Influence of Surface Treatment on Hydrogen Embrittlement of Commercial Vehicle Seat Slider Bracket

In the automotive industry, the selection of structural materials plays a critical role in ensuring component durability and performance. Among these, seats hold significant importance across all vehicle types—whether three-wheelers or four-wheelers. Based on the working mechanism, these seats can be hydraulic-based, spring-loaded or air suspension types, among others. Usually, high-strength steels are used to manufacture these seats. One such example is pneumatic seats, which are designed for multidirectional movement and are equipped with a seat slider assembly. Recently, we received a failed seat slider assembly from the plant. It was observed that the tooth of the locking bracket had completely broken off from the parent part.

The locking bracket features a toothed design which engages with the seat slider cavities to secure the seat position in multiple X-Y directions, ensuring movement of the seat. This part is manufactured using high-strength hot-rolled steel (SPFH 590 grade). Due to continuous engagement and repetitive cycles, the teeth are prone to wear; hence, carbonitriding surface treatment is applied to the toothed region to enhance surface hardness, achieving a hardness range of 625–725 HV.

During fractography analysis, intergranular fracture, along with multiple cracks, was observed. A study of the manufacturing and heat treatment process revealed that the part undergoes case hardening during carbonitriding, along with ammonium–hydrogen gas mixture exposure, which results in hydrogen pickup. This hydrogen diffuses inside the material and accumulates on the grain boundaries, eventually weakening them, which can further lead to intergranular fracture. To further mitigate the issue, the amount of diffusible hydrogen needs to be decreased, which can be achieved by increasing the tempering temperature to 250 °C–300 °C.