Quenching heat treatment method for large carburizing gear shaft forge piece

Abstract

The invention relates to the technical field of heat treatment, in particular to a quenching heat treatment method for a large carburized gear shaft forge piece. Comprising the following steps: S1, carburizing treatment; s2, hydrogen diffusion annealing; s3, quenching, heating and heating; s4, quenching, heat preservation and heating; s5, rapid cooling; s6, performing low-temperature tempering for the first time; and S7, secondary low-temperature tempering. According to the quenching heat treatment technology for the large carburizing steel gear shaft in metallurgical products after carburizing, it can be guaranteed that a tooth area of the large carburizing steel gear shaft after quenching reaches high hardness and hardened layer depth through the technology, meanwhile, the residual stress of the center area of the gear shaft can be remarkably reduced, and the gear shaft is effectively prevented from being fractured in the early stage.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment, in particular to a quenching heat treatment method for a large carburized gear shaft forging. Background Art

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

[0003] The main problem with carburizing and quenching of large gear shaft forgings (shaft diameter is 600mm~1300mm) is that the gear shaft parts are large in cross-section, and the metallurgical and forging quality is difficult to guarantee. Therefore, early cracking and fracture failure are prone to occur during use. At present, the main solution is to improve the quality of raw material forgings. Since the fracture of large gear shafts is also closely related to the hydrogen in the gear shaft and the stress distribution of the gear shaft after carburizing and quenching, and hydrogen and thermal stress in the heat treatment process are the most critical factors, the defects in the core of the gear shaft during use become crack sources under residual stress and large instantaneous working stress, and quickly expand in the core, resulting in fracture. At the same time, residual stress will also increase the accumulation of hydrogen and cause brittle fracture.

[0004] At present, according to the conventional heat treatment method, the gear shaft parts after conventional carburizing are quenched and heated by ordinary step-by-step heating, and the entire part is heated at the quenching temperature. During quenching, both the surface and the center area are cooled to a lower temperature, and finally two tempering treatments are performed to eliminate stress. After using the conventional heat treatment process, the stress distribution in the simulated center area is relatively high, which brings great risks to the use of the gear shaft. Therefore, it is necessary to study a quenching heat treatment method for large carburized gear shaft forgings to significantly reduce the residual stress in the center of the gear shaft and prevent the gear shaft from premature fracture, which has important social and economic significance. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a quenching heat treatment method for a large carburized gear shaft forging, which is used to reduce the residual stress in the central area of ​​the gear shaft.

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

[0007] S1. Carburizing treatment: Place the gear shaft in a heating furnace and heat it to 920-950℃. Carburizing treatment is performed after the gear shaft core reaches the temperature. During the carburizing treatment, the gear shaft is kept warm for ≥60h;

[0008] S2. Hydrogen expansion annealing: After the gear shaft is carburized, the gear shaft is cooled down to 630-680℃ and then kept warm. The hydrogen expansion annealing is performed during the warm-keeping process. The warm-keeping time is ≥12h. After the warm-keeping is completed, the gear shaft is cooled down to below 300℃ in the heating furnace and then removed from the heating furnace.

[0009] S3, quenching heating: place the gear shaft in a heating furnace and heat it to 600-650°C at a rate of 2°C-5°C / min, so that the gear shaft center reaches the temperature, and keep it warm for 6-10h. After the insulation is completed, heat it to the quenching temperature of 800-820°C at a rate of 1-3°C / min;

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

[0011] S5. Rapid cooling: The quenched and heated gear shaft is quenched and cooled in an oil tank after being taken out of the furnace, and the temperature is reduced to 100-200℃ on the surface and 200-350℃ in the center area.

[0012] S6. First low-temperature tempering: After the quenching of the gear shaft is completed, it is promptly put into the furnace for low-temperature tempering treatment. The gear shaft is heated to the tempering temperature of 200-220°C. After the heat preservation time is ≥20h, it 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, it is put into the furnace for the second tempering, heated to the tempering temperature of 200-220℃, and kept at this temperature for ≥16h before being 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 enriched gas is acetone, the carburizing temperature is 920-950°C, the carbon potential of the atmosphere during the intensive 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 hardened 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° C. and kept at 400° C. for 4-6 hours. Nitrogen is introduced at the beginning of the heating to replace the air in the furnace.

[0016] Further, in step S4, nitrogen is introduced throughout the entire heating process, and methanol is introduced when the temperature is heated to 760°C, and acetone is introduced when the temperature is heated to 800°C. The carbon potential target value of the atmosphere is set at 0.75-0.8CP during the insulation stage.

[0017] Further, in step S7, the hardness of the gear shaft tooth surface after the second low temperature tempering meets the requirement of HRC58-62.

[0018] The beneficial effects of the present invention are as follows: the present invention is a quenching heat treatment process for large carburized steel gear shafts in metallurgical products after carburizing. The process can ensure that the tooth area of ​​the large carburized steel gear shaft reaches a high hardness and hardened layer depth after quenching, and at the same time can significantly reduce the residual stress in the central area of ​​the gear shaft, effectively preventing the gear shaft from premature fracture. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0020] Figure 2 is a flow chart of the present invention;

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

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

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

[0024] Figure 6 is the stress distribution diagram of comparative example 1 along the axial direction;

[0025] Figure 7 is the stress distribution diagram of comparative example 1 along circumferential direction 1;

[0026] Figure 8 This is the stress distribution diagram of comparative example 1 along circumferential direction 2. DETAILED DESCRIPTION

[0027] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0028] like Figure 2 As shown, the present invention provides a quenching heat treatment method for a large carburized gear shaft forging, comprising the following steps:

[0029] S1. Carburizing treatment: Place the gear shaft in a heating furnace and heat it to 920-950℃. Carburize the gear shaft after the gear shaft reaches the temperature. Keep the gear shaft warm for ≥60h during the carburizing process. Carburize the gear shaft in a nitrogen-based atmosphere. The carrier gas is nitrogen and methanol, the enriched 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.

[0030] S2. Hydrogen expansion annealing: After the carburizing treatment of the gear shaft is completed, the gear shaft is cooled down. After cooling to 630-680℃, the gear shaft is kept warm. During the warm-keeping process, hydrogen expansion annealing is performed. The warm-keeping time is ≥12h. After the warm-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. During the cooling process, the gear shaft is kept warm at 630-680℃ for hydrogen expansion treatment, which can further realize the dehydrogenation of the gear shaft and improve the dehydrogenation efficiency.

[0031] S3, quenching heating: place the gear shaft in a heating furnace and heat it to 600-650℃ at a rate of 2℃-5℃ / min, so that the gear shaft core reaches the temperature, and keep it warm for 6-10h. After the insulation, heat it to the quenching temperature of 800-820℃ at a rate of 1-3℃ / min; during the heating process of the gear shaft, keep it warm at 600-650℃ to ensure that the central area is heated thoroughly to prevent the temperature difference between the surface and the central area from being too large when the temperature is raised to the quenching temperature, causing the thermal stress of the gear shaft to increase. The purpose of controlling the heating rate 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 and heat preservation heating: keep the quenching temperature at 800-820℃ for 6-10h, so that the tooth area of ​​the gear shaft and the area 50-100mm below the tooth root reach the quenching temperature, and the temperature of the central area of ​​the gear shaft is in the range of 650-770℃; the quenching and heat preservation time is 6-10h, so that the surface temperature of the tooth and the area 50-100mm below the tooth root reach the quenching temperature, so that the tooth area and the area 50-100mm below the tooth root can achieve the surface hardening effect, and the heat preservation time is 6-10h. During the heat preservation period, the temperature of the central area of ​​the gear shaft is 650-770℃, which is lower than the quenching temperature. The central area of ​​the gear shaft is the area from 200mm below the tooth root to the center of the gear shaft, thereby effectively reducing the tensile stress caused by thermal stress in the central area of ​​the gear shaft.

[0033] S5. Rapid cooling: The quenched and heated gear shaft is quenched and cooled in the oil tank after being taken out of the furnace. The tooth area and the area 50 to 100 mm below the tooth root are cooled to 100 to 200 ° C, while the central area is 200 to 350 ° C. Since the cooling amplitude and cooling rate of the gear shaft tooth area and the area 50 to 100 mm below the tooth root are higher than the cooling amplitude and cooling rate of the central area of ​​the gear shaft, the thermal stress in the central area of ​​the gear shaft is further reduced. During quenching, the gear shaft exchanges heat with the quenching oil, and the contact area between the edge of the gear shaft and the quenching oil is large, so the cooling rate of the edge of the gear shaft is fast. Since the center 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 tooth area and the area 50 to 100 mm below the tooth root and the temperature of the central area are all in a range of temperatures.

[0034] S6. First low-temperature tempering: After the quenching of the gear shaft is completed, it should be put into the furnace for low-temperature tempering in time. The gear shaft should be heated to the tempering temperature of 200-220℃. After the heat preservation time is ≥20h, it should be taken out of the furnace and air-cooled to room temperature. When the gear shaft tooth surface hardness requirement is ≥HRC57, the tempering temperature is 200-220℃, and it should be fully kept warm to reduce the residual tensile stress in the center area. When the tooth surface hardness requirement is lower than HRC57, the first tempering temperature can be further increased to reduce the tensile stress in the center area.

[0035] S7, Second low temperature tempering: After the first low temperature tempering of the gear shaft, it is put into the furnace for the second tempering, heated to the tempering temperature of 200-220℃, kept at temperature for ≥16h, and then taken out of the furnace and air cooled to room temperature. The second low temperature tempering can further reduce the stress caused by the change of the surface structure after the first tempering, and further reduce the residual tensile stress in the center area.

[0036] Further, after step S2 and before step S3, the gear shaft is heated to 400° C. and kept at 400° C. for 4 to 6 hours. Nitrogen is introduced at the beginning of the heating to replace the air in the furnace.

[0037] Further, in step S4, nitrogen is introduced throughout the heating process, methanol is introduced when the temperature reaches 760°C, and acetone is introduced when the temperature reaches 800°C, and the carbon potential target value of the atmosphere is set at 0.75-0.8CP during the heat preservation stage to prevent the surface hardness of the tooth from decreasing after quenching and tempering due to the decrease in surface carbon concentration.

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

[0039] Example 1

[0040] A factory conducted an engineering test according to the heat treatment method of the present invention. The material of the test forging was 17Cr2Ni2Mo. The cross-sectional size of the carburized and quenched part of the gear shaft of the test workpiece was Da=915mm. The heat treatment method of the forging was:

[0041] Step 1: Carburize the gear shaft by heating and high-temperature carburizing. The carburizing process uses nitrogen-based atmosphere carburizing, with nitrogen and methanol as carrier gas, acetone as enriched gas, carburizing temperature of 930°C, carbon potential of atmosphere in strong carburizing period of 1.08CP, carbon potential of atmosphere in diffusion period of 0.77CP, carburizing time of 140h at 930°C, and effective hardening layer depth of 5.5mm after "810°C oil quenching + 200°C tempering" of the furnace carburized sample, indicating that the carburized layer depth meets the requirements and can proceed to the subsequent steps.

[0042] Step 2: Cool down the gear shaft after carburizing and perform hydrogen expansion annealing. After the end of carburizing treatment, the temperature is lowered with the furnace, and after it drops to 660°C, it is kept warm for 15 hours. After the end of the warming, the furnace is cooled to below 300°C and then air-cooled.

[0043] Step 3: Carburize and hydrogenate the gear shaft for quenching and heating. After the carburizing and hydrogenate annealing of the gear shaft, quench the gear shaft and heat it to 400°C, keep it at 400°C for 5 hours, then heat it to 630°C at a rate of 3°C / min until the center of the gear shaft reaches the temperature, keep it at this temperature for 8 hours, and then heat it to the quenching temperature of 810°C at a rate of 2°C / min after the heat preservation.

[0044] Step 4: After quenching and heating, the gear shaft is quenched and heated. The quenching temperature is kept at 810°C for 8 hours, so that the gear shaft tooth area and the area 50-100mm below the tooth root reach the quenching temperature of 810°C, and the temperature of the central area of ​​the gear shaft (i.e., the area from 200mm below the tooth root to the gear shaft center) is in the range of 650-770°C. In this embodiment, the gear shaft axis position is 750°.

[0045] Step 5, quickly cool the gear shaft after quenching and heating: lift the quenched and heated gear shaft out of the furnace and quench it in an oil tank for 130 minutes. The gear shaft tooth area and the area 50 to 100 mm below the tooth root are cooled to 100 to 200°C, while the central area of ​​the gear shaft is at 200 to 350°C. In this embodiment, the tooth area is 120° and the gear shaft axis position is 330°.

[0046] Step 6: Perform the first low-temperature tempering treatment on the gear shaft after quenching and cooling. After the quenching of the gear shaft, it is promptly put into the furnace for low-temperature tempering treatment, heated to the tempering temperature of 210°C, and kept at this temperature for 26 hours. After the tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

[0047] Step 7, perform a second low-temperature tempering on the gear shaft forging after the first low-temperature tempering treatment: After the first low-temperature tempering of the gear shaft is completed, it is put into the furnace for the second tempering, heated to the tempering temperature of 210°C, and kept at this temperature for 20 hours. After the tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

[0048] The performance test of the 17Cr2Ni2Mo steel gear shaft forgings after the above process was carried out, and the results were: the surface Rockwell hardness was 58.2-58.6HRC, the gear core hardness was 40.2HRC, and the hardened layer depth Eht was 5.29mm-5.47mm. The stress simulation calculation test of the center area of ​​the gear shaft was carried out according to the above process method, see Figure 3-Figure 5 , the stress distribution simulation diagrams in the axial direction, circumferential direction 1, and circumferential direction 2 of the central area show that the maximum stress is located inside the gear shaft, reaching 345, 185, and 172Mpa respectively.

[0049] Example 2

[0050] A factory conducted an engineering test according to the heat treatment method of the present invention. The material of the test forging was 17Cr2Ni2Mo. The cross-sectional size of the carburized and quenched part of the gear shaft of the test workpiece was Da=915mm. The heat treatment method of the forging was:

[0051] Step 1: Carburize the gear shaft by heating and high-temperature carburizing. The carburizing process uses nitrogen-based atmosphere carburizing, with nitrogen and methanol as carrier gas, acetone as enriched gas, carburizing temperature of 920°C, carbon potential of atmosphere in strong carburizing period of 1.06CP, carbon potential of atmosphere in diffusion period of 0.75CP, carburizing time of 130h at 930°C, and effective hardening layer depth of 5.4mm after "810°C oil quenching + 200°C tempering" of the furnace carburized sample, indicating that the carburized layer depth meets the requirements and can proceed to the subsequent steps.

[0052] Step 2: Carburize and cool the gear shaft and perform hydrogen annealing. After the carburizing treatment, the temperature is lowered with the furnace, and the temperature is kept at 630℃ for 12 hours. After the temperature is kept, the furnace is cooled to below 300℃ and then air-cooled.

[0053] Step 3: Carburize and hydrogenate the gear shaft for quenching and heating. After the carburizing and hydrogenate annealing of the gear shaft is completed, the gear shaft is quenched and heated to 400°C, and kept at 400°C for 4 hours, then heated to 600°C at a rate of 2°C / min, so that the gear shaft reaches the temperature, and kept at this temperature for 6 hours. After the insulation is completed, it is heated to the quenching temperature of 800°C at a rate of 1°C / min;

[0054] Step 4: After quenching and heating, the gear shaft is quenched and heated. Keep the quenching temperature at 800°C for 6 hours, so that the gear shaft tooth area and the area 50-100mm below the tooth root reach the quenching temperature of 800°C, and the temperature of the central area of ​​the gear shaft (i.e., the area from 200mm below the tooth root to the center of the gear shaft) is 650-770°C. In this embodiment, the center position of the gear shaft is 740°.

[0055] Step 5, quickly cool the gear shaft after quenching and heating: lift the quenched and heated gear shaft out of the furnace and quench and cool it in the oil tank for 130 minutes. The gear shaft tooth area and the area 50 to 100 mm below the tooth root are cooled to 100 to 200°C, while the central area of ​​the gear shaft is 200 to 350°C. In this embodiment, the tooth area is 100° and the gear shaft axis position is 320°.

[0056] Step 6: Perform the first low-temperature tempering treatment on the gear shaft after quenching and cooling. After the quenching of the gear shaft is completed, it is promptly put into the furnace for low-temperature tempering treatment, heated to the tempering temperature of 200°C, and kept at this temperature for 20 hours. After the tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

[0057] Step 7, perform a second low-temperature tempering on the gear shaft forging after the first low-temperature tempering treatment: After the first low-temperature tempering of the gear shaft is completed, it is put into the furnace for the second tempering, heated to the tempering temperature of 200°C, and the heat preservation time is 16 hours. After the tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

[0058] The performance test of the 17Cr2Ni2Mo steel gear shaft forgings processed according to the above process showed that the surface Rockwell hardness was 58.3-58.6HRC, the gear core hardness was 40.1HRC, and the hardened layer depth Eht was 5.27mm-5.42mm. The stress simulation calculation test of the center area of ​​the gear shaft was carried out according to the above process method. The maximum stress was located inside the gear shaft, reaching 355, 189, and 177Mpa respectively.

[0059] Example 3

[0060] A factory conducted an engineering test according to the heat treatment method of the present invention. The material of the test forging was 17Cr2Ni2Mo. The cross-sectional size of the carburized and quenched part of the gear shaft of the test workpiece was Da=915mm. The heat treatment method of the forging was:

[0061] Step 1: Carburize the gear shaft at elevated temperature and high temperature. The carburizing process uses nitrogen-based atmosphere carburizing, with nitrogen and methanol as carrier gas, acetone as enriched gas, carburizing temperature of 950°C, carbon potential of atmosphere during strong carburizing period of 1.1CP, carbon potential of atmosphere during diffusion period of 0.8CP, carburizing time of 150h at 950°C, and effective hardening layer depth of 5.7mm after "810°C oil quenching + 200°C tempering" of the furnace carburized sample, indicating that the carburized layer depth meets the requirements and can proceed to the subsequent steps.

[0062] Step 2: Cool down the gear shaft after carburizing and perform hydrogen expansion annealing. After the end of carburizing treatment, the temperature is lowered with the furnace, and after it drops to 680°C, it is kept warm for 16 hours. After the end of the warming, the furnace is cooled to below 300°C and then air-cooled.

[0063] Step 3: Carburize and hydrogenate the gear shaft for quenching and heating. After the carburizing and hydrogenate annealing of the gear shaft is completed, the gear shaft is quenched and heated to 400°C, and kept at 400°C for 6 hours, and then heated to 650°C at a rate of 5°C / min to make the gear shaft reach the temperature, and kept at this temperature for 10 hours. After the insulation is completed, it is heated to the quenching temperature of 820°C at a rate of 3°C / min;

[0064] Step 4: After quenching and heating, the gear shaft is quenched and heated. Keep the quenching temperature at 820°C for 10 hours, so that the gear shaft tooth area and the area 50-100mm below the tooth root reach the quenching temperature of 820°C, and the temperature of the central area of ​​the gear shaft (i.e., the area from 200mm below the tooth root to the gear shaft center) is 650-770°C. In this embodiment, the gear shaft axis position is 758°.

[0065] Step 5, quickly cool the gear shaft after quenching and heating: lift the quenched and heated gear shaft out of the furnace and quench and cool it in the oil tank for 130 minutes. The gear shaft tooth area and the area 50 to 100 mm below the tooth root are cooled to 100 to 200°C, while the center area of ​​the gear shaft is 200 to 350°C. In this embodiment, the tooth area is 128° and the gear shaft axis position is 337°.

[0066] Step 6: Perform the first low-temperature tempering treatment on the gear shaft after quenching and cooling. After the quenching of the gear shaft, it is promptly put into the furnace for low-temperature tempering treatment, heated to the tempering temperature of 220°C, and kept at this temperature for 28 hours. After the tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

[0067] Step 7, perform a second low-temperature tempering on the gear shaft forging after the first low-temperature tempering treatment: After the first low-temperature tempering of the gear shaft is completed, it is put into the furnace for the second tempering, heated to the tempering temperature of 220°C, and kept at this temperature for 23 hours. After the tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

[0068] The performance test of the 17Cr2Ni2Mo steel gear shaft forgings processed according to the above process showed that the surface Rockwell hardness was 58.3-58.9HRC, the gear core hardness was 40.5HRC, and the hardened layer depth Eht was 5.30mm-5.49mm. The stress simulation calculation test of the center area of ​​the gear shaft was carried out according to the above process method. The maximum stress was located inside the gear shaft, reaching 338, 179, and 170Mpa respectively.

[0069] Comparative Example 1

[0070] A factory conducted an engineering test according to the heat treatment method of the present invention. The material of the test forging was 17Cr2Ni2Mo. The cross-sectional size of the carburized and quenched part of the gear shaft of the test workpiece was Da=915mm. The heat treatment method of the forging was:

[0071] Step 1: Carburize the gear shaft by heating and high-temperature carburizing. The carburizing process uses nitrogen-based atmosphere carburizing, with nitrogen and methanol as carrier gas, acetone as enriched gas, carburizing temperature of 930°C, carbon potential of atmosphere in strong carburizing period of 1.08CP, carbon potential of atmosphere in diffusion period of 0.77CP, carburizing time of 140h at 930°C, and effective hardening layer depth of 5.5mm after "810°C oil quenching + 200°C tempering" of the furnace carburized sample, indicating that the carburized layer depth meets the requirements and can proceed to the subsequent steps.

[0072] Step 2: Carburize the gear shaft and cool it down. After the heat preservation is finished, the temperature of the gear shaft is cooled down with the furnace until it is cooled to below 300℃ and then air-cooled.

[0073] Step 3: Carburize the gear shaft and heat it for quenching. After carburizing and hydrogen annealing, quench the gear shaft and heat it to 400°C. Keep it at 400°C for 5 hours, then heat it to 630°C at a rate of 3°C / min, keep it at this temperature for 8 hours, heat the center area thoroughly, and heat it to 810°C at a rate of 2°C / min after the heat preservation.

[0074] Step 4: After quenching and heating, the gear shaft is quenched and heated. Keep the quenching temperature at 810°C for 16 hours to allow the gear shaft to be heated to the quenching temperature of 810°C.

[0075] Step 5, quickly cool the gear shaft after quenching and heating: lift the quenched and heated gear shaft out of the furnace and quench it in an oil tank for 160 minutes to make the overall temperature of the gear shaft less than 150°C.

[0076] Step 6: Perform the first low-temperature tempering treatment on the gear shaft after quenching and cooling. After the quenching of the gear shaft, it is promptly put into the furnace for low-temperature tempering treatment, heated to the tempering temperature of 220°C, and kept at this temperature for 26 hours. After the tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

[0077] Step 7, perform a second low-temperature tempering on the gear shaft after the first low-temperature tempering treatment: after the first low-temperature tempering of the forging is completed, put it into the furnace for the second tempering, heat it to the tempering temperature of 220°C, keep it at this temperature for 20 hours, and after the tempering is completed, take it out of the furnace and air cool it to room temperature.

[0078] The gear shaft processed according to the above process is subjected to surface Rockwell hardness (generally three points on the gear shaft are tested), gear core hardness, hardened layer depth, shaft surface temperature and core temperature, and stress distribution simulation calculation in the axial direction, circumferential direction 1, and circumferential direction 2 of the central area. Figure 6-Figure 8 , the results are shown in Table 1 below.

[0079] Table 1 Comparison 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-term medium temperature insulation + surface heating at quenching temperature + quenching cooling to control the final cooling temperature + two tempering" in Example 1 and the 17Cr2Ni2Mo steel gear shaft forging treated by the traditional process method of "carburizing + quenching heating ordinary step heating + conventional overall heat transfer at quenching temperature + quenching cooling to a lower temperature + two tempering" in Example 1 are shown in Table 1. After treatment by the process method of the present invention, the tooth area can achieve the surface hardening effect, while the quenching temperature of the central area is low, thereby effectively reducing the tensile stress generated by thermal stress in the central area; shortening the cooling time and increasing the oil outlet temperature can keep the temperature of the central area of ​​the gear shaft at about 200-350°C, thereby effectively reducing the thermal stress in the low temperature stage, and finally achieving a significant reduction in the stress in the central area after quenching.

[0082] The embodiments of this specific implementation method are all preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Therefore, any equivalent changes made based on the structure, shape, and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A quenching heat treatment method for a large carburized gear shaft forging, characterized in that: The following steps are included: S1. Carburizing treatment: Place the gear shaft in a heating furnace and heat it to 920-950℃. Carburizing treatment is performed after the gear shaft core reaches the temperature. During the carburizing treatment, the gear shaft is kept warm for ≥60h; S2. Hydrogen expansion annealing: After the carburizing treatment of the gear shaft is completed, the gear shaft is cooled down. After the temperature is cooled down to 630-680℃, the gear shaft is kept warm. During the warming process, hydrogen expansion annealing is performed. The warming time is ≥12h. After the warming is completed, the gear shaft is cooled in the heating furnace to below 300℃, and then the gear shaft is removed from the heating furnace and air-cooled to room temperature. S3, quenching heating: place the gear shaft in a heating furnace and heat it to 600-650°C at a rate of 2°C-5°C / min, so that the gear shaft center reaches the temperature, and keep it warm for 6-10h. After the insulation is completed, heat it to the quenching temperature of 800-820°C at a rate of 1-3°C / min; S4, quenching and heat preservation heating: keep the quenching temperature at 800-820℃ for 6-10h, so that the tooth area of ​​the gear shaft and the area 50-100mm below the tooth root 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 that has been quenched and heated is quenched and cooled in the oil tank after being taken out of the furnace. The tooth area and the area 50-100mm below the tooth root are cooled to 100-200℃, while the central area is 200-350℃; S6. First low-temperature tempering: After the quenching of the gear shaft is completed, it is promptly put into the furnace for low-temperature tempering treatment. The gear shaft is heated to the tempering temperature of 200-220°C. After the heat preservation time is ≥20h, it is 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, it is put into the furnace for the second tempering, heated to the tempering temperature of 200-220℃, and kept at this temperature for ≥16h before being taken out of the furnace and air-cooled to room temperature.

2. A quenching heat treatment method for a large carburized gear shaft forging according to claim 1, characterized in that: In step S1, the carburizing treatment adopts nitrogen-based atmosphere carburizing, the carrier gas is nitrogen and methanol, the enriched gas is acetone, the carburizing temperature is 920-950°C, the carbon potential of the atmosphere during the intensive 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. The quenching heat treatment method of a large carburized gear shaft forging according to claim 1, characterized in that: After step S2 and before step S3, the gear shaft is heated to 400°C and kept at 400°C for 4 to 6 hours. Nitrogen is introduced to replace the air in the furnace when the temperature is raised.

4. The quenching heat treatment method for a large carburized gear shaft forging according to claim 1, characterized in that: In step S4, nitrogen is introduced throughout the entire heating process, and methanol is introduced when the temperature is heated to 760°C, and acetone is introduced when the temperature is heated to 800°C. The carbon potential target value of the atmosphere is set at 0.75-0.8CP during the insulation stage.

5. The quenching heat treatment method for a large carburized gear shaft forging according to claim 1, characterized in that: In step S7, the hardness of the gear shaft tooth surface after the second low-temperature tempering meets the requirement of HRC58-62.

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