Surface carburizing process for 18NiCrMo7-6 gear steel

Abstract

The application discloses a surface carburizing process for 18NiCrMo7-6 gear steel, which comprises the following steps: S1, placing the gear steel workpiece in a vacuum carbonization furnace, increasing the furnace temperature to 840-880 DEG C, and introducing 6-10 m 3 / h of acetylene and 1-3 m 3 / h of nitrogen to perform carburizing treatment; S2, maintaining the temperature unchanged, and introducing 1-3 m 3 / h of nitrogen; S3, introducing 6-10 m 3 / h of acetylene and 1-3 m 3 / h of nitrogen to perform carburizing treatment; S4, introducing 1-3 m 3 / h of nitrogen; repeating steps S3-S4, wherein steps S3-S4 are taken as a basic pulse unit, the time of each basic pulse unit is 5-7 min, the total treatment time is 6-10 h, and then the gear steel workpiece is subjected to segmented cooling, uniform temperature maintaining, high-temperature tempering, quenching and low-temperature tempering. The carburizing efficiency is improved without increasing the carburizing temperature, and a relatively thin carbide layer and a relatively thick diffusion layer can be obtained.

Description

Technical Field

[0001] This invention belongs to the field of steel surface treatment technology, and relates to the surface carburizing process of 18NiCrMo7-6 gear steel. Background Technology

[0002] 18CrNiMo7-6 carburizing steel, with a Cr content of 1.5-1.8% and a Ni content of 1.4-1.7%, is a low-carbon, high-alloy carburizing steel commonly used in gears. However, its surface hardness is insufficient, requiring carburizing treatment.

[0003] Carburizing is the process of infiltrating carbon atoms into the surface layer of a metallic material under high temperature and pressure. Carburizing can improve the wear resistance and hardness of the metal surface while maintaining the original internal toughness and plasticity of the metal. Traditional gas carburizing temperatures are generally 900-930℃. Due to the relatively low temperature, carbon atom diffusion is slow, resulting in a long carburizing cycle, which increases both production costs and energy waste. In engineering, the carburizing temperature is often increased to improve the diffusion rate and thus carburizing efficiency; however, increasing the carburizing temperature leads to grain coarsening, which in turn results in large martensite sizes after quenching, insufficient strength, and other problems, affecting the overall performance of the material.

[0004] Typically, the surface microstructure after carburizing consists of a carbide layer (also known as a bright white layer) and a diffusion layer. The carbide layer has high hardness and good wear resistance, but is brittle and should not be too thick. The diffusion layer has certain wear resistance and good toughness, resulting in good overall performance. Ideally, carburizing should yield a thin carbide surface layer and a thick diffusion layer. The high hardness of the carbide layer effectively improves wear resistance, while the sufficient toughness of the diffusion layer effectively improves fatigue strength and overall performance. To increase the thickness of the diffusion layer, the carbon potential and carburizing time are usually increased. However, increasing the carbon potential and carburizing time can lead to an excessively thick carbide surface layer. Furthermore, increasing the carbon potential and carburizing time causes carbide-forming elements in the diffusion layer to form carbides with carbon at grain boundaries. With increasing carburizing time, these carbides can form network carbides and rod-shaped carbides, which negatively impact the overall performance of the material and are difficult to eliminate. Patent 202210744139 proposes a gas pulse vacuum carburizing method, which employs repeated pulse intense carburizing treatment, followed by a diffusion process. However, it requires pre-carburizing heat treatment to achieve the desired effect, and the carburizing temperature is relatively high, ranging from 900 to 1000°C, with a relatively long carburizing cycle. Summary of the Invention

[0005] The purpose of this invention is to provide a surface carburizing process for 18NiCrMo7-6 gear steel, which improves carburizing efficiency, shortens the carburizing cycle, and obtains a thinner carbide layer and a thicker diffusion layer without increasing the carburizing temperature.

[0006] The technical solution adopted in this invention is a surface carburizing process for 18NiCrMo7-6 gear steel, comprising the following steps:

[0007] Step (a) Strong Infiltration Stage:

[0008] S1. Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carbonization furnace. Raise the temperature inside the vacuum carbonization furnace to 840-880℃ and introduce 6-10m³ of air. 3 / h of acetylene and 1-3m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0009] S2. Maintain constant temperature and introduce 1-3m... 3 / h of nitrogen;

[0010] S3. Maintain constant temperature and introduce 6-10m³ of gas. 3 / h of acetylene and 1-3m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0011] S4. Maintain constant temperature and introduce 1-3m... 3 / h of nitrogen;

[0012] Repeat steps S3-S4, which serve as basic pulse units. Each basic pulse unit lasts for 5-7 minutes, and the total processing time is 6-10 hours.

[0013] The ratio of carburizing time in step S3 to time in step S4 is 1:1 to 1:3.

[0014] Step (b) Segmented cooling and temperature equalization;

[0015] Step (c) High-temperature tempering;

[0016] Step (d) Quenching;

[0017] Step (e) Low-temperature tempering.

[0018] Specifically, in step (b), the segmented cooling and temperature equalization process is as follows: reduce the furnace temperature, cool the workpiece with the furnace to 750-800℃, hold it at that temperature, and then air-cool it to room temperature after removing it from the furnace.

[0019] Specifically, in step (c), the high-temperature tempering process involves placing the workpiece in a tempering furnace and heating it to 620-680°C, holding it at that temperature, and then air-cooling it to room temperature after removing it from the furnace.

[0020] Specifically, in step (d), the quenching process involves heating the workpiece to 800-850℃, holding it at that temperature, and then performing oil quenching.

[0021] Specifically, in step (e), the low-temperature tempering process involves placing the workpiece in a tempering furnace and heating it to 150-230°C for low-temperature tempering, holding it at that temperature, and then cooling the furnace to room temperature.

[0022] The beneficial effects of this invention are as follows:

[0023] This invention employs a multi-stage pulsed repetitive carburizing process for surface carburizing of 18NiCrMo7-6 gear steel workpieces. During the strong carburizing stage, nitrogen gas is supplied for buffering after each carburizing gas pulse. This nitrogen buffering during the pulsed carburizing stage is crucial. This process serves as a basic pulse unit, and this basic unit is repeatedly used for multi-stage pulsed repetitive carburizing. This method improves carburizing efficiency and shortens the carburizing cycle without increasing the carburizing temperature. The same carburizing performance can be achieved at 840-880℃ as at 900-1000℃, with a further reduction in carburizing time and a lower likelihood of forming rod-shaped or network-shaped carbides. This invention also yields a thinner carbide layer and a thicker diffusion layer. The effective diffusion layer of the resulting workpiece is 2.5-3.5 mm thick, and the surface carbide layer (bright white layer) thickness is 0.017-0.025 mm. Detailed implementation method:

[0024] Example 1:

[0025] A surface carburizing process for 18NiCrMo7-6 gear steel includes the following steps:

[0026] Step (a) Strong Infiltration Stage:

[0027] S1. Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carbonization furnace, raise the temperature inside the furnace to 840℃, and introduce 6m... 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0028] S2. Maintain constant temperature and introduce 2m... 3 / h of nitrogen;

[0029] S3. Maintain constant temperature and introduce 6m³ / h. 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0030] S4. Maintain constant temperature and introduce 2m... 3 / h of nitrogen;

[0031] Repeat steps S3-S4, which serve as the basic pulse unit. Each pulse unit lasts for 5 minutes, and the total processing time is 6 hours. The ratio of carburizing time in step S3 to time in step S4 is 1:1.

[0032] Step (b) Segmented cooling and temperature equalization: Reduce the furnace temperature, cool the workpiece with the furnace to 750°C, hold for 4 hours, and then air cool to room temperature after removal from the furnace.

[0033] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0034] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0035] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0036] Example 2:

[0037] A surface carburizing process for 18NiCrMo7-6 gear steel includes the following steps:

[0038] Step (a) Strong Infiltration Stage:

[0039] S1. Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carbonization furnace, raise the temperature inside the furnace to 880℃, and introduce 10m... 3 / h of acetylene and 3m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0040] S2. Maintain constant temperature and introduce 3m³ / h. 3 / h of nitrogen;

[0041] S3. Maintain constant temperature and introduce 10m... 3 / h of acetylene and 3m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0042] S4. Maintain constant temperature and introduce 3m³ / h. 3 / h of nitrogen;

[0043] Repeat steps S3-S4, which serve as the basic pulse unit. Each pulse unit lasts for 5 minutes, and the total processing time is 6 hours. The ratio of carburizing time in step S3 to time in step S4 is 1:3.

[0044] Step (b) Segmented cooling and temperature equalization: Reduce the furnace temperature, cool the workpiece with the furnace to 750°C, hold for 4 hours, and then air cool to room temperature after removal from the furnace.

[0045] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0046] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0047] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0048] Example 3:

[0049] A surface carburizing process for 18NiCrMo7-6 gear steel includes the following steps:

[0050] Step (a) Strong Infiltration Stage:

[0051] S1. Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carbonization furnace, raise the temperature inside the furnace to 880℃, and introduce 8m³ of air. 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0052] S2. Maintain constant temperature and introduce 2m... 3 / h of nitrogen;

[0053] S3. Maintain constant temperature and introduce 8m³ / h. 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0054] S4. Maintain constant temperature and introduce 2m... 3 / h of nitrogen;

[0055] Repeat steps S3-S4, which serve as the basic pulse unit. Each pulse unit lasts for 5 minutes, and the total processing time is 6 hours. The ratio of carburizing time in step S3 to time in step S4 is 1:3.

[0056] Step (b) Segmented cooling and temperature equalization: Reduce the furnace temperature, cool the workpiece with the furnace to 750°C, hold for 4 hours, and then air cool to room temperature after removal from the furnace.

[0057] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0058] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0059] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0060] Comparative Example 1:

[0061] A surface carburizing process for 18NiCrMo7-6 gear steel includes the following steps:

[0062] Step (a) Strong Carburization Stage: Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carburizing furnace and raise the temperature in the vacuum carburizing furnace to 840℃. The total processing time is 6 hours.

[0063] Step (b) Segmented cooling and temperature equalization: Reduce the furnace temperature, cool the workpiece with the furnace to 750°C, hold for 4 hours, and then air cool to room temperature after removal from the furnace.

[0064] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0065] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0066] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0067] Comparative Example 2:

[0068] A surface carburizing process for 18NiCrMo7-6 gear steel includes the following steps:

[0069] Step (a) Strong Infiltration Stage:

[0070] S1. Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carbonization furnace, raise the temperature inside the furnace to 840℃, and introduce 6m... 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0071] S2. Repeat step S1, using step S1 as the basic pulse unit. Each pulse unit lasts for 5 minutes, and the total processing time is 6 hours.

[0072] Step (b) Segmented cooling and temperature equalization: Reduce the furnace temperature, cool the workpiece with the furnace to 750°C, hold for 4 hours, and then air cool to room temperature after removal from the furnace.

[0073] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0074] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0075] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0076] Comparative Example 3:

[0077] A surface carburizing process for 18NiCrMo7-6 gear steel includes the following steps:

[0078] Step (a) Strong Infiltration Stage:

[0079] S1. Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carbonization furnace, raise the temperature inside the furnace to 840℃, and introduce 6m... 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0080] S2. Repeat step S1, using step S1 as the basic pulse unit. Each pulse unit lasts for 5 minutes, and the total processing time is 6 hours.

[0081] After carburizing in step (b), the workpiece continues to be heated in the heating chamber. Heating is stopped, nitrogen is introduced in a pulse manner, and the temperature is slowly reduced to 750°C. The temperature is held for 4 hours, and the workpiece is then removed from the furnace and air-cooled to room temperature.

[0082] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0083] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0084] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0085] Comparative Example 4:

[0086] A surface carburizing process for 42CrMo4 gear steel includes the following steps:

[0087] Step (a) Strong Infiltration Stage:

[0088] S1. Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carbonization furnace, raise the temperature inside the furnace to 840℃, and introduce 6m... 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0089] S2. Maintain constant temperature and introduce 2m... 3 / h of nitrogen;

[0090] S3. Maintain constant temperature and introduce 6m³ / h. 3 / h of acetylene and 2m 3 Nitrogen gas at a rate of / h is used for carburizing treatment;

[0091] S4. Maintain constant temperature and introduce 2m... 3 / h of nitrogen;

[0092] Repeat steps S3-S4, which serve as the basic pulse unit. Each pulse unit lasts for 5 minutes, and the total processing time is 6 hours. The ratio of carburizing time in step S3 to time in step S4 is 1:1.

[0093] Step (b) Segmented cooling and temperature equalization: Reduce the furnace temperature, cool the workpiece with the furnace to 750°C, hold for 4 hours, and then air cool to room temperature after removal from the furnace.

[0094] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0095] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0096] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0097] Comparative Example 5:

[0098] A surface carburizing process for 18NiCrMo7-6 gear steel includes the following steps:

[0099] Step (a) Strong Carburization Stage: Place the 18NiCrMo7-6 gear steel workpiece in a vacuum carburizing furnace, raise the temperature inside the vacuum carburizing furnace to 940℃, and continuously introduce 6m³ of oxygen. 3 / h of acetylene and 2m 3 Nitrogen gas is supplied at a rate of / h for carburizing treatment; the total treatment time is 6h. Step (b) Segmented cooling and temperature equalization: the furnace temperature is reduced, the workpiece is cooled to 750℃ along with the furnace, held at that temperature for 4h, and then air-cooled to room temperature after being removed from the furnace;

[0100] Step (c) High-temperature tempering: Place the workpiece in a tempering furnace and heat it to 620°C. Hold it at that temperature for 4 hours. Remove it from the furnace and air cool it to room temperature.

[0101] Step (d) Quenching: Heat the workpiece to 800°C, hold for 3 hours, and then perform oil quenching;

[0102] Step (e) Low-temperature tempering: Place the workpiece in a tempering furnace and heat it to 200°C for low-temperature tempering. Hold it at that temperature for 6 hours and then cool it to room temperature.

[0103] Comparative Example 6

[0104] The difference from Comparative Example 5 is that the total treatment time for the strong infiltration stage in step (a) was 10 hours, while the rest of the treatment process was the same.

[0105] The performance of the carburized gear steel obtained in Examples 1-3 and Comparative Examples 1-6 was tested, and the results are shown in Table 1; Test results of Examples 1-3: The surface hardness of the material was 59-64 HRC; the diffusion layer was 2.5-3.5 mm; the surface carbides were dispersed, and the thickness of the surface carbide layer (bright layer) was 0.017-0.025 mm. In summary, the carburizing result of the material was judged to be qualified.

[0106] From Example 1 and Comparative Examples 1-3, it can be seen that for the multi-stage pulsed repetitive carburizing treatment process provided by the present invention, during the strong carburizing stage, after each carburizing gas pulse, nitrogen is supplied for buffering. Buffering with nitrogen during the pulsed carburizing stage is very crucial. Using such a treatment method as the basic pulse unit, this basic unit is continuously repeated for multi-stage pulsed repetitive carburizing. By this method, without increasing the carburizing temperature, the carburizing efficiency can be improved, the carburizing cycle can be shortened, the same carburizing performance as that at 900-1000 °C can be obtained at 840-880 °C, and the carburizing time is shortened, and it is not easy to form rod-shaped and network carbides. The present invention can also obtain a thinner carbide layer and a thicker diffusion layer. The effective diffusion layer of the obtained workpiece is 2.5-3.5 mm, and the thickness of the surface carbide layer (bright layer) is 0.017-0.025 mm.

[0107] From Example 1 and Comparative Example 4, it can be seen that after the treatment process of the present invention is applied to 42CrMo4, which is also gear steel, the thickness of the surface carbide layer is too thick and the thickness of the diffusion layer is relatively low, which cannot meet the requirements. When applied to different steel grades, the process needs to be further improved and optimized.

[0108] The performance of Comparative Example 6 is equivalent to that of Example 1, but the carburizing temperature is 940 °C (840-880 °C in the present invention), the carburizing time is significantly increased, and the thickness of the surface carbide layer is easy to form rod-shaped and network carbides, which may become the source of fatigue cracks in the later stage. It can be seen that the present invention can improve the carburizing efficiency, shorten the carburizing cycle, obtain the same carburizing performance as that at 900-1000 °C at 840-880 °C without increasing the carburizing temperature, and the carburizing time is shortened, and it is not easy to form rod-shaped and network carbides.

[0109] Table 1 Test results of carburized gear steel

[0110]

[0111]

Claims

1.18NiCrMo7-6 gear steel surface carburizing process, characterized in that, Includes the following steps: Step (a) Strong Infiltration Stage: S1. Place the 18NICRMO7-6 gear steel workpiece in a vacuum carbonization furnace, raise the temperature in the vacuum carbonization furnace to 840-880℃, and introduce 6-10 m³ / h of acetylene and 1-3 m³ / h of nitrogen to carry out carburizing treatment. S2. Maintain a constant temperature and introduce nitrogen gas at a rate of 1-3 m³ / h. S3. Maintaining a constant temperature, introduce 6-10 m³ / h of acetylene and 1-3 m³ / h of nitrogen to carry out carburizing treatment; S4. Maintain a constant temperature and introduce nitrogen gas at a rate of 1-3 m³ / h. Repeat steps S3-S4, with each step S3-S4 serving as a basic pulse unit. Each basic pulse unit lasts for 5-7 minutes, and the total processing time is 6-10 hours. The ratio of carburizing time in step S3 to time in step S4 is 1:1 to 1:3; Step (b) Segmented cooling and temperature equalization; In step (b), the specific process of segmented cooling and temperature equalization is as follows: reduce the furnace temperature, cool the workpiece with the furnace to 750-800℃, keep it at the temperature, and then air cool it to room temperature after it is taken out of the furnace. Step (c) High-temperature tempering; In step (c), the specific process of high-temperature tempering is as follows: the workpiece is placed in a tempering furnace and heated to 620-680℃, held at that temperature, and then air-cooled to room temperature after being removed from the furnace. Step (d) Quenching; In step (d), the quenching process specifically involves heating the workpiece to 800-850℃, holding it at that temperature, and then performing oil quenching. Step (e) Low-temperature tempering; In step (e), the specific process of low-temperature tempering is as follows: the workpiece is placed in a tempering furnace and heated to 150-230°C for low-temperature tempering, held at that temperature, and then cooled to room temperature in the furnace.

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