A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails

Abstract

This invention belongs to the field of heat treatment technology and relates to a method for medium-frequency induction hardening of ultra-long cast iron bed guideways, particularly suitable for the hardening treatment of large-size, integrated ultra-long guideways. The method includes the following steps: aging treatment, machining, stress relief treatment, medium-frequency induction hardening, and tempering of the ultra-long cast iron bed guideway. The medium-frequency induction hardening employs a dual-inductor system: one for preheating and one for quenching. The distance between the two inductors is 15–20 mm, and the distance between the inductor surface and the quenching surface is 4–6 mm. By rationally controlling parameters such as the power and frequency of the induction power supply, the inductor distance, the moving speed, cooling, and airflow, the hardness of the bed guideway reaches 55–60 HRC, and the layer depth reaches 5.0–7.6 mm.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology and relates to a medium-frequency induction quenching method for ultra-long bed cast iron guide rails, which is particularly suitable for the quenching treatment of large-size, integrated ultra-long guide rails. Background Technology

[0002] Guide rails are the reference components that ensure the motion accuracy of machine tools. Compared with the steel-insulated guide rails or linear guide rails that are often used in medium and large high-precision machine tools, the integrated bed guide rails have obvious advantages in terms of vibration reduction, stability and overall rigidity.

[0003] Due to the large size of the machine tool guide rails, the most widely used heat treatment method to improve their mechanical properties is induction hardening. However, this method is prone to problems such as guide rail deformation, surface cracking, and uneven hardness, which seriously restricts the development of machine tool guide rails.

[0004] The aforementioned problems are more complex in ultra-long integrated bed guide rails, and urgently need to be improved by changing the heat treatment process. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a medium-frequency induction hardening method for ultra-long bed cast iron guide rails, suitable for the hardening treatment of large-size, integrated ultra-long guide rails, ensuring high hardness, deep layers, and uniform distribution of the ultra-long bed guide rails.

[0006] Technical solution:

[0007] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails includes the following steps:

[0008] Step 1: Perform stress-relieving aging treatment on the cast iron guide rails and the entire bed of the extra-long bed after casting;

[0009] Step 2: Machining the cast iron guide rails of the extra-long bed after stress relief and aging treatment, as well as the parts requiring machining;

[0010] Step 3: Perform stress relief treatment on the machined extra-long bed cast iron guide rail section and the entire bed;

[0011] Step 4: Perform medium-frequency induction hardening on the stress-relief treated extra-long cast iron bed guide rails;

[0012] Step 5: Perform a tempering treatment on the quenched extra-long cast iron guide rails and the entire bed;

[0013] Step Six: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering;

[0014] Step 7: Perform a secondary tempering treatment on the extra-long cast iron guide rails of the bed after rough grinding.

[0015] Furthermore, in step one, stress-relieving aging treatment is performed using tempering equipment. An aging treatment is performed after casting and before rough machining. The aging treatment process is as follows: the furnace is loaded at a temperature below 200°C, heated to 320-380°C at a heating rate of 50-80°C / h, and held for 2-3 hours; then heated to 520-580°C at a heating rate of 40-60°C / h, and held for 6-8 hours. The furnace is then cooled to below 250°C before being removed from the furnace.

[0016] Furthermore, after the machining process in step two, the surface roughness Ra of the cast iron guide rail part of the extra-long bed is less than 3.2, with no visible defects.

[0017] Furthermore, the stress relief treatment in step three is as follows: the furnace is loaded at a temperature below 200°C, heated to 340-360°C at a heating rate of 50-80°C / h, held for 1.5-2.5 hours, heated to 530-550°C at a heating rate of 40-60°C / h, held for 8-10 hours, and then cooled to below 200°C before being removed from the furnace.

[0018] Furthermore, the medium-frequency induction quenching in step four employs dual-inductor quenching: one inductor is a preheating inductor, which preheats the extra-long bed cast iron guide rail section before quenching without water spraying, with a maximum preheating temperature of 840–850°C; the other is a quenching inductor, equipped with its own quenching water spray and auxiliary cooling; the inductors are designed based on the overall or partial decomposition contouring of the quenching cross-section of the extra-long bed cast iron guide rail; the distance between the preheating inductor and the quenching inductor is 15–20 mm.

[0019] Furthermore, the distance between the sensor surface and the quenched surface of the extra-long bed cast iron guide rail is 4-6 mm, and the sensor size on the quenched surface exceeds the quenched cross-section width of the bed cast iron guide rail by 6-12 mm; the water pressure inside the sensor is 2-3 MPa, the water spray direction is opposite to the sensor's forward direction, and forms a 40-50° angle with the quenched surface of the extra-long bed cast iron guide rail; the air pressure of the blower is 6-8 MPa, and the air direction is parallel to the guide rail surface.

[0020] Furthermore, in step four, the medium-frequency induction hardening process parameters are as follows: output power 40-70kW, frequency 2-6kHz, transformer turns ratio 9:2, moving speed 180-240mm / min, induction heating temperature 880-930℃, cooling medium is water or water-based quenching agent, and continuous spray cooling method is adopted.

[0021] Furthermore, in step five, the tempering treatment should be completed within 12 hours after induction hardening, with a tempering temperature of 180–260°C and a tempering time of 6–8 hours.

[0022] Furthermore, in step seven, the guide rail after rough grinding is subjected to a secondary tempering treatment. The tempering treatment is completed within 24 hours after rough grinding, the tempering temperature is 180-260℃, and the tempering time is ≥6 hours.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows.

[0024] By performing stress-relief aging treatment on ultra-long cast iron bed guideways before machining, the casting stress in the guideways can be reduced, thus minimizing deformation caused by stress release during machining. Stress-relief aging treatment on the machined ultra-long cast iron bed guideways effectively releases machining stress and reduces deformation after heat treatment. Using dual induction heating—one induction preheating and the other quenching—increases the depth of the induction hardened layer. Tempering is performed shortly after induction hardening to prevent cracks in the guideways caused by quenching stress. The secondary tempering treatment after rough grinding further improves the stability of the microstructure, further releases quenching and grinding stresses, reduces the tendency for quenching cracks to form, stabilizes the microstructure, reduces the tendency and amount of metastable phase transformation during subsequent use, and improves dimensional accuracy maintenance. Detailed Implementation

[0025] The present invention will be further described below.

[0026] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails, the specific implementation of which is as follows:

[0027] Step 1: Perform stress-relief aging treatment on the extra-long cast iron guide rails and integral bed. A tempering device is used for aging treatment, performed after casting and before rough machining. The aging treatment process is as follows: Load the machine into the furnace at below 200℃, heat to 320-380℃ at a rate of 50-80℃ / h, hold for 2-3 hours, then heat to 520-580℃ at a rate of 40-60℃ / h, hold for 6-8 hours (holding time depends on the furnace load and the wall thickness of the thickest part), and cool with the furnace to below 250℃ before removing from the furnace. This stress-relief aging treatment reduces casting stress and minimizes dimensional changes caused by stress release during machining.

[0028] Step 2: Machin the extra-long cast iron guide rails of the bed after aging treatment to make the surface roughness Ra < 3.2 and visually free of defects.

[0029] Step 3: Stress relief treatment is performed on the machined extra-long cast iron bed guide rails. The rails are loaded into the furnace at below 200℃, heated to 340-360℃ at a rate of 50-80℃ / h, held for 1.5-2.5 hours, then heated to 530-550℃ at a rate of 40-60℃ / h, held for 8-10 hours, and finally cooled in the furnace to below 200℃ before being removed. This process removes machining stress and reduces deformation during subsequent induction hardening heat treatment.

[0030] Step 4: Perform medium-frequency induction hardening on the stress-relieved guide rails. The medium-frequency induction hardening process parameters are as follows: output power 40-70kW, frequency 2-6kHz, transformer turns ratio 9:2, moving speed 180-240mm / min, induction heating temperature 880-930℃, cooling medium water, and continuous spray cooling to achieve a hardness of 55-60HRC or higher and a layer depth of 5.0-7.6mm. A dual-inductor hardening method is used: one inductor is a preheating inductor (no water spray, preheating temperature 840-850℃), and the other is a quenching inductor. The system includes a quenching water spray function, with auxiliary cooling added as needed. The inductors should be designed based on the overall or partially disassembled shape of the guide rail cross-section. The distance between the inductor surface and the quenching surface is 4–6 mm. The inductor size on the quenching surface should be 6–12 mm wider than the guide rail cross-section. The water spray holes should be rationally set according to the inductor width, evenly distributed, with a water pressure of 2–3 MPa. The water spray direction is opposite to the inductor's forward direction and at a 40–50° angle to the quenching surface of the guide rail. The air blower pressure is 6–8 MPa, and the airflow direction is parallel to the guide rail surface to prevent water backflow. The distance between the two inductors is 15–20 mm. Heating with two inductors achieves both preheating and quenching, significantly increasing the depth of the hardened layer.

[0031] Step 5: Temper the extra-long cast iron bed guide rails after quenching. The tempering treatment should be completed within 12 hours after induction hardening, with a tempering temperature of 180-260℃ and a tempering time of 6-8 hours. Tempering within a short time after quenching helps to reduce quenching stress and suppress quenching cracks.

[0032] Step Six: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering.

[0033] Step 7: Perform secondary tempering on the extra-long cast iron guide rails and the entire bed after rough grinding. The tempering treatment should be completed within 24 hours after rough grinding, with a tempering temperature of 180-260℃ and a tempering time of ≥6 hours. The longer the tempering time, the more fully the grinding stress is reduced. Secondary tempering can reduce quenching stress, reduce the probability of crack formation, and promote the transformation of metastable phase structure, thereby improving the maintenance of guide rail accuracy.

[0034] Example 1.

[0035] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails is implemented as follows:

[0036] Step 1: Aging treatment of extra-long cast iron bed guide rails using HT300: After casting and before rough machining, an aging treatment is performed using tempering equipment. The aging treatment process is as follows: Load the furnace at 180℃, heat to 350℃ at a heating rate of 60℃ / h, hold for 2 hours, heat to 550℃ at a heating rate of 50℃ / h, hold for 6 hours, and then cool to 220℃ with the furnace before unloading.

[0037] Step 2: The extra-long cast iron guide rails of the bed after aging treatment are machined with HT300, with a surface roughness Ra=2.5 and no defects.

[0038] Step 3: The extra-long cast iron bed guide rails after machining are stress-relieved using HT300. The furnace is loaded at 180℃, heated to 350℃ at a heating rate of 50℃ / h, held for 2 hours, heated to 530℃ at a heating rate of 40℃ / h, held for 10 hours, and then cooled to 180℃ with the furnace before being taken out.

[0039] Step 4: Perform medium-frequency induction hardening on the stress-relieved guide rail using an HT300. The medium-frequency induction hardening process parameters are as follows: output power 50kW, frequency 4kHz, transformer turns ratio 9:2, moving speed 200mm / min, induction heating temperature 900℃, and continuous water spray cooling method; use dual inductor hardening, with a distance of 5mm between the inductor surface and the hardening surface, and the inductor size on the hardening surface should be 10mm wider than the guide rail cross-section. The water spray holes should be reasonably set according to the width of the inductors and evenly distributed, with a water pressure of 2MPa, the water spray direction opposite to the inductor's forward direction and at a 45° angle to the guide rail hardening surface, and a blower air pressure of 6MPa, with the air direction parallel to the guide rail surface, and a distance of 15mm between the two inductors.

[0040] Step 5: Temper the extra-long cast iron bed guide rails after quenching using HT300. The tempering process is carried out 8 hours after induction hardening, with a tempering temperature of 220℃ and a tempering time of 6 hours.

[0041] Step Six: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering.

[0042] Step 7: Perform a secondary tempering treatment on the rough-ground extra-long cast iron guide rails and the entire bed. The tempering temperature is 220℃, and the tempering time is 6 hours. Secondary tempering can reduce quenching stress, reduce the probability of crack formation, and promote the transformation of metastable phase structure, thereby improving the maintenance of guide rail accuracy.

[0043] After this quenching process, the hardness of HT300 for guide rails reaches 55-58 HRC, and the layer depth is 5.0 mm.

[0044] Example 2.

[0045] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails is implemented as follows:

[0046] Step 1: Aging treatment of extra-long bed cast iron guide rails using HT300: After casting and before rough machining, an aging treatment is performed using tempering equipment. The aging treatment process is as follows: Load the furnace at room temperature, heat to 380℃ at a heating rate of 50℃ / h, hold for 2 hours, heat to 520℃ at a heating rate of 40℃ / h, hold for 8 hours, and then cool to 250℃ with the furnace before unloading.

[0047] Step 2: The extra-long cast iron guide rails of the bed after aging treatment are machined with HT300, with a surface roughness Ra=3.0 and no defects visible to the naked eye.

[0048] Step 3: The extra-long cast iron bed guide rails after machining are stress-relieved using HT300. The furnace is loaded at room temperature and heated to 360℃ at a heating rate of 80℃ / h, held for 2 hours, then heated to 550℃ at a heating rate of 50℃ / h, held for 8 hours, and then cooled to 190℃ in the furnace before being taken out.

[0049] Step 4: Perform medium-frequency induction hardening on the stress-relieved guide rail using an HT300. The medium-frequency induction hardening process parameters are as follows: output power 40kW, frequency 4kHz, transformer turns ratio 9:2, moving speed 180mm / min, induction heating temperature 920℃, and continuous water spray cooling method; use dual inductor hardening, with a distance of 4mm between the inductor surface and the hardening surface, and the inductor size on the hardening surface should be 10mm wider than the guide rail cross-section. The water spray holes should be reasonably set according to the width of the inductors and evenly distributed, with a water pressure of 2MPa, the water spray direction opposite to the inductor's forward direction and at a 45° angle to the guide rail hardening surface, and a blower air pressure of 7MPa, with the air direction parallel to the guide rail surface, and a distance of 18mm between the two inductors.

[0050] Step 5: Temper the extra-long cast iron bed guide rails after quenching using HT300. The tempering treatment is carried out 8 hours after induction hardening, with a tempering temperature of 220℃ and a tempering time of 6 hours.

[0051] Step Six: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering.

[0052] Step 7: Perform a secondary tempering treatment on the rough-ground extra-long cast iron guide rails and the entire bed. The tempering temperature is 220℃, and the tempering time is 7 hours. Multiple temperings can reduce quenching stress, reduce the probability of crack formation, and promote the transformation of metastable phase structure, thereby improving the maintenance of guide rail accuracy.

[0053] After this quenching process, the hardness of HT300 for guide rails reaches 55-57 HRC, and the layer depth is 5.5 mm.

[0054] Example 3.

[0055] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails is implemented as follows:

[0056] Step 1: Aging treatment of extra-long cast iron bed guide rails using HT300: After casting and before rough machining, an aging treatment is performed using tempering equipment. The aging treatment process is as follows: Load the furnace at 190℃, heat to 360℃ at a heating rate of 80℃ / h, hold for 2 hours, heat to 580℃ at a heating rate of 60℃ / h, hold for 8 hours, and then cool to 180℃ with the furnace before unloading.

[0057] Step 2: The extra-long cast iron guide rails of the bed after aging treatment are machined with HT300, with a surface roughness Ra=2.0 and no defects visible to the naked eye.

[0058] Step 3: The extra-long cast iron bed guide rails after machining are stress-relieved using HT300. The furnace is loaded at room temperature and heated to 345℃ at a heating rate of 70℃ / h, held for 2 hours, then heated to 580℃ at a heating rate of 50℃ / h, held for 7 hours, and then cooled to 200℃ in the furnace before being removed from the furnace.

[0059] Step 4: Perform medium-frequency induction hardening on the stress-relieved guide rail using an HT300. The medium-frequency induction hardening process parameters are as follows: output power 55kW, frequency 4kHz, transformer turns ratio 9:2, moving speed 180mm / min, induction heating temperature 930℃, and continuous water spray cooling method; use dual inductor hardening, with a distance of 4.5mm between the inductor surface and the hardening surface, and the inductor size on the hardening surface should be 10mm wider than the guide rail cross-section. The water spray holes should be reasonably set according to the width of the inductors and evenly distributed, with a water pressure of 2MPa, the water spray direction opposite to the inductor's forward direction and at a 45° angle to the guide rail hardening surface, and a blower air pressure of 6.5MPa, with the air direction parallel to the guide rail surface, and a distance of 20mm between the two inductors.

[0060] Step 5: Temper the extra-long cast iron bed guide rails after quenching using HT300. The tempering treatment is carried out 8 hours after induction hardening, with a tempering temperature of 180℃ and a tempering time of 8 hours.

[0061] Step Six: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering.

[0062] Step 7: Perform a secondary tempering treatment on the rough-ground extra-long cast iron guide rails and the entire bed. The tempering temperature is 200℃, and the tempering time is 7 hours. Multiple temperings can reduce quenching stress, reduce the probability of crack formation, and promote the transformation of metastable phase structure, thereby improving the maintenance of guide rail accuracy.

[0063] After this quenching process, the hardness of the HT300 guide rail reaches 57-58 HRC, and the layer depth is 7mm.

[0064] Comparative Example 1.

[0065] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails is implemented as follows:

[0066] Step 1: The extra-long cast iron bed guide rails after aging treatment are machined with HT300, with a surface roughness Ra=3.2 and no visible defects.

[0067] Step 2: Perform ordinary medium-frequency induction hardening on the machined guide rail using an HT300. The medium-frequency induction hardening process parameters are as follows: output power 65.2kW, frequency 9.2kHz, transformer turns ratio 8:2, moving speed 180mm / min, induction heating temperature 900℃, continuous water spray cooling method, distance between the inductor surface and the hardening surface 3.5mm, inductor size on the hardening surface should be 10mm wider than the guide rail cross-section, water spray holes should be reasonably set according to the width of the inductor and evenly distributed, water pressure 2MPa, water spray direction opposite to the inductor's forward direction and at a 45° angle to the guide rail hardening surface, air blower pressure 6.5MPa, air direction parallel to the guide rail surface, and distance between the two inductors 20mm.

[0068] Step 3: Temper the extra-long cast iron bed guide rails after quenching using HT300. The tempering treatment is carried out 8 hours after induction hardening, with a tempering temperature of 180℃ and a tempering time of 8 hours.

[0069] Step 4: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering.

[0070] After this quenching process, the hardness of HT300 for guide rails reaches 50-52 HRC, and the layer depth is 2.5 mm.

[0071] Comparative Example 2.

[0072] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails is implemented as follows:

[0073] Step 1: The extra-long cast iron guide rail of the bed is machined with HT300, with a surface roughness Ra=2.5 and no defects.

[0074] Step 2: The extra-long cast iron bed guide rail after machining is stress-relieved using HT300. It is loaded into the furnace at 180℃, heated to 350℃ at a heating rate of 50℃ / h, held for 2 hours, heated to 530℃ at a heating rate of 40℃ / h, held for 10 hours, and then cooled to 180℃ in the furnace before being taken out.

[0075] Step 3: Perform medium-frequency induction hardening on the stress-relieved guide rail using an HT300. The medium-frequency induction hardening process parameters are as follows: output power 50kW, frequency 4kHz, transformer turns ratio 9:2, moving speed 200mm / min, induction heating temperature 900℃, and continuous water spray cooling method; use dual inductor hardening, with a distance of 5mm between the inductor surface and the hardening surface, and the inductor size on the hardening surface should be 10mm wider than the guide rail cross-section. The water spray holes should be reasonably set according to the width of the inductors and evenly distributed, with a water pressure of 2MPa, the water spray direction opposite to the inductor's forward direction and at a 45° angle to the guide rail hardening surface, and a blower air pressure of 6MPa, with the air direction parallel to the guide rail surface, and a distance of 15mm between the two inductors.

[0076] Step 4: Temper the extra-long cast iron bed guide rails after quenching using HT300. The tempering process is completed 8 hours after induction hardening, with a tempering temperature of 220℃ and a tempering time of 6 hours.

[0077] Step 5: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering.

[0078] Step Six: Perform a second tempering treatment on the extra-long cast iron guide rails and the entire bed after rough grinding. The tempering temperature is 220℃ and the tempering time is 6 hours.

[0079] After the quenching process, the hardness of HT300 for guide rails reaches 55-58HRC. Since the HT300 castings have not undergone stress relief aging treatment, the deformation is obvious after machining and heat treatment. After rough grinding, the hardened layer thickness of the guide rail is uneven, with a layer depth range of 3.5~5.5mm.

[0080] Comparative Example 3.

[0081] A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails is implemented as follows:

[0082] Step 1: Aging treatment of extra-long cast iron bed guide rails using HT300: After casting and before rough machining, an aging treatment is performed using tempering equipment. The aging treatment process is as follows: Load the furnace at 180℃, heat to 350℃ at a heating rate of 60℃ / h, hold for 2 hours, heat to 550℃ at a heating rate of 50℃ / h, hold for 6 hours, and then cool to 220℃ with the furnace before unloading.

[0083] Step 2: The extra-long cast iron guide rails of the bed after aging treatment are machined with HT300, with a surface roughness Ra=2.5 and no defects.

[0084] Step 3: Perform medium-frequency induction hardening on the guide rail from Step 2 using an HT300. The medium-frequency induction hardening process parameters are as follows: output power 50kW, frequency 4kHz, transformer turns ratio 9:2, moving speed 200mm / min, induction heating temperature 900℃, and continuous water spray cooling method; use dual inductor hardening, with a 5mm gap between the inductor surface and the hardening surface, and the inductor size on the hardening surface should be 10mm wider than the guide rail cross-section. The water spray holes should be reasonably set according to the width of the inductors and evenly distributed, with a water pressure of 2MPa, the water spray direction opposite to the inductor's forward direction and at a 45° angle to the guide rail hardening surface, and a blower air pressure of 6MPa, with the air direction parallel to the guide rail surface, and a 15mm gap between the two inductors.

[0085] Step 4: Temper the extra-long cast iron bed guide rails after quenching using HT300. The tempering process is completed 8 hours after induction hardening, with a tempering temperature of 220℃ and a tempering time of 6 hours.

[0086] Step 5: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering.

[0087] Step Six: Perform a second tempering treatment on the extra-long cast iron guide rails and the entire bed after rough grinding. The tempering temperature is 220℃ and the tempering time is 6 hours.

[0088] After this quenching process, the hardness of HT300 for guide rails reaches 55-58HRC. Since the HT300 castings have not undergone stress after rough machining, the deformation is obvious after heat treatment. After rough grinding, the hardened layer thickness of the guide rail is uneven, with a layer depth range of 4.5~5.0mm.

Claims

1. A method for medium-frequency induction hardening of ultra-long bed cast iron guide rails, characterized in that, Includes the following steps: Step 1: Perform stress-relieving aging treatment on the cast iron guide rails and the entire bed of the extra-long bed after casting; Step 2: Machining the cast iron guide rails of the extra-long bed after stress relief and aging treatment, as well as the parts requiring machining; Step 3: Perform stress relief treatment on the machined extra-long bed cast iron guide rail section and the entire bed; Step 4: Perform medium-frequency induction hardening on the stress-relief treated extra-long cast iron bed guide rails; Step 5: Perform a tempering treatment on the quenched extra-long cast iron guide rails and the entire bed; Step Six: Rough grinding is performed on the extra-long cast iron guide rails of the bed after tempering; Step 7: Perform a secondary tempering treatment on the extra-long cast iron guide rails of the bed after rough grinding; In step one, stress-relieving aging treatment is performed using tempering equipment. An aging treatment is performed after casting and before rough machining. The aging treatment process is as follows: the furnace is loaded at a temperature below 200°C, and the temperature is increased to 320-380°C at a heating rate of 50-80°C / h, and held for 2-3 hours; the temperature is increased to 520-580°C at a heating rate of 40-60°C / h, and held for 6-8 hours, and then cooled in the furnace to below 250°C before being removed from the furnace. The intermediate frequency induction quenching in step four employs a dual-inductor quenching process: one inductor is a preheating inductor, which preheats the extra-long cast iron guide rail section of the bed before quenching without water spraying, with a maximum preheating temperature of 840–850°C; the other is a quenching inductor, equipped with its own quenching water spray and auxiliary cooling; the inductors are designed based on the overall or partial decomposition contouring of the quenching cross-section of the extra-long cast iron guide rail; the distance between the preheating inductor and the quenching inductor is 15–20 mm; In step five, the tempering treatment should be completed within 12 hours after induction hardening, with a tempering temperature of 180-260℃ and a tempering time of 6-8 hours. In step seven, the guide rail after rough grinding is subjected to a second tempering treatment. The tempering treatment is completed within 24 hours after rough grinding, the tempering temperature is 180~260℃, and the tempering time is ≥6 hours.

2. The method for medium-frequency induction hardening of ultra-long bed cast iron guide rails according to claim 1, characterized in that, After the machining process in step two, the surface roughness Ra of the cast iron guide rail part of the extra-long bed is less than 3.2, and there are no visible defects.

3. The method for medium-frequency induction hardening of ultra-long bed cast iron guide rails according to claim 1, characterized in that, The stress relief treatment in step three is as follows: the furnace is loaded at a temperature below 200°C, heated to 340-360°C at a heating rate of 50-80°C / h, held for 1.5-2.5 hours, heated to 530-550°C at a heating rate of 40-60°C / h, held for 8-10 hours, and then cooled to below 200°C before being removed from the furnace.

4. The method for medium-frequency induction hardening of ultra-long bed cast iron guide rails according to claim 1, characterized in that, The distance between the sensor surface and the quenched surface of the extra-long bed cast iron guide rail is 4-6 mm, and the sensor size on the quenched surface exceeds the quenched cross-section width of the bed cast iron guide rail by 6-12 mm; the water pressure inside the sensor is 2-3 MPa, the water spray direction is opposite to the sensor's forward direction and forms a 40-50° angle with the quenched surface of the extra-long bed cast iron guide rail, the air pressure of the blower is 6-8 MPa, and the air direction is parallel to the guide rail surface.

5. The method for medium-frequency induction hardening of ultra-long bed cast iron guide rails according to claim 1, characterized in that, In step four, the medium-frequency induction quenching process parameters are as follows: output power 40-70kW, frequency 2-6kHz, transformer turns ratio 9:2, moving speed 180-240mm / min, induction heating temperature 880-930℃, cooling medium is water or water-based quenching agent, and continuous spray cooling method is adopted.