A small deformation vacuum carburizing method for synchronously strengthening and toughening the workpiece surface and matrix

Abstract

The present invention relates to a low-deformation vacuum carburizing method for simultaneously strengthening and toughening a workpiece's surface layer and substrate. This method combines vacuum carburizing with isothermal gas quenching to minimize deformation during the vacuum carburizing and quenching process without damaging the vacuum system. This method rapidly achieves a gradient structure with both high hardness and good toughness, achieving simultaneous strengthening of the surface layer and substrate, while also achieving environmentally friendly results. This method features a short process flow, high carburizing efficiency, clean and pollution-free operation, and strong practicality.

Description

Technical Field

[0001] The invention relates to the field of metal surface heat treatment, in particular to a small deformation vacuum carburizing method for synchronously strengthening and toughening a workpiece surface layer and a matrix. Background Art

[0002] In order to maintain structural integrity in harsh operating environments, transmission bearings and gears require excellent comprehensive surface properties. Research has shown that surface heat treatment technologies, such as carburizing, can effectively improve the surface hardness and wear resistance of shaft and gear parts, thereby greatly increasing their service life. Due to the long cycle and severe pollution of gas carburizing, it is gradually being replaced by clean and green vacuum carburizing to reduce national energy and resource consumption. Traditional vacuum carburizing mostly uses a carburizing + quenching + tempering process, which has many steps and large carburizing deformation, and cannot meet the carburizing deformation control requirements of large, precise, complex, and long-life components.

[0003] Conventional austempering techniques are often used for the overall heat treatment of specific steel grades to achieve a bainitic structure. For example, Chinese invention patent CN 114717392 A proposes an austempering process for Dievar steel. The elements in the carburized layer after surface treatment are distributed in a gradient, and the workpiece requires high surface hardness and high core toughness. Therefore, a gradient structure, from surface martensite to core bainite, is required, rather than a single bainitic structure. Conventional austempering processes are no longer suitable for carburized specimens.

[0004] In addition, traditional austempering is mostly performed using salt bath or oil quenching. For example, Chinese invention patent CN114369769 A uses salt bath for austempering. Salt bath or oil quenching can easily damage the vacuum system, causing pipe blockage and pump oil contamination, and cannot be directly combined with the vacuum carburizing process. Summary of the Invention

[0005] In response to the shortcomings of the prior art, the present invention aims to provide a low-deformation vacuum carburizing method that simultaneously strengthens and toughens the workpiece surface and matrix. The present invention combines vacuum carburizing with isothermal gas quenching. This method not only allows the entire vacuum carburizing process to be completed in one go without damaging the vacuum system, controlling the vacuum carburizing deformation rate to ≤0.1%, but also allows for the rapid acquisition of a gradient structure with both high hardness and good toughness. Specifically, the surface layer within a certain depth range is primarily martensitic, the interior of the carburized layer is a mixed structure of martensite and bainite, and the matrix is bainitic. Compared to traditional salt bath isothermal quenching and isothermal oil quenching processes, isothermal gas quenching replaces the constant temperature process with a temperature holding method that rapidly fluctuates up and down in a zigzag pattern within a certain temperature range.

[0006] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0007] A small deformation vacuum carburizing method for synchronously strengthening and toughening a workpiece surface layer and a substrate, characterized by comprising the following steps:

[0008] Step 1: heating and holding the workpiece in a carburizing chamber;

[0009] Step 2, at 750-980° C., carburizing gas is introduced into the carburizing chamber to perform carburizing operation;

[0010] Step 3: After carburizing is completed, cooling gas is introduced into the carburizing chamber to cool the workpiece surface to 150-350°C at a cooling rate of ≥10°C / s;

[0011] Step 4: When the workpiece surface is cooled to the target temperature of the isothermal gas quenching, the workpiece is subjected to an isothermal gas quenching operation, and the isothermal gas quenching operation time is more than 30 minutes.

[0012] Step 5: Cooling gas is introduced into the carburizing chamber to cool the workpiece to room temperature.

[0013] On the basis of the above scheme,

[0014] The heating and heat preservation treatment process described in step 1 is a process of more than two steps and the heat preservation temperature rises step by step.

[0015] On the basis of the above scheme,

[0016] The carburizing operation described in step 2 includes a carburizing cycle and a diffusion cycle performed successively.

[0017] On the basis of the above scheme,

[0018] The pressure in the carburizing chamber during carburizing is 200 to 2000 Pa; the ratio of the total carburizing cycle time to the total diffusion cycle time is 1:2 to 1:7; the duration of the carburizing operation is more than 30 minutes.

[0019] On the basis of the above scheme,

[0020] The isothermal gas quenching operation in step 4 is as follows: maintaining the surface temperature of the workpiece fluctuating around the isothermal gas quenching target temperature with a fluctuation range of ≤±25°C;

[0021] On the basis of the above scheme,

[0022] The fluctuation is of equal amplitude.

[0023] or:

[0024] The fluctuation includes two or more stages with different amplitudes, and the fluctuation in each stage is of equal amplitude.

[0025] On the basis of the above scheme,

[0026] The method to keep the workpiece surface temperature fluctuating around the target temperature of isothermal gas quenching is:

[0027] Intermittently fill the carburizing chamber with cooling gas and then intermittently heat it;

[0028] or:

[0029] Cooling gas is intermittently filled into the carburizing chamber.

[0030] On the basis of the above scheme,

[0031] The pressure of the cooling gas filled into the carburizing chamber is ≥2×10 5 Pa.

[0032] On the basis of the above scheme,

[0033] The carburizing gas is methane, acetylene or propane, and the cooling gas is nitrogen, argon or helium.

[0034] The small deformation vacuum carburizing method for synchronously strengthening and toughening the surface layer and the substrate of a workpiece according to the present invention has the following beneficial effects:

[0035] (1) The present invention uses high-pressure gas quenching technology to keep the vacuum carburized workpiece warm in the form of a zigzag wave near the Ms point in the carbon-rich area on the surface. Due to the rapid surface cooling rate, the surface layer within a certain depth range is mainly composed of acicular martensite. As the depth of the carburized layer increases, the cooling rate slows down and the bainite content increases. A mixed structure of martensite and bainite is quickly obtained inside the carburized layer, and the matrix cools the slowest to form a bainite structure, thereby simultaneously obtaining a gradient structure with a hard surface and a tough core. In addition, rapid temperature change near the Ms point is conducive to shortening the incubation period of bainite formation. After vacuum carburizing, the deformation rate of the workpiece is ≤0.1%, the surface carbide is level 1, the surface hardness reaches 760HV, and the core hardness is above 300HV.

[0036] (2) This patent proposes a new carburizing process that combines carburizing with vacuum austempering. It is green, environmentally friendly, pollution-free, will not damage the vacuum system, and requires low equipment. It can be completed in one go, without the need for austempering or low-temperature tempering after carburizing. It is easy to operate, shortens the production cycle, and has a wide range of applications. Therefore, it has important engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The present invention has the following accompanying drawings:

[0038] Figure 1 This is the vacuum low-pressure carburizing and isothermal quenching process curve of the present invention in Example 1;

[0039] Figure 2 This is the actual temperature measurement curve of the vacuum low-pressure carburizing and isothermal quenching process in the present invention in Example 1;

[0040] Figure 3 This is a SEM image of the vacuum carburized surface microstructure of the WLY-20CrMnTi driven gear in Example 1;

[0041] Figure 4 This is a SEM image of the microstructure of the inner half of the vacuum carburized layer of the WLY-20CrMnTi driven gear in Example 1;

[0042] Figure 5 This is a SEM image of the vacuum carburized matrix microstructure of the WLY-20CrMnTi driven gear in Example 1;

[0043] Figure 6 This is the hardness distribution curve of the WLY-20CrMnTi driven gear after vacuum carburizing in Example 1;

[0044] Figure 7 is the radial deformation rate of the WLY-20CrMnTi driven gear before and after vacuum carburizing in Example 1;

[0045] Figure 8 is the axial deformation rate of the WLY-20CrMnTi driven gear before and after vacuum carburizing in Example 1;

[0046] Figure 9 This is the vacuum low-pressure carburizing isothermal gas quenching process curve in Example 2;

[0047] Figure 10 The metallographic image of the vacuum carburized surface microstructure of the WLY-20CrMnTi driving gear in Example 2 (a) and the metallographic image of the matrix microstructure (b) DETAILED DESCRIPTION

[0048] The following is a clear and complete description of the technical solutions in the embodiments of the present invention, in conjunction with the accompanying drawings. It is obvious that the described embodiments are only some of the embodiments of the present invention, not all of them, and do not constitute a limitation of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0049] The invention discloses a small deformation vacuum carburizing method for synchronously strengthening and toughening a workpiece surface layer and a matrix, which comprises heating and heat preservation, carburizing, and isothermal gas quenching.

[0050] In the heating and holding stage, whether the heating process is divided into sections can be determined according to the material and size of the workpiece, and the heating temperature, heating time and holding time of each section can be set.

[0051] The carburizing operation includes a carburizing cycle and a diffusion cycle, and the temperature and time of the carburizing cycle and the diffusion cycle can be adjusted according to the carbon concentration gradient of the workpiece surface and the effective hardened layer depth;

[0052] During the carburizing operation, the temperature of the carburizing cycle and the diffusion cycle is 750 to 980°C, and the carburizing pressure is 200 to 2000 Pa. The carburizing cycle is performed first, followed by the diffusion cycle. The carburizing method can be vacuum carburizing or plasma carburizing. The carburizing operation can also be performed in a multi-stage manner, with alternating carburizing and diffusion cycles. The carbon concentration gradient and effective hardened layer depth of the workpiece surface can be adjusted by the number of alternating carburizing and diffusion cycles and the duration of each cycle. During the carburizing operation, methane, acetylene, or propane atmosphere is selected as the carburizing gas. The duration of the carburizing stage is ≥30 minutes.

[0053] The austempering process involves: after carburizing, the Ms point gradually decreases as the surface carbon content increases. The Ms point in the carbon-rich surface region is selected as the target austempering temperature (150-350°C). A certain pressure of cooling gas is introduced into the carburizing chamber to rapidly cool the workpiece surface to the target austempering temperature, at a cooling rate exceeding the critical cooling rate for martensite transformation (≥10°C / s). The temperature is then maintained within a certain temperature range (≤±25°C) above and below the target austempering temperature using a sawtooth-like rapid fluctuation pattern. Finally, the workpiece is rapidly cooled to room temperature. Due to the rapid surface cooling rate, the surface layer is primarily martensitic within a certain depth. As the depth of the carburized layer increases, the cooling rate slows, and the bainite content increases. Within the carburized layer, a mixed structure of martensite and bainite rapidly forms. The matrix cools the slowest, becoming bainite, resulting in a gradient structure with a hard surface and a tough core. Furthermore, rapid temperature changes near the Ms point help shorten the incubation period for bainite formation.

[0054] During the isothermal gas quenching operation, the pressure of the cooling gas charged into the carburizing chamber is ≥ 2×10 5 Pa, holding time ≥ 30min. This stage is carried out by intermittently filling the carburizing chamber with cooling gas and then intermittently heating. In the early stage, intermittent cooling gas is mainly blown into the heating chamber to prevent the surface temperature from excessively rising due to the latent heat inside the workpiece. In the later stage, intermittent heating is mainly used to prevent the workpiece temperature from further decreasing. The cooling gas filled into the carburizing chamber is selected from nitrogen, argon or helium.

[0055] Example 1:

[0056] Parts and materials: 20CrMnTi synchronizer sliding gear sleeve.

[0057] Technical requirements: surface hardness ≥ 60HRC; carbide level 1; martensite ≤ level 4; retained austenite ≤ level 3;

[0058] The hardened layer depth (CHD) is 0.7~1.0mm, and the deformation rate is ≤0.1%.

[0059] A small deformation vacuum carburizing method for simultaneously strengthening and toughening the surface layer and the matrix of a workpiece is carried out according to the following steps:

[0060] 1. Heating and holding stage: Heat the workpiece to 600°C within 10 minutes, keep it at this temperature for 20 minutes, then heat it to 930°C required for the subsequent pulse carburizing stage, and keep it at this temperature for 20 minutes;

[0061] Second, carburizing stage: At 930℃, acetylene is introduced as carburizing gas, and 15 pulses are performed at a carburizing pressure of 3000Pa, with a total carburizing time of 42 minutes. Then the carburizing gas is stopped and diffusion is carried out at the same temperature for 140 minutes.

[0062] Isothermal gas quenching stage: 8 bar nitrogen is filled into the furnace and forced to circulate using a fan, rapidly cooling the workpiece surface to 230°C at a cooling rate of 10°C / s. Then, the temperature is maintained by first fluctuating widely (±25°C) and then more narrowly (±5°C). For the first 20 minutes, due to the internal latent heat, the surface temperature will rise, so intermittent gas cooling is performed on the workpiece surface. For the last 40 minutes, intermittent heating is used to prevent the workpiece from cooling further. After the insulation is completed, 8 bar nitrogen is filled into the furnace and forced to circulate using a fan, cooling the workpiece to room temperature.

[0063] like Figure 1 FIG. 1 shows the vacuum carburizing process curve according to the embodiment 1, in which an isothermal gas quenching stage is added.

[0064] like Figure 2 As shown, this is the actual temperature measurement curve during the implementation of the vacuum carburizing process invented in Example 1. During the isothermal gas quenching stage, the furnace temperature first fluctuates greatly (±25°C) and then fluctuates slightly (±5°C) for insulation.

[0065] like Figure 3 As shown, the surface structure of the sample after being treated by the vacuum carburizing process invented in Example 1 is mainly acicular martensite, which satisfies martensite ≤ level 4 and retained austenite ≤ level 3; it also contains a small amount of dispersed fine carbides, level 1.

[0066] like Figure 4 As shown in FIG. 1 , the SEM photograph of the structure at 1 / 2 of the inner carburized layer of the sample after the vacuum carburizing process invented in Example 1 shows that the size of the martensite laths becomes larger, and bainite structure and a small amount of retained austenite appear.

[0067] like Figure 5 As shown, after the vacuum carburizing process invented in Example 1, the core matrix of the sample is mainly bainite and island-shaped retained austenite.

[0068] like Figure 6 As shown, after the vacuum carburizing process invented in Example 1, the surface hardness of the sample is 760HV, the core hardness is 300HV, and the depth of the hardened layer is 0.82mm.

[0069] like Figure 7 As shown, the radial deformation rate of the gear after being treated by the vacuum carburizing process invented in Example 1 is ≤0.09%.

[0070] like Figure 8 As shown, the axial deformation rate of the gear after being treated by the vacuum carburizing process invented in Example 1 is ≤0.03%.

[0071] Example 2:

[0072] Parts and materials: 20CrMnTi reducer gear.

[0073] Technical requirements: surface hardness ≥ 60HRC; carbide level 1; martensite ≤ level 4; retained austenite ≤ level 3;

[0074] The hardened layer depth (CHD) is 0.7~1.0mm, and the deformation rate is ≤0.1%.

[0075] A small deformation vacuum carburizing method for simultaneously strengthening and toughening the surface layer and the matrix of a workpiece is carried out according to the following steps:

[0076] 1. Heating and holding stage: Heat the workpiece to 600°C within 10 minutes, hold it for 20 minutes, and then heat it to 930°C required for the subsequent pulse carburizing stage;

[0077] 2. Carburizing stage: At 930°C, 12 carburizing cycles and diffusion cycles were performed alternately, with a total duration of 166 minutes. The carburizing time of the first carburizing cycle was 5.4 minutes, with 3 pulses. The second to twelfth carburizing cycles were 1.8 minutes, with 1 pulse. The durations of the second to twelfth diffusion cycles were 2.8 minutes, 4.0 minutes, 5.1 minutes, 6.0 minutes, 7.1 minutes, 8.3 minutes, 9.4 minutes, 10.4 minutes, 11.5 minutes, 12.6 minutes, 13.6 minutes, and 50 minutes, respectively. The carburizing gas was acetylene, with an acetylene flow rate of 40 L / min and a carburizing pressure of 1500 Pa.

[0078] Isothermal gas quenching: Fill the furnace with 8 bar of nitrogen and use a fan to force circulation, rapidly cooling the workpiece surface to 230°C at a cooling rate of 10°C / s. Maintain the temperature for 45 minutes using a constant amplitude fluctuation (±10°C). Then, fill the furnace with 8 bar of nitrogen and use a fan to force circulation, cooling the workpiece to room temperature.

[0079] Figure 9 This is the vacuum carburizing process curve invented in Example 2, wherein the isothermal gas quenching stage adopts a sawtooth-like constant-amplitude fluctuation form for heat preservation.

[0080] Figure 10 (a) After the vacuum carburizing process invented in Example 2, the surface structure of the sample is mainly acicular martensite, which meets the technical requirements of martensite ≤ level 4, retained austenite ≤ level 3, and most of the carbides are fine spherical and dispersed as level 1 carbides.

[0081] Figure 10 (b) After being treated by the vacuum carburizing process invented in Example 2, the surface structure of the sample is mainly bainite and island-shaped austenite.

[0082] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims. The information disclosed in the background technology section of this article is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or any form of implication that the information constitutes prior art already known to those skilled in the art.

Claims

1. A small deformation vacuum carburizing method for synchronously strengthening and toughening the surface layer and the matrix of a workpiece, characterized in that: The steps include: Step 1: heating and holding the workpiece in a carburizing chamber; Step 2, at 750-980° C., carburizing gas is introduced into the carburizing chamber to perform carburizing operation; Step 3: After carburizing is completed, cooling gas is introduced into the carburizing chamber to cool the workpiece surface to 150-350°C at a cooling rate of ≥10°C / s; Step 4: When the workpiece surface is cooled to the target temperature of the isothermal gas quenching, the workpiece is subjected to an isothermal gas quenching operation, and the isothermal gas quenching operation time is more than 30 minutes; Step 5, introducing cooling gas into the carburizing chamber to cool the workpiece to room temperature; The isothermal gas quenching operation in step 4 is to maintain the temperature within a temperature range of ±25°C above and below the isothermal gas quenching target temperature using a sawtooth-like fluctuation. The fluctuation is of constant amplitude, or the fluctuation includes two or more stages with different amplitudes, and the fluctuation in each stage is of constant amplitude; The steps of maintaining the temperature within the temperature range of ±25°C above and below the isothermal gas quenching target temperature by using a sawtooth-like fluctuation are as follows: The carburizing chamber is filled with cooling gas intermittently and then heated intermittently, or the carburizing chamber is filled with cooling gas intermittently.

2. A small deformation vacuum carburizing method for synchronously strengthening and toughening the surface layer and the substrate of a workpiece according to claim 1, characterized in that: The heating and heat preservation treatment process described in step 1 is a process of more than two steps and the heat preservation temperature rises step by step.

3. A small deformation vacuum carburizing method for synchronously strengthening and toughening the surface layer and the substrate of a workpiece according to claim 1, characterized in that: The carburizing operation described in step 2 includes a carburizing cycle and a diffusion cycle performed successively.

4. A small deformation vacuum carburizing method for synchronously strengthening and toughening the surface layer and the substrate of a workpiece according to claim 3, characterized in that: During the carburizing operation, the pressure of the carburizing chamber is 200 to 2000 Pa; the ratio of the total carburizing cycle time to the total diffusion cycle time is 1:2 to 1:7; and the duration of the carburizing operation is more than 30 minutes.

5. The small deformation vacuum carburizing method for synchronously strengthening and toughening the surface layer and the substrate of a workpiece according to claim 1, characterized in that: The pressure of the cooling gas filled into the carburizing chamber is ≥2×10 5 Pa.

6. A small deformation vacuum carburizing method for synchronously strengthening and toughening a workpiece surface layer and a substrate according to any one of claims 1 to 5, characterized in that: The carburizing gas is methane, acetylene or propane, and the cooling gas is nitrogen, argon or helium.

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