Quenching heat treatment method for large carburized gear shaft forging

Abstract

The present application relates to the technical field of heat treatment, in particular to a quenching heat treatment method for large carburized gear shaft forgings. The method comprises the following steps: S1, carburizing treatment; S2, hydrogen expansion annealing; S3, quenching heating; S4, quenching holding heating; S5, rapid cooling; S6, first low-temperature tempering; and S7, second low-temperature tempering. The present application is a quenching heat treatment process for large carburized steel gear shafts after carburizing in metallurgical products. The process can ensure that the gear teeth area of the large carburized steel gear shaft reaches high hardness and hardening layer depth after quenching, and can significantly reduce the residual stress in the center area of the gear shaft, effectively preventing early fracture of the gear shaft.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heat treatment, in particular to a quenching heat treatment method for large carburized gear shaft forgings. BACKGROUND

[0002] 17Cr2Ni2Mo, 20CrNi2Mo and other steels of Cr-Ni-Mo series are widely used in the manufacture of key components of large gears (shafts) in major equipment such as metallurgy, building materials, and water conservancy, etc. due to their good strength, plasticity, toughness and hardenability. The load-carrying capacity and fatigue strength of the gears are improved through carburizing and quenching heat treatment.

[0003] The main problem of carburizing and quenching of large gear shaft forgings (shaft diameter of 600mm-1300mm) is that the gear shaft parts are prone to early cracking and fracture failure in use due to the difficulty in guaranteeing the metallurgical and forging quality of the large cross-section. At present, the main solution is to improve the quality of the raw material forgings. The fracture of the large gear shaft is closely related to the hydrogen in the gear shaft and the stress distribution after carburizing and quenching of the gear shaft, and hydrogen and thermal stress in the heat treatment process are the most critical factors. The defects in the core of the gear shaft under the action of residual stress and large instantaneous working stress become crack sources and rapidly expand in the core, resulting in fracture. At the same time, the residual stress also increases the aggregation of hydrogen, leading to brittle fracture.

[0004] At present, according to the conventional heat treatment method, the gear shaft parts after conventional carburizing are quenched by ordinary step-up heating, and the whole body is heat-penetrated at the quenching temperature. The surface and the center area are cooled to a lower temperature during quenching, and finally two times of tempering are carried out to eliminate stress. After adopting the conventional heat treatment process, the stress distribution in the center area is high, which brings great risk to the use of the gear shaft. Therefore, it is necessary to study a quenching heat treatment method for large carburized gear shaft forgings, which can significantly reduce the residual stress in the center of the gear shaft and prevent early fracture of the gear shaft, and has important social and economic significance. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a quenching heat treatment method for large carburized gear shaft forgings, which can reduce the residual stress in the center of the gear shaft.

[0006] The technical solution adopted by the present application to solve the technical problem is a quenching heat treatment method for large carburized gear shaft forgings, comprising the following steps,

[0007] S1, carburizing treatment: placing the gear shaft in a heating furnace and heating to 920-950℃, and then performing carburizing treatment after the core of the gear shaft is heated, and the gear shaft is kept for ≥60h during the carburizing treatment;

[0008] S2, hydrogen expansion annealing: after the gear shaft carburizing treatment is completed, the gear shaft is cooled, and after being cooled to 630-680℃, the gear shaft is kept warm, and during the keeping warm process, hydrogen expansion annealing treatment is carried out, the keeping warm time is ≥12h, after the keeping warm is completed, the gear shaft is cooled to below 300℃ in the heating furnace, and then the gear shaft is moved out of the heating furnace;

[0009] S3, quenching heating: the gear shaft is placed in the heating furnace and heated to 600-650℃ at a speed of 2-5℃ / min, the gear shaft center is heated to the temperature, and the keeping warm time is 6-10h, after the keeping warm is completed, the gear shaft is heated to the quenching temperature 800-820℃ at a speed of 1-3℃ / min;

[0010] S4, quenching keeping warm: keeping warm at the quenching temperature 800-820℃ for 6-10h, so that the gear shaft tooth area and the area below the gear shaft tooth root 50-100mm reach the quenching temperature, and the temperature of the gear shaft center area is in the range of 650-770℃;

[0011] S5, rapid cooling: the gear shaft after quenching heating is taken out of the furnace and quenched in the oil tank, and the temperature is cooled to 100-200℃ on the surface, and the center area is at 200-350℃.

[0012] S6, first low temperature tempering: after the gear shaft quenching is completed, the gear shaft is timely put into the furnace for low temperature tempering treatment, the gear shaft is heated to the tempering temperature 200-220℃, the keeping warm time is ≥20h, and then the gear shaft is taken out of the furnace and air cooled to room temperature;

[0013] S7, second low temperature tempering: after the first low temperature tempering of the gear shaft is completed, the gear shaft is put into the furnace for the second time, heated to the tempering temperature 200-220℃, the keeping warm time is ≥16h, and then the gear shaft is taken out of the furnace and air cooled to room temperature.

[0014] Further, in step S1, the carburizing treatment adopts nitrogen-based atmosphere carburizing, the carrier gas is nitrogen and methanol, the rich gas is acetone, the carburizing temperature is 920-950℃, the atmosphere carbon potential during the strong carburizing period is 1.06-1.1CP, the atmosphere carbon potential during the diffusion period is 0.75-0.8CP, and the effective hardening layer depth of the carburized layer is Eht≥3.5mm.

[0015] Further, after step S2 and before step S3, the gear shaft is heated to 400℃, kept warm at 400℃ for 4-6h, and nitrogen is introduced to replace the air in the furnace as soon as the heating starts.

[0016] Further, in step S4, nitrogen is introduced during the entire heating process, methanol is introduced when the temperature reaches 760℃, acetone is introduced when the temperature reaches 800℃, and the target value of the atmosphere carbon potential during the keeping warm stage is set to 0.75-0.8CP.

[0017] Further, in step S7, the gear shaft tooth surface hardness after the second low-temperature tempering meets the requirement HRC 58-62.

[0018] The present application has the advantages that the present application is a quenching heat treatment process for large carburized steel gear shafts after carburization, which can ensure that the gear shaft tooth region after quenching has high hardness and hardening depth, while significantly reducing the residual stress in the center region of the gear shaft, effectively preventing early fracture of the gear shaft. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic diagram of the gear shaft of the present application;

[0020] Figure 2 is a flowchart of the present application;

[0021] Figure 3 is a stress distribution diagram along the axial direction of Example 1;

[0022] Figure 4 is a stress distribution diagram along the circumferential direction 1 of Example 1;

[0023] Figure 5 is a stress distribution diagram along the circumferential direction 2 of Example 1;

[0024] Figure 6 is a stress distribution diagram along the axial direction of Comparative Example 1;

[0025] Figure 7 is a stress distribution diagram along the circumferential direction 1 of Comparative Example 1;

[0026] Figure 8 is a stress distribution diagram along the circumferential direction 2 of Comparative Example 1. DETAILED DESCRIPTION

[0027] The embodiments of the present application will be described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals refer to the same or similar elements throughout. The embodiments described below by reference to the drawings are exemplary only, and are used only for the purpose of explaining the present application, and should not be construed as limiting the present application.

[0028] As shown in Figure 2 , the present application is a quenching heat treatment method for large carburized gear shaft forgings, comprising the following steps,

[0029] S1, carburizing treatment: the gear shaft is placed in a heating furnace to heat up to 920-950℃, and after the gear shaft core is heated, carburizing treatment is carried out, and the gear shaft is kept for ≥60h during the carburizing treatment; wherein, the carburizing treatment adopts nitrogen-based atmosphere carburizing, the carrier gas is nitrogen and methanol, the enrichment gas is acetone, the carburizing temperature is 920-950℃, the atmosphere carbon potential during the strong carburizing period is 1.06-1.1CP, the atmosphere carbon potential during the diffusion period is 0.75-0.8CP, and the effective hardening layer depth of the carburized layer is Eht≥3.5mm.

[0030] S2, hydrogen expansion annealing: after the gear shaft carburizing treatment is completed, the gear shaft is cooled, and after being cooled to 630-680℃, the gear shaft is kept, and the hydrogen expansion annealing treatment is carried out during the keeping process, the keeping time is ≥12h, and after the keeping is completed, the gear shaft is cooled to below 300℃ in the heating furnace, and then the gear shaft is removed from the heating furnace; the expansion hydrogen treatment is carried out during the keeping at 630-680℃ stage, which can further realize the dehydrogenation of the gear shaft and improve the dehydrogenation efficiency.

[0031] S3, quenching temperature heating: the gear shaft is placed in a heating furnace to heat to 600-650℃ at a speed of 2-5℃ / min, so that the gear shaft core is heated, and the keeping time is 6-10h, and after the keeping is completed, it is heated to quenching temperature 800-820℃ at a speed of 1-3℃ / min; the gear shaft is kept at 600-650℃ during the heating process to ensure that the center area is heated through, and to prevent the temperature difference between the surface layer and the center area from being too large when heating to the quenching temperature, which causes the thermal stress of the gear shaft to increase. The purpose of controlling the heating speed to 2-5℃ / min and 1-3℃ / min during the heating process is to reduce the overall stress of the gear shaft.

[0032] S4, quenching keeping heating: keeping at quenching temperature 800-820℃ for 6-10h, so that the gear tooth area and the area below the gear root 50-100mm reach the quenching temperature, while the center area temperature of the gear shaft is in the range of 650-770℃; the quenching keeping time is 6-10h, so that the gear tooth surface temperature and the area below the gear root 50-100mm reach the quenching temperature, which can make the gear tooth area and the area below the gear root 50-100mm reach the surface hardening effect, and the keeping time is 6-10h, and during the keeping period, the center area temperature of the gear shaft is 650-770℃, which is lower than the quenching temperature, and the center area of the gear shaft is the area from 200mm below the gear root to the center of the gear shaft, thereby effectively reducing the tensile stress generated by the thermal stress in the center area of the gear shaft.

[0033] S5, rapid cooling: the quenched and heated gear shaft is quenched and cooled in an oil tank, the gear region and the region 50-100 mm below the gear root are cooled to 100-200 °C, and the central region is at 200-350 °C. Since the cooling range and the cooling rate of the gear region and the region 50-100 mm below the gear root of the gear shaft are higher than those of the central region of the gear shaft, the thermal stress of the central region of the gear shaft is further reduced. During quenching, the gear shaft exchanges heat with the quenching oil, the contact area of the gear shaft edge with the quenching oil is large, so the cooling speed of the gear shaft edge is fast, and since the central position of the gear shaft is not directly in contact with the quenching oil, the temperature of the gear shaft from the outside to the inside presents a step change, so the temperature of the gear region and the region 50-100 mm below the gear root and the temperature of the central region are all in a range of temperature.

[0034] S6, first low-temperature tempering: after the gear shaft is quenched, it is promptly tempered in a furnace at a low temperature, the gear shaft is heated to a tempering temperature of 200-220 °C, and after a holding time of ≥20 h, it is taken out of the furnace and air-cooled to room temperature; when the gear surface hardness requirement of the gear shaft is ≥HRC57, the tempering temperature is 200-220 °C, and sufficient holding should be performed to reduce the residual tensile stress in the central region; when the gear surface hardness requirement is lower than HRC57, the first tempering temperature can be further increased to facilitate the reduction of the tensile stress in the central region.

[0035] S7, second low-temperature tempering: after the first low-temperature tempering of the gear shaft is completed, the gear shaft is tempered again in a furnace, heated to a tempering temperature of 200-220 °C, and after a holding time of ≥16 h, it is taken out of the furnace and air-cooled to room temperature. Through the second low-temperature tempering, the stress caused by the change in the surface layer structure after the first tempering can be further reduced, and the residual tensile stress in the central region can be further reduced.

[0036] Further, after step S2 and before step S3, the gear shaft is heated to 400 °C, and held at 400 °C for 4-6 h. Nitrogen is introduced to replace the air in the furnace as soon as the heating starts.

[0037] Further, in step S4, nitrogen is introduced throughout the heating process, and methanol is introduced when the temperature reaches 760 °C, and acetone is introduced when the temperature reaches 800 °C. The target carbon potential value of the atmosphere is set to 0.75-0.8 CP during the holding stage. This prevents the surface hardness of the gear from being reduced after quenching and tempering due to the reduction of the surface carbon concentration.

[0038] Further, in step S7, the gear surface hardness of the gear shaft after the second low-temperature tempering meets the requirement of HRC58-62.

[0039] Example 1

[0040] The engineering test was carried out in a factory according to the heat treatment method of the present application, the test forging material was 17Cr2Ni2Mo, the cross-sectional size of the carburized and quenched part of the gear shaft was Da=915mm, and the heat treatment method of the forging was as follows:

[0041] Step 1, the gear shaft was heated and high-temperature carburized. The carburizing process adopted nitrogen-based atmosphere carburizing, the carrier gas was nitrogen and methanol, the rich gas was acetone, the carburizing temperature was 930℃, the atmosphere carbon potential during the strong carburizing period was 1.08CP, the atmosphere carbon potential during the diffusion period was 0.77CP, the carburizing time at 930℃ was 140h, and the effective hardening layer depth of the carburized sample in the furnace after carburizing and after “810℃ oil quenching + 200℃ tempering” was detected as 5.5mm, indicating that the carburized layer depth met the requirements and the subsequent steps could be carried out.

[0042] Step 2, the gear shaft was cooled and hydrogen diffusion annealed after carburizing. After the carburizing treatment, the furnace was cooled to 660℃ and then the gear shaft was kept at this temperature for 15h, and then the gear shaft was cooled to below 300℃ in the furnace and then air-cooled.

[0043] Step 3, the gear shaft after “carburizing + hydrogen diffusion annealing” was quenched and heated. After the carburizing and hydrogen diffusion annealing of the gear shaft, the gear shaft was heated to 400℃ for quenching, kept at 400℃ for 5h, then heated to 630℃ at a speed of 3℃ / min, so that the center of the gear shaft reached the temperature, kept at this temperature for 8h, and then heated to the quenching temperature 810℃ at a speed of 2℃ / min.

[0044] Step 4, the gear shaft after quenching and heating was quenched and kept at temperature. The gear shaft was kept at the quenching temperature 810℃ for 8h, so that the gear shaft tooth area and the area below the tooth root 50-100mm reached the quenching temperature 810℃, and the center area of the gear shaft (i.e. the area below the tooth root 200mm to the center of the gear shaft) was in the range of 650-770℃, and in this embodiment, the center of the gear shaft was 750°.

[0045] Step 5, the gear shaft after quenching and heating was rapidly cooled. The quenched and heated gear shaft was lifted out of the furnace and then quenched and cooled in the oil tank, the cooling time was 130min, the gear shaft tooth area and the area below the tooth root 50-100mm were cooled to 100-200℃, and the center area of the gear shaft was in the range of 200-350℃, and in this embodiment, the gear shaft tooth area was 120° and the center of the gear shaft was 330°.

[0046] Step 6, the gear shaft after quenching and cooling was first low-temperature tempered. After the quenching of the gear shaft, the gear shaft was immediately put into the furnace for low-temperature tempering, heated to the tempering temperature 210℃, kept at temperature for 26h, and then taken out of the furnace and air-cooled to room temperature.

[0047] Step 7, the gear shaft forging after the first low-temperature tempering is subjected to second low-temperature tempering: after the first low-temperature tempering of the gear shaft is completed, the gear shaft is subjected to second tempering in the furnace, heated to a tempering temperature of 210℃, and held for 20h, and then taken out of the furnace and air-cooled to room temperature after the tempering is completed.

[0048] The 17Cr2Ni2Mo steel gear shaft forging after the foregoing process is subjected to performance testing, and the results are: the surface Rockwell hardness is 58.2-58.6HRC, the gear core hardness is 40.2HRC, and the hardened layer depth Eht is 5.29mm-5.47mm. The center region of the gear shaft is subjected to stress simulation calculation testing according to the foregoing process method, and the stress distribution simulation diagrams of the center region in the axial direction, the circumferential direction 1, and the circumferential direction 2 are shown in Figures 3-5 , and the maximum stress is located in the interior of the gear shaft, reaching 345Mpa, 185Mpa, and 172Mpa respectively.

[0049] Example 2

[0050] An engineering test is carried out in a factory according to the heat treatment method of the present application, the test forging is made of 17Cr2Ni2Mo, the cross-sectional size of the carburized and quenched part of the test workpiece gear shaft is Da=915mm, and the heat treatment method of the forging is as follows:

[0051] Step 1, the gear shaft is subjected to heating and high-temperature carburizing treatment. The carburizing treatment process adopts nitrogen-based atmosphere carburizing, the carrier gas is nitrogen and methanol, the enriched gas is acetone, the carburizing temperature is 920℃, the atmosphere carbon potential during the strong carburizing period is 1.06CP, the atmosphere carbon potential during the diffusion period is 0.75CP, the carburizing time at 930℃ is 130h, and the effective hardened layer depth of the carburized sample in the furnace after the carburizing is detected to be 5.4mm after “810℃ oil quenching + 200℃ tempering”, indicating that the carburized layer depth meets the requirements, and the subsequent steps can be carried out.

[0052] Step 2, the gear shaft is subjected to cooling and hydrogen diffusion annealing treatment after carburizing. After the carburizing treatment, the furnace is cooled to 630℃, and then held for 12h, and then cooled to below 300℃ in the furnace and air-cooled.

[0053] Step 3, the gear shaft subjected to “carburizing + hydrogen diffusion annealing” is subjected to quenching heating treatment. After the carburizing and hydrogen diffusion annealing of the gear shaft are completed, the gear shaft is heated to 400℃ for quenching, and held for 4h at 400℃, then heated to 600℃ at a speed of 2℃ / min, so that the core of the gear shaft is heated to the temperature, and held for 6h at the temperature, and then heated to the quenching temperature of 800℃ at a speed of 1℃ / min.

[0054] Step 4, quenching and holding heating of the gear shaft after quenching heating. The gear shaft is held at 800℃ for 6h to make the gear shaft tooth region and the region below the tooth root 50-100mm reach the quenching temperature 800℃, while the central region of the gear shaft (i.e. the region below the tooth root 200mm to the center of the gear shaft) is at 650-770℃, and the center of the gear shaft in this embodiment is at 740°.

[0055] Step 5, rapid cooling of the gear shaft after quenching heating: the quenched and heated gear shaft is lifted out of the furnace and quenched in an oil tank, the cooling time is 130 minutes, the gear shaft tooth region and the region below the tooth root 50-100mm are cooled to 100-200℃, and the central region of the gear shaft is at 200-350℃. In this embodiment, the tooth region is 100°, and the center of the gear shaft is 320°.

[0056] Step 6, first low-temperature tempering treatment of the gear shaft after quenching cooling. After the gear shaft is quenched, it is promptly put into the furnace for low-temperature tempering treatment, heated to a tempering temperature of 200℃, and held for 20h, and then cooled to room temperature after the tempering is completed.

[0057] Step 7, second low-temperature tempering of the gear shaft after the first low-temperature tempering: after the first low-temperature tempering of the gear shaft is completed, the gear shaft is put into the furnace for the second time for tempering, heated to a tempering temperature of 200℃, and held for 16h, and then cooled to room temperature after the tempering is completed.

[0058] The performance of the 17Cr2Ni2Mo steel gear shaft after the above process is tested, and the results are: the surface Rockwell hardness is 58.3-58.6HRC, the tooth core hardness is 40.1HRC, and the hardening layer depth Eht is 5.27mm-5.42mm. The stress simulation calculation test is carried out on the central region of the gear shaft according to the above process, and the maximum stress is located in the internal region of the gear shaft, which reaches 355, 189 and 177Mpa respectively.

[0059] Example 3

[0060] An engineering test was carried out in a factory according to the heat treatment method of the present application, the test forging material was 17Cr2Ni2Mo, the cross-sectional size of the carburized and quenched part of the test workpiece gear shaft was Da=915mm, and the heat treatment method of the forging was:

[0061] Step 1, the gear shaft is heated and high temperature carburizing treatment. Carburizing process using nitrogen-based atmosphere carburizing, carrier gas is nitrogen and methanol, rich gas is acetone, carburizing temperature 950℃, strong penetration period atmosphere carbon potential is 1.1CP, diffusion period atmosphere carbon potential is 0.8CP, carburizing time at 950℃ for 150h, after carburizing the furnace carburizing sample is detected after "810℃ oil quenching + 200℃ tempering" effective hardening layer depth is 5.7mm, indicating that the carburizing layer depth meets the requirements, can be carried out after the procedure.

[0062] Step 2, the gear shaft is cooled and hydrogen annealing treatment after carburizing. After the carburizing treatment, i.e. after the end of the heat preservation, the furnace is cooled to 680℃, then the heat preservation is carried out, the heat preservation time is 16h, after the end of the heat preservation, the furnace is cooled to below 300℃ and then the gear shaft is taken out and air cooled.

[0063] Step 3, the gear shaft is quenched and heated after "carburizing + hydrogen annealing". After the carburizing and hydrogen annealing of the gear shaft, the gear shaft is heated to 400℃, then the temperature is kept for 6h, then the temperature is increased to 650℃ at a speed of 5℃ / min, the center of the gear shaft is heated to the temperature, the temperature is kept for 10h, then the temperature is increased to quenching temperature 820℃ at a speed of 3℃ / min.

[0064] Step 4, the gear shaft is quenched and heated after the quenching. The temperature is kept for 10h at the quenching temperature 820℃, so that the gear shaft tooth area and the area below the tooth root 50-100mm reach the quenching temperature 820℃, while the center area of the gear shaft (i.e. the area below the tooth root 200mm to the center of the gear shaft) is at a temperature of 650-770℃. In this embodiment, the center of the gear shaft is at 758°.

[0065] Step 5, the gear shaft is rapidly cooled after quenching. After the quenching of the gear shaft, the gear shaft is taken out of the furnace and quenched in the oil tank, the cooling time is 130min, the gear shaft tooth area and the area below the tooth root 50-100mm are cooled to 100-200℃, while the center area of the gear shaft is at a temperature of 200-350℃. In this embodiment, the tooth area is at 128° and the center of the gear shaft is at 337°.

[0066] Step 6, the gear shaft is first low temperature tempered after quenching. After the quenching of the gear shaft, the gear shaft is immediately put into the furnace for low temperature tempering, the temperature is increased to 220℃, the heat preservation time is 28h, after the end of the tempering, the gear shaft is taken out of the furnace and air cooled to room temperature.

[0067] Step 7, the gear shaft is second low temperature tempered after the first low temperature tempering. After the first low temperature tempering of the gear shaft, the gear shaft is put into the furnace for the second time, the temperature is increased to 220℃, the heat preservation time is 23h, after the end of the tempering, the gear shaft is taken out of the furnace and air cooled to room temperature.

[0068] The performance of the 17Cr2Ni2Mo steel gear shaft forging after the foregoing process is tested, and the results are: the surface Rockwell hardness is 58.3-58.9 HRC, the gear core hardness is 40.5 HRC, and the hardening layer depth Eht is 5.30-5.49 mm. The stress simulation calculation test is performed on the center area of the gear shaft according to the foregoing process, and the maximum stress is located in the internal part of the gear shaft, which reaches 338, 179 and 170 MPa, respectively.

[0069] Comparative Example 1

[0070] Engineering tests were carried out in a certain factory according to the heat treatment method of the present application, the test forging material was 17Cr2Ni2Mo, the cross-sectional size of the carburizing and quenching part of the test workpiece gear shaft was Da = 915 mm, and the heat treatment method of the forging was as follows:

[0071] Step 1, the gear shaft is heated and high-temperature carburized. The carburizing process adopts nitrogen-based atmosphere carburizing, the carrier gas is nitrogen and methanol, the rich gas is acetone, the carburizing temperature is 930℃, the strong carburizing period atmosphere carbon potential is 1.08CP, the diffusion period atmosphere carbon potential is 0.77CP, the carburizing time at 930℃ is 140h, after carburizing, the effective hardening layer depth of the carburizing sample in the furnace is 5.5mm after "810℃ oil quenching + 200℃ tempering", which indicates that the carburizing layer depth meets the requirements and can proceed to the subsequent steps.

[0072] Step 2, the gear shaft is cooled after carburizing. After the carburizing treatment period, i.e. after the end of the holding, the furnace is cooled to below 300℃ and then the workpiece is taken out of the furnace and air-cooled.

[0073] Step 3, the gear shaft after carburizing is subjected to quenching and heating treatment. After the carburizing and hydrogen diffusion annealing of the gear shaft are completed, the gear shaft is heated to 400℃ for quenching, and then held at 400℃ for 5h, and then heated to 630℃ at a rate of 3℃ / min, and held at this temperature for 8h to make the center area heat-penetrated, and then heated to the quenching temperature of 810℃ at a rate of 2℃ / min.

[0074] Step 4, the gear shaft after quenching and heating is subjected to quenching and holding. The gear shaft is held at the quenching temperature of 810℃ for 16h to make the whole gear shaft heat-penetrated to the quenching temperature of 810℃.

[0075] Step 5, the gear shaft after quenching and heating is subjected to rapid cooling. The gear shaft after quenching and heating is lifted out of the furnace and then subjected to quenching cooling in an oil tank for 160 minutes to make the whole gear shaft temperature less than 150℃.

[0076] Step 6, the first low temperature tempering treatment is performed on the gear shaft after quenching and cooling. After quenching of the gear shaft, the low temperature tempering treatment is performed in the furnace in time, heating to the tempering temperature of 220℃, the holding time is 26h, after the tempering is finished, the furnace is discharged and air cooled to room temperature.

[0077] Step 7, the second low temperature tempering treatment is performed on the gear shaft after the first low temperature tempering treatment: after the first low temperature tempering treatment of the forging, the second tempering is performed in the furnace, heating to the tempering temperature of 220℃, the holding time is 20h, after the tempering is finished, the furnace is discharged and air cooled to room temperature.

[0078] The surface Rockwell hardness (generally three points on the gear shaft are detected), the gear tooth core hardness, the hardened layer depth, the shaft body surface temperature and the core temperature, the stress distribution simulation calculation of the central region in the axial direction, the circumferential direction 1 and the circumferential direction 2 of the gear shaft treated according to the foregoing process are performed, see Figures 6-8 , and the results are shown in Table 1.

[0079] Table 1: Comparison table of heat treatment performance and stress

[0080]

[0081] The heat treatment performance and stress comparison results of the 17Cr2Ni2Mo steel gear shaft forging treated by the process method of "carburizing + hydrogen expansion annealing + quenching heating and long time medium temperature holding + surface layer heating at quenching temperature + quenching cooling control of final cooling temperature + twice tempering" of Example 1 and the 17Cr2Ni2Mo steel gear shaft forging treated by the traditional process method of "carburizing + quenching heating ordinary stepwise heating + quenching temperature conventional whole heat penetration + quenching cooling to lower temperature + twice tempering" of Comparative Example 1 are shown in Table 1. After the process method of the application is used, the surface hardening effect can be achieved in the gear region, and the quenching temperature in the central region is low, thereby effectively reducing the tensile stress in the central region due to thermal stress; the cooling time is shortened, the oil outlet temperature is increased, the temperature in the central region of the gear shaft can be maintained at about 200-350℃, thereby effectively reducing the thermal stress in the low temperature stage, and finally achieving a significant reduction in the stress in the central region after quenching.

[0082] The embodiments of the specific implementation are the preferred embodiments of the application, and do not limit the protection scope of the application, so that: any equivalent changes made according to the structure, shape, principle of the application should be covered within the protection scope of the application.

Claims

1. A method of quenching heat treatment of a large carburized gear shaft forging, characterized by: It comprises the following steps, S1, carburizing treatment: the gear shaft is placed in a heating furnace for heating to 920-950℃, and after the gear shaft is heated, carburizing treatment is performed, and the gear shaft is kept for ≥60h during the carburizing treatment; S2, hydrogen expansion annealing: after the carburizing treatment of the gear shaft is completed, the gear shaft is cooled, and after being cooled to 630-680℃, the gear shaft is kept, and hydrogen expansion annealing is performed during the keeping, the keeping time is ≥12h, and after the keeping is completed, the gear shaft is cooled to below 300℃ in the heating furnace, and then the gear shaft is taken out of the heating furnace and air-cooled to room temperature; S3, quenching heating: the gear shaft is placed in a heating furnace and heated to 600-650℃ at a speed of 2-5℃ / min, the gear shaft is heated to the center, and kept for 6-10h, and then heated to a quenching temperature of 800-820℃ at a speed of 1-3℃ / min; S4, quenching keeping heating: kept for 6-10h at the quenching temperature of 800-820℃, so that the tooth area and the area below the tooth root of the gear shaft reach the quenching temperature, and the temperature of the central area of the gear shaft is in the range of 650-770℃; S5, rapid cooling: the gear shaft is taken out of the furnace and quenched in an oil tank, the tooth area and the area below the tooth root of the gear shaft are cooled to 100-200℃, and the central area is cooled to 200-350℃; S6, first low-temperature tempering: after the quenching of the gear shaft is completed, the gear shaft is promptly put into the furnace for low-temperature tempering treatment, heated to a tempering temperature of 200-220℃, kept for ≥20h, and then taken out of the furnace and air-cooled to room temperature; S7, second low-temperature tempering: after the first low-temperature tempering of the gear shaft is completed, the gear shaft is put into the furnace again for second tempering, heated to a tempering temperature of 200-220℃, kept for ≥16h, and then taken out of the furnace and air-cooled to room temperature.

2. A method of quenching heat treatment of a large carburized gear shaft forging as claimed in claim 1, characterized in that: In step S1, carburizing treatment is performed in a nitrogen-based atmosphere, the carrier gas is nitrogen and methanol, the enrichment gas is acetone, the carburizing temperature is 920-950℃, the carbon potential of the atmosphere during the strong carburizing period is 1.06-1.1CP, the carbon potential of the atmosphere during the diffusion period is 0.75-0.8CP, and the effective hardening layer depth of the carburized layer is Eht≥3.5mm.

3. A method of quenching heat treatment of a large carburized gear shaft forging as claimed in claim 1, characterized in that: After step S2 and before step S3, the gear shaft is heated to 400℃, and kept for 4-6h at 400℃, and nitrogen is introduced to replace the air in the furnace as soon as the heating starts.

4. A method of quenching heat treatment of a large carburized gear shaft forging as set forth in claim 1, characterized in that: In step S4, nitrogen is introduced during the entire heating process, and methanol is introduced when the temperature reaches 760℃, and acetone is introduced when the temperature reaches 800℃, and the target value of the carbon potential of the atmosphere during the keeping stage is set to 0.75-0.8CP.

5. A method of quenching heat treatment of a large carburized gear shaft forging as set forth in claim 1, characterized in that: In step S7, the gear shaft after the second low-temperature tempering has a tooth surface hardness that meets the requirements of HRC58-62.

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