In powder metallurgy structural parts, tooth deformation and performance degradation during heat treatment are key issues restricting their application, requiring coordinated optimization from multiple dimensions, including material properties, process design, and equipment control. The core lies in precisely controlling heat treatment parameters to reduce microscopic defects caused by porosity, density inhomogeneity, and stress concentration, thereby ensuring the dimensional accuracy and mechanical properties of the gears.
The porosity of powder metallurgy gears is the primary factor affecting the heat treatment effect. Due to the unavoidable formation of micropores during pressing and sintering, these pores become stress concentration points during heat treatment, leading to tooth deformation. For example, high-porosity areas are prone to uneven local shrinkage during quenching due to differences in cooling rates, resulting in tooth profile distortion. Therefore, it is necessary to optimize the sintering process to increase gear density, such as by using high-temperature long-time sintering or vacuum sintering technology, to reduce pore size and distribution density, thereby reducing the risk of deformation during heat treatment.
Precise control of heat treatment process parameters is crucial to avoiding tooth deformation. The quenching heating temperature needs to be set reasonably according to the material composition and gear size to avoid excessively high temperatures leading to grain coarsening or excessively low temperatures causing insufficient hardenability. For example, the quenching temperature of iron-based powder metallurgy gears usually needs to be controlled above the critical temperature to ensure sufficient austenitization, but overheating must be prevented to avoid surface decarburization. Simultaneously, the cooling rate must be matched with the gear thickness; thin-walled gears can use rapid cooling to reduce deformation, while thick-walled gears require segmented cooling to balance the internal and external temperature differences. Furthermore, the tempering process is crucial for eliminating quenching stress; the appropriate tempering temperature and time must be selected according to the gear's service conditions to balance hardness and toughness.
The choice of protective atmosphere directly affects the gear surface quality and performance. Powder metallurgy gears are prone to surface decarburization due to oxidation during heat treatment, which reduces wear resistance and fatigue strength. Therefore, inert gases or reducing atmospheres (such as nitrogen-based atmospheres or decomposed ammonia atmospheres) must be used for protection to prevent the gear surface from contacting oxygen. For high-precision gears, vacuum heat treatment technology can also be used to completely eliminate the risk of oxidation and decarburization, while improving the uniformity of the microstructure.
Preheating and staged quenching are effective means to reduce deformation. Preheating reduces the temperature difference between the inside and outside of the gear, thus reducing thermal stress; staged quenching, by first cooling the gear to an intermediate temperature (such as above the Ms point) before quenching in a cooling medium, makes the phase transformation process smoother, thereby reducing deformation. For example, for powder metallurgy gears with complex tooth profiles, oil bath staged quenching can be used, first holding the gear in high-temperature oil for a period of time, and then transferring it to low-temperature oil to complete the quenching, thus balancing hardness and deformation control.
Die design and pressing process have a decisive impact on the uniformity of gear density. A reasonable die structure can ensure that the powder is uniformly filled during pressing, avoiding heat treatment deformation caused by insufficient local density. For example, using a multi-stage die design combined with an elastic floating device can improve the density consistency between the gear end face and the teeth. In addition, the pressing pressure and holding time need to be optimized according to the material characteristics. Excessive pressure may cause powder particles to break, thus reducing density; while insufficient pressure cannot eliminate porosity.
Post-processing is indispensable for improving the overall performance of gears. Chemical heat treatments such as carburizing and nitriding can significantly improve the surface hardness and wear resistance of gears, but process parameters need to be adjusted according to the porosity characteristics of powder metallurgy gears. For example, during carburizing, the holding time needs to be extended to compensate for the hindering effect of pores on carbon diffusion, while a high carbon potential atmosphere is used to ensure the depth of the carburized layer. In addition, surface shot peening can introduce a compressive stress layer, inhibit crack propagation, and improve the fatigue life of the gears.
The heat treatment of powder metallurgy structural parts needs to balance deformation control and performance improvement. By optimizing the sintering process, precisely controlling heat treatment parameters, selecting a suitable protective atmosphere, adopting staged quenching and preheating technology, and improving mold design and post-processing, tooth deformation and performance degradation can be effectively reduced, meeting the stringent requirements of high-end equipment for gear precision and reliability.
