Method for preparing large-specification 9Cr18Mo stainless bearing steel through double-vacuum forging process

Abstract

The invention discloses a method for preparing large-specification 9Cr18Mo stainless bearing steel through a double-vacuum forging process. The defects in the prior art are overcome. A consumable steel ingot which is accurate in component, low in gas content, compact and uniform is obtained through VIM smelting and VAR counter electrode remelting; components of the cast ingot are homogenized through high-temperature diffusion treatment, and macrosegregation is effectively eliminated; carbides and crystal grains are fully crushed and uniformly distributed through the two-upsetting and two-drawing forging process; and after-forging annealing is carried out, carbides in a forging structure are spheroidized, the hardness is reduced, follow-up machining is facilitated, and a uniform original structure is prepared for final heat treatment. The method has the beneficial effects that (1) the purity is high, the oxygen content in the final forge piece is not greater than 10 * 10 <-6 >, and the inclusion level is below 1.0 level; (2) after heat treatment, the grain size of a phi 285 mm finished product forging material reaches grade 6, and the unevenness of eutectic carbides reaches grade 6; (3) the hardness (HRC) is greater than 58, and meanwhile, excellent corrosion resistance is achieved; (4) the process is stable and controllable.

Description

Technical Field

[0001] This invention belongs to the field of metal material manufacturing technology, specifically relating to a method for preparing large-size 9Cr18Mo stainless bearing steel using a double vacuum forging process. It is applicable to the smelting and forging of large-size, high-performance 9Cr18Mo stainless bearing steel for new aviation radar bearings. Background Technology

[0002] 9Cr18Mo is a high-carbon, high-chromium martensitic stainless steel. Its nominal chemical composition contains approximately 1.0% carbon and 18% chromium. The addition of molybdenum further enhances its corrosion resistance and hardenability. After heat treatment, this steel can achieve a high hardness (HRC) of not less than 58 and high wear resistance, while also possessing corrosion resistance. Therefore, it is widely used in the manufacture of corrosion-resistant bearings, precision measuring tools, and cutting tools. However, with the development of modern aviation and aerospace industries, the requirements for the reliability, lifespan, and size of bearing components are increasing, and the required steel specifications (Ф285mm) are constantly increasing. For large-sized 9Cr18Mo ingots and forgings, traditional manufacturing processes face severe challenges: (1) Purity issues: The oxygen content and non-metallic inclusions (such as oxides and sulfides) in the molten steel smelted in conventional electric arc furnaces are relatively high. These inclusions, as stress concentration sources, will significantly reduce the contact fatigue life of the bearings; (2) Macro and micro segregation: During the solidification process of large steel ingots, due to the slow cooling rate, alloying elements such as chromium, carbon, and molybdenum are prone to dendritic segregation, resulting in uneven composition inside the ingot, which in turn affects the subsequent hot working performance and the uniformity of the final product's structure; (3) Carbide control challenges: High carbon content inevitably leads to the formation of a large number of eutectic carbides. In the as-cast structure, these carbides are usually distributed in a coarse network or banded pattern, severely disrupting the matrix. If they cannot be fully broken and homogenized during the forging process, they will directly lead to early failure of the bearing during service. Currently, although there are reports of using single vacuum melting (such as VAR) or conventional forging to improve material properties, there is still a lack of a mature, efficient, and controllable integrated process scheme for the industrial and stable production of large-size 9Cr18Mo bearing steel with high purity, low segregation, and uniform structure. Therefore, developing a preparation method that can systematically solve the above problems has important industrial application value. Summary of the Invention

[0003] This invention discloses a method for preparing large-size 9Cr18Mo stainless bearing steel using a dual vacuum forging process. The purpose is to overcome the shortcomings of the prior art and provide a method for preparing large-size 9Cr18Mo stainless bearing steel that integrates pure melting, homogenization treatment and controllable plastic deformation.

[0004] The technical solution adopted in this invention includes the following steps: (1) Dual vacuum smelting: The process route of “vacuum induction melting (VIM) + vacuum consumable remelting (VAR)” is adopted. First, electrode rods with accurate composition and low gas content are obtained by VIM melting. Then, the electrodes are remelted by VAR process to further remove impurities and reduce gas content. A forced rapid cooling process (helium filling, 100ml / min~140ml / min) is adopted to obtain dense and uniform Φ508mm consumable steel ingots.

[0005] (2) High-temperature homogenization diffusion: VAR steel ingots are loaded into a homogenizing furnace, heated to 1200℃~1250℃, and held for more than 20 hours.

[0006] (3) Multiple forging: The homogenized ingot is heated to a forging temperature range of 1150℃~1180℃ and forged in a high-speed forging machine with "two upsetting and two drawing" forging; through multiple upsetting and drawing deformations, the upsetting height of the steel ingot in each forging is 30%~40% of the deformation of the steel ingot, so as to achieve full crushing of the as-cast dendrites and coarse carbides, and the fine forging adopts two frequency conversion production (90 times / min, 60 times / min) to further crush the structure.

[0007] (4) Post-forging annealing: Annealing is performed on the forgings. The specific process is as follows: heat to 800℃~850℃, hold for 15h~20h, and then slowly furnace cool (cooling rate not greater than 35℃ / h) to below 400℃ and then air cool.

[0008] The innovation of this invention lies in: (1) Electrode rods with precise composition and low gas content are obtained by VIM melting. Then, the electrodes are remelted by VAR process to further remove impurities, reduce gas content, and obtain dense and uniform consumable steel ingots.

[0009] (2) High-temperature diffusion treatment allows alloying elements in the ingot to diffuse fully, achieving homogenization of composition and effectively eliminating macrosegregation.

[0010] (3) The “two upsetting and two drawing” forging process enables the carbides and grains to be fully broken and evenly distributed.

[0011] (4) Post-forging annealing spheroidizes the carbides in the forging structure, reduces hardness, facilitates subsequent machining, and prepares a uniform original structure for the final heat treatment.

[0012] Advantages of this invention: (1) High purity, the oxygen content in the final forging is no more than 10×10⁻⁶. -6 The level of inclusions is below 1.0.

[0013] (2) The composition and structure are uniform. After heat treatment: 1060℃×45min oil cooling, -75℃×120min air cooling, 155℃×240min air cooling, the grain size of the Ф285mm finished forging material reaches level 6, and the eutectic carbide inhomogeneity is level 6.

[0014] (3) Excellent comprehensive performance: After the above heat treatment, the sample has a hardness (HRC) greater than 58 and also has excellent corrosion resistance.

[0015] (4) Stable and controllable process: The process parameters provided by this invention are clear and suitable for large-scale industrial production. It can stably manufacture large-size, high-purity, and highly corrosion-resistant stainless bearing steel forging bars. Attached Figure Description

[0016] Figure 1 Schematic diagram of 9Cr18Mo consumable ingot; Figure 2 Schematic diagram of upset bar stock; Figure 3 Schematic diagram of drawn and forged bar stock; Figure 4 Schematic diagram of the final forged bar stock; Figure 5 Low-magnification photograph of forged bar stock; Figure 6 Photographs of non-metallic inclusions in finished bar stock; Figure 7 Photo of carbides on finished bar stock after heat treatment; Figure 8 Photograph of the grain size of the finished bar stock. Detailed Implementation

[0017] The invention will be further described in detail below with reference to implementation examples.

[0018] Objective: To prepare a Ф285mm 9Cr18Mo stainless bearing steel forging bar for use in aerospace radar bearings, requiring it to have high purity, uniform microstructure and high comprehensive performance. Example 1

[0019] The preparation steps and process parameters are as follows: (1) Double vacuum smelting: ① Vacuum Induction Melting: High-quality raw materials are melted in a vacuum induction furnace, with precise control over the alloy composition (mass fraction / %), based on carbon: 1.0, chromium: 16.7, molybdenum: 0.5, with the balance being iron and unavoidable impurities), resulting in a low gas content and an oxygen content not exceeding 10 × 10⁻⁶. -6 And a uniformly composed electrode rod.

[0020] ② Vacuum consumable remelting: The electrode rods are remelted a second time in a vacuum consumable furnace. The melting rate is controlled at 4.0 kg / min, and a forced rapid cooling process (helium charging, 120 ml / min) is used to obtain consumable steel ingots with a diameter of Φ508 mm. Figure 1 As shown; through this process, the overall oxygen content of the steel ingot can be stably controlled to no more than 10 × 10⁻⁶. -6 .

[0021] (2) High-temperature homogenization diffusion treatment: The obtained consumable steel ingots are placed in a homogenizing furnace and heated to 1240°C. They are then held at this temperature for more than 16 hours to allow the segregated alloying elements such as chromium, molybdenum, and carbon in the ingots to fully diffuse.

[0022] The furnace temperature was then lowered to 1170℃ and held for 6 hours in preparation for subsequent forging.

[0023] (3) Multiple forging processes: The homogenized steel ingot is then forged using a "two-upsetting and two-drawing" process on a high-speed forging mill, as follows: ① First firing: such as Figure 2 As shown, the steel ingot is heated to 1170℃ for the first upsetting, and the deformation is controlled to be about 1 / 3 of the height of the steel ingot. Then it is drawn into an approximately octagonal ingot of 500mm.

[0024] ② Second upsetting: Return the forging to the furnace and hold it at 1150℃ for 2.5 hours for a second upsetting. The deformation is controlled to be about 2 / 5 of the ingot height. Then, it is drawn into an octagonal ingot of 450mm.

[0025] ③ Third firing: Return to the furnace, hold at 1140℃ for 1.5 hours, and finally draw to a length of 350mm for the octagonal ingot. Figure 3 As shown.

[0026] ④ Precision Forging: The ingot is reheated at 1130℃ for 1 hour, and then the 350mm octagonal ingot is forged using a radial forging mill. A variable frequency process is employed: first, forging is performed at a high frequency of 90 times / min to further break down carbides, then the frequency is switched to a lower frequency of 60 times / min for finishing, ultimately forging to a bar size of Ф300mm. Figure 4 As shown.

[0027] (4) Post-forging annealing: After forging, in order to obtain a uniform spheroidized structure to facilitate subsequent processing, the forging is annealed. The specific process is as follows: the forging is heated to 860℃ and held for 19 hours. Then, it is furnace cooled to below 400℃ at a cooling rate of 30℃ / h. Finally, it is air-cooled to room temperature after being taken out of the furnace and then machined to the finished size of Ф285mm.

[0028] (5) Performance and tissue testing results: ① The low-magnification tissue examination was satisfactory, with no defects such as white spots, dark spots, radial segregation, or ring-shaped patterns. Figure 5 .

[0029] ② Samples of Ф285mm 9Cr18Mo forged bars were analyzed, and the results are as follows: Purity: Oxygen content is 10 × 10⁻⁶ -6 ; The non-metallic inclusion ratings are shown in Table 1, and metallographic images are available in [link to table]. Figure 6 .

[0030] Table 1

[0031] ③ After final heat treatment (oil quenching at 1060℃ for 45 min + cryogenic treatment at -75℃ for 120 min + tempering at 175℃ for 240 min), the grain size is grade 7 to 6 (see...). Figure 7 The eutectic carbide inhomogeneity is grade 6 (see...). Figure 8 ).

[0032] ④ Mechanical properties: The hardness (HRC) after heat treatment is 58.7, which meets and exceeds the requirement of no less than 58 HRC for high hardness of materials in aerospace bearings. Example 2

[0033] The preparation steps and process parameters are as follows: (1) Double vacuum smelting: ① Vacuum Induction Melting: High-quality raw materials are melted in a vacuum induction furnace, and the alloy composition (mass fraction / %) is precisely controlled (carbon: 0.97, chromium: 17, molybdenum: 0.6, balance being iron and unavoidable impurities) to obtain alloys with low gas content and an oxygen content not exceeding 10×10⁻⁶. -6 And a uniformly composed electrode rod.

[0034] ② Vacuum consumable remelting: The electrode rods are remelted a second time in a vacuum consumable furnace. The melting rate is controlled at 4.0 kg / min, and a forced rapid cooling process (helium charging, 120 ml / min) is used to obtain consumable steel ingots with a diameter of Φ508 mm. After this process, the overall oxygen content of the steel ingot is stably controlled to be no more than 10 × 10⁻⁶. -6 .

[0035] (2) High-temperature homogenization diffusion treatment: The obtained consumable steel ingots are placed in a homogenizing furnace and heated to 1240°C. They are then held at this temperature for more than 18 hours to allow the segregated alloying elements such as chromium, molybdenum, and carbon in the ingots to fully diffuse.

[0036] The furnace temperature was then lowered to 1180℃ and held for 6 hours in preparation for subsequent forging.

[0037] (3) Multiple forging processes: The homogenized steel ingot is then forged using a "two-upsetting and two-drawing" process on a high-speed forging mill, as follows: ① First heating: The steel ingot is heated to 1180℃ and upset for the first time. The deformation is controlled to be about 1 / 3 of the height of the steel ingot. Then it is drawn into an approximately octagonal ingot of 500mm.

[0038] ② Second upsetting: Return the forging to the furnace and hold it at 1160℃ for 3 hours for a second upsetting. The deformation is controlled to be about 2 / 5 of the ingot height. Then draw it to a 450mm octagonal ingot.

[0039] ③ Third firing: Return to the furnace and hold at 1140℃ for 1.5 hours, then finally draw the octagonal ingot to 350mm.

[0040] ④ Precision forging: The ingot is reheated and held at 1130℃ for 1 hour. The 350mm octagonal ingot is then forged using a radial forging machine. A "frequency conversion" process is used: first, the forging is performed at a high frequency of 90 times / min to further break down the carbides, and then the frequency is switched to a lower frequency of 60 times / min for finishing, finally forging to a bar size of Ф300mm.

[0041] (4) Post-forging annealing: After forging, in order to obtain a uniform spheroidized structure to facilitate subsequent processing, the forging is annealed. The specific process is as follows: the forging is heated to 860℃ and held for 20h. Then, it is furnace cooled to below 400℃ at a cooling rate of 30℃ / h. Finally, it is air-cooled to room temperature after being taken out of the furnace and then machined to the finished size of Ф285mm.

[0042] (5) Performance and tissue testing results: ① Samples of Ф285mm 9Cr18Mo forged bars were analyzed, and the results are as follows: Purity: Oxygen content is 10 × 10⁻⁶ -6 ; The rating of non-metallic inclusions is shown in Table 2.

[0043] Table 2

[0044] ②After final heat treatment (1060℃×45min oil quenching + -75℃×120min cryogenic treatment + 175℃×240min tempering), the grain size is grade 6 and the eutectic carbide inhomogeneity is grade 6.5.

[0045] ③Mechanical properties: The hardness (HRC) after heat treatment is 58.8, which meets and exceeds the requirement of no less than 58 HRC for high hardness of materials in aerospace bearings.

[0046] in conclusion: This embodiment successfully prepared large-size, high-purity, uniformly structured, and high-performance 9Cr18Mo stainless bearing steel. The results show that the preparation method provided by this invention has well-defined, stable, and controllable process parameters, which can meet the stringent requirements of the aerospace and other fields for high-end bearing materials.

Claims

1. A method for preparing large-size 9Cr18Mo stainless bearing steel using a dual vacuum forging process, characterized in that, The method includes the following steps: (1) Dual vacuum smelting: The process route of "vacuum induction melting (VIM) + vacuum arc remelting (VAR)" is adopted. First, electrode rods with accurate composition and low gas content are obtained through VIM melting. Then, the electrodes are remelted through VAR process to further remove impurities and reduce gas content. Forced rapid cooling process is adopted: helium filling 100ml / min~140ml / min; to obtain dense and uniform Φ508mm arc remelting steel ingots. (2) High-temperature homogenization diffusion: VAR steel ingots are placed in a homogenizing furnace, heated to 1200℃~1250℃, and held at that temperature for more than 20 hours; (3) Multiple forging: The homogenized ingot is heated to a forging temperature range of 1150℃~1180℃ and forged in "two upsetting and two drawing" on a high-speed forging machine; through multiple upsetting and drawing deformations, the upsetting height of the steel ingot in each forging is 30%~40% of the deformation of the steel ingot, so as to achieve full crushing of the cast dendrites and coarse carbides, and the precision forging adopts two frequency conversion production: 90 times / min and 60 times / min; (4) Post-forging annealing: Annealing is performed on the forgings. The specific process is as follows: heat to 800℃~850℃, hold for 15h~20h, and then slowly furnace cool: the cooling rate is no more than 35℃ / h, and the forgings are removed from the furnace and air cooled when the temperature drops below 400℃.

2. The method for preparing large-size 9Cr18Mo stainless bearing steel using a double vacuum forging process according to claim 1, characterized in that, The method includes the following steps: (1) Double vacuum smelting: ① Vacuum Induction Melting: High-quality raw materials are melted in a vacuum induction furnace, with precise control over the alloy composition (mass fraction / %): Carbon: 1.0, Chromium: 16.7, Molybdenum: 0.5, with the balance being iron and unavoidable impurities, resulting in a low gas content: oxygen content not exceeding 10 × 10⁻⁶. -6 And a uniformly composed electrode rod; ② Vacuum consumable remelting: The electrode rods are remelted a second time in a vacuum consumable furnace, with the melting rate controlled at 4.0 kg / min and a forced rapid cooling process employed: helium charging at 120 ml / min, to obtain consumable steel ingots with a diameter of Φ508 mm. The overall oxygen content of the ingots is no greater than 10 × 10⁻⁶. -6 ; (2) High-temperature homogenization diffusion treatment: The obtained consumable steel ingots are placed in a homogenizing furnace and heated to 1240°C. They are then held at this temperature for more than 16 hours to allow the alloying elements such as chromium, molybdenum, and carbon that segregate in the ingots to fully diffuse. The furnace temperature was then lowered to 1170℃ and held for 6 hours in preparation for subsequent forging. (3) Multiple forging processes: The homogenized steel ingot is then forged using a "two-upsetting and two-drawing" process on a high-speed forging mill, as follows: ① First heating: The steel ingot is heated to 1170℃ and upsetting is carried out for the first time. The deformation is controlled to be about 1 / 3 of the height of the steel ingot. Then it is drawn into an approximately octagonal ingot of 500mm. ② Second upsetting: Return the forging to the furnace and hold it at 1150℃ for 2.5 hours for a second upsetting. The deformation is controlled to be about 2 / 5 of the ingot height. Then it is drawn into an octagonal ingot of 450mm. ③ Third firing: Return to the furnace again, hold at 1140℃ for 1.5 hours, and finally draw out to a length of 350mm octagonal ingot; ④ Precision forging: The ingot is reheated and held at 1130℃ for 1 hour. The 350mm octagonal ingot is then forged using a radial forging machine. A "frequency conversion" process is used: first, the forging is performed at a high frequency of 90 times / min to further break down the carbides, and then the frequency is switched to a lower frequency of 60 times / min for finishing, finally forging to a bar size of Ф300mm. (4) Post-forging annealing: After forging, in order to obtain a uniform spheroidized structure to facilitate subsequent processing, the forging is annealed. The specific process is as follows: the forging is heated to 860℃ and held for 19 hours. Then, it is furnace cooled to below 400℃ at a cooling rate of 30℃ / h. Finally, it is air-cooled to room temperature after being taken out of the furnace and then machined to the finished size of Ф285mm.

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