Take photos of samples showing clear austenite grain boundaries and complete microscopic image rating.

[0045] The display process of this embodiment includes: (1) Cut the medium carbon steel specimen into 15 cm × 15 cm pieces and then hot-mount the cut specimens; (2) After the specimen is mounted, it is ground flat, cleaned, and dried, and then automatically ground in five passes. The first pass is 180# sandpaper, 200s, 300 rpm, and 240N pressure. The second pass is 320# sandpaper, 200s, 300 rpm, and 220N pressure. The third pass is 600# sandpaper, 200s, 300 rpm, and 200N pressure. The fourth pass is 1000# sandpaper, 200s, 300 rpm, and 180N pressure.

[0046] (3) After grinding, perform three passes of automatic polishing: the first pass: 5 μm particle size polishing agent, time 200 s, speed 180 rpm, pressure 120 N; the second pass: 1 μm particle size polishing agent, time 200 s, speed 180 rpm, pressure 110 N; the third pass: 0.05 μm particle size polishing agent, time 1000 s, speed 160 rpm, pressure 100 N.

[0047] (4) After the last polishing, place the sample in a glass container filled with alcohol, clean it with ultrasonic waves for 30 seconds, and then blow it dry.

[0048] (5) Soak and corrode with 3% nitric acid alcohol for 5 seconds, clean the surface with alcohol and blow dry, and observe with a metallographic microscope. Figure 2 As shown in FIG, it is the austenite grain of the medium carbon steel shown in this embodiment. Figure 2 It can be seen that this method can efficiently and accurately display the clear grain boundaries of austenite grains.

[0049] S3) is graded according to the metal average grain size determination method, and the intercept method is used to calculate the austenite grain grade and grain size.

[0050] The standard grain size is divided into 12 levels, 1 to 4 are coarse grains, 5 to 8 are fine grains, and 9 to 12 are ultrafine grains.

[0051] The present invention is rated according to GB / T6394-2017 "Method for Determination of Average Grain Size of Metals" and uses imager analysis software to perform interception analysis. The specific analysis steps are as follows: S31) Randomly draw multiple circles of equal diameter on the microscopic image. The circle diameters must cover a sufficient number of grains to avoid duplicate counting. Preferably, the number of circles of equal diameter must be greater than or equal to 5.

[0052] S32) Record the number of intersections between each circle and the grain boundary. Specifically, the number of intersections between each circle and the grain boundary is counted as 1 if the circle intersects the grain boundary at the grain boundary, and as 0.5 if the circle intersects a grain corner. If the circle is located at the edge of the image, only the portion completely within the field of view is counted.

[0053] S33) Calculate the average number of intersections of all circles.

[0054] Specifically, the average number of intersections of all circles is calculated by the following formula Where, is the average number of intersections of all circles, P i is the number of intersection points of a single circle, N is the total number of circles.

[0055] S34) Calculate the average intercept length by the average number of intersections of all circles, and convert the average intercept length to ASTM grain size grade using the grain size grade formula.

[0056] Specifically, the average intercept length is calculated by Where, L m is the average intercept length, is the average number of intersections of all circles, D is the diameter of the circle, in mm.

[0057] The grain size grade formula is: G = -6.6359 log(L m ) + 12.641 Where, G is the grain size grade, L m is the average intercept length.

[0058] Figure 3 The intercept method is used in the embodiment of the present invention to calculate the austenite grain grade and grain size. Figure 3 In the figure, green represents the grain boundary, and the red dot represents the intersection of the concentric circle and the grain boundary. The calculation shows that the austenite grain size is 8.5 and the grain size is 19.2 m.

[0059] The method for measuring the original austenite grain size of medium carbon steel of the present invention adopts different special heat treatment processes for medium carbon steels with different carbon contents, and then performs fine polishing and conventional nitric alcohol etching methods to clearly show the original austenite grain boundaries and avoid the precipitation of cementite by using picric acid or alkaline picric acid as an etching agent. This not only solves the problem caused by the shortage of picric acid, but also can quickly and accurately obtain the effective size of the original austenite grain size of medium carbon steel.

[0060] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for measuring the original austenite grain size of medium carbon steel, characterized in that: The steps include: S1) austenite grain boundary formation; The medium carbon steel sample is heated to above the original austenitizing temperature and kept at this temperature for a period of time to allow carbides to precipitate from the original austenite grain boundaries, thereby facilitating the subsequent display of the austenite grain boundaries that existed during the original austenitizing process. Different heat treatment processes are used for medium carbon steel samples with different carbon contents, as follows: S11) For medium carbon steel samples with a carbon content of 0.25% to 0.40%, heat to Ac3+50~70℃, hold for 30 to 40 minutes, then quench and quickly cool to room temperature, then heat the medium carbon steel sample to 570~650℃ and hold for 60 minutes; S12) For medium carbon steel samples with a carbon content of 0.40% to 0.60%, heat the sample to Ac3+30~50℃ and hold it for 20 to 30 minutes, then quench and quickly cool it to room temperature. Then heat the medium carbon steel sample to 500~570℃ and hold it for 60 minutes. S2) austenite grain boundary display; The specific steps include: S21) cutting and mounting the heat-treated medium carbon steel sample to prepare a medium carbon steel metallographic sample; S22) Grinding and fine polishing of medium carbon steel metallographic samples; S23) using nitric acid to etch the medium carbon steel metallographic sample, then cleaning the surface of the medium carbon steel metallographic sample with alcohol and drying it until the austenite grain boundaries are revealed; S24) Take photos of austenite grain boundary samples and complete microscopic image rating; S3) is graded according to the metal average grain size determination method, and the intercept method is used to calculate the austenite grain grade and grain size.

2. The method for measuring the prior austenite grain size of medium carbon steel according to claim 1, wherein: In S22), the grinding of medium carbon steel metallographic samples is manual grinding; The manual grinding includes grinding from coarse to fine on different sandpapers. Each time the sandpaper is changed, the sample must be rotated 90 degrees. o Grind in a direction perpendicular to the old wear marks until the old wear marks disappear completely and the new wear marks are uniform. Each time, the sample must be cleaned and dried with water or ultrasound before proceeding to the next sample preparation procedure.

3. The method for measuring the prior austenite grain size of medium carbon steel according to claim 2, wherein: In S22), the medium carbon steel metallographic sample was ground through 4 passes. The sandpaper used in the 1st to 4th passes were 180#, 320#, 600#, and 1000#, respectively. The grinding pressure was 240N, 220N, 200N, and 180N, respectively. The grinding time was 180s, 220s, 260s, and 300s, respectively. The grinding speed increased from 200 rpm to 300 rpm, respectively.

4. The method for measuring the prior austenite grain size of medium carbon steel according to claim 1, wherein: In S22), the polishing of the medium carbon steel metallographic sample is mechanical polishing, and the mechanical polishing is manual polishing; The manual polishing includes lightly pressing the sample on the polishing disc and polishing back and forth along the diameter of the disc; controlling the humidity of the polishing cloth to avoid affecting the polishing quality.

5. The method for measuring the prior austenite grain size of medium carbon steel according to claim 4, wherein: In S22), the medium carbon steel metallographic sample was polished through three passes. The particle diameters of the polishing agents for the first to third passes were 5 mm, 1 mm, and 0.05 mm, respectively. The polishing pressures were 120 N, 110 N, and 100 N, respectively. The polishing times were 300 s, 750 s, and 1200 s, respectively. The polishing speeds were 160 rpm, 180 rpm, and 200 rpm, respectively.

6. The method for measuring the prior austenite grain size of medium carbon steel according to claim 5, wherein: S23) For medium carbon steel metallographic samples with a carbon content of 0.25% to 0.40%, 3% to 5% nitric acid alcohol immersion etching is used for 10 to 20 seconds; for medium carbon steel metallographic samples with a carbon content of 0.40% to 0.60%, 3% to 5% nitric acid alcohol immersion etching is used for 5 to 10 seconds.

7. The method for measuring the prior austenite grain size of medium carbon steel according to claim 1, wherein: In S3), according to GB / T6394-2017 "Method for Determination of Average Grain Size of Metals", the imager analysis software is used for interception analysis. The specific analysis steps are as follows: S31) Randomly draw multiple circles of equal diameter on the microscopic image. The circle diameter needs to cover enough grains to avoid duplicate counting. S32) Record the number of intersections between each circle and the grain boundary; S33) calculating the average number of intersections of all circles; S34) Calculate the average intercept length by the average number of intersections of all circles, and convert the average intercept length to ASTM grain size grade using the grain size grade formula.

8. The method for measuring the prior austenite grain size of medium carbon steel according to claim 7, wherein: In S31), the number of circles with equal diameter must be greater than or equal to 5.

9. The method for measuring the prior austenite grain size of medium carbon steel according to claim 8, wherein: In S32), the number of intersections between each circle and the grain boundary is counted as follows: if the circle intersects the grain boundary at the grain boundary, the count is 1; if the circle cuts through a grain corner, the count is 0.

5.

10. The method for measuring the prior austenite grain size of medium carbon steel according to claim 9, wherein: S33), the average number of intersections of all circles is calculated by the following formula Where, is the average number of intersections of all circles, P i is the number of intersection points of a single circle, N is the total number of circles, The average intercept length is calculated by the following formula Where, L m is the average intercept length, is the average number of intersections of all circles, D is the diameter of the circle, in mm; The grain size grade formula is: G = -6.6359 log(L m ) + 12.641 Where, G is the grain size grade, L m is the average intercept length.

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