Gear part and production method and application thereof

Abstract

The invention relates to a gear part and a production method and application thereof. The process route of the production method comprises the steps of smelting, rolling, forging, machining, hot charging, cleaning, pre-oxidizing, preheating, carburizing, step quenching and tempering. The carburizing technology comprises the steps that convection heating, vacuum heating, pulse carburizing and vacuum heat preservation are sequentially conducted on the gear part at the carburizing temperature, and then heat preservation diffusion is conducted at the temperature 20-50 DEG C lower than the carburizing temperature; the step quenching technology comprises the steps that high-pressure nitrogen is adopted for purging the gear part, and the gear part is cooled in two stages. By improving the 20CrMnTi steel and the quenching process after carburization, the content of bainite which is not easy to expand in a matrix is increased, so that the thermal deformation is reduced, the tooth grinding amount in subsequent processing is reduced, and the production cost is remarkably reduced.

Description

Technical Field

[0001] This invention relates to the field of gear part processing technology, and in particular to a gear part, its production method, and its application. Background Technology

[0002] Gear components are core parts of mechanical transmission. They are toothed cylindrical structures that transmit power and motion through tooth meshing. They are widely used in key systems such as automotive gearboxes and drive shafts. They must have high strength to resist wear and impact, and high precision to ensure transmission efficiency.

[0003] However, in actual production, heat treatment deformation has always been a key bottleneck restricting the improvement of the quality of shaft and gear parts. The essence of this problem stems from the difference in characteristics between the two microstructures of martensite and bainite formed during the carburizing and quenching process: Although martensite can give the surface of shaft and gear parts extremely high hardness, its phase transformation expansion rate is as high as 3% to 5%, which can easily lead to defects such as twisting and cracking in shaft and gear parts; In contrast, bainite has an expansion rate of only 0.5% to 1% and better toughness, which is theoretically more conducive to deformation control, but it has not been widely used because it is difficult to obtain stably by traditional processes. Traditional carburizing heat treatment processes (such as oil quenching, pressure quenching, and repeated tempering) often face a dilemma in practice. If rapid cooling methods such as oil quenching are used, although surface hardness can be guaranteed, a large amount of martensite will be generated, which will aggravate deformation. If the cooling rate is controlled by staged quenching or isothermal quenching, the cost will increase due to the complexity of the process and the extension of the production cycle, and some mechanical properties may even be sacrificed. It is difficult to achieve a balance between deformation control and production efficiency. Therefore, it is urgent to break through this dilemma by optimizing the composition design and process parameters of carburized gear steel.

[0004] In the current automotive industry, 20CrMnTi steel has become the mainstream choice for shaft and gear steel due to its unique properties: This material significantly improves hardenability by adding Cr (1.00%~1.30%) and Mn (0.80%~1.10%), ensuring a uniform microstructure from surface to core after quenching for shaft and gear parts with a cross-section ≤30mm. After carburizing and quenching followed by low-temperature tempering, a hard and wear-resistant tempered martensite layer (carburized layer depth 0.8~1.2mm) forms on the surface, while the core maintains a low-carbon martensite or bainite microstructure with a balance of strength and toughness (hardness 30-45HRC, impact energy ≥55J), perfectly matching the "hard outside, tough inside" requirements of shaft and gear parts. Furthermore, the addition of Ti (0.04%~0.10%) effectively refines the grains, suppresses grain growth during high-temperature carburizing, reduces overheat sensitivity, and, combined with the excellent machinability after normalizing, further ensures the machining accuracy and heat treatment stability of shaft and gear parts. However, existing 20CrMnTi shaft and gear parts still face insurmountable deformation problems in traditional carburizing processes. To pursue high efficiency, most production still uses oil quenching as the cooling method. While rapid cooling achieves surface hardening, it inevitably promotes the precipitation of supersaturated carbon atoms in the crystal lattice, forming a large amount of lamellar martensite. This leads to a surge in the volume expansion rate of the shaft and gear parts, with tooth profile errors reaching 0.05~0.15mm, far exceeding the tolerance requirements of high-precision shafts and gears. Therefore, companies have to add subsequent grinding and straightening processes for correction, which not only increases production costs by 15%~20%, but may also cause uneven carburized layer thickness due to secondary processing, reducing the fatigue life of the shaft and gear parts. This contradiction between "material performance advantages and process deformation defects" highlights the limitations of existing 20CrMnTi shaft and gear parts in high-precision transmission systems and provides a practical necessity for the research and application of bainitic matrix shaft and gear parts. Summary of the Invention

[0005] To address the problem of severe thermal deformation of existing 20CrMnTi gear parts after carburizing, this invention provides a gear part, its manufacturing method, and its application.

[0006] The technical solution provided by this invention is as follows: In a first aspect, the present invention provides a method for producing gear parts, wherein the chemical composition of the gear parts, by mass percentage, comprises: C: 0.19%~0.22%, Si: 0.17%~0.37%, Mn: 1.10%~1.25%, Cr: 1.20%~1.35%, S: 0.02%~0.04%, Ti: 0.01%~0.05%, Nb: 0.015%~0.03%, Cu: ≤0.2%, Ni: ≤0.30%, P: ≤0.015%, N: ≤0.008%, with the balance being Fe and unavoidable impurities; The production process is as follows: smelting → rolling → forging → machining → hot charging → cleaning → pre-oxidation → preheating → carburizing → graded quenching → tempering; wherein: The carburizing process includes: sequentially performing convection heating, vacuum heating, pulse carburizing, and vacuum heat preservation on gear parts at the carburizing temperature, followed by heat preservation and diffusion at a temperature 20~50℃ lower than the carburizing temperature. The graded quenching process includes: purging the gear parts with high-pressure nitrogen gas and cooling them in two stages; wherein: First cooling stage: Control the fan speed and nitrogen pressure to cool the gear parts to 587~798℃ within 120 seconds; The second cooling stage: controlling the fan speed and nitrogen pressure to cool the gear parts to 374°C for no less than 180 seconds.

[0007] In conjunction with the first aspect of the invention, some embodiments include: The carburizing process is carried out in a vacuum carburizing furnace, with three stages: convection heating, vacuum heating, and pulse carburizing, maintaining and controlling the furnace temperature at the carburizing temperature; wherein: Convection heating stage: Evacuate the furnace to a vacuum level of <10 mbar, then fill it with nitrogen to a pressure of 1000~1200 mbar for convection heating, and hold for 40~60 minutes; Vacuum heating stage: Evacuate the furnace to a vacuum level of <10mbar and hold for 15~30 minutes.

[0008] In conjunction with the first aspect of the invention, some embodiments include: Pulse carburizing stage: Acetylene is introduced into the furnace in 5-9 cycles, followed by a period of diffusion. Each acetylene injection takes 30-180 seconds, the acetylene flow rate is 2000-10000 L / h, and the waiting time is 3-14 minutes.

[0009] In conjunction with the first aspect of the present invention, in some embodiments: the carburizing temperature is 920°C to 1000°C.

[0010] In conjunction with the first aspect of the present invention, in some embodiments: the heat preservation time of the vacuum heat preservation stage is 30 to 120 minutes; and / or, the heat preservation time of the heat preservation diffusion stage is 30 to 120 minutes.

[0011] In conjunction with the first aspect of the invention, some embodiments include: The pressure of the high-pressure nitrogen gas is 10~18 bar; The purging time for the first cooling stage is 30-120 seconds, and the fan speed is 2400-3000 rpm; The second cooling stage has a purging time of 180-240 seconds and a fan speed of 800-2000 rpm.

[0012] In conjunction with the first aspect of the present invention, in some embodiments: the gear part is a shaft gear part.

[0013] In conjunction with the first aspect of the present invention, in some embodiments: the preheating process includes: placing the pre-oxidized gear parts in a vacuum environment and heating them to 650±10°C, and holding them at that temperature for 60~120 minutes.

[0014] Secondly, the present invention provides a gear part, which is produced by the above-described manufacturing method.

[0015] Thirdly, the present invention provides an automobile comprising the aforementioned gear parts.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: This invention improves the 20CrMnTi steel and the quenching process after carburizing, increasing the content of bainite, which is not easily expanded, in the matrix to reduce the amount of hot deformation, thereby reducing the amount of grinding in subsequent processing and significantly reducing production costs. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The image shows the metallographic structure of the core of the gear part sample obtained in Example 1.

[0019] Figure 2 The image shows the metallographic structure of the core of the gear part sample obtained in Example 2.

[0020] Figure 3 The image shows the metallographic structure of the core of the gear part sample obtained in Example 3.

[0021] Figure 4 This is a comparison chart of the accuracy of gear part samples from three embodiments. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0023] For simplicity, this paper only discloses some explicitly defined numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range. Similarly, any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit, combined with any other point or individual value, or combined with other lower or upper limits to form an undefined range.

[0024] It should be noted that, in this description, unless otherwise stated, "above" and "below" include the number itself, and "multiple" in "one or more" means two or more. Relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0025] In the description of this specification, the references to terms such as "any embodiment / mode," "one embodiment / mode," "some embodiments / modes," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment / mode or example is included in at least one embodiment / mode or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment / mode or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments / modes or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments / modes or examples described in this specification, as well as the features of different embodiments / modes or examples.

[0026] The above description of the invention is not intended to describe every disclosed embodiment or implementation. Instead, exemplary embodiments are described in more detail below. These embodiments can be used in various combinations. The examples listed in the various embodiments are representative only and should not be construed as exhaustive.

[0027] To improve the problem of hot deformation of 20CrMnTi steel after carburizing and quenching, this invention optimizes the composition of 20CrMnTi steel and the carburizing and quenching process, resulting in gear parts with less hot deformation.

[0028] In the method for producing gear parts provided by the present invention, the chemical composition of the gear parts, by mass percentage, includes: C: 0.19%~0.22%, Si: 0.17%~0.37%, Mn: 1.10%~1.25%, Cr: 1.20%~1.35%, S: 0.02%~0.04%, Ti: 0.01%~0.05%, Nb: 0.015%~0.03%, Cu: ≤0.2%, Ni: ≤0.30%, P: ≤0.015%, N: ≤0.008%, with the balance being Fe and unavoidable impurities; The manufacturing process for gear parts is as follows: smelting → rolling → forging → machining → hot fitting → cleaning → pre-oxidation → preheating → carburizing → graded quenching → tempering; where: The carburizing process includes: sequentially performing convection heating, vacuum heating, pulse carburizing, and vacuum heat preservation on gear parts at the carburizing temperature, followed by heat preservation and diffusion at a temperature 20~50℃ lower than the carburizing temperature. The graded quenching process includes: purging the gear parts with high-pressure nitrogen gas and cooling them in two stages; wherein: First cooling stage: Control the fan speed and nitrogen pressure to cool the gear parts to 587~798℃ within 120 seconds; The second cooling stage: controlling the fan speed and nitrogen pressure to cool the gear parts to 374°C for no less than 180 seconds.

[0029] This invention primarily aims to improve the quenching process after carburizing to reduce hot deformation. Traditional oil quenching can only rapidly cool gear parts to room temperature at a fixed cooling rate, making it difficult to control phase transformation and hot deformation during the cooling process. This invention employs vacuum segmented gas quenching, which differs from direct gas quenching. First, the gear parts are rapidly cooled at a relatively fast cooling rate to between the critical temperature for ferrite formation (798℃) and the critical temperature for bainite formation (587℃) to avoid high-temperature phase transformation. Then, they are slowly cooled to 374℃ at a lower cooling rate to generate more bainite. Since the expansion amount of bainite phase transformation is only one-sixth to one-eighth of that of martensite phase transformation, the amount of hot deformation of the parts is reduced.

[0030] To match the cooling rate in the graded quenching process, this invention employs a high hardenability 20CrMnTiH steel suitable for vacuum gas quenching: its lower C content ensures the weldability of gear parts; Si element can inhibit grain boundary oxidation and carbide precipitation and improve steel strength; its Mn and Cr content are appropriately increased compared to conventional 20CrMnTi steel, ensuring the hardenability of gear parts; while the Ti content is reduced compared to conventional 20CrMnTi steel, which can control the large-particle composite precipitates of Ti; the addition of a small amount of Nb element improves the stability of high-temperature austenite grains and refines Ti precipitates.

[0031] This invention also improves the heating method of the carburizing process by using segmented heating to eliminate stress caused by previous processing. In some embodiments, the carburizing process is carried out in a vacuum carburizing furnace, maintaining the furnace temperature at the carburizing temperature through three stages: convection heating, vacuum heating, and pulse carburizing. Specifically: in the convection heating stage, the furnace is evacuated to a vacuum level <10 mbar, and then nitrogen is introduced to a pressure of 1000~1200 mbar for convection heating, held for 40~60 minutes; in the vacuum heating stage, the furnace is evacuated to a vacuum level <10 mbar, held for 15~30 minutes. This invention achieves rapid and uniform heating through nitrogen circulation convection heating in a vacuum carburizing furnace; convection heating efficiency is higher than traditional radiation heating, which can shorten the process time and reduce the overall heat treatment cycle; it protects the surface of gear parts: the nitrogen environment avoids oxidation, ensures surface cleanliness before carburizing, and reduces carbon black adhesion. This invention then uses vacuum heating to eliminate impurity gases in the furnace, allowing the gear parts to be fully austenitized. A vacuum environment can reduce oxides and non-metallic inclusions, improve material purity, and reduce grain boundary brittleness; rapid austenitization ensures uniform composition between the carburized layer and the core, avoiding localized abnormal carbon concentration.

[0032] This invention also employs pulse carburizing by injecting acetylene into the furnace in 5-9 cycles to precisely control the carbon potential and carburized layer depth, thereby reducing carbon black formation. In some embodiments: during the pulse carburizing stage, acetylene is injected into the furnace in 5-9 cycles, each injection lasting 30-180 seconds, with an acetylene flow rate of 2000-10000 L / h, and a waiting time of 3-14 minutes per cycle. Injecting acetylene in 5-9 cycles improves carbon diffusion efficiency, avoids carbon black deposition caused by excessive acetylene, enhances the uniformity of the carburized layer, and reduces defects. The acetylene flow rate is mainly related to the loading capacity and surface area of ​​the part. The carburizing temperature primarily affects the heat treatment cycle time and the magnitude of thermal deformation. The number of pulses, the duration of each acetylene injection, and the subsequent holding time are mainly determined by the carburized layer depth and the required surface hardness. Simultaneously, reasonable control of the acetylene flow rate and the number of pulses can improve acetylene utilization and prevent carbon black formation. After pulse carburizing, a heat-diffusion holding process is performed for 30-120 minutes. The holding temperature also determines the magnitude of thermal deformation during quenching; therefore, an appropriate holding temperature needs to be selected based on the processing cycle and the magnitude of thermal deformation.

[0033] In some embodiments of the present invention, the carburizing temperature is 920℃~1000℃.

[0034] This invention employs a temperature-controlled diffusion process following pulse carburizing. This allows carbon elements penetrating the surface of the part to diffuse inward, mitigating stress concentration caused by the carbon concentration gradient and reducing the risk of quenching deformation. In some embodiments, the holding time during the diffusion stage is 30–120 minutes. The holding temperature during the diffusion stage needs to be selected based on the machining cycle time and thermal deformation control requirements.

[0035] Unlike direct gas quenching, the vacuum segmented gas quenching method of this invention first cools the gear parts to 587~798℃ at a relatively fast cooling rate, and then slowly cools them to below 374℃ at a lower cooling rate. Specifically, high-pressure nitrogen is used as the quenching medium, and the gear parts are purged from the top. In some embodiments, the pressure of the high-pressure nitrogen is 10~18 bar; the purging time of the first cooling stage is 30~120 seconds, and the fan speed is 2400~3000 rpm; the purging time of the second cooling stage is 180~240 seconds, and the fan speed is 800~2000 rpm.

[0036] In conjunction with the first aspect of the present invention, in some embodiments: the gear part is a shaft gear part.

[0037] This invention also includes preheating before carburizing to release residual stress generated by previous processing (forging, machining), eliminating and avoiding localized deformation caused by stress release during high-temperature carburizing; at the same time, it promotes the homogenization of material grains, providing a stable and uniform matrix structure for subsequent carburizing and quenching, and reducing the risk of intergranular ferrite formation. In some embodiments: the preheating process includes: placing the pre-oxidized gear parts in a vacuum environment and heating to 650±10℃, holding at that temperature for 60~120 minutes.

[0038] This invention also releases residual stress generated during quenching through thorough tempering and stabilizes the bainitic and martensitic structures. In some embodiments, the tempering temperature is 200-300°C, and the tempering time is 120-180 minutes. Preferably, the tempering temperature is 170-190°C.

[0039] This invention also includes cleaning and pre-oxidizing the parts before preheating. The purpose of cleaning is to remove oil stains from the surface of the parts after machining; pre-oxidation can remove residual oil stains from the surface of the parts through high temperature, improving the uniformity of carburizing; on the other hand, it can form a dense oxide layer on the surface of the parts, promoting subsequent carburizing.

[0040] To ensure uniform stress distribution on gear parts during heat treatment, this invention employs a three-point support vertical mounting method. The three-point support allows for more even stress distribution on the gear parts, resulting in more stable thermal deformation. Compared to the traditional horizontal mounting method, the vertical mounting significantly improves the resistance to bending deformation caused by gravity. Furthermore, due to its smaller footprint, the vertical mounting method can hold 48 to 360 parts per furnace.

[0041] In addition, to reduce the impact of heat treatment deformation on the post-heat processing, the tool feed rate and feed amount can be controlled to reduce residual stress generated on the surface of the part during the processing.

[0042] The gear parts prepared by this invention have a core structure that is mostly bainitic. The gear surface hardness is 710 HV, and the core hardness is 326 HV. The surface hardness meets the wear resistance requirements of automotive gears under high-speed and heavy-load conditions, reducing the decrease in transmission efficiency caused by tooth surface wear. The bainitic core structure (with better toughness than martensite) can effectively absorb impact loads, and the bainitic core structure reduces deformation. When assembled into automotive transmission components, it can improve transmission efficiency, reduce the amount of tooth grinding, and lower production costs.

[0043] In some embodiments, the method for producing gear parts provided by the present invention includes the following process route: hot fitting → cleaning → pre-oxidation → preheating → carburizing → graded quenching → tempering; wherein: Hot charging: Three-point support vertical charging is adopted, and 48 to 360 gear parts are charged in one furnace; Cleaning: Cleaning removes oil and dirt from the surface of gear parts after machining; Pre-oxidation treatment: The gear parts are fed into a continuous pre-oxidation furnace at a temperature of 380℃±30℃.

[0044] Preheating treatment: Heat the pre-oxidized gear parts to 650℃±10℃ and hold for 60~120 minutes.

[0045] Carburizing heat treatment consists of, in sequence, a convection heating stage, a vacuum heating stage, a pulse carburizing stage, and a heat-holding diffusion stage; among which: Convection heating stage: Temperature 920~1000℃, evacuate the furnace to a vacuum degree <10mbar, then fill with nitrogen to a pressure of 1000~1200mbar for convection heating, and hold for 40~60 minutes; Vacuum heating stage: Temperature 920~1000℃, evacuate the furnace to a vacuum degree <10mbar, and hold for 15~30 minutes.

[0046] Pulse carburizing stage: temperature 920~1000℃, waiting time 3~14 minutes each time, acetylene charging time 30~180 seconds each time, acetylene flow rate 2000~10000L / h; Heat preservation and diffusion stage: After cooling down by 20~50℃, vacuum heat preservation for 30~120 minutes; Staged quenching: Vacuum segmented gas quenching is adopted, and high-pressure nitrogen is used to purge the gear parts, cooling them to below 374℃ in two stages; wherein: First cooling stage: fan speed 2400~3000 rpm, purging time 30~120 seconds, high-pressure nitrogen pressure 10~18 bar; Second cooling stage: fan speed 800~2000 rpm, purging time 180~240 seconds, high-pressure nitrogen pressure 10~18 bar; Tempering: Temperature 200~300℃, time 120~180 minutes.

[0047] The following are embodiments of the present invention. These embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in the art or according to the product manual. Unless otherwise specified, the equipment or raw materials used are all conventional products that can be obtained commercially.

[0048] Example 1: The method for manufacturing gear parts provided in this embodiment specifically includes the following steps: 1. Alloying treatment during steel bar refining process Steel composition: C: 0.19%, Si: 0.23%, Mn: 1.10%, Cr: 1.15%, Ti: 0.040%, Ni: 0.02%, Nb: 0.025%, Cu: 0.03%, P: 0.017%, S: 0.019%, N: 0.0065%, with the remainder being Fe and unavoidable impurities.

[0049] 2. Forging process 2.1 Heating: The gear steel bar is heated to 1250℃ in a medium-frequency heating furnace, and the initial forging temperature is measured to be 1100℃.

[0050] 2.2 Upsetting: Upset once on an open press.

[0051] 2.3 Die forging: After upsetting, the billet is forged into a casting on a vertical die forging machine (once), and then clamped to a punching machine for punching.

[0052] 2.4 Cooling: Air cooling after drilling; concentrated normalizing after air cooling (divided into three zones): Zone 1: 850℃, Zone 2: 900℃, Zone 3: 920℃, each waiting time is 120 minutes; After normalizing, the upper and lower fans are simultaneously cooled: the upper fan frequency is 40Hz, the lower fan frequency is 20Hz, and the duration is 180s.

[0053] 3. Machining After normalizing, the forgings undergo rough turning, finish turning, gear hobbing, and other processes in sequence.

[0054] 4. Hot-loading method It adopts a three-point support vertical assembly, with 80 gear parts loaded in one furnace.

[0055] 5. Cleaning Clean the oil stains on the surface of gear parts after cleaning the machine to ensure uniform carburization.

[0056] 6. Pre-oxidation treatment The gear parts are fed into a continuous pre-oxidation furnace at a temperature of 380°C.

[0057] 7. Preheating treatment Transfer the pre-oxidized parts to a vacuum carburizing furnace, evacuate the furnace to a vacuum level below 10 mbar, heat to about 650°C, and hold for 90 minutes.

[0058] 8. Carburizing heat treatment 8.1 Convection heating stage (furnace temperature 950℃): Nitrogen gas is introduced into the vacuum carburizing furnace to a pressure of 1200mbar, and convection heating is carried out by circulating heating through a nitrogen protector for 50 minutes. 8.2 Vacuum heating stage (furnace temperature 950℃): Evacuate the furnace to a vacuum level of <10mbar and hold for 30 minutes.

[0059] 8.2 Pulse carburizing stage (furnace temperature 950℃): First time: 160 seconds of acetylene charging, acetylene flow rate 7300 L / h, wait 6 minutes; Second time: Charge acetylene for 60 seconds, acetylene flow rate 5000L / h, wait for 6 minutes; 3rd-4th times: Charge with acetylene for 60 seconds at an acetylene flow rate of 4900 L / h, then wait for 8 minutes. 5th-6th times: Charge with acetylene for 60 seconds at a flow rate of 4500 L / h, then wait 10 minutes. 7th time: 60 seconds of acetylene charging, acetylene flow rate 4700L / h, wait 12 minutes; 8th time: 30 seconds of acetylene charging, acetylene flow rate 5900L / h, wait 3 minutes.

[0060] 8.3 Diffusion and heat preservation: After carburizing, first keep it at 950℃ in vacuum for 40 minutes, then cool it down to 920℃ and keep it in vacuum for diffusion for 80 minutes.

[0061] 9. Staged quenching High-pressure nitrogen gas was used to purge the gear parts from the top, and the parts were cooled to room temperature in two stages; wherein: First cooling stage: nitrogen pressure 16 bar, purging for 45 seconds, fan speed 3000 rpm; Second cooling stage: nitrogen pressure 10 bar, purging for 255 seconds, fan speed 800 rpm.

[0062] 10. Tempering Temperature 180℃, time 165 minutes.

[0063] Test results: Hardness: 710 HV on the surface, 326 HV in the core.

[0064] The core structure of the gear part sample obtained in this embodiment is as follows: Figure 1 As shown, the core microstructure is mostly bainite, with a small amount of ferrite and martensite. Figure 1 Analysis revealed that the high-temperature phase transformation during the quenching process was not completely avoided. Some gear parts contained intergranular ferrite and blocky ferrite, which are microstructures that should be avoided during carburizing heat treatment. The small amount of ferrite resulted in uneven microstructure distribution in the gear parts, and even with an increase in the amount of bainite phase change, thermal deformation remained uneven.

[0065] Example 2: The method for manufacturing gear parts provided in this embodiment specifically includes the following steps: 1. Alloying treatment during steel bar refining process Steel composition: C: 0.19%, Si: 0.23%, Mn: 1.10%, Cr: 1.15%, Ti: 0.040%, Ni: 0.02%, Nb: 0.025%, Cu: 0.03%, P: 0.017%, S: 0.019%, N: 0.0065%, with the remainder being Fe and unavoidable impurities.

[0066] 2. Forging process 2.1 Heating: The gear steel bar is heated to 1250℃ in a medium-frequency heating furnace, and the initial forging temperature is measured to be 1100℃.

[0067] 2.2 Upsetting: Upset once on an open press.

[0068] 2.3 Die forging: After upsetting, the billet is forged into a casting on a vertical die forging machine (once), and then clamped to a punching machine for punching.

[0069] 2.4 Cooling: Air cooling after drilling; concentrated normalizing after air cooling (divided into three zones): Zone 1: 850℃, Zone 2: 900℃, Zone 3: 920℃, each waiting time is 120 minutes; After normalizing, the upper and lower fans are simultaneously cooled: the upper fan frequency is 40Hz, the lower fan frequency is 20Hz, and the duration is 180s.

[0070] 3. Machining After normalizing, the forgings undergo rough turning, finish turning, gear hobbing, and other processes in sequence.

[0071] 4. Hot-loading method It adopts a three-point support vertical assembly, with 80 gear parts loaded in one furnace.

[0072] 5. Cleaning Clean the oil stains on the surface of gear parts after cleaning the machine to ensure uniform carburization.

[0073] 6. Pre-oxidation treatment The gear parts are fed into a continuous pre-oxidation furnace at a temperature of 380°C.

[0074] 7. Preheating treatment Transfer the pre-oxidized parts to a vacuum carburizing furnace, evacuate the furnace to a vacuum level below 10 mbar, heat to about 650°C, and hold for 90 minutes.

[0075] 8. Carburizing heat treatment 8.1 Convection heating stage (furnace temperature 950℃): Nitrogen gas is introduced into the vacuum carburizing furnace to a pressure of 1200mbar, and convection heating is carried out by circulating heating through a nitrogen protector for 50 minutes. 8.2 Vacuum heating stage (furnace temperature 950℃): Evacuate the furnace to a vacuum level of <10mbar and hold for 30 minutes.

[0076] 8.2 Pulse carburizing stage (furnace temperature 950℃): First time: 160 seconds of acetylene charging, acetylene flow rate 7300 L / h, wait 6 minutes; Second time: Charge acetylene for 60 seconds, acetylene flow rate 5000L / h, wait for 6 minutes; 3rd-4th times: Charge with acetylene for 60 seconds at an acetylene flow rate of 4900 L / h, then wait for 8 minutes. 5th-6th times: Charge with acetylene for 60 seconds at a flow rate of 4500 L / h, then wait 10 minutes. 7th time: 60 seconds of acetylene charging, acetylene flow rate 4700L / h, wait 12 minutes; 8th time: 30 seconds of acetylene charging, acetylene flow rate 5900L / h, wait 3 minutes.

[0077] 8.3 Diffusion and heat preservation: After carburizing, first keep it at 950℃ in vacuum for 40 minutes, then cool it down to 920℃ and keep it in vacuum for diffusion for 80 minutes.

[0078] 9. Staged quenching High-pressure nitrogen gas was used to purge the gear parts from the top, and the parts were cooled to room temperature in two stages; wherein: First cooling stage: nitrogen pressure 16 bar, purging for 90 seconds, fan speed 3000 rpm; Second cooling stage: nitrogen pressure 10 bar, purging for 210 seconds, fan speed 800 rpm.

[0079] 10. Tempering Temperature 180℃, time 165 minutes.

[0080] Note: Compared to Example 1, Example 2 extends the purging time of the first cooling stage to completely avoid high-temperature phase transformation, promoting more phase transformation in the low-temperature region and forming low-temperature lath bainite. [Regarding the microstructure...] Figure 2 Analysis shows that under these cooling conditions, high-temperature phase transformation is almost completely avoided, and the main microstructure consists of lath bainite and martensite.

[0081] Test results: Hardness: Surface 706HV, core 347HV. Example 3: The method for manufacturing gear parts provided in this embodiment specifically includes the following steps: 1. Alloying treatment during steel bar refining process Steel composition: C: 0.19%, Si: 0.23%, Mn: 1.10%, Cr: 1.15%, Ti: 0.040%, Ni: 0.02%, Nb: 0.025%, Cu: 0.03%, P: 0.017%, S: 0.019%, N: 0.0065%, with the remainder being Fe and unavoidable impurities.

[0082] 2. Forging process 2.1 Heating: The gear steel bar is heated to 1250℃ in a medium-frequency heating furnace, and the initial forging temperature is measured to be 1100℃.

[0083] 2.2 Upsetting: Upset once on an open press.

[0084] 2.3 Die forging: After upsetting, the billet is forged into a casting on a vertical die forging machine (once), and then clamped to a punching machine for punching.

[0085] 2.4 Cooling: Air cooling after drilling; concentrated normalizing after air cooling (divided into three zones): Zone 1: 850℃, Zone 2: 900℃, Zone 3: 920℃, each waiting time is 120 minutes; After normalizing, the upper and lower fans are simultaneously cooled: the upper fan frequency is 40Hz, the lower fan frequency is 20Hz, and the duration is 180s.

[0086] 3. Machining After normalizing, the forgings undergo rough turning, finish turning, gear hobbing, and other processes in sequence.

[0087] 4. Hot-loading method It adopts a three-point support vertical assembly, with 80 gear parts loaded in one furnace.

[0088] 5. Cleaning Clean the oil stains on the surface of gear parts after cleaning the machine to ensure uniform carburization.

[0089] 6. Pre-oxidation treatment The gear parts are fed into a continuous pre-oxidation furnace at a temperature of 380°C.

[0090] 7. Preheating treatment Transfer the pre-oxidized parts to a vacuum carburizing furnace, evacuate the furnace to a vacuum level below 10 mbar, heat to about 650°C, and hold for 90 minutes.

[0091] 8. Carburizing heat treatment 8.1 Convection heating stage (furnace temperature 950℃): Nitrogen gas is introduced into the vacuum carburizing furnace to a pressure of 1200mbar, and convection heating is carried out by circulating heating through a nitrogen protector for 50 minutes. 8.2 Vacuum heating stage (furnace temperature 950℃): Evacuate the furnace to a vacuum level of <10mbar and hold for 30 minutes.

[0092] 8.2 Pulse carburizing stage (furnace temperature 950℃): First time: 160 seconds of acetylene charging, acetylene flow rate 7300 L / h, wait 6 minutes; Second time: Charge acetylene for 60 seconds, acetylene flow rate 5000L / h, wait for 6 minutes; 3rd-4th times: Charge with acetylene for 60 seconds at an acetylene flow rate of 4900 L / h, then wait for 8 minutes. 5th-6th times: Charge with acetylene for 60 seconds at a flow rate of 4500 L / h, then wait 10 minutes. 7th time: 60 seconds of acetylene charging, acetylene flow rate 4700L / h, wait 12 minutes; 8th time: 30 seconds of acetylene charging, acetylene flow rate 5900L / h, wait 3 minutes.

[0093] 8.3 Diffusion and heat preservation: After carburizing, first keep it at 950℃ in vacuum for 40 minutes, then cool it down to 920℃ and keep it in vacuum for diffusion for 80 minutes.

[0094] 9. Staged quenching High-pressure nitrogen gas was used to purge the gear parts from the top, and the parts were cooled to room temperature in two stages; wherein: First cooling stage: nitrogen pressure 16 bar, purging for 90 seconds, fan speed 3000 rpm; Second cooling stage: nitrogen pressure 10 bar, purging for 210 seconds, fan speed 800 rpm.

[0095] 10. Tempering Temperature 180℃, time 165 minutes.

[0096] Compared to Example 1, this example further extends the purging time of the first cooling stage to simulate the change in gear precision when the amount of martensite increases after the gear parts are fully quenched. From the microstructure... Figure 3 Analysis revealed a significant increase in martensite content and a decrease in bainite content.

[0097] Test results: Hardness: Surface 708HV, core 366HV. Figure 4The diagrams show the precision of the gear parts obtained from the three embodiments. The dimensional test results indicate that the staged quenching process in Embodiment 2 is optimal. In the first cooling stage, rapid cooling at 3000 rpm for 90 seconds is followed by a second cooling stage at 800 rpm to room temperature, while the nitrogen pressure is reduced from 16 bar to 10 bar. Under this staged quenching process, the tooth profile error dispersion, tooth direction error dispersion, and cumulative tooth direction error are all minimized. This is because when the first cooling stage is shortest, some high-temperature phase transformation products are generated, leading to poor microstructure uniformity and increased gear precision error. When the first cooling stage is longest, a large amount of martensite is generated, resulting in significant phase transformation expansion and poorer gear precision.

[0098] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for producing gear parts, characterized in that: The chemical composition of the gear parts, by mass percentage, includes: C: 0.19%~0.22%, Si: 0.17%~0.37%, Mn: 1.10%~1.25%, Cr: 1.20%~1.35%, S: 0.02%~0.04%, Ti: 0.01%~0.05%, Nb: 0.015%~0.03%, Cu: ≤0.2%, Ni: ≤0.30%, P: ≤0.015%, N: ≤0.008%, with the balance being Fe and unavoidable impurities; The production process is as follows: smelting → rolling → forging → machining → hot charging → cleaning → pre-oxidation → preheating → carburizing → graded quenching → tempering; wherein: The carburizing process includes: sequentially performing convection heating, vacuum heating, pulse carburizing, and vacuum heat preservation on gear parts at the carburizing temperature, followed by heat preservation and diffusion at a temperature 20~50℃ lower than the carburizing temperature. The graded quenching process includes: purging the gear parts with high-pressure nitrogen gas and cooling them in two stages; wherein: First cooling stage: Control the fan speed and nitrogen pressure to cool the gear parts to 587~798℃ within 120 seconds; The second cooling stage: controlling the fan speed and nitrogen pressure to cool the gear parts to 374°C for no less than 180 seconds.

2. The production method according to claim 1, characterized in that: The carburizing process is carried out in a vacuum carburizing furnace, with three stages: convection heating, vacuum heating, and pulse carburizing, maintaining and controlling the furnace temperature at the carburizing temperature; wherein: Convection heating stage: Evacuate the furnace to a vacuum level of <10 mbar, then fill it with nitrogen to a pressure of 1000~1200 mbar for convection heating, and hold for 40~60 minutes; Vacuum heating stage: Evacuate the furnace to a vacuum level of <10mbar and hold for 15~30 minutes.

3. The production method according to claim 1, characterized in that: Pulse carburizing stage: Acetylene is introduced into the furnace in 5-9 cycles, followed by a period of diffusion. Each acetylene injection takes 30-180 seconds, the acetylene flow rate is 2000-10000 L / h, and the waiting time is 3-14 minutes.

4. The production method according to claim 1, characterized in that: The carburizing temperature is 920℃~1000℃.

5. The production method according to claim 1, characterized in that: The heat preservation time during the vacuum heat preservation stage is 30~120 minutes; and / or, The heat preservation and diffusion stage requires 30 to 120 minutes of heat preservation time.

6. The production method according to claim 1, characterized in that: The pressure of the high-pressure nitrogen gas is 10~18 bar; The purging time for the first cooling stage is 30-120 seconds, and the fan speed is 2400-3000 rpm; The second cooling stage has a purging time of 180-240 seconds and a fan speed of 800-2000 rpm.

7. The production method according to claim 1, characterized in that: The gear part is a shaft gear part.

8. The production method according to claim 1, characterized in that: The preheating process includes: placing the pre-oxidized gear parts in a vacuum environment and heating them to 650±10℃, and holding them at that temperature for 60~120 minutes.

9. A gear part, characterized in that: It is produced by the production method described in claim 1.

10. A car, characterized in that: Includes the gear part as described in claim 9.

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