Method for displaying austenite grain boundaries of low carbon bearing steel

Abstract

The application discloses a method for displaying austenite grain boundaries of low-carbon bearing steel, and belongs to the technical field of physical and chemical detection of bearing materials. First, more second phases prone to corrosion are generated on the grain boundaries by adopting oxidation pretreatment, and the element segregation of the grain boundaries is intensified, so that the difference in corrosion resistance between the grain boundaries and the intracrystalline matrix is enhanced, and then the original austenite grain boundaries are displayed by adopting a chemical corrosion method. Specifically, (1) the sample after mechanical polishing treatment is subjected to oxidation treatment at 600-720 DEG C for 0.5-4 h; (2) the oxidation layer is removed by adopting mechanical grinding and polishing methods; (3) the sample is soaked in a corrosion agent containing 10 g of FeCl3, 40 ml of HCl, 40 ml of water and 0.3-0.8 g of sodium dodecyl benzene sulfonate; and (4) the sample is rinsed, dried and observed for the austenite grain boundaries. The method combining oxidation pretreatment and chemical corrosion is adopted, so that the corrosion contrast between the austenite grain boundaries and the intracrystalline matrix of the low-carbon bearing steel is more obvious.

Description

Technical Field

[0001] This invention relates to the field of physical and chemical testing technology for bearing materials, specifically to a method for displaying austenite grain boundaries in low-carbon bearing steel. Background Technology

[0002] Low-carbon bearing steel is widely used in my country due to its excellent microstructure and properties. Its typical chemical composition (weight percentage) is as follows: C: 0.19-0.23%, Si: 0.25-0.40%, Mn: 0.55-0.70%, Cr: 0.45-0.65%, Ni: 1.60-2.00%, Mo: 0.20-0.30%, Cu: ≤0.25%, P: ≤0.030%, S: ≤0.030%, with the balance being Fe.

[0003] Grain size is a crucial indicator of a material's overall performance, significantly impacting its strength, toughness, plasticity, and machinability. The presence of austenitic grain boundaries directly affects grain size statistics. For low-carbon bearing steel, the most commonly used etchant is picric acid + anhydrous ethanol + hydrochloric acid. However, the corrosion resistance difference between grain boundaries and intragranular structures in low-carbon bearing steel is relatively small, resulting in a low corrosion contrast and making the corrosion process difficult to control. Direct chemical etching easily leads to problems where grain boundaries and intragranular structures are either simultaneously uncorroded or corroded. Therefore, developing methods to improve the corrosion contrast between austenitic grain boundaries and intragranular structures in low-carbon bearing steel is meaningful. Summary of the Invention

[0004] The purpose of this invention is to provide a method for displaying austenite grain boundaries in low-carbon bearing steel. This method solves the problem that the austenite grain boundaries in low-carbon bearing steel are unclear due to the small difference in corrosion resistance between grain boundaries and intragranular structures.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for revealing austenitic grain boundaries in low-carbon bearing steel. This method first employs an oxidation pretreatment to form more easily corroded second phases on the grain boundaries and intensify elemental segregation at the grain boundaries, thereby enhancing the difference in corrosion resistance between the grain boundaries and the microstructure within them. Then, a chemical etching method is used to reveal the original austenitic grain boundaries. The method specifically includes the following steps:

[0007] (1) Sample preparation: Low carbon bearing steel was wire-cut to obtain samples, and then quenched. The quenched low carbon bearing steel samples were mechanically ground and polished, then the surface was cleaned with anhydrous ethanol and dried with a hair dryer to obtain the sample to be analyzed;

[0008] (2) Oxidation treatment: The low carbon bearing steel is kept at a temperature range (600℃~720℃) below the Ac1 temperature for 0.5h~4h, and the cooling method is air cooling;

[0009] (3) Sample surface treatment: The oxidized sample is mechanically ground and polished. During mechanical grinding, the grinding thickness of the sample surface must be controlled. Grinding should be stopped when a metallic luster is seen on the sample surface to ensure that only the thin gray-black oxide layer on the surface is removed. If the oxide layer is thin after oxidation treatment, mechanical grinding can be skipped and mechanical polishing can be performed directly.

[0010] (4) Chemical etching: Add 10g FeCl3, 0.3-0.8g sodium dodecylbenzenesulfonate, 40ml water and 40ml concentrated HCl (HCl concentration 36-38wt.%) to a beaker in sequence, and then stir thoroughly with a glass rod to obtain the etching agent. After drying the surface-treated sample, immerse it in the etching agent for 10-60s;

[0011] (5) Grain boundary observation: The chemically etched sample is first cleaned with water and then with anhydrous ethanol, then dried with a hair dryer, and finally the austenitic grain boundaries are observed using an optical microscope.

[0012] The beneficial effects of this invention are as follows:

[0013] The commonly used chemical etchant for bearing materials is a saturated trinitrophenol solution (picric acid) + anhydrous ethanol + hydrochloric acid. However, because the difference in corrosion resistance between the grain boundaries and the microstructure within the grain boundaries of low-carbon bearing steel is small, direct etching with this etchant results in low contrast and unclear austenite grain boundaries. Furthermore, the saturated trinitrophenol solution (picric acid) is unstable and has a certain potential for explosion during the etching process. Compared with traditional chemical etching methods, this invention uses oxidation pretreatment to precipitate an easily corroded second phase at the austenite grain boundaries of low-carbon bearing steel, exacerbating the segregation of grain boundary elements and enhancing the difference in corrosion resistance between the grain boundaries and the microstructure within the grain boundaries. Subsequent chemical etching then more easily reveals clear and complete austenite grain boundaries. This invention solves the problems of poor etching effect and unstable etchant in traditional etching methods for low-carbon bearing steel.

[0014] This invention provides a method for displaying austenitic grain boundaries in low-carbon bearing steel. Attached Figure Description

[0015] Figure 1 The austenitic grain boundaries of low-carbon bearing steel after oxidation at 660℃ for 3.5 hours and subsequent corrosion by a corrosive agent.

[0016] Figure 2 The austenitic grain boundaries of low-carbon bearing steel after oxidation at 720℃ for 2 hours and subsequent corrosion by a corrosive agent.

[0017] Figure 3 The austenitic grain boundaries of low-carbon bearing steel are obtained by directly etching with picric acid without oxidation.

[0018] Figure 4 The austenitic grain boundaries of the low-carbon bearing steel were obtained by direct FeCl3 etching without oxidation. Detailed Implementation

[0019] To further understand the present invention, the present invention is described below with reference to examples. However, the examples are only for further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention.

[0020] Example 1:

[0021] This embodiment provides a method for displaying austenite grain boundaries in low-carbon bearing steel, including the following steps:

[0022] (1) Sample preparation:

[0023] Low-carbon bearing steel samples meeting national standards were obtained by molybdenum wire cutting and then quenched. The quenching process involved holding the steel at 860℃ for 10 hours followed by oil quenching. The quenched bearing steel samples were then mechanically ground and polished, cleaned with water and anhydrous ethanol, and dried with a blower to obtain the samples for analysis.

[0024] (2) Oxidation treatment:

[0025] The oxidation temperature below the Ac1 temperature of low carbon bearing steel is selected as 660℃, the holding time is 3.5h, and the cooling method is air cooling.

[0026] (3) Sample surface treatment:

[0027] The oxidized samples are then subjected to mechanical grinding and polishing. During mechanical grinding, the grinding thickness needs to be controlled. Grinding is stopped when the thin, grayish-black oxide layer on the sample surface is removed, revealing a metallic luster. The sample is then mechanically polished.

[0028] (4) Chemical corrosion:

[0029] Add 10g FeCl3, 0.5g sodium dodecylbenzenesulfonate, 40ml deionized water, and 40ml concentrated HCl (HCl concentration 36-38 wt.%) to a beaker in sequence, and stir thoroughly with a glass rod to obtain the etchant. After drying the surface-treated sample, immerse it in the etchant for about 30 seconds.

[0030] (5) Grain boundary observation:

[0031] The etched samples were first cleaned with water and then with anhydrous ethanol, followed by drying with a hair dryer. Finally, the austenite grain boundaries were observed using a metallographic microscope. The etching effect is shown in the attached figure. Figure 1 As shown.

[0032] Example 2:

[0033] This embodiment describes a method for displaying austenite grain boundaries in low-carbon bearing steel, including the following steps:

[0034] (1) Sample preparation:

[0035] Low-carbon bearing steel with a composition meeting national standards was sampled by molybdenum wire cutting and then quenched. The quenching process involved holding the sample at 900℃ for 45 minutes, then at 820℃ for 20 minutes, followed by oil quenching. The quenched samples were then mechanically ground and polished. The surfaces were then cleaned with water and anhydrous ethanol, and finally dried with a blower to obtain the samples for testing.

[0036] (2) Oxidation treatment:

[0037] The oxidation temperature below the Ac1 temperature of low carbon bearing steel is selected as 720℃, the holding time is 2h, and the cooling method is air cooling.

[0038] (3) Sample surface treatment:

[0039] The oxidized samples are then subjected to mechanical grinding and polishing. During mechanical grinding, the grinding thickness needs to be controlled. Grinding is stopped when the thin, grayish-black oxide layer on the sample surface is removed, revealing a metallic luster. The sample is then mechanically polished.

[0040] (4) Chemical corrosion:

[0041] Add 10g FeCl3, 0.5g sodium dodecylbenzenesulfonate, 40ml deionized water, and 40ml concentrated HCl (HCl concentration 36-38 wt.%) to a beaker in sequence, and stir thoroughly with a glass rod to obtain the etchant. After drying the surface-treated sample, immerse it in the etchant for about 30 seconds.

[0042] (5) Grain boundary observation:

[0043] The etched samples were first cleaned with water and then with anhydrous ethanol, followed by drying with a hair dryer. Finally, the austenite grain boundaries were observed using a metallographic microscope. The etching effect is shown in the attached figure. Figure 2 As shown.

[0044] Comparative Example 1:

[0045] This example demonstrates a method for directly corroding the austenite grain boundaries of low-carbon bearing steel using picric acid, anhydrous ethanol, and hydrochloric acid. The method includes the following steps:

[0046] (1) Sample preparation

[0047] Low-carbon bearing steel whose composition meets national standards was sampled using molybdenum wire cutting, and then quenched. The quenching process involved holding the sample at 900℃ for 45 minutes, then at 820℃ for 20 minutes, followed by oil quenching. The quenched samples were then mechanically ground and polished.

[0048] (2) Sample corrosion:

[0049] Add 1g of picric acid, 1ml of hydrochloric acid, and 38ml of anhydrous ethanol to a beaker in sequence, and stir thoroughly with a glass rod to obtain the etching agent. Immerse the polished sample in the etching agent for about 15 seconds, and remove it when the sample surface turns grayish-black.

[0050] (3) Grain boundary observation:

[0051] The etched samples were first cleaned with water and then with anhydrous ethanol, followed by drying with a hair dryer. Finally, the austenite grain boundaries were observed using a metallographic microscope. The etching effect is shown in the attached figure. Figure 3 As shown.

[0052] Comparative Example 2:

[0053] This example demonstrates a method for directly corroding the austenitic grain boundaries of low-carbon bearing steel using FeCl3 + sodium dodecylbenzenesulfonate + deionized water + hydrochloric acid, including the following steps:

[0054] (1) Sample preparation

[0055] Low-carbon bearing steel whose composition meets national standards was sampled using molybdenum wire cutting, and then quenched. The quenching process involved holding the sample at 900℃ for 45 minutes, then at 820℃ for 20 minutes, followed by oil quenching. The quenched samples were then mechanically ground and polished.

[0056] (2) Sample corrosion:

[0057] Add 10g FeCl3, 0.5g sodium dodecylbenzenesulfonate, 40ml deionized water, and 40ml HCl to a beaker in sequence, and stir thoroughly with a glass rod to obtain the etching agent. After the polished sample is dried, immerse it in the etching agent for about 30 seconds.

[0058] (3) Grain boundary observation:

[0059] The etched samples were first cleaned with water and then with anhydrous ethanol, followed by drying with a hair dryer. Finally, the austenite grain boundaries were observed using a metallographic microscope. The etching effect is shown in the attached figure. Figure 4 As shown.

[0060] The method provided in this invention enhances the difference in corrosion resistance between grain boundaries and intragranular structures in low-carbon bearing steel through oxidation pretreatment. Therefore, the light contrast of the sample is improved after etching with the etchant, and the original austenite grain boundaries are clearly displayed, as shown below. Figure 1 and Figure 2 As shown.

Claims

1. A method for displaying austenitic grain boundaries in low-carbon bearing steel, characterized in that: This method first employs an oxidation pretreatment to form more easily corroded second phases on the grain boundaries and intensifies element segregation at the grain boundaries, thereby enhancing the difference in corrosion resistance between the grain boundaries and the microstructure within the grain boundaries; then, a chemical etching method is used to reveal the original austenitic grain boundaries. The method includes the following steps: (1) Sample preparation: First, the low carbon bearing steel is quenched, then the sample is mechanically ground and polished, and then the surface is cleaned and dried with anhydrous ethanol to obtain the sample to be analyzed. (2) Oxidation pretreatment: The sample to be analyzed is kept in dry air at a temperature of 720℃ for 0.5 to 4 hours and cooled by air cooling. (3) Sample surface treatment: After oxidation pretreatment, the oxide layer of the sample is removed by mechanical grinding and polishing in sequence; Alternatively, the oxide layer can be removed directly from the sample after oxidation pretreatment by polishing. (4) Chemical etching: After drying the sample that has undergone surface treatment in step (3), immerse it in the etchant for 10-60 seconds; the etchant is composed of FeCl3, sodium dodecylbenzenesulfonate, water and HCl; (5) Grain boundary observation: The etched sample is first cleaned with water and then with anhydrous ethanol, then dried with a hair dryer, and finally its austenite grain boundaries are observed. In step (4), the corrosive agent is obtained by sequentially adding 10g FeCl3, 0.3-0.8g sodium dodecylbenzenesulfonate, 40ml water, and 40ml concentrated HCl to a beaker, followed by thorough stirring. In step (2), the selected oxidation pretreatment temperature is 720℃ to avoid recrystallization and ensure that the oxidation pretreatment does not affect the austenite grain size of the steel. In step (2), the sample oxidation pretreatment is carried out at a temperature of 0.5 h to 4 h to allow the sample to undergo sufficient oxidation.

2. The method for displaying austenitic grain boundaries in low-carbon bearing steel according to claim 1, characterized in that: In step (3), the sample after surface treatment needs to be mechanically ground and polished. The grinding thickness needs to be controlled during grinding to ensure that only the thin gray-black oxide layer on the sample surface is ground off. Grinding should be stopped when the sample has a metallic luster. Polishing should be stopped when the sample has a metallic luster.

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