Bainite quenching method for bearing ring

Abstract

The invention relates to a bearing ring bainite quenching method, and belongs to the technical field of bearing heat treatment. The bainite quenching method for the bearing ring comprises the following steps that firstly, the bearing ring is subjected to quenching heating treatment at the temperature of 870-880 DEG C, then the bearing ring obtained after quenching heating treatment is placed in a salt bath to be subjected to quenching cooling treatment and isothermal treatment in sequence, and finally the bearing ring obtained after isothermal treatment is taken out of the salt bath to be subjected to tempering treatment. The temperature of the quenching cooling treatment is 205-235 DEG C, the temperature of the isothermal treatment is 230-245 DEG C, and the temperature of the tempering treatment is 230-240 DEG C. According to the bainite quenching method for the bearing ring, the content of retained austenite in the bearing ring can be effectively reduced, the grain size is refined, and the hardness of the bearing ring is improved.

Description

Technical Field

[0001] The invention relates to a bearing ring bainite quenching method and belongs to the technical field of bearing heat treatment. Background Art

[0002] With the development of the global wind power industry, wind power bearings have also developed rapidly. Wind power bearings have high requirements for the hardness, strength and toughness of materials. Therefore, in order to improve the performance of wind power bearings, they need to be heat treated. Heat treatment is a thermal processing process that heats metal or alloy to a certain temperature in the solid state, keeps the temperature for a certain period of time, and then cools it at different cooling rates to make it undergo a phase transformation of the crystal (metal) structure and form a variety of organizational structures, thereby obtaining the required performance.

[0003] For example, Chinese patent document CN108504846A discloses an environmentally friendly heat treatment process for a wind turbine bearing ring assembly. The heat treatment method disclosed in the patent document includes the following steps: heating the bearing ring assembly in a furnace to 600-750°C, keeping the temperature for 1.5-3 hours, taking out the bearing ring assembly for oil quenching treatment, the oil temperature is 220-300°C, keeping the temperature for 1-1.5 hours, and then cooling the oil to room temperature; then treating the quenched bearing ring assembly at a temperature of 450-550°C for 2 hours. After 0-30 minutes, cool to room temperature, then freeze the bearing ring assembly, and then place the bearing ring assembly in a tempering furnace for tempering, raise the temperature in the furnace to 100-150°C, keep it warm for 20-50 minutes, then raise the temperature in the furnace to 160-180°C, keep it warm for 20-30 minutes, then lower the temperature in the furnace to 100-120°C, keep it warm for 2-5 hours, and repeat the tempering treatment 2-4 times; after tempering, take out the bearing ring assembly from the tempering furnace and cool it naturally to room temperature. The heat treatment method disclosed in the patent document can make the bearing ring assembly have higher surface strength, significantly enhance the surface wear resistance of the bearing ring assembly, and improve the toughness of the material, but the heat treatment method has the problems of complex operation, high cost, unsuitable for large-scale application, and inability to effectively reduce the content of residual austenite in the ring. Summary of the invention

[0004] The purpose of the present invention is to provide a bearing ring bainite quenching method, which can solve the problem that the current wind power bearing ring heat treatment method cannot take into account the hardness of the core of the bearing ring and the residual austenite content.

[0005] In order to achieve the above-mentioned object, the technical solution adopted by the bearing ring bainite quenching method of the present invention is as follows: A method for quenching bainite of a bearing ring comprises the following steps: firstly subjecting the bearing ring to quenching heating treatment at 870-880°C, then placing the bearing ring after quenching heating treatment in a salt bath for quenching cooling treatment and isothermal treatment in sequence, and finally taking the bearing ring after isothermal treatment out of the salt bath for tempering treatment; the temperature of quenching cooling treatment is 205-235°C, the temperature of isothermal treatment is 230-245°C, and the temperature of tempering treatment is 230-240°C.

[0006] The bearing ring bainite quenching method of the present invention first performs a quenching heating treatment on the bearing ring, then performs a quenching cooling treatment and an isothermal treatment in a salt bath in sequence, and finally performs a tempering treatment, which can effectively reduce the residual austenite content in the ring, refine the grain size, and improve the hardness of the bearing ring.

[0007] The performance indicators of the wind power bearing ring obtained by the bearing ring bainite quenching method of the present invention can reach: surface hardness of 59-61HRC, core hardness of 57.5-60HRC, bainite structure <1 level, grain size of 9 level, and residual austenite content ≤5%.

[0008] Preferably, the carbon potential of the quenching and heating treatment is 0.6-1.0%.

[0009] Preferably, the total time of the isothermal treatment and the tempering treatment is 14.5 to 18.5 hours.

[0010] Preferably, the total time of the isothermal treatment and the tempering treatment is 15 to 18 hours.

[0011] Preferably, the salt bath consists of potassium nitrate and sodium nitrite in a mass ratio of 1:(0.9-1.05).

[0012] Preferably, the outer diameter of the bearing ring is 736~2056mm, the height is 41~438mm, the effective wall thickness is 16.9~78.7mm, the temperature of the quenching and heating treatment is 872~875℃, and the time is 18~390min; the temperature of the quenching and cooling treatment is 210~235℃, and the time is 7~50min; the temperature of the isothermal treatment is 232~235℃, and the time is 5~12h; the temperature of the tempering treatment is 232~235℃, and the time is 6~10h; the carbon potential of the quenching and heating treatment is 0.65~0.85%.

[0013] The beneficial effects of the present invention are as follows: The bearing ring bainite quenching method of the present invention first performs quenching and heating treatment on the bearing ring, then performs quenching and cooling treatment and isothermal treatment in a salt bath in sequence, and finally performs tempering treatment, which can effectively reduce the residual austenite content in the ring, refine the grain size, and improve the hardness of the bearing ring. The performance indicators of the wind power bearing ring obtained by the bearing ring bainite quenching method of the present invention can reach: surface hardness of 59~61HRC, core hardness of 57.5~60HRC, bainite structure <1 level, grain size of 9 level, and residual austenite content ≤5%. DETAILED DESCRIPTION

[0014] The bearing ring bainite quenching method of the present invention is a pioneering invention. The bearing ring bainite quenching method of the present invention comprises the following steps: firstly, the bearing ring is subjected to quenching and heating treatment at 870-880°C, then the bearing ring after quenching and heating treatment is placed in a salt bath for quenching and cooling treatment and isothermal treatment in sequence, and finally, the bearing ring after isothermal treatment is taken out from the salt bath for tempering treatment; the quenching and cooling treatment temperature is 205-235°C, the isothermal treatment temperature is 230-245°C, and the tempering treatment temperature is 230-240°C.

[0015] The bearing ring bainite quenching method of the present invention first performs a quenching heating treatment on the bearing ring, then performs a quenching cooling treatment and an isothermal treatment in a salt bath in sequence, and finally performs a tempering treatment, which can effectively reduce the residual austenite content in the ring, refine the grain size, and improve the hardness of the bearing ring.

[0016] The performance indicators of the wind power bearing ring obtained by the bearing ring bainite quenching method of the present invention are as follows: surface hardness is 59-61HRC, core hardness is 57.5-60.5HRC, bainite structure is less than level 1, grain size is level 9, and residual austenite content is ≤5%.

[0017] In some preferred embodiments, the carbon potential of the quenching heating treatment is 0.6-1.0%.

[0018] In some preferred embodiments, the quenching and heating treatment is carried out in a pit furnace with a carbon potential of 0.75-1.0%; or the quenching and heating treatment is carried out on a continuous furnace production line with a carbon potential of 0.6-0.8%.

[0019] In some preferred embodiments, the temperature of the isothermal treatment is not less than the temperature of the quenching cooling treatment.

[0020] In some preferred embodiments, the temperature of the isothermal treatment is equal to the temperature of the tempering treatment.

[0021] In some preferred embodiments, the total time of the isothermal treatment and the tempering treatment is 14.5 to 18.5 hours.

[0022] In some preferred embodiments, the total time of the isothermal treatment and the tempering treatment is 15 to 18 hours.

[0023] In the present invention, the time for quenching heating treatment, quenching cooling treatment, isothermal treatment and tempering treatment can be determined according to the size of the bearing ring and the actual charging amount. In some preferred embodiments, the outer diameter of the bearing ring is 736~2056mm, the height is 41~438mm, the effective wall thickness is 16.9~78.7mm, the temperature of quenching heating treatment is 875℃, and the time is 18~390min; the temperature of quenching cooling treatment is 210~235℃, and the time is 7~50min; the temperature of isothermal treatment is 235℃, and the time is 5~12h; the temperature of tempering treatment is 235℃, and the time is 6~10h.

[0024] The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0025] Example 1 The bearing ring bainite quenching method of this embodiment specifically includes the following steps: (1) The bearing ring is placed in a pit quenching furnace for quenching and heating treatment. The quenching and heating treatment temperature is 875°C and the time is 390 minutes. During the quenching and heating treatment, carburizing gas is introduced into the pit quenching furnace (the carburizing gas is composed of nitrogen, natural gas and methanol, and the flow rate of nitrogen is 6~7m 3 / h, the flow rate of methanol is 9~10L / h, and the flow rate of natural gas is ≤6m 3 / h, methanol is used as a carrier in the carburizing process to play a dilution role, which can exclude the air in the furnace at the initial stage of carburizing to maintain a positive pressure in the furnace and stabilize the carbon potential), the carbon potential in the pit quenching furnace is 0.8%; the bearing ring in this embodiment is a certain large wind power bearing ring, with an outer diameter of 2056mm, a height of 185mm, an effective wall thickness (the effective wall thickness is specified in Appendix A of GB / T34891-2017, which means the wall thickness at H / 3 from the large end face of the ring) of 78.7mm, and the material is 100CrMnMoSi8-4-6; (2) Transfer the bearing ring after quenching and heating treatment from the pit quenching furnace to a salt bath in a salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1) for quenching and cooling treatment. The quenching and cooling treatment temperature is 235°C and the time is 30 minutes. (3) After quenching and cooling treatment, the bearing ring is further subjected to isothermal treatment (i.e., heat preservation treatment) in a salt bath in a salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1). The isothermal treatment temperature is 235°C and the time is 5 hours. (4) The bearing ring after isothermal treatment is taken out from the salt tank, and then immediately transferred to the pit-type tempering furnace for tempering treatment to ensure that the ring temperature is maintained within the lower bainite phase transformation temperature range (when the bearing ring is transferred into the pit-type tempering furnace, the temperature of the bearing ring measured by the infrared thermometer is 232°C). The tempering temperature is 235°C and the time is 10 hours. During the tempering treatment, a salt receiving tray is installed on the loading rack to prevent the salt liquid attached to the loading rack and the ring from being lost and damaging the furnace lining. At the same time, the salt on the salt receiving tray can be collected and reused to reduce the pollution caused by emissions to the environment.

[0026] The performance index requirements of the large wind turbine bearing ring of this embodiment are as follows: surface hardness is 58-62HRC, bainite structure B≤1 level, grain size reaches 8 level or finer, core hardness ≥55HRC, and retained austenite content ≤5%.

[0027] Example 2 The bearing ring bainite quenching method of this embodiment specifically includes the following steps: (1) The bearing ring is placed in a pit quenching furnace for quenching and heating treatment. The quenching and heating treatment temperature is 875°C and the time is 360 minutes. During the quenching and heating treatment, carburizing gas is introduced into the pit quenching furnace (the carburizing gas is composed of nitrogen, natural gas and methanol, and the flow rate of nitrogen is 6~7m 3 / h, the flow rate of methanol is 9~10L / h, and the flow rate of natural gas is ≤6m 3 / h, methanol is used as a carrier in the carburizing process to play a diluting role, which can exclude the air in the furnace at the initial stage of carburizing to maintain a positive pressure in the furnace and stabilize the carbon potential), the carbon potential in the pit quenching furnace is 0.85%; the bearing ring in this embodiment is a certain large wind power bearing ring, with an outer diameter of 1500mm, a height of 438mm, an effective wall thickness of 45.5mm, and a material of 100CrMnMoSi8-4-6; (2) Transfer the bearing ring after quenching and heating treatment from the pit quenching furnace to a salt bath in a salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1) for quenching and cooling treatment. The quenching and cooling treatment temperature is 230°C and the time is 20 minutes; (3) After quenching and cooling treatment, the bearing ring is further subjected to isothermal treatment (i.e., heat preservation treatment) in a salt bath in a salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1). The isothermal treatment temperature is 235°C and the time is 5 hours. (4) The bearing ring after isothermal treatment is taken out from the salt tank, and then immediately transferred to the pit-type tempering furnace for tempering treatment to ensure that the ring temperature is maintained within the lower bainite phase transformation temperature range (when the bearing ring is transferred into the pit-type tempering furnace, the temperature of the bearing ring measured by the infrared thermometer is 235°C). The tempering temperature is 235°C and the time is 10 hours. During the tempering treatment, a salt receiving tray is installed on the loading rack to prevent the salt liquid attached to the loading rack and the ring from being lost and damaging the furnace lining. At the same time, the salt on the salt receiving tray can be collected and reused to reduce the pollution caused by emissions to the environment.

[0028] The performance index requirements of the large wind turbine bearing ring of this embodiment are as follows: surface hardness is 58-62HRC, bainite structure B≤1 level, grain size reaches 8 level or finer, core hardness ≥55HRC, and retained austenite content ≤5%.

[0029] Example 3 The bearing ring bainite quenching method of this embodiment specifically includes the following steps: (1) The bearing ring is placed in a continuous quenching furnace for quenching and heating treatment. The quenching and heating treatment temperature is 875°C, the feeding cycle is 55 minutes, and the quenching and heating treatment time is 275 minutes. During the quenching and heating treatment, carburizing gas is introduced into the continuous quenching furnace (the carburizing gas is composed of nitrogen, natural gas and methanol, and the flow rate of nitrogen is 36~38m 3 / h, the flow rate of methanol is 3~4L / h, and the flow rate of natural gas is ≤3m 3 / h, methanol is used as a carrier in the carburizing process to play a diluting role, which can exclude the air in the furnace at the initial stage of carburizing to maintain a positive pressure in the furnace and stabilize the carbon potential), the carbon potential in the pit quenching furnace is 0.65%; the bearing ring in this embodiment is a certain large wind power bearing ring, with an outer diameter of 1143mm, a height of 411.6mm, an effective wall thickness of 59.3mm, and a material of 100CrMnMoSi8-4-6; (2) After the quenching and heating treatment, the bearing ring is removed from the continuous quenching furnace and transferred to the salt bath in the quenching and cooling salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1) for quenching and cooling treatment. The quenching and cooling treatment temperature is 210°C and the time is 50 minutes; (3) After quenching and cooling treatment, the bearing ring is transferred to a salt bath in an isothermal treatment salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1) for isothermal treatment (i.e., heat preservation treatment). The isothermal treatment temperature is 235°C and the time is 12 hours; (4) The bearing ring after isothermal treatment is taken out from the isothermal treatment salt tank, and then immediately transferred to the pit-type tempering furnace for tempering treatment to ensure that the ring temperature is maintained within the lower bainite phase transformation temperature range (when the bearing ring is transferred into the pit-type tempering furnace, the temperature of the bearing ring measured by the infrared thermometer is 232°C). The tempering temperature is 235°C and the time is 6 hours. During the tempering treatment, a salt receiving tray is installed on the loading rack to prevent the salt liquid attached to the loading rack and the ring from being lost and damaging the furnace lining. At the same time, the salt on the salt receiving tray can be collected and reused to reduce the pollution caused by emissions to the environment.

[0030] The performance index requirements of the large wind turbine bearing ring of this embodiment are as follows: surface hardness is 58-62HRC, bainite structure B≤1 level, grain size reaches 8 level or finer, core hardness ≥55HRC, and retained austenite content ≤5%.

[0031] Example 4 The bearing ring bainite quenching method of this embodiment specifically includes the following steps: (1) The bearing ring is placed in a continuous quenching furnace for quenching and heating treatment. The quenching and heating treatment temperature is 875°C, the feeding cycle is 18 minutes, and the quenching and heating treatment time is 90 minutes. During the quenching and heating treatment, carburizing gas is introduced into the continuous quenching furnace (the carburizing gas is composed of nitrogen, natural gas and methanol, and the flow rate of nitrogen is 36~38m 3 / h, the flow rate of methanol is 3~4L / h, and the flow rate of natural gas is ≤3m 3 / h, methanol is used as a carrier in the carburizing process, plays a diluting role, can exclude the air in the furnace at the initial stage of carburizing to maintain a positive pressure in the furnace, and stabilize the carbon potential), the carbon potential in the pit quenching furnace is 0.75%; the bearing ring in this embodiment is a certain large wind power bearing ring, with an outer diameter of 736.6mm, a height of 41.275mm, an effective wall thickness of 16.9mm, and a material of 100CrMnMoSi8-4-6; (2) After quenching and heating treatment, the bearing ring is removed from the continuous quenching furnace and transferred to a salt bath in a quenching and cooling salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1) for quenching and cooling treatment. The quenching and cooling treatment temperature is 210°C and the time is 7 minutes. (3) After quenching and cooling treatment, the bearing ring is transferred to a salt bath in an isothermal treatment salt tank (the salt bath is composed of potassium nitrate and sodium nitrite in a mass ratio of 1:1) for isothermal treatment (i.e., heat preservation treatment). The isothermal treatment temperature is 235°C and the time is 12 hours; (4) The bearing ring after isothermal treatment is taken out from the isothermal treatment salt tank, and then immediately transferred to the pit-type tempering furnace for tempering treatment to ensure that the ring temperature is maintained within the lower bainite phase transformation temperature range (when the bearing ring is transferred into the pit-type tempering furnace, the temperature of the bearing ring measured by the infrared thermometer is 232°C). The tempering temperature is 235°C and the time is 6 hours. During the tempering treatment, a salt receiving tray is installed on the loading rack to prevent the salt liquid attached to the loading rack and the ring from being lost and damaging the furnace lining. At the same time, the salt on the salt receiving tray can be collected and reused to reduce the pollution caused by emissions to the environment.

[0032] The performance index requirements of the large wind turbine bearing ring of this embodiment are as follows: surface hardness is 58-62HRC, bainite structure B≤1 level, grain size reaches 8 level or finer, core hardness ≥55HRC, and retained austenite content ≤5%.

[0033] Example 5 The difference between the bearing ring bainite quenching method of the present embodiment and the bearing ring bainite quenching method of embodiment 2 is that the salt bath used in step (2) and step (3) of the bearing ring bainite quenching method of the present embodiment consists of potassium nitrate and sodium nitrite in a mass ratio of 1:0.9, and the temperature of the isothermal treatment in step (3) is 245°C.

[0034] Comparative Example 1 The difference between the bearing ring bainite quenching method of this comparative example and the bearing ring bainite quenching method of Example 2 is that the temperature of the isothermal treatment in step (3) and the temperature of the tempering treatment in step (4) of the bearing ring bainite quenching method of this comparative example are both 215°C.

[0035] Comparative Example 2 The difference between the bearing ring bainite quenching method of this comparative example and the bearing ring bainite quenching method of Example 2 is that the tempering treatment time in step (4) of the bearing ring bainite quenching method of this comparative example is 0, that is, step (4) is omitted.

[0036] Comparative Example 3 The difference between the bearing ring bainite quenching method of this comparative example and the bearing ring bainite quenching method of Example 2 is that the temperature of the quenching cooling treatment in step (2) and the temperature of the isothermal treatment in step (3) of the bearing ring bainite quenching method of this comparative example are both 239°C.

[0037] Comparative Example 4 The difference between the bearing ring bainite quenching method of this comparative example and the bearing ring bainite quenching method of Example 2 is that the time for quenching cooling treatment in step (2) of the bearing ring bainite quenching method of this comparative example is 0, that is, step (2) is omitted, and the bearing ring after quenching heating treatment is directly transferred from the pit quenching furnace to the salt bath in the salt tank for isothermal treatment.

[0038] Comparative Example 5 The difference between the bearing ring bainite quenching method of this comparative example and the bearing ring bainite quenching method of Example 2 is that the salt bath used in step (2) and step (3) of the bearing ring bainite quenching method of this comparative example consists of potassium nitrate and sodium nitrite in a mass ratio of 1:1.1, and the temperature of the isothermal treatment in step (3) is 225°C.

[0039] Experimental example In order to evaluate the treatment effect of each embodiment and comparative example on the bearing ring, the surface hardness, bainite structure grade, grain size grade, core hardness and retained austenite content of the bearing ring obtained by the treatment of each embodiment and comparative example were tested respectively. The results are shown in Table 1.

[0040] Table 1 Surface hardness, bainite structure grade, grain size grade, core hardness and retained austenite content of the bearing rings obtained from the treatment of each embodiment and comparative example Quenching method Surface hardness (HRC) Core hardness (HRC) Bainite grade Grain size grade Retained austenite content (%) Example 1 59~60 57.5~58.5 <Level 1 Level 9 3.07 Example 2 59~60 58.2~59.0 <Level 1 Level 9 2.61 Example 3 59~60 58.1~58.6 <Level 1 Level 9 4.68 Example 4 59~61 59.0~60.0 <Level 1 Level 9 4.81 Example 5 59~60 57.8~58.6 <Level 1 Level 9 3.29 Comparative Example 1 58~59 57.5~58.0 <Level 1 Level 9 10.16 Comparative Example 2 58~59 57.5~58.0 <Level 1 Level 9 8.91 Comparative Example 3 59~60 56.1~57.2 <Level 1 Level 9 4.58 Comparative Example 4 59~60 54.3~55.1 <Level 1 Level 9 3.79 Comparative Example 5 59~60 58.1~58.9 <Level 1 Level 9 7.75 As shown in Table 1, the surface hardness, bainite structure grade, grain size grade, core hardness and retained austenite content of the bearing rings processed by Examples 1-5 of the present invention all meet the requirements. The retained austenite content of the bearing rings processed by Comparative Examples 1-2 and 5 is too high and does not meet the requirements. Although the bearing rings processed by Comparative Examples 3-4 have a lower retained austenite content, the lower limit of the hardness is too small and does not meet the use requirements.

[0041] In order to investigate the influence of the salt bath composition on the experimental results, the experiment was repeated according to the method of Example 2, except that the mass ratio of potassium nitrate to sodium nitrite in the salt bath used in step (2) and step (3) was replaced with 1:0.95 or 1:1.05, or the temperature of the quenching heat treatment was adjusted from 875°C to 872°C, or the temperature of the isothermal treatment was adjusted from 235°C to 232°C, or the temperature of the tempering treatment was adjusted from 235°C to 232°C, and then the surface hardness, bainite structure grade, grain size grade, core hardness and retained austenite content of the bearing ring processed after the parameters were adjusted were tested according to the method of the experimental example. The results showed that the performance of the bearing ring processed after the parameters were adjusted was close to that of the bearing ring processed in Example 2.

Claims

1. A method for quenching bainite of a bearing ring, characterized in that: The following steps are involved: First, the bearing ring is subjected to quenching and heating treatment at 870-880°C, and then the quenched and heated bearing ring is placed in a salt bath for quenching and cooling treatment and isothermal treatment in sequence, and finally the isothermal treated bearing ring is taken out of the salt bath for tempering treatment; the quenching and cooling treatment temperature is 205-235°C, the isothermal treatment temperature is 230-245°C, and the tempering treatment temperature is 230-240°C.

2. The method for quenching bainite of a bearing ring according to claim 1, characterized in that: The carbon potential of the quenching and heating treatment is 0.6-1.0%.

3. The method for quenching bainite of a bearing ring according to claim 1 or 2, characterized in that: The total time of the isothermal treatment and the tempering treatment is 14.5 to 18.5 hours.

4. The method for quenching bainite of a bearing ring according to claim 3, characterized in that: The total time of the isothermal treatment and the tempering treatment is 15 to 18 hours.

5. The method for quenching bainite of a bearing ring according to claim 1 or 2, characterized in that: The salt bath consists of potassium nitrate and sodium nitrite in a mass ratio of 1:(0.9~1.05).

6. The method for quenching bainite of a bearing ring according to claim 1 or 2, characterized in that: The outer diameter of the bearing ring is 736~2056mm, the height is 41~438mm, the effective wall thickness is 16.9~78.7mm, the temperature of the quenching and heating treatment is 872~875℃, and the time is 18~390min; the temperature of the quenching and cooling treatment is 210~235℃, and the time is 7~50min; the temperature of the isothermal treatment is 232~235℃, and the time is 5~12h; the temperature of the tempering treatment is 232~235℃, and the time is 6~10h; the carbon potential of the quenching and heating treatment is 0.65~0.85%.