High temperature carburizing process for different media quenched gear steels

Abstract

The present application belongs to the field of metal materials, and particularly relates to a high-temperature carburizing process method of gear steel quenched by different media. The present application develops the carburizing process of gear alloy carburizing steel by finite element simulation mode, and proposes a double-medium quenching process to improve the phase organization of the core of the sample, reduce the martensite content of the core, increase the bainite content, thereby reduce the hardness of the core, and further improve the toughness of the core of the sample, on the basis of ensuring the control of production cost, without increasing the process flow and improving the use performance. The carburized surface is still martensite organization, and the high hardness of the surface layer is maintained.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, specifically relating to a high-temperature carburizing process for gear steel quenched in different media. Background Technology

[0002] Carburizing is a chemical heat treatment method used in the hot working of workpieces such as gears and bearings. Its purpose is to increase the carbon content on the workpiece surface to a certain depth, thereby obtaining a high-hardness surface and a strong, tough core. Carburizing mainly involves heating and holding the workpiece in a carburizing medium, allowing carbon atoms to diffuse into the surface layer, forming a carbon concentration gradient with the core. When the effective hardened layer depth reaches 3mm or more, it can be called deep carburizing. Deep carburizing is the most effective surface strengthening method for heavy-duty gears both domestically and internationally. Currently, the carburizing process for heavy-duty gears is carried out at around 930℃, which has disadvantages such as long process cycle, high energy consumption, and high labor intensity. Increasing the carburizing temperature is an effective means to improve the carbon diffusion rate and accelerate carburizing efficiency. After obtaining a deep carburized layer through high-temperature carburizing, heavy-duty gears undergo high-temperature tempering, quenching, cold treatment, and low-temperature tempering heat treatment processes to obtain a martensitic structure on the surface, thus obtaining a deep, effectively hardened layer.

[0003] Carburizing gear steel at high temperatures leads to coarse austenite grains, reducing mechanical properties. As the carburizing temperature increases, the grains in the gear core inevitably grow larger, potentially causing excessively high core hardness and reduced toughness after quenching. Therefore, after high-temperature carburizing, gear steel typically employs methods such as multiple quenching, raw material micro-alloying, preheating, and pre-carburizing to control grain growth, thereby improving performance while enhancing carburizing efficiency.

[0004] Currently, the quenching of high-temperature carburized heavy-duty gears is generally carried out using only oil as a medium, which easily leads to excessively high core hardness and reduced toughness after quenching. This technology aims to use both oil and air as media during the quenching heat treatment process of high-temperature carburized heavy-duty gears, thereby reducing the martensite content and increasing the bainite content in the core to a certain extent, and improving the core toughness.

[0005] In the prior art, application number 202211053387.8 discloses a Nb-Ti-B microalloyed high-temperature resistant, low-internal-oxidation carburizing gear steel and its manufacturing method; application number CN201811471734.2 discloses a heat treatment process for heavy-duty gears with large carburization layers; application number 201810943344.4 discloses a gear carburizing process and its application; application number CN201510916396.9 discloses a heat treatment process for shallow-deep carburized gears with hardness gradient; and application number CN201510799731.1 discloses a carburizing heat treatment process for 8620H gear steel.

[0006] While the aforementioned existing technologies, through multiple quenching processes, can refine the grain size of gear steel and improve the overall performance of the material, they significantly exacerbate gear deformation, increase grinding allowance, and cause the gear to lose beneficial surface residual compressive stress, optimal metallographic structure, and excellent wear resistance. Microalloying of raw materials is also an effective method to suppress the coarsening of the original austenite grains in gear steel; however, there is currently no unified standard for heavy-duty gear steel, its application is not mature, and the quality of small-batch steelmaking is difficult to control stably, resulting in high raw material costs for gear manufacturing. Summary of the Invention

[0007] Existing technologies generally do not involve high-temperature rapid carburizing, nor can they further improve the overall performance while ensuring the original performance.

[0008] Current research mainly focuses on improving the overall performance of materials by slightly modifying the heat treatment process after high-temperature carburizing. The purpose of this invention is to provide a high-temperature carburizing process for gear steel quenched in different media and its application, to improve the problems of existing technologies that require multiple quenchings, microalloying of raw materials, preheating, and precarburizing, resulting in a cumbersome process and high manufacturing costs for new materials. This invention improves the phase composition of the sample core, reduces martensite content, increases bainite content, and reduces core hardness to further improve the toughness of the sample core, while the carburized surface remains martensitic and maintains high hardness.

[0009] To address the aforementioned technical problems, this application provides the following technical solution:

[0010] This invention provides a high-temperature carburizing process for gear steel quenched in different media, comprising the following steps:

[0011] S1: Carburize the gear steel workpiece at 970℃ and hold for 40 hours, then air cool it.

[0012] S2: Perform two high-temperature tempering processes on the gear steel workpiece after air cooling in step S1;

[0013] S3: Heat the gear steel workpiece that has undergone two high-temperature temperings in step S2 to 810-830℃, oil cool for 32s, and then air cool to room temperature (25±5℃).

[0014] S4: After the gear steel workpiece that has been air-cooled to room temperature in step S3 is cold-treated, it is heated to 190-210℃ and tempered for 7.5-8.5 hours, then removed from the furnace and air-cooled to room temperature.

[0015] Preferably, in step S1, after carburizing and heat preservation, the furnace is cooled and then air-cooled after exiting the furnace.

[0016] Furthermore, the furnace is cooled to 810-830°C before being removed and air-cooled.

[0017] Preferably, in step S2, the high-temperature tempering method involves heating the gear steel workpiece and then air-cooling it to room temperature.

[0018] Furthermore, the heating temperature is 620-640℃.

[0019] Furthermore, the heating and heat preservation time is 3.5-4.5 hours.

[0020] Preferably, in step S3, the heating time is 60-70 minutes.

[0021] Preferably, in step S4, the temperature for cold treatment is -80 to -70°C.

[0022] Preferably, in step S4, the cold treatment time is 90-150 minutes.

[0023] The quenching process following high-temperature carburizing is a complex physical process involving the interaction of temperature, microstructure, and stress fields. During quenching, there is a significant temperature difference between the surface and core of the sample. This invention uses 18Cr2Ni4W alloy steel as an example. Figure 1 This is a temperature distribution cloud map of the sample after quenching in oil for 32 seconds. At this moment, the surface temperature of the sample is approximately 270℃, while the core temperature is approximately 430℃.

[0024] The phase transformation temperatures of 18Cr2Ni4W alloy steel under different carbon contents were obtained by calculation and simulation using JMatPro software. The results are as follows: Figure 2 As shown. Figure 2 The two horizontal dashed lines in the middle represent Figure 1 Temperatures of the core and surface of the sample. The carbon concentration in the core is approximately 0.14 wt.%, and after 32 seconds of oil quenching, the core temperature is higher than the martensite initiation temperature; therefore, the core still retains an austenitic structure. Changing the quenching medium at this point and reducing the cooling rate of the core would help reduce the martensite content and increase the bainite content.

[0025] Based on the above analysis, the dual-medium quenching method proposed in this invention involves oil quenching for 32 seconds followed by air cooling to room temperature. The specific carburizing heat treatment process flow chart is as follows: Figure 3 As shown. Oil has a much higher heat transfer capacity than air. Oil quenching for 32 seconds can cause the surface carburized layer to undergo martensitic transformation. Air cooling can reduce the cooling rate of the core, allowing more bainite to form in the core, thereby improving the toughness of the core.

[0026] The present invention also provides gear steel prepared by the above-mentioned high-temperature carburizing process of gear steel quenched in different media.

[0027] The technical solution of the present invention has the following advantages compared with the prior art:

[0028] This invention utilizes finite element method (FEM) simulation to develop a carburizing process for gear alloy carburized steel. While ensuring controlled production costs, without increasing process steps, and improving performance, it proposes a dual-medium quenching process to improve the phase structure of the sample core, reducing the martensite content and increasing the bainite content, thereby reducing core hardness and further improving core toughness. Meanwhile, the carburized surface retains a martensitic structure, maintaining high surface hardness. Attached Figure Description

[0029] Figure 1 The temperature distribution cloud map of the sample after 32s oil quenching.

[0030] Figure 2 The diagram shows the phase transformation temperatures of 18Cr2Ni4W alloy steel with different carbon contents.

[0031] Figure 3 This is a process flow diagram of the present invention.

[0032] Figure 4 The following are simulation results of carbon concentration distribution: (a) carbon concentration distribution cloud map; (b) carbon concentration distribution curve.

[0033] Figure 5 Figures show the simulation results of the microstructure and properties of quenching in a single oil medium; (a) microstructure distribution cloud map; (b) microstructure distribution curve; (c) comparison of simulated and experimental hardness distribution.

[0034] Figure 6 Figures show the simulation results of the microstructure and properties of dual-medium quenching; (a) microstructure distribution cloud map; (b) microstructure distribution curve; (c) comparison of hardness distribution between single-medium quenching and dual-medium quenching.

[0035] Figure 7 The microstructure of 18Cr2Ni4WA steel after dual-medium quenching is shown in the diagram; (a) is the carburized layer; and (b) is the core. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0037] Example 1

[0038] Carburizing: The gear steel sample was carburized at 970℃ and held for 40 hours, then cooled in the furnace to 820℃ and then air-cooled.

[0039] Two high-temperature tempering processes: The sample was heated to 630°C and held at that temperature for 4 hours before being air-cooled, and this process was repeated twice.

[0040] Quenching: Heat the sample to 820℃ and hold for 60 minutes, then oil cool for 32 seconds, and then air cool to room temperature.

[0041] Cryogenic treatment: The sample is cryogenically treated at -75℃ for 2 hours.

[0042] Low-temperature tempering: After the sample temperature returns to room temperature, heat it to 200℃ for tempering, then remove it from the furnace and air cool it to room temperature.

[0043] Example 2

[0044] Carburizing: The gear steel sample was carburized at 970℃ and held for 40 hours, then cooled in the furnace to 820℃ and then air-cooled.

[0045] Two high-temperature tempering processes: The sample was heated to 630℃ and held at that temperature for 4.5 hours before being air-cooled, and this process was repeated twice.

[0046] Quenching: Heat the sample to 810℃ and hold for 70 minutes, then oil cool for 32 seconds, and then air cool to room temperature.

[0047] Cryogenic treatment: The sample is cryogenically treated at -80℃ for 2 hours.

[0048] Low-temperature tempering: After the sample temperature returns to room temperature, heat it to 190°C for tempering, then remove it from the furnace and air cool it to room temperature.

[0049] Example 3

[0050] Carburizing: The gear steel sample was carburized at 970℃ and held for 40 hours, then cooled in the furnace to 820℃ and then air-cooled.

[0051] Two high-temperature tempering processes: The sample was heated to 630°C and held at that temperature for 4 hours before being air-cooled, and this process was repeated twice.

[0052] Quenching: Heat the sample to 830℃ and hold for 60 minutes, then oil cool for 32 seconds, and then air cool to room temperature.

[0053] Cryogenic treatment: The sample is cryogenically treated at -70℃ for 2.5 hours.

[0054] Low-temperature tempering: After the sample temperature returns to room temperature, heat it to 210℃ for tempering, then remove it from the furnace and air cool it to room temperature.

[0055] Effect Evaluation 1

[0056] Firstly, this invention uses finite element method (FEM) simulation to study the dual-medium quenching process and verify its feasibility. The carburizing heat treatment process is then simulated using a finite element model, and the simulated carbon concentration distribution results obtained through the heat treatment process of this invention are as follows: Figure 4As shown in the carbon concentration distribution cloud map, after the intense carburizing period and diffusion period of high-temperature carburizing, the sample surface obtains a high carbon concentration carburized layer with a thickness of approximately 5 mm. The carbon concentration distribution of the carburized layer on the sample surface is monotonically decreasing. After 30 hours of intense carburizing, the surface carbon concentration of the sample is approximately 1.24 wt.%, and the carbon concentration at a distance of 5 mm from the surface is approximately 0.17 wt.%. However, after 10 hours of diffusion, the surface carbon concentration of the sample is approximately 0.84 wt.%, and the carbon concentration at a distance of 5 mm from the surface is approximately 0.24 wt.%. This indicates that the diffusion period significantly reduces the carbon concentration on the sample surface, thereby increasing the thickness of the carburized layer. This is because the intense carburizing period with high carbon potential is the process of carbon atoms being adsorbed onto the workpiece surface, while the diffusion period with low carbon potential is the process of carbon atoms diffusing into the sample under the driving force of the carbon concentration gradient.

[0057] Figure 5 Simulation results of microstructure and properties after single-medium quenching. The microstructure obtained after single-medium quenching includes martensite, bainite, and retained austenite. The main microstructure of the sample is martensite, while bainite and retained austenite are mainly distributed in the core and carburized layer, respectively. Figure 5 (a) The martensite content increases and then decreases from the surface to the core, reaching a maximum at approximately 5 mm. The bainite content gradually increases from the surface to the core, while the retained austenite content gradually decreases, such as... Figure 5 (b) After carburizing and quenching, the martensite, retained austenite, and bainite contents obtained on the sample surface were 87.7%, 11.9%, and 0.4%, respectively. At a distance of 5 mm from the surface, the martensite, retained austenite, and bainite contents were 96.8%, 1.7%, and 1.5%, respectively. At the core of the sample, the martensite, retained austenite, and bainite contents were 79.4%, 0.1%, and 20.5%, respectively. The higher carbon concentration at the sample surface lowers the martensite initiation temperature, thereby increasing the stability of austenite and the incubation period for bainite transformation. Therefore, after carburizing and quenching, the sample surface has a certain amount of retained austenite, while the bainite content is almost nonexistent. As the carbon concentration and cooling rate decrease, austenite begins to transform into bainite. Therefore, the bainite content gradually increases from 5 mm towards the core, while the martensite content gradually decreases. However, most of the austenite still transforms into martensite. Furthermore, influenced by variations in carbon concentration and microstructure, the sample hardness exhibits a distinct gradient distribution from the surface to the core. The simulated hardness distribution agrees well with the experimental measurements, such as... Figure 5 (c)

[0058] The finite element method (FEM) simulation of the dual-medium quenching heat treatment process was performed on alloy steel. The simulation results after dual-medium carburizing and quenching are as follows: Figure 6 As shown. The microstructure obtained from the sample after dual-medium quenching includes martensite, bainite, and retained austenite, such as... Figure 6 (a) After carburizing and quenching, the contents of martensite, retained austenite, and bainite obtained on the sample surface were 90.0%, 8.0%, and 2.0%, respectively. The contents of martensite, retained austenite, and bainite obtained in the sample core were 54.3%, 0%, and 45.7%, respectively. Figure 6 (b) Compared with single-medium quenching, the bainite content after dual-medium quenching first increases and then decreases from the surface to the core, reaching a maximum of 51.6% at approximately 5.5 mm. This indicates that bainite transformation first occurs at this location under the combined effects of carbon concentration and cooling rate. The volume fraction of the phase structure on the sample surface does not change significantly after dual-medium quenching, while the bainite content in the core increases from 20.5% to 45.7%, an increase of 25.2%. This demonstrates that the dual-medium quenching heat treatment process can significantly reduce the martensite content and increase the bainite content in the core. Furthermore, compared with single-medium quenching, the dual-medium quenching process not only improves the surface hardness of the sample but also reduces the core hardness to a certain extent. Figure 6 (c) This is because the dual-medium quenching process of oil cooling + air cooling reduces the cooling rate at different locations of the sample, thereby reducing the content of residual austenite on the surface and martensite in the core.

[0059] Subsequently, experiments were conducted using a dual-medium quenching process. The carburized layer and core microstructure of the carburized and quenched samples were then observed and analyzed using scanning electron microscopy. Figure 7 As shown. The microstructure within 2 mm of the carburized layer from the surface, besides acicular high-carbon martensite and retained austenite, also contains fine carbides, such as... Figure 7 (a); The residual austenite content at this location was determined to be 8%, consistent with the calculated result. The core microstructure consists of lath-like low-carbon martensite and bainite, such as... Figure 7 (b) The results obtained meet the process requirements.

[0060] This invention optimizes the quenching heat treatment process after high-temperature carburizing. By replacing the single oil medium quenching process with 32-second oil quenching + air cooling, the cooling rate of different positions of the sample is reduced without increasing the complexity of the process or controlling costs. This reduces the content of residual austenite on the surface and martensite in the core, thereby achieving a decrease in core hardness and an increase in toughness.

[0061] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A high-temperature carburizing process for gear steel quenched in different media, characterized in that, Includes the following steps: S1: The gear steel workpiece is carburized at 970℃ and held for 40 h, then air-cooled; the gear steel workpiece is 18Cr2Ni4W gear steel. S2: Perform two high-temperature tempering processes on the gear steel workpiece after air cooling in step S1; in step S2, the high-temperature tempering method is to heat the gear steel workpiece and then air cool it to room temperature, the heating temperature is 620-640℃, and the heating holding time is 3.5-4.5h. S3: Heat the gear steel workpiece that has undergone two high-temperature temperings in step S2 to 810-830℃, oil cool for 32 seconds, and then air cool to room temperature; S4: After the gear steel workpiece that has been air-cooled to room temperature in step S3 is cold-treated, it is heated to 190-210℃ and tempered for 7.5-8.5 hours, then removed from the furnace and air-cooled to room temperature.

2. The high-temperature carburizing process for gear steel quenched in different media as described in claim 1, characterized in that, In step S1, after carburizing and heat preservation, the furnace is cooled and then air-cooled after exiting the furnace.

3. The high-temperature carburizing process for gear steel quenched in different media as described in claim 2, characterized in that, After being cooled to 810-830℃ in the furnace, the furnace is removed and air-cooled.

4. The high-temperature carburizing process for gear steel quenched in different media as described in claim 1, characterized in that, In step S3, the heating time is 60-70 minutes.

5. The high-temperature carburizing process for gear steel quenched in different media as described in claim 1, characterized in that, In step S4, the temperature for cold treatment is -80 to -70°C.

6. The high-temperature carburizing process for gear steel quenched in different media as described in claim 1, characterized in that, In step S4, the cold treatment time is 90-150 minutes.

7. A gear steel prepared by a high-temperature carburizing process for gear steel quenched in different media as described in any one of claims 1-6.

Images