Gas nitriding process of gear

Abstract

The invention belongs to the field of nitriding processes, and particularly relates to a gas nitriding process of a gear, which comprises the following steps: a) placing the gear in a well type gas nitriding furnace, and carrying out heating pre-oxidation treatment; b) after the pre-oxidation treatment is finished, introducing ammonia gas into the furnace, and heating to a nitriding strong nitriding stage 1; c) after the nitriding strong nitriding stage 1 is finished, heating to a nitriding diffusion stage 1; d) after the nitriding diffusion stage 1 is finished, cooling to a nitriding strong nitriding stage 2; e) after the nitriding strong nitriding stage 2 is finished, heating to a nitriding diffusion stage 2; f) after the nitriding diffusion stage 2 is finished, cooling to a nitriding strong nitriding stage 3; g) after the nitriding strong nitriding stage 3 is finished, heating to a nitriding diffusion stage 3; h) after the nitriding diffusion stage 3 is finished, ammonia gas is continuously introduced into the furnace, and cooling is performed; i) after the temperature is reduced to the preset temperature, the gas introduced into the furnace is switched from ammonia gas to nitrogen gas; and then, the nitriding furnace is further cooled, and the treated gear is taken out of the furnace.

Description

Technical Field

[0001] This invention belongs to the field of nitriding process, and particularly relates to a gas nitriding process for gears. Background Technology

[0002] Nitriding is a chemical heat treatment method in which nitrogen atoms diffuse into the surface of steel parts, forming a nitride-based layer. Nitriding not only improves the surface hardness, wear resistance, and fatigue performance of workpieces, but also enhances their corrosion resistance in corrosive media, while also providing resistance to galling and scratching. Due to its low temperature and minimal workpiece distortion, nitriding has been widely used in production practices in the machinery industry.

[0003] Currently, conventional gas nitriding processes mainly include single-stage nitriding, two-stage nitriding, and three-stage nitriding. Although these processes can meet the gas nitriding requirements of most workpieces with deep nitriding layers, they have long production times and low economic benefits. Furthermore, as production time increases, the deformation of the workpiece in the furnace also increases, which is detrimental to workpiece quality. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a gas nitriding process for gears, which can effectively shorten the nitriding time, reduce production costs, improve economic efficiency, and at the same time reduce the tendency of increased deformation of gear parts caused by prolonged nitriding time.

[0005] This invention provides a gas nitriding process for gears, comprising the following steps:

[0006] a) Place the gears in a pit-type gas nitriding furnace for pre-oxidation treatment by heating;

[0007] b) After the pre-oxidation treatment is completed, ammonia gas is introduced into the furnace and the temperature is raised to the nitriding strong percolation stage 1;

[0008] The nitriding and strong nitriding stage 1 has a strong nitriding temperature of 500-540℃, a temperature equalization time of 10-30h, an ammonia decomposition rate of 20-45%, and a furnace pressure of 0.8-1.2MPa.

[0009] c) After the nitriding strong penetration stage 1 is completed, the temperature is raised to the nitriding diffusion stage 1;

[0010] The diffusion temperature of the first stage of nitriding diffusion is 520-560℃, the temperature averaging time is 20-40h, the ammonia decomposition rate is 45-65%, and the furnace pressure is 0.6-1MPa.

[0011] d) After the first stage of nitriding diffusion is completed, the temperature is lowered to the second stage of strong nitriding diffusion;

[0012] The nitriding and strong nitriding stage 2 has a strong nitriding temperature of 490–530℃, a temperature equalization time of 10–30 h, an ammonia decomposition rate of 20–40%, and a furnace pressure of 0.8–1.2 MPa.

[0013] e) After the nitriding strong penetration stage 2 is completed, the temperature is raised to the nitriding diffusion stage 2;

[0014] The diffusion temperature of the second stage of nitriding diffusion is 510-550℃, the temperature equalization time is 20-40h, the ammonia decomposition rate is 40-60%, and the furnace pressure is 0.6-1MPa.

[0015] f) After the nitriding diffusion stage 2 is completed, the temperature is reduced to the strong nitriding diffusion stage 3;

[0016] The nitriding and strong nitriding stage 3 has a strong nitriding temperature of 480-520℃, a temperature equalization time of 10-20h, an ammonia decomposition rate of 20-40%, and a furnace pressure of 0.8-1.2MPa.

[0017] g) After the nitriding strong penetration stage 3 is completed, the temperature is raised to the nitriding diffusion stage 3;

[0018] The diffusion temperature of the three-stage nitriding diffusion process is 500-540℃, the temperature equalization time is 20-30h, the ammonia decomposition rate is 40-55%, and the furnace pressure is 0.6-1MPa.

[0019] h) After the nitriding diffusion stage 3 is completed, continue to introduce ammonia gas into the furnace and cool it down;

[0020] i) After cooling to the preset temperature, the gas introduced into the furnace is switched from ammonia to nitrogen; then, the nitriding furnace is further cooled, and the processed gear is taken out of the furnace.

[0021] Preferably, in step a), the temperature of the pre-oxidation treatment is 300-400°C, and the temperature uniformity time is 1-2 hours.

[0022] Preferably, in step b), the heating rate is ≤70℃ / h.

[0023] Preferably, in step c), the heating rate is ≤50℃ / h.

[0024] Preferably, in step d), the cooling method is furnace-side cooling, and the cooling rate is ≤100℃ / h.

[0025] Preferably, in step e), the heating rate is ≤50℃ / h.

[0026] Preferably, in step f), the cooling method is furnace-side cooling, and the cooling rate is ≤100℃ / h.

[0027] Preferably, in step g), the heating rate is ≤50℃ / h.

[0028] Preferably, in step h), the cooling method is furnace-side cooling, the cooling rate is ≤80℃ / h, the final temperature of the cooling is 350~400℃, and the furnace pressure during the cooling period is controlled at 0.2~0.6MPa.

[0029] Preferably, in step i), the preset temperature is 350-400℃; the furnace pressure during nitrogen gas introduction is controlled at 0.2-0.6MPa; and the furnace temperature when the gear is removed is ≤150℃.

[0030] Compared with the prior art, the present invention provides a gas nitriding process for gears, comprising the following steps: a) placing the gear in a pit-type gas nitriding furnace for pre-oxidation treatment; b) after the pre-oxidation treatment, introducing ammonia gas into the furnace and raising the temperature to the strong nitriding stage 1; the strong nitriding stage 1 has a strong nitriding temperature of 500-540℃, a uniform temperature time of 10-30h, an ammonia decomposition rate of 20-45%, and a furnace pressure of 0.8-1.2MPa; c) after the strong nitriding stage 1 is completed, The temperature is increased to the nitriding diffusion stage 1; the diffusion temperature of the nitriding diffusion stage 1 is 520-560℃, the averaging time is 20-40h, the ammonia decomposition rate is 45-65%, and the furnace pressure is 0.6-1MPa; d) After the nitriding diffusion stage 1 is completed, the temperature is decreased to the strong nitriding diffusion stage 2; the strong nitriding diffusion stage 2 has a strong nitriding temperature of 490-530℃, an averaging time of 10-30h, an ammonia decomposition rate of 20-40%, and a furnace pressure of 0.8-1.2MPa; e) Strong nitriding diffusion... After stage 2, the temperature is increased to the nitriding diffusion stage 2; the diffusion temperature of the nitriding diffusion stage 2 is 510–550℃, the averaging time is 20–40 h, the ammonia decomposition rate is 40–60%, and the furnace pressure is 0.6–1 MPa; f) After the nitriding diffusion stage 2, the temperature is decreased to the strong nitriding diffusion stage 3; the strong nitriding diffusion stage 3 has a strong nitriding diffusion temperature of 480–520℃, a averaging time of 10–20 h, an ammonia decomposition rate of 20–40%, and a furnace pressure of 0.8–1.2 MPa. g) After the strong nitriding stage 3, the temperature is raised to the diffusion nitriding stage 3; the diffusion temperature of the diffusion nitriding stage 3 is 500-540℃, the temperature equalization time is 20-30h, the ammonia decomposition rate is 40-55%, and the furnace pressure is 0.6-1MPa; h) After the diffusion nitriding stage 3, ammonia gas is continued to be introduced into the furnace and the temperature is lowered; i) After the temperature is lowered to the preset temperature, the gas introduced into the furnace is switched from ammonia to nitrogen; subsequently, the nitriding furnace is further cooled, and the processed gear is taken out from the furnace. Based on the iron-nitrogen phase diagram, this invention obtains an ideal nitriding layer phase composition by controlling the nitriding temperature and the ammonia decomposition rate (nitrogen potential) and through reasonable segmentation. This process can effectively shorten the nitriding time, reduce production costs, and improve economic efficiency without changing the production equipment and nitriding atmosphere, while also reducing the tendency for increased part deformation caused by prolonged nitriding time. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] This invention provides a gas nitriding process for gears, comprising the following steps:

[0033] a) Place the gears in a pit-type gas nitriding furnace for pre-oxidation treatment by heating;

[0034] b) After the pre-oxidation treatment is completed, ammonia gas is introduced into the furnace and the temperature is raised to the nitriding strong percolation stage 1;

[0035] c) After the nitriding strong penetration stage 1 is completed, the temperature is raised to the nitriding diffusion stage 1;

[0036] d) After the first stage of nitriding diffusion is completed, the temperature is lowered to the second stage of strong nitriding diffusion;

[0037] e) After the nitriding strong penetration stage 2 is completed, the temperature is raised to the nitriding diffusion stage 2;

[0038] f) After the nitriding diffusion stage 2 is completed, the temperature is reduced to the strong nitriding diffusion stage 3;

[0039] g) After the nitriding strong penetration stage 3 is completed, the temperature is raised to the nitriding diffusion stage 3;

[0040] h) After the nitriding diffusion stage 3 is completed, continue to introduce ammonia gas into the furnace and cool it down;

[0041] i) After cooling to the preset temperature, the gas introduced into the furnace is switched from ammonia to nitrogen; then, the nitriding furnace is further cooled, and the processed gear is taken out of the furnace.

[0042] In the gas nitriding process provided by the present invention, in step a), the material of the gear includes, but is not limited to, 42CrMoA; the temperature of the pre-oxidation treatment is preferably 300-400℃, specifically 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃ or 400℃; the temperature uniformity time of the pre-oxidation treatment is preferably 1-2h, specifically 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2h.

[0043] In the gas nitriding process provided by this invention, in step b), the heating rate is preferably ≤70℃ / h, specifically 50℃ / h, 55℃ / h, 60℃ / h, 65℃ / h, or 70℃ / h; the strong nitriding temperature in the first stage of strong nitriding is 500~540℃, specifically 500℃, 505℃, 510℃, 515℃, 520℃, 525℃, 530℃, 535℃, or 540℃; the temperature equalization time (holding time) in the first stage of strong nitriding is 10~30h, specifically 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h, 25h, 26h, 27h, 28h, 29h, or 30h. The ammonia decomposition rate (the degree to which ammonia introduced into the furnace decomposes into hydrogen and active nitrogen atoms during gas nitriding, expressed as a percentage) in the first stage of strong nitriding is 20%–45%, specifically 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, or 45%; the furnace pressure in the first stage of strong nitriding is 0.8–1.2 MPa, specifically 0.8 MPa, 0.85 MPa, 0.9 MPa, 0.95 MPa, 1 MPa, 1.05 MPa, 1.1 MPa, 1.15 MPa, or 1.2 MPa.

[0044] In the gas nitriding process provided by this invention, in step c), the heating rate is preferably ≤50℃ / h, specifically 30℃ / h, 35℃ / h, 40℃ / h, 45℃ / h, or 50℃ / h; the diffusion temperature of the first nitriding diffusion stage is 520~560℃, specifically 520℃, 525℃, 530℃, 535℃, 540℃, 545℃, 550℃, 555℃, or 560℃; the temperature homogenization time of the first nitriding diffusion stage is 20~40h, specifically 20h, 21h, 22h, 23h, 24h, 25h, 26h, 27h, 28h, 29h, 30h, 31h, 32h, 33h, or 3... The time frame for the nitriding diffusion stage is 4h, 35h, 36h, 37h, 38h, 39h, or 40h; the ammonia decomposition rate in the first stage of nitriding diffusion is 45%–65%, specifically 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, or 65%; the furnace pressure in the first stage of nitriding diffusion is 0.6–1MPa, specifically 0.6MPa, 0.65MPa, 0.7MPa, 0.75MPa, 0.8MPa, 0.85MPa, 0.9MPa, 0.95MPa, or 1MPa.

[0045] In the gas nitriding process provided by this invention, in step d), the cooling method is preferably furnace-side cooling, and the cooling rate is ≤100℃ / h; the strong nitriding temperature in the second stage of strong nitriding is 490~530℃, specifically 490℃, 495℃, 500℃, 505℃, 510℃, 515℃, 520℃, 525℃ or 530℃; the temperature equalization time in the second stage of strong nitriding is 10~30h, specifically 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h, 25h, 26h, 2 The time frame for the nitriding strong permeation stage is 7h, 28h, 29h, or 30h; the ammonia decomposition rate in the second stage of nitriding is 20-40%, specifically 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%; the furnace pressure in the second stage of nitriding strong permeation is 0.8-1.2MPa, specifically 0.8MPa, 0.85MPa, 0.9MPa, 0.95MPa, 1MPa, 1.05MPa, 1.1MPa, 1.15MPa, or 1.2MPa.

[0046] In the gas nitriding process provided by this invention, in step e), the heating rate is preferably ≤50℃ / h, specifically 30℃ / h, 35℃ / h, 40℃ / h, 45℃ / h, or 50℃ / h; the diffusion temperature of the second stage of nitriding diffusion is 510~550℃, specifically 510℃, 515℃, 520℃, 525℃, 530℃, 535℃, 540℃, 545℃, or 550℃; the temperature homogenization time of the second stage of nitriding diffusion is 20~40h, specifically 20h, 21h, 22h, 23h, 24h, 25h, 26h, 27h, 28h, 29h, 30h, 31h, 32h, 33h, or 30h. The time frame for the nitriding diffusion stage is 4h, 35h, 36h, 37h, 38h, 39h, or 40h; the ammonia decomposition rate in the second stage of nitriding diffusion is 40-60%, specifically 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, or 60%; the furnace pressure in the second stage of nitriding diffusion is 0.6-1MPa, specifically 0.6MPa, 0.65MPa, 0.7MPa, 0.75MPa, 0.8MPa, 0.85MPa, 0.9MPa, 0.95MPa, or 1MPa.

[0047] In the gas nitriding process provided by this invention, in step f), the cooling method is preferably furnace-side cooling, and the cooling rate is ≤100℃ / h; the strong nitriding temperature of the third stage of strong nitriding is 480~520℃, specifically 480℃, 485℃, 490℃, 495℃, 500℃, 505℃, 510℃, 515℃ or 520℃; the temperature equalization time of the third stage of strong nitriding is 10~20h, specifically 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h or 20h; the strong nitriding temperature of the third stage of strong nitriding is ≤100℃ / h. The ammonia decomposition rate in the third nitriding stage is 20%–40%, specifically 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%; the furnace pressure in the third nitriding stage is 0.8–1.2 MPa, specifically 0.8 MPa, 0.85 MPa, 0.9 MPa, 0.95 MPa, 1 MPa, 1.05 MPa, 1.1 MPa, 1.15 MPa, or 1.2 MPa.

[0048] In the gas nitriding process provided by this invention, in step g), the heating rate is preferably ≤50℃ / h, specifically 30℃ / h, 35℃ / h, 40℃ / h, 45℃ / h, or 50℃ / h; the diffusion temperature of the third stage of nitriding diffusion is 500~540℃, specifically 500℃, 505℃, 510℃, 515℃, 520℃, 525℃, 530℃, 535℃, or 540℃; the temperature homogenization time of the third stage of nitriding diffusion is 20~30h, specifically 20h, 21h, 22h, 23h, 24h, 25h, or 26h. The time for each of the three stages of nitriding diffusion is 27h, 28h, 29h, or 30h; the ammonia decomposition rate during the three stages is 40-55%, specifically 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, or 55%; the furnace pressure during the three stages of nitriding diffusion is 0.6-1MPa, specifically 0.6MPa, 0.65MPa, 0.7MPa, 0.75MPa, 0.8MPa, 0.85MPa, 0.9MPa, 0.95MPa, or 1MPa.

[0049] In the gas nitriding process provided by this invention, in step h), the cooling method is preferably furnace-side cooling, and the cooling rate is ≤80℃ / h; the final temperature of the cooling is preferably 350~400℃, specifically 350℃, 380℃ or 400℃; the furnace pressure during the cooling period is preferably controlled at 0.2~0.6MPa, specifically 0.2MPa, 0.23MPa, 0.25MPa, 0.27MPa, 0.3MPa, 0.32MPa, 0.35MPa, 0.37MPa, 0.4MPa, 0.42MPa, 0.45MPa, 0.47MPa, 0.5MPa, 0.52MPa, 0.55MPa, 0.57MPa or 0.6MPa.

[0050] In the gas nitriding process provided by this invention, in step i), the preset temperature is preferably 350–400°C, specifically 350°C, 380°C, or 400°C; the furnace pressure control during nitrogen introduction is preferably 0.2–0.6 MPa, specifically 0.2 MPa, 0.23 MPa, 0.25 MPa, 0.27 MPa, 0.3 MPa, 0.32 MPa, 0.35 MPa, or 0.37 MPa. The furnace temperature is preferably ≤150℃ when the gear is removed, specifically 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃ or 150℃.

[0051] In the gas nitriding process provided by the present invention, the treated gear is taken out of the furnace and air-cooled to room temperature.

[0052] The nitriding process provided by this invention is based on the iron-nitrogen phase diagram. By controlling the nitriding temperature and the decomposition rate (nitrogen potential) of the nitriding ammonia, and through reasonable segmentation, an ideal nitriding layer phase composition is obtained. This process can effectively shorten the nitriding time, reduce production costs, and improve economic efficiency without changing the production equipment and nitriding atmosphere. At the same time, it reduces the tendency for increased deformation of gear parts caused by prolonged nitriding time.

[0053] For clarity, the following examples and comparative models will be used to provide a detailed description.

[0054] Example 1

[0055] The nitriding process for gear components made of 42CrMoA material is as follows:

[0056] a) Place the gear in a pit-type gas nitriding furnace for pre-oxidation treatment at a temperature of 360℃ and a uniform temperature time of 1 hour;

[0057] b) After the pre-oxidation stage is completed, open the ammonia pipeline valve, introduce ammonia into the furnace, and raise the temperature (heating rate 60℃ / h) to the nitriding strong permeation stage 1. The strong permeation temperature of the nitriding strong permeation stage 1 is 500℃, the uniform temperature time is 20h, the ammonia decomposition rate is 30-40%, and the furnace pressure is 1.2MPa.

[0058] c) After the first stage of strong nitriding is completed, the temperature is increased (heating rate 50℃ / h) to the first stage of nitriding diffusion. The diffusion temperature of the first stage of nitriding diffusion is 520℃, the uniform temperature time is 40h, the ammonia decomposition rate is 50-60%, and the furnace pressure is 0.8MPa.

[0059] d) After the first stage of nitriding diffusion is completed, the furnace is cooled down to the second stage of strong nitriding diffusion. The strong nitriding diffusion stage has a strong nitriding temperature of 490℃, a uniform temperature time of 20h, an ammonia decomposition rate of 25-35%, and a furnace pressure of 1.2MPa.

[0060] e) After the nitriding strong diffusion stage 2 is completed, the temperature is increased (heating rate 50℃ / h) to the nitriding diffusion stage 2. The diffusion temperature of the nitriding diffusion stage 2 is 520℃, the uniform temperature time is 40h, the ammonia decomposition rate is 50-60%, and the furnace pressure is 0.8MPa.

[0061] f) After the second stage of nitriding diffusion is completed, the furnace is cooled down to the third stage of strong nitriding diffusion. The strong nitriding diffusion stage has a strong nitriding temperature of 490℃, a uniform temperature time of 10h, an ammonia decomposition rate of 25-35%, and a furnace pressure of 1.2MPa.

[0062] g) After the nitriding strong diffusion stage 3 is completed, the temperature is increased (heating rate 50℃ / h) to the nitriding diffusion stage 3. The diffusion temperature of the nitriding diffusion stage 3 is 520℃, the uniform temperature time is 20h, the ammonia decomposition rate is 45-55%, and the furnace pressure is 1.0MPa.

[0063] h) After the nitriding diffusion stage 3 is completed, continue to introduce ammonia gas into the furnace and cool the furnace to 380°C with a furnace pressure of 0.4 MPa;

[0064] i) When the furnace temperature drops to 380°C, close the ammonia pipeline valve, open the nitrogen pipeline valve, introduce nitrogen into the furnace at a furnace pressure of 0.4 MPa, and turn on the fast cooling fan in the nitriding equipment until the furnace temperature drops below 150°C. Then, lift the gear out and air-cool it to room temperature.

[0065] The nitrided samples were tested according to GB / T 11354-2005 standard. The results were as follows: nitriding layer depth NHD400HV0.3 = 0.85 mm, nitride = grade 1, porosity = grade 2, brittleness = grade 1, and surface hardness 662HV5.

[0066] Comparative Example 1

[0067] The nitriding process for gear components made of 42CrMoA material is as follows:

[0068] a) Place the gear in a pit-type gas nitriding furnace for pre-oxidation treatment at a temperature of 360℃ and a uniform temperature time of 1 hour;

[0069] b) After the pre-oxidation stage, the temperature is increased (heating rate 60℃ / h) to the nitriding strong nitriding stage. The strong nitriding stage has a strong nitriding temperature of 500℃, a uniform temperature time of 80h, an ammonia decomposition rate of 30-45%, and a furnace pressure of 1.2MPa.

[0070] c) After the nitriding strong nitriding stage is completed, the temperature is increased (heating rate 50℃ / h) to the nitriding diffusion stage. The diffusion temperature of the nitriding diffusion stage is 520℃, the temperature uniformity time is 100h, the ammonia decomposition rate is 50-60%, and the furnace pressure is 0.8MPa.

[0071] d) After the nitriding diffusion stage is completed, continue to introduce ammonia gas into the furnace and cool it down to 380°C with the furnace pressure of 0.4 MPa;

[0072] e) When the furnace temperature drops to 380°C, close the ammonia pipeline valve, open the nitrogen pipeline valve, introduce nitrogen into the furnace at a furnace pressure of 0.4 MPa, and turn on the fast cooling fan in the nitriding equipment until the furnace temperature drops below 150°C. Then, lift the gear out and air-cool it to room temperature.

[0073] The nitrided samples were tested according to GB / T 11354-2005 standard. The results were as follows: nitrided layer depth NHD400HV0.3 = 0.83 mm, nitride level = 2, porosity level = 2, brittleness level = 2, surface hardness 611HV5.

[0074] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A process for gas nitriding a gear, characterized in that, The method comprises the following steps: a) placing the gear into a pit gas nitriding furnace for temperature rising pre-oxidation treatment; b) after the pre-oxidation treatment, ammonia gas is introduced into the furnace and the temperature is raised to the first stage of nitriding strong nitriding; the temperature of the first stage of nitriding strong nitriding is 500-540℃, the temperature holding time is 10-30h, the ammonia decomposition rate is 20-45%, and the furnace pressure is 0.8-1.2MPa; c) after the first stage of nitriding strong nitriding, the temperature is raised to the first stage of nitriding diffusion; the diffusion temperature of the first stage of nitriding diffusion is 520-560℃, the temperature holding time is 20-40h, the ammonia decomposition rate is 45-65%, and the furnace pressure is 0.6-1MPa; d) after the first stage of nitriding diffusion, the temperature is lowered to the second stage of nitriding strong nitriding; the temperature of the second stage of nitriding strong nitriding is 490-530℃, the temperature holding time is 10-30h, the ammonia decomposition rate is 20-40%, and the furnace pressure is 0.8-1.2MPa; e) after the second stage of nitriding strong nitriding, the temperature is raised to the second stage of nitriding diffusion; the diffusion temperature of the second stage of nitriding diffusion is 510-550℃, the temperature holding time is 20-40h, the ammonia decomposition rate is 40-60%, and the furnace pressure is 0.6-1MPa; f) after the second stage of nitriding diffusion, the temperature is lowered to the third stage of nitriding strong nitriding; the temperature of the third stage of nitriding strong nitriding is 480-520℃, the temperature holding time is 10-20h, the ammonia decomposition rate is 20-40%, and the furnace pressure is 0.8-1.2MPa; g) after the third stage of nitriding strong nitriding, the temperature is raised to the third stage of nitriding diffusion; the diffusion temperature of the third stage of nitriding diffusion is 500-540℃, the temperature holding time is 20-30h, the ammonia decomposition rate is 40-55%, and the furnace pressure is 0.6-1MPa; h) after the third stage of nitriding diffusion, ammonia gas is continuously introduced into the furnace and the temperature is lowered; i) after the temperature is lowered to a preset temperature, the gas introduced into the furnace is switched from ammonia to nitrogen; then, the nitriding furnace is further cooled, and the treated gear is taken out from the furnace.

2. The gas nitriding process according to claim 1, characterized in that, In step a), the temperature of the pre-oxidation treatment is 300-400℃, and the temperature holding time is 1-2h.

3. The gas nitriding process of claim 1, wherein, In step b), the temperature raising rate is ≤70℃ / h.

4. The gas nitriding process of claim 1 wherein, In step c), the temperature raising rate is ≤50℃ / h.

5. The gas nitriding process of claim 1 wherein, In step d), the temperature lowering mode is furnace temperature lowering, and the temperature lowering rate is ≤100℃ / h.

6. The gas nitriding process of claim 1 wherein, In step e), the temperature raising rate is ≤50℃ / h.

7. The gas nitriding process of claim 1 wherein, In step f), the temperature lowering mode is furnace temperature lowering, and the temperature lowering rate is ≤100℃ / h.

8. The gas nitriding process of claim 1 wherein, In step g), the temperature raising rate is ≤50℃ / h.

9. The gas nitriding process of claim 1 wherein, In step h), the temperature lowering mode is furnace temperature lowering, the temperature lowering rate is ≤80℃ / h, the final temperature of the temperature lowering is 350-400℃, and the furnace pressure during the temperature lowering is controlled at 0.2-0.6MPa.

10. The gas nitriding process of claim 1 wherein, In step i), the preset temperature is 350-400℃, the furnace pressure during the introduction of nitrogen is controlled at 0.2-0.6MPa, and the temperature in the furnace when the gear is taken out is ≤150℃.