A heat treatment process for measuring the decarburization layer of water-penetrating bearing steel

Abstract

The present invention relates to the technical field of bearing steel decarburization layer depth detection, and in particular to a heat treatment process for measuring the decarburization layer of water-penetrating bearing steel. The process comprises the following steps: sampling the water-penetrating bearing steel, placing the sample into a carburizing furnace for quantitative carburization treatment, grinding, polishing, and etching the surface carburized sample transversely, and then observing the prepared sample through an optical microscope; determining the decarburization layer depth of the water-penetrating bearing steel by observing the reduction of carbide content relative to the matrix; the present invention transforms the water-penetrating structure (tempered martensite structure) into an equilibrium structure pearlite structure or pearlite + carbide structure by performing quantitative carburization heat treatment on the sample with a carbon potential of 0.70%±0.05%, thereby facilitating subsequent measurement of the decarburization layer depth, providing strong data support for production, and improving production efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of bearing steel decarburization layer depth detection, in particular to a heat treatment process for measuring the decarburization layer of water-penetrating bearing steel. Background Art

[0002] Reticular carbides form when high-carbon chromium bearing steel cools after rolling in the Acm-Ar1 temperature range. Excessive carbon concentrations precipitate as carbides along austenite grain boundaries, surrounding the austenite grains and forming a network-like structure under a microscope. These carbides exhibit virtually zero plasticity and toughness, and are extremely brittle. To control the formation of reticular carbides, many bearing steel manufacturers subject the rolled bearing steel to a water-penetrating treatment during production. This treatment causes martensite to form on the surface of the specimen. As rolling continues, the surface temperature rises, causing the previously formed martensite to auto-temper, forming tempered martensite. The GB / T 224-2019 standard stipulates that metallographic measurement of decarburization is based on the structural changes from the surface to the matrix as the carbon content changes. High-carbon chromium bearing steel is a hypereutectoid steel, and decarburization is determined by the decrease in carbide content relative to the matrix. However, specimens with tempered martensite on the surface cannot be metallographically determined for decarburization depth.

[0003] In the prior art, Chinese patent application number 201410532941.X provides a method for quickly measuring the decarburization layer after water penetration, which includes the following steps: (1) sampling, preparing, etching (immersing in 3% nitric acid alcohol solution for 12±5 seconds), and observing the bearing steel after water penetration; (2) directly measuring the tempered martensite layer and the transition layer of 1 / 2 of the water-penetrating bearing steel. The sum of the depths of the two regions is the decarburization layer depth of the water-penetrating bearing steel; (3) then using the decarburization layer depth of the water-penetrating bearing steel after normalizing to verify whether the decarburization layer depth measured by the above measurement method is consistent. This technical solution can only indicate that the decarburization layer depth of this batch of water-penetrating bearing steel after normalizing is exactly consistent with the result measured by its measurement method. If the water-penetrating bearing steel is deeper after the water penetration ring, the carbon content in the water penetration layer is already consistent with the carbon content of the bearing steel matrix, and this method cannot be used for measurement, resulting in inaccurate decarburization layer depth measurement. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: in order to overcome the problem in the prior art that tempered martensite structure is formed on the surface of bearing steel after water penetration treatment, which makes it impossible to detect the depth of the decarburization layer by metallographic method, a heat treatment process for measuring the decarburization layer of water-penetrating bearing steel is provided.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solution: a heat treatment process for measuring the decarburized layer of water-penetrating bearing steel, comprising the following steps:

[0006] S1. Sampling of water-penetrating bearing steel rolled products: intercepting and cutting to obtain samples in accordance with the sampling requirements specified in GB / T 224-2019;

[0007] S2. Place the cut sample into the carburizing furnace for quantitative carburizing treatment: After the sample is heated to 755℃, nitrogen and methanol are introduced, and the ratio of the two is adjusted to 1:0.9 by the flow meter. The temperature of 755℃ is the temperature at which methanol is completely decomposed, and nitrogen is introduced at a rate of "m 3 / h" is used as the unit, and methanol is used as the unit of "L / h". Under high temperature environment, it decomposes into 15.8% CO, 52.6% N2, and 31.6% H2 according to this ratio. The carbon potential in the furnace is measured by the oxygen probe to be 0.70%±0.05% at this time. The temperature is raised to 925℃±2℃ and kept warm for 3h; it is cooled to 880℃±2℃ with the furnace and kept warm for 2h; it is then cooled to 850℃±2℃ with the furnace, kept warm for 0.5h, and then air-cooled after being taken out of the furnace; the carbon potential is controlled at 0.70%±0.05% to ensure that the water-penetrating area with a carbon content lower than this carbon potential of the bearing steel can be carburized to this carbon potential range. It is impossible to produce network carbides at a carbon potential of about 0.75%; and the water-penetrating area with a carbon content higher than this carbon potential cannot be carburized, only the microstructure morphology is changed, and the normal microstructure of the matrix remains unchanged.

[0008] S3. Prepare the sample after heat treatment: the cross section is subjected to rough grinding → fine grinding → polishing → etching;

[0009] S4. Place the eroded sample under an optical microscope for observation, and measure the depth of the decarburized layer on the sample surface, which is the depth of the decarburized layer on the surface of the water-penetrating bearing steel.

[0010] The above technical solution utilizes quantitative carburizing heat treatment on the sample, and the carbon potential is 0.70% ± 0.05%, ensuring that the water-penetrating area of the bearing steel with a carbon content lower than this carbon potential can be carburized within this carbon potential range. A carbon potential of about 0.75% will not produce network carbides, while the water-penetrating area with a carbon content higher than this carbon potential cannot be carburized. Only the tissue morphology is changed, and the normal structure of the matrix remains unchanged. The quantitative carburizing heat treatment transforms the water-penetrating structure (tempered martensite structure) into a balanced structure pearlite structure or pearlite + carbide structure, which facilitates the subsequent measurement of the decarburization layer depth, provides strong data support for production, and improves production efficiency.

[0011] Furthermore, in step S1, a grinding wheel cutter is used to prepare the sample, and the cutting fluid needs to be kept to cool the sample at all times during the sample processing process to prevent the sample from being burned and affecting decarburization.

[0012] Furthermore, in step S3, the heat-treated sample is coarsely ground by a spectral grinding machine, and then finely ground by 180-mesh, 600-mesh, and 1000-mesh metallographic sandpaper in sequence, and finally polished. The entire process requires cooling with water to prevent the grinding surface of the sample from overheating.

[0013] Furthermore, in step S3, the etchant is a 2% nitric acid alcohol solution, and the polished surface of the sample is immersed in the etchant for about 20 seconds. During the etch process, the sample is slightly shaken to maintain etch uniformity.

[0014] The beneficial effects of the present invention are as follows: the present invention samples the decarburized layer of the water-penetrating bearing steel, and then performs a quantitative carburizing heat treatment on the sample (carbon potential 0.70%±0.05%), and ensures that the water-penetrating area of the bearing steel with a carbon content lower than this carbon potential can be carburized to within this carbon potential range, and a carbon potential of about 0.75% will not produce network carbides, while the water-penetrating area with a carbon content higher than this carbon potential cannot be carburized, only the tissue morphology is changed, and the normal matrix tissue remains unchanged. The quantitative carburizing heat treatment transforms the water-penetrating tissue (tempered martensite tissue) into an equilibrium tissue pearlite tissue or pearlite+carbide tissue, which facilitates the subsequent measurement of the decarburized layer depth, provides strong data support for production, and improves production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will be further described below with reference to the accompanying drawings and examples.

[0016] Figure 1 This is a photograph of the depth of the decarburized layer of water-penetrating bearing steel after quantitative carburizing heat treatment observed under a microscope in Example 1 of the present invention;

[0017] Figure 2 This is a photo of the depth morphology of the decarburized layer of the water-penetrating bearing steel after normalizing heat treatment observed under a microscope in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0018] The present invention will now be described in further detail with reference to the accompanying drawings.

[0019] Example 1:

[0020] The present invention provides a heat treatment process for measuring the decarburized layer of water-penetrating bearing steel, comprising the following steps:

[0021] S1. Sampling of water-penetrating bearing steel rolled products: Samples are cut and intercepted according to the sampling requirements specified in GB / T 224-2019. A grinding wheel cutter is used for cutting. During the sample processing, the cutting fluid must be kept cooling at all times to prevent sample burns.

[0022] S2. Place the cut sample into the carburizing furnace for quantitative carburizing treatment: After the sample is heated to 755℃, nitrogen and methanol are introduced and the ratio of the two is adjusted to 1:0.9 by the flow meter. Nitrogen is at "m 3 / h" is used as the unit, and methanol is used as the unit of "L / h". Under high temperature environment, it decomposes into 15.8% CO, 52.6% N2, and 31.6% H2 according to this ratio. The carbon potential in the furnace is measured by the oxygen probe to be 0.70%±0.05% at this time. The temperature is raised to 925℃±2℃ and kept warm for 3h; it is cooled to 880℃±2℃ with the furnace and kept warm for 2h; it is then cooled to 850℃±2℃ with the furnace, kept warm for 0.5h, and then air-cooled after being taken out of the furnace; the carbon potential is controlled at 0.70%±0.05% to ensure that the water-penetrating area with a carbon content lower than this carbon potential of the bearing steel can be carburized to this carbon potential range. It is impossible to produce network carbides at a carbon potential of about 0.75%; and the water-penetrating area with a carbon content higher than this carbon potential cannot be carburized, only the microstructure morphology is changed, and the normal microstructure of the matrix remains unchanged.

[0023] S3. Prepare the sample after heat treatment: the cross section is subjected to coarse grinding → fine grinding → polishing → etching. The heat-treated sample is coarsely ground with a spectral grinding machine, then finely ground with 180 mesh, 600 mesh, and 1000 mesh metallographic sandpaper in sequence, and finally polished. The entire process requires water cooling to prevent the sample grinding surface from overheating. The etchant is 2% nitric acid alcohol solution. The polished surface of the sample is immersed in the etchant for about 20 seconds. The sample is slightly shaken during the etching process to maintain etching uniformity.

[0024] S4. Place the eroded sample under an optical microscope for observation and measure the depth of the decarburized layer on the sample surface, which is the depth of the decarburized layer on the surface of the water-penetrating bearing steel. The decarburized layer depth morphology photo is shown in Figure 1 .

[0025] Comparative Example 1:

[0026] A method for detecting the depth of a decarburized layer of water-penetrating bearing steel after normalizing heat treatment comprises the following steps:

[0027] S1. Sampling of water-penetrating bearing steel rolled products: intercepting and cutting to obtain samples in accordance with the sampling requirements specified in GB / T 224-2019;

[0028] S2. Place the cut sample into a heat treatment furnace, heat the sample to 870°C, keep it at this temperature for 1 hour, and then take it out of the furnace and air cool it.

[0029] S3. Prepare the sample after heat treatment: the cross section is subjected to rough grinding → fine grinding → polishing → etching;

[0030] S4. Place the eroded sample under an optical microscope for observation and measure the depth of the decarburized layer on the sample surface, which is the depth of the decarburized layer on the surface of the water-penetrating bearing steel. The decarburized layer depth morphology photo is shown in Figure 2 .

[0031] The above description of the preferred embodiments of the present invention is intended to serve as a guide. Based on the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A heat treatment process for measuring the decarburization layer of water-penetrating bearing steel, characterized by: The following steps are involved: S1. Sampling of water-penetrating bearing steel rolled products: intercepting and cutting to obtain samples in accordance with the sampling requirements specified in GB / T 224-2019; S2. Place the cut sample into the carburizing furnace for quantitative carburizing treatment: After the sample is heated to 755℃, nitrogen and methanol are introduced in a ratio of 1:0.

9. Nitrogen is heated at a rate of "m 3 / h" is used as the unit, methanol is used as "L / h". At this time, the carbon potential in the furnace is 0.70%±0.05%. Heat to 925℃±2℃ and keep warm for 3h; cool with the furnace to 880℃±2℃ and keep warm for 2h; then cool with the furnace to 850℃±2℃, keep warm for 0.5h and then take out of the furnace for air cooling; S3. Prepare the sample after heat treatment: the cross section is subjected to rough grinding → fine grinding → polishing → etching; S4. Place the eroded sample under an optical microscope for observation and measure the depth of the decarburized layer on the sample surface.

2. A heat treatment process for measuring the decarburized layer of water-penetrating bearing steel according to claim 1, characterized in that: In step S1, a grinding wheel cutter is used to prepare the sample. During the sample processing, the cutting fluid must be kept to cool the sample at all times to prevent the sample from being burned.

3. The heat treatment process for measuring the decarburized layer of water-penetrating bearing steel according to claim 1, characterized in that: In step S3, the heat-treated sample is coarsely ground by a spectral grinding machine, then finely ground by 180-mesh, 600-mesh, and 1000-mesh metallographic sandpaper in sequence, and finally polished. The entire process requires water cooling to prevent the sample grinding surface from overheating.

4. The heat treatment process for measuring the decarburized layer of water-penetrating bearing steel according to claim 1, characterized in that: In step S3, the etchant is a 2% nitric acid alcohol solution, and the polished surface of the sample is immersed in the etchant for 20 seconds. During the etch process, the sample is slightly shaken to maintain etch uniformity.

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