Low-carbon alloy steel carbonitriding continuous furnace system and heat treatment method

Abstract

The invention relates to the technical field of alloy heat treatment, in particular to a low-carbon alloy steel carbonitriding continuous furnace system and a heat treatment method. According to the specific technical scheme, the interior of a continuous furnace is sequentially divided into a pre-oxidation zone, a preheating zone, a strong carburizing zone, a diffusion zone, an intercooling zone, a secondary heating zone and a cooling zone, the tail end of the cooling zone is connected with double oil tanks for quenching, outlets of the double oil tanks are communicated with an oil blowing zone, and the oil blowing zone is communicated with an oil inlet. An outlet of the oil blowing area is sequentially communicated with the rear cleaning area, the tempering area and the air cooling area. The continuous furnace and intercooling process is adopted for carbonitriding, the production efficiency is effectively improved, the production cost is reduced, meanwhile, grain refinement can be conducted on workpieces with poor original structures, and quality improvement is achieved.

Description

Technical Field

[0001] This invention relates to the field of alloy heat treatment technology, specifically to a continuous carbonitriding furnace system and heat treatment method for low-carbon alloy steel. Background Technology

[0002] When performing carbonitriding in a conventional multi-purpose furnace, the lower carburizing temperature and longer time result in higher costs than normal high-temperature carburizing. Moreover, conventional carbonitriding lacks an intermediate cooling process; that is, it is directly cooled and quenched after carbonitriding. This can lead to workpieces with poor original microstructure having grain sizes that are coarser than grade 5 or exhibiting mixed grains, ultimately affecting the workpiece's lifespan.

[0003] Therefore, the present invention uses a continuous furnace + intercooling process for carbonitriding, which effectively improves production efficiency, reduces production costs, and can also refine the grains of workpieces with poor original structure, thereby improving quality. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a continuous carbonitriding furnace system and heat treatment method for low-carbon alloy steel.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention discloses a continuous carbonitriding furnace system for low-carbon alloy steel. The continuous furnace is divided into a pre-oxidation zone, a preheating zone, a strong infiltration zone, a diffusion zone, a medium cooling zone, a secondary heating zone, and a cooling zone. The tail end of the cooling zone is connected to a double oil tank for quenching. The outlet of the double oil tank is connected to an oil blowing zone. The outlet of the oil blowing zone is connected to a post-cleaning zone, a tempering zone, and an air cooling zone.

[0007] Preferably, the entrance and exit of the pre-oxidation zone are connected to the pre-cleaning zone.

[0008] Preferably, partition doors are provided between the pre-oxidation zone, preheating zone, strong permeation zone, diffusion zone, intermediate cooling zone, secondary heating zone, and cooling zone. A partition door is also provided between the cooling zone and the double oil tank. An insulation layer is provided on the outer wall of the continuous furnace.

[0009] Preferably, each of the two oil tanks is equipped with an online oil circulation filter device.

[0010] Accordingly, a heat treatment method based on the aforementioned low-carbon alloy steel carbonitriding continuous furnace system involves processing the low-carbon alloy steel material into workpieces with a stable P and F crystal phase structure. The workpieces are then sent to a pre-cleaning zone for cleaning at 80°C for 1 hour, followed by a continuous furnace process. In the pre-oxidation zone, they are heated to 500°C for 1.5 hours; then sent to a preheating zone for 1.5 hours at 880°C; then to a strong carbonitriding zone for 6 hours at 880°C with a controlled carbon potential of 1.1%; then to a diffusion zone for 3 hours at 880°C with a controlled carbon potential of 0.8%; then to a cooling zone for 2 hours at 500°C; then to a secondary heating zone for 3 hours at 880°C with a controlled carbon potential of 0.8%; finally, to a cooling zone for 2 hours at 810°C with a controlled carbon potential of 0.8%; and finally, quenched in a double oil bath to a temperature of 80-150°C.

[0011] Preferably, the quenched workpiece is sent to the oil blowing zone for treatment at room temperature for 0.5 hours, and then sequentially sent to the post-cleaning zone, tempering zone and air-cooling zone for treatment; in the post-cleaning zone, it is cleaned at 80°C for 1 hour; in the tempering zone, it is treated at 200°C for 4 hours; in the air-cooling zone, it is treated at room temperature for 0.5 hours.

[0012] The present invention has the following beneficial effects:

[0013] This invention employs a continuous furnace + intercooling process for carbonitriding, which effectively improves production efficiency, reduces production costs, and also refines the grains of workpieces with poor original microstructure, thereby improving quality. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the continuous furnace system disclosed in this invention;

[0015] In the diagram: 1. Pre-oxidation zone; 2. Preheating zone; 3. Strong penetration zone; 4. Diffusion zone; 5. Intercooling zone; 6. Secondary heating zone; 7. Cooling zone; 8. Double oil tank quenching zone; 9. Oil blowing zone; 10. Post-cleaning zone; 11. Tempering zone; 12. Air cooling zone; 13. Pre-cleaning zone; 14. Loading and unloading zone; 15. Material tray rotation zone. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Unless otherwise specified, the technical means used in the implementation examples are conventional means well known to those skilled in the art.

[0018] This invention discloses a continuous carbonitriding furnace system for low-carbon alloy steel. The continuous furnace is sequentially divided into a pre-oxidation zone 1, a preheating zone 2, a strong carbonitriding zone 3, a diffusion zone 4, an intermediate cooling zone 5, a secondary heating zone 6, and a cooling zone 7. The tail end of the cooling zone 7 is connected to a double oil tank for quenching. The outlet of the double oil tank is connected to an oil blowing zone 9. The outlet of the oil blowing zone 9 is sequentially connected to a post-cleaning zone 10, a tempering zone 11, and an air-cooling zone 12. The inlet and outlet of the pre-oxidation zone 1 are connected to a pre-cleaning zone 13.

[0019] like Figure 1 As shown, the entire system of the present invention has a ring-shaped structure, consisting of the following areas in sequence: loading and unloading area 14, material tray rotation area 15, pre-cleaning area 13, pre-oxidation area 1, preheating area 2, strong penetration area 3, diffusion area 4, intermediate cooling area 5, secondary heating area 6, cooling area 7, double oil tank quenching area 8, oil blowing area 9, post-cleaning area 10, tempering area 11, and air cooling area 12.

[0020] The pre-oxidation zone 1, preheating zone 2, strong penetration zone 3, diffusion zone 4, intermediate cooling zone 5, secondary heating zone 6, and cooling zone 7 are all equipped with partition doors 16. The cooling zone 7 is also equipped with a partition door 16 between it and the double oil tank. The outer wall of the continuous furnace is equipped with a heat insulation layer. At the same time, the outer wall of the tempering furnace (corresponding to the tempering zone) is also equipped with a heat insulation layer.

[0021] Furthermore, each of the dual oil tanks is equipped with an online oil circulation filtration device. It should be understood that the dual oil tanks consist of two oil tanks, each equipped with an online oil circulation filtration device. In this invention, unless otherwise specified, other disclosed devices or corresponding functional areas should be understood as existing equipment, such as the pre-cleaning area, post-cleaning area, oil blowing area, tempering area, and air-cooling area. Only equipment capable of performing the corresponding functions needs to be selected; for example, a tempering furnace can be selected for the tempering area, an oil blowing machine or air blowing device can be selected for the oil blowing area, and an air-cooling box can be selected for the air-cooling area, etc.

[0022] In this invention, the function of each zone is as follows:

[0023] (1) The pre-cleaning zone is set up to clean the workpiece and prevent oil or other impurities from entering the equipment and causing quality and safety risks.

[0024] (2) The pre-oxidation zone is used to dry the water on the surface of the cleaned workpiece, and at the same time, the temperature is appropriately increased to reduce the heating pressure in the preheating zone. There is a water vapor vent at the top to avoid excessive furnace pressure caused by water vaporization during the heating process.

[0025] (3) The preheating zone is used to heat the workpiece to near or reach the carburizing temperature, while nitrogen is used for deep gas replacement to remove oxygen from mixing in in order to avoid internal oxidation, so that the workpiece is heat treated in a nitrogen atmosphere.

[0026] (4) The strong carburizing zone is used to force carbon in the atmosphere into the surface of the workpiece, shortening the time of the entire carburizing process.

[0027] (5) The diffusion zone is used to distribute the carbon that has penetrated the workpiece surface more evenly to the subsurface layer of the workpiece while keeping the workpiece surface from decarburizing.

[0028] (6) The intermediate cooling zone is used to return the austenitized workpiece to the pearlite and ferrite state, refine the grains, make the structure more uniform, and make the grain size finer.

[0029] (7) The secondary heating zone is used to re-homogenize the austenite in the workpiece that has been refined by grain refinement.

[0030] (8) The cooling zone is used to lower the temperature of the workpiece before quenching, reduce the stability of austenite, reduce the amount of residual austenite, and minimize the deformation caused by the martensitic phase transformation of the workpiece. Among them, the workpiece is heat-treated in the strong penetration zone, diffusion zone, secondary heating zone and cooling zone under the atmosphere of nitrogen and a mixed gas containing active carbon atoms. The active carbon atoms are obtained by a complex cracking reaction between propane and methanol.

[0031] (9) The dual-oil-tank quenching zone is used for workpiece quenching and can store two different quenching oils simultaneously, adapting to workpieces with different structures and requirements, making it more flexible than single-oil-tank equipment. An online oil circulation and filtration device is also provided. The filtration device circulates and filters the oil according to the set interval and working time to remove solid suspended matter such as residual carbon and iron filings mixed in the oil, keeping the oil clean and extending its service life.

[0032] (10) In the oil blowing area, a high-pressure blower blows air onto the workpiece to accelerate the dripping process of the quenching oil. This reduces the amount of quenching oil adhering to the workpiece surface within the same time frame, saving oil costs, reducing the pressure of post-cleaning and the generation of waste oil, and protecting the environment. The system mainly uses a motor-controlled air nozzle to blow high-pressure air from the air pump at different angles onto the workpiece, causing the quenching oil adhering to the workpiece to drip off quickly. The dripped oil is collected in a dedicated recycling box for centralized processing.

[0033] (11) The post-cleaning area is used to clean the residual quenching oil on the workpiece to avoid excessive oil fumes during tempering, which could cause safety and environmental problems.

[0034] (12) The tempering zone is used for low-temperature tempering of workpieces after quenching.

[0035] (13) The air-cooled zone is used to quickly cool tempered workpieces to room temperature.

[0036] (14) The loading and unloading area is used by workers to hoist the loaded materials onto the tray or to lift the heat-treated workpieces off the tray.

[0037] (15) The material tray rotation area is used to rotate the material tray 90° to avoid deformation of the material tray caused by long-term unilateral force, which may eventually lead to premature failure and the risk of material jamming. At the same time, this station can realize automatic weighing function to avoid overloading and affecting the service life of the equipment.

[0038] This invention discloses a heat treatment method based on the aforementioned continuous carbonitriding furnace system for low-carbon alloy steel. The heat treatment of workpieces is performed in the aforementioned continuous furnace system, which is mainly applicable to low-carbon alloy steel materials such as SAE4320H and 20CrMoH.

[0039] The specific heat treatment steps are as follows:

[0040] (1) Process low carbon alloy steel material into workpieces. At this time, the crystal phase structure of the workpiece is stable P (pearlite) and F (ferrite). Send the workpiece to the pre-cleaning zone and clean it at 80°C for 1 hour. Then send it to the continuous furnace and heat it to 500°C in the pre-oxidation zone for 1.5 hours.

[0041] (2) Then it is sent to the preheating zone and treated at 880℃ for 1.5h. At this time, all the pearlite and ferrite in the workpiece are transformed into austenite structure.

[0042] (3) Then it is sent to the strong penetration zone and treated at 880℃ for 6 hours, and the carbon potential is controlled at 1.1% to force carbon into the surface of the workpiece.

[0043] (4) Then enter the diffusion zone and treat at 880℃ for 3 hours, and control the carbon potential to 0.8%. Under the premise of keeping the workpiece surface from decarburizing, the carbon that has penetrated the workpiece surface is more evenly distributed to the subsurface layer of the workpiece.

[0044] (5) Then it is sent to the intermediate cooling zone and treated at 500℃ for 2 hours to allow the austenitized workpiece to return to the pearlite and ferrite state, refine the grains, make the structure more uniform and the grain size finer.

[0045] (6) The workpiece is then sent to the secondary heating zone and treated at 880°C for 3 hours, with the carbon potential controlled at 0.8%, so that the workpiece that has completed grain refinement can be re-homogenized with austenite.

[0046] (7) It was then sent to the cooling zone and treated at 810℃ for 2 hours, with the carbon potential controlled at 0.8% to reduce the stability of austenite;

[0047] (8) Subsequently, the workpiece is sent to a double oil bath for quenching, and the temperature is reduced to 80-150℃. The quenching time is 0.5h. The microstructure of the workpiece after quenching is transformed from austenite to martensite (quenched martensite), with only a small amount of residual austenite remaining. Among them, quenched martensite is formed directly by rapid cooling (quenching) of austenite. Due to the excessively fast cooling rate, carbon atoms cannot diffuse, resulting in a supersaturated solid solution structure with a large number of dislocations and twins. The physical and chemical characteristics of this structure are extremely high hardness, but high brittleness, high residual stress, and easy cracking.

[0048] Regarding the presence of retained austenite, iron-based materials begin to transform into austenite above the AC1 temperature, and completely transform into austenite at the AC3 temperature. When the temperature rapidly decreases to the MS temperature (martensite transformation temperature), many martensite needles or blocks are instantaneously generated within the austenite. Because martensite has a body-centered cubic lattice while austenite has a face-centered cubic lattice, the volume of martensite is larger than that of austenite. This causes micro-expansion of the entire matrix, generating internal compressive stress, which hinders the transformation of austenite to martensite, thus resulting in retained austenite.

[0049] (9) The quenched workpiece is sent to the oil blowing zone for 0.5 hours at room temperature, and then sequentially sent to the post-cleaning zone, tempering zone, and air-cooling zone for further processing. In the post-cleaning zone, it is cleaned at 80°C for 1 hour. In the tempering zone, it is treated at 200°C for 4 hours. At this time, the microstructure of the workpiece is tempered martensite with a small amount of residual austenite. In the air-cooling zone, it is treated at room temperature for 0.5 hours, and finally discharged. Among them, due to the defects of quenched martensite, it is necessary to temper the quenched martensite. The tempered quenched martensite is called tempered martensite. After tempering, some carbon atoms precipitate to form fine carbides, the residual stress is reduced, but the acicular morphology is still retained. Tempered martensite has a two-phase microstructure (α-iron matrix + dispersed ε-carbides). The supersaturation of carbon is reduced, the hardness is reduced, but the toughness is significantly improved, the internal stress is reduced, and the overall performance is better.

[0050] The workpiece treated by the above heat treatment method according to the present invention has a grain size grade of 8, a carbide grade of 1-2, a martensite grade of 1-3, and a retained austenite grade of 1-3.

[0051] Simultaneously, using the same heat treatment method, 100 samples were tested. The final results showed that the probability of the grain size grade being 8 was 100% for the 100 treated workpieces, the probability of the carbide grade being 1 was 97%, and the probability of grade 2 was 3%; the probability of the martensite grade being 1 was 1%, the probability of grade 2 was 22%, and the probability of grade 3 was 77%; the probability of the retained austenite grade being 1 was 24%, the probability of grade 2 was 72%, and the probability of grade 3 was 4%. The results indicate that after the heat treatment disclosed in this invention, the grain size of workpieces made of low-carbon alloy steel reaches grade 8, thus improving the quality of the workpieces.

[0052] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A continuous carbonitriding furnace system for low-carbon alloy steel, characterized in that: The continuous furnace is divided into a pre-oxidation zone, a preheating zone, a strong infiltration zone, a diffusion zone, a medium cooling zone, a secondary heating zone, and a cooling zone. The end of the cooling zone is connected to a double oil tank for quenching. The outlet of the double oil tank is connected to an oil blowing zone. The outlet of the oil blowing zone is connected to a post-cleaning zone, a tempering zone, and an air cooling zone.

2. The continuous carbonitriding furnace system for low-carbon alloy steel according to claim 1, characterized in that: The pre-oxidation zone is connected to the pre-cleaning zone at its entrance and exit.

3. The continuous carbonitriding furnace system for low-carbon alloy steel according to claim 1, characterized in that: Isolation doors are provided between the pre-oxidation zone, preheating zone, strong permeation zone, diffusion zone, intermediate cooling zone, secondary heating zone, and cooling zone. Isolation doors are also provided between the cooling zone and the double oil tank. An insulation layer is provided on the outer wall of the continuous furnace.

4. The continuous carbonitriding furnace system for low-carbon alloy steel according to claim 1, characterized in that: Each of the two oil tanks is equipped with an online oil circulation and filtration device.

5. A heat treatment method based on the low-carbon alloy steel carbonitriding continuous furnace system according to any one of claims 1-4, characterized in that: Low-carbon alloy steel material is processed into workpieces with a stable P and F crystal phase structure. The workpieces are sent to the pre-cleaning zone and cleaned at 80℃ for 1 hour, then sent to the continuous furnace and heated to 500℃ in the pre-oxidation zone for 1.5 hours; sent to the preheating zone and treated at 880℃ for 1.5 hours; sent to the strong diffusion zone and treated at 880℃ for 6 hours, with the carbon potential controlled at 1.1%; sent to the diffusion zone and treated at 880℃ for 3 hours, with the carbon potential controlled at 0.8%; sent to the intermediate cooling zone and treated at 500℃ for 2 hours; sent to the secondary heating zone and treated at 880℃ for 3 hours, with the carbon potential controlled at 0.8%; sent to the cooling zone and treated at 810℃ for 2 hours, with the carbon potential controlled at 0.8%; and finally sent to the double oil bath for quenching, cooling to 80-150℃.

6. The carbonitriding heat treatment method for low-carbon alloy steel according to claim 5, characterized in that: The quenched workpieces were sent to the oil blowing zone and treated at room temperature for 0.5 hours. Then they were sent to the post-cleaning zone, tempering zone and air-cooling zone for treatment in sequence. In the post-cleaning zone, the workpieces were cleaned at 80°C for 1 hour. In the tempering zone, the workpieces were treated at 200°C for 4 hours. In the air-cooling zone, the workpieces were treated at room temperature for 0.5 hours.

Images