A heat treatment method for improving mechanical properties of a rare earth M50NiL steel after carburizing

Abstract

This invention relates to the field of heat treatment technology for M50NiL bearing steel used in aero-engines, specifically a heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing. The chemical composition and mass percentage of the rare earth M50NiL steel are as follows: C 0.11–0.15 wt.%, Si 0.10–0.25 wt.%, Cr 4.00–4.25 wt.%, Mo 4.00–4.50 wt.%, V 1.13–1.33 wt.%, Ni 3.20–3.60 wt.%, Mn 0.15–0.35 wt.%, Ce and La total ≤0.02 wt.%, P ≤0.01 wt.%, S ≤0.01 wt.%, with the balance being Fe. First, the cast steel ingots meeting the composition requirements are heated and held at a certain temperature, then subjected to three-dimensional forging, normalizing, spheroidizing annealing, and machining, followed by surface carburizing. Then, they undergo high-temperature tempering, quenching, cryogenic treatment, and three tempering heat treatments in sequence. This invention addresses the heat treatment process of rare earth M50NiL steel after carburizing for aero-engines. By using cryogenic treatment at different temperatures, the microstructure from the surface to the core is improved, significantly enhancing the mechanical properties of the carburized rare earth M50NiL steel and meeting the requirements for its use in aero-engine bearings.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for M50NiL bearing steel for aero-engines, specifically a heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing. Background Technology

[0002] Currently, M50NiL steel is the second-generation bearing steel used in aero-engine main shaft bearings. It was developed based on M50 steel by reducing the carbon (C) and increasing the nickel (Ni) content. The lower carbon content allows for a finer, more dispersed distribution of carbides, solving the problem of large-sized carbides in M50 steel. Simultaneously, it increases fracture toughness by more than two times and exhibits "external hardness and internal toughness" after carburizing, thus broadening its application prospects. During service, M50NiL steel has a surface carbon content of 0.85–1.10 wt.%, a surface hardness of 700–800 HV (60–64 HRC), a core hardness of 430–490 HV (43–48 HRC), and an effective hardened layer thickness of 1.8–2.2 mm. The hardness distribution of M50NiL bearings abroad is shown below. Figure 4 (2) As shown. However, due to the high C and alloy content of the carburized layer, the austenite is very stable. Therefore, a large amount of residual austenite will exist in the carburized layer structure after quenching. Numerous studies have also shown that a large amount of residual austenite still exists in the carburized layer after tempering. Therefore, M50NiL steel has problems such as unstable structure, poor dimensional stability, low hardness, and insufficient wear resistance after carburizing, which seriously affects the service life of bearing steel.

[0003] Therefore, the most direct way to reduce the retained austenite content in the carburized layer and improve the performance and service life of M50NiL steel parts is through cryogenic treatment. This involves further cooling the material, which has been rapidly cooled to room temperature, to an even lower temperature, providing continued supercooling to transform the retained austenite and thus improve the performance and dimensional stability of the parts. Current research on M50NiL steel largely focuses on the carburizing process and subsequent quenching and tempering processes, but research on the application and impact of cryogenic treatment is limited. Cryogenic treatment may be an important way to improve the mechanical properties of carburized M50NiL steel. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention aims to propose a heat treatment method to improve the mechanical properties of rare earth M50NiL steel after carburizing. Based on the design concept of cryogenic treatment, which can further cool materials that have been rapidly cooled to room temperature to an even lower temperature to provide additional supercooling to transform the retained austenite, thereby improving the performance and dimensional stability of the parts, the heat treatment process improves the microstructure of carburized rare earth M50NiL steel from the surface to the core, effectively reducing the content of retained austenite and significantly improving the mechanical properties of carburized rare earth M50NiL steel, thus meeting the requirements for the use of M50NiL steel in bearings for aero-engines.

[0005] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0006] A heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing, wherein the chemical composition and mass percentage of the rare earth M50NiL steel are as follows: C 0.11-0.15 wt.%, Si 0.10-0.25 wt.%, Cr 4.00-4.25 wt.%, Mo 4.00-4.50 wt.%, V 1.13-1.33 wt.%, Ni 3.20-3.60 wt.%, Mn 0.15-0.35 wt.%, Ce and La total ≤0.02 wt.%, P ≤0.01 wt.%, S ≤0.01 wt.%, with the balance being Fe;

[0007] The heat treatment method includes forging, normalizing, spheroidizing annealing, machining, surface carburizing, high-temperature tempering, quenching, cryogenic treatment, three-stage tempering, and final testing. The specific steps are as follows:

[0008] (1) Forging: The steel ingot is made by a double vacuum smelting method of vacuum induction + vacuum self-consumption, and is subjected to three-dimensional forging. The forging temperature is 900~1120℃. After forging, the surface temperature is air-cooled until it drops to 500~600℃, and then furnace-cooled to room temperature to eliminate stress.

[0009] (2) Normalizing: The steel ingot is normalized after forging. The heating temperature is 1100±20℃. After holding at the temperature for 1 to 5 hours, it is air-cooled to room temperature.

[0010] (3) Spheroidizing annealing: After normalizing, the steel ingot is first heated to 650±20℃ at a heating rate of 100~200℃ / h for 0.5~1h preheating and holding, then heated to 850±20℃ at a heating rate of 100~200℃ / h for 4~5h, and cooled to 740±20℃ at a rate of 20~40℃ / h for 6~8h, and then cooled to room temperature in the furnace.

[0011] (4) Machining: After spheroidizing annealing, the steel ingot is cut into samples of specified size by wire cutting. The surface of the sample is rough ground to facilitate surface carburization.

[0012] (5) Surface carburizing: After machining, gas carburizing is carried out in a carburizing atmosphere created by methanol cracking. The carburizing temperature is 930±20℃, the carbon potential range is 0.8~1.2%C, and the carburizing is carried out for 30~40h by strong carburizing + diffusion carburizing. Finally, the furnace is cooled to room temperature.

[0013] (6) High temperature tempering: After surface carburizing treatment, high temperature tempering is carried out. The tempering temperature is 750±20℃, the holding time is 4~6h, the furnace is cooled to 550±20℃, and then the furnace is removed and air-cooled to room temperature.

[0014] (7) Quenching: After high-temperature tempering, first heat to 650±20℃ at a heating rate of 100~200℃ / h for 0.5~1h preheating and holding, then heat to 850±20℃ at a heating rate of 100~200℃ / h for 0.5~1h preheating and holding; finally heat to quenching temperature of 1100±10℃ at a heating rate of 100~200℃ / h, holding for 0.5~1h, and oil cool to room temperature;

[0015] (8) Deep cryogenic treatment: After quenching, deep cryogenic treatment is carried out, cooling down to -196 to -40℃ at a cooling rate of 25 to 100℃ / h, and holding at this temperature for 2 to 4 hours. By lowering the temperature, the unstable residual austenite is further transformed into stable martensite, and then the temperature is raised to room temperature at the same rate.

[0016] (9) Three tempering: After cryogenic treatment, three tempering treatments are performed. Each heating temperature is 500-580℃ and the holding time is 1-3h. Then, the mixture is air-cooled to room temperature.

[0017] The heat treatment method described above for improving the mechanical properties of rare earth M50NiL steel after carburizing has a total Ce and La content of 0.01 to 0.02 wt.%, and a Ce to La mass fraction ratio of 1:2.

[0018] In the heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing, during the final testing process, the 0-1mm carburized layer is regarded as the machining allowance according to the surface carbon content requirements, and the 1mm subsurface is regarded as the actual surface.

[0019] In the heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing, in step (2), during the normalizing heating process, a stepped preheating and heat preservation process is carried out: preheating and heat preservation is carried out at 650±20℃ for 0.5~1h, and preheating and heat preservation is carried out at 850±20℃ for 0.5~1h.

[0020] In the heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing, in step (5), the rare earth M50NiL steel is ultrasonically cleaned with ethanol before surface carburizing, and the high-temperature tempering, quenching and three tempering processes after surface carburizing are all carried out in a vacuum heat treatment furnace.

[0021] In the heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing, in step (5), the strong carburizing time is 10-20h and the carbon potential is 1.0-1.2%C; the diffusion carburizing time is 15-25h and the carbon potential is 0.8-1.0%C.

[0022] The design concept of this invention is:

[0023] After carburizing, M50NiL steel has a high C and alloy content in the carburized layer. The decrease in the martensitic transformation temperature makes the austenite very stable, resulting in a large amount of retained austenite in the carburized layer after quenching. This invention addresses this issue by utilizing the idea that unstable retained austenite transforms into stable martensite at low temperatures during cryogenic treatment, thereby improving the mechanical properties of the parts. First, the cast steel ingot, meeting the composition requirements, is heated and held at a certain temperature before undergoing triaxial forging, normalizing, spheroidizing annealing, and machining, followed by surface carburizing. Then, it undergoes high-temperature tempering, quenching, cryogenic treatment, and three tempering heat treatments in sequence. This invention focuses on the heat treatment process of rare earth M50NiL steel for aero-engines after carburizing. By using cryogenic treatment at different temperatures, the microstructure from the surface to the core is improved, effectively reducing the content of retained austenite and significantly improving the mechanical properties of the carburized rare earth M50NiL steel. Simultaneously, the addition of rare earth elements Ce and La does not change the material's heat treatment process. The research findings have significant value for practical industrial applications.

[0024] The advantages and beneficial effects of this invention are:

[0025] 1. This invention adds small amounts of rare earth elements Ce and La to the original M50NiL steel, and performs performance heat treatment on the carburized rare earth M50NiL steel, especially deep cryogenic treatment at different temperatures. This involves further cooling the material, which has been rapidly cooled to room temperature, to even lower temperatures to provide additional supercooling and facilitate the transformation of retained austenite. This comprehensively improves the hardness, impact energy, tensile strength, and other mechanical properties of the rare earth M50NiL steel. This invention aligns with the current development trend of bearing steel for aero-engines and has a very broad application prospect.

[0026] 2. This invention involves heat treating carburized rare earth M50NiL steel to achieve a surface hardness of 800–900 HV. 0.1 The core hardness value is 440-490 HV. 0.1The effective hardened layer thickness is stable at 1.8–2.4 mm. Compared with foreign M50NiL bearings, the surface hardness and effective hardened layer thickness are improved, meeting the requirements for the use of M50NiL steel in aero-engine bearings. Attached Figure Description

[0027] Figure 1 This figure shows the carbon content distribution of rare earth M50NiL steel after carburizing. The horizontal axis (Distance from the surface) represents the distance from the surface, and the vertical axis (Carbon content) represents the carbon content.

[0028] Figure 2 SEM image of the carburized layer at a distance of 0.7 mm from the surface after quenching (Q) and cryogenic treatment at -80℃ (DC80) on carburized rare earth M50NiL steel.

[0029] Figure 3 The figure shows the distribution of residual austenite content in the carburized layer after quenching (Q) and cryogenic treatment at -80℃ (DC80) on carburized rare earth M50NiL steel. The horizontal axis (Distance from the surface) represents the distance from the surface, and the vertical axis (RA content) represents the residual austenite content.

[0030] Figure 4 (1) The microhardness distribution of the carburized layer after quenching (Q) and deep cryogenic treatment at -80℃ (DC80) on rare earth M50NiL steel after carburizing. Figure 4 (2) The microhardness distribution of rare earth M50NiL steel after carburizing was compared with that of foreign M50NiL bearings after undergoing deep cryogenic treatment at -80℃ and non-deep cryogenic treatment, followed by three tempering processes (replaced by DC80-TTT and TTT respectively). In the figure, the horizontal axis (Distance from the surface) represents the distance from the surface, and the vertical axis (Hardness) represents the microhardness.

[0031] Figure 5 The mechanical properties of carburized rare earth M50NiL steel after cryogenic treatment at -80℃ and without cryogenic treatment, followed by three tempering processes (denoted as DC80-TTT and TTT respectively), are compared. The figure includes the yield strength R. p0.2 (MPa), tensile strength Rm (MPa), impact energy KW2 (J), surface hardness (HV) 0.1 ), Effective hardened layer thickness (mm). Detailed Implementation

[0032] In practical implementation, the heat treatment method of the present invention for improving the mechanical properties of rare earth M50NiL steel after carburizing is as follows:

[0033] The chemical composition and mass percentage of rare earth M50NiL steel are as follows: C 0.11~0.15wt.%, Si 0.10~0.25wt.%, Cr 4.00~4.25wt.%, Mo 4.00~4.50wt.%, V 1.13~1.33wt.%, Ni 3.20~3.60wt.%, Mn 0.15~0.35wt.%, Ce and La total ≤0.02wt.%, P≤0.01wt.%, S≤0.01wt.%, with the balance being Fe. The mass fraction ratio of Ce to La is 1:2.

[0034] The heat treatment method includes forging, normalizing, spheroidizing annealing, machining, surface carburizing, high-temperature tempering, quenching, cryogenic treatment, three-stage tempering, and final testing. The specific steps are as follows:

[0035] (1) Forging: The steel ingot is made by a double vacuum smelting method of vacuum induction + vacuum self-consumption, and then the three-dimensional forging process is adopted. The forging processing temperature is 900~1120℃. After forging, the surface temperature is air-cooled until it drops to 500~600℃, and then furnace-cooled to room temperature to eliminate stress.

[0036] (2) Normalizing: After forging, the steel ingot is normalized at a heating temperature of 1100±20℃ and held at that temperature for 1 to 5 hours before being air-cooled to room temperature. During the normalizing process, a stepped preheating and holding process is performed: preheating and holding at 650±20℃ for 0.5 to 1 hour, and preheating and holding at 850±20℃ for 0.5 to 1 hour, to ensure uniform temperature of the material during the heat treatment process.

[0037] (3) Spheroidizing annealing: After normalizing, the steel ingot is first heated to 650±20℃ at a heating rate of 100~200℃ / h for 0.5~1h preheating and holding, then heated to 850±20℃ at a heating rate of 100~200℃ / h for 4~5h, then cooled to 740±20℃ at a rate of 20~40℃ / h for 6~8h, and then cooled to room temperature in the furnace.

[0038] (4) Machining: After spheroidizing annealing, the steel ingot is cut into samples of specified dimensions by wire cutting. Then, the surface of the sample is rough ground to facilitate surface carburizing.

[0039] (5) Surface carburizing: After machining, gas carburizing is carried out in a carburizing atmosphere created by methanol cracking. The carburizing temperature is 930±20℃, the carbon potential range is 0.8~1.2%C, and the carburizing is carried out for 30~40h by strong carburizing + diffusion carburizing. Finally, the furnace is cooled to room temperature.

[0040] (6) High-temperature tempering: After surface carburizing treatment, high-temperature tempering is carried out at a temperature of 750±20℃ and a holding time of 4 to 6 hours. Then, the furnace is cooled to 550±20℃ and then air-cooled to room temperature. On the one hand, the residual stress during surface carburizing can be eliminated, and on the other hand, the hardness can be reduced to remove excess carburized layer.

[0041] (7) Quenching: After high-temperature tempering, heat to 650±20℃ at a heating rate of 100~200℃ / h and preheat for 0.5~1h. Then heat to 850±20℃ at a heating rate of 100~200℃ / h and preheat for 0.5~1h. Finally, heat to quenching temperature of 1100±10℃ at a heating rate of 100~200℃ / h and hold for 0.5~1h to dissolve the carbides. Then oil cool to room temperature.

[0042] (8) Deep cryogenic treatment: After quenching, deep cryogenic treatment is carried out, cooling down to -196 to -40℃ at a cooling rate of 25 to 100℃ / h, and holding at this temperature for 2 to 4 hours. By lowering the temperature, the unstable residual austenite is further transformed into stable martensite, and then the temperature is raised to room temperature at the same rate.

[0043] (9) Three tempering: After deep cooling, three tempering treatments are performed, with each heating temperature being 500-580℃ and the holding time being 2h, followed by air cooling to room temperature.

[0044] Before carburizing, rare earth M50NiL steel was ultrasonically cleaned with ethanol. After carburizing, high-temperature tempering, quenching, and three tempering processes were all carried out in a vacuum heat treatment furnace. During surface carburizing, the strong carburizing time was 10-20 hours, and the carbon potential was 1.0-1.2%C. The diffusion carburizing time was 15-25 hours, and the carbon potential was 0.8-1.0%C. In the final testing process, according to the surface carbon content requirements, the 0-1 mm carburized layer was regarded as the machining allowance, and the 1 mm subsurface was regarded as the actual surface.

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the following embodiments are provided to further illustrate this invention, thereby providing a clearer and more explicit definition of the scope of protection of this invention.

[0046] Example 1

[0047] In this embodiment (numbered TTT), a heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing is determined, wherein:

[0048] The chemical composition and mass percentage of rare earth M50NiL steel are as follows: C 0.13wt.%, Si 0.23wt.%, Cr 4.19wt.%, Mo 4.27wt.%, V 1.18wt.%, Ni 3.38wt.%, Mn 0.28wt.%, Ce and La total 0.012wt.% (Ce to La mass fraction ratio is 1:2), P 0.005wt.%, S 0.0011wt.%, with the balance being Fe, for a total of 100wt.%.

[0049] (1) First, the ingredients are prepared according to the above chemical composition mass ratio, and steel ingots are made by double vacuum smelting method of vacuum induction + vacuum self-consumption; then, the three-dimensional forging process is adopted, the steel ingot is heated to 1120°C at a heating rate of 100°C / h and fully burned through before forging, the final forging temperature is 900°C, the final cross-sectional size is 75mm×75mm, after forging, the surface temperature is air-cooled to 530°C, and then furnace-cooled to room temperature to eliminate stress.

[0050] (2) Perform normalizing treatment. Heat to 650℃ at a heating rate of 180℃ / h and preheat for 0.5h. Then heat to 850℃ at a heating rate of 100℃ / h and hold for 0.5h. The final temperature is 1100℃. After holding for 3h, air cool to room temperature.

[0051] (3) Perform spheroidizing annealing. First heat to 650℃ at a heating rate of 180℃ / h and hold for 0.5h. Then heat to 850℃ at a heating rate of 100℃ / h and hold for 4h. Cool to 740℃ at a rate of 30℃ / h and hold for 6h. Cool to room temperature with the furnace.

[0052] (4) During machining, the material is cut into a sample with a cross-sectional size of 13mm×13mm, and then the sample surface is rough ground to 12mm×12mm to facilitate surface carburization.

[0053] (5) Before carburizing, the sample was ultrasonically cleaned with ethanol. Then, gas carburizing was carried out in a carburizing atmosphere created by methanol cracking. The carburizing temperature was 930℃, the carbon potential range was 0.8% to 1.18%C, and the carburizing time was 16h for strong carburizing (strong carburizing potential was 1.0% to 1.18%C) + 18h for diffusion carburizing (diffusion carburizing potential was 0.8% to 1.0%C), for a total of 34h. Finally, the sample was cooled to room temperature with the furnace.

[0054] (6) After carburizing, high-temperature tempering is carried out in a vacuum heat treatment furnace. The tempering temperature is 750℃, the holding time is 5h, the furnace is cooled to 550℃, and then the furnace is air-cooled to room temperature.

[0055] (7) Quenching is carried out in a vacuum heat treatment furnace. During heating, a stepped heat preservation method is used: first, the temperature is raised to 650℃ at a heating rate of 180℃ / h and held for 0.5h, then the temperature is raised to 850℃ at a heating rate of 100℃ / h and held for 0.5h, and finally the temperature is raised to 1100℃ at a heating rate of 100℃ / h and held for 0.5h, and then oil-cooled to room temperature.

[0056] (8) The effect of deep cryogenic treatment on mechanical properties is compared between quenching without deep cryogenic treatment and deep cryogenic treatment.

[0057] (9) Perform three tempering treatments, each heating temperature is 540℃, the holding time is 2h, and then air cool to room temperature.

[0058] (10) During the final testing process, the 0-1mm carburized layer is regarded as the machining allowance according to the surface carbon content requirements, and the 1mm subcutaneous layer is regarded as the actual surface.

[0059] Example 2

[0060] In this embodiment (DC80-TTT), a heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing is determined, wherein:

[0061] The chemical composition and mass percentage of rare earth M50NiL steel are as follows: C 0.13wt.%, Si 0.23wt.%, Cr 4.19wt.%, Mo 4.27wt.%, V 1.18wt.%, Ni 3.38wt.%, Mn 0.28wt.%, Ce and La total 0.012wt.% (Ce to La mass fraction ratio is 1:2), P 0.005wt.%, S 0.0011wt.%, with the balance being Fe, for a total of 100wt.%.

[0062] (1) First, the ingredients are prepared according to the above chemical composition mass ratio, and steel ingots are made by double vacuum smelting method of vacuum induction + vacuum self-consumption; then, the three-dimensional forging process is adopted, the steel ingot is heated to 1120°C at a heating rate of 100°C / h and fully burned through before forging, the final forging temperature is 900°C, the final cross-sectional size is 75mm×75mm, after forging, the surface temperature is air-cooled to 530°C, and then furnace-cooled to room temperature to eliminate stress.

[0063] (2) Perform normalizing treatment. Heat to 650℃ at a heating rate of 180℃ / h and preheat for 0.5h. Then heat to 850℃ at a heating rate of 100℃ / h and hold for 4h. Cool to 740℃ at a rate of 30℃ / h and hold for 6h. Cool to room temperature with the furnace.

[0064] (4) During machining, the material is cut into a sample with a cross-sectional size of 13mm×13mm, and then the sample surface is rough ground to 12mm×12mm to facilitate surface carburization.

[0065] (5) Before carburizing, the sample was ultrasonically cleaned with ethanol. Then, gas carburizing was carried out in a carburizing atmosphere created by methanol cracking. The carburizing temperature was 930℃, the carbon potential range was 0.8% to 1.18%C, and the carburizing time was 16h for strong carburizing (strong carburizing potential was 1.0% to 1.18%C) + 18h for diffusion carburizing (diffusion carburizing potential was 0.8% to 1.0%C), for a total of 34h. Finally, the sample was cooled to room temperature with the furnace.

[0066] (6) After carburizing, high-temperature tempering is carried out in a vacuum heat treatment furnace. The tempering temperature is 750℃, the holding time is 5h, the furnace is cooled to 550℃, and then the furnace is air-cooled to room temperature.

[0067] (7) Quenching is carried out in a vacuum heat treatment furnace. During heating, a stepped heat preservation method is used: first, the temperature is raised to 650℃ at a heating rate of 180℃ / h and held for 0.5h, then the temperature is raised to 850℃ at a heating rate of 100℃ / h and held for 0.5h, and finally the temperature is raised to 1100℃ at a heating rate of 100℃ / h and held for 0.5h, and then oil-cooled to room temperature.

[0068] (8) Perform cryogenic treatment, cool down to -80°C at a cooling rate of 50°C / h, and hold at this temperature for 2 hours. By lowering the temperature, the unstable residual austenite is further transformed into stable martensite, and then the temperature is raised to room temperature at the same rate.

[0069] (9) The furnace is subjected to three tempering treatments in a vacuum heat treatment furnace. Each heating temperature is 540℃ and the holding time is 2h. The furnace is then air-cooled to room temperature.

[0070] (10) During the final testing process, the 0-1mm carburized layer is regarded as the machining allowance according to the surface carbon content requirements, and the 1mm subcutaneous layer is regarded as the actual surface.

[0071] Figure 1 The carbon content distribution of rare earth M50NiL steel after carburizing is shown. It can be seen that the carbon content gradually decreases with the increase of distance from the surface after carburizing, and reaches the required carbon content value at 1 mm from the surface.

[0072] Figure 2 SEM images of the carburized layer at a depth of 0.7 mm from the surface of rare earth M50NiL steel after quenching (Q) and cryogenic treatment at -80℃ (DC80) show that the microstructure of the carburized layer is martensite (M) and retained austenite (A). RIt consists of a small amount of carbides and contains large-sized retained austenite after quenching. However, after cryogenic treatment, the size of the retained austenite is refined and the content is reduced.

[0073] Figure 3 The figure shows the distribution of residual austenite content in the carburized layer of rare earth M50NiL steel after quenching (Q) and cryogenic treatment at -80℃ (DC80). As can be seen from the figure, the residual austenite content in the carburized layer is reduced in all places after cryogenic treatment.

[0074] Figure 4 (1) The microhardness distribution of the carburized layer after quenching (Q) and cryogenic treatment at -80℃ (DC80) on rare earth M50NiL steel after carburizing. It can be seen that the hardness of the carburized layer first increases and then decreases due to the high content of residual austenite after quenching, while the hardness of the carburized layer increases significantly after cryogenic treatment at -80℃.

[0075] Figure 4 (2) The microhardness distribution of rare earth M50NiL steel after carburizing was compared with that of foreign M50NiL bearings after undergoing three tempering processes (DC80-TTT and TTT respectively) and deep cryogenic treatment at -80℃. It can be seen that the surface hardness value after three tempering processes is 800-900 HV. 0.1 The core hardness value is 440-490 HV. 0.1 The effective hardened layer thickness is stable at 1.8–2.4 mm. Compared with foreign M50NiL bearings, the surface hardness and effective hardened layer thickness have reached the same level.

[0076] like Figure 5 As shown, the mechanical properties of rare earth M50NiL steel after carburizing were compared after undergoing deep cryogenic treatment at -80℃ and no deep cryogenic treatment, followed by three tempering processes (replaced by DC80-TTT and TTT respectively). It can be seen that, compared with the group without deep cryogenic treatment, all mechanical properties increased after deep cryogenic treatment at -80℃.

[0077] In summary, as shown in the attached figures of the specification, the experimental results demonstrate that cryogenic treatment can effectively reduce the content of retained austenite, comprehensively improve the mechanical properties of rare earth M50NiL steel after carburizing, and meet the requirements for the use of M50NiL steel in bearings for aero-engines.

Claims

1. A heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing, characterized in that, The chemical composition and mass percentage of rare earth M50NiL steel are as follows: C 0.11~0.15 wt.%, Si 0.10~0.25 wt.%, Cr 4.00~4.25 wt.%, Mo 4.00~4.50 wt.%, V 1.13~1.33 wt.%, Ni 3.20~3.60 wt.%, Mn 0.15~0.35 wt.%, Ce and La total 0.01~0.02 wt.% with Ce to La mass fraction ratio of 1:2, P≤0.01 wt.%, S≤0.01 wt.%, balance Fe; The heat treatment method includes forging, normalizing, spheroidizing annealing, machining, surface carburizing, high-temperature tempering, quenching, cryogenic treatment, three-stage tempering, and final testing. The specific steps are as follows: (1) Forging: The steel ingot is made by a double vacuum smelting method of vacuum induction + vacuum self-consumption. It is forged in three directions. The forging temperature is 900-1120℃. After forging, it is air-cooled until the surface temperature drops to 500-600℃, and then furnace-cooled to room temperature to eliminate stress. (2) Normalizing: The steel ingot is normalized after forging. During the heating process of normalizing, a stepped preheating and heat preservation is carried out: preheating and heat preservation at 650±20℃ for 0.5~1h, preheating and heat preservation at 850±20℃ for 0.5~1h, and the final heating temperature is 1100±20℃. After heat preservation for 1~5h, it is air-cooled to room temperature. (3) Spheroidizing annealing: After normalizing, the steel ingot is first heated to 650±20℃ at a heating rate of 100~200℃ / h for 0.5~1h preheating and holding, then heated to 850±20℃ at a heating rate of 100~200℃ / h for 4~5h, and cooled to 740±20℃ at a rate of 20~40℃ / h for 6~8h, and then cooled to room temperature in the furnace. (4) Machining: After spheroidizing annealing, the steel ingot is cut into samples of specified size by wire cutting. The surface of the sample is rough ground to facilitate surface carburizing. (5) Surface carburizing: After machining, rare earth M50NiL steel is ultrasonically cleaned with ethanol before surface carburizing. Then, gas carburizing is carried out in a carburizing atmosphere created by methanol cracking. The carburizing temperature is 930±20℃, and the carbon potential range is 0.8~1.2%C. Carburizing is carried out by strong carburizing + diffusion carburizing for 30~40h. Finally, the furnace is cooled to room temperature. Among them, the strong carburizing time is 10~20h, and the carbon potential is 1.0~1.2%C; the diffusion carburizing time is 15~25h, and the carbon potential is 0.8~1.0%C. (6) High-temperature tempering: After surface carburizing treatment, high-temperature tempering is carried out at a temperature of 750±20℃ and a holding time of 4 to 6 hours. The furnace is cooled to 550±20℃ and then air-cooled to room temperature. (7) Quenching: After high-temperature tempering, first heat to 650±20℃ at a heating rate of 100~200℃ / h for 0.5~1h preheating and holding, then heat to 850±20℃ at a heating rate of 100~200℃ / h for 0.5~1h preheating and holding; finally heat to quenching temperature of 1100±10℃ at a heating rate of 100~200℃ / h, holding for 0.5~1h, and oil cool to room temperature; (8) Deep cryogenic treatment: After quenching, deep cryogenic treatment is carried out, cooling down to -196 to -40℃ at a cooling rate of 25 to 100℃ / h, and holding at this temperature for 2 to 4 hours. By lowering the temperature, the unstable residual austenite is further transformed into stable martensite, and then the temperature is raised to room temperature at the same rate. (9) Three tempering: After cryogenic treatment, three tempering treatments are carried out. Each heating temperature is 500-580℃ and the holding time is 1-3h. Then, the temperature is cooled to room temperature.

2. The heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing according to claim 1, characterized in that, During the final testing process, based on the surface carbon content requirements, the 0-1mm carburized layer was considered as the machining allowance, and the 1mm subsurface was considered as the actual surface area.

3. The heat treatment method for improving the mechanical properties of rare earth M50NiL steel after carburizing according to claim 1, characterized in that, The high-temperature tempering, quenching, and tertiary tempering processes following surface carburizing are all carried out in a vacuum heat treatment furnace.

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