Method for preparing double martensite through low-pressure variable-frequency carbonitriding

Abstract

The invention discloses a method for preparing double martensite through low-pressure variable-frequency carbonitriding, and relates to a method for preparing double martensite through carbonitriding. The purpose is to solve the problem that wear resistance and fatigue performance cannot meet use requirements due to low hardness of components subjected to conventional carburizing and quenching treatment. The method comprises the following steps: firstly, carrying out carburizing treatment on a metal component by adopting a carburizing technology to obtain a high-carbon-concentration layer on the metal surface, then, treating the carburized component by adopting a low-pressure variable-frequency carbonitriding technology, and enabling carbon atoms and nitrogen atoms to rapidly enter a carburized layer in an alternating manner at lower air pressure to obtain proper C and N concentrations on the surface of the carburized layer, so that the carburized layer is subjected to high-pressure carburizing treatment. And finally, quenching treatment is carried out, so that a large amount of carbon / nitrogen-containing austenite is converted into double martensite, and a double martensite layer with excellent performance is prepared on the surface of the component. Compared with a carburizing layer obtained through conventional carburizing treatment and carbonitriding treatment, the surface hardness of the carburizing layer is at least improved by 3-5 HRC; meanwhile, due to the existence of double martensite, the pressure stress of the infiltrated layer is greatly improved, so that the bearing resistance of the infiltrated layer is improved.

Description

Technical Field

[0001] This invention relates to a method for preparing bismartensite by carbonitriding and nitrogen percolation. Background Technology

[0002] In fields such as machinery manufacturing, automotive industry, and aerospace, surface strengthening treatment of metallic materials (such as alloy steel, stainless steel, and titanium alloys) is a key technology for improving the wear resistance, fatigue resistance, and corrosion resistance of parts. In traditional surface modification processes, carburizing can form a high-hardness carburized layer (such as martensitic structure) on the metal surface, significantly improving the material's wear resistance and contact fatigue strength.

[0003] However, the hardness of conventional steel components (gears, bearings, molds, connectors) after carburizing and quenching is only 58-62 HRC, which cannot meet the requirements of practical applications. Current research uses low-temperature nitriding after carburizing to improve surface hardness. However, low-temperature nitriding after carburizing results in a thin and brittle nitrided layer. Microcracks or compositional segregation appear at the carbon-nitrogen composite layer interface, and the nitrided layer is prone to peeling off. For most components, nitriding after carburizing and quenching reduces the core hardness due to the increased temperature, failing to meet the core hardness requirements. Therefore, how to improve the hardness of carburized components has become an urgent technical problem to be solved.

[0004] Carbonitriding technology can introduce nitrogen atoms to generate nitrogen-containing compounds on the basis of carburizing, thus providing a possibility to improve the hardness of carburized components. However, conventional carbonitriding treatment results in a thin carburized layer and the presence of retained austenite, limiting the improvement in hardness. Currently, how to significantly improve the surface hardness and wear resistance of carburized components has become a key research issue. Summary of the Invention

[0005] The purpose of this invention is to solve the problem that the low hardness of components after conventional carburizing and quenching treatment leads to insufficient wear resistance and fatigue performance to meet the application requirements. A method for preparing double martensite by low-pressure variable frequency carbonitriding is proposed.

[0006] The method for preparing double martensite using low-pressure variable frequency carbonitriding of the present invention is carried out according to the following steps:

[0007] 1. The component is placed in a carburizing furnace for carburizing, and then quenched to obtain a carburized component with a high carbon concentration on the surface.

[0008] The component is made of steel with a carbon content of less than 0.7 wt.%.

[0009] 2. The carburized component with a high carbon concentration on the surface is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer is 0.15mm to 1mm.

[0010] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0011] The process for the strong permeation treatment is as follows:

[0012] Alternating carburizing and nitriding are performed under low pressure, with carburizing and nitriding temperatures ranging from 820°C to 880°C. The carburizing gas is acetylene, acetone, propane, or methane, and the carburizing pressure is 300–800 Pa. The nitriding gas is ammonia or a mixture of ammonia and nitrogen, and the nitriding pressure is 200–500 Pa. The alternation frequency of carburizing and nitriding is 0.001–1 Hz.

[0013] The diffusion treatment process is as follows:

[0014] Introduce ammonia gas and maintain the pressure at 200–800 Pa for 3–5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 5–10 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 10–30 minutes.

[0015] 3. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0016] During quenching, the carbon-nitrogen double-diffusion component is quenched to the Ms point, which transforms the carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point, which transforms the nitrogen-containing austenite into nitrogen-containing martensite.

[0017] The advantages of this invention over the prior art are as follows:

[0018] This invention first employs carburizing technology to treat metal components, obtaining a high carbon concentration layer on the metal surface. Then, it uses low-pressure variable-frequency carbonitriding technology to treat the carburized components, allowing carbon and nitrogen atoms to rapidly and alternately enter the carburized layer at a lower pressure, resulting in suitable C and N concentrations on the surface. Finally, quenching transforms a large amount of carbon / nitrogen-containing austenite into double martensite (α′1 martensite and α′2 martensite), creating a high-performance double martensite layer on the component surface. This layer has a fine-grained structure, and the double martensite is lath martensite. Compared to the carburized layers obtained by conventional carburizing and carbonitriding treatments (composed of twinned martensite and carbides, as well as carbon / nitrides), the surface hardness of this invention is increased by at least 3–5 HRC. Simultaneously, the presence of double martensite significantly increases the compressive stress of the carburized layer, thereby enhancing its load-bearing capacity.

[0019] The method of this invention for metal modification treatment to obtain a carburized layer hardness gradient will not result in a phenomenon of low surface hardness and high internal hardness. It is applicable not only to shallow carburizing layers but also to deep carburizing layers. It can be applied to heavy-duty environments such as heavy-duty gear bearings, and solves the problem of insufficient surface hardness and wear resistance of conventional carburized layers under heavy-duty conditions.

[0020] Another method for preparing double martensite using low-pressure variable frequency carbonitriding according to the present invention is carried out according to the following steps:

[0021] 1. The component is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer is 0.15mm to 1mm.

[0022] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0023] The component is made of steel with a carbon content greater than 0.7 wt.%.

[0024] The process for the strong permeation treatment is as follows:

[0025] Alternating carburizing and nitriding are performed under low pressure, with carburizing and nitriding temperatures ranging from 820°C to 880°C. The carburizing gas is acetylene, acetone, propane, or methane, and the carburizing pressure is 300–800 Pa. The nitriding gas is ammonia or a mixture of ammonia and nitrogen, and the nitriding pressure is 200–500 Pa. The alternation frequency of carburizing and nitriding is 0.001–1 Hz.

[0026] The diffusion treatment process is as follows:

[0027] Introduce ammonia gas and maintain the pressure at 200–800 Pa for 3–5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 5–10 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 10–30 minutes.

[0028] 2. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0029] During quenching, the carbonitrided component is quenched to Ms to transform carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below Mf to transform nitrogen-containing austenite into nitrogen-containing martensite.

[0030] The cooling rate during the cold treatment is 5-10℃ / min. After cooling to the Mf point, the temperature is maintained for 20-40 minutes, and then the temperature is raised to room temperature.

[0031] This invention targets steels with a carbon content greater than 0.7 wt.% and a shallow carburizing layer requirement. It employs low-pressure variable-frequency carbonitriding technology to treat carburized components. Under lower pressure, carbon and nitrogen atoms rapidly and alternately enter the carburized layer, achieving suitable C and N concentrations on the surface. Finally, quenching transforms a large amount of carbon / nitrogen austenite into double martensite (α′1 martensite and α′2 martensite), creating a high-performance double martensite layer on the component surface. This layer has a fine-grained structure, and the double martensite is lath martensite. Compared to conventional carburizing and carbonitriding processes that produce carburized layers (composed of twinned martensite and carbides, as well as carbon / nitrides), this invention increases surface hardness by at least 3–5 HRC. Simultaneously, the presence of double martensite significantly increases the compressive stress of the carburized layer, thereby enhancing its load-bearing capacity.

[0032] The method of this invention for metal modification treatment to obtain a carburized layer hardness gradient will not result in a phenomenon of low surface hardness and high internal hardness. It is applicable not only to shallow carburizing layers but also to deep carburizing layers. It can be applied to heavy-duty environments such as heavy-duty gear bearings, and solves the problem of insufficient surface hardness and wear resistance of conventional carburized layers under heavy-duty conditions. Attached Figure Description

[0033] Figure 1 The XRD pattern of the furnace-fed sample after low-pressure frequency conversion carbonitriding treatment in Example 1;

[0034] Figure 2 The images are metallographic images of the furnace-carburized samples after low-pressure frequency conversion carbonitriding treatment in Example 1. a is the carburized sample obtained in step one, and b is the carburized + carbonitriding sample obtained in step three. Detailed Implementation

[0035] The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any reasonable combination of the specific embodiments.

[0036] Specific Implementation Method 1: The method for preparing double martensite using low-pressure variable frequency carbonitriding in this implementation method is carried out according to the following steps:

[0037] 1. The component is placed in a carburizing furnace for carburizing, and then quenched to obtain a carburized component with a high carbon concentration on the surface.

[0038] The component is made of steel with a carbon content of less than 0.7 wt.%.

[0039] 2. The carburized component with a high carbon concentration on the surface is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer is 0.15mm to 1mm.

[0040] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0041] The process for the strong permeation treatment is as follows:

[0042] Alternating carburizing and nitriding are performed under low pressure, with carburizing and nitriding temperatures ranging from 820°C to 880°C. The carburizing gas is acetylene, acetone, propane, or methane, and the carburizing pressure is 300–800 Pa. The nitriding gas is ammonia or a mixture of ammonia and nitrogen, and the nitriding pressure is 200–500 Pa. The alternation frequency of carburizing and nitriding is 0.001–1 Hz.

[0043] The diffusion treatment process is as follows:

[0044] Introduce ammonia gas and maintain the pressure at 200–800 Pa for 3–5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 5–10 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 10–30 minutes.

[0045] 3. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0046] During quenching, the carbon-nitrogen double-diffusion component is quenched to the Ms point, which transforms the carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point, which transforms the nitrogen-containing austenite into nitrogen-containing martensite.

[0047] This embodiment has the following beneficial effects:

[0048] This embodiment first employs carburizing technology to treat the metal component, obtaining a high carbon concentration layer on the metal surface. Then, it uses low-pressure variable-frequency carbonitriding technology to treat the carburized component, allowing carbon and nitrogen atoms to rapidly and alternately enter the carburized layer at a lower pressure, resulting in suitable C and N concentrations on the surface. Finally, quenching transforms a large amount of carbon / nitrogen-containing austenite into double martensite (α′1 martensite and α′2 martensite), creating a high-performance double martensite layer on the component surface. This layer has a fine-grained structure, and the double martensite is lath martensite. Compared to the carburized layers obtained by conventional carburizing and carbonitriding treatments (composed of twinned martensite and carbides, as well as carbon / nitrides), this embodiment increases the surface hardness by at least 3–5 HRC. Simultaneously, the presence of double martensite significantly increases the compressive stress of the carburized layer, thereby enhancing its load-bearing capacity.

[0049] The method described in this embodiment, when used for metal modification treatment to obtain a carburized layer hardness gradient, avoids the phenomenon of low surface hardness and high internal hardness. It is applicable not only to shallow carburizing layers but also to deep carburizing layers, and can be applied to heavy-duty environments such as heavy-load gear bearings. It solves the problem of insufficient surface hardness and wear resistance of conventional carburized layers under heavy-load conditions.

[0050] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the carburizing temperature in step one is 890℃~1050℃, and the carburized layer depth is 0.5mm~5mm.

[0051] Specific Implementation Method 3: This implementation method differs from Specific Implementation Method 1 or 2 in that: the strong infiltration process described in step 2 is as follows: alternating carburizing and nitriding are performed under low pressure, with the carburizing and nitriding temperatures being 860°C; the carburizing gas is acetylene, the carburizing gas pressure is 500Pa, the nitriding gas is a mixture of ammonia and nitrogen, the nitriding gas pressure is 300Pa, and the alternation frequency of carburizing and nitriding is 0.02Hz.

[0052] The diffusion treatment process described in step two is as follows: ammonia gas is introduced and maintained at a pressure of 250 Pa for 4 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced and maintained at a pressure of 250 Pa for 5 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced and maintained at a pressure of 250 Pa for 10 minutes.

[0053] Specific Implementation Method Four: This implementation method differs from one of the specific implementation methods one to three in that the quenching process described in step three is oil quenching, gas quenching, or water quenching.

[0054] Specific Implementation Method 5: This implementation method differs from Specific Implementation Methods 1 to 4 in that: during the cold treatment described in step 3, the cooling rate is 5 to 10°C / min, and after cooling to the Mf point, it is kept warm for 20 to 40 minutes, and then heated to room temperature.

[0055] Specific Implementation Method Six: The method for preparing double martensite using low-pressure variable frequency carbonitriding in this implementation method is carried out according to the following steps:

[0056] 1. The component is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer is 0.15mm to 1mm.

[0057] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0058] The component is made of steel with a carbon content greater than 0.7 wt.%.

[0059] The process for the strong permeation treatment is as follows:

[0060] Alternating carburizing and nitriding are performed under low pressure, with carburizing and nitriding temperatures ranging from 820°C to 880°C. The carburizing gas is acetylene, acetone, propane, or methane, and the carburizing pressure is 300–800 Pa. The nitriding gas is ammonia or a mixture of ammonia and nitrogen, and the nitriding pressure is 200–500 Pa. The alternation frequency of carburizing and nitriding is 0.001–1 Hz.

[0061] The diffusion treatment process is as follows:

[0062] Introduce ammonia gas and maintain the pressure at 200–800 Pa for 3–5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 5–10 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 10–30 minutes.

[0063] 2. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0064] During quenching, the carbonitrided component is quenched to the Ms point to transform carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point to transform nitrogen-containing austenite into nitrogen-containing martensite.

[0065] The cooling rate during the cold treatment is 5-10℃ / min. After cooling to the Mf point, the temperature is maintained for 20-40 minutes, and then the temperature is raised to room temperature.

[0066] This embodiment targets steel with a carbon content greater than 0.7 wt.% and a shallow carburized layer requirement. It further employs low-pressure variable-frequency carbonitriding technology to treat the carburized component. Under lower pressure, carbon and nitrogen atoms rapidly and alternately enter the carburized layer, achieving suitable C and N concentrations on the surface. Finally, quenching transforms a large amount of carbon / nitrogen austenite into double martensite (α′1 martensite and α′2 martensite), creating a high-performance double martensite layer on the component surface. This layer has a fine-grained structure, and the double martensite is lath martensite. Compared to conventional carburizing and carbonitriding processes that produce carburized layers (composed of twinned martensite and carbides, as well as carbon / nitrides), this embodiment increases the surface hardness by at least 3–5 HRC. Simultaneously, the presence of double martensite significantly increases the compressive stress of the carburized layer, thereby enhancing its load-bearing capacity.

[0067] The method described in this embodiment, when used for metal modification treatment to obtain a carburized layer hardness gradient, avoids the phenomenon of low surface hardness and high internal hardness. It is applicable not only to shallow carburizing layers but also to deep carburizing layers, and can be applied to heavy-duty environments such as heavy-load gear bearings. It solves the problem of insufficient surface hardness and wear resistance of conventional carburized layers under heavy-load conditions.

[0068] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Six in that the process of strong permeation treatment described in step one is as follows:

[0069] Alternating carburizing and nitriding were performed under low pressure at a temperature of 860℃. The carburizing gas was acetylene at a pressure of 500 Pa, and the nitriding gas was a mixture of ammonia and nitrogen at a pressure of 600 Pa. The alternation frequency of carburizing and nitriding was 0.02 Hz.

[0070] The diffusion treatment process described in step one is as follows:

[0071] Introduce ammonia gas and maintain the pressure at 250 Pa for 4 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 250 Pa for 5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 250 Pa for 10 minutes.

[0072] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Six in that: during the cold treatment described in step two, the cooling rate is 8℃ / min, the temperature is lowered to the Mf point and then kept warm for 30 minutes before being raised to room temperature.

[0073] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Six in that: during the cold treatment described in step two, the cooling rate is 5-10℃ / min, the temperature is lowered to the Mf point and then kept warm for 30 minutes before being raised to room temperature.

[0074] Specific Implementation Method 10: This implementation method differs from Specific Implementation Method 6 in that the quenching process described in step 2 is oil quenching, gas quenching, or water quenching.

[0075] Example 1

[0076] The method for preparing double martensite using low-pressure variable frequency carbonitriding in this embodiment is carried out according to the following steps:

[0077] 1. Gears made of 18Cr2Ni4W material were placed in a carburizing furnace for 12 hours and then quenched to obtain carburized components with high carbon concentration on the surface. The surface hardness of the carburized components was measured to be 62.2 HRC.

[0078] The carburizing treatment temperature is 930℃, and the carburized layer depth is 3.2mm;

[0079] 2. The carburized component with a high carbon concentration on the surface is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer is 0.4 mm.

[0080] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0081] The process of the intensive carburizing treatment is as follows: alternating carburizing and nitriding are performed under low pressure.

[0082] The carburizing and nitriding temperatures were 860℃; the carburizing gas was acetylene, the carburizing pressure was 500Pa, the nitriding gas was ammonia, the nitriding pressure was 300Pa, and the alternation frequency of carburizing and nitriding was 0.02Hz, with alternation occurring for 4 hours.

[0083] The diffusion treatment process described in step two is as follows: ammonia gas is introduced and maintained at a pressure of 250 Pa for 4 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced and maintained at a pressure of 250 Pa for 5 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced and maintained at a pressure of 250 Pa for 10 minutes.

[0084] 3. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0085] During quenching, the carbonitrided component is quenched to the Ms point to transform carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point to transform nitrogen-containing austenite into nitrogen-containing martensite.

[0086] The quenching process is oil quenching;

[0087] The cooling rate during the cold treatment is 7℃ / min. After cooling to the Mf point, the temperature is held for 20 minutes, and then the temperature is raised to room temperature.

[0088] Figure 1 The XRD pattern of the furnace-fed sample after low-pressure frequency conversion carbonitriding treatment in Example 1 is shown below. Figure 1 It can be seen that the surface of the treated sample is mainly martensitic, with almost no residual austenite and carbon / nitrogen compound phases, which indicates that a double martensitic phase layer (α′1 and α′2) has been prepared. Figure 2 The image shows the metallographic images of the furnace-carburized samples after low-pressure frequency conversion carbonitriding treatment in Example 1. In the image, a is the carburized sample obtained in step one, and b is the carburized + carbonitriding sample obtained in step three. Figure 2 It can be seen that after quenching, the carburized sample in step one mainly consists of carbonaceous martensite, retained austenite, and a small amount of fine carbides. After carbonitriding in step three, the sample microstructure mainly consists of martensite, a small amount of retained austenite, and a small amount of fine carbides. The surface hardness of the component tested in Example 1 can reach 65.3 HRC.

[0089] Example 2

[0090] The method for preparing double martensite using low-pressure variable frequency carbonitriding in this embodiment is carried out according to the following steps:

[0091] 1. The gear made of 20Cr2Ni4 material was placed in a carburizing furnace for carburizing. After carburizing, it was quenched to obtain a carburized component with a high carbon concentration on the surface. The surface hardness of the carburized component was measured to be 62.8 HRC.

[0092] The carburizing treatment temperature is 920℃, and the carburized layer depth is 1.80mm;

[0093] 2. The carburized component with a high carbon concentration on the surface is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer is 0.2 mm.

[0094] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0095] The process of the strong carburizing treatment is as follows: alternating carburizing and nitriding are carried out under low pressure; the temperature of carburizing and nitriding is 830℃; the carburizing gas is acetylene, the carburizing gas pressure is 500Pa, the nitriding gas is ammonia, the nitriding gas pressure is 300Pa, the alternation frequency of carburizing and nitriding is 0.005Hz, and the alternation is carried out for 2 hours.

[0096] The diffusion process is as follows: ammonia gas is introduced, and the pressure is maintained at 280 Pa for 3 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced. The pressure is maintained at 280 Pa for 5 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced. The pressure is maintained at 280 Pa for 10 minutes.

[0097] 3. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0098] During quenching, the carbonitrided component is quenched to the Ms point to transform carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point to transform nitrogen-containing austenite into nitrogen-containing martensite.

[0099] The quenching process is oil quenching;

[0100] The cooling rate during the cold treatment is 8℃ / min. After cooling to the Mf point, the temperature is held for 20 minutes, and then the temperature is raised to room temperature.

[0101] Example 2 shows that the surface hardness of the component can reach 66.2 HRC.

[0102] Example 3

[0103] The method for preparing double martensite using low-pressure variable frequency carbonitriding in this embodiment is carried out according to the following steps:

[0104] 1. Gears made of 18CrNiMo7-6 material were placed in a carburizing furnace for carburizing. After carburizing, they were quenched to obtain carburized components with a high carbon concentration on the surface. The surface hardness of the carburized components was measured to be 62.6 HRC.

[0105] The carburizing treatment temperature is 940℃, and the carburized layer depth is 1.80 mm;

[0106] 2. Place the carburized components with high carbon concentration on the surface into a carburizing furnace for low-pressure variable frequency carbonitriding treatment;

[0107] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0108] The process of the strong infiltration treatment is as follows: alternating carburizing and nitriding are performed under low pressure to obtain a carbonitrided layer with a depth of 0.28 mm.

[0109] The carburizing and nitriding temperatures were 840℃; the carburizing gas was methane, the carburizing pressure was 800Pa, the nitriding gas was a mixture of ammonia and nitrogen in a 1:1 ratio, the nitriding pressure was 300Pa, the alternation frequency of carburizing and nitriding was 0.05Hz, and the alternation was carried out for 2 hours.

[0110] The diffusion treatment process is as follows: ammonia gas is introduced and maintained at a pressure of 260 Pa for 4 minutes, the gas in the furnace is extracted and new ammonia gas is introduced and maintained at a pressure of 260 Pa for 6 minutes, the gas in the furnace is extracted and new ammonia gas is introduced and maintained at a pressure of 260 Pa for 10 minutes.

[0111] 3. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0112] During quenching, the carbonitrided component is quenched to the Ms point to transform carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point to transform nitrogen-containing austenite into nitrogen-containing martensite.

[0113] The quenching process is oil quenching;

[0114] The cooling rate during the cold treatment is 8℃ / min. After cooling to the Mf point, the temperature is held for 40 minutes, and then the temperature is raised to room temperature.

[0115] Example 3 shows that the surface hardness of the component can reach 65.8 HRC. Table 1 compares the contact fatigue life of 18CrNiMo7-6 steel gears. It can be seen that the contact fatigue life is greatly improved after carbonitriding treatment. Compared with conventional carburizing (the conventional carburizing process is: gas carburizing at 940℃ for 16 hours, with a carburized layer thickness of 1.98 mm), the fatigue life of the gear is improved by nearly an order of magnitude.

[0116] Table 1

[0117]

[0118] Example 4

[0119] The method for preparing double martensite using low-pressure variable frequency carbonitriding in this embodiment is carried out according to the following steps:

[0120] 1. H13 steel is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer is 0.5 mm.

[0121] The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment;

[0122] The process of the strong carburizing treatment is as follows: alternating carburizing and nitriding are carried out under low pressure, the temperature of carburizing and nitriding is 840℃; the carburizing gas is acetylene, the carburizing gas pressure is 500Pa, the nitriding gas is ammonia, the nitriding gas pressure is 260Pa, the alternation frequency of carburizing and nitriding is 0.005Hz, and the alternation is carried out for 6 hours.

[0123] The diffusion process is as follows: ammonia gas is introduced, and the pressure is maintained at 280 Pa for 5 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced. The pressure is maintained at 280 Pa for 8 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced. The pressure is maintained at 280 Pa for 30 minutes.

[0124] 3. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment.

[0125] During quenching, the carbon-nitrogen double-diffusion component is quenched to below Ms point, and during cold treatment, the component temperature is lowered to below Mf point, so that carbon / nitrogen austenite is transformed into carbon / nitrogen double martensite.

[0126] The quenching process is oil quenching;

[0127] The cooling rate during the cold treatment is 8℃ / min. After cooling to the Mf point, the temperature is held for 20 minutes, and then the temperature is raised to room temperature.

[0128] Example 4 shows that the surface hardness of the component can reach 67.5 HRC.

Claims

1. A method for preparing double martensite using low-pressure variable frequency carbonitriding, characterized in that: The method for preparing double martensite using low-pressure variable frequency carbonitriding is carried out according to the following steps:

1. The component is placed in a carburizing furnace for carburizing, and then quenched to obtain a carburized component with a high carbon concentration on the surface. The component is made of steel with a carbon content of less than 0.7 wt.%.

2. Place the carburized components with high carbon concentration on the surface into a carburizing furnace for low-pressure variable frequency carbonitriding treatment; The low-pressure variable frequency carbonitriding treatment includes strong infiltration treatment and diffusion treatment; The process for the strong permeation treatment is as follows: Alternating carburizing and nitriding are performed under low pressure, with carburizing and nitriding temperatures ranging from 820°C to 880°C. The carburizing gas is acetylene, acetone, propane, or methane, and the carburizing pressure is 300–800 Pa. The nitriding gas is ammonia or a mixture of ammonia and nitrogen, and the nitriding pressure is 200–500 Pa. The alternation frequency of carburizing and nitriding is 0.001–1 Hz. The diffusion treatment process is as follows: Introduce ammonia gas and maintain the pressure at 200–800 Pa for 3–5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 5–10 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 10–30 minutes. The carbonitriding layer depth obtained by the low-pressure frequency conversion carbonitriding is 0.15 mm to 1 mm; Third, the components after carbonitriding and nitriding treatment are quenched and then cold-treated in sequence. During quenching, the carbon-nitrogen double-diffusion component is quenched to the Ms point, which transforms the carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point, which transforms the nitrogen-containing austenite into nitrogen-containing martensite.

2. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: The carburizing temperature in step one is 890℃~1050℃, and the carburized layer depth is 0.5mm~5mm.

3. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: The process of the strong infiltration treatment described in step two is as follows: alternating carburizing and nitriding are carried out under low pressure; the temperature of carburizing and nitriding is 860℃; the carburizing gas is acetylene, the carburizing gas pressure is 500Pa; the nitriding gas is a mixture of ammonia and nitrogen, the nitriding gas pressure is 300Pa; and the alternation frequency of carburizing and nitriding is 0.02Hz. The diffusion treatment process described in step two is as follows: ammonia gas is introduced and maintained at a pressure of 250 Pa for 4 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced and maintained at a pressure of 250 Pa for 5 minutes. The gas in the furnace is then extracted and new ammonia gas is introduced and maintained at a pressure of 250 Pa for 10 minutes.

4. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: The quenching process described in step three is oil quenching, gas quenching, or water quenching.

5. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: In step three, the cooling rate during the cold treatment is 5-10℃ / min. After cooling to the Mf point, the temperature is maintained for 20-40 minutes, and then the temperature is raised to room temperature.

6. A method for preparing double martensite using low-pressure variable frequency carbonitriding, characterized in that: The method for preparing double martensite using low-pressure variable frequency carbonitriding is carried out according to the following steps:

1. The component is placed in a carburizing furnace for low-pressure variable frequency carbonitriding treatment, and the depth of the carbonitrided layer obtained is 0.15mm to 1mm; the low-pressure variable frequency carbonitriding treatment includes strong carburizing treatment and diffusion treatment; The component is made of steel with a carbon content greater than 0.7 wt.%. The process for the strong permeation treatment is as follows: Alternating carburizing and nitriding are performed under low pressure, with carburizing and nitriding temperatures ranging from 820°C to 880°C. The carburizing gas is acetylene, acetone, propane, or methane, and the carburizing pressure is 300–800 Pa. The nitriding gas is ammonia or a mixture of ammonia and nitrogen, and the nitriding pressure is 200–500 Pa. The alternation frequency of carburizing and nitriding is 0.001–1 Hz. The diffusion treatment process is as follows: Introduce ammonia gas and maintain the pressure at 200–800 Pa for 3–5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 5–10 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 200–800 Pa for 10–30 minutes.

2. The components after carbonitriding and nitriding treatment are then subjected to quenching and cold treatment. During quenching, the carbonitriding component is quenched to the Ms point to transform the large carbon-containing austenite into carbon-containing martensite. During cold treatment, the component temperature is lowered to below the Mf point to transform the nitrogen-containing austenite into nitrogen-containing martensite. The cooling rate during the cold treatment is 5-10℃ / min. After cooling to the Mf point, the temperature is maintained for 20-40 minutes, and then the temperature is raised to room temperature.

7. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: The process of strong permeation treatment described in step one is as follows: Alternating carburizing and nitriding were performed under low pressure at a temperature of 860℃. The carburizing gas was acetylene at a pressure of 500 Pa, and the nitriding gas was a mixture of ammonia and nitrogen at a pressure of 600 Pa. The alternation frequency of carburizing and nitriding was 0.02 Hz. The diffusion treatment process described in step one is as follows: Introduce ammonia gas and maintain the pressure at 250 Pa for 4 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 250 Pa for 5 minutes. Then, extract the gas from the furnace and introduce new ammonia gas. Maintain the pressure at 250 Pa for 10 minutes.

8. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: In step two, the cooling rate during the cold treatment is 8°C / min. After cooling to the Mf point, the temperature is held for 30 minutes, and then the temperature is raised to room temperature.

9. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: In step two, the cooling rate during the cold treatment is 5-10℃ / min. After cooling to the Mf point, the temperature is maintained for 30 minutes, and then the temperature is raised to room temperature.

10. The method for preparing double martensite using low-pressure variable frequency carbonitriding according to claim 1, characterized in that: The quenching process described in step two is oil quenching, gas quenching, or water quenching.

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