Effect of Ultrasonic Shot Peening Pretreatment on Carburizing Heat Treatment Process of 20CrMnTi Gear Steel

Ultrasonic shot peening (USP) strengthening represents an emerging and efficient surface mechanical reinforcement technique. 20CrMnTi gear steel is commonly employed engineering material for gear components within vehicular power transmission systems. In this study, a gradient nanostructured (GN) surface layer was successfully fabricated in 20CrMnTi gear steel at room temperature using USP technology. Furthermore, an integrated process combining pre-USP treatment with subsequent carburizing heat treatment (USP-carburized) was implemented, resulting in an approximate increase of 50 HV in surface hardness and an augmentation of the hardened layer depth by roughly 50 μm compared to samples subjected solely to carburizing. OM and EBSD analyses reveal that the pre-USP treatment nearly doubles the carbide density at the surface, while concurrently reducing both the carbide and martensite dimensions by approximately 10%. Additionally, SEM and EDS results demonstrates that the pre-USP process significantly elevates the internal carbon content in 20CrMnTi gear steel after carburizing. The microstructural evolution and promoting mechanism of the GN during the CHT is elucidated in the study. TEM observations indicate that pre-USP treatment induces a pronounced surface nanocrystallization phenomenon where carbon atoms preferentially diffuse into the high angle grain boundaries (HAGBs, θ ≤ 15°) during carburizing, resulting in the carbides inside the carburized 20CrMnTi gear steel distribute along grain boundaries (GBs) accompanied by a high density of dislocations. Consequently, the GN effectively promotes rapid carbon diffusion. This study significantly advances the understanding of the mechanisms by which GN influence the efficacy of carburizing treatments.