Heat treatment method for improving structure property of G20CrNi2MoA material

Abstract

The invention discloses a heat treatment method for improving the structure property of a G20CrNi2MoA material, which comprises the following steps: step 1, preparing a bearing turning part made of the G20CrNi2MoA material in an annealed state; secondly, carburizing, nitriding and quenching treatment are conducted, the bearing vehicle part is carburized in a carburizing furnace, the temperature of the bearing vehicle part is increased to 800-900 DEG C along with the furnace at the heating rate of 10-30 DEG C per minute, heat preservation is conducted for 20-60 min, then the temperature of the bearing vehicle part is increased to 940-980 DEG C at the heating rate of 10-30 DEG C per minute, pulse carburizing is started, the carburizing atmosphere is acetylene, the gas flow is 1200-2500 L / h, the pressure is 2-7 mbar, the pulse frequency is 25-35, the total carburizing time is 15-25 h, and the carburizing time is 15-25 h; after carburization is completed, the temperature is reduced to 820-860 DEG C at the cooling rate of 10-30 DEG C per minute, heat preservation is conducted for 30-60 min, ammonia gas is introduced, the flow of the ammonia gas is 0.2-0.5 m < 3 > / h, the carbon potential is set to be 0.80-0.95%, finally, nitrogen is used for cooling, the inflation pressure is 2-6 bar, the inflation time is 1000-1500 s, air cooling is conducted to the room temperature after gas quenching, and then discharging is conducted. Thirdly, high-temperature annealing treatment is conducted; and 4, quenching and low-temperature tempering treatment. The wear resistance, the grain size and the tempering resistance of the material can be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment of metal materials, in particular to a heat treatment method for improving the structural properties of a G20CrNi2MoA material. Background Art

[0002] G20CrNi2MoA material plays a key role in the preparation of railway axle box bearings. The railway operating environment is complex, and axle box bearings need to withstand frequent and strong impact loads, which places strict requirements on material performance. After the carburizing heat treatment process, its surface microstructure shows the characteristics of martensite plus a small amount of carbide. The depth of the carburizing layer is precisely controlled between 1.5 and 2.0 mm. This depth range can effectively improve the wear resistance and contact fatigue strength of the bearing surface, and the corresponding hardness reaches 59 to 62HRC, giving the surface excellent wear resistance. The core structure is low-carbon martensite plus a small amount of ferrite, with a hardness of 40 to 48HRC, which ensures the overall toughness and strength of the bearing. However, for axle box bearings operating at higher speeds, the problem of roller slippage is more prominent. This will cause a sharp temperature rise in local areas, thereby accelerating wear. To address this fact, it is necessary to further improve the wear resistance of the material surface, refine the grain size to enhance the density and uniformity of the material, and at the same time strengthen the anti-tempering performance to ensure that stable hardness and mechanical properties can be maintained at high temperatures, thereby significantly extending the service life of the axle box bearing and ensuring the safety and stability of railway operations. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention provides a heat treatment method for improving the microstructure and properties of a G20CrNi2MoA material, which can improve the wear resistance, grain size and tempering resistance of the G20CrNi2MoA material after heat treatment.

[0004] To achieve the above object, the present invention provides a heat treatment method for improving the microstructure and properties of G20CrNi2MoA material, comprising the following steps: Step 1, preparing a bearing machined part made of annealed G20CrNi2MoA material; Step 2: Carburizing, nitriding and quenching treatment. Carburize the bearing parts in a low-pressure vacuum carburizing furnace. First, heat the furnace to 800-900°C at a heating rate of 10-30°C per minute, keep warm for 20-60 minutes, and then heat to 940-980°C at a heating rate of 10-30°C per minute. Start pulse carburizing. The carburizing atmosphere is acetylene, the gas flow rate is 1200-2500L / h, the pressure is 2-7mbar, and the pulse The number of strokes is 25~35 times, and the total carburizing time is 15~25h. After carburizing, the temperature is cooled to 820~860℃ at a cooling rate of 10~30℃ per minute, and the temperature is kept for 30~60min. Ammonia is introduced, and the ammonia flow rate is 0.2~0.5m3 / h. The carbon potential is set at 0.80~0.95%. Finally, nitrogen is used for cooling, and the inflation pressure is 2~6bar. The inflation time is 1000~1500s. After gas quenching, it is air-cooled to room temperature and then taken out of the furnace; Step three, high temperature annealing treatment; Step 4: quenching and low temperature tempering treatment.

[0005] As a further configuration of the present invention, the high temperature annealing in step three is to heat the bearing parts to 600-700°C in the furnace, keep them warm for 3-6 hours, and then cool them with nitrogen. The inflation pressure is 2-5 bar, the inflation time is 1000-1400 seconds, and then air-cool them to room temperature before taking them out of the furnace.

[0006] As a further configuration of the present invention, the quenching in step four is to heat treat the bearing parts in a roller furnace, first heating the furnace to 800~820°C, setting the carbon potential to 0.75~0.85%, keeping warm for 100~150 minutes, and then quenching, the oil temperature is 50~100°C, the quenching time is 2~5 minutes, and the bearing parts are washed.

[0007] As a further configuration of the present invention, the low-temperature tempering in step 4 is to temper the cleaned bearing parts at 150-200° C. for 5-10 hours.

[0008] The beneficial effect of this setting is that the internal structure of the annealed material is relatively uniform, the early processing stress is eliminated, and it is conducive to the diffusion and reorganization of atoms in the subsequent process. This carburizing method promotes the formation of fine and dispersed carbides and nitrides on the surface, greatly refines the surface grains, and significantly improves the wear resistance. At the same time, the incorporation of carbon and nitrogen elements changes the martensitic structure and enhances the anti-tempering stability. High-temperature annealing treatment can eliminate the internal stress generated by quenching and further homogenize the internal structure of the material. The quenching and low-temperature tempering treatment in step four stabilizes the internal structure of the material, further improves the balance between hardness and toughness, consolidates the wear resistance, grain size and anti-tempering performance, and fully guarantees the performance of G20CrNi2MoA material in axle box bearing applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is the microstructure diagram under the existing process; Figure 2 It is the grain size diagram under the existing process; Figure 3 A microstructure diagram of an embodiment of the present invention; Figure 4 is a grain size diagram of an embodiment of the present invention; Figure 5 It is a hardness gradient data diagram of the embodiment of the present invention and the prior art; Figure 6 It is a hardness gradient data diagram of the embodiment of the present invention and the prior art after tempering at 250°C. DETAILED DESCRIPTION

[0010] The present invention provides an embodiment of a heat treatment method for improving the microstructure and properties of G20CrNi2MoA material, such as Figures 3 to 6 As shown, the material is a railway axle box bearing of G20CrNi2MoA that meets the Q / CR 592 standard, with a maximum outer diameter of φ250mm and a thickest section size of 18mm. The heat treatment steps are: Step 1: Prepare the bearing parts made of G13Cr4Mo4Ni4V material.

[0011] Step 2: Carburizing was performed in a low-pressure vacuum carburizing furnace for the first time. The temperature was first raised to 850°C at a heating rate of 20°C, and kept at this temperature for 50 minutes. The temperature was then raised to 960°C at a heating rate of 20°C, and pulse carburizing was started. The carburizing atmosphere was acetylene, the gas flow rate was 2000L / h, the pressure was 5mbar, the pulse number was 30, and the total carburizing time was 20h. Then cool to 840℃, keep warm for 45min, and introduce ammonia at a flow rate of 0.4 m 3 / h, carbon potential 0.80%.

[0012] Then it is cooled with nitrogen, the inflation pressure is 3 bar, the inflation time is 1200s, and after gas quenching, it is air-cooled to room temperature and then taken out of the furnace.

[0013] Step three, after carburizing, high temperature annealing treatment is carried out in a low-pressure vacuum carburizing furnace, the temperature is raised to 650°C with the furnace, kept warm for 5 hours, and then cooled with nitrogen. The inflation pressure is 3 bar and the inflation time is 1200 seconds. After gas quenching, it is air-cooled to room temperature and taken out of the furnace.

[0014] Step 4: After high temperature annealing, quenching treatment is carried out in a roller furnace. First, heat it to 810°C with the furnace, carbon potential 0.80%, keep it warm for 120 minutes, then quench it in 70°C oil for 3 minutes, after cleaning, temper it at 180°C, and keep it warm for 7 hours.

[0015] The final inspection uses the existing carburizing heat treatment process. The carburizing layer depth of the part is 1.8mm, the surface hardness is 60.5~61.2HRC, the core hardness is 43.5~44.5HRC, and the hardness gradient data is as follows Figure 5 As shown, the surface microstructure is Figure 1 and Figure 2 As shown, the surface is composed of needle-shaped martensite and residual martensite, and there are coarse martensite and mixed crystals in some parts. The carburizing heat treatment process of this technical solution has a carburizing layer depth of 1.8mm, a surface hardness of 62.1~62.8HRC, a core hardness of 44.5~45.4HRC, and a hardness gradient data as shown in Figure 5 As shown in the figure, the surface hardness is about 2HRC higher than that before the improvement, mainly because of the increase of nitrogen-containing carbides. The surface microstructure is as follows Figure 3 and Figure 4 As shown in the figure, a large number of fine and uniform carbides are evenly distributed, which improves the surface hardness and wear resistance. The hardness gradient after tempering at 250℃ is as follows Figure 6 As shown, the hardness remains high.

[0016] The above example is only one of the preferred specific examples of the present invention. The common changes and substitutions made by those skilled in the art within the scope of the technical solution of the present invention are all included in the protection scope of the present invention.

Claims

1. A heat treatment method for improving the microstructure and properties of G20CrNi2MoA material, characterized in that: The following steps are included: Step 1, preparing a bearing machined part made of annealed G20CrNi2MoA material; Step 2: Carburizing, nitriding and quenching treatment. Carburize the bearing parts in a low-pressure vacuum carburizing furnace. First, heat the furnace to 800-900°C at a heating rate of 10-30°C per minute, keep warm for 20-60 minutes, and then heat to 940-980°C at a heating rate of 10-30°C per minute. Start pulse carburizing. The carburizing atmosphere is acetylene, the gas flow rate is 1200-2500L / h, the pressure is 2-7mbar, and the pulse The number of strokes is 25~35 times, and the total carburizing time is 15~25h. After carburizing, the temperature is cooled to 820~860℃ at a cooling rate of 10~30℃ per minute, and the temperature is kept for 30~60min. Ammonia is introduced, and the ammonia flow rate is 0.2~0.5m3 / h. The carbon potential is set at 0.80~0.95%. Finally, nitrogen is used for cooling, and the inflation pressure is 2~6bar. The inflation time is 1000~1500s. After gas quenching, it is air-cooled to room temperature and then taken out of the furnace; Step three, high temperature annealing treatment; Step 4: quenching and low temperature tempering treatment.

2. The heat treatment method for improving the microstructure and properties of G20CrNi2MoA material according to claim 1, characterized in that: The high temperature annealing in step three is to heat the bearing parts to 600-700°C in the furnace, keep them warm for 3-6 hours, and then cool them with nitrogen. The inflation pressure is 2-5 bar and the inflation time is 1000-1400 seconds. After air cooling, the parts are air cooled to room temperature and then taken out of the furnace.

3. The heat treatment method for improving the microstructure and properties of G20CrNi2MoA material according to claim 1, characterized in that: The quenching in step 4 is to heat treat the bearing parts in a roller furnace, first raise the temperature to 800-820°C with the furnace, set the carbon potential to 0.75-0.85%, keep warm for 100-150 minutes, then quench, the oil temperature is 50-100°C, the quenching time is 2-5 minutes, and the bearing parts are washed.

4. The heat treatment method for improving the microstructure and properties of G20CrNi2MoA material according to claim 3, characterized in that: The low temperature tempering in step 4 is to temper the cleaned bearing parts at 150-200° C. for 5-10 hours.

Images