Process for high hardness and high residual austenite of high carbon chromium bearing steel by carbonitriding

Abstract

A carbonitriding process for achieving high hardness and high residual austenite in high-carbon chromium bearing steel comprises 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, aging treatment to thermally stabilize the residual austenite inside the high-carbon chromium bearing steel; S6, cold treatment to improve the surface hardness of the high-carbon chromium bearing steel; and S7, tempering to achieve high hardness. The beneficial effects of this invention are: by introducing a synergistic process route of "carbonitriding-aging-cold treatment-tempering," this invention effectively solves the contradiction that traditional processes struggle to simultaneously achieve high hardness, high residual austenite, high wear resistance, and good impact resistance.

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 high residual austenite content. Background Technology

[0002] Carbonitriding of high-carbon chromium bearing steel is widely used in the manufacture of bearing parts that operate under harsh conditions such as impact loads, contaminated lubrication, and high contact stress, such as wind turbine main shaft bearings, aero-engine main shaft bearings, and heavy-duty bearings for mining machinery. A typical process involves placing pre-heat-treated high-carbon chromium bearing steel parts in a furnace containing a mixture of carburizing atmosphere and ammonia, and holding them at an austenitizing temperature range of 800–860°C for an extended period. This allows carbon and nitrogen atoms to co-diffuse into the surface layer, forming a carbonitrided layer of a certain depth. The parts are then directly oil-quenched or gas-quenched and subjected to low-temperature tempering. By adjusting parameters such as temperature, atmospheric carbon and nitrogen potentials, and holding time, a high-carbon-content martensitic structure can be obtained on the surface while retaining some residual austenite, with minimal impact on the core strength and toughness.

[0003] However, the microstructure of the carburized layer obtained by existing carbonitriding processes often fails to simultaneously achieve high hardness, high wear resistance, and good impact resistance. On the one hand, if the amount of retained austenite is too low in pursuit of high hardness, although the quenching hardness can be improved, the surface toughness will be reduced, making the parts prone to cracking or even spalling failure under large impacts or heavy loads. On the other hand, if more retained austenite is retained to improve impact resistance, it often leads to insufficient surface hardness and wear resistance, making it difficult to meet more stringent long-life service requirements. Therefore, existing processes cannot simultaneously improve surface hardness and wear resistance while maintaining a high and stable retained austenite content in the carburized layer, limiting the application of high-carbon chromium bearing steel in harsher environments. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a carbonitriding process for high-carbon chromium bearing steel that can maintain high toughness and high residual austenite while improving surface hardness and wear resistance.

[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 high residual austenite content, 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, aged treatment, thermally stabilizes the residual austenite inside the high-carbon bearing steel, with a surface residual austenite content of 15-35% and a core residual austenite content ≤15%; S6, cold treatment to remove high carbon chromium bearing steel and increase the hardness of the parts; S7, tempering, brings the high-carbon bearing steel to 63-68 HRC.

[0006] The beneficial effects of this invention are as follows: By introducing a synergistic process route of "carbonitriding-aging-cold treatment-tempering," this invention effectively solves the contradiction that traditional processes struggle to simultaneously achieve high hardness, high wear resistance, and good impact resistance. Specifically, the S3 carbonitriding step forms a hardened layer with a certain carbon-nitrogen concentration gradient; the S5 aging treatment does not simply remove retained austenite, but rather stabilizes it through thermal activation, improving the mechanical stability of the retained austenite during subsequent cooling or service; the S6 cold treatment promotes the transformation of some unstable austenite into martensite, significantly increasing hardness; finally, the S7 tempering process finely controls the microstructure and eliminates stress. This entire process allows the final austenite layer to achieve a hardness 2 HRC higher than conventional processes while still retaining a suitable amount of stable retained austenite. This stabilized retained austenite layer can absorb energy through a phase transformation-induced plasticity effect when the part is subjected to impact loads, hindering crack initiation and propagation, significantly improving surface toughness, and effectively preventing spalling failure. This results in superior service performance of the part under heavy loads, impacts, and other harsh conditions. As a preferred method, the S5 aging treatment can be carried out in a furnace equipped with a hot air circulation system. The treatment temperature is controlled within a range higher than the subsequent cold treatment temperature but lower than the temperature range where significant microstructure decomposition occurs. This causes carbon atoms to segregate at dislocations and phase interfaces, thereby pinning the phase interfaces and achieving thermal stabilization of the retained austenite. As another preferred method, the S6 cold treatment uses a cryogenic chamber with a programmed cooling rate. By slowly cooling down to reduce thermal stress, and then maintaining this low-temperature environment for a sufficient time, most of the unstable retained austenite is transformed into fine lamellar martensite under controllable conditions, laying the foundation for obtaining a high-hardness and uniformly structured matrix through subsequent tempering.

[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 range 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] This step effectively limits grain growth at high temperatures by controlling the strong carburizing temperature below 890℃, providing a prerequisite for obtaining a strong and tough matrix with a fine grain structure. Simultaneously, maintaining both the carburizing potential and diffusion carbon potential at a low level of no more than 1.5% inhibits the formation of network carbides along grain boundaries, allowing carbonitrides to precipitate as fine, dispersed particles. This second phase morphology significantly improves surface hardness and wear resistance while minimizing the tearing effect on matrix toughness. In the quenching process, the oil temperature is set at 60-140℃, and isothermal or rapid quenching oil is flexibly selected based on the part's wall thickness (isothermal quenching oil is used for wall thicknesses below 6mm, and rapid quenching oil is used for thicknesses above 6mm, with a quenching time of 10-30 minutes), allowing for precise control of the martensitic phase transformation rate. For parts with large differences in wall thickness, using isothermal quenching oil can reduce the temperature difference between the core and the surface, resulting in near-synchronous internal microstructure transformation. This significantly reduces the risk of quenching distortion and cracking, and helps form a multiphase structure of more lath martensite and stable retained austenite, providing ideal microstructure preparation for subsequent aging and cold treatment. As a preferred method, the atmosphere setup in S31 involves injecting liquid nitrogen-containing organic compounds, such as formamide, into a pyrolysis furnace, causing it to decompose at high temperature into gaseous components containing active carbon and nitrogen atoms, which are then introduced into the furnace to more precisely control the ratio of nitrogen and carbon potentials. As another preferred method, the quenching stage in S33 can employ a two-liquid quenching method. This involves first briefly cooling the part in a high-temperature rapid quenching oil to avoid the pearlite transformation zone, and then immediately transferring it to a higher-temperature isothermal quenching oil bath to complete the martensitic transformation, thus balancing the requirements of high hardness and low deformation.

[0009] Furthermore, 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.

[0010] This feature clarifies multiple specific implementation methods for atmosphere composition. Nitrogen serves as a safety gas and diluent, while methanol decomposes into carbon monoxide and hydrogen at high temperatures, working in conjunction with propane to construct a precisely controllable carbon potential environment. Alternatively, a combination of RX gas and propane can be used. RX gas itself is an endothermic, controllable atmosphere with stable gas production and constant composition, and its linkage with propane can efficiently respond to carbon potential fluctuations. These multiple options allow the process to flexibly adapt to the existing gas supply conditions and resource endowments of different plants, lowering the threshold for process implementation and reducing gas costs while ensuring uniform and controllable percolation layer quality. When propane supply is unavailable, natural gas can be directly switched to, as it also provides highly active CH4 groups as a carbon raiser, ensuring process continuity and economy.

[0011] Furthermore, step S5 is a low-temperature tempering process, with a temperature of 50-150°C and a processing time of 1-4 hours.

[0012] The aging process is defined as a low-temperature tempering at 50-150℃ for 1-4 hours. This gentle thermal activation process aims to promote short-range diffusion of carbon and nitrogen atoms, forming Cotillard atmospheres at dislocation lines and the phase interface between martensite and retained austenite. These atmospheres exert a strong pinning effect on dislocation movement, mechanically stabilizing the retained austenite at the microscopic level. After this treatment, the retained austenite will not easily undergo phase transformation and fracture under subsequent slight temperature rises or mechanical friction, but retains its intrinsic advantages of absorbing energy through phase transformation and improving crack propagation resistance under high contact stress or impact. This represents a precise control that significantly improves the overall service life of materials. As a preferred method, this aging process can be carried out in a hot oil bath, utilizing the rapid heat transfer and uniform temperature characteristics of the liquid medium to ensure that all micro-regions of high-precision bearing parts achieve the same stabilization effect. Alternatively, a tempering furnace can be used to slowly raise the temperature to the target aging temperature using a programmed temperature control method and hold it there, extending the thermal diffusion time to achieve more complete microstructural stabilization.

[0013] Furthermore, step S6 involves cold water or liquid nitrogen for cooling treatment at a temperature of -150 to 20°C for 1 to 4 hours.

[0014] This cryogenic treatment step significantly increases the driving force of martensite phase transformation by cryogenically cooling the workpiece to extreme low temperatures of -20°C to -150°C, prompting the further transformation of unaged, carbon- and nitrogen-rich retained austenite with extremely low Ms points into martensite. This process significantly improves the overall hardness of the surface and subsurface layers of the part. Simultaneously, the fresh martensite generated by the phase transformation precipitates finer, more dispersed carbides during subsequent tempering, contributing to secondary hardening. Meanwhile, the retained austenite stabilized by S5 aging is retained, precisely achieving the microstructure control of "removing the unstable and retaining the beneficial." The treatment time is set to 1-4 hours, sufficient to ensure a full balance between the core temperature of the part and the medium temperature, resulting in a thorough and uniform microstructure transformation and avoiding uneven hardness caused by transformation lag. As a preferred method, a liquid nitrogen cryogenic chamber combined with a liquid nitrogen spraying and gas-phase circulation system is used. By controlling the amount of liquid nitrogen injected, the temperature inside the chamber and the cooling rate are precisely adjusted to prevent thermal shock cracking. As another preferred approach, for parts with extremely high dimensional accuracy requirements, a low-temperature pre-tempering can be added between the S6 cold treatment and the S7 tempering to buffer structural stress.

[0015] Furthermore, alkaline cleaning agents or hydrocarbon cleaning agents can be used for cleaning in steps S1 and S4.

[0016] Both the pre-cleaning S1 and the post-co-diffusion cleaning S4 explicitly use alkaline or hydrocarbon cleaning agents to ensure surface cleanliness control throughout the entire process. The pre-cleaning effectively removes residual rust-preventive oil, cutting fluid, and other contaminants from the processing, preventing their decomposition during high-temperature co-diffusion to produce carbon black, acid radicals, and other substances that could negatively impact the furnace atmosphere and workpiece surface, thus ensuring the stability and repeatability of the diffusion layer concentration gradient. The post-cleaning removes residual oil adhering to the part surface after quenching and oil cooling, providing a clean surface for subsequent aging and cold treatment processes, and preventing residual oil from coking and sintering during subsequent heating, which would affect the uniformity of the chemical heat treatment. Furthermore, compared to traditional chlorine-containing solvent cleaning, alkaline and hydrocarbon cleaning agents are more environmentally friendly, reducing health hazards to operators and emissions of volatile organic compounds. As a preferred method, step S1 can employ ultrasonic-assisted alkaline cleaning, using the cavitation effect generated by high-frequency oscillation to peel away stubborn contaminants in deep holes and tiny grooves, achieving high cleanliness requirements. As another preferred method, step S4 uses a vacuum hydrocarbon cleaning machine, which uses hydrocarbon solvents to vaporize and liquefy repeatedly to immerse and steam wash the parts under vacuum, and realizes the recycling of solvents through a recovery distillation system, achieving zero-emission clean production.

[0017] Furthermore, the tempering temperature in step S7 is set to 160-400℃, and the holding time is 2 hours.

[0018] Setting the final tempering temperature within the range of 160-400℃ effectively eliminates the macroscopic and microscopic internal stresses accumulated during quenching and cold treatment, significantly reducing the brittleness of the steel. Within this temperature range, the supersaturated fresh martensite formed during cold treatment precipitates a large number of dispersed submicron-sized transition carbides, achieving dispersion strengthening or secondary hardening effects. This effectively compensates for the excessive decrease in matrix hardness that may be caused by high-temperature tempering, and is a key technical guarantee for ultimately obtaining the aforementioned high hardness. Simultaneously, this temperature range avoids significant decomposition of the stabilized residual austenite, thus completely preserving the designed multiphase microstructure. The wide range of temperature selection allows for final fine-tuning of toughness and hardness according to the service conditions of the final part: a lower tempering temperature can be used if extremely high hardness and resistance to abrasive wear are required; if optimized toughness to withstand severe impacts is needed, tempering at the upper limit temperature can be used, providing flexible microstructure control and strong process adaptability. As a preferred method, this step can be performed in a tempering furnace with a protective atmosphere, through which pure nitrogen is introduced to prevent the workpiece surface from oxidizing at temperatures above 160°C, thus avoiding the formation of tempering discoloration that could affect the appearance and subsequent dimensional stability. Alternatively, this tempering step can be performed in two or more stages, employing multi-stage tempering. After the first tempering, the workpiece is cooled to room temperature before undergoing a second tempering to more thoroughly eliminate stress and stabilize residual austenite, thereby optimizing dimensional accuracy and hardness uniformity. Attached Figure Description

[0019] 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

[0020] This embodiment provides a process for carbonitriding high-carbon chromium bearing steel to achieve high hardness and high residual austenite. This process, through the synergistic combination of specific cleaning, pre-oxidation, carbonitriding, aging, cold treatment and tempering steps, aims to significantly improve the surface hardness of high-carbon chromium bearing steel parts and optimize their residual austenite state, ultimately obtaining high-performance parts with a hardness increase of more than 2 HRC while retaining a high level of beneficial residual austenite.

[0021] The process begins with pre-cleaning step S1, the purpose of which is to ensure that the surface of the high-carbon chromium bearing steel parts is clean and free of oil. In specific operation, alkaline cleaning agents or hydrocarbon cleaning agents can be selected to thoroughly clean the parts, depending on the type of contaminants that adhered to them during the machining process, providing a clean surface foundation for subsequent heat treatment processes.

[0022] After the initial cleaning, the parts undergo a pre-oxidation step S2. The main purpose of this step is to remove any water stains that may remain on the surface of the parts from the previous process and to pre-form a thin and uniform oxide film. The selectable temperature is 300-650℃. This oxide film can effectively activate the surface in the initial stage of subsequent carbonitriding, promoting the uniformity of the carburizing and nitriding processes.

[0023] The next step is the core carbonitriding process, S3, which aims to significantly improve the surface hardness and wear resistance of steel by simultaneously infiltrating carbon and nitrogen into the surface. Specifically, the S31 atmosphere is established and maintained by introducing ammonia gas into the furnace to provide nitrogen, supplemented by a protective atmosphere. This protective atmosphere can be a combination of nitrogen, methanol, and propane, or a combination of RX gas (i.e., an endothermic atmosphere) and propane; propane can be replaced with natural gas as an equivalent, depending on the specific circumstances. Under a stable carbonitriding atmosphere, the S32 strong infiltration-diffusion process is performed. The strong infiltration stage is carried out at a temperature below 890°C, with a maximum infiltration time of 6 hours. The strong infiltration carbon potential is strictly controlled within a range not exceeding 1.5% to achieve rapid infiltration of carbon and nitrogen atoms. The diffusion stage then begins, with the diffusion carbon potential also controlled within a range not exceeding 1.5% to promote the diffusion of infiltrated atoms into the inner layer, forming an ideal concentration gradient and infiltration depth. After the carbonitriding treatment is completed, the S33 quenching operation is performed directly. The quenching oil temperature is set within the range of 60℃ to 140℃. Process engineers can flexibly choose to use isothermal quenching oil or rapid quenching oil according to the different wall thicknesses and distortion requirements of the parts. 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 to ensure that the parts obtain high hardness while minimizing the risk of quenching distortion and cracking.

[0024] After carbonitriding and quenching, a post-cleaning step S4 is performed. This step is similar to the pre-cleaning step S1, and alkaline or hydrocarbon cleaning agents can be used to thoroughly remove residual quenching oil and other contaminants from the surface of the parts, restoring the surface to a clean state and preparing it for subsequent processes.

[0025] After cleaning, the parts undergo aging treatment step S5, which involves a low-temperature tempering. This treatment is carried out at a temperature of 50°C to 150°C for 1 to 4 hours. The core purpose of this step is not to adjust the hardness, but to thermally stabilize the unstable residual austenite after quenching through thermal activation, allowing it to transform more smoothly and effectively during the subsequent cold treatment process.

[0026] Following aging, a cryogenic treatment step S6 is performed to further enhance surface hardness. This treatment uses cold water or liquid nitrogen as the cooling medium, subjecting the parts to deep cryogenic treatment at a low temperature of -150°C to -20°C for 1 to 4 hours. Under these cryogenic conditions, most of the thermally stabilized residual austenite in the parts will continue to transform into martensite, thereby significantly increasing the final hardness of the parts.

[0027] Finally, tempering step S7 is performed, where the part is heated to a temperature range of 160°C to 400°C for tempering. This step aims to eliminate the micro-stress generated during cold treatment and, through the secondary hardening effect of precipitated dispersed carbides, enable the part to achieve the required high hardness, while stabilizing the microstructure and obtaining an ideal combination of strength and toughness.

[0028] The working principle of this embodiment is as follows: The working principle of this process revolves around the microstructure transformation and stress control of high-carbon chromium bearing steel. First, through the fine cleaning in step S1 and the pre-oxidation in step S2, optimal surface conditions are created for the carbonitriding in step S3, ensuring the uniformity of the diffusion layer. In step S3, by precisely controlling the co-diffusion temperature below 890°C, the protective atmosphere composed of ammonia and nitrogen / methanol / propane or RX / propane, and the carbon potential not exceeding 1.5% in both the strong diffusion and diffusion stages, controllable co-diffusion of carbon and nitrogen atoms is achieved, resulting in ideal surface composition and microstructure. In the subsequent step S33, isothermal or rapid quenching is performed in hot oil at 60-140°C, transforming the high-carbon, high-nitrogen austenite on the surface into high-hardness martensite, while retaining a significant amount of residual austenite.

[0029] The subsequent steps S5 to S7 are designed to maintain high hardness while controlling the beneficial effects of retained austenite. Step S5 involves aging at 50-150℃ for 1-4 hours to thermally stabilize the unstable retained austenite after quenching, preventing disordered transformation and microcracks during the -150℃ to -20℃ cryogenic treatment in step S6. The aged retained austenite then smoothly transforms into martensite during cryogenic treatment, resulting in improved surface hardness beyond that of conventional carbonitriding. Finally, the S7 tempering treatment at 160-400℃ eliminates the stress generated by cryogenic treatment and produces a secondary hardening effect through the precipitation of ultrafine carbides, further solidifying the high hardness. Meanwhile, after tempering in this temperature range, some of the thermally stable retained austenite is retained. When the part is subjected to stress during use, this retained austenite can undergo phase transformation, absorb energy, and delay crack propagation, thereby improving the contact fatigue life and wear resistance of the part. This is the core mechanism by which this process enables the part to retain a high level of beneficial retained austenite while achieving ultra-high hardness.

[0030] 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.

[0031] This embodiment employs the above-described process and selects... Figure 1 The specific parameters are as follows. The product parameter indicators are obtained as follows: Figure 2-3 As shown.

[0032] 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 high residual austenite content, 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, aged treatment, thermally stabilizes the residual austenite inside the high-carbon bearing steel, with a surface residual austenite content of 15-35% and a core residual austenite content ≤15%; S6, cold treatment to remove high carbon chromium bearing steel and increase the hardness of the parts; S7, tempering, brings the high-carbon bearing steel to 63-68 HRC.

2. The process for carbonitriding high-carbon chromium bearing steel to achieve high hardness and high residual austenite content according to 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%, and diffusion carbon potential is no more than 1.5%; S33, quenching, oil temperature set at 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 high residual austenite content according to 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 high residual austenite content according to claim 1, characterized in that: Step S5 is a low-temperature tempering process, with a temperature of 50-150℃ and a processing time of 1-4 hours.

5. The process method for carbonitriding high-carbon chromium bearing steel to achieve high hardness and high residual austenite content according to claim 1, characterized in that: Step S6 involves cold water or liquid nitrogen for cooling treatment at a temperature of -150 to 20°C for 1 to 4 hours.

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

7. The process method for carbonitriding high-carbon chromium bearing steel to achieve high hardness and high residual austenite content according to claim 1, characterized in that: The tempering temperature in step S7 is set to 160-400℃, and the holding time is 2 hours.

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