Investigation of the effects of a laser-induced integrated gradient structure on the wear resistance of EX30 bearing steel

The gradient structure can enhance the wear resistance of low carbon steel while maintaining its impact toughness through the heterogeneous synergy between the surface layer and the interior layer region. However, the conventional surface single-gradient composition structure is increasingly failing to meet the service requirements of EX30 bearing steel. Therefore, an integrated gradient structure is proposed to further enhance the wear resistance of EX30 bearing steel under oil-lubricated conditions, and the friction and wear mechanisms of the steel with this integrated gradient structure are investigated. A carbon composition gradient was formed through carburizing heat treatment, and a nanocrystalline gradient layer was further induced by laser shock peening (LSP). The results indicate that the carburizing heat treatment formed a compositional gradient layer approximately 1.5 mm thick on the material surface. The plastic deformation induced by LSP eliminated the texture of the surface layer in specific orientations, triggered intense dislocation motion, and resulted in defect features such as dislocation accumulation and slip. Ultimately, a gradient nanocrystalline layer approximately 200 μm thick was formed. The single-composition gradient structure tends to generate localized stress concentration during reciprocating friction, leading to micro-cutting and plastic deformation. In contrast, the richer local defect characteristics and the abundance of nanograins in the integrated gradient structure promote the dispersion of surface strain over a wider stress range, significantly alleviating material damage and crack propagation caused by stress localization. Furthermore, the integrated gradient structure reduced the wear rate by 53.9% and 54.3% across different sliding frequencies compared to the single-gradient structure.