Cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high-temperature carburizing

Abstract

The invention belongs to the technical field of bearing heat treatment, and particularly relates to a carbon concentration gradient and high-temperature carburizing combined cylindrical roller carburizing heat treatment process, a cylindrical roller adopts carburizing steel as a raw material, carburizing treatment is performed on the cylindrical roller, and primary oil quenching, high-temperature tempering, secondary quenching and low-temperature tempering are performed on the cylindrical roller after carburizing treatment; the carburizing treatment comprises a first-stage strong carburizing process, the temperature is 950 DEG C + / -20 DEG C, the carbon potential is 1.35 + / -0.05%, and the time is 60 hours; in the second-stage diffusion process, the temperature is 950 + / -20 DEG C, the carbon potential is 1.0 + / -0.2%, and the time is 15 hours; and in the third cooling process, the temperature is 870 + / -20 DEG C, the carbon potential is 0.9 + / -0.2%, and the time is 2 hours. In the first stage, the carburizing process is conducted at the high temperature of 960 DEG C and the high carbon potential of 1.35%, the machining time is shortened, and the cost of raw materials is reduced. In the carburizing process, a long-time diffusion section is adopted, so that a smooth carbon concentration difference is formed between a hardened layer and an original structure of a core part, and the stress difference, caused by the structure difference, between areas with different carbon contents is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of bearing heat treatment, and in particular relates to a cylindrical roller carburizing heat treatment process combining carbon concentration gradient with high-temperature carburizing. Background Art

[0002] Ordinary wind turbine bearings are large in size, and the raw material and production costs are high when processed by carburizing. Generally, high-carbon bearing steel is used as the raw material and ordinary heating quenching is performed. However, the disadvantage is that the core structure formed after ordinary heating quenching is basically a mechanical mixture of martensite and troostite, with poor mechanical properties, and the bearings often have poor impact resistance. Summary of the invention

[0003] In view of the defects of the above-mentioned prior art, the purpose of the present invention is to provide a carburizing heat treatment process for cylindrical bearing rolling elements that combines carbon concentration gradient with high-temperature carburizing, which is mainly used in applications with harsh working conditions and requirements of high wear resistance, long life and large load, such as wind power components.

[0004] To achieve the above-mentioned purpose, the technical scheme adopted by the present invention is: a cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high-temperature carburizing, the cylindrical roller adopts carburizing steel as raw material, the cylindrical roller is carburized, and the cylindrical roller is subjected to oil quenching, high-temperature tempering, secondary quenching and low-temperature tempering after carburizing; the carburizing treatment includes a first stage of strong carburizing process, with a temperature of 950℃±20℃, a carbon potential of 1.35±0.05%, and a time of 60h; a second stage of diffusion process, with a temperature of 950℃±20℃, a carbon potential of 1.0±0.2%, and a time of 15h; and a third stage of cooling process, with a temperature of 870℃±20℃, a carbon potential of 0.9±0.2%, and a time of 2h.

[0005] Furthermore, the methanol flow rate during the carburizing process is 3.6-4.0L / h, and the process nitrogen flow rate is 4.0-4.4m 3 / h, propane flow rate is 0-2.0L / h.

[0006] Furthermore, the oil quenching process adopts segmented cooling, and the segmented cooling adjusts the cooling rate through different stirring frequencies. The first cooling stirring time is 40 minutes, the stirring frequency is 1800rmp, the second cooling stirring time is 10 minutes, the stirring frequency is 600rmp, the quenching oil temperature is 30-80℃, and the cylindrical roller is oil quenched to 50℃.

[0007] Furthermore, the high temperature tempering is carried out at a temperature of 620° C. for 12 hours, then the temperature is raised to 650° C. for 12 hours, and then the temperature is lowered to room temperature by furnace cooling and air cooling.

[0008] Furthermore, the secondary quenching is firstly heated to 815° C., and then cooled by salt bath quenching, and the salt bath quenching process is performed in stages.

[0009] Furthermore, the quenching temperature during the secondary quenching is 175° C., the first stage stirring time and frequency are 10 / 1300 rpm, the second stage stirring time and frequency are 7 / 650 rpm, and the total heating time for the secondary quenching is 120 minutes.

[0010] Furthermore, after the secondary quenching, the temperature is lowered to room temperature and then low-temperature tempering is performed. The temperature of the low-temperature tempering process is 200° C. and the holding time is 15 hours.

[0011] Furthermore, before the carburizing treatment, a predetermined carburizing temperature and carbon potential are reached by gradient temperature increase and gradient carbon potential increase.

[0012] Furthermore, before the carburizing treatment, the temperature is first raised to 860°C, the carbon potential is 0.8%, and kept warm for 60 minutes, then raised to 930°C, the carbon potential is raised to 1.20%, and kept warm for 3 hours, and finally raised to 950°C, the carbon potential is raised to 1.35%.

[0013] The beneficial effects of the present invention are as follows: the cylindrical roller carburizing heat treatment process combining carbon concentration gradient with high temperature carburizing of the present invention innovatively adopts a high temperature and high carbon potential of 960°C and a carbon potential of 1.35% for the carburizing process in the first stage, which shortens the processing time and reduces the cost of raw materials. At the same time, the present invention adopts a relatively long diffusion stage during the carburizing process to form a gentle carbon concentration difference between the hardened layer and the original core structure, thereby reducing the stress difference between different carbon content areas caused by the difference in structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The process flow chart of the cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing of the present invention; Figure 2 This is the cylindrical roller product drawing. DETAILED DESCRIPTION

[0015] In order to make the structure and function of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0016] See attached Figure 1, a carburizing heat treatment process for cylindrical rollers combining carbon concentration gradient and high-temperature carburizing, wherein the cylindrical rollers are carburized using carburizing steel as raw material, and then subjected to primary quenching, high-temperature tempering, secondary quenching and low-temperature tempering after carburizing; the carburizing treatment comprises a first stage of strong carburizing process, at a temperature of 950°C±20°C, a carbon potential of 1.35±0.05%, and a time of 60h; a second stage of diffusion process, at a temperature of 950°C±20°C, a carbon potential of 1.0±0.2%, and a time of 15h; a third stage of cooling process, at a temperature of 870°C±20°C, a carbon potential of 0.9±0.2%, and a time of 2h; a methanol flow rate of 3.6-4.0L / h is introduced into the carburizing process, and a process nitrogen flow rate of 4.0-4.4m 3 / h, propane flow rate is 0-2.0L / h, propane is adjusted according to the carbon potential; then it is taken out of the furnace for oil quenching (one quenching) to 50℃, and the quenching process adopts segmented cooling. The segmented cooling adjusts the cooling rate through different stirring frequencies. The first cooling stirring time is 40 minutes, the stirring frequency is 1800rmp, the second cooling stirring time is 10 minutes, the stirring frequency is 600rmp, and the quenching oil temperature is 30-80℃.

[0017] Furthermore, before the carburizing treatment, a gradient temperature increase and a gradient carbon potential increase are used to reach the predetermined temperature and carbon potential of carburizing. Specifically, the temperature is first increased to 860°C, the carbon potential is 0.8%, and the temperature is kept for 60 minutes, then the temperature is increased to 930°C, the carbon potential is increased to 1.20%, and the temperature is kept for 3 hours, and finally the temperature is increased to 950°C, and the carbon potential is increased to 1.35%. The gradient temperature increase ensures that the temperature of the surface and the core of the product can be consistent at different stages of temperature, reducing the thermal stress deformation caused by the temperature difference between the surface and the core. The gradient increase in carbon potential is conducive to the decomposition of the enriched gas and carrier gas dripping into the furnace, preventing the formation of carbon black, affecting the carburizing of the product and causing malfunctions to the equipment. First, the carbon potential is kept at 0.8% for 60 minutes and then increased to 1.20%, and finally the carbon potential is increased to 1.35%.

[0018] Based on the above technical solution, this wind turbine bearing uses carburizing steel as raw material, and innovatively uses a high temperature and high carbon potential of 960℃ with a carbon potential of 1.35% for the carburizing process in the first stage, which shortens the processing time and reduces the cost of raw materials. For products with large size and thick wall such as wind turbine bearings, it is necessary to increase the carburizing speed. The present invention uses a gradient heating and carbon potential increase method to strictly control the carburizing speed of the first stage carburizing process, and uses high temperature and high carbon potential to ensure that the first stage carburizing obtains a faster speed. The second stage uses a carbon potential 0.2-0.4 higher than the required carbon content for diffusion to obtain a stable carbon concentration.

[0019] Furthermore, the high temperature tempering is carried out at a temperature of 620° C. for 12 hours, then the temperature is raised to 650° C. for 12 hours, and then the temperature is lowered to room temperature by furnace cooling and air cooling.

[0020] Furthermore, the secondary quenching is first heated to 815°C, and then cooled by salt bath quenching. The salt bath quenching process is cooled in stages. The quenching temperature during the secondary quenching is 175°C, the first stage stirring time and frequency are 10 / 1300rmp, the second stage stirring time and frequency are 7 / 650rmp, and the total heating time for the secondary quenching is 120 minutes.

[0021] Based on the above technical solution, the quenching temperature of 175°C means that the workpiece enters the quenching tank again for quenching after the heating and insulation process, and the surface temperature of the workpiece reaches 175°C after quenching.

[0022] Furthermore, after the secondary quenching, the temperature is lowered to room temperature and then low-temperature tempering is performed. The temperature of the low-temperature tempering process is 200° C. and the holding time is 15 hours.

[0023] Based on the above technical solution, due to the difference in carbon content between the surface and the core of the product caused by carburization, there is a difference in the specific volume of the structure formed after cooling transformation, and the temperature of the carburization process is high, there is a large degree of undercooling during the cooling process, and the quenching process is prone to produce convexity and cracks under the combined action of tissue stress and thermal stress. Therefore, in order to reduce the surface convexity after heat treatment and prevent cracking, the present invention makes the following improvements to the heat treatment process.

[0024] a. Adjust the carbon concentration gradient curve In the carburizing process of the present invention, a relatively long diffusion period is used to form a gentle carbon concentration difference between the hardened layer and the original core structure, thereby reducing the stress difference between regions with different carbon contents caused by the difference in structures.

[0025] b. Reduce supercooling The present invention performs segmented cooling in the oil bath primary cooling and the salt bath secondary cooling, adjusts the cooling rate by different stirring frequencies to achieve segmented cooling, and keeps each temperature section warm for a certain period of time to ensure that the product temperature is slowly reduced, reduce the degree of supercooling in the quenching process, and reduce thermal stress.

[0026] The physical test results of the product are as follows: The product diameter is 70x80mm, and the required carbon content is 0.85-1.05%; 1. Carbon concentration gradient detection: 0.3mm (depth from product surface): 0.89% (carbon concentration), 0.5mm: 0.82%, 1.0mm: 0.75%, 1.5mm: 0.70%, 2.0mm: 0.67%, 2.5mm: 0.64%, 3.0mm: 0.60%, 3.5mm: 0.56%, 4.0mm: 0.50%, 4.5mm: 0.44%, 5.0mm: 0.38%; 2. Martensite structure: Level 3; 3. Decarburization depth: 0.03mm; 4. Surface hardness: 60.2HRC, 59.6HRC, 59.8HRC (three positions corresponding to 120° detection on the surface of the ferrule); 5. 550HV1: 5.2mm, 58HRC depth: 3.89mm; 6. Surface carbon content: 0.89%; 7. Grain size: 8 levels; 8. Retained austenite: 17.2%.

[0027] It should be noted that the parts not described in detail in the present invention are prior art.

[0028] The above examples are only the best embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and many variations are possible. All variations that can be directly derived or associated with the contents disclosed by a person skilled in the art should be considered as the protection scope of the present invention.

Claims

1. Cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing, characterized by: The cylindrical roller adopts carburizing steel as raw material, and the cylindrical roller is carburized. After the carburizing treatment, the cylindrical roller is subjected to oil quenching, high temperature tempering, secondary quenching and low temperature tempering; the carburizing treatment includes a first stage of strong carburizing process, a temperature of 950℃±20℃, a carbon potential of 1.35±0.05%, and a time of 60h; a second stage of diffusion process, a temperature of 950℃±20℃, a carbon potential of 1.0±0.2%, and a time of 15h; a third stage of cooling process, a temperature of 870℃±20℃, a carbon potential of 0.9±0.2%, and a time of 2h.

2. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1 is characterized in that: The methanol flow rate during carburizing is 3.6-4.0L / h, and the process nitrogen flow rate is 4.0-4.4m 3 / h, propane flow rate is 0-2.0L / h.

3. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1, characterized in that: The primary oil quenching process adopts segmented cooling, and the segmented cooling adjusts the cooling rate through different stirring frequencies. The first cooling stirring time is 40 minutes, the stirring frequency is 1800rmp, the second cooling stirring time is 10 minutes, the stirring frequency is 600rmp, the quenching oil temperature is 30-80℃, and the cylindrical roller is oil quenched to 50℃.

4. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1, characterized in that: The high temperature tempering is performed at a temperature of 620° C. for 12 hours, then the temperature is raised to 650° C. for 12 hours, and then the temperature is lowered to room temperature by furnace cooling and air cooling.

5. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1, characterized in that: The secondary quenching is first heated to 815° C., and then cooled by salt bath quenching, and the salt bath quenching process is cooled in sections.

6. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1 or 5, characterized in that: The quenching temperature during the secondary quenching is 175° C., the first stage stirring time and frequency are 10 / 1300 rpm, the second stage stirring time and frequency are 7 / 650 rpm, and the total heating time for the secondary quenching is 120 minutes.

7. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1, characterized in that: After the secondary quenching, the temperature is lowered to room temperature and then low-temperature tempering is performed. The low-temperature tempering process temperature is 200° C. and the insulation time is 15 hours.

8. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1, characterized in that: Before the carburizing treatment, the predetermined carburizing temperature and carbon potential are reached by gradient temperature increase and gradient carbon potential increase.

9. The cylindrical roller carburizing heat treatment process combining carbon concentration gradient and high temperature carburizing according to claim 1 or 8, characterized in that: Before the carburizing treatment, the temperature is first raised to 860° C., the carbon potential is 0.8%, and kept warm for 60 minutes, then raised to 930° C., the carbon potential is raised to 1.20%, and kept warm for 3 hours, and finally raised to 950° C., the carbon potential is raised to 1.35%.

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