Determination method for original austenite grain size of low-carbon hydro-electric steel

Abstract

The invention discloses a method for measuring the original austenite grain size of low-carbon hydro-electric steel, which comprises the following steps: sequentially carrying out pretreatment, heat treatment, grinding and polishing and corrosion on a to-be-measured sample, and then displaying the austenite grain size of the to-be-measured sample by adopting a ferrite network method, according to heat treatment, the heat treatment temperature is 880-900 DEG C, the heat preservation time is 0.5-1 h, and then water cooling is conducted after air cooling is conducted to 640-650 DEG C; the corrosion liquid is prepared from 5-8 g of citric acid, 4-8 drops of nitric acid and 100 mL of water. According to the method, the original austenite of the low-carbon hydroelectric steel can be clearly displayed, and the grade of the austenite grain size is accurately evaluated.

Description

Technical Field

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

[0002] According to the relevant provisions of the GB / T 6394-2017 standard, when using the ferrite network method to display austenite grain size for carbon steel and alloy steel with a carbon content of 0.20% to 0.60%, the sample must first be heat treated. The standard stipulates that samples with a carbon content (mass fraction) of 0.20% or less and less than 0.35% should be heated at 890°C ± 10°C for at least 30 minutes. In order to precipitate ferrite as a fine and continuous network at the austenite grain boundaries, thereby clearly displaying the original austenite grains, the cooling method must be adjusted for different steel grades. However, the standard does not specify how to handle and prepare the samples later. It is recommended that low-carbon steel (carbon content approximately 0.20%) samples be heated to 890°C, held at this temperature for 30 minutes, then transferred to a furnace at 730°C to 790°C, held there for 3 minutes to 5 minutes, and then water-cooled. Medium-carbon steel (carbon content approximately 0.50%) is suitable for furnace cooling. However, low-carbon hydropower steel samples with a carbon content of 0.10% to 0.15% are not well suited to the sample preparation methods mentioned in the above standards. The oxide layer produced by heat treatment on the sample surface is very thin, generally only around 30μm, and even the slightest mishandling can render the previous work useless. When picric acid is used as the raw material, obtaining an accurate austenite grain size number is greatly difficult. Austenite grain size is closely related to the physical and mechanical properties of the material. Accurate grain size number is closely related to material production and heat treatment processes, and its importance cannot be underestimated. Summary of the Invention

[0003] The present invention aims to provide a method for measuring the original austenite grain size of low-carbon hydropower steel, which can accurately evaluate the grade of the austenite grain size.

[0004] To achieve the above object, the present invention provides the following technical solutions: A method for determining the original austenite grain size of low-carbon hydropower steel comprises the following steps: pre-treating, heat treating, grinding and polishing, and corroding the sample to be tested, and then displaying the austenite grain size using a ferrite mesh method.

[0005] Furthermore, the pretreatment of the present invention specifically includes: grinding the surface of the sample to be tested with 180#, 400#, 800# and 1000# sandpaper in sequence, and then polishing it to a mirror surface with a silk polishing cloth on a polishing machine.

[0006] Furthermore, in the heat treatment of the present invention, the heat treatment temperature is 880-900° C., the holding time is 0.5-1 hour, and then the heat treatment is air-cooled to 640-650° C. and then water-cooled.

[0007] Furthermore, the grinding and polishing of the present invention uses 1000# sandpaper for 10-15 seconds to remove the oxide layer on the surface of the sample, and then uses a silk polishing cloth sprinkled with 2.5μm diamond polishing agent to polish at a speed of 290-300 rpm for 5-8 seconds.

[0008] Furthermore, in the corrosion of the present invention, the corrosion liquid is prepared by 5-8g of citric acid, 4-8 drops of nitric acid and 100mL of water.

[0009] Furthermore, after the etching is completed, the surface is rinsed with water and anhydrous ethanol in sequence and then blown dry.

[0010] Furthermore, the carbon content of the low-carbon hydroelectric steel of the present invention is 0.10% to 0.15%.

[0011] The inventive principle of the technical solution of the present invention is as follows:

[0012] According to the lever principle of the iron-carbon phase diagram, when low-carbon steel is slowly cooled from austenite to 640-650℃, a small amount of ferrite is preferentially precipitated at the grain boundaries. Then, it is water-cooled, and the ferrite precipitates in a network along the austenite grain boundaries. That is, the original austenite grain morphology of the low-carbon steel can be outlined by the fine network of ferrite to characterize the austenite grain morphology.

[0013] The etching solution of the present invention is composed of citric acid, nitric acid, and deionized water. Citric acid is an organic substance, while nitric acid is an inorganic substance. Using deionized water as a solvent can not only perfectly dissolve the citric acid, but also weaken the corrosiveness of nitric acid, ensuring the clear display of the ferrite network at the grain boundaries. The etching solution of the present invention can also suppress the display of intracrystalline structure, ultimately achieving accurate characterization of grain size.

[0014] The beneficial technical effects of the technical solution of the present invention are as follows:

[0015] The method of the present invention improves the characterization method and process of measuring the austenite grain size of low-carbon hydropower steel with a carbon content of 0.10% to 0.15% by the ferrite mesh method, and can accurately evaluate the austenite grain size grade. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a photo of the ferrite network microstructure of the sample in Example 1 displayed under a metallographic microscope at 500X; Figure 2 This is a photo of the ferrite network microstructure of the sample in Example 2 displayed under a metallographic microscope at 500X; Figure 3 This is a photo of the ferrite network microstructure of the sample in Example 3 displayed under a metallographic microscope at 500X. DETAILED DESCRIPTION

[0017] The present application will be further described below in conjunction with specific examples. It will be understood that the following examples are intended only to illustrate the present application and are not intended to limit the scope of the present application. Experimental methods in the following examples where specific conditions are not indicated are generally based on national standards. If there are no corresponding national standards, then the methods are carried out in accordance with general international standards or according to the conditions required by the manufacturer. Example 1

[0018] The carbon content of the low-carbon hydroelectric steel in this embodiment is 0.10%. The method for determining the original austenite grain size includes pre-treating, heat treating, grinding and polishing, and etching the test sample in sequence, and then using the ferrite network method to display the austenite grain size. The specific operation is as follows: (1) Pretreatment: Use 180#, 400#, 800# and 1000# sandpaper to grind the surface of the sample on the grinding and polishing machine. When the grinding marks on the surface are neat and consistent, use silk polishing cloth to polish it to a mirror surface.

[0019] (2) Heat treatment: Heat the sample to be tested to 880℃, keep it warm for 1 hour, take it out, air cool it to 650℃ and then water cool it.

[0020] (3) Grinding and polishing: Grind on 1000# sandpaper for 10 seconds to remove the oxide layer on the surface of the sample, and then polish with a silk polishing cloth sprinkled with 2.5μm diamond polishing agent at a speed of 300 rpm for 8 seconds.

[0021] (4) Corrosion: Use a citric acid and nitric acid aqueous solution (concentration ratio: 8g citric acid, 4 drops of nitric acid, 100ml deionized water) to corrode the surface of the sample to be tested until it turns silver-gray, then rinse with running water and anhydrous ethanol in turn, and blow dry with a hair dryer.

[0022] (5) The ferrite network method is used to display the austenite grain size of low carbon hydropower steel. The ferrite network microstructure photo is as follows: Figure 1 shown by Figure 1 It can be seen that ferrite is generated along the grain boundaries, forming a ferrite network that is clearly visible. After measuring the grain size, the accurate result of the austenite grain size can be obtained. Example 2

[0023] The carbon content of the low-carbon hydroelectric steel in this embodiment is 0.13%. The method for determining the original austenite grain size includes pre-treating, heat treating, grinding and polishing, and etching the test sample in sequence, and then using the ferrite network method to display the austenite grain size. The specific operation is as follows: (1) Pretreatment: Use 180#, 400#, 800# and 1000# sandpaper to grind the surface of the sample on the grinding and polishing machine. When the grinding marks on the surface are neat and consistent, use silk polishing cloth to polish it to a mirror surface.

[0024] (2) Heat treatment: Heat the sample to be tested to 890℃, keep it warm for 45 minutes, take it out, air cool it to 640℃, and then water cool it.

[0025] (3) Grinding and polishing: Grind on 1000# sandpaper for 12 seconds to remove the oxide layer on the surface of the sample, and then polish with a silk polishing cloth sprinkled with 2.5μm diamond polishing agent at a speed of 300 rpm for 6 seconds.

[0026] (4) Corrosion: Use a citric acid and nitric acid aqueous solution (concentration ratio: 6g citric acid, 6 drops of nitric acid, 100ml deionized water) to corrode the surface of the sample to be tested until it turns silver-gray, then rinse with running water and anhydrous ethanol in turn, and blow dry with a hair dryer.

[0027] (5) The ferrite network method is used to display the austenite grain size of low carbon hydropower steel. The ferrite network microstructure photo is as follows: Figure 2 shown by Figure 2 It can be seen that ferrite is generated along the grain boundaries, forming a ferrite network that is clearly visible. After measuring the grain size, the accurate result of the austenite grain size can be obtained. Example 3

[0028] The carbon content of the low-carbon hydroelectric steel in this embodiment is 0.15%. The method for determining the original austenite grain size includes pre-treating, heat treating, grinding and polishing, and etching the test sample in sequence, and then using the ferrite network method to display the austenite grain size. The specific operation is as follows: (1) Pretreatment: Use 180#, 400#, 800# and 1000# sandpaper to grind the surface of the sample on the grinding and polishing machine. When the grinding marks on the surface are neat and consistent, use silk polishing cloth to polish it to a mirror surface.

[0029] (2) Heat treatment: Heat the sample to be tested to 900℃, keep it warm for 30 minutes, take it out, air cool it to 645℃, and then water cool it.

[0030] (3) Grinding and polishing: Grind on 1000# sandpaper for 12 seconds to remove the oxide layer on the surface of the sample, and then polish with a silk polishing cloth sprinkled with 2.5μm diamond polishing agent at a speed of 300 rpm for 5 seconds.

[0031] (4) Corrosion: Use a citric acid and nitric acid aqueous solution (concentration ratio: 5g citric acid, 8 drops of nitric acid, 100ml deionized water) to corrode the surface of the sample to be tested until it turns silver-gray, then rinse with running water and anhydrous ethanol in turn, and blow dry with a hair dryer.

[0032] (5) The ferrite network method is used to display the austenite grain size of low carbon hydropower steel. The ferrite network microstructure photo is as follows: Figure 3 shown by Figure 3It can be seen that ferrite is generated along the grain boundaries, forming a ferrite network that is clearly visible. After measuring the grain size, the accurate result of the austenite grain size can be obtained.

Claims

1. A method for measuring the original austenite grain size of low-carbon hydropower steel, characterized in that: The method includes pre-treatment, heat treatment, grinding and polishing, and etching of the test sample in sequence, and then using the ferrite network method to display its austenite grain size.

2. The method for determining the original austenite grain size of low-carbon hydropower steel according to claim 1, characterized in that: The pretreatment comprises the following steps: grinding the surface of the sample to be tested with 180#, 400#, 800# and 1000# sandpaper in sequence, and then polishing it to a mirror surface with a silk polishing cloth on a polishing machine.

3. The method for measuring the original austenite grain size of low-carbon hydropower steel according to claim 1, characterized in that: The heat treatment temperature is 880-900° C., the heat preservation time is 0.5-1 hour, and then the heat is cooled to 640-650° C. by air and then water-cooled.

4. The method for measuring the original austenite grain size of low-carbon hydropower steel according to claim 1, characterized in that: The grinding and polishing is carried out by using 1000# sandpaper for 10-15 seconds, and then using a silk polishing cloth sprinkled with 2.5 μm diamond polishing agent for 5-8 seconds at a rotation speed of 290-300 rpm.

5. The method for measuring the original austenite grain size of low-carbon hydropower steel according to claim 1, characterized in that: The corrosion solution is prepared from 5-8 g of citric acid, 4-8 drops of nitric acid and 100 mL of water.

6. A method for measuring the original austenite grain size of low-carbon hydropower steel according to claim 5, characterized in that: After the etching is completed, the surface is rinsed with water and anhydrous ethanol in sequence and blown dry.

7. The method for measuring the original austenite grain size of low-carbon hydropower steel according to claim 1, characterized in that: The carbon content of the low-carbon hydroelectric steel is 0.10% to 0.15%.

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