Carburizing and quenching heat treatment process

Abstract

The invention relates to a carburizing and quenching heat treatment process which comprises the following steps: carburizing, diffusing and preserving heat of medium alloy steel in sequence, directly quenching, and tempering at low temperature. The carburizing and quenching heat treatment process for the medium alloy steel is provided for solving the problems that an existing carburizing and quenching process for the medium alloy steel is long in production period, low in efficiency and complex in operation, and it can be guaranteed that a martensite structure on the surface of a steel infiltrated layer is finer, the depth of a non-martensite structure layer is shallower, and heat treatment deformation is smaller through the process; the service life of the steel is greatly prolonged, the process repeatability is good, and the product quality is stable.

Description

Technical Field

[0001] The present application relates to the field of alloy heat treatment, and in particular to a carburizing and quenching heat treatment process. Background Art

[0002] 17CrNiMo6 material is mainly used for gears that transmit large power and bear large loads. It has been widely used in various industrial fields such as engineering machinery, ships, and wind power generation. Due to the high content of Cr and Ni elements and good hardenability, 17CrNiMo6 has excellent mechanical and mechanical properties. Its yield strength and tensile strength are 487MPa and 698MPa respectively.

[0003] As for the heat treatment process of the material, the prior art, such as a carburizing and quenching method of a medium alloy carburizing steel disclosed in Chinese patent document CN 115287584 B, comprises: carburizing, equalizing, air cooling, and high-temperature tempering the steel, and then directly heating it to the quenching temperature without cooling for quenching and low-temperature tempering; the carbon potential of the carburizing is 0.7-1.3%C; methanol is added in advance during the heating to the high-temperature tempering process before the high-temperature tempering after the air cooling to provide atmosphere protection during the high-temperature tempering process; the introduction temperature of the methanol is 600-640°C; the temperature is directly raised to the quenching temperature without cooling after the high-temperature tempering; isopropanol is introduced during the heating to the quenching temperature after the high-temperature tempering to establish the carbon potential in advance; the introduction temperature of the isopropanol is 760-780°C; the temperature of the high-temperature tempering is 600-680°C, the temperature of the quenching is 800-860°C, and the carbon potential of the quenching is 0.6-1.0%C.

[0004] The above scheme and the traditional heat treatment process are all heat treatment processes of carburizing slow cooling + high temperature tempering + quenching + low temperature tempering, where the purpose of high temperature tempering is to obtain uniform and fine carbides for subsequent quenching and reduce the content of residual austenite. This traditional carburizing process is relatively conservative, with a long production cycle, low efficiency and complex operation. Therefore, it is urgent to improve the carburizing and quenching process of the material 17CrNiMo6. Summary of the invention

[0005] In view of the long production cycle, low efficiency and complicated operation of the existing carburizing and quenching process for medium alloy steel, the present invention provides a carburizing and quenching heat treatment process for medium alloy steel. The process can ensure that the martensitic structure on the surface of the steel carburized layer is finer, the non-martensitic structure layer is shallower, the heat treatment deformation is smaller, etc., thereby greatly improving the service life of the steel. In addition, the process has good repeatability and stable product quality.

[0006] The solution of the present invention is a carburizing and quenching heat treatment process, comprising:

[0007] The medium alloy steel is carburized, diffused, and kept warm in sequence, then directly quenched and then tempered at a low temperature.

[0008] Preferably, the temperatures of the carburizing and diffusion treatments are both 935°C±5°C, the carbon potential of carburizing is 1.20%±0.05%, the carbon potential of diffusion is 0.70%±0.05%, there is no interval between carburizing and diffusion, and the ratio of carburizing to diffusion duration is 1:0.5.

[0009] Preferably, the temperature of the medium alloy steel during quenching treatment is 820°C±5°C, the carbon potential of quenching is 0.60%±0.05%, and the temperature of the quenching medium is 60°C±10°C.

[0010] Preferably, the heat preservation treatment is to stop heating at the temperature of the diffusion period, cool down naturally with the furnace to 820°C±5°C and then keep warm, and the ratio of the duration of carburizing to that of heat preservation is 1:0.5.

[0011] Preferably, after the medium alloy steel is cooled in the quenching medium, it is placed in the air for several hours to further naturally cool to room temperature and then low-temperature tempering is performed, and the temperature of the low-temperature tempering treatment is 190°C±10°C.

[0012] Preferably, the duration of the carburizing and diffusion treatment is proportional to the thickness of the hardened layer of the medium alloy steel and the module size of the gear.

[0013] Preferably, the duration of the low temperature tempering treatment is proportional to the size of the medium alloy steel.

[0014] Preferably, the material of the medium alloy steel is 17CrNiMo6, and the durations of carburizing, diffusion, insulation and low-temperature tempering are 16 hours, 8 hours, 3 hours and 8 hours respectively.

[0015] Preferably, the quenching medium is oil.

[0016] Preferably, the carburizing, diffusion and insulation stages are all carried out in the same pit furnace.

[0017] By adopting the above technical scheme, the medium alloy steel is directly cooled and quenched after carburizing, which reduces the process of entering the furnace, heating, and exhausting once, improves the non-martensitic structure, and reduces the risk of grinding cracking. When the temperature is directly cooled and quenched after carburizing, the quenching is placed in the air for more than several hours, and most of the supercooled austenite on the surface is transformed into martensite. The remaining supercooled austenite (surface and core) is cooled to below the Ms point during the placement process and slowly transformed into martensite. The structure is completely transformed, and finally martensite and a small amount of residual austenite are obtained. During the placement process of the gear parts processed by the medium alloy steel after quenching in the process of the present invention, the temperature of the surface and the core of the parts is basically the same, which reduces the temperature difference between the inside and outside, reduces the thermal stress and the structural stress, and controls the deformation of the outer circle of the shaft during heat treatment to about 0.15mm.

[0018] The heat treatment process of the present invention mainly controls the carbon potential and heating temperature of the medium alloy steel during the carburizing process, especially the carbon potential and quenching temperature in the later stage of carburizing, and direct quenching after carburizing is feasible. The direct cooling and quenching process of the medium alloy steel after carburizing has stable process quality, shortened production cycle, reduced production cost, and significant economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic flow chart of an embodiment of the heat treatment process of the present invention.

[0020] Figure 2 This is a comparison diagram of the martensite structure of the tooth carburized layer surface of the conventional process and the embodiment of the present invention, wherein Figure 2 a is a micrograph of the martensitic structure of the tooth surface in the conventional process at level 3 at a magnification of 400 times. Figure 2 b is a micrograph of the martensitic structure of level 2 on the surface of the tooth carburized layer of an embodiment of the present invention at a magnification of 400 times.

[0021] Figure 3 This is a comparison diagram of the non-martensite structure of the tooth carburized layer surface between the conventional process and the embodiment of the present invention, wherein Figure 3 a is a micrograph of the non-martensitic structure of the tooth surface in the conventional process with a thickness of 0.02 mm at a magnification of 400 times. Figure 3 b is a micrograph of the non-martensitic structure of the tooth diffusion layer surface with a thickness of 0.01 mm at a magnification of 400 times in an embodiment of the present invention. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of this application more clear, the following Figure 1 It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0023] The embodiments of the present application are further described in detail below in conjunction with the drawings in the specification.

[0024] Taking 17CrNiMo6 material as an example, we can see the characteristics of alloy steel. Since it is a carburizing steel with high Cr and Ni content, the carbon content of the eutectoid component is reduced, and carbides are easily formed on the surface during carburizing, making the carbon content in the surface austenite lower than that in the subsurface. By studying the CCT curve, it can be seen that with the increase of carbon content, the hardenability also increases, and the C curve shifts to the right. In this way, if air cooling is performed after carburizing, a troostite structure may be formed on the surface, so air cooling after carburizing is prone to surface cracks. After carburizing and direct quenching, it is easy to have carbide level deviation, martensite and residual austenite level deviation, and low hardness. Since the ratio of Cr and Ni content in 17CrNiMo6 material is close to 1:1, the hardenability of the material is very high. According to the characteristics of this material, we have controlled the carbon concentration of the carburized surface layer and the carbon concentration gradient of the entire carburized layer within an appropriate range through multiple tests and analyses, and at the same time selected a reasonable quenching temperature and quenching medium, so it is feasible to realize direct carburizing and quenching of 17CrNiMo6. In this new treatment process, the key is to control the surface carbon potential and quenching temperature, especially the carbon potential during the diffusion period and the insulation process before quenching. The process flow can be summarized as carburizing, diffusion, and insulation of the medium alloy steel in sequence, then quenching directly and then low-temperature tempering.

[0025] The solution is described as follows:

[0026] A pit furnace is used as the process equipment, and the carburizing, diffusion and insulation stages are all carried out in the same pit furnace. After starting the equipment, the heating rate is appropriately controlled, and the carburizing stage is entered after the temperature is raised to 935℃ for 20 minutes. The temperature of the carburizing and diffusion treatments are both 935℃±5℃, the carbon potential of carburizing is 1.20%C±0.05%C, and the carbon potential of diffusion is 0.70%C±0.05%C. There is no interval between the carburizing and diffusion, and the ratio of the duration of carburizing to diffusion is 1:0.5. The subsequent insulation treatment is to stop heating at the temperature of the diffusion period, and keep the temperature naturally lowered to 820℃±5℃ with the furnace. The ratio of the duration of carburizing to insulation is 1:0.5. The temperature during quenching treatment is 820℃±5℃, the carbon potential of quenching is 0.60%C±0.05%C, and the temperature of the quenching medium is 60℃±10℃. After the medium alloy steel is cooled in the quenching medium, it is placed in the air for several hours to further cool naturally to room temperature and then low-temperature tempering is performed. The temperature of the low-temperature tempering treatment is 190°C ± 10°C. The specific process is as follows Figure 1 shown.

[0027] Other medium oils can also be used as quenching medium oil. Quenching with medium oil can quickly cool down and isolate air to ensure the quenching effect. The duration of the carburizing and diffusion treatment is proportional to the thickness of the hardened layer of the medium alloy steel and the module size of the gear. The duration of the low-temperature tempering treatment is proportional to the size of the medium alloy steel. That is, the relationship between the duration of the carburizing and diffusion treatment and the thickness of the hardened layer of the low-carbon alloy steel is determined according to the designed module size of the gear. The larger the module, the deeper the hardened layer, and vice versa. The relationship between the duration of the low-temperature tempering treatment and the size of the low-carbon alloy steel is that the larger the effective size of the part, the longer the tempering treatment time, and vice versa.

[0028] The medium alloy steel material is taken as a specific embodiment of 17CrNiMo6 material, and further compared and explained by comparing it with gear parts processed by 17CrNiMo6 material under traditional processing.

[0029] A pit furnace is used as the process equipment, and the carburizing, diffusion and insulation stages are all carried out in the same pit furnace. After starting the equipment, the heating rate is properly controlled. After heating to 935°C and averaging for 20 minutes, the carburizing stage begins. The duration of carburizing is 16 hours, and the carbon potential Cp is maintained at 1.20% during the carburizing period. The duration of the diffusion period is 8 hours, and the carbon potential Cp is maintained at 0.70%. After the diffusion period, the temperature is lowered to 820°C with the furnace, and the carbon potential Cp is maintained at 0.60%. The insulation is completed after 3 hours, and it is cooled in 60°C Houghton K-2000 quenching oil. After cooling, it is placed in the air for several hours, and low-temperature tempering is carried out in a 190°C device for 8 hours.

[0030] As a comparative example, the 17CrNiMo6 material was treated by the traditional heat treatment process:

[0031] Carburizing and air cooling: The carburizing temperature is about 920℃, the heating rate is 50-90℃ / h, and the carbon potential Cp is 1.2%; after carburizing, the temperature of the parts is reduced to 840℃ and kept for 4h (carbon potential Cp is 0.6-0.8%), and then the gear parts are lifted out of the furnace for air cooling.

[0032] The intermediate gear shaft is hoisted into the furnace, the furnace temperature is raised to the high return temperature of 650℃, and after keeping warm for 3 to 5 hours, it is taken out of the furnace and cooled to room temperature. When the temperature is raised to 780℃, methanol is introduced into the furnace at a heating rate of 50 to 90℃ / h, and the quenching temperature is directly raised to about 820℃. After keeping warm for 5 to 8 hours, it is quenched with oil. The oil temperature is 60±10℃, and the carbon potential Cp is 0.6 to 0.8%.

[0033] Low temperature tempering: The low temperature tempering temperature is 200℃, the heating temperature is 50~90℃ / h, the tempering time is 10h, and then it is taken out of the furnace and air cooled.

[0034] In the above embodiments and comparative examples, the gear shaft module m of the gear part is 16, the number of teeth Z is 15, the tooth surface is carburized and quenched, the process layer depth CHD is required to be 2.0-2.5 mm, the tooth surface hardness is 58-62 HRC, the core hardness is 30-45 HRC, the carbide is ≤ level 3, the surface martensite and residual austenite are ≤ level 3, and the ferrite is ≤ level 4.

[0035] The gears after carburizing and quenching in the embodiment of the present invention were tested, and the test results showed that the tooth surface hardness of the part was 62HRC (the test method was GB / T 17394, GB / T 230.1, and the test standard was GB / T 3480.5), and the core hardness was 40-41HRC (GB / T230.1); the metallographic sample test (GB / T 25744) CHD=2.16mm (GB / T 9450), the carbide was level 1-2, the surface martensite and residual austenite were level 1-2, and the ferrite was level 1 (the test standard was JB / T 6141), and the part indicators were excellent.

[0036] The gears after carburizing and quenching of the conventional heat treatment process were tested. The test results showed that the tooth surface hardness of the part was 60HRC (test method GB / T 17394, GB / T 230.1, test standard GB / T 3480.5), the core hardness was 38HRC (GB / T 230.1); metallographic sample test (GB / T 25744) CHD = 2.05mm (GB / T 9450), carbide was level 2, surface martensite and residual austenite were level 2, ferrite was level 1 (test standard JB / T 6141), and the parts were qualified.

[0037] The enlarged comparison diagram of the martensite structure of the surface of the tooth carburized layer of the gear parts of the embodiment of the present invention and the comparative example is shown in Figure 2 , see the penetration depth comparison chart Figure 3 Compared with the two, the direct cooling and quenching of 17CrNiMo6 material after carburizing in the present invention can significantly improve the metallographic structure, increase the surface hardness, reduce non-martensite, reduce heat treatment deformation, and the carbon potential control in the carburizing diffusion and quenching stages is lower than that of the traditional process, and the performance parameters are significantly better than those of the traditional process.

[0038] In the present invention, the 17CrNiMo6 material is directly cooled and quenched after carburizing, which reduces the process of entering the furnace, heating, and exhausting once, improves the non-martensitic structure, and reduces the risk of grinding cracking. When the quenching is directly cooled and quenched after carburizing, the quenching is placed in the air for more than several hours, and most of the supercooled austenite on the surface is transformed into martensite. The remaining supercooled austenite (surface and core) is cooled to below the Ms point during the placement process and slowly transformed into martensite. The structure is completely transformed, and finally martensite and a small amount of residual austenite are obtained. During the placement process of the gear parts processed by 17CrNiMo6 material after quenching by the process of the present invention, the temperature of the surface and the core of the parts is basically the same, which reduces the temperature difference between the inside and outside, reduces the thermal stress and the structural stress, and controls the deformation of the outer circle of the shaft during heat treatment to about 0.15mm.

[0039] This heat treatment process mainly controls the carbon potential and heating temperature of 17CrNiMo6 materials during carburizing, especially the carbon potential and quenching temperature in the later stage of carburizing. It is feasible to directly quench after carburizing. The direct cooling and quenching process of 17CrNiMo6 materials after carburizing has stable process quality, shortened production cycle, reduced production cost, and significant economic benefits.

[0040] Those skilled in the art can clearly understand that for the convenience and simplicity of description, only the division of the above-mentioned functional modules is used as an example. In actual applications, the above-mentioned functions can be assigned to different functional modules as needed, that is, the internal structure of the device is divided into different functional modules to complete all or part of the functions described above. The specific working process of the system, device and unit described above can refer to the corresponding process in the aforementioned method embodiment, and will not be repeated here.

Claims

1. A carburizing and quenching heat treatment process, characterized in that: include: The medium alloy steel is carburized, diffused, and kept warm in sequence, then directly quenched and then tempered at a low temperature.

2. A carburizing and quenching heat treatment process according to claim 1, characterized in that: The steps include: The temperatures of the carburizing and diffusion treatments are both 935°C±5°C, the carbon potential of carburizing is 1.20%±0.05%, the carbon potential of diffusion is 0.70%±0.05%, there is no interval between carburizing and diffusion, and the ratio of the duration of carburizing to diffusion is 1:0.

5.

3. A carburizing and quenching heat treatment process according to claim 2, characterized in that: The temperature of the medium alloy steel during quenching treatment is 820°C±5°C, the carbon potential of quenching is 0.60%±0.05%, and the temperature of the quenching medium is 60°C±10°C.

4. A carburizing and quenching heat treatment process according to claim 3, characterized in that: The heat preservation treatment is to stop heating at the temperature of the diffusion period, cool down naturally with the furnace to 820°C±5°C and then keep the temperature, and the ratio of the duration of carburization to that of heat preservation is 1:0.

5.

5. A carburizing and quenching heat treatment process according to claim 3, characterized in that: After the medium alloy steel is cooled in the quenching medium, it is placed in the air for several hours to further naturally cool to room temperature and then low-temperature tempering is performed. The temperature of the low-temperature tempering treatment is 190°C±10°C.

6. A carburizing and quenching heat treatment process according to claim 3, characterized in that: The duration of the carburizing and diffusion treatment is proportional to the thickness of the hardened layer of the medium alloy steel and the module size of the gear.

7. A carburizing and quenching heat treatment process according to claim 5, characterized in that: The duration of the low temperature tempering treatment is proportional to the size of the medium alloy steel.

8. A carburizing and quenching heat treatment process according to any one of claims 1 to 7, characterized in that: The material of the medium alloy steel is 17CrNiMo6, and the durations of carburizing, diffusion, heat preservation and low-temperature tempering are 16 hours, 8 hours, 3 hours and 8 hours respectively.

9. A carburizing and quenching heat treatment process according to claim 8, characterized in that: The quenching medium is oil.

10. A carburizing and quenching heat treatment process according to any one of claims 1 to 7, characterized in that: The carburizing, diffusion and heat preservation stages are all carried out in the same pit furnace.

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