A low-pressure vacuum carburizing method for 16crni4mo steel

Abstract

The application discloses a low-pressure vacuum carburizing method for 16CrNi4Mo steel, and comprises the following steps: S1, plating a protective layer on a non-carburizing area of a 16CrNi4Mo steel workpiece, and performing sandblasting cleaning on a carburizing area; S2, placing the 16CrNi4Mo steel workpiece in a carburizing furnace to perform low-pressure vacuum carburizing treatment; and S3, sequentially performing first high-temperature tempering treatment, second high-temperature tempering treatment, quenching treatment, cold treatment and tempering treatment on the 16CrNi4Mo steel workpiece. The application has the advantages of stable heat treatment quality, controllable carburizing layer depth and controllable hardness.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal heat treatment, and particularly relates to a low-pressure vacuum carburizing method for 16CrNi4Mo steel. BACKGROUND

[0002] The 16CrNi4Mo steel is a widely used British system carburizing steel in China, has good hardenability, high strength and good wear resistance, and is suitable for important structural parts such as transmission shafts and gears, and is a main material for engine transmission parts.

[0003] A certain type of engine transmission gear adopts the 16CrNi4Mo steel, and the carburizing heat treatment requirement is that the effective hardening layer depth is 0.3-0.5mm, the surface hardness is greater than or equal to 80HRA (58HRC), and the core hardness is 41-46HRC. When the material is processed by using the traditional carburizing process, the carbon potential fluctuation cannot be well controlled, the net-shaped carbide is easily formed, and the heat treatment quality is unstable. The existing pulse low-pressure vacuum carburizing process of other materials cannot make the 16CrNi4Mo steel meet the above requirements because the austenitizing temperature of the material is different, and the alloy composition and the combination characteristics of carbon atoms are also different. SUMMARY

[0004] The present application solves the technical problems of overcoming the deficiencies of the prior art, and provides a low-pressure vacuum carburizing method for 16CrNi4Mo steel, which has stable heat treatment quality, controllable carburizing layer depth and hardness.

[0005] To solve the above technical problems, the present application adopts the following technical scheme:

[0006] A low-pressure vacuum carburizing method for 16CrNi4Mo steel, comprising the following steps:

[0007] S1, a protective layer is plated on the non-carburizing area of the 16CrNi4Mo steel workpiece, and the carburizing area is subjected to sand blasting cleaning;

[0008] S2, the 16CrNi4Mo steel workpiece is placed in a carburizing furnace for low-pressure carburizing treatment; the low-pressure carburizing treatment specifically comprises the following steps:

[0009] S201, vacuumizing and heating to a carburizing temperature;

[0010] S202, a carburizing atmosphere is introduced at a flow rate v1 for a time t1 for the first time carburizing treatment, and then a carburizing atmosphere is introduced at a flow rate v2 for a time t2 for carburizing treatment, and v1>v2 and t1

[0011] S203, after vacuumizing, nitrogen gas diffusion treatment is conducted;

[0012] S204, repeating steps S201 to S203;

[0013] S205, final nitrogen gas diffusion treatment is conducted, and cooling is conducted;

[0014] S3, the 16CrNi4Mo steel workpiece is sequentially subjected to first tempering treatment, second tempering treatment, quenching treatment, cold treatment and third tempering treatment.

[0015] As a further improvement of the above technical solution:

[0016] In the step S202, v1 is 1000-3000 NL / H, t1 is 10-20 s, v2 is 300-800 NL / H, and t2 is 30-300 s.

[0017] In the step S2, the carburizing temperature is 880-950 DEG C, the acetylene pressure during carburizing treatment is 1000-3000 mbar, and the partial pressure during nitrogen gas diffusion treatment is 3000-6000 mbar.

[0018] In the step S205, the nitrogen gas pressure is 1200-1800 mbar.

[0019] In the step S1, the number of repetitions in the step S205 is 7-20.

[0020] In the step S3, the temperature of the first tempering treatment and the second tempering treatment is 660-690 DEG C, and the holding time is 180-210 min.

[0021] In the step S3, the temperature of the quenching treatment is 800-840 DEG C, the holding time is 70-210 min, and the oil temperature is 40-80 DEG C.

[0022] In the step S3, the temperature of the cold treatment is -70 DEG C to -90 DEG C, and the holding time is 120-180 min.

[0023] In the step S3, the temperature of the third tempering treatment is 150-200 DEG C, and the holding time is 180-240 min.

[0024] In the step S1, the thickness of the protective layer is 40-60 mu m; the sandblasting sand is corundum sand, the mesh number is greater than or equal to 100 mesh, and the pressure is less than or equal to 0.2 MPa.

[0025] Compared with the prior art, the present application has the following advantages:

[0026] This invention discloses a low-pressure vacuum carburizing method for 16CrNi4Mo steel. During carburizing, the carburizing temperature remains constant, and the carburizing atmosphere is introduced in stages. The first stage involves a high flow rate and a short duration of carburizing atmosphere, instantly filling the furnace with the carburizing atmosphere and ensuring sufficient carbon source on the workpiece surface. The second stage involves a low flow rate and a long duration of carburizing atmosphere, allowing carbon atoms to diffuse slowly. The small flow rate of atmosphere gradually replenishes the carbon source, preventing the accumulation of carbon ash. After processing, the surface of the 16CrNi4Mo steel workpiece can obtain a uniformly dispersed distribution of carbides. The microstructure is stable after heat treatment, with an effective hardened layer depth of 0.3–0.5 mm, a surface hardness ≥58 HRC, and a core hardness of 35–46 HRC. Attached Figure Description

[0027] Figure 1 This is a simplified process diagram of the present invention.

[0028] Figure 2 This is an organizational diagram of Embodiment 1 of the present invention.

[0029] Figure 3 This is a tissue diagram of Comparative Example 1 of the present invention. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Unless otherwise specified, the instruments or materials used in the present invention are commercially available.

[0031] Example 1

[0032] The requirements for carburizing heat treatment of transmission gears of a certain type of engine are: effective hardened layer depth of 0.3 to 0.5 mm, surface hardness ≥ 80 HRA (58 HRC), and core hardness of 41 to 46 HRC.

[0033] like Figure 1 As shown, the specific implementation steps of the low-pressure vacuum carburizing method for 16CrNi4Mo steel in this embodiment include:

[0034] 1. After the workpiece is copper-plated as a whole, the carburized area is machined in the machining process: the copper layer thickness is required to be 50μm, and there should be no defects such as blistering, peeling, or pinholes.

[0035] 2. Blow corundum sand into the carburizing area of ​​the workpiece. The sand particle size is 100 mesh, and the pressure is 0.20 MPa.

[0036] 3. Carburizing of the workpiece: The carburizing equipment is a low-pressure vacuum carburizing furnace with a carburizing temperature of 880℃; acetylene pressure of 2500mbar and partial pressure of 4000mbar.

[0037] 3-1, the workpiece is vacuumized in the furnace at room temperature to below 100 mbar, nitrogen is filled, and the temperature is raised from room temperature to 800°C at a rate of 10°C / min, and the temperature is kept uniform for 20 min, and then the temperature is raised to the carburizing temperature of 880°C, and then kept uniform for 20 min again;

[0038] 3-2: vacuumizing for 60 s to below 0 mbar, and then starting to pass in acetylene for carburizing treatment.

[0039] 3-3, acetylene is passed in in two stages for carburizing treatment. The first-stage acetylene is passed in for 15 s at a flow rate of 2000 NL / H; the second-stage acetylene is passed in for 160 s at a flow rate of 450 NL / H;

[0040] 3-4, after vacuumizing, nitrogen is passed in for diffusion treatment, and the nitrogen pressure is 4000 mbar.

[0041] 3-5, steps 3-2 to 3-4 are repeated 7 times. In step 3-3, the acetylene passing-in time t2 is gradually decreased by 10 s to 20 s each time, and the range is 60-180 s, with 175 s for the first time, 160 s for the second time, 140 s for the third time, 120 s for the fourth time, 100 s for the fifth time, 80 s for the sixth time, and 60 s for the seventh time; in step 3-4, the nitrogen diffusion treatment time is gradually increased by 100 s to 300 s each time, and the time range is 300 s-1800 s, and in this embodiment, the first time is 300 s, the second time is 450 s, the third time is 700 s, the fourth time is 850 s, the fifth time is 1050 s, the sixth time is 1350 s, and the seventh time is 1650 s.

[0042] 3-6 final diffusion Figure 1 (mid-pressure diffusion): nitrogen is passed in at a pressure of 1200 mbar for 1800 s.

[0043] 3-7, after the workpiece is cooled to room temperature and taken out of the furnace, nitrogen is passed in for air cooling at a pressure of 1800 mbar.

[0044] In step 3-1, acetylene is passed in in two stages for carburizing treatment. The first-stage acetylene is passed in at a large flow rate for a short time, so that the furnace is filled with acetylene instantaneously, and the workpiece surface obtains sufficient carbon source; the second-stage acetylene is passed in at a small flow rate for a long time, so that carbon atoms slowly diffuse, and the small-flow atmosphere slowly replenishes the carbon source, preventing the accumulation of carbon ash.

[0045] In step 3-4, nitrogen is passed in because after acetylene is passed in for carburizing treatment, the workpiece surface has absorbed enough carbon atoms, and nitrogen needs to be passed in to press the carbon atoms into the workpiece interior during diffusion, which is beneficial to the diffusion of carbon atoms.

[0046] 4. The workpiece undergoes two high-temperature tempering processes. The high-temperature tempering temperature is 680℃ and the holding time is 3.5h. After the first high-temperature tempering is cooled to room temperature, the temperature is raised to the high-temperature tempering temperature for the second high-temperature tempering treatment. The hardness after tempering is ≤38HRC.

[0047] 5. The workpiece is quenched as a whole at a temperature of 820℃. The quenching equipment is a vacuum furnace. The temperature is raised to the quenching temperature at a rate of 15℃ / min and held for 90 minutes. Oil quenching is used at an oil temperature of 60℃. After quenching, the workpiece is cooled for 1 hour.

[0048] 6. The workpiece undergoes cold treatment at -80℃ for 150 minutes; after air cooling to room temperature, a third tempering treatment is performed 2 hours later.

[0049] 7. The workpiece undergoes a third tempering treatment at a tempering temperature of 160℃ and a tempering holding time of 210 minutes, followed by air cooling to room temperature.

[0050] Physical and chemical testing: Fine carbides can be uniformly and diffusely distributed on the surface of the workpiece. Figure 2 The white spots are evenly distributed (rated 1-3), such as... Figure 2 As shown, the microstructure is stable and qualified after heat treatment, with an effective hardened layer depth of 0.45 mm (measured to 550 HV), a surface hardness of 83 HRA (62 HRC), and a core hardness of 43 HRC. It has excellent wear resistance and fatigue resistance. All indicators of the 16CrNi4MoA steel workpiece processed by the method of this invention meet the technical requirements.

[0051] Comparative Example 1

[0052] The carburizing method for the 16CrNi4Mo steel in this comparative example is largely the same as that in Example 1, mainly differing in the carburizing and quenching steps. The specific implementation steps include:

[0053] 1. After the workpiece is copper-plated as a whole, the carburized area is machined in the machining process: the copper layer thickness is required to be 50μm, and there should be no defects such as blistering, peeling, or pinholes.

[0054] 2. Blow corundum sand into the carburizing area of ​​the workpiece. The sand particle size is 100 mesh, and the pressure is 0.20 MPa.

[0055] 3. Carburizing of the workpiece: The carburizing equipment is a multi-purpose furnace. The carburizing temperature is 880℃, the carbon potential is (1.15±0.05)%, and the time is (60~90)min. Carburizing is carried out according to the conventional traditional carburizing process: the room temperature is raised to 800℃, the carbon potential is set to 0.85%, and the temperature is uniform for 30min. Then the temperature is raised to 880℃, the carbon potential is set to 1.15%, and the temperature is uniform for 30min. After the temperature and carbon potential meet the requirements, acetylene is introduced to start carburizing. The carburizing time is 180min.

[0056] 4. The workpiece undergoes two high-temperature tempering processes. The high-temperature tempering temperature is 680℃ and the holding time is 3.5h. After the first high-temperature tempering is cooled to room temperature, the temperature is raised to the high-temperature tempering temperature for the second high-temperature tempering treatment. The hardness after tempering is ≤38HRC.

[0057] 5. Overall quenching of the workpiece: The quenching temperature is 820℃. First, the workpiece is heated from room temperature to 580℃ and held for 30 minutes, then transferred to 820℃ and held for 45 minutes, and then transferred to 580℃ and held for 30 minutes. Oil quenching is used, and the oil temperature is 40℃.

[0058] 6. The workpiece undergoes cold treatment at -80℃ for 150 minutes; after air cooling to room temperature, a third tempering treatment is performed 2 hours later.

[0059] 7. The workpiece undergoes a third tempering treatment at a tempering temperature of 160℃ and a tempering holding time of 210 minutes, followed by air cooling to room temperature.

[0060] Physicochemical testing showed that the carbides were coarse and distributed in a blocky or semi-continuous network pattern, with a rating of 5-6. Figure 3 As shown, the effective hardened layer depth is 0.37 mm (measured to 550 HV), the surface hardness is 79 HRA, and the core hardness is 41.5 HRC, indicating that the workpiece prepared in this comparative example is unqualified.

[0061] Comparative Example 2

[0062] The carburizing method for the 16CrNi4Mo steel in this comparative example is largely the same as that in Example 1, except that the carburizing treatment is different. The specific implementation steps include:

[0063] 1. After the workpiece is copper-plated as a whole, the carburized area is machined in the machining process: the copper layer thickness is required to be 50μm, and there should be no defects such as blistering, peeling, or pinholes.

[0064] 2. Blow corundum sand into the carburizing area of ​​the workpiece. The sand particle size is 100 mesh, and the pressure is 0.20 MPa.

[0065] 3. Carburizing of the workpiece: The carburizing equipment is a low-pressure vacuum carburizing furnace with a carburizing temperature of 880℃; acetylene pressure of 2000mbar and partial pressure of 3000mbar.

[0066] 3-1. The workpiece is evacuated to below 100 mbar in the furnace at room temperature, then nitrogen is introduced and the temperature is raised from room temperature to 800℃ at a rate of 10℃ / min. The temperature is uniformized for 20 min, then the temperature is raised to the carburizing temperature of 880℃, and then uniformized for another 20 min.

[0067] 3-2 Evacuate for 60 seconds until the pressure drops below 0 mbar, then begin acetylene carburizing.

[0068] 3-3, one-stage carburizing treatment by passing in acetylene. Acetylene flow rate: 600 NL / H, time: 150 s to 300 s.

[0069] 3-4, nitrogen diffusion treatment after vacuumizing. Nitrogen pressure: 1200 mbar.

[0070] 3-5, steps 3-2 to 3-4 are repeated 7 times. The carburizing time of each stage of acetylene in step 3-3 is gradually decreased by 10 s to 80 s each time, and the carburizing time is in the range of 150 s to 300 s, such as 300 s for the first time, 240 s for the second time, 230 s for the third time, 210 s for the fourth time, 190 s for the fifth time, 170 s for the sixth time, and 150 s for the seventh time. The nitrogen passing-in time in step 3-4 is gradually increased by 200 s to 300 s each time, and the nitrogen passing-in time is in the range of 300 s to 1800 s. In this example, the nitrogen passing-in time is 500 s for the first time, 800 s for the second time, 1000 s for the third time, 1200 s for the fourth time, 1400 s for the fifth time, 1600 s for the sixth time, and 1800 s for the seventh time.

[0071] 3-6 final diffusion Figure 1 (mid-pressure diffusion): nitrogen is passed in at a pressure of 1200 mbar for a time of 1800 s.

[0072] 3-7, after the workpiece is cooled to room temperature, nitrogen is passed in for air cooling at a pressure of 1800 mbar.

[0073] 4, the workpiece is subjected to two high-temperature tempering treatments. The high-temperature tempering temperature is 680℃, and the holding time is 3.5 h. After the first high-temperature tempering treatment is cooled to room temperature, the workpiece is heated to the high-temperature tempering temperature again for the second high-temperature tempering treatment. After tempering, the hardness is ≤38HRC.

[0074] 5, the workpiece is integrally quenched. The quenching temperature is 820℃, the quenching equipment is a vacuum furnace, the holding time is 90 min, and oil quenching is adopted at an oil temperature of 60℃. The workpiece is cold treated 1 h after quenching.

[0075] 6, the workpiece is cold treated at a cold treatment temperature of -80℃ for a cold treatment holding time of 150 min. The workpiece is air cooled to room temperature and then subjected to a third tempering treatment 2 h later.

[0076] 7, the workpiece is subjected to a third tempering treatment at a tempering temperature of 160℃ for a tempering holding time of 210 min, and is air cooled to room temperature.

[0077] Physical and chemical testing shows that the surface carbides are in a network shape, the microstructure is unqualified, the effective hardening layer depth is 0.54 mm (measured to 550 HV), the surface hardness is 56HRC, and the core hardness is 44HRC, indicating that the workpiece prepared in this example is unqualified.

[0078] Although the present application has been described in connection with the preferred embodiment thereof with reference to the drawings, it is not to be limited to the details described therein but can be modified in various ways which are within the scope of the present application and can be apparent to those skilled in the art without departing from the spirit of the application.

Claims

1. A low-pressure vacuum carburizing method for 16CrNi4Mo steel, characterized in that: Includes the following steps: S1. Apply a protective layer to the non-carburized areas of the 16CrNi4Mo steel workpiece, and clean the areas to be carburized by sandblasting. S2. Place the 16CrNi4Mo steel workpiece in a carburizing furnace for low-pressure carburizing treatment; the low-pressure carburizing treatment specifically includes the following steps: S201. Vacuum pump and heat to the carburizing temperature; S202. The first carburizing treatment is performed by introducing a carburizing atmosphere at a flow rate of v1 and holding it for a time of t1. Then, the carburizing treatment is performed by introducing a carburizing atmosphere at a flow rate of v2 and holding it for a time of t2, satisfying v1 > v2 and t1 < t2. S203. After vacuuming, nitrogen gas is introduced for diffusion treatment. S204. Repeat steps S201 to S203; S205, nitrogen gas is introduced for final diffusion treatment, followed by cooling; S3. The 16CrNi4Mo steel workpiece is subjected to the first tempering treatment, the second tempering treatment, the quenching treatment, the cold treatment, and the third tempering treatment in sequence. In step S202, v1 is 1000-3000 NL / H, t1 is 10-20s, v2 is 300-800 NL / H, and t2 is 30-300s; In step S2, the carburizing temperature is 880–950°C, the acetylene pressure during carburizing is 1000–3000 mbar, and the partial pressure during nitrogen diffusion is 3000–6000 mbar.

2. The low-pressure vacuum carburizing method according to claim 1, characterized in that: In step S205, the nitrogen pressure is 1200-1800 mbar.

3. The low-pressure vacuum carburizing method according to claim 1 or 2, characterized in that: In step S1 and step S205, the number of repetitions is 7 to 20 times.

4. The low-pressure vacuum carburizing method according to claim 1 or 2, characterized in that: In step S3, the temperature of the first tempering treatment and the second tempering treatment is 660-690℃, and the holding time is 180-210min.

5. The low-pressure vacuum carburizing method according to claim 1 or 2, characterized in that: In step S3, the quenching temperature is 800–840℃, the holding time is 70–210 min, and the oil temperature is 40–80℃.

6. The low-pressure vacuum carburizing method according to claim 1 or 2, characterized in that: In step S3, the temperature of the cold treatment is -70 to -90 ℃, and the holding time is 120 to 180 min.

7. The low-pressure vacuum carburizing method according to claim 1 or 2, characterized in that: In step S3, the temperature of the third tempering treatment is 150–200 °C, and the holding time is 180–240 min.

8. The low-pressure vacuum carburizing method according to claim 1 or 2, characterized in that: In step S1, the thickness of the protective layer is 40-60 μm; the sand particles used in the sandblasting process are corundum sand with a mesh size ≥100 and a pressure ≤0.2MPa.

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