Sample preparation method for testing austenite grain size in steel through oxidation method

Abstract

The invention belongs to the technical field of metal grain size detection, and particularly relates to a preparation method of a sample for detecting austenite grain size in steel by an oxidation method, which comprises the following steps: S1, grinding and polishing the detection surface of the sample until no macroscopic scratch exists, and immersing the sample in a container filled with electrochemical coke; s2, putting a container filled with the sample and the electrochemical coke into a heating furnace, heating to 860 + / -10 DEG C along with the furnace, keeping the temperature for 0.5-1 hour, then taking out the container, pouring the sample and the electrochemical coke on the ground at the same time, enabling the polished surface of the sample to be fully exposed in the air, and rapidly performing water quenching within 5 seconds; s3, dividing the primary polishing surface of the oxidized sample subjected to heat treatment into two parts, and grinding and polishing one part; and S4, corroding the secondary polished surface of the sample for 8-10 seconds by adopting a 15% hydrochloric acid alcoholic solution, cleaning by using analytically pure alcohol, and finally drying by blowing. The method has the advantage that the accuracy and reliability of the austenite grain size test result are ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of metal grain size detection, and in particular relates to a sample preparation method for testing austenite grain size in steel using an oxidation method. Background Art

[0002] Austenite grain size has a significant impact on material properties. Specifically, the finer the austenite grains, the higher the material's strength and hardness, and the better its plasticity, low-temperature toughness, impact toughness, corrosion resistance, fatigue resistance, and wear resistance. Strictly controlling austenite grain size can significantly optimize a material's overall performance. Therefore, accurately assessing austenite grain size in metal materials is a crucial inspection item.

[0003] The oxidation method for testing austenite grain size, as specified in GBT / 6394-2017, "Methods for Determination of Average Grain Size of Metals," involves polishing the austenite grain size specimen, heating it in an air-heated furnace with the polished surface facing upward for 1 hour, then cooling it with water. Depending on the oxidation state, the specimen is then tilted 10° to 15° for grinding and polishing, leaving an oxide layer on a portion of the test surface. After etching with 10% hydrochloric acid, the austenite grain size is measured directly under a microscope. This method is prone to forming a decarburized layer on the test surface, resulting in inaccurate austenite grain size results. GBT / 6394-2017 specifies that this method is suitable for carbon and alloy steels with a carbon content of 0.25% to 0.60%. However, B3 steel, with a carbon content greater than 0.60%, is susceptible to forming a decarburized layer in the austenite region. Therefore, B3 steel is not suitable for the oxidation method specified in the standard for determining austenite grain size. B3 steel has high Si and Cr contents and good corrosion resistance. However, the direct hardening method cannot fully and clearly show the austenite grain boundaries. Therefore, it is necessary to develop a test method suitable for testing the austenite grain size of B3 steel. Summary of the Invention

[0004] The purpose of the present invention is to provide a sample preparation method for testing the austenite grain size in steel by an oxidation method, so as to solve the problem that the method in the GBT / 6394-2017 standard cannot clearly display the austenite grain size, so that the test surface can clearly display the austenite grain boundary after heat treatment, thereby improving the accuracy of the austenite grain size test results.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] A method for preparing a sample for testing austenite grain size in steel by oxidation method comprises the following steps:

[0007] S1, grind and polish the test surface of the sample until there are no visible scratches, then immerse it in a container filled with electrochemical coke;

[0008] S2. Place the container containing the sample and electrochemical coke into a heating furnace. Raise the temperature to 860±10℃ and keep it warm for 0.5 to 1 hour. Then take out the container and pour the sample and electrochemical coke onto the ground at the same time so that the polished surface of the sample is fully exposed to the air. Then quickly quench with water within 5 seconds.

[0009] S3, dividing the initially polished surface of the heat-treated and oxidized sample into two parts, and grinding and polishing one part;

[0010] S4, use 15% hydrochloric acid alcohol solution to erode the secondary polished surface of the sample for 8 to 10 seconds, then clean it with analytical pure alcohol and finally blow it dry.

[0011] In step S1, the test surface of the sample is polished with 40#, 240#, 500#, and 1000# metallographic sandpaper in sequence, and then polished with tweed cloth and 1-2.5μm diamond polishing paste or polishing agent until there are no visible scratches.

[0012] The container is a straight cylindrical container made of iron or high-temperature resistant alloy with a sealed bottom and an open top; the diameter of the container is 30-40 mm and the height is 80-100 mm.

[0013] In step S1, a 5-10 mm thick layer of electrified coke is first added to the container, and then the polished surface of the sample is placed perpendicular to the electrified coke layer on the bottom of the container. Then, electrified coke is continued to be added to the container until the electrified coke completely covers the sample and fills the container, and the electrified coke layer covering the upper surface of the sample is not less than 30 mm deep.

[0014] The indicators of the electrochemical coke are: w(C)>96%, volatile matter≤1.0%, w(S)≤0.5%, w(water)≤0.55%; and the particle size of the electrochemical coke is 1-5 mm.

[0015] In step S3, the oxidized primary polished surface of the sample is first divided into two equal parts with a marker pen, and then polished with 1000# metallographic sandpaper until one part appears metallic in color, and the black oxide film on the other part of the oxidized primary polished surface of the sample is still visible to the naked eye. Finally, polish until there are no visible scratches on the metallic part.

[0016] In step S4, the sample is etched with a 15% hydrochloric acid alcohol solution for the second polishing surface to fully reveal the austenite grain boundaries, the grain boundary oxide network is observed with a metallographic microscope, and the grain size of the sample austenite is measured in accordance with GB / T6394-2017.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The method of the present invention provides a method for clearly displaying austenite grain boundaries in steel, laying a good foundation for evaluating austenite grain size in steel and ensuring the accuracy and reliability of austenite grain size test results.

[0019] 2. The method of the present invention uses dry, clean, moderately sized (1-5 mm) and highly reducing electrochemical coke as the covering agent for preparing the sample, which can effectively avoid the generation of a decarburized layer during the heat treatment process of the polished surface of the austenite grain sample, and is easy to operate and environmentally friendly.

[0020] 3. The method of the present invention only grinds away a portion of the initially polished surface of the oxidized sample, thereby solving the problem of difficult operation of the traditional grinding method of tilting the sample 10° to 15° for grinding and polishing, and avoiding the problem of improper grinding that causes the failure of the test by completely grinding away the oxide network, thereby improving the operability of sample preparation.

[0021] 4. Standardized process: The present invention immerses the polished surface of the test sample in a container filled with electrochemical coke and then performs heat treatment. The oxidized initial polished surface of the sample is divided into two parts and only one part is polished. This achieves the standardization of the sample preparation process for the oxidation method test of austenite grain size, which is conducive to maintaining consistent test results between different batches and different operators, and improves the controllability and consistency of the production process.

[0022] 5. Reduce the risk of test failure: By immersing the polished surface in a container filled with electrochemical coke and then performing heat treatment and controlling the grinding and polishing process of the polished surface after heat treatment, the probability of test failure caused by improper operation is reduced, saving time and material costs and improving overall work efficiency.

[0023] 6. The method of the present invention can be used to test the austenite grain size of B3 steel and other steels with high silicon contents, such as 27SiMn and 30CrMnSi. It can also be used to test the austenite grain size of carbon steel and alloy steels with a normal silicon content and a carbon content of 0.25% to 0.60%. This method solves a series of problems in austenite grain size display caused by the purchase restrictions on picric acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a metallographic diagram of the decarburization morphology of the austenite grain size test surface of the sample displayed using the method in GB / T6394-2017.

[0025] Figure 2 1 is a metallographic diagram of the austenite grain size of B3 steel displayed by the method of the present invention in Example 1.

[0026] Figure 3 This is a metallographic diagram of the austenite grain size of 30CrMnSi steel displayed using the method of the present invention in Example 2.

[0027] Figure 4 This is a metallographic diagram of the austenite grain size of 42CrMo steel displayed using the method of the present invention in Example 3.

[0028] Figure 5 This is the metallographic diagram of the austenite grain size of 20Cr steel shown in Example 4 using the method of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0030] A method for preparing a sample for testing austenite grain size in steel by oxidation method comprises the following steps:

[0031] S1, grind and polish the test surface of the sample until there are no visible scratches, then immerse it in a container filled with electrochemical coke;

[0032] Grinding and polishing is to polish the test surface of the sample with 40#, 240#, 500#, and 1000# metallographic sandpaper in sequence, and then change the cloth and polish it with 1-2.5μm diamond polishing paste or polishing agent until there are no scratches visible to the naked eye.

[0033] The container is a straight cylindrical container made of iron or high-temperature resistant alloy (such as nickel-based alloy) with a sealed bottom and an open top. The diameter of the container is 30-40 mm and the height is 80-100 mm.

[0034] Electrochemical coke is made from calcined petroleum coke and is characterized by high fixed carbon content and low ash, volatile matter, and impurities such as sulfur, phosphorus, and nitrogen. Its composition is: w(C)>96%, volatile matter ≤1.0%, w(S)≤0.5%, and w(moisture) ≤0.55%. Electrochemical coke is dry, clean, and has a moderate particle size (1-5 mm). Due to its strong reducing properties, it can be used as a covering agent for sample preparation, effectively preventing the formation of a decarburized layer on polished surfaces during heat treatment. It is also easy to operate and environmentally friendly.

[0035] When placing the sample into the container, first add 5 to 10 mm thick electrochemical coke into the container, then place the polished surface of the sample perpendicular to the electrochemical coke layer on the bottom of the container, and then continue to add electrochemical coke into the container until the electrochemical coke completely covers the sample and fills the container, and the depth of the electrochemical coke layer covering the upper surface of the sample is not less than 30 mm.

[0036] S2. Place the container containing the sample and electrochemical coke into a heating furnace. Raise the temperature to 860±10℃ and keep it warm for 0.5 to 1 hour. Then take out the container and pour the sample and electrochemical coke onto the ground at the same time so that the polished surface of the sample is fully exposed to the air. Then quickly quench with water within 5 seconds.

[0037] S3, dividing the initially polished surface of the heat-treated and oxidized sample into two parts, and grinding and polishing one part;

[0038] Specifically: first, use a marker to divide the oxidized initial polished surface of the sample into two equal parts, then use 1000# metallographic sandpaper to polish one part until it appears metallic in color, and the black oxide film on the other part of the oxidized initial polished surface of the sample is still visible to the naked eye, and finally polish until there are no visible scratches on the metallic part.

[0039] S4: Etch the secondary polished surface of the sample with a 15% hydrochloric acid alcohol solution for 8-10 seconds, then clean it with analytical pure alcohol and finally blow dry it to fully display the austenite grain boundaries. Observe the grain boundary oxide network with a metallographic microscope, and determine the austenite grain size of the sample according to GB / T6394-2017.

[0040] Example 1

[0041] A method for preparing a sample for testing the austenite grain size of B3 steel by an oxidation method comprises the following steps:

[0042] 1) The inspection surface of the austenite grain size sample is polished in sequence with 40#, 240#, 500#, and 1000# metallographic sandpaper. Then 2.5μm polishing paste is evenly applied on the polishing cloth, and then polished on a polishing machine until there are no visible scratches on the polished surface.

[0043] 2) Add 10mm thick electrolytic coke into a container with a diameter of 30mm and a height of 100mm. Then place the polished surface of the sample vertically on the electrolytic coke layer in the container. Then continue to add electrolytic coke into the iron drum until the electrolytic coke completely covers the sample and fills the container. The depth of the electrolytic coke layer covering the upper surface of the sample is not less than 30mm.

[0044] 3) Place the container containing the sample and electrochemical coke into the heating furnace. Heat the furnace to 860±5℃ and keep it warm for 0.5 to 1 hour. Take out the container, pour the sample and electrochemical coke on the ground so that the polished surface of the sample is fully exposed to the air, and then quickly quench with water within 5 seconds.

[0045] 4) First, use a marker to divide the initial polished surface of the sample into two equal parts, then use 1000# metallographic sandpaper to gently grind until one part appears metallic color, but the black oxide film of the other part is still visible to the naked eye, and finally polish until there are no visible scratches on the metallic part.

[0046] 5) Etch the secondary polished surface of the sample with a 15% hydrochloric acid alcohol solution for 10 seconds to fully display the austenite grain boundaries. Observe the grain boundary oxide network with a metallographic microscope and determine the austenite grain size of the test sample in accordance with GB / T6394-2017.

[0047] Example 2

[0048] A method for preparing a sample for testing the austenite grain size of 30CrMnSi by an oxidation method comprises the following steps:

[0049] 1) The inspection surface of the austenite grain size sample is polished in sequence with 40#, 240#, 500#, and 1000# metallographic sandpaper. Then 2.5μm polishing paste is evenly applied on the polishing cloth, and then polished on a polishing machine until there are no visible scratches on the polished surface.

[0050] 2) Add 10mm thick electrolytic coke into a container with a diameter of 30mm and a height of 100mm. Then place the polished surface of the sample vertically on the electrolytic coke layer in the container. Then continue to add electrolytic coke into the iron drum until the electrolytic coke completely covers the sample and fills the container. The depth of the electrolytic coke layer covering the upper surface of the sample is not less than 30mm.

[0051] 3) Place the container containing the sample and electrochemical coke into the heating furnace. Heat the furnace to 890℃±10℃ and keep it warm for 0.5 to 1 hour. Take out the iron drum, pour the sample and electrochemical coke on the ground so that the polished surface of the sample is fully exposed to the air, and then quickly quench with water within 5 seconds.

[0052] 4) First, use a marker to divide the initial polished surface of the sample into two equal parts, then use 1000# metallographic sandpaper to gently grind until one part appears metallic color, but the black oxide film of the other part is still visible to the naked eye, and finally polish until there are no visible scratches on the metallic part.

[0053] 5) Etch the secondary polished surface of the sample with a 15% hydrochloric acid alcohol solution for 10 seconds to fully display the austenite grain boundaries. Observe the grain boundary oxide network with a metallographic microscope and determine the austenite grain size of the test sample in accordance with GB / T6394-2017.

[0054] Example 3

[0055] A method for preparing a sample for testing the austenite grain size of 42CrMo by oxidation method comprises the following steps:

[0056] 1) Sample: The test surface of the austenite grain size sample is polished in sequence with 40#, 240#, 500#, and 1000# metallographic sandpaper. Then, 2.5μm polishing paste is evenly applied on the polishing cloth, and then ground and polished on a polishing machine until there are no visible scratches on the test surface.

[0057] 2) Add 10mm thick electrolytic coke into a container with a diameter of 30mm and a height of 100mm. Then place the polished surface of the sample vertically on the electrolytic coke layer in the container. Then continue to add electrolytic coke into the iron drum until the electrolytic coke completely covers the sample and fills the container. The depth of the electrolytic coke layer covering the upper surface of the sample is not less than 30mm.

[0058] 3) Place the container containing the sample and electrochemical coke into the heating furnace. Heat the furnace to 860℃±10℃ and keep it warm for 0.5 to 1 hour. Take out the container, pour the sample and electrochemical coke on the ground so that the polished surface of the sample is fully exposed to the air, and then quickly quench with water within 5 seconds.

[0059] 4) First, use a marker to divide the initial polished surface of the sample into two equal parts, then use 1000# metallographic sandpaper to gently grind until one part appears metallic color, but the black oxide film of the other part is still visible to the naked eye, and finally polish until there are no visible scratches on the metallic part.

[0060] 5) Etch the secondary polished surface of the sample with a 15% hydrochloric acid alcohol solution for 10 seconds to fully display the austenite grain boundaries. Observe the grain boundary oxide network with a metallographic microscope and determine the austenite grain size of the test sample in accordance with GB / T6394-2017.

[0061] Example 4

[0062] A method for preparing a sample for testing the grain size of 20Cr austenite by an oxidation method comprises the following steps:

[0063] 1) Sample: The test surface of the austenite grain size sample is polished in sequence with 40#, 240#, 500#, and 1000# metallographic sandpaper. Then, 2.5μm polishing paste is evenly applied on the polishing cloth, and then ground and polished on a polishing machine until there are no visible scratches on the test surface.

[0064] 2) Add 10mm thick electrolytic coke into a container with a diameter of 30mm and a height of 100mm. Then place the polished surface of the sample vertically on the electrolytic coke layer in the container. Then continue to add electrolytic coke into the iron drum until the electrolytic coke completely covers the sample and fills the container. The depth of the electrolytic coke layer covering the upper surface of the sample is not less than 30mm.

[0065] 3) Place the container containing the sample and electrochemical coke into the heating furnace. Heat the furnace to 890℃±10℃ and keep it warm for 0.5 to 1 hour. Take out the container, pour the sample and electrochemical coke on the ground so that the polished surface of the sample is fully exposed to the air, and then quickly quench with water within 5 seconds.

[0066] 4) First, use a marker to divide the initial polished surface of the sample into two equal parts, then use 1000# metallographic sandpaper to gently polish until one part appears metallic color, but the black oxide film of the other part is still visible to the naked eye, and finally polish until there are no visible scratches on the metallic part.

[0067] 5) Etch the secondary polished surface of the sample with a 15% hydrochloric acid alcohol solution for 10 seconds to fully display the austenite grain boundaries. Observe the grain boundary oxide network with a metallographic microscope and determine the austenite grain size of the test sample in accordance with GB / T6394-2017.

[0068] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing a sample for testing austenite grain size in steel by oxidation method, characterized in that: The following steps are involved: S1, grind and polish the test surface of the sample until there are no visible scratches, then immerse it in a container filled with electrochemical coke; S2. Place the container containing the sample and electrochemical coke into a heating furnace. Raise the temperature to 860±10℃ and keep it warm for 0.5 to 1 hour. Then take out the container and pour the sample and electrochemical coke onto the ground at the same time so that the polished surface of the sample is fully exposed to the air. Then quickly quench with water within 5 seconds. S3, dividing the initially polished surface of the heat-treated and oxidized sample into two parts, and grinding and polishing one part; S4, use 15% hydrochloric acid alcohol solution to erode the secondary polished surface of the sample for 8 to 10 seconds, then clean it with analytical pure alcohol and finally blow it dry.

2. The method for preparing a sample for testing austenite grain size in steel by oxidation method according to claim 1, characterized in that: In step S1, the test surface of the sample is polished with 40#, 240#, 500#, and 1000# metallographic sandpaper in sequence, and then polished with tweed cloth and 1-2.5μm diamond polishing paste or polishing agent until there are no visible scratches.

3. The method for preparing a sample for testing austenite grain size in steel by oxidation method according to claim 1, characterized in that: The container is a straight cylindrical container made of iron or high-temperature resistant alloy with a sealed bottom and an open top; the diameter of the container is 30-40 mm and the height is 80-100 mm.

4. The method for preparing a sample for testing austenite grain size in steel by oxidation method according to claim 1, characterized in that: In step S1, a 5-10 mm thick layer of electrified coke is first added to the container, and then the polished surface of the sample is placed perpendicular to the electrified coke layer on the bottom of the container. Then, electrified coke is continued to be added to the container until the electrified coke completely covers the sample and fills the container, and the electrified coke layer covering the upper surface of the sample is not less than 30 mm deep.

5. The method for preparing a sample for testing austenite grain size in steel by oxidation method according to claim 1, characterized in that: The indicators of the electrochemical coke are: w(C)>96%, volatile matter≤1.0%, w(S)≤0.5%, w(water)≤0.55%; and the particle size of the electrochemical coke is 1-5 mm.

6. The method for preparing a sample for testing austenite grain size in steel by oxidation method according to claim 1, characterized in that: In step S3, the oxidized primary polished surface of the sample is first divided into two equal parts with a marker pen, and then polished with 1000# metallographic sandpaper until one part appears metallic in color, and the black oxide film on the other part of the oxidized primary polished surface of the sample is still visible to the naked eye. Finally, polish until there are no visible scratches on the metallic part.

7. The method for preparing a sample for testing austenite grain size in steel by oxidation method according to claim 1, characterized in that: In step S4, the sample is etched with a 15% hydrochloric acid alcohol solution for the second polishing surface to fully reveal the austenite grain boundaries, the grain boundary oxide network is observed with a metallographic microscope, and the grain size of the sample austenite is measured in accordance with GB / T6394-2017.

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