Process for deep hardening of low hardenability bearing steel

Abstract

The application discloses a heat treatment method for deep quenching of low quenching bearing steel, and comprises the following steps: S1, performing spheroidizing annealing treatment on bearing steel material; S2, performing turning processing on the bearing steel material subjected to the spheroidizing annealing treatment and processing the bearing steel material into a part; S3, performing first cleaning and first drying on the part; S4, performing heating quenching on the part; S5, cooling the part by using saturated calcium chloride water-based quenching liquid; S6, performing second cleaning and second drying on the cooled part; and S7, performing tempering treatment on the part subjected to the second cleaning and the second drying. The application solves the problem that large parts with a diameter or wall thickness greater than 20 mm are difficult to be integrally quenched by using oil quenching and nitrate salt quenching due to poor quenching of traditional GCr15 bearing steel.

Description

Technical Field

[0001] This invention relates to the field of bearing heat treatment technology, specifically a heat treatment method for deep hardening of low-hardenability bearing steel. Background Technology

[0002] Rolling bearings are core components of various mechanical equipment, and GCr15 is currently the most widely used and lowest-cost standard high-carbon chromium bearing steel in the industrial field. It conforms to the standard GB / T 18254 and, due to its high hardness and wear resistance after quenching, is widely used in the manufacture of key parts such as bearing rings and rolling elements. However, GCr15 is a typical low-hardenability bearing steel, lacking sufficient amounts of molybdenum, manganese, and other alloying elements to improve hardenability, which significantly limits the application of conventional heat treatment processes. Furthermore, the mainstream quenching media for existing GCr15 parts are either rapid quenching oil or nitrate salts, both of which have insurmountable technical shortcomings. 1. Insufficient cooling capacity of quenching oil limits the cross-sectional dimensions of parts: The cooling rate of quenching oil in the unstable medium-high temperature range of supercooled austenite is relatively low, making it difficult for the core of the workpiece to reach the critical quenching rate. This easily leads to the decomposition of supercooled austenite, generating harmful structures such as troostite and lamellar pearlite, making it impossible to achieve overall hardening. Conventional oil quenching processes in the industry can only stably process small bearing parts with a wall thickness / diameter of less than 20mm. For medium-to-large cross-section workpieces with a diameter of φ50mm, oil quenching results in low core hardness, excessive microstructure, and high residual austenite content, leading to poor dimensional stability and making the bearing prone to fatigue failure and precision degradation during service. To produce large cross-section bearing parts, the industry can only use high-alloy bearing steels such as GCr15SiMn and GCr18Mo as substitutes. This not only significantly increases raw material procurement costs but also makes the smelting of high-alloy steel difficult, resulting in severe segregation of carbides and alloying elements, making it difficult to control the uniformity of the raw material structure and further increasing production management costs.

[0003] 2. Quenching oil and nitrate salt media have environmental and safety drawbacks: When using quenching oil in production, the immersion of high-temperature workpieces in the oil generates a large amount of oil fumes, which pollutes the workshop air and poses a fire hazard. Furthermore, quenching oil is prone to aging and deterioration with long-term use, resulting in high costs for replacement and waste disposal. While isothermal or staged quenching with nitrate salts can slightly improve the quenching depth, nitrate salts are highly toxic and corrosive. High-temperature salt spray corrodes equipment and harms the health of operators. Moreover, the treatment process for nitrate salt waste is complex, requiring high environmental protection investment. Long-term use significantly increases the company's costs for safe production and environmental governance.

[0004] 3. Conventional water-based quenching media are prone to workpiece deformation and cracking: Ordinary water and low-salt water have acceptable cooling rates in the high-temperature zone, but the cooling rate in the martensitic phase transformation zone is too fast, which causes the quenching stress of GCr15 high-carbon steel to increase sharply. Large cross-section parts are very prone to deformation and cracking defects, so they are rarely used directly for batch quenching of bearing steel in the industry. Existing PAG polymer water-based quenching fluids do not have matching rust-preventive and mildew-preventive additives. Long-term use is prone to mold growth and workpiece corrosion. Moreover, there is a lack of complete annealing, temperature control, cleaning and drying processes suitable for deep hardening of large GCr15 parts, making it difficult to stably achieve overall hardening of large-size parts. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a heat treatment method for deep hardenability of bearing steel with low hardenability, which solves the problem that traditional GCr15 bearing steel has poor hardenability and large parts with diameter or wall thickness greater than 20mm are difficult to harden through as a whole by oil quenching or salt nitrate quenching.

[0006] To achieve the above objectives, the present invention provides a heat treatment method for deep hardening of bearing steel with low hardenability, comprising the following steps: S1. Prepare bearing steel billet. The bearing steel billet is GCr15 material conforming to GB / T 18254 standard. The bearing steel billet is subjected to spheroidizing annealing treatment in a spheroidizing annealing furnace. The spheroidizing annealing treatment is to heat the bearing steel billet from room temperature to 810°C at a rate of ≤150°C / h and hold it at that temperature for 6-8h. Then, it is cooled in the furnace to 760°C and held at that temperature for 12-15h. Finally, it is cooled in the furnace to 550°C and then removed from the furnace and air-cooled. S2. After the bearing steel billet is cooled by air after being taken out of the furnace, it is turned to remove the surface defects of the bearing steel billet, and then machined into parts. S3. Perform the first cleaning and first drying of the parts; S4. The parts after the first cleaning and first drying are heated and quenched in a roller heat treatment furnace. The effective heating zone temperature deviation of the roller heat treatment furnace is ±5°C, the temperature control deviation of the roller heat treatment furnace is ±1.5°C, the carbon potential parameter of the roller heat treatment furnace is 0.6%, the temperature of the roller heat treatment furnace is 850°C, and the heating and quenching time is 120 min. S5. After removing the part from the heat treatment furnace in step S4, place it in a quenching tank. Add saturated calcium chloride water-based quenching liquid to the quenching tank to cool the part. The saturated calcium chloride water-based quenching liquid is composed of anhydrous calcium chloride, sodium benzoate, triethanolamine, borax and deionized water. The temperature of the saturated calcium chloride water-based quenching liquid in the quenching tank is controlled at 60-80°C. The stirring frequency of the quenching tank is 100-500 rpm. S6. Perform a second cleaning and a second drying on the parts that have been cooled in step S5. S7. Tempering treatment of parts after the second cleaning and second drying.

[0007] The present invention further specifies that the saturated calcium chloride water-based quenching solution in step S5 is composed of 50% anhydrous calcium chloride, 1% sodium benzoate, 3% triethanolamine, 1% borax, and the balance deionized water, and the density parameter of the calcium chloride water-based quenching solution is 1.4-1.5 g / cm³. 3 .

[0008] The present invention further specifies that the first cleaning is performed by washing with alkaline water at 90°C for 20 minutes, and the first drying is performed by drying with hot air at 150°C for 20 minutes.

[0009] The present invention further specifies that the second cleaning is performed by washing with alkaline water at 30°C for 20 minutes, and the second drying is performed by drying with hot air at 120°C for 20 minutes.

[0010] The present invention further specifies that the tempering temperature in step S6 is 160-180°C and the tempering time is 2-4 hours.

[0011] The advantages of adopting the above technical solution are as follows: In step S1, the two-stage stepped spheroidizing annealing process of long-term holding at 810℃ + ultra-long isothermal holding at 760℃ is more effective than traditional short-time spheroidizing annealing. This process can fully break down the lamellar pearlite and network carbides in the raw material, promote the uniform spheroidization and refinement of carbides, and thus significantly alleviate the problem of segregation of primary carbides and alloying elements in GCr15 steel. This allows the uniform and fine spheroidized carbides to be uniformly dissolved into the matrix during the subsequent austenitization stage, ensuring consistent carbon content from the surface to the core of the part. This solves the defects of insufficient hardening and uneven hardness in the core of large cross-section parts from the perspective of raw material microstructure, while also improving the machinability of the billet and reducing the wear of machining tools.

[0012] In step S4 above, the effective heating zone temperature difference, temperature control accuracy and temperature of the roller heat treatment furnace are limited to eliminate the problem of local overheating in the furnace; the carbon potential is limited to 0.6% to inhibit decarburization or carbonization of the surface layer of the parts; and the heating and quenching time is controlled at 120 minutes to ensure that the carbides in the core of medium and large cross-section parts are fully dissolved and the austenite composition is uniform, so as to solve the pain points of incomplete austenitization in the core of traditional short-heat-holding large parts, shallow hardened layer and excessive troostite in the core, and ensure the consistency of the microstructure from the surface layer to the core of the workpiece.

[0013] In step S5 above, saturated calcium chloride water-based quenching fluid replaces quenching oil and nitrates. In the medium-high temperature range where the stability of supercooled austenite is lowest, the inorganic polymers in the saturated calcium chloride water-based quenching fluid create a film-bursting effect, resulting in a higher cooling rate and inhibiting the decomposition of supercooled austenite. Simultaneously, near the martensitic transformation initiation temperature, the boiling point is higher than water, leading to a higher convection initiation temperature. Furthermore, the viscosity is higher than water, resulting in poorer heat transfer, thus slowing the cooling rate and reducing quenching stress, preventing deformation. It prevents rust formation and cracking; and avoids pollution from the volatilization of quenching oil and the hazards of toxic nitrates; the above-mentioned saturated calcium chloride water-based quenching fluid is composed of 50% anhydrous calcium chloride, 1% sodium benzoate, 3% triethanolamine, 1% borax and the balance deionized water. The saturated calcium chloride plays the main role in quenching, while sodium benzoate inhibits the growth of mold and deterioration of the water-based medium during long-term storage, thus extending the service life of the quenching fluid. Triethanolamine and borax work together to provide short-term rust prevention, making up for the defect of simple calcium chloride aqueous solution that easily corrodes workpieces.

[0014] By processing bearing steel materials through the above-mentioned process steps, there is no need to replace high-alloy, high-hardenability bearing steels such as GCr15SiMn and GCr18Mo. Low-cost conventional GCr15 can be used directly to produce medium and large cross-section bearing parts, thereby avoiding the problems of segregation in the smelting of high-alloy steel, high procurement price, and high cost of flaw detection and control. Saturated calcium chloride raw material is readily available and inexpensive. The medium does not age easily and has a much longer replacement cycle than quenching oil. It can replace quenching oil and nitrate salts, eliminating high-temperature oil fume pollution and fire safety hazards, and eliminating the hazards of highly toxic corrosion and carcinogenic salt spray from nitrate salts. This significantly reduces the cost of hazardous waste liquid and waste salt disposal, lowers the investment in environmental protection and safety protection in the workshop, and improves the production and operation environment in the heat treatment workshop. Attached Figure Description

[0015] Fig. 1 This is a comparison diagram of the hardness gradient between the embodiments and comparative examples of the present invention; Fig. 2 This is a comparison diagram of the residual austenite in the embodiments and comparative examples of the present invention; Fig. 3 This is a comparison image of the microstructures of the embodiments and comparative examples of the present invention. Detailed Implementation

[0016] This invention provides a heat treatment method for deep hardenability of bearing steel with low hardenability. A specific embodiment is as follows: S1. Prepare bearing steel billet, wherein the bearing steel billet is GCr15 material conforming to GB / T 18254 standard. The bearing steel billet is subjected to spheroidizing annealing treatment in a spheroidizing annealing furnace. The spheroidizing annealing treatment is to heat the bearing steel billet from room temperature to 810°C at a rate of ≤150°C / h and hold it at that temperature for 6-8h, then cool it in the furnace to 760°C and hold it at that temperature for 12-15h, and then cool it in the furnace to 550°C before air cooling. S2. After the bearing steel billet is cooled by air after being taken out of the furnace, it is turned to remove the surface defects of the bearing steel billet, and then machined into a part with a surface roughness of φ50*50mm. S3. Perform a first cleaning and a first drying on the parts. The first cleaning is performed by washing with alkaline water at 90°C for 20 minutes, and the first drying is performed by drying with hot air at 150°C for 20 minutes. S4. The parts after the first cleaning and first drying are heated and quenched in a roller heat treatment furnace. The effective heating zone temperature deviation of the roller heat treatment furnace is ±5°C, the temperature control deviation of the roller heat treatment furnace is ±1.5°C, the carbon potential parameter of the roller heat treatment furnace is 0.6%, the temperature of the roller heat treatment furnace is 850°C, and the heating and quenching time is 120 min. S5. After removing the part from the heat treatment furnace in step S4, place it in a quenching tank. Add saturated calcium chloride water-based quenching liquid to the quenching tank to cool the part. The saturated calcium chloride water-based quenching liquid is composed of anhydrous calcium chloride, sodium benzoate, triethanolamine, borax and deionized water. The temperature of the saturated calcium chloride water-based quenching liquid in the quenching tank is controlled at 60-80°C. The stirring frequency of the quenching tank is 100-500 rpm. S6. The parts cooled in step S5 are cleaned and dried a second time. The second cleaning is performed by washing with alkaline water at 30°C for 20 minutes, and the second drying is performed by drying with hot air at 120°C for 20 minutes. S7. Tempering treatment of the parts after the second cleaning and second drying, wherein the tempering temperature is 160-180°C and the tempering time is 2-4 hours.

[0017] Comparative example: S1. Prepare bearing steel billet. The bearing steel billet is GCr15 material conforming to GB / T 18254 standard. The bearing steel billet is subjected to spheroidizing annealing treatment in a spheroidizing annealing furnace. The spheroidizing annealing treatment is to heat the bearing steel billet from room temperature to 810°C at a rate of ≤150°C / h and hold it at that temperature for 6-8h. Then, it is cooled in the furnace to 760°C and held at that temperature for 12-15h. Finally, it is cooled in the furnace to 550°C and then removed from the furnace and air-cooled. S2. After the bearing steel billet is cooled by air after being taken out of the furnace, it is turned to remove the surface defects of the bearing steel billet, and then machined into a part with a surface roughness of φ50*50mm. S3. Perform a first cleaning and a first drying on the parts. The first cleaning is performed by washing with alkaline water at 90°C for 20 minutes, and the first drying is performed by drying with hot air at 150°C for 20 minutes. S4. The parts after the first cleaning and first drying are heated and quenched in a roller heat treatment furnace. The effective heating zone temperature deviation of the roller heat treatment furnace is ±5°C, the temperature control deviation of the roller heat treatment furnace is ±1.5°C, the carbon potential parameter of the roller heat treatment furnace is 0.6%, the temperature of the roller heat treatment furnace is 850°C, and the heating and quenching time is 120 min. S5. After removing the part from the heat treatment furnace in step S4, place it into the quenching tank. Add rapid quenching oil to the quenching tank, control the temperature at 60-80°C, and stir the quenching tank at a frequency of 100-500 rpm. S6. The parts cooled in step S5 are cleaned and dried a second time. The second cleaning is performed by washing with alkaline water at 30°C for 20 minutes, and the second drying is performed by drying with hot air at 120°C for 20 minutes. S7. Tempering treatment of the parts after the second cleaning and second drying, wherein the tempering temperature is 160-180°C and the tempering time is 2-4 hours.

[0018] Hardness gradient, retained austenite, and microstructure analysis were simultaneously performed on the parts of the examples and the comparative examples, referring to the appendix of the instruction manual. Figs. 1-3 It can be known that: The comparative example shows that the core was not fully quenched, with excessive amounts of troostite and lamellar pearlite, and the desired martensite structure was not obtained. The surface hardness is 60.5-61.5 HRC, and the residual austenite content is 13-15%.

[0019] In the example, the surface and core are thoroughly hardened to obtain the desired martensitic structure, the core has a grade 1 troostite structure, no pearlite structure is observed, the surface hardness is 61.5-62.5 HRC, and the residual austenite content is 8-10%.

[0020] Comparative tests were conducted on GCr15 parts of the same specification (φ50×50mm) using the saturated calcium chloride water-based quenching process of this invention (example) and the traditional quenching oil process (comparative example). The results showed that under the traditional oil quenching method, the core of the parts could not be fully quenched, resulting in a large amount of unqualified structures such as troostite and lamellar pearlite. The surface hardness was only 60.5~61.5HRC, and the residual austenite content was as high as 13%~15%, making it difficult to meet the GB / T34891 standard. In contrast, the parts treated with the process of this invention achieved overall quenching from the surface to the core, with only Grade 1 troostite and no pearlite structures in the core. The surface hardness increased to 61.5~62.5HRC, and the residual austenite content decreased to 8%~10%. The overall microstructure was finer and more uniform, resulting in better dimensional stability, wear resistance, and fatigue performance. Furthermore, it avoided the oil fume pollution, fire hazards, and cost issues associated with quenching oil, as well as the need for expensive high-alloy bearing steel for large-sized workpieces.

[0021] The foregoing has shown and described the basic principles and main features of the present invention, as well as its advantages. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the present invention. Various changes and modifications can be made to the present invention without departing from its spirit and scope. All such changes and modifications fall within the scope of the present invention as claimed, which is defined by the appended claims and their equivalents.

Claims

1. A heat treatment method for deep hardening of low-hardenability bearing steel, characterized in that: Includes the following steps: S1. Prepare bearing steel billet. The bearing steel billet is GCr15 material conforming to GB / T 18254 standard. The bearing steel billet is subjected to spheroidizing annealing treatment in a spheroidizing annealing furnace. The spheroidizing annealing treatment is to heat the bearing steel billet from room temperature to 810°C at a rate of ≤150°C / h and hold it at that temperature for 6-8h. Then, it is cooled in the furnace to 760°C and held at that temperature for 12-15h. Finally, it is cooled in the furnace to 550°C and then removed from the furnace and air-cooled. S2. After the bearing steel billet is cooled by air after being taken out of the furnace, it is turned to remove the surface defects of the bearing steel billet, and then machined into parts. S3. Perform the first cleaning and first drying of the parts; S4. The parts after the first cleaning and first drying are heated and quenched in a roller heat treatment furnace. The effective heating zone temperature deviation of the roller heat treatment furnace is ±5°C, the temperature control deviation of the roller heat treatment furnace is ±1.5°C, the carbon potential parameter of the roller heat treatment furnace is 0.6%, the temperature of the roller heat treatment furnace is 850°C, and the heating and quenching time is 120 min. S5. After removing the part from the heat treatment furnace in step S4, place it in a quenching tank. Add saturated calcium chloride water-based quenching liquid to the quenching tank to cool the part. The saturated calcium chloride water-based quenching liquid is composed of anhydrous calcium chloride, sodium benzoate, triethanolamine, borax and deionized water. The temperature of the saturated calcium chloride water-based quenching liquid in the quenching tank is controlled at 60-80°C. The stirring frequency of the quenching tank is 100-500 rpm. S6. Perform a second cleaning and a second drying on the parts that have been cooled in step S5. S7. Tempering treatment of parts after the second cleaning and second drying.

2. The heat treatment method for deep hardening of low hardenability bearing steel according to claim 1, characterized in that: The saturated calcium chloride water-based quenching solution in step S5 consists of 50% anhydrous calcium chloride, 1% sodium benzoate, 3% triethanolamine, 1% borax, and the balance deionized water. The density parameter of the calcium chloride water-based quenching solution is 1.4-1.5 g / cm³. 3 .

3. The heat treatment method for deep hardening of low hardenability bearing steel according to claim 1, characterized in that: The first cleaning was performed using alkaline water at 90°C for 20 minutes, and the first drying was performed using hot air at 150°C for 20 minutes.

4. The heat treatment method for deep hardening of low hardenability bearing steel according to claim 1, characterized in that: The second cleaning is performed by washing with alkaline water at 30°C for 20 minutes, and the second drying is performed by drying with hot air at 120°C for 20 minutes.

5. The heat treatment method for deep hardening of low hardenability bearing steel according to claim 1, characterized in that: In step S6, the tempering temperature is 160-180°C and the tempering time is 2-4 hours.

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