Carbonitriding process for high-carbon chromium bearing steel to achieve high hardness and low residual austenite content

Abstract

A process for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite content is disclosed, comprising the following steps: S1, pre-cleaning to ensure the surface of the high-carbon chromium bearing steel is clean and free of oil; S2, pre-oxidation to ensure the surface of the high-carbon chromium bearing steel is free of water stains; S3, carbonitriding to improve the surface hardness and wear resistance of the high-carbon chromium bearing steel; S4, post-cleaning to ensure the surface of the high-carbon chromium bearing steel is clean and free of oil; S5, cold treatment to retain low residual austenite inside the high-carbon chromium bearing steel; S6, tempering to achieve high hardness of the high-carbon chromium bearing steel. The beneficial effects of this invention are: this process organically combines pre-oxidation, carbonitriding, cold treatment, and tempering to form a high-hardness, high-wear-resistant carbonitrided layer on the surface of the bearing steel, and the cold treatment effectively controls the residual austenite content, suppressing dimensional changes due to aging.

Description

Technical Field

[0001] This invention relates to a heat treatment process, and more particularly to a process for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite. Background Technology

[0002] In the bearing manufacturing industry, carbonitriding is widely used for surface strengthening of materials such as high-carbon chromium bearing steel, especially in aerospace, precision machine tool, and high-speed heavy-load applications where strict requirements for wear resistance, fatigue resistance, and dimensional stability are necessary. This process is typically carried out in a controlled atmosphere furnace, where the workpiece is heated to the austenitizing temperature range of 800°C to 900°C, and an atmosphere containing carbon and nitrogen is introduced, allowing carbon and nitrogen atoms to simultaneously diffuse into the workpiece surface, forming a carbonitrided layer of a certain depth. Subsequent quenching and low-temperature tempering treatments yield a high-hardness martensitic structure and dispersed carbonitrides on the surface, thereby improving surface hardness and wear resistance, while the core retains good toughness.

[0003] However, existing carbonitriding processes still have significant shortcomings in practical applications, especially when processing high-carbon chromium bearing steel. It is difficult to achieve high surface hardness and excellent wear resistance while simultaneously maintaining a stable low level of retained austenite content. While excessively high retained austenite content can improve toughness, it leads to decreased dimensional stability, making the bearing prone to micro-deformation during long-term use or temperature fluctuations, affecting precision fit accuracy. Existing technologies often require a trade-off between hardness and retained austenite content, failing to simultaneously achieve the high dimensional stability required for a balance between high hardness, high wear resistance, and low retained austenite content. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a process method for achieving high hardness and low residual austenite in high-carbon chromium bearing steel through carbonitriding.

[0005] To achieve the above objectives, the technical solution of this invention is as follows: A process for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite, implemented according to the following steps: S1, pre-cleaning, to ensure the surface of high carbon chromium bearing steel is clean and free of oil stains; S2, pre-oxidation, to ensure that the surface of high carbon bearing steel is free of water stains; S3, carbonitriding, to improve the surface hardness and wear resistance of high carbon bearing steel; S4, post-cleaning, to ensure the surface of the high-carbon chromium bearing steel is clean and free of oil stains; S5, cold-treated to ensure that the residual austenite content inside the high-carbon bearing steel is ≤15%; S6 tempering brings the high-carbon bearing steel to a value of 63 HRC or higher.

[0006] The beneficial effects of this invention are as follows: This process organically combines pre-oxidation, carbonitriding, cryogenic treatment, and tempering to form a high-hardness, high-wear-resistant carbonitrided layer on the surface of bearing steel. Cryogenic treatment effectively controls the retained austenite content and suppresses dimensional changes due to aging. The pre-oxidation step forms a uniform and dense oxide film on the surface, preventing localized overheating oxidation during subsequent heating. Furthermore, this oxide film is reduced by the atmosphere in the early stages of carbonitriding, activating the surface state and promoting rapid and uniform penetration of carbon and nitrogen atoms. While improving surface hardness and wear resistance, carbonitriding also stabilizes the supercooled austenite through nitrogen penetration, allowing it to be partially retained during subsequent quenching. Cryogenic treatment further transforms this retained austenite into martensite, thus balancing hardness and dimensional stability. As a preferred method, during the S2 pre-oxidation step, the workpiece is held at 400℃ in an oxygen-containing atmosphere for 40 minutes to form a uniform Fe3O4 film. During subsequent carbonitriding, this film is reduced to active iron in an ammonia decomposition atmosphere, providing numerous nucleation sites and resulting in a more uniform carbonitriding concentration distribution. Alternatively, in the S5 cryogenic treatment step, the workpiece is placed directly from room temperature into a -80℃ cryogenic bath for 2 hours. This utilizes the martensitic phase transformation driving force to induce the transformation of most of the retained austenite, reducing the volume fraction of retained austenite, increasing hardness, and stabilizing dimensions.

[0007] Furthermore, the specific implementation of step S3 is as follows: S31, Atmosphere establishment and maintenance, using ammonia as the nitrogen element to provide the atmosphere (ammonia ratio of 1%-20%), supplemented by a protective atmosphere; S32, strong infiltration-diffusion process is implemented, strong infiltration temperature is less than 890℃, strong infiltration time is no more than 6 hours, strong infiltration carbon potential is no more than 1.5%, and diffusion carbon potential is no more than 1.5%; S33, quenching, oil temperature set to 60-140℃, select isothermal quenching oil or rapid quenching oil according to different wall thicknesses.

[0008] Through step S3 above, ammonia decomposes in the carbonitriding atmosphere to produce active nitrogen atoms, which, together with the protective atmosphere, form a carbonitriding layer with a good concentration gradient on the workpiece surface, avoiding the formation of a brittle white compound layer. The strong carburizing stage controls the carbon potential within a reasonable range, ensuring the carburizing rate while inhibiting carbide precipitation along grain boundaries. The diffusion stage further adjusts the carbonitriding concentration distribution, making the hardness gradient gentler. The quenching oil temperature is flexibly selected according to the wall thickness; isothermal quenching oil is used for thick-walled parts to reduce quenching deformation, while rapid quenching oil is used for thin-walled parts to obtain a higher hardened layer depth. As a preferred method, a protective atmosphere combining ammonia and nitrogen-methanol-propane is used in S31. Methanol decomposes to produce carbon monoxide and hydrogen, maintaining positive pressure in the furnace and participating in the carburizing reaction. Propane is periodically pulsed in to quickly restore carbon potential fluctuations, working synergistically with the nitrogen atoms from ammonia decomposition to form a high-hardness carbonitriding compound diffusely distributed carburized layer on the workpiece surface, improving both the matrix hardness and refining the microstructure.

[0009] Furthermore, the protective atmosphere is nitrogen, methanol and propane or a combination of RX gas and propane, wherein the propane can be replaced by natural gas.

[0010] Limiting the protective atmosphere to the above combination allows for precise control of carbon and nitrogen potentials within the furnace, preventing the formation of carbon black and carbide networks. Methanol provides the basic carbon source, while propane or natural gas acts as an enrichment gas, rapidly responding to carbon potential fluctuations. RX gas is an endothermic atmosphere pre-prepared in the generator, exhibiting stable composition and low gas consumption, which is beneficial for large-scale continuous production. Replacing propane with natural gas further reduces raw material costs. Simultaneously, natural gas undergoes cracking in the furnace to generate a large number of active hydrocarbon groups, which, along with nitrogen atoms, diffuse in, improving the uniformity of nitrogen content in the diffusion layer. As a preferred approach, a combination of RX gas and natural gas is used. RX gas is produced by the incomplete combustion of propane and air in a catalytic reactor and is introduced into the furnace as a carrier gas. Natural gas, as an enrichment gas, is automatically replenished according to the carbon potential signal, stabilizing the furnace gas carbon potential at the set value and thus controlling the fluctuation of the co-diffusion layer depth within a small range.

[0011] Furthermore, step S5 involves cold water or liquid nitrogen for cooling treatment at a temperature of -150 to 20°C for 1 to 4 hours.

[0012] Cold treatment of carbonitrided workpieces within this temperature range allows the residual austenite to continue transforming into martensite, minimizing the residual austenite content to a low level while preventing cracking. The treatment time is controlled between 1 and 4 hours, flexibly adjustable according to the effective cross-sectional size of the workpiece to ensure the core also reaches the full transformation temperature. Water cooling equipment is simple and low-cost, suitable for standard-sized bearing parts; liquid nitrogen cryogenic treatment can achieve even lower temperatures and stronger driving forces, resulting in a more thorough elimination of residual austenite. As a preferred method, a liquid nitrogen atomization cooling chamber is used, placing the workpiece in circulating liquid nitrogen vapor and cooling it from room temperature to -100°C at a rate of 2-3°C / min, holding it at that temperature for 2.5 hours. The gentle cooling rate avoids microcracks caused by excessive thermal stress, while the sufficient holding time allows the residual austenite to fully transform into high-hardness twinned martensite, further improving overall hardness and dimensional stability.

[0013] Furthermore, alkaline cleaning agents or hydrocarbon cleaning agents can be used for cleaning in steps S1 and S4.

[0014] Cleaning workpieces with alkaline or hydrocarbon cleaning agents before and after carbonitriding can thoroughly remove residual processing grease, rust-preventive oil, and quenching oil from the surface, preventing these contaminants from decomposing at high temperatures and forming carbon black, which can lead to uneven diffusion layers or soft spots. Alkaline cleaning agents have a strong ability to remove polar dirt and can quickly clean the surface when combined with spray or ultrasonic cleaning. Hydrocarbon cleaning agents have extremely low surface tension, allowing them to penetrate into delicate structures such as bearing raceways, and leave no residue after evaporation, avoiding interference with atmosphere stability caused by solvent residue. As a preferred method, hydrocarbon cleaning agents are used in conjunction with a vacuum ultrasonic cleaner in the pre-S1 cleaning stage. The workpiece is immersed in the hydrocarbon solvent under vacuum, and the grease in the tiny pores is removed by the ultrasonic cavitation effect. Subsequently, it is rapidly dried under reduced pressure, ensuring that the surface reaches molecular-level cleanliness before entering the furnace, providing an ideal surface condition for subsequent pre-oxidation and carbonitriding.

[0015] Furthermore, the tempering temperature in step S6 is set to 160-400℃, and the holding time is 2 hours.

[0016] Tempering workpieces after carbonitriding and cryogenic treatment within this temperature range effectively eliminates micro-stress generated during quenching and deep cryogenic treatment, reduces material brittleness, and allows supersaturated carbon and nitrogen atoms in the quenched martensite to diffuse and aggregate appropriately, forming dispersed carbonitrides. This, in turn, improves toughness and wear resistance while maintaining high hardness. The wide temperature range of 160~400℃ allows for flexible selection based on different operating conditions: lower temperature tempering achieves higher surface hardness, meeting heavy-duty wear resistance requirements; higher temperature tempering slightly reduces hardness but significantly improves microstructure stability and dimensional aging accuracy. As a preferred method, for precision spindle bearings, long-term tempering at 180℃ is used to stabilize surface hardness within a high range, while retaining almost no rebound of austenite, ensuring the bearing maintains its initial accuracy even under repeated temperature fluctuations. As another preferred method, for bearings operating in high-temperature environments, tempering at 350℃ promotes the precipitation and spheroidization of some carbonitrides, imparting good thermal stability to the surface microstructure while maintaining high hardness. Attached Figure Description

[0017] Figure 1 The carbonitriding process curve is shown in the embodiment of the present invention. Figure 2 This is a comparison diagram of surface hardness in an embodiment of the present invention; Figure 3 This is a comparison chart of the residual austenite content in embodiments of the present invention. Detailed Implementation

[0018] An embodiment of the present invention provides a process for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite content. Figure 1-2 As shown: First, the high-carbon chromium bearing steel parts undergo pre-cleaning to remove surface oil and impurities, ensuring a clean surface. Pre-cleaning can be performed using alkaline or hydrocarbon cleaning agents through immersion or spraying, followed by drying. Then, the parts are placed in a low-temperature tempering furnace for pre-oxidation treatment, heated to 300-650℃ in air and held for a certain time to remove residual water stains and form a uniform thin oxide film, providing good surface activity for subsequent carbonitriding.

[0019] After pre-oxidation, the parts are transferred to a carbonitriding furnace for carbonitriding treatment. The carbonitriding process includes atmosphere establishment and maintenance, strong infiltration-diffusion process execution, and quenching. First, ammonia gas is introduced into the furnace to provide active nitrogen elements, while a protective atmosphere is introduced to maintain positive pressure and adjust the carbon potential. The protective atmosphere can be a combination of nitrogen, methanol, and propane, or a combination of an RX endothermic atmosphere and propane, where propane can be replaced by natural gas. After the atmosphere stabilizes, the strong infiltration stage begins, with the strong infiltration temperature controlled below 890℃; the strong infiltration carbon potential is set to no higher than 1.5%; and the strong infiltration time does not exceed 6 hours, allowing carbon and nitrogen atoms to fully penetrate the surface of the parts. After strong infiltration, the diffusion stage begins, reducing the carbon potential to no higher than 1.5%. After diffusion, quenching is performed directly. The quenching oil temperature can be selected as isothermal quenching oil or rapid quenching oil according to the wall thickness of the part. Specifically, isothermal quenching oil is used for wall thicknesses below 6mm, and rapid quenching oil is used for thicknesses above 6mm. The quenching time is 10-30 minutes, and the oil temperature is set in the range of 60℃~140℃. Rapid cooling transforms the diffusion layer structure into high-hardness martensite.

[0020] After quenching, the parts undergo post-cleaning using alkaline or hydrocarbon cleaning agents to remove surface oil and ensure cleanliness. After cleaning and drying, the parts are immediately subjected to cryogenic treatment, using chilled water or liquid nitrogen as the cooling medium. The treatment temperature is between -150℃ and -20℃ to convert retained austenite into martensite as much as possible, reducing the retained austenite content in the microstructure. After cryogenic treatment, the parts are allowed to naturally return to room temperature in air or a furnace, followed by tempering at a temperature between 160℃ and 400℃ to eliminate stresses generated during quenching and cryogenic treatment, stabilize the microstructure, and ultimately achieve uniform high hardness and good dimensional stability.

[0021] After processing according to the above process, the surface hardness of high carbon chromium bearing steel parts is increased by about 2 HRC compared with conventional carbonitriding process, while the content of residual austenite in the surface layer is significantly reduced, and the dimensional stability is guaranteed.

[0022] The working principle of this embodiment is as follows: Pre-cleaning and pre-oxidation ensure the steel surface is clean and activated, facilitating the adsorption and diffusion of active atoms during carbonitriding. During carbonitriding, ammonia decomposes to produce active nitrogen atoms, which, together with active carbon atoms provided by the protective atmosphere, diffuse into the steel surface, forming a carbonitrided layer of a certain depth. Because nitrogen diffusion improves the hardenability of the diffusion layer and refines carbides, higher hardness can be achieved under lower heating temperatures and milder carbon potential conditions. The strong diffusion stage controls carbon potential and temperature to ensure sufficient carbon and nitrogen concentrations in the diffusion layer. The diffusion stage reduces carbon potential to smooth the concentration gradient and avoid defects such as network carbides. During quenching, a suitable oil temperature is selected to achieve a high-hardness martensitic structure in the diffusion layer while controlling deformation. Subsequent cold treatment utilizes low temperatures to further transform retained austenite into martensite, significantly reducing the amount of retained austenite and minimizing dimensional changes caused by the aging transformation of austenite, thus improving the dimensional stability of the parts. Finally, low-temperature tempering is used to eliminate quenching stress and cold treatment stress, stabilize the microstructure, slightly reduce the hardness but increase the toughness, and ultimately obtain high-carbon chromium bearing steel parts with high hardness, good wear resistance and long-term dimensional stability.

[0023] This embodiment uses the above-mentioned specific process parameters and time selection. Figure 1 The parameters in the table. The resulting product parameter indicators are as follows: Figure 2-3 As shown.

[0024] The above embodiments are merely one preferred embodiment of the present invention. Ordinary variations and substitutions made by those skilled in the art within the scope of the technical solution of the present invention are all included within the protection scope of the present invention.

Claims

1. A process for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite, comprising the following steps: S1, pre-cleaning, to ensure the surface of high carbon chromium bearing steel is clean and free of oil stains; S2, pre-oxidation, to ensure that the surface of high carbon bearing steel is free of water stains; S3, carbonitriding, to improve the surface hardness and wear resistance of high carbon bearing steel; S4, post-cleaning, to ensure the surface of the high-carbon chromium bearing steel is clean and free of oil stains; S5, cold-treated to ensure that the residual austenite content inside the high-carbon bearing steel is ≤15%; S6 tempering brings the high-carbon bearing steel to a value of 63 HRC or higher.

2. The process method for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite as described in claim 1, characterized in that: The specific implementation method of step S3 is as follows: S31, Atmosphere establishment and maintenance, using ammonia as the nitrogen element to provide the atmosphere, supplemented by a protective atmosphere; S32, strong infiltration-diffusion process is implemented, strong infiltration temperature is less than 890℃, strong infiltration time is no more than 6 hours, strong infiltration carbon potential is no more than 1.5%, diffusion carbon potential is no more than 1.5%; S33, quenching, oil temperature set to 60-140℃, select isothermal quenching oil or rapid quenching oil according to different wall thicknesses.

3. The process method for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite as described in claim 2, characterized in that: The protective atmosphere is a combination of nitrogen, methanol and propane, or RX gas and propane, wherein the propane can be replaced by natural gas.

4. The process method for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite as described in claim 1, characterized in that: Step S5 involves cold water or liquid nitrogen for cooling treatment at a temperature of -150 to 20°C for 1 to 4 hours.

5. The process method for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite as described in claim 1, characterized in that: In steps S1 and S4, alkaline cleaning agents or hydrocarbon cleaning agents can be used for cleaning.

6. The process method for carbonitriding high-carbon chromium bearing steel to achieve high hardness and low residual austenite as described in claim 1, characterized in that: The tempering temperature in step S6 is set to 160-400℃, and the holding time is 2 hours.

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