Counterfeit method for reducing uneven vacuum carburization

Abstract

The invention discloses a countermeasure method for reducing uneven vacuum carburization, and belongs to the technical field of metal material heat treatment. Comprising the following steps: integrally heating a workpiece to be subjected to heat treatment in a heating chamber to a preset carburizing temperature, and carburizing; in the carburizing step, carburizing gas is continuously supplied into the heating chamber, and steps A and B are alternately and repeatedly executed, the step A comprises the steps that an exhaust valve used for exhausting gas in the heating chamber is closed, and exhausting stopping time continues to be preset; the step B comprises the steps that an exhaust valve used for exhausting gas in the heating chamber is opened, and the preset exhaust time is continued. According to the countermeasure method for reducing uneven vacuum carburization, gas can be continuously introduced under the condition that low pressure is kept in the heating chamber, so that it is guaranteed that high-purity carburization gas is always subjected to carburization reaction, exhaust is periodically stopped, the carburization gas is promoted to spread all over the heating chamber, and therefore the carburization uniformity is improved.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and specifically relates to a method for reducing uneven vacuum carburizing. Background Technology

[0002] In practical applications of vacuum carburizing technology, when faced with high-density furnace loading, i.e., a large number of products to be processed and a high packing density, the traditional low-pressure constant flow carburizing method and pulse carburizing method have revealed some problems that urgently need to be solved. In low-pressure constant-flow carburizing, the process operates within a low-pressure molecular flow domain, with a relatively fixed gas flow path. During high-density loading, the workpieces being processed create complex disturbances to the gas flow. Due to the positional relationship between the gas inlet and outlet, areas within the furnace inevitably experience insufficient carburizing gas supply. Furthermore, because this method cannot flexibly adjust the gas flow rate and path to compensate for these deficiencies, areas closer to the gas inlet receive a high concentration of carburizing gas, while areas further away have lower concentrations and purity. This uneven gas distribution makes it difficult to achieve uniform carburizing. Especially in the initial stages of carburizing, before the workpiece temperature reaches the Acm point, a large amount of carburizing gas is required to initiate the carburizing reaction. In areas with scarce carburizing gas, the reaction is sluggish, ultimately resulting in shallower carburizing depth and lower carbon concentration on the workpiece surface, severely impacting product quality consistency. While pulse carburizing improves gas distribution to some extent—specifically, when exhaust stops, the carburizing gas can better distribute throughout the densely packed products in a relatively static environment, resulting in more uniform gas distribution—its drawbacks become apparent once the carburizing gas supply stops and exhaust resumes. At this point, the gas flow within the furnace changes drastically. Similar to the low-pressure constant-flow method, under complex gas flow conditions, it becomes difficult to guarantee a continuous and uniform supply of carburizing gas to all products. Especially in areas far from the supply inlet and where gas flow is obstructed, insufficient carburizing gas can occur, leading to uneven carburizing and affecting product quality. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method to reduce the unevenness of vacuum carburizing. This method involves continuously introducing carburizing gas to maintain a low pressure in the heating chamber and periodically venting the gas from the heating chamber to achieve uniform diffusion of the carburizing gas. This, in turn, controls the matching relationship between the workpiece loading density and the supply and exhaust ports of the carburizing gas, thereby reducing the deviation in carburizing depth and the unevenness of surface carbon concentration.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is: a method for reducing uneven vacuum carburizing, comprising: The workpiece to be heat-treated is placed in the heating chamber and heated to the preset carburizing temperature before the carburizing process is carried out. In the carburizing process, while continuously supplying carburizing gas to the heating chamber, steps A and B are alternately and repeatedly executed, wherein; Step A includes closing the exhaust valve used to discharge gas from the heated room and continuing for a preset stop time; Step B includes opening the exhaust valve for venting gas from the heated room and continuing to vent for a preset time.

[0005] Optionally, after the carburizing process is completed, a diffusion process is performed; the diffusion process includes stopping the supply of carburizing gas to the heating chamber while continuously opening the exhaust valve.

[0006] Optionally, the workpiece to be heat-treated shall be subjected to at least one carburizing-diffusion cycle. The carburizing-diffusion cycle process includes performing the carburizing process once, and performing the diffusion process after the carburizing process is completed.

[0007] Optionally, after the diffusion process is completed, the workpiece to be heat-treated is transferred to a cooling chamber for quenching.

[0008] Optionally, after the diffusion process is completed, the workpiece to be heat-treated is slowly cooled to the quenching temperature and subjected to homogenization heat treatment until the temperature of the workpiece to be heat-treated remains uniform at the quenching temperature before quenching treatment is performed.

[0009] Optionally, in the carburizing process, the supply flow rate of the carburizing gas is increased or decreased according to the total surface area of ​​the workpiece to be heat-treated, and the supply flow rate of the carburizing gas remains constant before the surface temperature of the workpiece to be heat-treated reaches point Acm.

[0010] Optionally, in the carburizing process, after the surface temperature of the workpiece to be heat-treated reaches the Acm point, the supply flow rate of the carburizing gas is kept constant or reduced to 2 / 3 or less of the supply flow rate.

[0011] Optionally, in step A, the preset stop time is greater than 1 second.

[0012] Optionally, in step B, the preset exhaust time is longer than the time it takes for the pressure inside the heating chamber to drop to the baseline pressure.

[0013] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: (1) During the carburizing process, by continuously introducing carburizing gas and venting the vacuum, the gas in the heating chamber is periodically stopped, thereby breaking the fixed gas flow path and allowing all workpieces to come into contact with sufficient carburizing gas to improve the uniformity of carburizing. (2) By alternately performing carburizing and diffusion processes on the workpiece to be heat-treated, the precipitation of carbides on the protrusions of the workpiece with complex shape can be reduced. Attached Figure Description

[0014] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0015] Figure 1 This is a cross-sectional structural schematic diagram of a carburizing furnace derived from atmosphere carburizing in a preferred embodiment of the present invention; Figure 2 This is a schematic diagram of the spatial arrangement of the gas exhaust port and gas inlet port relative to the heating chamber in a preferred embodiment of the present invention; Figure 3 This is a flow chart of the heat treatment process using pulse carburizing, low-pressure constant-flow carburizing, and pulse exhaust carburizing in a preferred embodiment of the present invention. Figure 4 This is an example diagram illustrating the implementation time of the pulse carburizing method and the low-pressure constant-flow carburizing method (pulse exhaust carburizing method) in the preferred embodiments of the present invention; Figure 5 This is a schematic diagram of the sample loading position in a preferred embodiment of the present invention; Figure 6 This is a graph showing the carbon concentration data of the sample surface after heat treatment using the pulse carburizing method in a preferred embodiment of the present invention. Figure 7 This is a hardness distribution data diagram of a sample after heat treatment using the pulse carburizing method in a preferred embodiment of the present invention. Figure 8 This is a hardness distribution data diagram of a sample after heat treatment using the low-pressure constant flow method in a preferred embodiment of the present invention. Figure 9 This is a hardness distribution data diagram of a sample after heat treatment using the pulse exhaust carburizing method in a preferred embodiment of the present invention. Figure 10 This is a comparison diagram of the carbon concentration on the surface of samples after heat treatment using different preset stop-and-release times and pulse carburizing or pulse exhaust carburizing methods in a preferred embodiment of the present invention. Figure 11 This is a graph showing the hardness distribution deviation-standard deviation of samples after heat treatment using different preset stop times, pulse carburizing method or pulse exhaust carburizing method in a preferred embodiment of the present invention. Figure 12 This is a graph showing the carbon concentration-standard deviation data of the sample surface after heat treatment using different preset stop-release times, pulse carburizing method or pulse exhaust carburizing method in a preferred embodiment of the present invention. Figure 13 This is a comparison chart of the sample hardness distribution deviation - standard deviation before and after the increase of the base pressure in a preferred embodiment of the present invention; Figure 14 This is a graph showing the comparison of carbon concentration-standard deviation on the sample surface before and after the increase in base pressure in a preferred embodiment of the present invention. The components include: 1. heating chamber; 2. cooling chamber; 3. gas inlet; and 4. gas outlet. Detailed Implementation

[0016] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. These drawings are simplified schematic diagrams, which are only used to illustrate the basic structure of the present invention and therefore only show the components relevant to the present invention.

[0017] It should be noted that if directional indicators (such as up, down, bottom, top, etc.) are involved in this embodiment, these directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature. Unless otherwise explicitly specified and limited, the terms "set," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances. Example 1

[0018] A method for reducing uneven vacuum carburizing includes: The workpiece to be heat-treated is placed in heating chamber 1 and heated to the preset carburizing temperature before the carburizing process is carried out. Specifically, after several workpieces to be heat-treated are evenly stacked on carburizing trays and carburizing carts, they can be pre-sent into cooling chamber 2, which is interconnected with heating chamber 1. After the workpieces are placed inside, cooling chamber 2 and heating chamber 1 cut off the airflow path to the external environment, allowing vacuum exhaust in both chambers under the action of vacuum pumps, exhaust valves, vacuum gauges, and other devices to achieve a preset vacuum level (≤50 Pa), significantly reducing the content of oxidizing gases such as oxygen and water vapor in heating chamber 1. Simultaneously, after the workpieces are placed into heating chamber 1, heating chamber 1 cuts off the airflow path except for gas inlet 3 and gas outlet 4. Heating elements within heating chamber 1 can then meet the heating treatment requirements of the workpieces until the temperature of heating chamber 1 and the workpieces gradually rises to the carburizing temperature, satisfying the environmental requirements for vacuum carburizing. It should be noted that after the temperature in the heating chamber 1 reaches the carburizing temperature, the temperature in the heating chamber 1 can be maintained within the carburizing temperature range by heating elements, heat insulation devices, etc., and last for 90 to 150 minutes, preferably 120 minutes, so that the temperature of all workpieces placed in the heating chamber 1 can reach the carburizing temperature.

[0019] It should be noted that the carburizing gas in this embodiment is preferably acetylene gas, the carburizing temperature is 900℃~1050℃, preferably 930℃, and the heating chamber 1 can be preheated (to a temperature of 750℃~800℃) before the workpiece to be processed is transferred into the heating chamber 1, so as to reduce the time for the heating chamber 1 to heat to the carburizing temperature and improve the efficiency of vacuum carburizing heat treatment of the workpiece.

[0020] During the carburizing process, while continuously supplying carburizing gas into the heating chamber, steps A and B are executed alternately and repeatedly. Step A includes closing the exhaust valve used to discharge gas from heating chamber 1 and maintaining this position for a preset pause time. During this process, the pressure inside heating chamber 1 gradually increases due to the continuous introduction of carburizing gas, allowing the carburizing gas to diffuse randomly to every corner of heating chamber 1. This prevents differences in carburizing depth and surface carburite concentration in workpieces located at different positions within heating chamber 1 due to uneven distribution of carburizing gas concentration.

[0021] Step B includes opening the exhaust valve for venting the gas in the heating chamber and continuously venting for a preset time to reduce the content of the carburizing reaction tail gas (composed of substances produced by the cracking reaction of carburizing gas) in the heating chamber 1 and reduce the pressure in the heating chamber 1, thereby reducing the risk of condensed carbides on the surface of the workpiece.

[0022] As described above, the opening and closing of the exhaust valve is related to the start and stop of the vacuum pump. When the exhaust valve is open, the vacuum pump can remain in the start state, and its output end can be connected to the gas exhaust port 4 through the exhaust valve, thereby discharging the gas in the heating chamber 1. When the exhaust valve is closed, it can block the connection between the vacuum pump and the gas exhaust port 4, thereby blocking the path of gas discharge in the heating chamber 1. At this time, the vacuum pump can enter the standby state to wait for the exhaust valve to open.

[0023] It is important to note that during the carburizing process, the supply flow rate of the carburizing gas remains constant until the surface temperature of the workpiece reaches the Acm point. This supply flow rate needs to be increased or decreased accordingly based on the surface area of ​​the workpiece. After the surface temperature reaches the Acm point, the supply flow rate of the carburizing gas can remain constant or be reduced to 2 / 3 or less of the supply flow rate, but it must still meet the carbon requirements for the diffusion of carbon atoms from the workpiece surface into its interior. The Acm point on the workpiece surface is a key concept in the iron-carbon alloy phase diagram; it represents the temperature at which secondary cementite begins to precipitate from austenite in equilibrium.

[0024] Furthermore, during step A, the pressure inside heating chamber 1 gradually increases, while during step B, the pressure inside heating chamber 1 gradually decreases from its maximum value reached in step A to the base pressure. The base pressure refers to the equilibrium pressure between a fixed gas volume (in this embodiment, continuously supplied carburizing gas to heating chamber 1) and the vacuum pump's exhaust capacity. In step A, the preset stop-exhaust time is greater than 1 second. In step B, the preset exhaust time is greater than the time it takes for the pressure inside the heating chamber to drop to the base pressure; that is, within the preset exhaust time range, after the pressure inside heating chamber 1 drops to the base pressure, the pressure inside heating chamber 1 remains constant for a period of time.

[0025] After the carburizing process, a diffusion process is performed. This process involves stopping the supply of carburizing gas into heating chamber 1 while continuously keeping the exhaust valve open. This is primarily because carbides will form on the surface of the workpiece if the carbon concentration is above the Acm point. Therefore, the carbon concentration needs to be reduced to a target concentration, typically below 0.75%. The diffusion process stops the replenishment of carbon atoms to the surface of the workpiece, controlling the carbide content in heating chamber 1 to zero. This causes the carbon atoms enriched on the surface of the workpiece to diffuse towards the core of the workpiece under the influence of the concentration gradient, thus reducing the surface carbon concentration to a reasonable range. This prevents abnormal carbide precipitation, reduces the surface brittleness of the workpiece, and improves the overall toughness of the carburized layer.

[0026] It is important to note that in this embodiment, the workpiece to be heat-treated needs to undergo at least one carburizing-diffusion cycle. The carburizing-diffusion cycle consists of first performing a carburizing process on the workpiece, followed by a diffusion process after the carburizing process. In this embodiment, the carburizing and diffusion processes can be performed alternately on the workpiece to be heat-treated, which can reduce carbide precipitation on protrusions of complex-shaped workpieces. Increasing the number of carburizing-diffusion cycles can further reduce carbide precipitation on protrusions of complex-shaped workpieces, thereby meeting the carburizing requirements of the workpiece. After the diffusion process, the workpiece to be heat-treated needs to be transferred to cooling chamber 2 for quenching treatment. Quenching treatment includes, but is not limited to, one or more of various quenching methods such as oil cooling, water cooling, salt bath cooling, and gas cooling.

[0027] It is important to note that after the diffusion process, the workpiece to be heat-treated can be slowly cooled to the quenching temperature and then subjected to homogenization treatment until the temperature of the workpiece remains uniform at the quenching temperature before quenching. The homogenization treatment should last at least 30 minutes to ensure the effectiveness of the quenching. The quenching temperature is 830℃~870℃, preferably 850℃. Example 2

[0028] like Figures 1-14 As shown, based on Example 1, a Φ20×H50mm sample was used for evaluation. The material was 20CrMo. The evaluation method was to measure the surface carbon concentration on the end face of the round bar and to measure the hardness distribution by cutting it open at the center of the bar at a height of 50mm. The equipment used was an atmosphere carburizing derivative type carburizing furnace, the structure of which is as follows: Figure 1 As shown, the heating chamber 1, which is associated with carburizing, has a single outlet for venting carburizing gas, namely a single gas exhaust port 4, located below the longitudinal front side of the heating chamber 1. Gas inlets are located on both longitudinal sides of the heating chamber 1. However, the arrangement of the gas exhaust port 4 and gas inlets can lead to insufficient carburizing gas supply to the workpiece to be heat-treated. Therefore, a gas inlet is added in the middle of the longitudinal rear side of the heating chamber 1. The spatial arrangement of the gas exhaust port 4 and gas inlet relative to the heating chamber 1 is as follows: Figure 2 As shown.

[0029] The vacuum carburizing furnace was used, and the pulse carburizing method disclosed in the technology was used for the experiment. The test conditions for the pulse carburizing method are as follows: Figure 4 As shown, the preset stop time is set to 10 seconds, and the preset exhaust time is set to 140 seconds. The sample loading position is as follows. Figure 5As shown in the figure, the numbers within the circles represent the sample numbers. The workpieces to be heat-treated are stacked in layers on the carburizing tray of the carburizing cart, arranged from top to bottom as the first, second, third, fourth, and fifth layers. Each layer has several workpieces to be heat-treated evenly spaced. The total surface area of ​​the workpieces to be heat-treated is approximately 12 m², and samples 9, 10, and 11 are located in the third layer.

[0030] The surface carbon concentration results are as follows Figure 6 As shown, the surface carbon concentration of samples 1, 2, and 10 is relatively low, which is clearly affected by the exhaust position: because the gas exhaust port 4 is located in the center of the lower front of the heating chamber 1, the carburizing gas in the lower front will concentrate towards the gas exhaust port 4, so the carburizing gas supplied to the locations of samples 1 and 2 will become relatively scarce. The same situation applies to sample 10. However, the surface carbon concentration of samples 5 and 6 is not so low. This is because the resistance to the flow of carburizing gas around the locations of samples 5 and 6 is relatively small, and the carburizing gas can easily flow to the locations of samples 5 and 6.

[0031] Based on this result, samples 1, 2, and 10, which showed a decrease in surface carbon concentration, and sample 11, which was placed in a similar location to sample 10, were compared. Additionally, sample 7, which had a high surface carbon concentration and was symmetrical to sample 2, was used as a comparison point. The hardness distribution results are as follows: Figure 7 As shown in the figure. Figure 10a, 10b, and 10c represent the hardness distribution data of three cross-sections of sample 10 cut at 120-degree intervals with the central axis as the midpoint, serving as a comparative test. Similarly, Figure 11a, 11b, and 11c also represent the hardness distribution data of three cross-sections of sample 10 cut at 120-degree intervals with the central axis as the midpoint.

[0032] Because the equipment cannot set the preset stop time to 0 seconds, the preset stop time is set to 1 second and the preset exhaust time to 140 seconds, simulating the low-pressure constant flow method in the disclosed technology (hereinafter referred to as the low-pressure constant flow carburizing method). The flow chart of the heat treatment process using the pulse carburizing method, the low-pressure constant flow carburizing method, and the countermeasures for reducing uneven vacuum carburizing described in Example 1 (hereinafter referred to as the pulse exhaust carburizing method) is as follows: Figure 3 As shown.

[0033] Subsequently, pulse carburizing and pulse degassing carburizing methods were used for testing under the following conditions: Condition 1: Set the preset stop and exhaust times to 5 seconds, 10 seconds, 30 seconds, 60 seconds, and 99 seconds respectively, and set the preset exhaust time to 140 seconds. The carburizing gas in the carburizing process is supplied to the heating chamber 1 at a supply flow rate of 28L / min. Condition 2: Set the preset stop and exhaust times to 10 seconds and 60 seconds respectively, and the preset exhaust time to 140 seconds. The carburizing gas in the carburizing process is supplied to the heating chamber 1 at a supply flow rate of 36L / min. At this time, the basic pressure in the heating chamber 1 during the carburizing process is about 280Pa.

[0034] It should be noted that the base pressure of heating chamber 1 during the carburizing process in condition 1 is about 180 Pa, and the base pressure of heating chamber 1 during the carburizing process in condition 2 is about 280 Pa. Limiting the base pressure of heating chamber 1 to less than 300 Pa in the above conditions can significantly reduce the non-uniformity in the carburizing process. This point is mentioned in the Japanese patent with publication number JP2963869B2 entitled Vacuum Carburizing Method and Equipment and Carburizing Products.

[0035] The experiment was conducted under conditions one and two, and the results are as follows: Figures 10-14 As shown in the figure, "pulse carburizing (a / b)" indicates that the pulse carburizing method is used, the preset stop time is set to a seconds, and the preset exhaust time is set to b seconds. "Pulse exhaust (c / d)" indicates that the pulse exhaust carburizing method is used, the preset stop time is set to c seconds, and the preset exhaust time is set to d seconds. When "pulse carburizing (a / b)" and "pulse exhaust (c / d)" have the suffix "280Pa", it means that the basic pressure in heating chamber 1 is about 280Pa. When "pulse carburizing (a / b)" and "pulse exhaust (c / d)" have no suffix, it means that the basic pressure in heating chamber 1 is 180Pa.

[0036] When the preset stop time is 1 second (simulating the low-pressure constant flow method in the publicly available technology), the preset exhaust time is set to 140 seconds, and the carburizing gas supply flow rate is 28 L / min, it is represented as "pulse exhaust (1 / 140)", and the result is as follows: Figure 10 , Figure 11 As shown, samples 1, 2, and 10 exhibit lower surface carbon concentrations and larger hardness deviations. Furthermore, while this trend also occurred when using the pulsed venting carburizing method with a preset pause time of 5 seconds and a carburizing gas supply flow rate of 28 L / min, the differences between samples 1, 2, and 10 and other samples began to decrease. However, when using the pulsed venting carburizing method with a carburizing gas supply flow rate of 28 L / min and a preset pause time of more than 10 seconds, this trend in surface carbon concentration did not occur in samples 1, 2, and 10. Specifically, as shown... Figure 12 As shown, the standard deviation of the overall surface carbon concentration of the sample exhibits a considerably decreasing trend. And as... Figure 11As shown, the standard deviation of hardness distribution also exhibits a similar decreasing trend. Specifically, the standard deviation of surface carbon concentration refers to the degree of deviation between the surface carbon concentration at different test points and the overall average carbon concentration, while the standard deviation of hardness distribution refers to the degree of deviation between the hardness at different test points and the overall average hardness.

[0037] When using the pulse carburizing method, the carburizing gas supply flow rate is 28 L / min and the preset stop time is 10 seconds, the same trend will occur. The reason is that when the pulse carburizing method is used, a large amount of carburizing gas will be introduced into the heating chamber 1 when the preset stop time stage is entered, which will cause the pressure in the heating chamber 1 to rise rapidly to 1-3.5 kPa. Although the operation in this stage will promote the supply of carburizing gas to all parts during high-density loading, there is a risk that the workpiece to be heat-treated will have surface carbides (carbon deposits). As the pulse carburizing method proceeds to the next stage (stopping the introduction of carburizing gas into heating chamber 1 and starting to exhaust the gas from heating chamber 1), the solidification of the gas flow path causes the gas in heating chamber 1 (including carburizing gas and carburizing reaction tail gas) to flow towards the gas exhaust port 4. This results in a reduction in the flow of carburizing gas away from the gas exhaust port 4. Consequently, processed samples not located on the gas flow path will experience a slow carburizing reaction due to the lack of carburizing gas flow and short contact time with the carburizing gas at their locations (samples 1, 2, and 10). In addition, the low-pressure constant-flow carburizing method, under high-density loading conditions, will also create areas where carburizing is difficult to achieve due to the fixed gas flow path, such as locations far from the exhaust port with high loading density (sample 10) and locations far from the exhaust port (samples 1 and 2).

[0038] However, as Figures 10-14 As shown, when using the pulse exhaust carburizing method for the carburizing process, the uniformity of carbon concentration and surface hardness of the workpiece to be heat-treated can be effectively improved by continuously supplying carburizing gas into the heating chamber 1 and periodically stopping the exhaust of gas from the heating chamber 1. Based on the above results, it can be clearly known that when the preset stop time is set to more than 10 seconds, the deviation of carbon concentration and hardness of the sample obtained by the pulse exhaust carburizing method is smaller.

[0039] Furthermore, such as Figures 10-14As shown, when using the pulse exhaust carburizing method, under the same conditions, the lower the base pressure in heating chamber 1, the lower the standard deviation of the surface carbon concentration and hardness distribution deviation. Furthermore, regarding base pressure, when the total surface area of ​​the workpiece to be heat-treated is large, the amount of carburizing gas needs to be increased accordingly, and the base pressure in heating chamber 1 will also rise during the carburizing process. Based on this, when the base pressure is too low or too high, it is sufficient to use a condition where the standard deviation of the surface carbon concentration and hardness distribution deviation is relatively small. Therefore, comparing the results with a preset stop time of 10 seconds and a preset stop time of 60 seconds, the difference in the standard deviation of the base pressure in heating chamber 1 is small at 180 Pa and 280 Pa when the preset stop time is 60 seconds. Thus, under these conditions, a longer preset stop time is more effective when the base pressure rises due to high-density loading. However, extending the preset stop time will increase the furnace pressure, which can easily lead to carbon buildup, so arbitrarily extending the preset stop time carries risks. This point is also described in the Japanese patent with publication number JP2963869B2, entitled "Vacuum Carburizing Method and Equipment and Carburizing Products".

[0040] Therefore, based on the analysis of the results obtained from the above specific experiments, the pulse exhaust carburizing method in this embodiment (the countermeasure method for reducing uneven vacuum carburizing described in Example 1) is adopted. It is preferred that the preset stop time is 10 seconds and the preset exhaust time is set to 140 seconds. When the carburizing gas in the carburizing process is supplied at a supply flow rate of 28L / min, the unevenness of carbon concentration and hardness distribution on the surface of the workpiece to be heat-treated can be effectively reduced.

[0041] Working principle: During the carburizing process, while continuously introducing carburizing gas into the heating chamber 1, the gas in the heating chamber 1 is periodically stopped from being discharged. This ensures that even with high-density loading, the carburizing gas will be distributed throughout all the workpieces being treated, thereby effectively improving the uniformity of carbon concentration and surface hardness on the surface of the workpieces to be heat-treated.

[0042] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for reducing uneven vacuum carburizing, characterized in that, include: The workpiece to be heat-treated is placed in the heating chamber and heated to the preset carburizing temperature before the carburizing process is carried out. In the carburizing process, while continuously supplying carburizing gas to the heating chamber, steps A and B are alternately and repeatedly executed, wherein; Step A includes closing the exhaust valve used to discharge gas from the heated room and continuing for a preset stop time; Step B includes opening the exhaust valve for venting gas from the heated room and continuing to vent for a preset time.

2. The method for reducing uneven vacuum carburizing according to claim 1, characterized in that: After the carburizing process is completed, a diffusion process is performed; the diffusion process includes stopping the supply of carburizing gas into the heating chamber while continuously opening the exhaust valve.

3. The method for reducing uneven vacuum carburizing according to claim 2, characterized in that: Perform at least one carburizing-diffusion cycle on the workpiece to be heat-treated; The carburizing-diffusion cycle process includes performing the carburizing process once, and performing the diffusion process after the carburizing process is completed.

4. The method for reducing uneven vacuum carburizing according to claim 2 or claim 3, characterized in that: After the diffusion process is completed, the workpiece to be heat-treated is transferred to the cooling chamber for quenching.

5. The method for reducing uneven vacuum carburizing according to claim 2 or claim 3, characterized in that: After the diffusion process is completed, the workpiece to be heat-treated is slowly cooled to the quenching temperature and subjected to homogenization heat treatment until the temperature of the workpiece to be heat-treated remains uniform at the quenching temperature, and then quenching treatment is performed.

6. The method for reducing uneven vacuum carburizing according to claim 1, characterized in that: In the carburizing process, the supply flow rate of carburizing gas increases or decreases accordingly based on the total surface area of ​​the workpiece to be heat-treated, and the supply flow rate of carburizing gas remains constant before the surface temperature of the workpiece reaches point Acm.

7. The method for reducing uneven vacuum carburizing according to claim 6, characterized in that: In the carburizing process, after the surface temperature of the workpiece to be heat-treated reaches the Acm point, the supply flow rate of the carburizing gas remains constant or is reduced to 2 / 3 or less of the supply flow rate.

8. The method for reducing uneven vacuum carburizing according to claim 1, characterized in that: In step A, the preset stop time is greater than 1 second.

9. The method for reducing uneven vacuum carburizing according to claim 1, characterized in that: In step B, the preset exhaust time is longer than the time it takes for the pressure inside the heating chamber to drop to the baseline pressure.

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