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What are the different heat treatment methods for gear forgings?
What are the different heat treatment methods for gear forgings?

10

2022

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10

What are the different heat treatment methods for gear forgings?

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During the manufacturing process, gear forgings all require heat treatment, which involves artificial heating, holding at a specific temperature, and controlled cooling to achieve the desired performance characteristics. Typically, the heat treatment methods for gear forgings are as follows: 1. Surface Hardening of Gear Forgings: This method is commonly used for medium-carbon steels such as 45 and 40Cr, as well as other medium-carbon alloy steels. After surface hardening, the hardness of the tooth surfaces usually falls within the range of 40 to 55 HRC. Such gear forgings exhibit excellent resistance to fatigue pitting, strong resistance to galling, and superior wear resistance. Since the core of the gear remains unhardened, these forgings retain sufficient toughness and can withstand relatively small impact loads. 2. Carburizing and Quenching of Gear Forgings: This method is often applied to low-carbon steels and materials such as 20 and 20Cr.
　　During the manufacturing process, gear forgings all require heat treatment, which is achieved through artificial heating, holding at a specific temperature, and controlled cooling to meet the desired performance requirements. Typically, the heat treatment methods for gear forgings are as follows:
　　1. Surface Hardening of Gear Forgings: This process is commonly applied to medium-carbon steels such as 45 and 40Cr steels, as well as other medium-carbon alloy steels. Typically, after surface hardening, the hardness of the tooth surfaces falls within the range of 40 to 55 HRC. Such gear forgings exhibit excellent resistance to fatigue pitting, strong resistance to galling, and superior wear resistance. Since the core of the gear remains unhardened, the forging retains sufficient toughness and can withstand relatively small impact loads.
　　2. Carburizing and quenching of gear forgings: This process is commonly used for low-carbon steels and low-carbon alloy steels such as 20 and 20Cr. In critical gear transmissions, the tooth surfaces treated by carburizing hardening can achieve a hardness of 56 to 62 HRC while still maintaining high toughness. As a result, gear forgings treated this way exhibit high bending strength and excellent contact strength on the tooth surfaces, along with good wear resistance, making them frequently employed in important gear transmissions. However, gear forgings subjected to carburizing hardening tend to undergo significant tooth deformation, so they should be subsequently ground to ensure precise tooth geometry.
　　3. Nitriding of gear forgings: Nitriding is a surface chemical heat treatment. After nitriding, the surface hardness of the gear teeth can reach 700–900 HV. Due to its high hardness, low processing temperature, and minimal deformation, nitriding is particularly suitable for internal gears and gears that are difficult to grind. It is commonly applied to nitrided steels containing alloying elements such as chromium, copper, and lead, such as 38CrMoAlA.
　　4. Adjusting the quality of gear forgings: These are typically made from medium-carbon steels and medium-carbon alloy steels, such as 45, 40Cr, and 35SiMn steel. After quenching and tempering, the surface hardness of the gear teeth usually ranges from 220 to 280 HBS. Due to their relatively low hardness, the gear tooth forgings can undergo precision finishing after heat treatment.
　　5. Normalized gear forgings: Normalizing can relieve internal stresses, refine grain size, and improve mechanical and machinability properties. Medium-carbon steel can be normalized; for large gear forgings, cast steel forgings can also be normalized.
　　When forging transmission gears, to reduce the bonding force and ensure the service life of the gears, the tooth surface hardness of the pinion is typically 30 to 50 times higher than that of the gear. For transmissions involving high speeds, heavy loads, or critical gear forgings, hard-toothed gear combinations can be used, where the tooth surface hardness of both gears is roughly the same.

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