Preparation method of anti-fatigue fine-grain carburized layer of carburized steel

Abstract

The invention discloses a preparation method of an anti-fatigue fine-grain carburized layer of carburized steel, and aims to improve the anti-fatigue performance of carburized bearing steel. The preparation method of the anti-fatigue fine-grain carburizing layer comprises the steps that firstly, carburizing treatment is conducted on carburizing steel in a low-pressure vacuum atmosphere, a multi-section pulse process is adopted in the carburizing treatment, the carburizing temperature is controlled to be 950-960 DEG C, supplementary diffusion treatment is conducted, and after nitrogen cooling, the carburizing steel with the carburizing layer is obtained; secondly, the carburizing steel with the carburizing layer is subjected to high-temperature quenching, the quenching temperature is controlled to be 1050-1120 DEG C, and the submicron grain carburizing steel with the carburizing layer is obtained; and thirdly, subzero treatment is conducted on the carburizing steel with the submicron grains of the carburizing layer at the temperature ranging from-70 DEG C to-80 DEG C, and finally heat preservation tempering treatment is conducted multiple times at the temperature ranging from 540 DEG C to 550 DEG C. Through preparation of submicron crystals, grain refinement and promotion of dispersed precipitation of carbides are realized, the anti-fatigue performance of a carburized layer is improved, and the ultimate fatigue strength is improved by about 50 MPa.

Description

Technical Field

[0001] The invention relates to a method for preparing submicron grains of a carburized layer based on an optimized carburizing heat treatment process to improve the fatigue resistance of the carburized layer. Background Art

[0002] As a surface modification technology, carburizing improves the surface hardness and fatigue resistance of steel while maintaining the original toughness and ductility of steel by forming a gradient distribution of carbon content on the carburized layer. It is widely used in vulnerable parts such as gears and bearings. G13Cr4Mo4Ni4V bearing steel can meet the stringent requirements of stable operation of aviation components under high temperature loads through surface carburizing technology, and is an ideal material for modern aircraft engine main shaft bearings.

[0003] The carburizing process usually uses a temperature above the austenite transformation point AC3 because the austenite phase region has a high carbon solubility and can produce a thicker carburized layer in a shorter time. G13Cr4Mo4Ni4V is a high alloy steel, in which the Mo, V and Cr elements are very easy to combine with C to form carbides. The formation of a dispersed second phase is one of the main mechanisms of carburizing strengthening. Affected by the enrichment of alloy elements at the grain boundary, G13Cr4Mo4Ni4V steel is prone to form large-sized M during long-term carburization. 23 C6 and M7C3 carbides, and even network carbides, reduce the fatigue resistance of the material. Therefore, high temperature quenching and subsequent deep cooling, tempering and other treatments (collectively referred to as carburizing heat treatment process) are used after carburizing to improve the structure and performance after carburizing.

[0004] Improving the mechanical properties of materials by refining grains is a common method. In recent years, scholars have found that second-phase particles, especially particles larger than 1 μm, can accelerate recrystallization kinetics through particle stimulated nucleation mechanism (PSN mechanism), refine grain structure, reduce texture strength, and improve the organization and mechanical properties of materials. In the carburizing process, due to the combined effect of phase transformation stress and lattice expansion caused by C atoms penetrating the surface, high stress is generated in the carburized layer. The growth of carbides and the expansion of austenite occupy each other's space and produce high interface stress. Summary of the invention

[0005] The purpose of the present invention is to improve the fatigue resistance of carburized bearing steel and to provide a method for refining carburized layer grains by optimizing the carburizing heat treatment process.

[0006] The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel of the present invention is implemented by the following steps:

[0007] 1. Preparation of large-sized carbides in carburized layer:

[0008] Carburizing steel is carburized in a low-pressure vacuum atmosphere. The carburizing process adopts a multi-stage pulse process, and the carburizing temperature is controlled to be 950-960°C. The multi-stage pulse process is alternately performed by a strong carburizing cycle and a diffusion cycle. A mixed gas of acetylene and nitrogen is introduced during the strong carburizing cycle, and the carburizing time is controlled to be 25-30 hours. Finally, a supplementary diffusion treatment is performed. After the carburizing is completed, nitrogen is introduced for cooling to obtain a carburized steel with a carburized layer.

[0009] 2. Quenching to prepare submicron crystals:

[0010] Carburized steel with a carburized layer is subjected to high temperature quenching, the quenching temperature is controlled to be 1050-1120°C, and the temperature is kept for 35-45 minutes. After the temperature is kept, nitrogen cooling is performed to obtain carburized steel with submicron crystals in the carburized layer;

[0011] 3. Deep cooling and tempering process:

[0012] The carburized steel with submicron carburized layer is cryogenically treated at -70℃~-80℃ for 1.5~2.5h, and finally subjected to heat preservation and tempering treatment at 540~550℃ for multiple times to obtain fatigue-resistant fine-grained carburized layer on the surface of the carburized steel.

[0013] The present invention provides a method for refining the grains of the carburized layer by optimizing the carburizing heat treatment process, and increases the storage stress of the carburized layer while preparing large-sized carbides by extending the carburizing time. Subsequently, high-temperature quenching is used, and the carburizing storage stress release is used as a driving force to promote the PSN of carbides to stimulate recrystallization nucleation and intracrystalline dislocation movement, prepare carburized layer recrystallization and subgrains, and refine the grains of the carburized layer. In the subsequent deep cooling and tempering process, submicron crystals refine the size of martensite and promote the dispersion and precipitation of carbides. Through friction and wear tests, it is proved that the preparation of submicron crystals improves the fatigue resistance of the carburized layer by nearly 50%.

[0014] Compared with the prior art, the fine-grained preparation method for improving fatigue resistance of the carburized layer of carburized steel of the present invention has the following beneficial effects:

[0015] The present invention proposes a method for preparing fine grains for improving the fatigue resistance of the carburized layer of carburized steel. The method prepares recrystallization and subgrains by optimizing the carburizing heat treatment process, and refines the grains of the carburized layer to the submicron level. The traditional process for preparing recrystallization and subgrains is to heat up after cold deformation or hot deformation. The method of the present invention prepares recrystallization without applying external stress, saving time and process costs. Through the preparation of submicron crystals, the grains are refined and the dispersion and precipitation of carbides are promoted. The fatigue resistance of the carburized layer is greatly improved, and the wear amount is reduced by nearly 50%. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1This is an electron microscope image of the submicron crystals formed after quenching in step 2 of the embodiment;

[0017] Figure 2 It is the martensite transmission morphology image of the non-submicron crystal sample and the submicron crystal sample in the embodiment;

[0018] Figure 3 These are test diagrams of the effect of submicron crystals on fatigue resistance in the embodiments, where (a) is a SN curve diagram and (b) is a test diagram of fatigue limit tested by the lifting method. DETAILED DESCRIPTION

[0019] Specific implementation method 1: The fine grain preparation method for improving the fatigue resistance of the carburized layer of carburized steel in this implementation method is implemented according to the following steps:

[0020] 1. Preparation of large-sized carbides in carburized layer:

[0021] Carburizing steel is carburized in a low-pressure vacuum atmosphere. The carburizing process adopts a multi-stage pulse process, and the carburizing temperature is controlled to be 950-960°C. The multi-stage pulse process is alternately performed by a strong carburizing cycle and a diffusion cycle. A mixed gas of acetylene and nitrogen is introduced during the strong carburizing cycle, and the carburizing time is controlled to be 25-30 hours. Finally, a supplementary diffusion treatment is performed. After the carburizing is completed, nitrogen is introduced for cooling to obtain a carburized steel with a carburized layer.

[0022] 2. Quenching to prepare submicron crystals:

[0023] Carburized steel with a carburized layer is subjected to high temperature quenching, the quenching temperature is controlled to be 1050-1120°C, and the temperature is kept for 35-45 minutes. After the temperature is kept, nitrogen cooling is performed to obtain carburized steel with submicron crystals in the carburized layer;

[0024] 3. Deep cooling and tempering process:

[0025] The carburized steel with submicron carburized layer is cryogenically treated at -70℃~-80℃ for 1.5~2.5h, and finally subjected to heat preservation and tempering treatment at 540~550℃ for multiple times to obtain fatigue-resistant fine-grained carburized layer on the surface of the carburized steel.

[0026] In this embodiment, long-term carburizing treatment is used to prepare large-sized carbides that can produce PSN mechanism. Quenching releases the carburized stored stress, providing a driving force for recrystallization and the generation of subgrains. Quenching increases the carbon content of the matrix, and a stable austenite phase is formed after quenching. After deep cooling and tempering, the submicron crystal area of ​​the carburized layer produces fine high-carbon martensite and a large number of dispersed carbides.

[0027] This embodiment proposes a method for preparing micro-nano crystals: by extending the carburizing time to promote the generation of large-sized M7C3 phases, using the interface energy stored between carbides and the matrix during the carburizing process as a driving force to promote the PSN process of the M7C3 phase in the quenching austenitization temperature range to prepare micro-nano crystals near the surface of the carburized layer. This method realizes the preparation of carburized micro-nano crystal areas by adjusting the heat treatment process without applying external force, thereby improving the fatigue resistance of the carburized layer.

[0028] Specific implementation method 2: The difference between this implementation method and specific implementation method 1 is that the carburizing steel in step 1 is G13Cr4Mo4Ni4V carburizing steel.

[0029] Specific implementation method three: The difference between this implementation method and specific implementation method one or two is that in step one, the carburized steel is preliminarily subjected to stress relief annealing treatment, and the stress relief annealing treatment is carried out at 680° C. for 3.5 to 5 hours.

[0030] Specific implementation method 4: This implementation method is different from any one of specific implementation methods 1 to 3 in that the carburizing temperature in step 1 is controlled to be 960°C.

[0031] Specific implementation mode 5: This implementation mode is different from specific implementation modes 1 to 4 in that the flow ratio of acetylene to nitrogen in the acetylene and nitrogen mixed gas in step 1 is 1:1.

[0032] Specific implementation method 6: The difference between this implementation method and specific implementation methods 1 to 5 is that the time for the supplementary diffusion treatment in step 1 is 4 to 6 hours.

[0033] Specific embodiment 7: The difference between this embodiment and any one of specific embodiments 1 to 6 is that the residual stress on the surface of the carburized steel with the carburized layer in step 1 is 750-850 MPa.

[0034] Specific implementation eight: This implementation differs from specific implementations one to seven in that the nitrogen ejection pressure in step two is controlled to be 2 bar during the nitrogen cooling process.

[0035] Specific embodiment 9: The difference between this embodiment and any one of specific embodiments 1 to 8 is that in step 3, the carburized steel with submicron crystals in the carburized layer is cryogenically treated at a temperature of -75°C, and the cryogenic treatment time is 2 hours.

[0036] Specific embodiment ten: This embodiment differs from specific embodiments one to nine in that in step three, the temperature is kept at 545° C. for 2 hours, cooled to room temperature for tempering treatment, and the tempering treatment is repeated three times.

[0037] Embodiment: The fine grain preparation method for improving fatigue resistance of carburized layer of carburized steel in this embodiment is implemented according to the following steps:

[0038] 1. Preparation of large-sized carbides in carburized layer:

[0039] G13Cr4Mo4Ni4V carburizing steel was carburized in a low-pressure vacuum atmosphere. The carburizing process used a multi-stage pulse process, and the carburizing temperature was controlled at 960°C. The multi-stage pulse process was alternately performed by a strong carburizing cycle and a diffusion cycle. The time of the strong carburizing cycle and the diffusion cycle is shown in Table 1.

[0040] In each strong carburizing cycle, a mixed gas of acetylene and nitrogen is introduced, the flow ratio of acetylene to nitrogen is 1:1, the total flow rate is 33.33sccm, the carburizing treatment time is controlled to be 25h, the air intake is closed in each diffusion cycle, and a high-power mechanical pump is used to discharge the carburizing gas. Finally, the diffusion treatment is supplemented at 960℃ for 5h to prevent the formation of network carbides and brittle cracking. After the carburizing is completed, 1.5bar high-pressure nitrogen is introduced for cooling to obtain a carburized steel with a carburized layer. The residual stress on the surface after carburizing can reach 781.2±15MPa.

[0041] 2. Quenching to prepare submicron crystals:

[0042] The carburized steel with carburized layer was subjected to high temperature quenching. The quenching temperature was first increased to 850°C and kept for 30 minutes to ensure uniform heating of the sample. Then the temperature was increased to 1100°C and kept for 40 minutes. Nitrogen cooling was performed after the end of the holding period. The nitrogen pressure was controlled to be 2 bar. After the quenching, the stored stress of the carburized layer was released, driving the M7C3 phase PSN mechanism to stimulate the formation of recrystallization, promoting the movement of dislocations within the crystal to be entangled by the pinning effect of nano-scale carbides within the crystal to form sub-grain boundaries, and obtaining carburized steel with submicron grains of carburized layer.

[0043] 3. Deep cooling and tempering process:

[0044] The carburized steel with submicron crystals in the carburized layer was cryogenically treated at -75°C for 2 hours, and then kept at 545°C for 2 hours, cooled to room temperature for tempering, and the tempering treatment was repeated three times. The cryogenic and tempering reduced the residual austenite content to less than 5%, avoiding the influence of the residual austenite transformation on the dimensional accuracy of the bearing steel. The submicron crystals refined the size of the martensite and promoted the dispersion and precipitation of carbides during the tempering process, completing the fine grain preparation method for improving the carburized layer of the carburized steel.

[0045] Table 1 Carburizing and diffusion time

[0046] Strong penetration diffusion Strong penetration diffusion Strong penetration diffusion Strong penetration diffusion Strong penetration diffusion 113s 10s 112s 130s 81s 190s 74s 250s 71s 310s Strong penetration diffusion Strong penetration diffusion Strong penetration diffusion Strong penetration diffusion Strong penetration diffusion 69s 370s 68s 430s 67s 490s 66s 550s 66s 610s Strong penetration diffusion Strong penetration diffusion ...... diffusion 66s 670s 66s 730s ...... 5h

[0047] In this embodiment, the depth of the submicron crystal layer after carburizing heat treatment is 420μm. Therefore, there are different grinding allowances before carburizing heat treatment, which are 200μm and 500μm respectively, to ensure that submicron crystals exist on the surface of one sample and no submicron crystals exist on the surface of the other sample. Two types of rotary bending fatigue specimens with submicron crystals on the surface and without submicron crystals on the surface are prepared.

[0048] A simply supported beam rotary bending testing machine of model QBWP-6000 was used to test fatigue life and ultimate fatigue strength. The speed range of the equipment is 1500-6000r / min (corresponding frequency 25-100Hz), and the maximum loading stress is 2500MPa. The temperature of the test process is 25±1℃, the stress ratio is -1, and the speed is 5000r / min (83.33Hz). The residual stress on the surface of the rotary bending specimen was tested. The surface residual stress of the specimen with submicron crystals is -743.2MPa, and the surface residual stress of the specimen without submicron crystals is -749.3MPa. The stress of the two is equivalent, so the effect of residual stress on fatigue performance can be ignored. The SN curve test is carried out by the throwing point method. When the specimen is fatigued, it stops. The number of cycles exceeds 107 to pass. The lifting method is used to measure the fatigue limit. The fatigue limit is calculated according to the formula:

[0049]

[0050] Among them, σ 50 is the final fatigue strength, in MPa; m is the total number of valid tests, in this test m=13, n is the test horizontal stress level; υ i The corresponding stress is σ i The number of samples in the level; σ i is the stress level of the ith level, in MPa.

[0051] Figure 1 These are submicron crystals formed after quenching. It can be seen that the submicron crystals formed by quenching after carburizing include recrystallization at the grain boundary and subgrains within the crystal.

[0052] Figure 2 This is the transmission morphology of martensite after tempering. It can be seen that due to the influence of grain refinement in the submicron crystal region after quenching, the size of the martensite transformation process is restricted, and the size of the martensite is refined.

[0053] Figure 3 The test diagram of the effect of submicron crystals on fatigue resistance, where (a) is the SN curve and (b) is the fatigue limit tested by the lifting method. The preparation of submicron crystals improves the fatigue life of the material. After calculation, the ultimate fatigue life of the sample with fine grain structure is 1130.8MPa, and the ultimate fatigue life of the sample without fine grain structure is 1080.8MPa. The ultimate fatigue strength is increased by nearly 50MPa.

[0054] In summary, the present invention promotes the formation of large-sized second phase particles (M7C3) near the grain boundary by extending the carburizing time, and increases the storage stress of the carburized layer. High temperature quenching is then carried out, which accelerates the release of storage stress to provide driving force on the one hand, and promotes the particle stimulation nucleation mechanism and intracrystalline dislocation movement of the M7C3 phase on the other hand, and successfully prepares recrystallization and subgrains in the carburized layer, and the grains are refined to submicron level. Subsequent deep cooling process is adopted to promote the transformation of austenite into high-strength high-carbon martensite, and submicron grains refine the martensite size after deep cooling. After deep cooling, tempering process is adopted to promote the transformation of retained austenite and the precipitation of dispersed carbides, and a large number of grain boundaries and subgrain boundaries in the submicron crystal region promote the precipitation of tempered carbides. By the above-mentioned carburizing heat treatment process, the carburized layer grain size is refined, and the overall fatigue life of G13Cr4Mo4Ni4V steel is successfully improved, and the ultimate fatigue strength is improved by nearly 50MPa.

Claims

1. A method for preparing a fatigue-resistant fine-grained carburized layer of carburized steel, characterized in that The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel is implemented by the following steps:

1. Preparation of large-sized carbides in carburized layer: Carburizing steel is carburized in a low-pressure vacuum atmosphere. The carburizing process adopts a multi-stage pulse process, and the carburizing temperature is controlled to be 950-960°C. The multi-stage pulse process is alternately performed by a strong carburizing cycle and a diffusion cycle. A mixed gas of acetylene and nitrogen is introduced during the strong carburizing cycle, and the carburizing time is controlled to be 25-30 hours. Finally, a supplementary diffusion treatment is performed. After the carburizing is completed, nitrogen is introduced for cooling to obtain a carburized steel with a carburized layer.

2. Quenching to prepare submicron crystals: Carburized steel with a carburized layer is subjected to high temperature quenching, the quenching temperature is controlled to be 1050-1120°C, and the temperature is kept for 35-45 minutes. After the temperature is kept, nitrogen cooling is performed to obtain carburized steel with submicron crystals in the carburized layer; 3. Deep cooling and tempering process: The carburized steel with submicron carburized layer is cryogenically treated at -70℃~-80℃ for 1.5~2.5h, and finally subjected to heat preservation and tempering treatment at 540~550℃ for multiple times to obtain fatigue-resistant fine-grained carburized layer on the surface of the carburized steel.

2. The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel according to claim 1, characterized in that In step 1, the carburizing steel is G13Cr4Mo4Ni4V carburizing steel.

3. The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel according to claim 1, characterized in that In step 1, the carburized steel is subjected to stress relief annealing treatment in advance, and the stress relief annealing treatment is carried out at 680° C. for 3.5 to 5 hours.

4. The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel according to claim 1, characterized in that In step 1, the carburizing temperature is controlled to be 960°C.

5. The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel according to claim 1, characterized in that In step 1, the flow ratio of acetylene to nitrogen in the acetylene and nitrogen mixed gas is 1:

1.

6. The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel according to claim 1, characterized in that The time for the supplementary diffusion treatment in step 1 is 4 to 6 hours.

7. The method for preparing a carburized steel fatigue-resistant fine-grained carburized layer according to claim 1, characterized in that The residual stress on the surface of the carburized steel with the carburized layer in step 1 is 750-850 MPa.

8. The method for preparing the fatigue-resistant fine-grained carburized layer of carburized steel according to claim 1, characterized in that During the nitrogen cooling process in step 2, the nitrogen ejection pressure is controlled to be 2 bar.

9. The method for preparing a carburized steel fatigue-resistant fine-grained carburized layer according to claim 1, characterized in that In step 3, the carburized steel with submicron grains in the carburized layer is cryogenically treated at a temperature of -75°C, and the cryogenic treatment time is 2 hours.

10. The method for preparing a fatigue-resistant fine-grained carburized layer of carburized steel according to claim 1, characterized in that In step 3, the temperature is kept at 545° C. for 2 hours, cooled to room temperature for tempering, and the tempering treatment is repeated three times.

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