After the sintering of the powder metallurgy iron-based structural parts, in order to prevent rust and facilitate subsequent processing, such as finishing, grinding, etc., the oil immersion treatment is carried out. In order to prevent carbon deposits on the surface of the product after heat treatment, affecting the appearance and surface quality, the product is heated and degreased before heat treatment: the product is heated at 300-400 ° C for 30-60 min.
The bicycle electric wheel set gear was originally processed by 40Cr forging machine. The hardness indexes of the main and driven gears were HRC38~42 and HRC33~37, respectively. Due to the good wear resistance and fatigue resistance of Fe-0.5Mo-X-0.6C sintered steel products, after strict examination by pilot production, installation test and road test, the user adjusts the technical requirements of product hardness to HRC33~ 38 and HRC 27 ~ 32.
Based on the above conditions, according to the on-site production process conditions, the heat treatment process is finally determined to be a conventional bright heat treatment for quenching and tempering of the mesh belt furnace. The heating temperature is 850-870 ° C, the heating time is 30-60 min, the quenching medium is the oil with better cooling characteristics; after quenching, it is tempered by oil bath of 160-200 ° C to reduce quenching stress, reduce brittleness and maintain high strength.
Heat treatment results and analysis of metallurgical structure of quenched metallographic bicycle electric wheel hub gear quenching parts: the surface is tempered martensite, and the inner part is troostite, showing the surface as compressive stress and the core as a good state of tensile stress. Its tensile strength is higher than that of the sintered state, reaching 720-780MPa.
Because the mesh belt type continuous bright heat treatment furnace is used, the heating and cooling of the workpiece are relatively uniform. At the same time, by adding appropriate amount of alloying elements such as Mn and Mo, the critical temperature of quenching is lowered, the hardenability is improved, and the supercooled austenite is increased. Stability, good heat treatment effect.
The dimensional change of the 59Z gear before and after heat treatment is a key problem as far as the cracking of the gear is concerned, which directly affects the cost of production. Because powder metallurgy gears have pores, low thermal conductivity and uneven heating, stress concentration is easy to form at sharp corners, cracks are easy to form and expand. At the same time, due to the speciality of the gear structure, the thickness of the gears is uneven. The density distribution is uneven or “filling up†phenomenon, and when the quenching is heated, the thermal conductivity of the gear is different, which causes a significant difference between thermal stress and tissue stress, and is prone to cracking. In the actual production, after repeated exploration, the following measures can be taken to effectively suppress the generation of cracks, so that the product qualification rate is above 96%: the mixture is sufficient, and the powder is added during the pressing to make the density distribution uniform; heat treatment Reasonably control the heating temperature and time, avoid heating too fast, reduce thermal shock; use mild quenching medium with better cooling characteristics.
Conclusion 3 shows the case of using Fe-0.5Mo-X-0.6C sintered alloy steel gears in bulk installation. It can be seen that: 1. After the quenching and tempering of the powder metallurgy gears for electric bicycle hubs by quenching and tempering, the comprehensive mechanical properties are effectively improved, and the deformation and crack problems are solved, which can completely replace the 40Cr forgings, thereby reducing the cost. Reduce noise and extend service life.
2. After the quenching and tempering of the powder metallurgy gear of the electric bicycle hub by the bright heat treatment process, the metallographic structure of the gear is: the surface is tempered martensite, the core is troostite; the hardness: the driving gear is HRC33~38 The driven gear is HRC28～33, and the tensile strength reaches 750～800Mpa.
The bicycle electric wheel set gear was originally processed by 40Cr forging machine. The hardness indexes of the main and driven gears were HRC38~42 and HRC33~37, respectively. Due to the good wear resistance and fatigue resistance of Fe-0.5Mo-X-0.6C sintered steel products, after strict examination by pilot production, installation test and road test, the user adjusts the technical requirements of product hardness to HRC33~ 38 and HRC 27 ~ 32.
Based on the above conditions, according to the on-site production process conditions, the heat treatment process is finally determined to be a conventional bright heat treatment for quenching and tempering of the mesh belt furnace. The heating temperature is 850-870 ° C, the heating time is 30-60 min, the quenching medium is the oil with better cooling characteristics; after quenching, it is tempered by oil bath of 160-200 ° C to reduce quenching stress, reduce brittleness and maintain high strength.
Heat treatment results and analysis of metallurgical structure of quenched metallographic bicycle electric wheel hub gear quenching parts: the surface is tempered martensite, and the inner part is troostite, showing the surface as compressive stress and the core as a good state of tensile stress. Its tensile strength is higher than that of the sintered state, reaching 720-780MPa.
Because the mesh belt type continuous bright heat treatment furnace is used, the heating and cooling of the workpiece are relatively uniform. At the same time, by adding appropriate amount of alloying elements such as Mn and Mo, the critical temperature of quenching is lowered, the hardenability is improved, and the supercooled austenite is increased. Stability, good heat treatment effect.
The dimensional change of the 59Z gear before and after heat treatment is a key problem as far as the cracking of the gear is concerned, which directly affects the cost of production. Because powder metallurgy gears have pores, low thermal conductivity and uneven heating, stress concentration is easy to form at sharp corners, cracks are easy to form and expand. At the same time, due to the speciality of the gear structure, the thickness of the gears is uneven. The density distribution is uneven or “filling up†phenomenon, and when the quenching is heated, the thermal conductivity of the gear is different, which causes a significant difference between thermal stress and tissue stress, and is prone to cracking. In the actual production, after repeated exploration, the following measures can be taken to effectively suppress the generation of cracks, so that the product qualification rate is above 96%: the mixture is sufficient, and the powder is added during the pressing to make the density distribution uniform; heat treatment Reasonably control the heating temperature and time, avoid heating too fast, reduce thermal shock; use mild quenching medium with better cooling characteristics.
Conclusion 3 shows the case of using Fe-0.5Mo-X-0.6C sintered alloy steel gears in bulk installation. It can be seen that: 1. After the quenching and tempering of the powder metallurgy gears for electric bicycle hubs by quenching and tempering, the comprehensive mechanical properties are effectively improved, and the deformation and crack problems are solved, which can completely replace the 40Cr forgings, thereby reducing the cost. Reduce noise and extend service life.
2. After the quenching and tempering of the powder metallurgy gear of the electric bicycle hub by the bright heat treatment process, the metallographic structure of the gear is: the surface is tempered martensite, the core is troostite; the hardness: the driving gear is HRC33~38 The driven gear is HRC28～33, and the tensile strength reaches 750～800Mpa.
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