A method of heat treating a gear

Abstract

The application relates to the technical field of metal material heat treatment, in particular to a gear heat treatment method, which comprises the following steps: pre-oxidizing the gear, loading the processed gear into an oxidation furnace for pre-oxidation, loading the gear into a carburizing furnace after the pre-oxidation is completed, performing carburizing treatment on the gear, loading the pre-oxidized gear into the carburizing furnace, performing carburizing treatment, quenching the gear, quickly taking out the gear from the carburizing furnace and sinking into molten salt for quenching. The method can avoid thermal stress concentration caused by sudden temperature change, reduce the risk of gear deformation or cracking, remove the impurities such as oxides and oil stains on the gear surface, ensure that carbon atoms can be smoothly adsorbed and diffused in the subsequent carburizing process, slow down the cooling speed through molten salt pre-quenching, avoid excessive hardening of surface martensite, and provide buffer time for core organization transformation.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal material heat treatment, in particular to a heat treatment method of a gear. BACKGROUND

[0002] Gear is one of the most widely used parts in mechanical transmission, and its function is to transmit power and movement according to the specified speed ratio. In work, its stress condition is relatively complex, the gear root is subjected to alternating bending stress, the tooth surface is subjected to large contact stress and produces strong friction, and the gear is also subjected to certain impact load when shifting, starting and poor engagement. The main failure modes of the gear are fatigue tooth breakage, fatigue pitting and excessive wear of the tooth surface. According to the stress condition and failure analysis of the gear, the gear generally needs to be subjected to appropriate heat treatment to improve the bearing capacity and prolong the service life.

[0003] The related art can refer to Chinese patent application for invention with publication number CN105986103A, which discloses a gear heat treatment process, including the following steps: 1) placing the gear into an acid pickling tank, adding corrosion inhibitor in the acid pickling tank according to the proportion of 0.1-0.5 grams per liter, installing an acid pickling agitator above the acid pickling tank and stirring in the acid pickling tank, acid pickling time is 10-45 min, acid pickling temperature is 40-65℃, acid concentration is 15-24%, and after taking out, washing twice with high-pressure water; 2) placing the gear into a heating furnace for preheating treatment, the furnace temperature is raised to 230-250℃, nitrogen gas is introduced as protective gas, and the temperature is kept for 5 hours; 3) raising the furnace temperature to 800-810℃, keeping for 2 hours; 4) after taking out, cooling for 2-5 min, placing the gear into quenching oil at 900-920℃ for quenching cooling, and the time is 1 hour; 5) placing the gear into a tempering furnace for tempering treatment, raising the furnace temperature to 680-710℃, and then keeping the temperature for 5 hours with a temperature drop of 80-90 degrees per hour, and finally taking out and air cooling to room temperature.

[0004] However, for high-precision parts such as gears and transmission shafts, the current main processes are carburizing and induction hardening, which have problems such as large deformation, poor dimensional stability, easy cracking, surface peeling under heavy load, etc. SUMMARY

[0005] In order to improve the strength of the gear after heat treatment, the present application provides a heat treatment method of a gear.

[0006] The present application adopts the following technical scheme:

[0007] A heat treatment method of a gear, including the following steps:

[0008] S1, pre-oxidizing the gear;

[0009] The processed gear is put into an oxidation furnace for pre-oxidation, and after the pre-oxidation is completed, the gear is put into a carburizing furnace;

[0010] S2, the gear is subjected to carburizing treatment;

[0011] The pre-oxidized gear is put into the carburizing furnace for carburizing treatment;

[0012] S3, the gear is subjected to quenching;

[0013] The gear subjected to the carburizing treatment is quickly taken out from the carburizing furnace and sunk into molten salt for quenching.

[0014] Optionally, the pre-oxidation temperature is 400-450℃, and the pre-oxidation time is 40-80 minutes.

[0015] Optionally, the pre-oxidation temperature is 400℃, and the pre-oxidation time is 40 minutes.

[0016] Optionally, the step S2 specifically comprises:

[0017] S21, a preheating stage, the temperature is 820-850℃, the carbon potential is 0.8-1.0%, and the time is 0-40 minutes;

[0018] S22, a strong carburizing stage, the temperature is 880-950℃, the carbon potential is 1.0-1.4%, and the time is 6-90 hours;

[0019] S23, a diffusion stage, the temperature is 850-920℃, the carbon potential is 0.8-1.2%, and the time is 1-70 hours;

[0020] S24, a holding stage, the temperature is 820-850℃, and the time is 40-90 minutes.

[0021] Optionally, after the diffusion stage in the step S24 ends, the step specifically comprises the following steps:

[0022] S25, a slow cooling stage, air cooling to room temperature;

[0023] S26, a second temperature rising stage, the temperature is 820-850℃, and the time is 40-90 minutes.

[0024] Optionally, the step S3 specifically comprises:

[0025] S31, a pre-quenching stage, the gear subjected to the carburizing treatment is quickly taken out from the carburizing furnace and sunk into molten salt, the temperature of the molten salt is 200-220℃, and the time is 15-1200 seconds;

[0026] S32, isothermal quenching stage, the pre-quenched gear in step S31 is put into new molten salt, the temperature of the molten salt is 220-260℃, and the time is 2-20 hours.

[0027] Optionally, the molten salt in step S3 comprises potassium nitrate and sodium nitrite; the content of potassium nitrate in the molten salt is 40-60%, and the rest is sodium nitrite.

[0028] In summary, the present application comprises at least one of the following beneficial technical effects:

[0029] 1. In the carburizing process, the gear is uniformly heated through the preheating stage, so that the gear slowly rises from room temperature to the carburizing temperature, avoiding the concentration of thermal stress caused by sudden temperature change and reducing the risk of gear deformation or cracking; further removing impurities such as oxides and oil stains on the surface of the gear to ensure that carbon atoms can be smoothly adsorbed and diffused in the subsequent carburizing process; through low-temperature preheating at 820-850℃, the gear surface is in an active state, preparing for carbon atom adsorption in the strong carburizing stage; avoiding the problem of deformation caused by uneven thermal expansion when the gear directly enters the high-temperature strong carburizing stage at 880-950℃ from room temperature;

[0030] 2. Through the molten salt pre-quenching at 200-220℃, the cooling speed can be slowed down to avoid excessive hardening of the gear surface martensite, and at the same time provide buffer time for the core organization transformation. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is the gear heat treatment process diagram of 20CrNiMo material and 2mm carburized bainite layer depth;

[0032] Figure 2 is the metallographic diagram of the gear of 20CrNiMo material and 2mm carburized bainite layer depth;

[0033] Figure 3 is the gear heat treatment process diagram of 18CrNiMo7-6 material and 6mm carburized bainite layer depth;

[0034] Figure 4 is the metallographic diagram of the gear of 18CrNiMo7-6 material and 6mm carburized bainite layer depth. DETAILED DESCRIPTION

[0035] The present application will be further described in detail below in combination with all the drawings.

[0036] The present application discloses a gear heat treatment method, specifically as follows:

[0037] S1, pre-oxidizing the gear.

[0038] The processed gear is loaded into an oxidation furnace for pre-oxidation, and after the pre-oxidation is completed, the gear is loaded into a carburizing furnace.

[0039] The pre-oxidation temperature is 400-450°C, preferably 400°C, and the pre-oxidation time is 40-80 minutes, preferably 40 minutes.

[0040] The purpose of pre-oxidation is to form a dense oxide film on the surface of the gear, which is reduced to primary iron by reducing substances (such as hydrogen) in the carburizing atmosphere during the subsequent carburizing process. This primary iron has very high chemical activity and can significantly enhance the adsorption capacity of carbon atoms, thereby accelerating the adsorption and diffusion speed of carbon atoms on the surface of the gear and improving the carburizing efficiency. Secondly, it helps to reduce the impurities such as residual oil and cleaning agent on the surface of the gear, which can hinder the adsorption of carbon elements during the carburizing process, resulting in uneven thickness of the carburized layer. Through pre-oxidation treatment, these residues can be completely removed, making the carburized layer thickness more uniform and improving the carburizing quality of the gear. And to a certain extent, it can reduce the deformation of the gear during the carburizing process.

[0041] S2, carburizing treatment of the gear.

[0042] The pre-oxidized gear is loaded into a carburizing furnace for carburizing treatment.

[0043] In step S2, the following steps are included:

[0044] S21, preheating stage, temperature 820-850°C, carbon potential 0.8-1.0%, time 0-40 minutes.

[0045] The purpose of the preheating stage is to uniformly heat the gear, slowly raising the gear from room temperature to carburizing temperature, avoiding the concentration of thermal stress due to sudden temperature change, reducing the risk of gear deformation or cracking. Secondly, further remove impurities such as residual oxides and oil stains on the surface of the gear to ensure smooth adsorption and diffusion of carbon atoms during the subsequent carburizing process; through low-temperature preheating, the gear surface is in an active state, preparing for the adsorption of carbon atoms in the strong carburizing stage; avoiding the problem of deformation caused by uneven thermal expansion when the gear directly enters the high-temperature strong carburizing stage from room temperature.

[0046] S22, strong carburizing stage, temperature 880-950°C, carbon potential 1.0-1.4%, time 6-90 hours.

[0047] The purpose of the strong carburizing stage is to quickly carburize carbon atoms into the surface of the gear under high temperature and high carbon potential conditions, forming a high-carbon concentration area. Through long-time carburizing, carbon atoms diffuse into the gear, forming a certain depth of carburized layer, improving the surface hardness and wear resistance; the growth of austenite grains at high temperature provides a suitable organizational basis for the subsequent diffusion stage, while avoiding overburning or grain boundary carbide aggregation.

[0048] S23, diffusion stage, temperature 850-920℃, carbon potential 0.8-1.2%, time 1-70 hours.

[0049] The purpose of the diffusion stage is to diffuse the surface high-carbon area to the interior by reducing the carbon potential and temperature, reduce the carbon concentration gradient, avoid surface over-carburization or insufficient carbon content in the core; and help carbon atoms to be more evenly distributed in the carburized layer, improve the stability of the quality of the carburized layer; promote the decomposition of residual austenite, improve the hardness and wear resistance of the carburized layer.

[0050] S24, holding stage, temperature 820-850℃, time 40-90 minutes.

[0051] The purpose of the holding stage is to relieve the thermal stress of the gear due to rapid cooling, reduce the risk of cracking, eliminate part of the residual stress by holding, and reduce the dimensional change of the gear in subsequent processing or use.

[0052] S3, quenching the gear.

[0053] The gear is quickly taken out from the carburizing furnace and immersed in molten salt for quenching.

[0054] The molten salt includes potassium nitrate and sodium nitrite, the content of potassium nitrate in the molten salt is 40-60%, and the rest of the molten salt is sodium nitrite.

[0055] S31, pre-quenching stage, the gear is quickly taken out from the carburizing furnace and immersed in molten salt, the temperature of the molten salt is 200-220℃, and the time is 15-1200 seconds.

[0056] After the gear is taken out from the high-temperature (820-850℃) carburizing furnace, there is a temperature gradient between the surface and the core. If it is cooled directly to room temperature, the surface will quickly form high-hardness martensite due to high-carbon content (carburized layer), while the core low-carbon area may form coarse martensite or bainite, leading to internal stress concentration and cracking risk; through pre-quenching in 200-220℃ molten salt, the cooling speed can be slowed down, avoiding excessive hardening of surface martensite, and providing buffer time for core microstructure transformation.

[0057] Secondly, rapid cooling is easy to cause thermal stress, leading to deformation or cracking of the gear. In the pre-quenching stage, by controlling the cooling rate, the temperature of the gear is uniformly reduced, reducing the accumulation of thermal stress, which is particularly important for the dimensional stability of complex-shaped gears (such as thin-walled, long-shaft gears).

[0058] S32, isothermal quenching stage, the gear is placed in new molten salt, the temperature of the molten salt is 220-260℃, and the time is 2-20 hours.

[0059] The austenite slowly transforms into lower bainite in the temperature range of 220-260℃. The lower bainite has high hardness (58-62HRC), high toughness and good wear resistance, significantly better than coarse martensite or upper bainite.

[0060] The residual austenite in the carburized layer partially transforms into bainite during isothermal process, reducing the residual austenite content (usually <5%), avoiding dimensional changes or performance degradation due to phase transformation during subsequent use.

[0061] The gear structure is completed by isothermal quenching at a constant temperature, reducing internal stress caused by rapid cooling and improving the fatigue resistance and dimensional stability of the gear.

[0062] It should be noted that for some products, in step S2, step S24 specifically includes the following steps:

[0063] S25, slow cooling stage, air cooling to room temperature.

[0064] During slow cooling, part of the high-carbon martensite decomposes into residual austenite, providing a more stable organizational basis for subsequent quenching or tempering.

[0065] S26, secondary heating stage, temperature 820-850℃, then holding for 40-90 minutes.

[0066] Reheat the gear to austenitizing temperature (820-850℃) to dissolve the carbides in the carburized layer, forming a uniform austenite structure, enhancing the surface strength of the gear and reducing the possibility of surface peeling under heavy load. After slow cooling, the gear may have minor stress, and the secondary heating relieves stress through thermal plastic deformation, reducing quenching deformation.

[0067] Example 1

[0068] Referring to Figure 1 and Figure 2 , this example takes a gear with a carburized bainite layer depth of 2mm as an example, and sequentially performs "pre-oxidation-preheating-strong carburization-diffusion-holding-pre-quenching-isothermal quenching" treatment.

[0069] 1) Pre-oxidation treatment of the gear, temperature 400℃, time 40 minutes;

[0070] 2) Carburizing treatment, the pre-oxidized gear is loaded into the carburizing furnace for carburizing treatment; the specific process is as follows:

[0071] 21) Preheating stage: temperature rises to 840℃, carbon potential is controlled at 0.8-1.0%, and temperature is uniform for 50 minutes;

[0072] 22) strong infiltration stage: 920℃ holding, carbon potential control 1.0-1.4%, strong infiltration for 480-660 minutes;

[0073] 23) diffusion stage: 860℃ holding, carbon potential control 0.8-1.2%, strong infiltration for 460-800 minutes;

[0074] 24) holding stage: 820℃ holding, carbon potential control 0.8-0.9%, holding for 45 minutes;

[0075] 3) quenching the gear, the gear after carburizing in the carburizing furnace is quickly taken out and sunk into the molten salt for quenching; the specific process is as follows

[0076] 31) pre-quenching stage, the gear after carburizing in the carburizing furnace is quickly taken out and sunk into the molten salt, the temperature of the molten salt is 210℃, and the time is 1-15 minutes;

[0077] 32) isothermal quenching stage, the gear is put into new molten salt, the temperature of the molten salt is 235℃, and the time is 2-20 hours.

[0078] Finally, the core structure is martensite with a hardness of 30-48HRC, and the surface structure is bainite structure with a hardness of 56-65HRC.

[0079] The detection results of the gear treated in the embodiment are shown in Table 1

[0080] Table 1

[0081]

[0082] From the test results of Table 1, it can be seen that the material processed by the heat treatment method in the examples is 20CrNiMo, the carburized bainite layer depth of the gear is 2mm, the core is low carbon martensite, the hardness is moderate, and the toughness is good; the core ferrite grade is 1, the ferrite is uniformly distributed in the form of lath, the non-martensite organization is avoided, and the core toughness is ensured; the tensile strength is 2200-2600MPa, the yield strength is 1500-2000MPa, the core strength and toughness are balanced, can withstand high impact load, and prevent brittle fracture; the core toughness is optimized, and the bending fatigue crack propagation is inhibited; the hardening layer depth is 2.1-2.4mm, which exceeds the carburized bainite layer depth (2mm), ensuring that the hardness gradient from the surface to the core is gentle, and avoiding sudden hardness drop; the hardening layer and the core have good transition, reducing the risk of spalling caused by stress concentration; the core toughness ensures that the core toughness prevents brittle fracture under impact load or alternating stress, ensuring the reliability of the gear under heavy load working conditions; the surface high hardness and core toughness are coordinated: the surface wear resistance is improved, the contact fatigue life is prolonged, the core toughness inhibits the bending fatigue crack propagation, and the overall fatigue performance is excellent; the residual austenite content is less than 3%, avoiding volume change caused by phase change, and further prolonging the fatigue life.

[0083] Example 2

[0084] Referring to Figure 3 and Figure 4 , this example takes the material 18CrNiMo7-6 and the gear with a carburized bainite layer depth of 6mm as an example, and sequentially performs "pre-oxidation-preheating-strong penetration-diffusion-slow cooling-secondary heating-pre-quenching-isothermal quenching".

[0085] 1) The gear is pre-oxidized at a temperature of 400℃ for 40 minutes;

[0086] 2) Carburizing treatment: the pre-oxidized gear is loaded into a carburizing furnace for carburizing treatment;

[0087] 21) Preheating stage: the temperature is raised to 840℃, the carbon potential is controlled at 0.8%, and the temperature is kept uniform for 50 minutes;

[0088] 22) Strong penetration stage: 950℃ for 75 hours, carbon potential control 1.0-1.4%;

[0089] 23) Diffusion stage: 860℃ for 30 hours, carbon potential control 0.8-1.2%;

[0090] 24) Slow cooling stage: the gear is air cooled to room temperature;

[0091] 25) Secondary heating stage: the gear is heated from room temperature to 830℃ and kept at this temperature, the carbon potential is controlled at 0.8-1.0%, and the holding time is 1 hour;

[0092] It should be noted that the slow cooling stage is air-cooled to room temperature, which is equivalent to placing the gear outside and standing for a certain period of time without specific time limit, and then performing secondary heating treatment, therefore, Figure 3 The process curve from the end of the slow cooling stage to the secondary heating stage is disconnected, so that the slow cooling stage and the secondary heating stage can be operated separately.

[0093] 3) Quenching the gear, quickly taking the gear from the carburizing furnace and sinking into the molten salt for quenching;

[0094] 31) Pre-quenching stage, quickly taking the gear from the carburizing furnace and sinking into the molten salt, the temperature of the molten salt is 205℃, and the time is 1-15 minutes;

[0095] 32) Isothermal quenching stage, placing the gear into new molten salt, the temperature of the molten salt is 250℃, and the time is 2-20 hours.

[0096] The detection results of the gear treated in this embodiment are shown in Table 2

[0097] Table 2

[0098]

[0099] As can be seen from the detection results in Table 2, the material 18CrNiMo7-6 processed by the heat treatment method in the embodiment, the gear with a carburized bainite layer depth of 6mm, forms high-carbon martensite and bainite on the surface through the carburizing process, the hardness reaches HRC58-62, and the wear resistance is significantly improved; the wear amount is 0.32-0.63mm³ / m, the wear amount is extremely low in the simulation test, indicating that the surface wear resistance is excellent, and it is suitable for high friction environment; the carbide grade is 1, the carbide is uniformly distributed in the form of fine particles, avoiding the brittleness caused by network carbide, and further improving the wear resistance; the core organization hardness is 44-47HRC, the core is low-carbon martensite, the hardness is moderate, and the toughness is good; the core ferrite grade is 1, the ferrite is uniformly distributed in the form of lath, avoiding non-martensite organization, and ensuring the toughness of the core; the tensile strength is 2300-2750MPa, and the yield strength is 1800-2100MPa, the strength and toughness of the core are balanced, can withstand high impact load, and prevent brittle fracture; the hardening layer depth is 6.2-6.5mm, which exceeds the carburized bainite layer depth (6mm), ensuring that the hardness gradient from the surface to the core is gentle, and avoiding sudden hardness drop; the residual austenite content is <3%, the residual austenite content is extremely low, reducing the volume change caused by phase change, reducing the risk of contact fatigue damage, avoiding the initiation of fatigue cracks caused by phase change, and prolonging the fatigue life; the surface hardness and core toughness are coordinated, the surface wear resistance is improved, the contact fatigue life is prolonged, the core toughness inhibits the propagation of bending fatigue cracks, and the overall fatigue performance is significantly improved.

[0100] It should be noted that: conventional carburizing quenching is a process for obtaining martensite as the main metallographic structure, the core purpose of which is to control carbide and residual austenite in the metallographic structure, and martensite transformation is very fast, resulting in very large residual stress, which is hard and brittle, so a tempering process is needed.

[0101] The present application does not use conventional carburizing quenching, but uses isothermal quenching to obtain a metallographic structure mainly of lower bainite (more than 95%); lower bainite transformation is very slow, and the residual stress is much smaller than that of martensite, which is hard and not brittle, and there will be dispersed carbide precipitation in the metallographic structure transformation process, forming pinning effect, which can reflect better mechanical properties, so there is no need for a tempering process, which can also save costs.

[0102] The above are preferred embodiments of the present application, which do not limit the protection scope of the present application, so: any equivalent changes made in structure, shape, principle, etc. according to the present application should be covered within the protection scope of the present application.

Claims

1. A heat treatment method for gears, characterized in that, The gear is made of 20CrNiMo or 18CrNiMo7-6 material, and the process includes the following steps: S1, Pre-oxidize the gears; The machined gears are loaded into an oxidation furnace for pre-oxidation, and after pre-oxidation, the gears are loaded into a carburizing furnace. S2, Carburizing treatment is performed on the gears; The pre-oxidized gears are loaded into a carburizing furnace for carburizing treatment; S3, quench the gear; The carburized gears are quickly removed from the carburizing furnace and immersed in molten salt for quenching. The process of loading the pre-oxidized gears into a carburizing furnace for carburizing treatment specifically includes: S21, preheating stage, temperature is 820-850℃, carbon potential is 0.8-1.0%, time is 0-40 minutes; S22, the strong infiltration stage, with a temperature of 880-950℃, a carbon potential of 1.0-1.4%, and a time of 6-90 hours; S23, diffusion stage, temperature 850-920℃, carbon potential 0.8-1.2%, time 1-70 hours; S24, heat preservation stage, temperature is 820-850℃, time is 40-90 minutes; The process of rapidly removing the carburized gear from the carburizing furnace and immersing it in molten salt for quenching includes: S31, pre-quenching stage: The carburized gear is quickly taken out of the carburizing furnace and immersed in molten salt at a temperature of 200-220℃ for 15-1200 seconds. S32, isothermal quenching stage: the gears pre-quenched in step S31 are placed in new molten salt at a temperature of 220-260℃ for 2-20 hours. The molten salt comprises potassium nitrate and sodium nitrite; the potassium nitrate content in the molten salt is 40-60%, and the remainder is sodium nitrite.

2. The heat treatment method for gears according to claim 1, characterized in that, The pre-oxidation temperature is 400-450℃, and the pre-oxidation time is 40-80 minutes.

3. The heat treatment method for gears according to claim 2, characterized in that, The pre-oxidation temperature is 400℃ and the pre-oxidation time is 40 minutes.

4. The heat treatment method for gears according to claim 1, characterized in that, Step S24 specifically includes: S25, slow cooling phase, air cooling to room temperature; S26, the second heating stage, the temperature is 820-850℃, and then the temperature is kept for 40-90 minutes.

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