Large cross-section difference step shaft and heat treatment method

Abstract

A stepped shaft with a large cross-sectional difference and a heat treatment method are disclosed. The stepped shaft includes a large-diameter section in the middle and small-diameter sections at both ends along the axial direction. The outer diameter of the large-diameter section is 2700-2900 mm, and the outer diameter of the small-diameter sections is 500-550 mm. The diameter ratio of the large-diameter section to the small-diameter section is >4. The chemical composition of the stepped shaft, by mass fraction, includes: 0.26%-0.30% C, 0.18%-0.35% Si, 0.30%-0.50% Mn, 1.40%-1.90% Cr, 3.30%-3.80% Ni, 0.28%-0.58% Mo, 0.07%-0.10% V, with the remainder being Fe and unavoidable impurities. The heat treatment methods include normalizing, mist cooling, air cooling, quenching, water cooling, air cooling, and tempering. Ultimately, the tensile strength of the large-section stepped shaft can reach more than 1200MPa, which has excellent comprehensive mechanical properties and is not prone to cracking, thus meeting the design requirements and ensuring high process safety.

Description

Technical Field

[0001] This invention relates to the field of stepped shafts with large cross-section differences, and in particular to a stepped shaft with large cross-section differences and a heat treatment method thereof. Background Technology

[0002] Large cross-section difference stepped shafts typically refer to stepped shafts where the diameters or cross-sectional areas of adjacent steps differ significantly (e.g., diameter ratio ≥ 2:1 or significant abrupt changes in cross-section). Depending on the requirements of their operating scenarios, stepped shafts are usually designed with significant differences in the diameters or cross-sectional areas of adjacent steps (e.g., diameter ratio ≥ 2:1 or significant abrupt changes in cross-section). They are used in scenarios requiring high torque transmission, segmented stiffness matching, or segmented bearing / gear positioning within a finite length, such as clutch shafts and gearbox shafts in automotive transmission systems, machine tool spindles / reducers, and wind turbine spindles and drive shafts. With the increasing demands for power density and lightweighting in high-end equipment, large cross-section difference designs are becoming more widely used in transmission path optimization and structural weight reduction.

[0003] Large-section stepped shafts are core components of rotating machinery (such as motors, generators, turbines, and pumps), with applications spanning energy, industrial manufacturing, transportation, aerospace, and other fields. These fields place high demands on the performance of stepped shafts. For example, in thermal / gas turbines, they must withstand the impact of high-temperature, high-pressure gas and transmit enormous torque to drive generators, requiring high strength and resistance to high-temperature creep (such as nickel-based alloys or superalloys). In thermal generators, they connect the impeller and generator, requiring resistance to water corrosion and fatigue, especially in impulse turbines where they must withstand high-frequency water flow impacts. In wind turbines, lightweight construction (carbon fiber composites or high-strength steel) is required to reduce tower load while ensuring wind load resistance and fatigue life.

[0004] The large cross-section of stepped shafts can lead to stress concentration and uncontrollable deformation, affecting dimensional stability and service reliability. The high difficulty in controlling the step transition fillets, surface roughness, and heat treatment quality makes them prone to becoming fatigue weak points. Furthermore, the complexity of consistency and traceability management under multi-material / multi-process collaboration increases the risk of quality fluctuations. Simultaneously, with the development of industries such as machinery manufacturing, aerospace, and automotive, the performance requirements for high-end materials are increasingly demanding, further increasing the difficulty of heat treatment for stepped shafts with large cross-section differences. Summary of the Invention

[0005] To address the performance deficiencies of stepped shafts with large cross-sectional differences, this invention provides a stepped shaft with large cross-sectional differences and a heat treatment method therein.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is: a stepped shaft with large cross-section difference, including a large diameter part located in the middle and small diameter parts at both ends of the axial direction. The outer diameter of the large diameter part is 2700-2900mm, the outer diameter of the small diameter part is 500-550mm, and the diameter ratio of the large diameter part to the small diameter part is >4. The chemical composition of the stepped shaft, by mass fraction, includes: 0.26%-0.30% C, 0.18%-0.35% Si, 0.30%-0.50% Mn, 1.40%-1.90% Cr, 3.30%-3.80% Ni, 0.28%-0.58% Mo, 0.07%-0.10% V, with the remainder being Fe and unavoidable impurities; After heat treatment, the stepped shaft has a yield strength >1100MPa, tensile strength >1250MPa, elongation >11%, reduction of area >40%, and impact energy at room temperature >40J.

[0007] Preferably, it contains 0.29% C, 0.25% Si, 0.39% Mn, 1.80% Cr, 3.65% Ni, 0.50% Mo, 0.09% V, with the remainder being Fe and unavoidable impurities.

[0008] Preferably, it contains 0.26% C, 0.35% Si, 0.30% Mn, 1.90% Cr, 3.30% Ni, 0.58% Mo, 0.07% V, with the remainder being Fe and unavoidable impurities.

[0009] Preferably, it contains 0.30% C, 0.18% Si, 0.50% Mn, 1.40% Cr, 3.80% Ni, 0.28% Mo, 0.10% V, with the remainder being Fe and unavoidable impurities.

[0010] The above-mentioned heat treatment method for a stepped shaft with large cross-section difference includes the following steps: S1. Normalizing: Heat the stepped shaft to 880-920℃ and hold for a duration of {(1.5~1.8)×H / 100} hours, where H is the maximum diameter of the major diameter section in mm; S2. Fogging: Use a spray device to spray and cool the surface of the stepped shaft after S1 insulation until the temperature at the junction of adjacent steps drops to 300-350℃. S3, air cooling, allowing the stepped shaft after S2 mist cooling to cool naturally in the air until the temperature at the junction of adjacent steps drops to ≤150℃; S4. Quenching: Heat the stepped shaft after air cooling in S3 to 810-840℃ and hold it at that temperature for {(1.5~1.8)×H / 100} hours. S5. Water cooling: First, allow the stepped shaft after S4 insulation to cool naturally in the air for (2~3) minutes. Then, immerse the stepped shaft in a water tank for water cooling for {(3.0~3.3)×H / 100} minutes. Then allow the stepped shaft to cool naturally in the air for 2-3 minutes, and then immerse the stepped shaft in a water tank for water cooling for 1.5-1.8 × H / 100 minutes. Then allow the stepped shaft to cool naturally in the air for 2-3 minutes, and then immerse the stepped shaft in a water tank for water cooling for 1.2-1.5 × H / 100 minutes. S6. Air cooling: Use a blower to cool the surface of the stepped shaft after water cooling in S5 until the temperature at the junction of adjacent steps drops to ≤150℃. S7. Tempering: Heat the stepped shaft after air cooling in S6 to 500-550℃ and hold it at that temperature for {(2.1~2.5)×H / 100} hours, then allow the stepped shaft to cool naturally to room temperature in the air.

[0011] Preferably, in S2, four or more spray devices are used to spray and cool the surface of the stepped shaft.

[0012] Preferably, in S6, four or more fans are used to blow air onto the surface of the stepped shaft for cooling.

[0013] According to the above technical solution, the beneficial effects of the present invention are: The large-section stepped shaft of this invention improves both the chemical composition and heat treatment methods, optimizing the heating, holding, and cooling processes for normalizing, quenching, and tempering. The normalizing holding temperature is higher than the quenching holding temperature, ensuring the complete transformation of the forging's initial microstructure (including any remaining coarse microstructure) into uniform austenite. This provides more driving forces and nucleation sites for austenite grains, achieving high-temperature recrystallization and complete austenitization. The cooling rate of the normalizing treatment is lower than that of the quenching treatment, maintaining a moderate cooling rate to suppress austenite grain growth. This avoids grain coarsening caused by excessively slow cooling and prevents the introduction of excessive stress as in quenching. This ensures that austenite does not have sufficient time to grow before the cooling transformation begins, resulting in fine ferrite and pearlite microstructures after the phase transformation. This breaks the microstructure inheritance, yielding a completely new, fine starting microstructure. Simultaneously, by heating to above the austenitizing temperature and holding at that temperature, carbon atoms can diffuse sufficiently, making the chemical composition of the entire cross-section more uniform. Subsequent air cooling will also make the hardness distribution more uniform. Uniform composition and hardness mean that during quenching, the entire workpiece can obtain a uniform austenitization effect, thereby forming a uniform martensitic structure, avoiding problems such as soft spots, insufficient hardness or inconsistent deformation caused by uneven structure.

[0014] The quenching and holding temperature is designed to ensure that all ferrite and cementite are completely dissolved through austenitization to form a uniform austenite, preparing for the subsequent martensitic transformation. At the same time, it avoids excessively high holding temperatures that could lead to overheating, which would cause a sharp increase in the driving force for austenite grain growth and enhanced atomic diffusion, preventing the originally fine austenite grains from rapidly merging and growing into abnormally large austenite grains. This results in a fine, uniform, strong, and tough martensitic structure. After tempering, the tensile strength of the large-section stepped shaft can reach over 1200 MPa, exhibiting excellent comprehensive mechanical properties and being less prone to cracking, thus meeting design requirements and ensuring high process safety. Detailed Implementation

[0015] A stepped shaft with large cross-section difference includes a large diameter section located in the middle and small diameter sections at both ends of the axial direction. The outer diameter of the large diameter section is 2700-2900mm, the outer diameter of the small diameter section is 500-550mm, and the diameter ratio of the large diameter section to the small diameter section is >4.

[0016] The chemical composition of the stepped shaft, by mass fraction, includes: 0.26%-0.30% C, 0.18%-0.35% Si, 0.30%-0.50% Mn, 1.40%-1.90% Cr, 3.30%-3.80% Ni, 0.28%-0.58% Mo, 0.07%-0.10% V, with the remainder being Fe and unavoidable impurities.

[0017] After heat treatment, the stepped shaft has a yield strength >1100MPa, tensile strength >1250MPa, elongation >11%, reduction of area >40%, and impact energy at room temperature >40J.

[0018] The above-mentioned heat treatment method for a stepped shaft with large cross-section difference includes the following steps: S1. Normalizing: Heat the stepped shaft to 880-920℃ and hold for a duration of {(1.5~1.8)×H / 100} hours, where H is the maximum diameter of the major diameter section in mm.

[0019] S2. Mist cooling: Use more than 4 spray devices to spray and cool the surface of the stepped shaft after S1 insulation until the temperature at the junction of adjacent steps drops to 300-350℃.

[0020] S3, air cooling, allows the stepped shaft after S2 mist cooling to cool naturally in the air until the temperature at the junction of adjacent steps drops to ≤150℃.

[0021] S4. Quenching: Heat the stepped shaft after air cooling in S3 to 810-840℃ and hold it at that temperature for {(1.5~1.8)×H / 100} hours.

[0022] S5. Water cooling: First, allow the stepped shaft after S4 insulation to cool naturally in the air for (2~3) minutes. Then, immerse the stepped shaft in a water tank for water cooling for {(3.0~3.3)×H / 100} minutes.

[0023] Then allow the stepped shaft to cool naturally in the air for 2-3 minutes, and then immerse it in a water tank for water cooling for 1.5-1.8 minutes.

[0024] Then allow the stepped shaft to cool naturally in the air for 2-3 minutes, and then immerse it in a water tank for water cooling for 1.2-1.5 minutes.

[0025] S6. Air cooling: Use more than 4 fans to blow air onto the surface of the stepped shaft after S5 water cooling until the temperature at the junction of adjacent steps drops to ≤150℃.

[0026] S7. Tempering: Heat the stepped shaft after air cooling in S6 to 500-550℃ and hold it at that temperature for {(2.1~2.5)×H / 100} hours, then allow the stepped shaft to cool naturally to room temperature in the air.

[0027] Example 1: A stepped shaft with a large cross-section difference includes a large diameter portion located in the middle and small diameter portions at both ends of the axial direction. The outer diameter of the large diameter portion is 2840 mm, and the outer diameter of the small diameter portion is 530 mm.

[0028] The chemical composition of the stepped shaft, by mass fraction, includes: 0.29% C, 0.25% Si, 0.39% Mn, 1.80% Cr, 3.65% Ni, 0.50% Mo, 0.09% V, with the remainder being Fe and unavoidable impurities.

[0029] The above-mentioned heat treatment method for a stepped shaft with large cross-section difference includes the following steps: S1. Normalizing: Heat the stepped shaft to 900℃ and hold for 44 hours.

[0030] S2. Mist cooling: Use more than 4 spray devices to spray and cool the surface of the insulated step shaft until the temperature at the junction of adjacent steps drops to 320℃.

[0031] S3. Air cooling: Allow the stepped shaft after fogging to cool naturally in the air until the temperature at the junction of adjacent steps drops to 130℃.

[0032] S4. Quenching: Heat the air-cooled stepped shaft to 810℃ and hold for 43 hours.

[0033] S5. Water cooling: First, allow the stepped shaft to cool naturally in the air for 3 minutes, then immerse the stepped shaft in a water tank for water cooling for 85.2 minutes.

[0034] Then, allow the stepped shaft to cool naturally in the air for 3 minutes, and then immerse it in a water tank for water cooling for 42.6 minutes.

[0035] Then, allow the stepped shaft to cool naturally in the air for 3 minutes, and then immerse it in a water tank for water cooling for 42.6 minutes.

[0036] S6. Air cooling: Use more than 4 fans to blow air onto the surface of the water-cooled stepped shaft until the temperature at the junction of adjacent steps drops to 120℃.

[0037] S7. Tempering: Heat the stepped shaft after air cooling in S6 to 530℃ and hold for 66 hours, then allow the stepped shaft to cool naturally to room temperature in the air.

[0038] Example 2: A stepped shaft with a large cross-section difference, comprising a large diameter portion in the middle and small diameter portions at both ends of the axial direction. The outer diameter of the large diameter portion is 2800mm, and the outer diameter of the small diameter portion is 510mm.

[0039] The chemical composition of the stepped shaft by mass fraction includes: 0.26% C, 0.35% Si, 0.30% Mn, 1.90% Cr, 3.30% Ni, 0.58% Mo, 0.07% V, with the remainder being Fe and unavoidable impurities.

[0040] The above-mentioned heat treatment method for a stepped shaft with large cross-section difference includes the following steps: S1. Normalizing: Heat the stepped shaft to 920℃ and hold for 43 hours.

[0041] S2. Mist cooling: Use more than 4 spray devices to spray and cool the surface of the insulated step shaft until the temperature at the junction of adjacent steps drops to 330℃.

[0042] S3. Air cooling: Allow the stepped shaft after fogging to cool naturally in the air until the temperature at the junction of adjacent steps drops to 135℃.

[0043] S4. Quenching: Heat the air-cooled stepped shaft to 830℃ and hold for 42 hours.

[0044] S5. Water cooling: First, allow the stepped shaft to cool naturally in the air for 2.5 minutes, then immerse the stepped shaft in a water tank for water cooling for 84 minutes.

[0045] Then allow the stepped shaft to cool naturally in the air for 2.5 minutes, and then immerse it in a water tank for water cooling for 42 minutes.

[0046] Then allow the stepped shaft to cool naturally in the air for 2.5 minutes, and then immerse it in a water tank for water cooling for 42 minutes.

[0047] S6. Air cooling: Use more than 4 fans to blow air onto the surface of the water-cooled stepped shaft until the temperature at the junction of adjacent steps drops to 125℃.

[0048] S7. Tempering: Heat the stepped shaft after air cooling in S6 to 520℃ and hold for 62 hours, then allow the stepped shaft to cool naturally to room temperature in the air.

[0049] Example 3: A stepped shaft with a large cross-section difference includes a large diameter section in the middle and small diameter sections at both ends of the axial direction. The outer diameter of the large diameter section is 2800 mm, and the outer diameter of the small diameter section is 510 mm.

[0050] The chemical composition of the stepped shaft by mass fraction includes: 0.30% C, 0.18% Si, 0.50% Mn, 1.40% Cr, 3.80% Ni, 0.28% Mo, 0.10% V, with the remainder being Fe and unavoidable impurities.

[0051] The above-mentioned heat treatment method for a stepped shaft with large cross-section difference includes the following steps: S1. Normalizing: Heat the stepped shaft to 880℃ and hold for 43 hours.

[0052] S2. Mist cooling: Use more than 4 spray devices to spray and cool the surface of the insulated step shaft until the temperature at the junction of adjacent steps drops to 310℃.

[0053] S3. Air cooling: Allow the stepped shaft after fogging to cool naturally in the air until the temperature at the junction of adjacent steps drops to 140℃.

[0054] S4. Quenching: Heat the air-cooled stepped shaft to 840℃ and hold for 42 hours.

[0055] S5. Water cooling: First, allow the stepped shaft to cool naturally in the air for 3 minutes, then immerse the stepped shaft in a water tank for water cooling for 84 minutes.

[0056] Then allow the stepped shaft to cool naturally in the air for 3 minutes, and then immerse it in a water tank for water cooling for 42 minutes.

[0057] Then allow the stepped shaft to cool naturally in the air for 3 minutes, and then immerse it in a water tank for water cooling for 42 minutes.

[0058] S6. Air cooling: Use more than 4 fans to blow air onto the surface of the water-cooled stepped shaft until the temperature at the junction of adjacent steps drops to 115℃.

[0059] S7. Tempering: Heat the stepped shaft after air cooling in S6 to 525℃ and hold for 62 hours, then allow the stepped shaft to cool naturally to room temperature in the air.

[0060] Table 1 shows the measured performance results of the three embodiments after heat treatment. All indicators can reach the design target value, and no cracking occurred.

[0061] Table 1 Mechanical Performance Test Results In summary, this invention enables the tensile strength of stepped shafts with large cross-section differences to reach over 1200 MPa, exhibiting excellent comprehensive mechanical properties and being less prone to cracking, thus meeting design requirements and demonstrating high process safety.

Claims

1. A stepped shaft with a large cross-section difference, comprising a large-diameter portion located in the middle and small-diameter portions at both axial ends, characterized in that: The outer diameter of the major diameter section is 2700-2900mm, the outer diameter of the minor diameter section is 500-550mm, and the diameter ratio of the major diameter section to the minor diameter section is >4; The chemical composition of the stepped shaft, by mass fraction, includes: 0.26%-0.30% C, 0.18%-0.35% Si, 0.30%-0.50% Mn, 1.40%-1.90% Cr, 3.30%-3.80% Ni, 0.28%-0.58% Mo, 0.07%-0.10% V, with the remainder being Fe and unavoidable impurities; After heat treatment, the stepped shaft has a yield strength >1100MPa, tensile strength >1250MPa, elongation >11%, reduction of area >40%, and impact energy at room temperature >40J.

2. The stepped shaft with large cross-section difference according to claim 1, characterized in that: The chemical composition of the stepped shaft, by mass fraction, includes: 0.29% C, 0.25% Si, 0.39% Mn, 1.80% Cr, 3.65% Ni, 0.50% Mo, 0.09% V, with the remainder being Fe and unavoidable impurities.

3. The stepped shaft with large cross-section difference according to claim 1, characterized in that: The chemical composition of the stepped shaft, by mass fraction, includes: 0.26% C, 0.35% Si, 0.30% Mn, 1.90% Cr, 3.30% Ni, 0.58% Mo, 0.07% V, with the remainder being Fe and unavoidable impurities.

4. The stepped shaft with large cross-section difference according to claim 1, characterized in that: The chemical composition of the stepped shaft, by mass fraction, includes: 0.30% C, 0.18% Si, 0.50% Mn, 1.40% Cr, 3.80% Ni, 0.28% Mo, 0.10% V, with the remainder being Fe and unavoidable impurities.

5. The heat treatment method for a stepped shaft with a large cross-section difference according to claim 1, characterized in that, Includes the following steps: S1. Normalizing: Heat the stepped shaft to 880-920℃ and hold for a duration of {(1.5~1.8)×H / 100} hours, where H is the maximum diameter of the major diameter section in mm; S2. Fogging: Use a spray device to spray and cool the surface of the stepped shaft after S1 insulation until the temperature at the junction of adjacent steps drops to 300-350℃. S3, air cooling, allowing the stepped shaft after S2 mist cooling to cool naturally in the air until the temperature at the junction of adjacent steps drops to ≤150℃; S4. Quenching: Heat the stepped shaft after air cooling in S3 to 810-840℃ and hold it at that temperature for {(1.5~1.8)×H / 100} hours. S5. Water cooling: First, allow the stepped shaft after S4 insulation to cool naturally in the air for (2~3) minutes. Then, immerse the stepped shaft in a water tank for water cooling for {(3.0~3.3)×H / 100} minutes. Then allow the stepped shaft to cool naturally in the air for 2-3 minutes, and then immerse the stepped shaft in a water tank for water cooling for 1.5-1.8 × H / 100 minutes. Then allow the stepped shaft to cool naturally in the air for 2-3 minutes, and then immerse the stepped shaft in a water tank for water cooling for 1.2-1.5 × H / 100 minutes. S6. Air cooling: Use a blower to cool the surface of the stepped shaft after water cooling in S5 until the temperature at the junction of adjacent steps drops to ≤150℃. S7. Tempering: Heat the stepped shaft after air cooling in S6 to 500-550℃ and hold it at that temperature for {(2.1~2.5)×H / 100} hours, then allow the stepped shaft to cool naturally to room temperature in the air.

6. The heat treatment method for a stepped shaft with a large cross-section difference according to claim 5, characterized in that: In S2, more than four spray devices are used to spray and cool the surface of the stepped shaft.

7. The heat treatment method for a stepped shaft with large cross-section difference according to claim 5, characterized in that: In S6, more than four fans are used to blow air onto the surface of the stepped shaft for cooling.

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