As a core component for adjusting seat position, the car seat transmission pin's comprehensive mechanical properties directly impact its reliability and lifespan. Heat treatment processes, by altering the internal metallographic structure of the material, can significantly optimize key properties of gears such as hardness, toughness, wear resistance, and fatigue strength. However, multiple heat treatments can induce changes in microstructure, leading to performance fluctuations or even degradation. Therefore, a thorough analysis is needed from three aspects: the cumulative effect of processes, the laws governing microstructure evolution, and the performance balancing mechanism.
The core processes of multiple heat treatments include quenching, tempering, carburizing, or nitriding. Their cumulative effect alters the phase composition and distribution within the material. For example, the initial quenching can form high-hardness martensite on the gear surface, while retaining a tough structure in the core. If quenched again, temperature control deviations may lead to excessive coarsening of the surface martensite, reducing toughness. Simultaneously, the core structure may generate residual stress due to repeated phase transformations, increasing the risk of deformation or cracking. Repeated tempering processes can further soften the surface hardness. Improper tempering temperature or time control can easily disrupt the balance between hardness and toughness, affecting the gear's wear resistance and fatigue resistance.
Repeated surface strengthening processes such as carburizing or nitriding alter the carbon/nitrogen concentration gradient and compound layer thickness on the gear surface. Initial carburizing can form a uniform high-carbon martensite layer, improving wear resistance; however, repeated carburizing may lead to excessively high carbon concentration, forming a network of carbides, reducing surface toughness and increasing the risk of brittle fracture. Repeated nitriding can thicken the surface nitride layer, increasing hardness, but an excessively thick nitride layer may peel off due to insufficient bonding strength with the matrix, thus shortening gear life. Furthermore, repeated formation and removal of the surface strengthening layer (such as reprocessing after grinding) can damage the original microstructure, leading to performance instability.
The impact of repeated heat treatments on the gear core microstructure is equally significant. The toughness of the core microstructure is crucial for resisting impact loads, and repeated heating and cooling can cause grain coarsening or abnormal phase transformations. For example, repeated quenching may cause the core microstructure to transform from fine sorbite to coarse lath martensite, reducing toughness; repeated tempering processes may soften the core microstructure, weakening its load-bearing capacity. Furthermore, the accumulation of residual stress in the core can lead to dimensional deformation, affecting the assembly accuracy of gears and transmission systems, and causing noise or jamming problems.
Fatigue performance is a core indicator for measuring gear life, and multiple heat treatments can affect fatigue strength by altering crack initiation and propagation mechanisms. Initial heat treatment can improve fatigue life by optimizing microstructure uniformity and introducing surface compressive stress; however, repeated treatments may accelerate fatigue crack initiation due to microstructure coarsening, residual stress accumulation, or increased surface defects. For example, repeated formation of surface strengthening layers may introduce microcracks or pores, becoming fatigue sources; softening or coarsening of the core microstructure may reduce crack propagation resistance and shorten fatigue life.
To balance the impact of multiple heat treatments on performance, process parameters and sequence need to be optimized. For example, using a combination of quenching and low-temperature tempering can reduce surface brittleness; increasing diffusion time after carburizing can reduce the surface carbon concentration gradient and avoid the formation of network carbides; pretreatment before nitriding (such as shot peening) can improve surface activity and promote uniform nitride formation. Furthermore, controlling the number of heat treatments and the intervals between them to avoid excessive microstructural evolution is also crucial for ensuring stable performance.
In practical applications, the number of heat treatments for car seat transmission pins needs to be determined comprehensively based on material properties, usage conditions, and cost. For gears subjected to high loads and high frequencies, a single-stage optimization process (such as vacuum carburizing + high-pressure gas quenching) is recommended to ensure performance. For gears requiring repair or modification, localized heat treatment (such as laser hardening) can reduce the impact on overall performance. In the future, with the development of intelligent heat treatment technology, precise control of temperature, time, and medium can further reduce the risks of multiple heat treatments and improve the overall performance of gears.
