As a core component of precision transmission devices, the heat treatment process of the internal gears in a high-precision planetary reducer directly affects the equipment's accuracy, load-bearing capacity, and service life. Considering the structural characteristics and high-precision requirements of planetary gears, mainstream heat treatment processes include surface hardening, carburizing and quenching, nitriding, and overall hardening. Each process optimizes gear performance through different physical or chemical mechanisms.
Surface hardening is a common method to improve the surface hardness of gears. It involves rapidly heating the gear surface to above the critical temperature using high-frequency induction heating or flame heating, followed by immediate water or oil cooling to form a high-hardness martensitic layer. This process is suitable for high-frequency hardening of small-sized gears and flame hardening of large-sized gears. The hardened layer depth typically covers the bottom of the tooth root, and the tooth surface hardness can reach 45-55 HRC. Its advantages include simple process, low cost, and preservation of the gear core's toughness, avoiding increased brittleness caused by overall hardening. However, the surface hardened layer is relatively thin, and under long-term heavy loads, wear may lead to a decrease in hardness. Therefore, it is mostly used in medium-to-low load applications.
Carburizing and quenching is a core process for improving the load-bearing capacity of gears, especially suitable for high-precision planetary reducers for servo gears. This process involves placing the gear in a carburizing medium, where carbon atoms diffuse to the gear surface at high temperatures, forming a high-carbon layer (0.8%-1.2% carbon content). This is followed by quenching and low-temperature tempering to obtain a high-hardness martensitic structure. The carburized layer depth is typically 0.8-2.5 mm, and the tooth surface hardness can reach 58-62 HRC, while the core retains low-carbon martensite or bainite, combining high strength and toughness. To eliminate heat treatment deformation, gear grinding is required after carburizing and quenching to ensure the gear precision reaches IT5-6 grade. Although this process is more expensive, it significantly improves the bending fatigue strength and contact fatigue strength of gears, meeting the requirements of high-load, high-precision transmission.
Nitriding treatment forms a nitride layer on the gear surface through nitrogen atom diffusion, achieving surface hardening and improved wear resistance. Compared to carburizing and quenching, nitriding involves a lower process temperature (typically 500-580℃), resulting in minimal gear deformation. After heat treatment, gears can be directly assembled without further machining, making it particularly suitable for hardening internal gears or gears with complex structures. The nitrided layer can achieve a hardness exceeding 1000 HV, exhibiting superior wear resistance compared to carburized layers. Furthermore, residual compressive stress on the surface can inhibit crack propagation and improve fatigue life. However, the nitrided layer is thinner (typically 0.1-0.6 mm), resulting in lower load-bearing capacity compared to carburized and quenched gears. It is primarily used in applications requiring light loads, high precision, or low noise.
Integral quenching (through-hardening) involves heating the entire gear to above its critical temperature, holding it at that temperature, and then rapidly cooling it, causing a martensitic structure to form from the surface to the core. This process is suitable for low-precision, small-module gears, significantly improving the overall hardness and strength of the gear. However, it results in significant deformation, necessitating subsequent finishing to correct dimensions. Because high-precision planetary reducers for servo gears have extremely high precision requirements, overall quenching is rarely used alone. It is often used as a pretreatment process combined with other processes (such as carburizing) to balance hardness and precision requirements.
In practical applications, the selection of heat treatment processes for high-precision planetary reducers for servo gears requires comprehensive consideration of load, precision, cost, and process feasibility. For example, the Asaoka planetary reducer uses vacuum carburizing, precisely controlling the carbon concentration gradient and quenching cooling rate to achieve a gear surface hardness of 62 HRC and maintain a core toughness above 38 HRC. Simultaneously, heat treatment deformation is controlled at the micrometer level, ensuring high meshing precision even under high loads. This type of process optimization enables the reducer to achieve efficient and stable transmission in high-end equipment such as industrial robots and CNC machine tools.
