Metal heat treatment nitriding process

Abstract

The invention discloses a metal heat treatment nitriding process, and belongs to the technical field of metal heat treatment. The invention relates to a metal heat treatment nitriding process, which comprises the following steps: carrying out nitriding pretreatment on metal, namely cleaning the surface, carrying out preheating treatment, preparing a required salt bath, putting potassium chloride, urea, ammonium chloride, sodium carbonate and lithium carbonate into a stainless steel basin, uniformly mixing, heating to enable the mixture to be in a molten state, forming a salt bath, putting the pretreated metal into the salt bath, and carrying out a nitriding process. According to the method, through pretreatment, various impurities on the surface of the metal are effectively removed, the contact area of the metal and the nitrogen-containing medium is increased, the permeation efficiency and uniformity of the nitrogen-containing medium are improved, and the prepared salt bath can provide the stable and sufficient nitrogen-containing medium, so that the metal is uniformly heated in the salt bath, and meanwhile, the nitrogen-containing medium is uniformly heated. And a uniform, compact and high-hardness nitriding layer can be formed on the surface, so that the hardness, the wear resistance and the service life of the metal are effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal heat treatment, and more specifically, to a nitriding process for metal heat treatment. Background Art

[0002] Bearings are important basic components in various locomotives and various machinery, and are widely used in industries such as aerospace, military, machinery, and automobile manufacturing. Since the usage environment of bearings is generally very harsh, the performance requirements for bearing steel among special steels are extremely strict. Currently, the world-recognized level of bearing steel is an important symbol of a country's metallurgical level. With the development of various high and new technologies at home and abroad, the current usage status and scenarios of bearings are becoming increasingly harsh. Therefore, there are also increasingly high requirements for all aspects of the performance of the materials used to manufacture bearings. Considering resource conservation and the characteristic requirements for the bearing contact surface, further improving the surface performance of bearing steel is one of the goals for the development of high-performance bearing steel in the future. Composite heat treatment can ensure the good toughness of the core layer of bearing steel to the greatest extent, and at the same time, by performing case treatment, the wear resistance and corrosion resistance of bearing steel materials can be greatly improved, thereby greatly increasing the service life of bearings and further improving the comprehensive performance of the main equipment body. Summary of the Invention

[0003] A nitriding process for metal heat treatment, characterized in that: a nitriding process for metal heat treatment, the process includes nitriding pretreatment of the metal, including surface cleaning and preheating treatment, then preparing the required salt bath, putting potassium chloride, urea, ammonium chloride, sodium carbonate, and lithium carbonate into a stainless steel basin and mixing them evenly, heating to make it in a molten state to form a salt bath, and placing the pretreated metal therein for the nitriding process.

[0004] Preferably, the metal is low-carbon CrNiMoV series bearing steel.

[0005] Preferably, the surface cleaning is to ultrasonically clean with an alkaline solution for 10 - 15 minutes, the pH of the alkaline solution is 9 - 11, to remove grease residues, and then perform sandblasting treatment with 120 - 180 mesh quartz sand to make the surface roughness Ra ≤ 0.8μm.

[0006] Preferably, the preheating treatment is to place the metal in a box-type resistance furnace, first hold it at 900 - 1300°C for 30 - 60 minutes, perform oil quenching, then perform air cooling for 1 - 3 hours at -60 - 100°C for the first time, then hold it at 400 - 600°C for 1 - 3 hours, perform oil quenching, and then perform air cooling for 1 - 3 hours at -60 - 100°C for the first time.

[0007] Preferably, the composition of the salt bath is 30 - 60 parts of potassium chloride, 22 - 35 parts of urea, 10 - 18 parts of ammonium chloride, 15 - 25 parts of sodium carbonate, and 15 - 25 parts of lithium carbonate.

[0008] Preferably, the condition for heating the salt bath to the molten state is to heat it at 500 - 660 °C for 20 - 40 minutes.

[0009] Preferably, the nitriding process is that the metal after pretreatment is kept in the salt bath for 1 - 3 h and then directly quenched in oil for 2 - 4 h, and then air - cooled to room temperature.

[0010] Compared with the prior art, the advantages of the present invention are as follows: (1) In the present invention, ultrasonic cleaning of the metal with an alkaline solution having a pH of 9 - 11 can remove grease, dust and other organic pollutants, making the metal surface cleaner. During the nitriding process, gas can penetrate uniformly and effectively, so that the reaction between the metal and the nitrogen - containing medium is more sufficient, improving the nitriding effect, making the nitriding layer more uniform, reducing surface defects. Moreover, the alkaline cleaning solution is more environmentally friendly, and the waste liquid after treatment is easy to neutralize or further process, helping to reduce the environmental burden and meeting the requirements of green manufacturing.

[0011] (2) In the present invention, sandblasting the metal with 120 - 180 - mesh quartz sand to make the surface roughness Ra ≤ 0.8 μm can remove oxides, rust layers and other impurities on the metal surface, improve the surface roughness of the metal, form a tiny surface uneven structure, increase the contact area with the nitrogen - containing medium, help nitrogen atoms to penetrate and diffuse to the metal surface, improve the uniformity and depth of nitriding, thereby improving the hardness, wear resistance and corrosion resistance of the metal, and significantly increasing the service life of the parts.

[0012] (3) In the present invention, quenching the metal first and then tempering it improves the hardness, wear resistance and toughness of the metal. At the same time, by eliminating internal stress, stabilizing the structure, controlling deformation and cracks, it promotes the homogenization of the internal molecular structure of the metal. Oil quenching can quickly cool the metal surface and interior to form a fine martensite structure, and air cooling can further transform the martensite grains, further increasing the surface hardness and wear resistance of the metal, while improving the uniformity and depth of the penetration of the nitrogen - containing medium, ensuring the high performance and reliability of the parts, and significantly increasing the service life of the parts.

[0013] (4) In the present invention, the components of the salt bath are 30-60 parts of potassium chloride, 22-35 parts of urea, 10-18 parts of ammonium chloride, 15-25 parts of sodium carbonate, and 15-25 parts of lithium carbonate. Urea and ammonium chloride form a multi-nitrogen source, which will decompose to generate ammonia and other nitrogen-containing active substances at high temperatures, providing a sufficient and stable nitrogen source for nitriding, facilitating the formation of a uniform and dense nitrided layer. Potassium chloride and other salts have excellent thermal conductivity, enabling precise temperature control, so that the metal is heated evenly in the salt bath, ensuring uniform nitriding. At the same time, sodium carbonate and lithium carbonate can adjust the alkalinity of the system, assist in controlling the activity of nitrogen, further improve the nitriding effect, enabling the formation of a uniform, dense and high-hardness nitrided layer on the metal surface, thereby improving the wear resistance, corrosion resistance and service life of the metal. Moreover, the raw material prices are relatively low and easy to configure. While having high process effects, it can control costs and reduce energy consumption.

[0014] (5) In the present invention, heating at 500-660 °C for 20-40 minutes causes the salt bath to melt sufficiently. Placing the pre-treated metal therein can completely wrap the metal with the salt bath, enabling it to be heated completely and evenly, and allowing the metal surface to come into full contact with the nitrogen-containing medium in the salt bath. With sufficient nitriding time, a uniform and dense nitrided layer can be formed on the metal surface, improving the wear resistance, corrosion resistance and service life of the metal. After the salt bath, direct quenching and oil cooling can rapidly reduce the workpiece temperature, facilitating the formation of a refined martensite structure, increasing the hardness and wear resistance of the metal surface, while reducing thermal stress and deformation risk. Subsequent air cooling can make the internal temperature of the metal tend to balance, further relieve residual stress, ensure the stability of the metal, and improve the overall mechanical properties. Specific embodiments

[0015] Example 1: A metal heat treatment nitriding process. Place low-carbon CrNiMoV series bearing steel metal in an alkaline solution with a pH of 9 for ultrasonic cleaning for 10 minutes, then perform sandblasting treatment with 120-mesh quartz sand, and then place it in a box-type resistance furnace. First, hold at 900 °C for 30 minutes, perform oil quenching, then air cool once at -60 °C for 1 h, then hold at 400 °C for 1 h, perform oil quenching, and then air cool once at -60 °C for 1 h. Then take 30 parts of potassium chloride, 12 parts of urea, 10 parts of ammonium chloride, 15 parts of sodium carbonate, and 15 parts of lithium carbonate, place them in a stainless steel basin and mix evenly. Heat the evenly mixed materials to a molten state at 500 °C for 20 minutes to form a salt bath. Place the pre-treated metal in the salt bath, hold for 1 h, then directly quench and oil cool for 2 h, and then air cool to room temperature.

[0016] Example 2: A metal heat treatment nitriding process. Place low-carbon CrNiMoV series bearing steel metal in an alkaline solution with a pH of 9.5 for ultrasonic cleaning for 11 minutes, then perform sandblasting treatment with 135-mesh quartz sand. Then place it in a box-type resistance furnace, first keep it at 1000 °C for 37 minutes, perform oil quenching, then perform one-time air cooling at -70 °C for 1.5 h, then keep it at 450 °C for 1.5 h, perform oil quenching, and then perform one-time air cooling at -70 °C for 1.5 h. Then take 37.50 parts of potassium chloride, 15 parts of urea, 12 parts of ammonium chloride, 17.5 parts of sodium carbonate, and 17.5 parts of lithium carbonate, place them in a stainless steel basin and mix evenly. Heat the evenly mixed materials to a molten state at 540 °C for 25 minutes to form a salt bath. Place the metal that has completed the pretreatment in the salt bath, keep it warm for 1.5 h, then directly quench and cool in oil for 2.5 h, and then air cool to room temperature.

[0017] Example 3: A metal heat treatment nitriding process. Place low-carbon CrNiMoV series bearing steel metal in an alkaline solution with a pH of 10 for ultrasonic cleaning for 12 minutes, then perform sandblasting treatment with 150-mesh quartz sand. Then place it in a box-type resistance furnace, first keep it at 1100 °C for 45 minutes, perform oil quenching, then perform one-time air cooling at -80 °C for 2 h, then keep it at 500 °C for 2 h, perform oil quenching, and then perform one-time air cooling at -80 °C for 2 h. Then take 45 parts of potassium chloride, 18 parts of urea, 14 parts of ammonium chloride, 20 parts of sodium carbonate, and 20 parts of lithium carbonate, place them in a stainless steel basin and mix evenly. Heat the evenly mixed materials to a molten state at 580 °C for 30 minutes to form a salt bath. Place the metal that has completed the pretreatment in the salt bath, keep it warm for 2 h, then directly quench and cool in oil for 3 h, and then air cool to room temperature.

[0018] Example 4: A metal heat treatment nitriding process. Place low-carbon CrNiMoV series bearing steel metal in an alkaline solution with a pH of 10.5 for ultrasonic cleaning for 14 minutes, then perform sandblasting treatment with 165-mesh quartz sand. Then place it in a box-type resistance furnace, first keep it at 1200 °C for 52 minutes, perform oil quenching, then perform one-time air cooling at -90 °C for 2.5 h, then keep it at 550 °C for 2.5 h, perform oil quenching, and then perform one-time air cooling at -90 °C for 2.5 h. Then take 52.5 parts of potassium chloride, 21 parts of urea, 16 parts of ammonium chloride, 22.5 parts of sodium carbonate, and 22.5 parts of lithium carbonate, place them in a stainless steel basin and mix evenly. Heat the evenly mixed materials to a molten state at 620 °C for 35 minutes to form a salt bath. Place the metal that has completed the pretreatment in the salt bath, keep it warm for 2.5 h, then directly quench and cool in oil for 3.5 h, and then air cool to room temperature.

[0019] Example 5: A metal heat treatment nitriding process involves placing low-carbon CrNiMoV series bearing steel metal in an alkaline solution with a pH of 11 for ultrasonic cleaning for 15 minutes, then performing sandblasting treatment with 180-mesh quartz sand. Subsequently, it is placed in a box-type resistance furnace, first held at 1300 °C for 60 minutes, oil quenched, and then air cooled once at -100 °C for 3 h. Then it is held at 600 °C for 3 h, oil quenched, and air cooled once at -100 °C for 3 h. Then, 60 parts of potassium chloride, 24 parts of urea, 18 parts of ammonium chloride, 25 parts of sodium carbonate, and 25 parts of lithium carbonate are placed in a stainless-steel basin and mixed evenly. The mixed materials are heated to a molten state at 660 °C for 40 minutes to form a salt bath. The metal that has completed the pretreatment is placed in the salt bath, held for 3 h, directly quenched in oil and cooled for 4 h, and then air cooled to room temperature.

[0020] Performance testing Metal hardness detection For the metals of Examples 1 - 5 that have completed the nitriding process, representative areas (including the complete nitrided layer and the substrate) are selected, and the workpieces are cut into small specimens with dimensions approximately 20 mm × 20 mm using a precision wire saw or a low-speed cutting machine. The cut specimens are successively ground with 180-mesh, 400-mesh, 800-mesh, and 1200-mesh sandpapers, and then finally polished using a metallographic polishing machine in combination with a polishing cloth and a special polishing agent (such as alumina suspension) until the specimen surface is smooth and free of scratches. Then, ultrasonic cleaning or chemical cleaning methods (such as alcohol cleaning) are used to remove surface oil stains and grinding residues to ensure a clean surface. The pretreated specimens are firmly installed on the test platform, test points are selected and marked, and a HR30N type Rockwell hardness tester is used to detect their hardness. Referring to the national standard GB / T 4340.1, the following table shows the test results: Metal nitrided layer thickness detection For the metals of Examples 1-5 that have completed the nitriding process, select representative areas (including the complete nitrided layer and the substrate), and use a precision wire saw or a low-speed cutting machine to cut the workpiece into small specimens with dimensions approximately 20 mm × 20 mm. Then, successively grind the cut specimens with 180-mesh, 400-mesh, 800-mesh, and 1200-mesh sandpapers, and finally polish them using a metallographic polishing machine in combination with a polishing cloth and a special polishing agent (such as alumina suspension) until the specimen surface is smooth and free of scratches. Select a suitable alkaline etching solution to etch the specimen for a short time to make the interface between the nitrided layer and the substrate clearer. Then, use ultrasonic cleaning or chemical cleaning methods (such as alcohol cleaning) to remove the surface oil and grinding residues to ensure the surface is clean. Select a metallographic microscope with a magnification between 100× and 500× for observation. Place the treated specimen on the microscope stage, adjust the focus and light source, find the interface between the nitrided layer and the substrate, and take a clear cross-sectional image of the nitrided layer under the microscope to ensure that the nitrided layer and the substrate can be clearly distinguished in the image. In the microscope or image analysis system, select appropriate measurement points along the direction perpendicular to the nitrided layer interface, and use the built-in scale of the microscope or calibrated image analysis software to measure the thickness of the nitrided layer. Referring to the national standard GB / T 13298, the following table shows the test results: Detection of Adhesion of Metal Nitrided Layer For the metals of Examples 1-5 that have completed the nitriding process, select representative areas (including the complete nitrided layer and the substrate), and use a precision wire saw or a low-speed cutting machine to cut the workpiece into small specimens with dimensions approximately 20 mm × 20 mm. Then, successively grind the cut specimens with 180-mesh, 400-mesh, 800-mesh, and 1200-mesh sandpapers, and finally polish them using a metallographic polishing machine in combination with a polishing cloth and a special polishing agent until the specimen surface is smooth and free of scratches. Then, use ultrasonic cleaning or chemical cleaning methods (such as alcohol cleaning) to remove the surface oil and grinding residues to ensure the surface is clean. Firmly mount the pretreated specimen on the test platform, select and mark the test points, and use an HCTC-10 type metal coating adhesion tester to detect the coating adhesion. Referring to the national standard GB / T 9286, the following table shows the test results: Detection of Metal Wear Resistance For the metals of Examples 1-5 that have completed the nitriding process, select representative areas (including the complete nitrided layer and the substrate), and use a precision wire saw or a low-speed cutting machine to cut the workpiece into disc specimens with a size of approximately 10 mm in diameter. Gradually grind the cut specimens with 180-mesh, 400-mesh, 800-mesh, and 1200-mesh sandpapers in sequence, and then use a metallographic polishing machine in combination with a polishing cloth and a special polishing agent (such as alumina suspension) for final polishing until the surface of the specimen is smooth and free of scratches. Then, use ultrasonic cleaning or chemical cleaning methods (such as alcohol cleaning) to remove surface oil stains and grinding residues to ensure a clean surface. Firmly mount the pretreated specimen on the test platform, select and mark the test points, and use a T-09 type wear testing machine to conduct wear resistance testing on it. The friction force is 5 N, the friction speed is 1 m / s, and the friction time is 1 h. Referring to the national standard GB / T 3744-2014, the following table shows the test results: Metal Corrosion Resistance Testing For the metals of Examples 1-5 that have completed the nitriding process, select representative areas (including the complete nitrided layer and the substrate), and use a precision wire saw or a low-speed cutting machine to cut the workpiece into small specimens with a size of approximately 20 mm × 20 mm. Gradually grind the cut specimens with 180-mesh, 400-mesh, 800-mesh, and 1200-mesh sandpapers in sequence, and then use a metallographic polishing machine in combination with a polishing cloth and a special polishing agent (such as alumina suspension) for final polishing until the surface of the specimen is smooth and free of scratches. Then, use ultrasonic cleaning or chemical cleaning methods (such as alcohol cleaning) to remove surface oil stains and grinding residues to ensure a clean surface. Firmly mount the pretreated specimen on the test platform, select and mark the test points, and use a Q-FOG type salt spray testing machine to conduct corrosion resistance testing on it. The salt solution concentration is 5% NaCl solution, the temperature is 35°C, the spray pressure is approximately 1.5 bar, and the corrosion duration is 72 h. Referring to the national standard GB / T 10125-2012, the following table shows the test results: Metal Surface Roughness Testing For the metals of Examples 1-5 that have completed the nitriding process, representative regions (including the complete nitrided layer and the substrate) are selected, and the workpieces are cut into disk specimens with a size of approximately 10 mm in diameter using a precision wire saw or a low-speed cutting machine. The cut specimens are successively ground with 180-mesh, 400-mesh, 800-mesh, and 1200-mesh sandpapers, and then finally polished using a metallographic polishing machine in combination with a polishing cloth and a special polishing agent (such as alumina suspension) until the surface of the specimen is smooth and free of scratches. Then, ultrasonic cleaning or chemical cleaning methods (such as alcohol cleaning) are used to remove surface oil stains and grinding residues to ensure a clean surface. The pretreated specimens are firmly mounted on the test platform, test points are selected and marked, and a Mitutoyo SJ-410 surface roughness meter is used to detect the surface roughness. The measurement path is 5 mm, and the measurement speed is 1 mm / s. Referring to the national standard GB / T 10064-2015, the following table shows the test results:

Claims

1. A metal heat treatment nitriding process, characterized in that: A metal heat treatment nitriding process, the process includes nitriding pretreatment of the metal, including surface cleaning, preheating treatment, and then configuring the required salt bath, potassium chloride, urea, ammonium chloride, sodium carbonate, and lithium carbonate are put into a stainless steel basin and mixed evenly, heated to make them in a molten state to form a salt bath, and the pretreated metal is placed in the salt bath to carry out the nitriding process.

2. The metal heat treatment nitriding process according to claim 1, characterized in that: The metal is low-carbon CrNiMoV bearing steel.

3. The metal heat treatment nitriding process according to claim 1, characterized in that: The surface cleaning is performed by ultrasonic cleaning with an alkaline solution for 10-15 minutes, wherein the pH of the alkaline solution is 9-11, to remove grease residues, and then sandblasting is performed with 120-180 mesh quartz sand to make the surface roughness Ra≤0.8 μm.

4. The metal heat treatment nitriding process according to claim 3, characterized in that: The preheat treatment is to place the metal in a box-type resistance furnace, first keep it at 900-1300°C for 30-60 minutes, oil quench it, air cool it at minus 60-100°C for 1-3 hours, then keep it at 400-600°C for 1-3 hours, and oil quench it, air cool it at minus 60-100°C for 1-3 hours.

5. The metal heat treatment nitriding process according to claim 1, characterized in that: The salt bath comprises 30-60 parts of potassium chloride, 12-24 parts of urea, 10-18 parts of ammonium chloride, 15-25 parts of sodium carbonate and 15-25 parts of lithium carbonate.

6. The metal heat treatment nitriding process according to claim 1, characterized in that: The condition for heating the salt bath to a molten state is heating at 500-660° C. for 20-40 minutes.

7. The metal heat treatment nitriding process according to claim 1, characterized in that: The nitriding process is to heat the pretreated metal in a salt bath for 1-3 hours, then directly quench and oil cool for 2-4 hours, and then air cool to room temperature.

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