Nanoengineering steel's durability: creating gradient nanostructured spheroidal carbides and lath-shaped nano-martensite via ultrasonic shot peening

Surface nanoengineering can significantly improve the mechanical properties and performance of metals, such as strength, hardness, fatigue, wear resistance, etc. In this work, we tailored the surface microstructure of GCr15 bearing steel within a thickness of approximately 800 μm using room temperature ultrasonic shot peening (USP) technology. Microstructure characterization studies reveal the formation of gradient nanosized spheroidal carbides and lath-shaped nano-martensite in the GCr15 bearing steel during the USP process. The micro-sized martensite structure (∼1.8 μm) in the as-received GCr15 bearing steel was refined to lath-shaped nano-martensite with a thickness of ∼10 nm. The USP-induced transformation of carbides and the segregation of carbon elements within the peened surface layer of the GCr15 bearing steel were clearly observed. The sizes of carbides were reduced from ∼800 nm to ∼460 nm during the USP treatment. These phenomena were primarily attributed to the interactions between dislocations and carbides, as well as the interfacial wear occurring between the carbides and the matrix materials. The segregation of carbon elements within the USP-treated sample was ascribed to the interplay between dislocations concentrated at grain boundaries and the ensuing carbon diffusion within the cementite phase. As a result of surface nanoengineering, the microhardness of the USP-treated GCr15 bearing steel was improved by approximately 30% compared to its as-received counterparts and a significant enhancement in wear resistance was achieved. The wear rate coefficient for USP-treated GCr15 bearing steel is reduced by 43.19% and 84.62% after 4 and 8 minutes of peening, respectively. This study paves the way for exciting advancements in nanoengineering metals for superior durability.