Method for determining the brittleness of medium alloy steel under vacuum tempering

Abstract

A method for determining vacuum tempering brittleness in medium alloy steel involves dividing selected test samples into test and control groups, and pre-treating the test samples. The pre-treated test samples are then placed in a vacuum furnace for quenching. Subsequently, the test samples from the test and control groups, after heat treatment, are placed in a vacuum furnace and an air furnace, respectively, for tempering. After tempering, the samples are cooled. The test group samples are cooled with gases of different pressures, while the control group samples are cooled to room temperature using air, water, and oil cooling media. By analyzing the performance changes of the control group samples, the relationship between gas pressure and cooling rate in the test group is calculated. Finally, based on the performance changes of the control group samples and the relationship between gas pressure and cooling rate in the test group, the pressure range at which tempering brittleness occurs is determined. Using the relationship between tempering brittleness and cooling rate, the gas pressure inside the vacuum furnace is controlled to prevent the occurrence of type II tempering brittleness.

Description

Technical Field

[0001] This invention relates to the field of vacuum heat treatment technology, and in particular to a method for determining the brittleness of medium alloy steel during vacuum tempering. Background Technology

[0002] Medium alloy steel is prone to high-temperature temper brittleness (also known as type II temper brittleness) when tempered at 450~550℃, which can lead to fatigue fracture during use. The key to high-temperature temper brittleness lies in the segregation of impurity elements such as Sb, P, Sn, and As at the grain boundaries, which easily form Cothausen gas clusters on dislocation lines. The longer the holding time, the more Cothausen gas clusters are formed, and the greater the brittleness. Chinese patent CN113897474B discloses a method for reducing the second type of temper embrittlement in Cr-Mo steel, including the following steps: vacuum induction melting, forging, rolling, and heat treatment. The method requires that the Mg alloying content be controlled at 50–100 ppm and the O content in the steel be controlled at 3 ppm or below. This patent utilizes the advantages of Mg in grain boundary segregation and grain boundary state in iron-based alloys through a trace Mg alloying method, reducing the embrittlement caused by the segregation of harmful impurity elements such as P and S, and effectively reducing the tendency of 2.25Cr1Mo steel to exhibit the second type of temper embrittlement. However, due to the diverse types of alloy materials used in heat treatment processes, this method lacks universality. Existing research has found that air cooling or faster cooling rates can effectively suppress the decrease in impact toughness caused by temper embrittlement, but currently there is no method based on the correlation between temper embrittlement and cooling rate to avoid the second type of temper embrittlement.

[0003] At present, various heat treatment standards, such as aviation standards, national standards, and enterprise standards, only specify cooling methods such as oil cooling, water cooling, and air cooling during tempering. However, due to the limitations of some vacuum tempering furnaces, if the furnace is opened immediately after the heat preservation is completed, air will be introduced. At this time, the furnace is in a high-temperature state, which will lead to the risk of oxidation and damage to the components inside the vacuum tempering furnace. Therefore, it can only be cooled with the furnace or cooled by introducing high-purity inert gas. This limits the cooling method of vacuum tempering furnaces to meet the relevant standards and makes it difficult to pass the requirements of various audits. Summary of the Invention

[0004] The technical problem solved by this invention is to provide a method for judging the vacuum tempering brittleness of medium alloy steel, so as to solve the problems in the background art mentioned above.

[0005] The technical problem solved by this invention is achieved by the following technical solution:

[0006] The specific steps for determining the vacuum tempering brittleness of medium alloy steel are as follows:

[0007] S1. Select several test samples made of 30CrMnSiA medium alloy steel, and randomly divide the test samples into test and control groups in equal quantities. Simultaneously, pre-treat the test samples:

[0008] S2. The samples from both the test group and the control group were placed in a vacuum furnace for quenching treatment.

[0009] S3. The samples of the test group and the control group were placed in a vacuum tempering furnace and an air furnace, respectively, for tempering and heat preservation treatment.

[0010] S4. The test group samples were cooled using media gases at different pressures; the control group samples were cooled to room temperature using air, water, and oil cooling media, respectively.

[0011] S5. The cooling rates of the test group and the control group were tested using a cryostat, the microstructure and performance changes of the control group samples were analyzed, and the relationship between gas pressure and cooling rate in the test group was calculated.

[0012] S6. Based on the performance change relationship between the test group and the control group samples and the relationship between tempering gas pressure and cooling rate in the test group, determine the pressure range in which tempering brittleness occurs. Utilize the relationship between tempering brittleness and cooling rate to control the gas pressure inside the vacuum furnace to prevent the workpiece from developing second-type tempering brittleness. At the same time, determine a reasonable vacuum tempering gas pressure range and find the correspondence between gas pressure and air furnace cooling method during vacuum tempering to meet standard requirements and the needs of vacuum tempered parts production.

[0013] In this invention, step S1) involves selecting and preprocessing the test samples as follows:

[0014] S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples.

[0015] S12. Calculate the J value for each rod-shaped test sample, and select rod-shaped test samples that meet the standard J value as the test samples. The J value is calculated by measuring the weight percentage w of Si, Mn, P, and S in each test sample. Si %、w Mn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn )×(w P +w S )×10 4 ;

[0016] S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

[0017] In this invention, in step S2), the quenching temperature in the vacuum furnace is 900±10℃, and the holding time is 30~60min.

[0018] In this invention, in step S3), the tempering temperature in the air furnace is 510±10°C and the holding time is 50 min to 90 min; the tempering temperature in the vacuum tempering furnace is 510±10°C and the holding time is 1.5 times that of the air furnace.

[0019] In this invention, in step S4), the medium gas is nitrogen or argon, and the filling pressure ranges from 0.8 bar to 6 bar.

[0020] In this invention, step S5) involves analyzing the microstructure and performance changes of the control group sample, including microstructure observation, performance testing, and pendulum test. The microstructure observation characterizes the content of precipitates in the control group sample, the performance test detects the surface and core hardness of the control group sample, and the pendulum test measures the impact toughness of the control group sample.

[0021] Microscopic observations of the organization are as follows:

[0022] Microstructure observations were performed on the original sample, the quenched sample, and the tempered sample using metallographic microscopy and SEM.

[0023] Performance testing and verification are as follows:

[0024] The performance test results of hardness testing of the original sample, the quenched sample, and the tempered sample using a Vickers hardness tester were used to further obtain the correspondence between furnace temperature, cooling rate, precipitated phase, and hardness.

[0025] Based on the results of the pendulum test, the minimum cooling rate to avoid temper brittleness is as follows:

[0026] Fracture surface scanning was performed on the samples after the impact toughness test to observe the morphology of the fracture surface and determine the characteristics of the fracture mode of the samples after the impact toughness test. Based on the results of the pendulum test, the minimum cooling rate that does not produce temper brittleness was obtained.

[0027] In this invention, in step S6), the pressure range at which temper brittleness occurs is specifically determined as follows:

[0028] Based on microstructure observation, performance test results, and pendulum test results, the relationship between gas pressure and cooling rate in the test group was verified and the error was adjusted to determine the pressure range in which temper brittleness occurs.

[0029] Beneficial Effects: This invention tempers quenched test samples in both a vacuum tempering furnace and an air-cooled electric furnace, cooling them under different media and pressures. It establishes the correlation between gas pressure and cooling methods such as water cooling, oil cooling, and air cooling during vacuum tempering. The relationship between furnace temperature, cooling rate, precipitated phases, and hardness is obtained, along with the minimum cooling rate to prevent temper brittleness and the relationship between gas pressure and cooling rate. This determines the pressure range within which temper brittleness occurs. By utilizing the relationship between temper brittleness and cooling rate, the gas pressure within the vacuum furnace is controlled to prevent the occurrence of second-type temper brittleness in the workpiece, effectively improving the impact toughness of medium alloy steel. Simultaneously, a reasonable range of vacuum tempering gas pressure is determined to meet standard requirements and the needs of vacuum-tempered parts production. Attached Figure Description

[0030] Figure 1 This is a flowchart illustrating a preferred embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram of the fitting of the cooling rate and hardness curve of medium alloy steel in a preferred embodiment of the present invention.

[0032] Figure 3 This is a schematic diagram showing the curve fitting of the relationship between the hardness of medium alloy steel and the pressure of vacuum tempering cooling gas in a preferred embodiment of the present invention.

[0033] Figure 4 This is a schematic diagram showing the curve fitting of the relationship between the impact toughness of medium alloy steel and vacuum gas pressure in a preferred embodiment of the present invention.

[0034] Figure 5 This is a schematic diagram of the curve fitting between the impact toughness and cooling rate of medium alloy steel in a preferred embodiment of the present invention.

[0035] Figure 6 This is a schematic diagram showing the fitting of the relationship between vacuum tempering gas pressure and cooling rate for medium alloy steel in a preferred embodiment of the present invention. Detailed Implementation

[0036] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific illustrations. Example 1

[0037] The specific steps for determining the vacuum tempering brittleness of medium alloy steel are as follows:

[0038] S1. Select several test samples made of 30CrMnSiA medium alloy steel, and randomly divide the test samples into test and control groups in equal quantities. Simultaneously, pre-treat the test samples:

[0039] S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples.

[0040] S12. Calculate the J value for each rod-shaped test sample, and select rod-shaped test samples that meet the standard J value as the test samples. The J value is calculated by measuring the weight percentage w of Si, Mn, P, and S in each test sample. Si %、w Mn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn )×(w P +w S )×10 4 ;

[0041] S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

[0042] S2. Place the samples of both the test group and the control group in a vacuum furnace and quench them at 900±10℃ for 30~60min.

[0043] S3. Place the samples of the test group and the control group in a vacuum tempering furnace and an air furnace, respectively, and temper them at 510±10°C. The holding time in the air furnace is 50 min to 90 min. The holding time in the vacuum tempering furnace is 1.5 times that of the air furnace.

[0044] S4. The test group samples were cooled by filling with medium gas at different pressures. The medium gas was nitrogen or argon, and the filling pressure ranged from 0.8 bar to 6 bar. Different pressure values ​​were selected from this range. The control group samples were cooled to room temperature under air, water and oil cooling media, respectively.

[0045] S5. The cooling rates of the test group and the control group were tested using a cryostat, the microstructure and performance changes of the control group samples were analyzed, and the relationship between gas pressure and cooling rate in the test group was calculated.

[0046] The specific changes in the microstructure and properties of the analyzed samples are as follows:

[0047] S51. The samples were subjected to microstructure observation, performance testing and pendulum test respectively. The microstructure observation was used to characterize the content of precipitates in the sample, the performance test was used to detect the surface and core hardness of the sample, and the pendulum test was used to measure the impact toughness of the sample.

[0048] Microscopic observations of the organization are as follows:

[0049] Microstructure observations were performed on the original sample, the quenched sample, and the tempered sample using metallographic microscopy and SEM.

[0050] Performance testing and verification are as follows:

[0051] The performance test results of hardness testing of the original sample, the quenched sample, and the tempered sample using a Vickers hardness tester were used to further obtain the correspondence between furnace temperature, cooling rate, precipitated phase, and hardness.

[0052] S52. Based on the results of the pendulum test, the minimum cooling rate to prevent temper brittleness is obtained as follows:

[0053] Fracture surface scanning was performed on the samples after the impact toughness test to observe the morphology of the fracture surface and determine the characteristics of the fracture mode of the samples after the impact toughness test. The minimum cooling rate that does not produce temper brittleness was obtained based on the results of the pendulum test.

[0054] S6. Based on the performance change relationship between the test group and the control group samples, and the relationship between gas pressure and cooling rate in the test group, determine the pressure range in which temper brittleness occurs, specifically:

[0055] Based on microstructure observation, performance test results, and pendulum test results, the relationship between gas pressure and cooling rate in the test group was verified and the error was adjusted to determine the pressure range in which temper brittleness occurs. Example 2

[0056] The specific steps for determining the vacuum tempering brittleness of medium alloy steel are as follows:

[0057] S1. Select several test samples made of 30CrMnSiA medium alloy steel, and randomly divide the test samples into test and control groups in equal quantities. Simultaneously, pre-treat the test samples:

[0058] S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples.

[0059] S12. Calculate the J value for each rod-shaped test sample, and select rod-shaped test samples that meet the standard J value as the test samples. The J value is calculated by measuring the weight percentage w of Si, Mn, P, and S in each test sample. Si %、w Mn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn)×(w P +w S )×10 4 ;

[0060] S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

[0061] S2. The samples of both the test group and the control group were placed in a vacuum furnace and quenched at 900±10℃ for 30~60min. The cooling method was oil cooling.

[0062] S3. Place the samples of the test group and the control group in a vacuum tempering furnace and an air furnace, respectively, and temper them at 510±10°C. The holding time in the air furnace is 50 min to 90 min. The holding time in the vacuum tempering furnace is 1.5 times that of the air furnace.

[0063] S4. The samples in the test group were cooled by a medium gas filled with different pressures, and the medium gas was nitrogen or argon; the samples in the control group were cooled to room temperature by air, water and oil cooling media respectively.

[0064] S5. The samples of the test group and the control group were observed under metallographic microscope and SEM to obtain the microstructure. Regardless of the tempering cooling method, the microstructure after tempering was tempered sorbite, ferrite and carbides. Example 3

[0065] The specific steps for determining the vacuum tempering brittleness of medium alloy steel are as follows:

[0066] S1. Select several test samples, the material of which is 30CrMnSiA medium alloy steel. Randomly divide the test samples into test groups and control groups in equal quantities, and pre-treat the test samples at the same time.

[0067] S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples.

[0068] S12. Calculate the J value for each rod-shaped test sample, and select rod-shaped test samples that meet the standard J value as the test samples. The J value is calculated by measuring the weight percentage w of Si, Mn, P, and S in each test sample. Si %、w Mn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn )×(w P+w S )×10 4 ;

[0069] S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

[0070] S2. Place all samples in a vacuum furnace and quench them at 900±10℃ for 30~60min. The cooling method is oil cooling.

[0071] S3. The quenched test samples were tempered in an air furnace at a temperature of 510±10℃ and a holding time of 50 min to 90 min. Vacuum cooling oil, water and PAG coolant of different concentrations were used for cooling.

[0072] S4. Use a chiller to test the maximum cooling rate of the medium after the sample has been tempered.

[0073] S5. Test the surface and core hardness of the control group samples;

[0074] S6. Perform curve fitting based on the cooling rate and sample hardness values ​​obtained from the control group samples.

[0075] S7. Obtain the curve showing the relationship between hardness and cooling rate.

[0076] Through this embodiment, the following is obtained: Figure 2 As shown, the curves of hardness versus cooling rate of alloy steel in 30CrMnSiA are presented. The obtained curves are then verified and error adjusted to prepare for further deduction of the relationship between cooling rate and gas pressure. Example 4

[0077] The specific steps for determining the vacuum tempering brittleness of medium alloy steel are as follows:

[0078] S1. Select several test samples, the material of which is 30CrMnSiA medium alloy steel. Randomly divide the test samples into test groups and control groups in equal quantities, and pre-treat the test samples at the same time.

[0079] S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples.

[0080] S12. Calculate the J value for each rod-shaped test sample, and select rod-shaped test samples that meet the standard J value as the test samples. The J value is calculated by measuring the weight percentage w of Si, Mn, P, and S in each test sample. Si %、wMn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn )×(w P +w S )×10 4 ;

[0081] S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

[0082] S2. All samples to be tested are placed in a vacuum furnace and quenched at 900±10℃ for 30~60min. The cooling method is oil cooling.

[0083] S3. The quenched test samples were tempered in a vacuum tempering furnace at a tempering temperature of 510±10℃ and a holding time of 75 min to 135 min. Different medium gases at different pressures were introduced for cooling. The medium gas was nitrogen or argon.

[0084] S4. Test the surface and core hardness of the test group samples;

[0085] S5. Correspond one-to-one with the gas pressure in the vacuum tempering furnace and the hardness of the test group samples under the tempering and cooling at that gas pressure, perform curve fitting on these points, and deduce the curve relationship between gas pressure and hardness.

[0086] Through this embodiment, the following is obtained: Figure 3 As shown, the curves of the relationship between the hardness of the alloy steel in 30CrMnSiA and the vacuum tempering gas pressure are presented. The obtained curves of the relationship between hardness and gas pressure are then verified and the error is adjusted. Combined with the relationship between hardness and cooling rate obtained in Example 3, the relationship between cooling rate and gas pressure is further deduced. Example 5

[0087] The specific steps for determining the vacuum tempering brittleness of medium alloy steel are as follows:

[0088] S1. Select several test samples made of 30CrMnSiA medium alloy steel, and randomly divide the test samples into test and control groups in equal quantities. Simultaneously, pre-treat the test samples:

[0089] S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples.

[0090] S12. Calculate the J value for each rod-shaped test sample, and select rod-shaped test samples that meet the standard J value as the test samples. The J value is calculated by measuring the weight percentage w of Si, Mn, P, and S in each test sample. Si %、w Mn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn )×(w P +w S )×10 4 ;

[0091] S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

[0092] S2. The samples of both the test group and the control group were placed in a vacuum furnace and quenched at 900±10℃ for 30~60min. The cooling method was oil cooling.

[0093] S3. The samples of the test group and the control group were placed in a vacuum tempering furnace and a regular air furnace, respectively, and tempered at 510±10℃. The holding time in the air furnace was 50 min to 90 min. The holding time in the vacuum tempering furnace was 1.5 times that of the air furnace.

[0094] S4. The samples in the test group were cooled to room temperature by filling them with medium gases at different pressures, and pressure test points were selected. The medium gas was either nitrogen or argon. The samples in the control group were cooled to room temperature in air, water, or oil cooling media, respectively.

[0095] S5. Perform pendulum tests on the samples of the test group and the control group to obtain the impact toughness values ​​of the samples of the test group and the control group respectively; the impact toughness of the samples with different cooling media and different vacuum air pressure tempering is matched one by one, and curve fitting is performed on these points to infer the curve relationship between the cooling medium and the air pressure and the impact toughness during vacuum tempering furnace cooling.

[0096] S6. Perform fracture surface scanning on the sample after the impact toughness test, observe the morphology of the fracture surface, and determine the characteristics of the fracture mode of the sample after the impact toughness test.

[0097] This embodiment demonstrates the relationship between the impact toughness of alloy steel in 30CrMnSiA and vacuum gas pressure, such as... Figure 4As shown, the obtained impact toughness versus gas pressure curve was then verified and the error was adjusted. Combined with the hardness versus cooling rate and hardness versus gas pressure obtained in Examples 3 and 4, the relationship between cooling rate and gas pressure was further deduced.

[0098] This embodiment establishes the relationship between gas pressure during vacuum tempering and the cooling methods (water cooling, oil cooling, and air cooling) of the air furnace, based on the impact toughness values ​​of the test and control group samples. The details are as follows:

[0099] When using a vacuum furnace for tempering, if the standard requires air cooling, it is recommended to use air cooling at 0.8 to 2 Bar (excluding 2 Bar); if the standard requires oil cooling, it is recommended to use air cooling at 2 to 4 Bar (excluding 4 Bar); if the standard requires water cooling, it is recommended to use air cooling at 3 to 6 Bar (excluding 6 Bar).

[0100] This provides reference and guidance for vacuum heat treatment in practical engineering applications. Example 6

[0101] The specific steps for determining the vacuum tempering brittleness of medium alloy steel are as follows:

[0102] S1. Select several test samples, the material of which is 30CrMnSiA medium alloy steel. Randomly divide the test samples into test groups and control groups in equal quantities, and pre-treat the test samples at the same time.

[0103] S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples.

[0104] S12. Calculate the J value for each rod-shaped test sample, and select rod-shaped test samples that meet the standard J value as the test samples. The J value is calculated by measuring the weight percentage w of Si, Mn, P, and S in each test sample. Si %、w Mn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn )×(w P +w S )×10 4 ;

[0105] S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

[0106] S2. Place all samples in a vacuum furnace and quench them at 900±10℃ for 30-60 minutes. The cooling method is oil cooling.

[0107] S3. The quenched sample is tempered in an air furnace at a temperature of 510±10℃ and a holding time of 50 min to 90 min. Vacuum cooling oil, water and PAG coolant of different concentrations are used for cooling.

[0108] S4. The maximum cooling rate of the medium was measured after tempering of the control group sample using a cold tester.

[0109] S5. Test the impact toughness of the control group samples;

[0110] S6. Based on the cooling rate and impact toughness values ​​of the control group samples, curve fitting was performed to deduce the relationship curve between impact toughness and cooling rate.

[0111] This embodiment obtains the relationship between impact toughness and cooling rate of alloy steel in 30CrMnSiA. The obtained impact toughness vs. cooling rate curve is then verified and error adjusted. Combined with the relationships between hardness and cooling rate, hardness and gas pressure, and impact toughness and gas pressure obtained in Examples 3, 4, and 5, the relationship between impact toughness and cooling rate is obtained, as follows: Figure 5 As shown, by taking the average value and reducing the error, the corrected relationship curve between air pressure and cooling rate is obtained, as shown below. Figure 6 As shown.

[0112] From Examples 1 to 6 above, the relationships between furnace temperature, cooling rate, precipitated phase, hardness, and impact toughness were obtained, as well as the minimum cooling rate to prevent temper brittleness and the relationship between gas pressure and cooling rate. This led to the determination of the pressure range where temper brittleness occurs. Utilizing the relationship between temper brittleness and cooling rate, the gas pressure inside the vacuum furnace was controlled to prevent the workpiece from developing second-type temper brittleness. Simultaneously, a reasonable range of vacuum tempering gas pressure was determined, and the corresponding relationships between gas pressure during vacuum tempering and cooling methods such as water cooling, oil cooling, and air cooling in air-fired furnaces were identified to meet standard requirements and the needs of vacuum-tempered parts production.

[0113] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the vacuum tempering brittleness of medium alloy steel, characterized in that, The specific steps are as follows: S1. Select several test samples and randomly divide them into test groups and control groups in equal quantities. Simultaneously, pre-process the test samples: S2. The samples from both the test group and the control group were placed in a vacuum furnace for quenching treatment. S3. The samples of the test group and the control group were placed in a vacuum tempering furnace and an air furnace, respectively, for tempering and heat preservation treatment. S4. The test group samples were cooled using media gases at different pressures; the control group samples were cooled to room temperature using air, water, and oil cooling media, respectively. S5. The cooling rate of the test group and the control group samples were tested using a cold tester. The microstructure and performance changes of the control group samples were analyzed, including microstructure observation, performance testing and pendulum test. Among them, the microstructure observation was used to characterize the content of precipitates in the control group samples, the performance test was used to detect the surface and core hardness of the control group samples, and the pendulum test was used to measure the impact toughness of the control group samples. Then the relationship between gas pressure and cooling rate in the test group was calculated. Microscopic observations of the organization are as follows: Microstructure observations were performed on the original sample, the quenched sample, and the tempered sample using metallographic microscopy and SEM. Performance testing and verification are as follows: The performance test results of hardness testing of the original sample, the quenched sample, and the tempered sample using a Vickers hardness tester were used to further obtain the correspondence between furnace temperature, cooling rate, precipitated phase, and hardness. Based on the results of the pendulum test, the minimum cooling rate to avoid temper brittleness is as follows: Fracture surface scanning was performed on the samples after the impact toughness test to observe the morphology of the fracture surface and determine the characteristics of the fracture mode of the samples after the impact toughness test. The minimum cooling rate that does not produce temper brittleness was obtained based on the results of the pendulum test. S6. Based on the performance changes of the test and control group samples and the relationship between tempering gas pressure and cooling rate in the test group, determine the pressure range in which tempering brittleness occurs, specifically: The relationship between gas pressure and cooling rate in the test group was verified and the error was adjusted based on microstructure observation, performance test results and pendulum test results. Then, by utilizing the relationship between temper brittleness and cooling rate, the second type of temper brittleness in the workpiece is avoided by controlling the gas pressure inside the vacuum furnace; at the same time, a reasonable range of vacuum tempering gas pressure is determined, and the correspondence between gas pressure and air furnace cooling method during vacuum tempering is found to meet standard requirements and the needs of vacuum tempered parts production.

2. The method for determining the vacuum tempering brittleness of medium alloy steel according to claim 1, characterized in that, In step S1), the test sample is 30CrMnSiA medium alloy steel.

3. The method for determining the vacuum tempering brittleness of medium alloy steel according to any one of claims 1 to 2, characterized in that, In step S1), the selection and pretreatment of the test samples are as follows: S11. Take equal amounts of 30CrMnSiA alloy steel material and divide it into test group and control group, and make the 30CrMnSiA alloy steel material into rod-shaped test samples. S12. Calculate the J value of each rod-shaped test sample, and select the rod-shaped test sample that meets the standard J value as the test sample; S13. Place the sample to be tested in an ultrasonic cleaner and clean it in the following order: clean twice with ethanol, clean twice with acetone, and clean once with distilled water. Each cleaning should last 3 to 8 minutes.

4. The method for determining the vacuum tempering brittleness of medium alloy steel according to claim 3, characterized in that, In step S12), the J value is calculated as follows: the weight percentages w of Si, Mn, P, and S in each sample to be tested are measured respectively. Si %、w Mn %、w P % and w S %, and calculate the J value for each sample to be tested, J = (w Si +w Mn )×(w P +w S )×10 4 .

5. The method for determining the vacuum tempering brittleness of medium alloy steel according to claim 1, characterized in that, In step S2), the quenching temperature in the vacuum furnace is 900±10℃, the holding time is 30~60min, and the cooling method is oil cooling.

6. The method for determining the vacuum tempering brittleness of medium alloy steel according to claim 1, characterized in that, In step S3), the tempering temperature in the air furnace is 510±10°C and the holding time is 50 min to 90 min; the tempering temperature in the vacuum tempering furnace is 510±10°C and the holding time is 1.5 times that of the air furnace.

7. The method for determining the vacuum tempering brittleness of medium alloy steel according to claim 1, characterized in that, In step S4), the medium gas is nitrogen or argon, and the filling pressure ranges from 0.8 bar to 6 bar.

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