All-metal stator internal spiral cavity surface hardening method and obtained stator

Abstract

The invention belongs to the field of oil-gas exploration and development and drilling engineering technical equipment, and particularly relates to a surface hardening method for an internal spiral cavity of an all-metal stator and the obtained hardened all-metal stator. The pre-oxidation catalysis treatment is added, so that the process period is shortened, and energy conservation and consumption reduction are realized; the nitriding atmosphere pressure in the nitriding furnace is properly improved, so that the nitriding hardness of different parts of the internal spiral surface of the all-metal stator and the depth uniformity of a hardened layer are improved; according to the technology, the low-temperature and ultrahigh-ammonia-decomposition-rate nitrogen removal technology process is adopted finally, the surface structure is adjusted, the brittle compound phase of a nitriding layer is reduced, the brittleness of the nitriding layer is reduced, and therefore the abrasion resistance of the all-metal motor is improved; the process atmosphere guiding device is additionally arranged, the nitriding hardness and the layer depth uniformity of the surface of the inner spiral cavity are improved, local abnormal rapid abrasion is overcome, and then the overall service life of the all-metal motor is prolonged.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas exploration, development, and drilling engineering technical equipment, and specifically relates to a method for hardening the surface of a spiral cavity inside an all-metal stator and the obtained hardened all-metal stator. Background Art

[0002] The normal temperature stator shell is made of rolled long cylindrical shell steel that has been quenched and tempered, and then a spiral-shaped cavity rubber is injected into the inner cavity of the shell.

[0003] In the prior art, such as Figure 1 As shown, 1-3 stator metal shell and 1-2 stator rubber cavity are bonded together to form a stator. Stator injection molding process: 1-3 metal stator shell is coated with adhesive, and the glue injection machine injects hydrogenated nitrile and other rubber into 1-3 metal stator shell by injection, and the rubber is bonded and cured on 1-3 metal stator shell to form 1-2 stator rubber cavity.

[0004] The screw drilling tool motor is a positive displacement motor, which consists of a stator and a rotor. The function of the 1-2 stator rubber cavity is that the rubber spiral surface cavity and the spiral surface of the rotor mesh with each other to form a spatial conjugate surface, thereby forming multiple closed sealed cavities that can accommodate the flow of high-pressure drilling fluid. The high-pressure cavity and the low-pressure cavity are constantly and dynamically converted, which is conducive to the hydrostatic pressure of the drilling fluid to drive the rotor to perform planetary motion in the stator, generating speed and torque.

[0005] 1-3 The metal stator shell needs to be tempered. The conventional tempering process is as follows:

[0006] Description of the quenching and tempering process of the stator metal shell 1-3 blank:

[0007] High-strength stator metal shell 1-3 containing chromium, nickel, molybdenum and other alloys Figure 2 The high temperature heating furnace shown is heated to 860℃±5℃ through the process (2-1), and the internal structure of the stator metal shell 1-3 is fully austenitized after the process (2-2) of 860℃±5℃ insulation time for 3 hours. Then, it is quickly transferred to the rapid quenching oil or water-based quenching medium for rapid cooling process (2-3), so that the stator metal shell 1-3 is quickly cooled to room temperature, so that the internal structure of the steel is transformed from austenite structure to martensite structure to the greatest extent. Thereafter, it is transferred to the tempering furnace within a maximum of 2 hours, heated to 580℃±5℃ through the process (2-4), and after the insulation process (2-5) of 580℃±5℃ insulation time for 3.5 hours, the martensite structure is transformed into the troostite structure as completely as possible, and then it is quickly transferred to the cooling water tank with a water temperature below 30℃ for process (2-6) to cool to room temperature, so as to reduce the segregation of impurities in the alloy at the grain boundary and better improve the impact toughness and strength of the stator metal shell 1-3.

[0008] However, when the stator of the screw drill bit injected with hydrogenated nitrile or other rubber is placed in a temperature environment exceeding 200°C, the adhesive bonding the stator rubber will quickly fail. At the same time, the stator rubber will quickly age and fail, causing the glue to fall off and debond, causing the entire drill bit to fail quickly. Therefore, conventional motors cannot adapt to high-temperature downhole environments exceeding 200°C.

[0009] In order to adapt to the high temperature underground environment with a temperature exceeding 200℃, it is necessary to develop materials that can replace hydrogenated nitrile and other rubbers as screw drill bushings. However, there is currently no similar material that can replace hydrogenated nitrile and other rubbers as screw drill bushings. Figure 3 As shown, the rubber cavity of 1-2 is eliminated and a stator formed entirely of metal is directly used, which is equivalent to combining the metal shell of the stator 1-3 and the rubber cavity of the stator 1-2 into a 3-1 all-metal stator, which together with the metal rotor of 1-1 form an all-metal motor.

[0010] Since conventional rules such as Figure 1 As shown, the hydrogenated nitrile rubber bushing 1-2 stator rubber cavity and the 1-1 metal rotor work together, and the 1-1 metal rotor rotates in the hydrogenated nitrile rubber bushing 1-2 stator rubber cavity. The hydrogenated nitrile rubber bushing is an elastomer and wears slowly. Figure 3 As shown, the all-metal screw drill motor is a 1-1 metal rotor directly matched with a 1-3 all-metal stator, which is a mutual meshing movement between metals. In order to enhance the inner spiral surface of the all-metal stator (such as Figure 4 As shown in the figure, the wear resistance and corrosion resistance of the 1-3 all-metal stator are improved, and the wear of the 1-3 all-metal stator is reduced. The 1-3 all-metal stator needs to harden the surface of the inner spiral cavity. The surface treatment process on the market is currently limited by many processing techniques, and it is difficult to harden the surface of the inner spiral cavity of the 1-3 all-metal stator (such as Figure 4 As shown in the figure, the internal surface wear-resistant treatment process of the all-metal stator is selected as the all-metal screw drill that can withstand 300℃ after repeated demonstrations. Finally, the internal spiral surface of the 3-1 all-metal stator is selected for gas nitriding treatment.

[0011] Compared with the conventional gas nitriding process of other similar steel products, Figure 5 :

[0012] Conventional gas nitriding process description:

[0013] The workpiece is degreased, derusted and cleaned before entering the furnace;

[0014] The workpiece is loaded vertically into the furnace and heated up along with the furnace (5-1);

[0015] According to the size of the workpiece, uniform heating is performed for a certain period of time (5-2). In this embodiment, uniform heating is performed for 2.5 hours;

[0016] Control the heating rate to less than 50°C / h and heat to the nitriding temperature of 540°C (5-3);

[0017] A certain flow rate of ammonia decomposition gas with a decomposition rate of 25% is introduced to carry out strong nitriding for 25 hours (5-4);

[0018] At a nitriding process temperature of 540°C, ammonia decomposition gas with a decomposition rate of 60% is introduced at a properly reduced flow rate for 35 hours of diffusion nitriding (5-5);

[0019] Stop the gas and heating, and let the workpiece cool down with the furnace until the temperature drops below 150℃ before being air-cooled (5-6).

[0020] Disadvantages of conventional gas nitriding process:

[0021] The process takes a long time, consumes a lot of energy, and the workpiece is deformed greatly. The deformation is difficult to correct, making it difficult to assemble the all-metal motor. Even if it is assembled, it will increase the abnormal rapid wear of the local area. At the same time, the size of the 1-1 metal rotor and the 1-3 all-metal stator is out of tolerance, causing drilling fluid leakage. It is difficult for the all-metal motor to reach the designed speed and torque output;

[0022] Inner spiral cavity surface ( Figure 4 ) The nitriding hardness is uneven. The nitriding surface hardness of the two ends is about 580-620HV (material 4145H steel, 40CrNiMoA steel), and the hardened layer depth is 0.60mm, while the nitriding surface hardness of the middle part is 570HV (material 4145H steel, 40CrNiMoA steel), and the hardened layer depth is 0.43mm. That is, the closer to the middle of the stator, the more uneven the nitriding of the inner spiral surface, the lower the hardness, and the lower the hardened layer depth. The nitriding layer is loose at level 2, the nitride level in the nitriding layer is level 2, and the brittleness of the nitriding layer surface is level 2. This result will reduce the overall wear resistance;

[0023] The nitrided surface is relatively brittle, which accelerates wear and shortens the service life of the workpiece;

[0024] If the process time is too long, the matrix structure of the stator metal shell 1-3 will become coarser and the matrix strength of the stator metal shell 1-3 will be reduced. Summary of the invention

[0025] In order to solve the problems in the prior art that the conventional nitriding process has the advantages of long process time, high energy consumption, large deformation of the workpiece, and difficulty in correcting the deformation, which makes it difficult to assemble the all-metal motor, the all-metal stator size is out of tolerance, causing drilling fluid leakage, and the all-metal motor is difficult to reach the designed speed and torque output, increasing local abnormal rapid wear; the workpiece has a short service life; the matrix strength is low and is not suitable for the all-metal stator, and the existing surface hardening process cannot meet the use requirements of the all-metal screw drill, the present invention provides a method for hardening the surface of the spiral cavity inside the all-metal stator and the obtained hardened all-metal stator.

[0026] The technical solution provided by the present invention is as follows:

[0027] A method for hardening the surface of a spiral cavity in an all-metal stator, comprising at least the following steps:

[0028] 1) The workpiece is derusted, cleaned and degreased before entering the furnace;

[0029] 2) The workpiece is hoisted vertically into the furnace and heated as the furnace heats up;

[0030] 3) After the furnace is heated up, uniform heating is performed;

[0031] 4) Control the heating rate and continue heating to the pre-oxidation temperature;

[0032] 5) pre-oxidation at a constant temperature at a pre-oxidation temperature;

[0033] 6) After the pre-oxidation is completed, control the heating rate and continue heating to the strong nitriding temperature;

[0034] 7) Performing strong nitriding at strong nitriding temperature and under the conditions of ammonia decomposition gas with low ammonia decomposition rate;

[0035] 8) Increase the nitriding temperature after the strong nitriding is completed;

[0036] 9) Diffusion nitriding is carried out under ammonia decomposition gas with a medium ammonia decomposition rate;

[0037] 10) Cooling down after diffusion nitriding;

[0038] 11) Carry out nitriding surface structure adjustment and denitrification treatment under ammonia decomposition gas with high ammonia decomposition rate;

[0039] 12) Stop the gas and heating, and let the workpiece cool down with the furnace before being air-cooled.

[0040] In the above technical solution:

[0041] Adding pre-oxidation catalytic treatment can shorten the process cycle, save energy and reduce consumption;

[0042] Finally, the process uses a low-temperature, ultra-high ammonia decomposition rate surface denitrification process to adjust the surface nitride structure, reduce the brittle compound phase and white bright layer ε phase of the nitriding layer surface by about 50%, reduce the brittleness of the nitriding layer, and improve the erosion resistance of the workpiece surface, thereby improving the wear resistance of the all-metal motor.

[0043] Specifically, the material of the workpiece is 4145H steel, 40CrNiMoA steel or 38CrMoAl steel, preferably 4145H steel.

[0044] Specifically, in step 3), the absorptive heating temperature is 190-210° C., preferably 200° C.; the absorptive heating time is 1-3 h, which can be determined according to the effective wall thickness of the workpiece, preferably 2 h.

[0045] Specifically, in step 4), the heating rate is less than 60°C / h; the pre-oxidation temperature is 290-310°C, preferably 300°C;

[0046] Specifically, in step 5), the pre-oxidation time is 1-2 hours, preferably 1 hour. If the time is too short, the penetration-enhancing effect is not obvious, and after 1 hour, the penetration-enhancing effect is significantly weakened, and after more than 2 hours, the penetration-enhancing effect no longer increases.

[0047] Specifically, in step 6), the heating rate is <50°C / h; the strong nitriding temperature is 470-490°C, preferably 480°C;

[0048] The above technical solution appropriately increases the nitriding atmosphere pressure in the nitriding furnace, which can improve the uniformity of the nitriding hardness and hardened layer depth of different parts of the inner spiral surface of the all-metal stator.

[0049] Specifically, in step 7), the ammonia decomposition rate is 18-22%, preferably 20%; the nitriding atmosphere pressure in the furnace is 5500-6500Pa, preferably 6000Pa; and the strong nitriding time is 16-20h, preferably 18h.

[0050] The above technical solution appropriately increases the nitriding atmosphere pressure in the nitriding furnace, which can improve the uniformity of the nitriding hardness and hardened layer depth of different parts of the inner spiral surface of the all-metal stator.

[0051] Specifically, in step 8), the temperature is increased to 510-530° C., preferably 520° C.;

[0052] Specifically, in step 9), the ammonia decomposition rate is 55-65%, preferably 60%; the nitriding atmosphere pressure in the furnace is 4500-5500Pa, preferably 5000Pa; and the strong nitriding time is 18-22h, preferably 20h.

[0053] The above technical solution appropriately increases the nitriding atmosphere pressure in the nitriding furnace, which can improve the uniformity of the nitriding hardness and hardened layer depth of different parts of the inner spiral surface of the all-metal stator.

[0054] Specifically, in step 10), the temperature is lowered to 490-500°C, preferably 495°C.

[0055] Specifically, in step 11), the ammonia decomposition rate is 85-95%, preferably 90%; the nitriding atmosphere pressure in the furnace is 750-850Pa, preferably 800Pa; and the strong nitriding time is 1-3h, preferably 2h.

[0056] Furthermore, the furnace body is provided with a nitriding atmosphere sealed flow guide device, which at least includes a gas source, an air inlet duct, a cavity air inlet hood, a cavity air outlet hood and an air outlet duct; the gas source, the air inlet duct, the cavity air inlet hood and the air inlet of the cavity are connected in sequence, and the air outlet of the cavity, the cavity air outlet hood and the air outlet duct are connected in sequence.

[0057] In the above technical scheme, in order to improve the nitriding hardness and uniformity of the hardened layer depth of different parts of the inner spiral surface of the all-metal stator, a nitriding atmosphere guide device is added to ensure that the nitriding atmosphere is in full contact with the inner spiral surface of the all-metal stator and has sufficient time for penetration reaction. Adding a process atmosphere guide device can improve the nitriding hardness and layer depth uniformity of the inner spiral cavity surface, overcome local abnormal rapid wear, and thereby improve the overall service life of the all-metal motor.

[0058] When multiple cavities are connected in series, the cavities can be connected through a corrugated air guide tube.

[0059] Specifically, in order to reduce deformation during the nitriding process of the all-metal stator, the nitriding process is carried out by hoisting in a pit-type gas nitriding furnace.

[0060] Compared with the conventional gas nitriding process, the present invention has the following beneficial effects:

[0061] 1) 200℃ uniform temperature stage: Compared with the conventional gas nitriding process, the uniform temperature is different, uniform temperature is 200℃, and the furnace is preheated to prepare for pre-oxidation; finally, the depth of the nitriding layer in different parts is more uniform;

[0062] 2) 300℃ pre-oxidation catalytic infiltration stage: Compared with the conventional nitriding process, the 300℃ pre-oxidation catalytic infiltration treatment is added to form a dense ferroferric oxide film on the nitriding surface, which greatly shortens the nitriding cycle by 15%-30%, thus significantly improving production efficiency and saving energy;

[0063] 3) 480℃ strong nitriding stage: Compared with the nitriding temperature of conventional gas nitriding process, the strong nitriding temperature is lowered, the atmosphere pressure is increased, the cycle is shortened, the nitriding speed is increased while maintaining the high hardness of the nitrided workpiece surface and reducing the coarsening of the internal structure. Due to the use of pre-oxidation catalysis, the cycle is shorter than the conventional process;

[0064] 4) 520℃ diffusion stage: Compared with the conventional gas nitriding process, the diffusion cycle is shortened due to the addition of surface denitriding process ( Figure 6 The atmosphere pressure is appropriately increased during the 6-10 process stage of the present invention. The purpose of high-temperature diffusion (relative to the temperature of the strong nitriding stage of the gas nitriding process of the present invention) is to increase the depth of the nitriding layer, make the hardness gradient more gentle, and be more conducive to improving the wear resistance of the workpiece. Also, due to the use of pre-oxidation catalysis, the cycle is shorter than that of the conventional process;

[0065] 5) 495℃ denitriding stage: At the end of the process, the surface structure adjustment and surface denitriding are added at 495℃ for 2 hours. Compared with the conventional gas nitriding process, the temperature is lowered to adjust the structure and denitriding. The purpose is to make the surface of the nitrided layer obtain a higher hardness than the conventional nitriding process while reducing the surface brittleness, improving the erosion resistance, and thus improving the wear resistance.

[0066] 6) After nitriding, the nitriding surface hardness of the two ports is about 585-620HV (material 4145H steel, 40CrNiMoA steel), and the hardened layer depth is 0.65mm, while the surface nitriding hardness of the middle part is 580HV (material 4145H steel, 40CrNiMoA steel), and the hardened layer depth is 0.58mm. The nitriding surface hardness and nitriding layer depth of the upper, middle and lower parts of the specimen are relatively consistent. The nitriding layer is loose at level 2, the nitride level in the nitriding layer is level 1, and the surface brittleness of the nitriding layer is level 1. The surface brittleness of the nitriding layer is reduced, which is conducive to improving the overall wear resistance. When the mechanical strength requirements are not high, the surface nitriding hardness of up to about 1100HV can be obtained by using 38CrMoAl steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 It is a cross-sectional view of the stator and rotor of a conventional screw drilling tool motor in the prior art.

[0068] Figure 2 The present invention is a process flow chart of quenching and tempering the metal stator of a conventional screw drill motor in the prior art.

[0069] Figure 3 It is a cross-sectional view of an all-metal motor in the prior art.

[0070] Figure 4 It is an axial cross-sectional view of an all-metal stator of an all-metal motor in the prior art.

[0071] Figure 5 This is a conventional gas nitriding process flow chart in the prior art.

[0072] Figure 6 The present invention provides a gas nitriding process flow chart.

[0073] Figure 7 It is a structural schematic diagram of the nitriding atmosphere guide device provided by the present invention.

[0074] Attached Figure 7 The structures represented by the labels are listed below:

[0075] 1. Air inlet, 2. Air inlet duct, 3. Cavity air inlet hood, 4. Corrugated air duct, 5. Cavity air outlet hood, 6. Air outlet duct, 7. Air outlet, 8. Nitriding furnace tank. DETAILED DESCRIPTION

[0076] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0077] Example 1

[0078] This embodiment uses 4145H steel, and the processing flow of the workpiece is as follows:

[0079] Hot-rolled bars—straightening—drilling—rough turning of outer diameter—quenching and tempering heat treatment—magnetic particle inspection—ultrasonic inspection—rough and fine machining of inner hole—rough and fine turning of outer diameter—electrolytic machining—removal of process end—cleaning—nitriding—hardening layer and deformation detection—warehousing;

[0080] Quenching and tempering process: After heating to 860℃ in a pit-type resistance high temperature furnace and keeping it for 3 hours, cool it to room temperature with a water-based quenching medium, quickly move it to a pit-type resistance tempering furnace, heat it to 580℃ and keep it for 3.5 hours, and then cool it to room temperature with water.

[0081] The surface hardness of the workpiece is 300-320HB (the surface hardness of the upper port is 300HB, the surface hardness of the middle part is 310HB, and the surface hardness of the lower port is 320HB).

[0082] After electrolytic processing in the effective heating area: The pit type pre-vacuum gas nitriding furnace is used for gas nitriding treatment.

[0083] Combination Figure 6 , the gas nitriding surface hardening process is as follows:

[0084] 1. Derust, clean and degrease the workpiece before entering the furnace;

[0085] 2. To reduce the all-metal stator ( Figure 3 3-1) The nitriding process is deformed, and the nitriding process is loaded into the furnace by hoisting.

[0086] The airtight flow guide device made of ammonia corrosion-resistant heat-resistant steel pipe in the furnace is sealed and connected to the inlet and outlet holes of the furnace body. The air inlet is connected to the ammonia decomposition gas pipeline outside the furnace to disconnect the ammonia decomposition gas source and connect it with deoiled and dehydrated compressed air instead, with a pressure of 0.15MPa.

[0087] The workpiece starts to heat up along with the furnace (6-1 stage);

[0088] 3. The temperature rises to 200°C, and uniform heating is performed for 2 hours (6-2 stage) according to the workpiece dimensions of this embodiment (external diameter of the stator housing D = 172 mm, total length L = 4000 mm, major diameter of the spiral cavity d1 = 140 mm, minor diameter d2 = 112 mm, lead 1200 mm);

[0089] 4. Control the heating rate to 50℃ / h and heat to 300℃ (6-3 stage);

[0090] 5. Control the temperature to 300°C, keep warm for 1 hour, and perform pre-oxidation treatment (6-4 stage);

[0091] 6. Turn off the compressed air source and restore the sealed connection to the original ammonia decomposition gas source. Control the heating rate to 50℃ / h and raise the temperature to 480℃ (6-5 stages);

[0092] 7. At a process temperature of 480°C, introduce 1200 ml / min (average value, actually variable) of ammonia decomposition gas with a decomposition rate of 20%, and the nitriding atmosphere pressure in the furnace is about 6000 Pa, and carry out strong nitriding for 18 hours (6-6 stage);

[0093] 8. Increase the nitriding temperature to 520°C (6-7 stages);

[0094] 9. At the process temperature of 520°C, appropriately reduce the flow rate to 1000ml / min (average value) and introduce ammonia decomposition gas with a decomposition rate of 60%. The nitriding atmosphere pressure in the furnace is about 5000Pa, and diffuse nitriding is carried out for 20 hours (6-8 stages);

[0095] 10. Cool down to 495℃ (6-9 stages);

[0096] 11. At the process temperature of 495°C, the flow rate is appropriately reduced to 800ml / min (average value), and ammonia decomposition gas with a decomposition rate greater than 90% is introduced. The nitriding atmosphere pressure in the furnace is about 800Pa, and the temperature is kept for 2 hours to adjust the nitriding surface structure and denitrification treatment (6-10 stages);

[0097] 12. Stop the gas and heating, and let the workpiece cool down with the furnace until the temperature drops below 150℃, then take it out of the furnace and air cool it (stages 6-11).

[0098] The test results of the specimen after nitriding are as follows: the surface hardness of the two end parts is about 585HV, the hardened layer depth is 0.65mm, while the surface hardness of the middle part is 580HV, the hardened layer depth is 0.58mm, and the surface hardness and nitriding layer depth of the upper, middle and lower parts of the specimen are relatively consistent. The nitriding layer is loose at level 2, the nitride level in the nitriding layer is level 1, and the surface brittleness of the nitriding layer is level 1. The surface brittleness of the nitriding layer is reduced, which is conducive to improving the overall wear resistance.

[0099] Comparative Example 1

[0100] After repeated tests of the pre-oxidation process, it was found that increasing the pre-oxidation temperature to 400°C and extending the pre-oxidation time to 2 hours, the specific results were: at the same nitriding temperature and process time, the end nitriding layer depth was 0.62-0.65mm, and the middle nitriding layer depth was 0.56-0.58mm, which did not increase significantly. By comparing with the embodiment, it can be seen that the purpose of increasing the depth of the nitriding layer cannot be achieved, but the loose level of the nitriding layer reaches 2-3 levels, the nitride level in the nitriding layer is 2-3 levels, and the surface performance of the nitriding layer is deteriorated.

[0101] Comparative Example 2

[0102] The gas nitriding process is the same as in Example 1, except that the pre-oxidation treatment (6-4) in step 5) is not performed. The specific results of hardening are as follows:

[0103] Under the same nitriding process (same nitriding temperature, time and pressure), the depth of the nitriding layer is: 0.50-0.53mm at both ends and 0.45-0.48mm in the middle.

[0104] Comparative Example 3

[0105] The gas nitriding process is the same as in Example 1, except that the nitriding surface structure adjustment and denitriding treatment (6-10) in step 11) are not performed. The specific results of hardening are as follows:

[0106] Under the same nitriding process (same nitriding temperature, time and pressure), the thickness of the ε phase of the brittle white bright layer on the surface of the workpiece without surface denitriding treatment is about 20μm. After the surface denitriding process is added, the thickness of the ε phase of the brittle white bright layer on the surface of the workpiece is less than 10μm, and about 50% is converted into γ' phase.

[0107] Example 2

[0108] like Figure 7 As shown, the furnace body is provided with a nitriding atmosphere flow guide device, which at least includes a gas source, an air inlet guide pipe 2, a cavity air inlet hood 3, a cavity air outlet hood 5 and an air outlet guide pipe 6. The gas source, the air inlet guide pipe 2, the cavity air inlet hood 3, and the air inlet of the cavity are connected in sequence, and the air outlet of the cavity, the cavity air outlet hood 5 and the air outlet guide pipe 6 are connected in sequence. The two cavities are connected through a corrugated air guide pipe 4. The air inlet 1 of the air inlet guide pipe 2 and the air outlet 7 of the air outlet guide pipe 6 respectively penetrate the tank wall of the nitriding furnace tank body 8.

[0109] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for hardening the surface of the inner spiral cavity of an all-metal stator, characterized in that: At least the following steps are included: 1) The workpiece should be derusted, cleaned and degreased before entering the furnace; 2) The workpiece is hoisted vertically into the furnace and heated as the furnace heats up; 3) After the furnace is heated up, uniform heating is performed; 4) Control the heating rate and continue heating to the pre-oxidation temperature; 5) pre-oxidation at a constant temperature at a pre-oxidation temperature; 6) After the pre-oxidation is completed, control the heating rate and continue heating to the strong nitriding temperature; 7) Performing strong nitriding at strong nitriding temperature and under the conditions of ammonia decomposition gas with low ammonia decomposition rate; 8) Increase the nitriding temperature after the strong nitriding is completed; 9) Diffusion nitriding is carried out under ammonia decomposition gas with a medium ammonia decomposition rate; 10) Cooling down after diffusion nitriding; 11) Carry out nitriding surface structure adjustment and denitrification treatment under ammonia decomposition gas with high ammonia decomposition rate; 12) Stop the gas and heating, and let the workpiece cool down with the furnace before being air-cooled.

2. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to claim 1, characterized in that: In step 3), the absorptive heating temperature is 190-210° C. and the absorptive heating time is 1-3 h.

3. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to claim 2, characterized in that: In step 4), the heating rate is less than 60°C / h; the pre-oxidation temperature is 290-310°C; In step 5), the pre-oxidation time is 1-2h.

4. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to claim 3, characterized in that: In step 6), the heating rate is <50°C / h; the strong nitriding temperature is 470-490°C; In step 7), the ammonia decomposition rate is 18-22%; the nitriding atmosphere pressure in the furnace is 5500-6500 Pa; and the strong nitriding time is 16-20 hours.

5. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to claim 4, characterized in that: In step 8), the temperature is increased to 510-530°C; In step 9), the ammonia decomposition rate is 55-65%; the nitriding atmosphere pressure in the furnace is 4500-5500 Pa; and the strong nitriding time is 18-22 hours.

6. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to claim 5, characterized in that: In step 10), the temperature is lowered to 490-500°C.

7. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to claim 6, characterized in that: In step 11), the decomposition rate of ammonia is 85-95%; the pressure of the nitriding atmosphere in the furnace is 750-850 Pa; and the surface denitriding time is 1-3 hours.

8. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to any one of claims 1 to 7, characterized in that: The furnace body is provided with a nitriding atmosphere sealed flow guide device, which at least includes a gas source, an air inlet guide pipe, a cavity air inlet hood, a cavity air outlet hood and an air outlet guide pipe. The gas source, the air inlet guide pipe, the cavity air inlet hood and the air inlet of the cavity are connected in sequence, and the air outlet of the cavity, the cavity air outlet hood and the air outlet guide pipe are connected in sequence.

9. The method for hardening the surface of the inner spiral cavity of the all-metal stator according to claim 8, characterized in that: The material of the workpiece is 4145H steel, 40CrNiMoA or 38CrMoAl.

10. A hardened all-metal stator obtained by the method according to any one of claims 1 to 9, characterized in that: The nitrided surface hardness is 580-1100HV; The hardened layer depth is 0.58-0.65mm.

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