Carburizing bearing steel and heat treatment method thereof

Abstract

The invention relates to the technical field of machining heat treatment, in particular to carburized bearing steel and a heat treatment method thereof.The carburized bearing steel is in the thickness direction and 0.1 mm away from the surface of the carburized bearing steel, the volume fraction of retained austenite is 3-10%, the volume fraction of carbide is 20-35%, and the length of acicular martensite is 5-7 microns; the carbide with the diameter smaller than 1 micrometer accounts for 65-75% of the total number of the carbide, the carbide with the diameter smaller than 1 micrometer is spherical or granular, and the carburizing bearing steel avoids excessive growth of the carbide on the surface and is excellent in corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of machining and heat treatment technology, specifically to a carburized bearing steel and its heat treatment method. Background Technology

[0002] High-alloy carburizing steel, as a new type of bearing and gear steel, forms a gradient microstructure from the surface to the core after a series of heat treatments such as carburizing and quenching. The surface microstructure exhibits high hardness, high wear resistance, and high temperature resistance, while the core has good toughness and can withstand high-load impacts. Although increasing the surface carbon content helps to increase the surface carbide content and significantly improve surface hardness, high-alloy steel is prone to excessive retained austenite or coarse carbides, or even network carbides, under high-carbon concentration carburizing conditions, which reduces hardness and fatigue strength. At the same time, excessively high carbon content also causes a sharp decline in the material's corrosion resistance.

[0003] For critical components such as gears and bearings, the morphology, distribution, size, and quantity of carbides in the carburized layer are crucial for improving the service performance of these parts. High-alloy steels, with an alloy content exceeding 30 wt%, contain a large number of carbide-forming elements. After carburizing, a large number of carbides are formed in the carburized layer, making the optimization of carbide morphology a significant challenge. Currently, the conventional heat treatment process of carburizing + quenching + low-temperature tempering is usually adopted to obtain a martensitic + carbide structure, making the carbides fine, round and dispersed, the surface martensite structure fine and the austenite content reduced. However, this process is obviously not suitable for this high alloy carburizing steel. The existing carburizing heat treatment process for carburizing steel is carburizing + quenching + deep cooling + tempering. Chinese patent CN201811480006.8 discloses a low-pressure vacuum carburizing method for CSS-42L material, Chinese patent CN201811396268.6 discloses a nitriding method after carburizing for third-generation carburizing steel, and Chinese patent CN201611097539.9 discloses a softening method for the carburized area after carburizing CSS-42L gear steel. In the end, the surface of the workpiece still has strip-shaped, irregular, or even network-shaped carbides, so the effect of the carburizing process is not maximized. The existing process lacks optimization of the carbide structure.

[0004] To address the problem that increasing the surface carbon concentration in high-alloy carburized steel increases surface hardness but leads to excessive growth of surface carbide particles and a decrease in corrosion resistance, this invention is proposed. This process improves the morphology and microstructure of carbides in the carburized layer, while simultaneously achieving carbide spheroidization and reducing the content of retained austenite. The process parameters are precisely controlled locally in the carburized layer to avoid a decrease in core hardness, thereby maximizing hardness under the same carbon concentration or carburizing process and preventing excessive growth of surface carbides and a significant decrease in corrosion resistance.

[0005] Therefore, there is an urgent need to provide a high-alloy carburizing steel and its heat treatment method that avoids excessive growth of surface carbides and improves corrosion resistance. Summary of the Invention

[0006] The present invention aims to solve the technical problem of how to provide a high-alloy carburizing steel that avoids excessive growth of surface carbides and improves corrosion resistance, as well as a heat treatment method thereof.

[0007] To achieve the above objectives, a first aspect of the present invention provides a carburized bearing steel, wherein the components and their weight percentages in the carburized bearing steel are as follows:

[0008] The C content is 0.10-0.16%;

[0009] The Cr content is 13.75-15.00%;

[0010] The Co content is 5.00-14.00%;

[0011] The Ni content is 4.75-5.25%;

[0012] The Mo content is 3.00-5.00%;

[0013] Si content ≤ 0.05%;

[0014] Mn content ≤ 0.05%;

[0015] S content ≤ 0.01%;

[0016] P content ≤ 0.015%;

[0017] The content of other impurity elements is ≤0.03%;

[0018] The total content of other impurity elements is ≤0.15%;

[0019] The balance is Fe;

[0020] The sum of the contents of Cr, Co, Mo, and Ni is ≥25.00 wt%;

[0021] In the thickness direction, at a distance of 0.1 mm from the surface of the carburized bearing steel, the volume fraction of retained austenite is 3-10%, the volume fraction of carbides is 20-35%, and the length of acicular martensite is 5-7 μm.

[0022] Among them, carbides with a diameter of less than 1 μm account for 65-75% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular.

[0023] A second aspect of the present invention provides a heat treatment method for the above-mentioned carburized bearing steel, wherein the heat treatment method includes:

[0024] S1. Carburize the workpiece at 930-980℃. The effective hardened layer depth of the carburized layer is 0.6-1.2mm, the surface carbon concentration is 0.8-1.2wt.%, the carburizing atmosphere is acetylene, and after carburizing, cool it to 20-30℃ with nitrogen pressure of 0.1-0.2MPa. The average cooling rate is 15-40℃ / min.

[0025] S2. Place the workpiece obtained in S1 in a vacuum heat treatment furnace for vacuum spheroidization;

[0026] S3. Place the workpiece obtained in S2 into an atmosphere quenching furnace and heat it. After heating, remove it from the furnace and cool it by passing nitrogen gas through it.

[0027] S4. Place the workpiece obtained in S3 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting.

[0028] S5. Place the workpiece obtained in S4 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After the holding period, remove it from the furnace and air cool it in a vacuum environment.

[0029] S6. Place the workpiece obtained in S5 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting.

[0030] S7. Place the workpiece obtained in S6 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After holding, remove it from the furnace and air cool it in a vacuum environment to obtain the carburized bearing steel.

[0031] The beneficial effects of the present invention are:

[0032] Compared with existing processes, the carburized bearing steel provided by this invention exhibits superior surface carbide growth and corrosion resistance. The heat treatment method provided by this invention, after carburizing, uses a cooling rate greater than air cooling but less than quenching to rapidly precipitate carbides on the surface, preventing carbide growth. Before quenching, a spheroidizing process is added to allow sufficient time for the surface carbides to precipitate and spheroidize again, adjusting the carbon element distribution and preparing the microstructure for quenching. Simultaneously, it achieves carbide spheroidization and reduces the content of retained austenite. Precise control of process parameters in the local carburized layer prevents a decrease in core hardness. This process improves the morphology and microstructure of carbides in the carburized layer, achieving maximum hardness under the same carbon concentration or carburizing process. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the surface structure (×1000) of Embodiment 1 of the present invention.

[0034] Figure 2 This is a schematic diagram of the surface structure (×1000) of Comparative Example 1 of the present invention.

[0035] Figure 3 This is a hardness gradient diagram of Embodiment 1 of the present invention.

[0036] Figure 4 This is a hardness gradient diagram of Comparative Example 1 of the present invention. Detailed Implementation

[0037] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0038] In existing technologies, the carburizing process for carburized bearing steel has not achieved its maximum effect, and there is a lack of optimization of the carbide structure.

[0039] In this invention, by improving the heat treatment process, the morphology of carbides in carburized bearing steel can be optimized and its corrosion resistance improved.

[0040] To achieve this goal, the present invention attempts to optimize the microstructure, content, and processing technology of carburized bearing steel. By using specific microstructure, content, and heat treatment processes, the above objectives can be achieved.

[0041] The first aspect of this invention provides a carburized bearing steel, wherein the components and their weight percentages in the carburized bearing steel are as follows:

[0042] The C content is 0.10-0.16%;

[0043] The Cr content is 13.75-15.00%;

[0044] The Co content is 5.00-14.00%;

[0045] The Ni content is 4.75-5.25%;

[0046] The Mo content is 3.00-5.00%;

[0047] Si content ≤ 0.05%;

[0048] Mn content ≤ 0.05%;

[0049] S content ≤ 0.01%;

[0050] P content ≤ 0.015%;

[0051] The content of other impurity elements is ≤0.03%;

[0052] The total content of other impurity elements is ≤0.15%;

[0053] The balance is Fe;

[0054] The sum of the contents of Cr, Co, Mo, and Ni is ≥25.00 wt%;

[0055] In the thickness direction, at a distance of 0.1 mm from the surface of the carburized bearing steel, the volume fraction of retained austenite is 3-10%, the volume fraction of carbides is 20-35%, and the length of acicular martensite is 5-7 μm.

[0056] Among them, carbides with a diameter of less than 1 μm account for 65-75% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular.

[0057] In this invention, the carburized bearing steel provides a carburized layer with smaller carbides, an increased proportion of small carbide sizes, an increased proportion of carbides with a diameter of less than 1 μm, and a carbide morphology that tends to be spherical and granular. The metallographic structure of the carburized layer is refined. By analyzing the XRD surface of the carburized bearing steel sample, it was found that the proportion of residual austenite in the surface layer decreased, the carbide content increased, and the length of acicular martensite decreased from grade 3 martensite to grade 2 martensite.

[0058] According to the present invention, the diameter of the spherical carbide with a diameter of less than 1 μm is 0.1-0.8 μm;

[0059] The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 1-2:1.

[0060] According to the present invention, the carburized bearing steel includes any one of CSS42, 15Cr14Co12Mo5NiW, and 14Cr14Co13Mo4NiW.

[0061] A second aspect of the present invention provides a heat treatment method for the above-mentioned carburized bearing steel, wherein the heat treatment method includes:

[0062] S1. Carburize the workpiece at 930-980℃. The effective hardened layer depth of the carburized layer is 0.6-1.2mm, the surface carbon concentration is 0.8-1.2wt.%, the carburizing atmosphere is acetylene, and after carburizing, cool it to 20-30℃ with nitrogen pressure of 0.1-0.2MPa. The average cooling rate is 15-40℃ / min.

[0063] S2. Place the workpiece obtained in S1 in a vacuum heat treatment furnace for vacuum spheroidization;

[0064] S3. Place the workpiece obtained in S2 into an atmosphere quenching furnace and heat it. After heating, remove it from the furnace and cool it by passing nitrogen gas through it.

[0065] S4. Place the workpiece obtained in S3 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting.

[0066] S5. Place the workpiece obtained in S4 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After the holding period, remove it from the furnace and air cool it in a vacuum environment.

[0067] S6. Place the workpiece obtained in S5 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting.

[0068] S7. Place the workpiece obtained in S6 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After holding, remove it from the furnace and air cool it in a vacuum environment to obtain the carburized bearing steel.

[0069] In this invention, S1 is a vacuum pulse carburizing process in which the workpiece undergoes multiple alternating strong infiltration / diffusion to achieve a defined surface carbon concentration. Each pulse process involves first evacuating the vacuum, then introducing acetylene for strong infiltration, then evacuating the vacuum again, and then introducing nitrogen for carbon diffusion. The process is adjusted according to the number of workpieces, their surface area, and their placement. The surface carbon concentration and carbon concentration gradient are adjusted by adjusting the ratio of strong infiltration / diffusion time. The diffusion time / strong infiltration time of each pulse is ≥20, and the time of the next pulse is ≥ the time of the previous pulse. The accelerated cooling process after carburizing prevents carbide growth and surface oxidation.

[0070] According to the present invention, the time for strong permeation by introducing acetylene is ≤120s.

[0071] According to the present invention, the diffusion time / intense penetration time of each pulse is ≥20, and the time of the next pulse is ≥ the time of the previous pulse.

[0072] In this invention, carbon atoms are precipitated during the vacuum spheroidization process of S2, forming fine and dispersed carbides, which improves the morphology of the carbides and makes the infiltrated layer structure more uniform.

[0073] In this invention, S3 is gas quenching. After spheroidization treatment, the diffusion layer contains a large number of dispersed carbides, which can hinder the growth of austenite grains, laying the foundation for obtaining good quenched structure and properties, and promoting the formation of acicular martensite. The dissolution of spheroidal carbides is more gradual, effectively inhibiting the growth of austenite grains, reducing the overheating sensitivity of the material, and obtaining a finer martensite structure. The fine initial structure and uniform austenitization make the thermal stress and structural stress of each part of the workpiece smaller during quenching and cooling, significantly reducing the risk of deformation and cracking of the workpiece.

[0074] In this invention, the deep cooling and defrosting of S4 and S6, after spheroidizing pretreatment and uniform austenitization followed by quenching, result in more uniform martensite and more consistent composition and stability of the retained austenite. During the deep cooling process, the retained austenite can be transformed into martensite more synchronously and thoroughly, avoiding the problem of incomplete transformation caused by uneven microstructure.

[0075] In this invention, the high-temperature tempering of S5 and S7, and the spheroidized surface microstructure, make it easier to obtain fine acicular martensite after quenching and deep cooling. The precipitation and aggregation growth of carbides during tempering are slower and more uniform, resulting in finer and more uniform carbide particles distributed on the martensitic matrix. This microstructure is key to the excellent combination of strength and toughness.

[0076] According to the present invention, in S2, the vacuum environment is a vacuum degree of less than 20 Pa.

[0077] According to the present invention, in S2, the conditions for vacuum spheroidization include heating to 700-800°C, holding at that temperature in a vacuum environment for 2-4 hours, and then cooling to 600°C at a cooling rate of 3-5°C / min, followed by air cooling in a vacuum environment.

[0078] According to some preferred embodiments of the present invention, a second vacuum spheroidization is performed after S2, and the conditions for the second vacuum spheroidization are the same as those for the vacuum spheroidization in S2.

[0079] According to the present invention, in S3, the pressure of the nitrogen gas is 0.3-0.6 MPa.

[0080] According to the present invention, in S3, the heating conditions include: first heating to 500-600°C at a heating rate of 10-20°C / min, holding at that temperature for 30-90 min, and then heating to 1030-1070°C at a heating rate of 5-10°C / min, holding at that temperature for 30-90 min.

[0081] According to the present invention, in S3, the cooling conditions include: charging with nitrogen gas at 0.4-0.5 MPa, cooling at a rate of 25-50 °C / min, and cooling to 25-50 °C.

[0082] According to the present invention, in S4, the heat preservation time is 2-4 hours, and the defrosting conditions include: a defrosting temperature of 50-80°C and a defrosting heat preservation time of 20-40 minutes.

[0083] According to the present invention, in S6, the heat preservation time is 2-4 hours, and the defrosting conditions include: a defrosting temperature of 50-80°C and a defrosting heat preservation time of 20-40 minutes.

[0084] According to the present invention, in S5, the heat preservation time is 2-4 hours.

[0085] According to the present invention, in S7, the heat preservation time is 2-4 hours.

[0086] The cross-sectional gradient microhardness test method was performed according to GB / T 4340.1-2009 "Vickers Hardness Test for Metallic Materials". The Vickers hardness gradient was tested along the cross-section of the sample's diffusion layer using a Vickers hardness tester with a load of 1 kg. The unit is HV. Twenty-five points were selected for each test, and three tests were performed. The average value was used to ensure data accuracy.

[0087] The hardened layer depth test method shall be in accordance with GB / T9450-2005 "Determination and Verification of Hardened Layer Depth of Carburized and Quenched Steel Parts".

[0088] Metallographic structure testing methods were performed in accordance with GB / T25744 "Metallographic Inspection of Carburized, Quenched and Tempered Steel Parts".

[0089] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Obviously, the embodiments described herein are only some embodiments of the present invention and are not intended to limit the present invention. All other embodiments implemented by those skilled in the art based on the embodiments of the present invention without creative improvements are within the protection scope of the present invention.

[0090] Example 1

[0091] The experimental forging was made of 15Cr14Co12Mo5Ni material, with the following components and their weight percentages: C: 0.15%; Cr: 14%; Co: 13%; Ni: 2%; Mo: 5%; Si: 0.02%; Mn: 0.03%; S: 0.005%; P: 0.001%; ​​the content of other impurity elements was ≤0.03% for each individual element and ≤0.15% for the total content of other impurity elements; the balance was Fe; the total alloy content was 34 wt%.

[0092] The heat treatment method is as follows:

[0093] S1. Carburize the workpiece at 980℃. The effective hardened layer depth of the carburized layer is 1.1mm, the surface carbon concentration is 0.9wt.%, the carburizing atmosphere is acetylene, the strong carburizing time of acetylene is ≤120s, the diffusion time / strong carburizing time of each pulse is ≥20, the time of the next pulse is ≥ the time of the previous pulse, and after carburizing, cool to 25℃ with nitrogen pressure of 0.12MPa. The average cooling rate is 35℃ / min.

[0094] S2. Place the workpiece obtained in S1 in a vacuum heat treatment furnace, evacuate to a vacuum level of 10 Pa, heat to 760°C at a heating rate of ≤150°C / h, hold for 2 hours, after holding, cool to 600°C with the furnace at a cooling rate of 3°C / min, rapidly cool to room temperature by introducing 0.1 MPa nitrogen gas, then heat to 760°C at a heating rate of ≤150°C / h, hold for 2 hours, after holding, cool to 600°C with the furnace at a cooling rate of 3°C / min, rapidly cool to room temperature by introducing 0.1 MPa nitrogen gas, and then remove from the furnace.

[0095] S3. Place the workpiece obtained in S2 in an atmosphere quenching furnace and heat it. First, heat it to 600℃ at a heating rate of 15℃ / h and hold it for 30min. After holding, heat it to the quenching temperature of 1070℃ at a heating rate of 10℃ / min and hold it at the quenching temperature for 1h. After holding, send the workpiece out of the heating chamber and push it into the quenching cooling chamber. Fill it with 0.5MPa of nitrogen gas and cool it to 25℃ at an average rate of 40℃ / min. Then stop cooling.

[0096] After S4 and S3 quenching, the workpieces that have undergone atmosphere quenching are placed in a cryogenic chamber and purged with liquid nitrogen for cryogenic treatment at a temperature of -80℃. After the workpieces reach the cryogenic treatment temperature, they are kept at that temperature for 2 hours. After cryogenic treatment, the workpieces are placed in a 50℃ forced-air drying oven for defrosting for 30 minutes.

[0097] S5. The defrosted workpiece obtained in S4 is sent into a vacuum heat treatment furnace for high-temperature tempering at a furnace temperature of 500℃ for 2 hours. After tempering, the workpiece is taken out of the furnace and air-cooled to 25℃ in a vacuum environment.

[0098] S6. Perform a second deep cryogenic-defrosting process on the tempered workpiece obtained in S5. The process and specific parameters are as follows in step S4.

[0099] S7. The workpiece obtained in S6 after the second cryogenic defrosting process is subjected to a second tempering process. The process and specific parameters are the same as in step S5 to obtain carburized bearing steel.

[0100] In the thickness direction, at a distance of 0.1 mm from the surface, the retained austenite volume ratio of the carbide bearing steel is 5%, the carbide volume content is 30%, and the length of the acicular martensite is 5 μm.

[0101] Among them, carbides with a diameter of less than 1 μm account for 75% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular, and the diameter of the spherical carbides with a diameter of less than 1 μm is 0.1-0.8 μm;

[0102] The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 1:1.

[0103] The maximum surface hardness is 780 HV.

[0104] Example 2

[0105] The heat treatment method of Example 1 is different in that, in step S1, the workpiece is carburized at 930°C, the effective hardened layer depth of the carburized layer of the workpiece is 0.6 mm, the surface carbon concentration is 0.8 wt.%, the carburizing atmosphere is acetylene, and after carburizing, it is cooled to 20°C with nitrogen pressure of 0.1 MPa and the cooling rate is 15°C / min.

[0106] The prepared carburized bearing steel has a retained austenite ratio of 10%, a carbide volume content of 20%, and a acicular martensite length of 7μm at a distance of 0.1mm from the surface in the thickness direction.

[0107] Among them, carbides with a diameter of less than 1 μm account for 65% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular, and the diameter of the spherical carbides with a diameter of less than 1 μm is 0.1-0.8 μm;

[0108] The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 1.2:1.

[0109] The maximum surface hardness is 740 HV.

[0110] Example 3

[0111] The heat treatment method of Example 1 is different in that, in step S1, the workpiece is carburized at 930°C, the effective hardened layer depth of the carburized layer of the workpiece is 1.0 mm, the surface carbon concentration is 1.2 wt.%, the carburizing atmosphere is acetylene, and after carburizing, it is cooled to 30°C with a nitrogen pressure of 0.2 MPa and a cooling rate of 40°C / min.

[0112] The prepared carburized bearing steel has a retained austenite ratio of 8%, a carbide volume content of 28%, and a acicular martensite length of 6μm at a distance of 0.1mm from the surface in the thickness direction.

[0113] Among them, carbides with a diameter of less than 1 μm account for 68% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular; the diameter of the spherical carbides with a diameter of less than 1 μm is 0.1-0.8 μm;

[0114] The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 1.6:1.

[0115] The maximum surface hardness is 805 HV.

[0116] Example 4

[0117] The heat treatment method of Example 1 is followed, except that in step S2, the workpiece obtained in S1 is placed in a vacuum heat treatment furnace, evacuated to a vacuum, heated to 760°C at a heating rate of ≤150°C / h, held at that temperature for 2 hours, and then cooled to 600°C with the furnace at a cooling rate of 3°C / min. Nitrogen gas at 0.1 MPa is then introduced for rapid cooling to room temperature, after which the workpiece is removed from the furnace. No secondary spheroidization occurs.

[0118] The prepared carburized bearing steel has a retained austenite ratio of 7%, a carbide volume content of 25%, and a acicular martensite length of 5μm at a distance of 0.1mm from the surface in the thickness direction.

[0119] Among them, carbides with a diameter of less than 1 μm account for 70% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular, and the diameter of the spherical carbides with a diameter of less than 1 μm is 0.1-0.8 μm;

[0120] The aspect ratio of the granular carbides with a diameter of less than 1 μm is 1.3:1.

[0121] The maximum surface hardness is 760 HV.

[0122] Comparative Example 1

[0123] The heat treatment method of Example 1 is different except that step S2 is omitted.

[0124] The prepared carburized bearing steel has a retained austenite ratio of 15%, a carbide volume content of 20%, and a acicular martensite length of 7μm at a distance of 0.1mm from the surface in the thickness direction.

[0125] Among them, carbides with a diameter of less than 1 μm account for 50% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular.

[0126] The aspect ratio of the granular carbides with a diameter of less than 1 μm is 2.1:1.

[0127] The maximum surface hardness is 750 HV.

[0128] Comparative Example 2

[0129] The heat treatment method of Example 1 is different in that, in step S1, the workpiece is carburized at 860°C, the effective hardened layer depth of the carburized layer of the workpiece is 0.5 mm, the surface carbon concentration is 0.6 wt.%, the carburizing atmosphere is acetylene, and nitrogen is not introduced after carburizing, and the workpiece is directly cooled to 25°C in a vacuum environment.

[0130] The prepared carburized bearing steel has a retained austenite ratio of 20%, a carbide volume content of 18%, and a acicular martensite length of 10μm at a distance of 0.1mm from the surface in the thickness direction.

[0131] Among them, carbides with a diameter of less than 1 μm account for 45% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular.

[0132] The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 3:1.

[0133] The maximum surface hardness is 680 HV.

[0134] Comparative Example 3

[0135] The heat treatment method of Example 1 is different in that, in step S1, the workpiece is carburized at 1050°C, the effective hardened layer depth of the carburized layer of the workpiece is 1.1 mm, the surface carbon concentration is 1.2 wt.%, the carburizing atmosphere is acetylene, and after carburizing, it is cooled to 25°C with nitrogen pressure of 0.4 MPa, and the average cooling rate is 60°C / min.

[0136] The prepared carburized bearing steel has a retained austenite ratio of 15%, a carbide volume content of 32%, and a acicular martensite length of 8μm at a distance of 0.1mm from the surface in the thickness direction.

[0137] Among them, carbides with a diameter of less than 1 μm account for 40% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular, and there are network carbides.

[0138] The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 2:1;

[0139] The network carbides are characterized by a large number of carbides with an aspect ratio ≥ 3 and are interconnected.

[0140] The maximum surface hardness is 740 HV.

[0141] The electrochemical corrosion performance of the materials in Examples 1-4 and Comparative Examples 1-3 was characterized by polarization curves and AC impedance spectroscopy. A 3.5 wt.% NaCl aqueous solution was used as the electrolyte. Both tests employed a conventional three-electrode system. A saturated calomel electrode was used as the reference electrode, a platinum sheet electrode as the auxiliary electrode, and the test sample was used as the working electrode. When the material prepared in Example 1 of this invention was used, the corrosion potential of the infiltrated layer increased from -0.53253 V in Comparative Example 1 to -0.40728 V in Example 1, and its corrosion current density also changed from 3.3 × 10⁻⁶ V. -5 A / cm 2 Monotonically decreasing to 1.1 × 10 -5 A / cm 2The corrosion potential reflects the ease with which a material corrodes, while the corrosion current reflects the rate of corrosion. Generally, the higher the corrosion potential and the lower the corrosion current density, the better the material's corrosion resistance. Experimental comparisons and corrosion image observations show that spheroidization treatment results in a more uniform final microstructure, reducing the occurrence of severe localized corrosion. Furthermore, the replacement of network carbides with dispersed carbides eliminates the possibility of network carbides acting as rapid corrosion pathways, reducing the risk of intergranular corrosion and preventing the rapid spread of corrosion along the network texture, leading to brittle fracture or surface peeling. Therefore, this invention can effectively reduce the risk of corrosion performance deterioration while ensuring increased hardness.

[0142] Instruction manual illustrations Figure 1 and Figure 2 This indicates that the carburized bearing steel provided by the present invention precipitates smaller carbides in the carburized layer, with an increased proportion of small carbide sizes. The proportion of carbides with a diameter less than 1 μm increases from 50% to 70%, and the carbide morphology tends to be spherical and granular. The metallographic structure of the carburized layer is refined. Analysis of the XRD surface of samples from both processes revealed that the proportion of retained austenite in the surface layer decreased from 15% to 5%, the carbide content increased from 20% to 30%, and the length of acicular martensite decreased from 7 μm of grade 3 martensite to 5 μm of grade 2 martensite. (See attached drawings.) Figure 3 The maximum surface hardness reaches 780 HV, as shown in the attached diagram in the instruction manual. Figure 4 The maximum surface hardness in Comparative Example 1 was 750 HV. The surface hardness of the workpiece treated by the present invention increased by 30 HV, the effective hardness of the carburized layer increased, and the heat treatment quality of 15Cr14Co12Mo5Ni high alloy carburized steel was greatly improved.

[0143] By comparing the examples and comparative examples, it can be seen that the heat treatment method provided by the present invention optimizes the heat treatment process of carbide morphology in the carburized layer of high alloy carburized steel, so as to achieve the dispersed distribution of spherical carbides in the carburized layer and improve the carburizing heat treatment effect.

[0144] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A carburized bearing steel, characterized in that, The components and their weight percentages in the carburized bearing steel are as follows: The C content is 0.10-0.16%; The Cr content is 13.75-15.00%; The Co content is 5.00-14.00%; The Ni content is 4.75-5.25%; The Mo content is 3.00-5.00%; Si content ≤ 0.05%; Mn content ≤ 0.05%; S content ≤ 0.01%; P content ≤ 0.015%; The content of other impurity elements is ≤0.03%; The total content of other impurity elements is ≤0.15%; The balance is Fe; The sum of the contents of Cr, Co, Mo, and Ni is ≥25.00 wt%; In the thickness direction, at a distance of 0.1 mm from the surface of the carburized bearing steel, the volume fraction of retained austenite is 3-10%, the volume fraction of carbides is 20-35%, and the length of acicular martensite is 5-7 μm. Among them, carbides with a diameter of less than 1 μm account for 65-75% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular.

2. The carburized bearing steel according to claim 1, characterized in that, The diameter of the spherical carbides with a diameter of less than 1 μm is 0.1-0.8 μm; The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 1-2:

1.

3. The carburized bearing steel according to claim 1, characterized in that, The carburized bearing steel includes any one of CSS42, 15Cr14Co12Mo5NiW, and 14Cr14Co13Mo4NiW.

4. A heat treatment method for carburized bearing steel according to any one of claims 1-3, characterized in that, The heat treatment method includes: S1. Carburize the workpiece at 930-980℃. The effective hardened layer depth of the carburized layer is 0.6-1.2mm, the surface carbon concentration is 0.8-1.2wt.%, the carburizing atmosphere is acetylene, and after carburizing, cool it to 20-30℃ with nitrogen pressure of 0.1-0.2MPa. The average cooling rate is 15-40℃ / min. S2. Place the workpiece obtained in S1 in a vacuum heat treatment furnace for vacuum spheroidization; S3. Place the workpiece obtained in S2 into an atmosphere quenching furnace and heat it. After heating, remove it from the furnace and cool it by passing nitrogen gas through it. S4. Place the workpiece obtained in S3 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting. S5. Place the workpiece obtained in S4 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After the holding period, remove it from the furnace and air cool it in a vacuum environment. S6. Place the workpiece obtained in S5 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting. S7. Place the workpiece obtained in S6 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After holding, remove it from the furnace and air cool it in a vacuum environment to obtain the carburized bearing steel.

5. The heat treatment method according to claim 4, characterized in that, In S2, the vacuum environment is a vacuum degree of less than 20 Pa.

6. The method according to claim 4, characterized in that, In S2, the vacuum spheroidization conditions include heating to 700-800℃, holding at that temperature in a vacuum environment for 2-4 hours, and then cooling to 600℃ at a cooling rate of 3-5℃ / min, followed by air cooling in a vacuum environment. Preferably, a second vacuum spheroidization is performed after S2, and the conditions for the second vacuum spheroidization are the same as those for the vacuum spheroidization in S2.

7. The method according to claim 4, characterized in that, In S3, the pressure of the nitrogen gas is 0.3-0.6 MPa.

8. The method according to claim 4, characterized in that, In S3, the heating conditions include: first heating to 500-600℃ at a heating rate of 10-20℃ / min, holding at that temperature for 30-90min, and then heating to 1030-1070℃ at a heating rate of 5-10℃ / min, holding at that temperature for 30-90min. In S3, the cooling conditions include: charging with nitrogen gas at 0.4-0.5 MPa, a cooling rate of 30-40 °C / min, and cooling to 25-50 °C.

9. The method according to claim 4, characterized in that, In S4, the heat preservation time is 2-4 hours, and the defrosting conditions include: defrosting temperature of 50-80℃ and defrosting heat preservation time of 20-40 minutes. In S6, the heat preservation time is 2-4 hours, and the defrosting conditions include: defrosting temperature of 50-80℃ and defrosting heat preservation time of 20-40 minutes.

10. The method according to claim 4, characterized in that, In S5, the heat preservation time is 2-4 hours; In S7, the heat preservation time is 2-4 hours.

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