Ultrastrong gradient nanostructured CSS-42L bearing steel and its enhanced wear resistance at elevated temperature

A gradient nanostructured surface layer was successfully fabricated on the CSS-42L(15Cr14Co12Mo5Ni) aerospace bearing steel by ultrasonic shot peening (USP) technology at room temperature. The original austenite in the as-received (AR) CSS-42L bearing steel was fully transferred into lath martensite by quenching and two cryogenic tempering cycles. The lath martensitic structure at the surface of the bearing steel was refined into nanoscale martensite phases with an average size of 33.9 nm after USP treatment. The microhardness of the gradient heterogeneous nanostructured CSS-42L bearing steel increased by approximately 39 % compared to that of the tempered bearing steel. The mechanisms of martensite refinement during the USP process were analyzed and discussed using advanced materials characterization methods. Dry sliding wear tests at elevated temperatures (500 °C) were conducted on the tempered and USPed CSS-42L bearing steel samples, respectively. The wear resistance of the USPed sample was significantly enhanced compared to that of its untreated coarse-grained counterpart. The wear rate coefficient of the USPed CSS-42L was reduced by 80.42 % compared to the AR sample. The superior wear resistance of the USPed CSS-42L bearing steel was attributed to the formation of an ultrastrong surface layer with a gradient heterogeneous nanostructure fabricated by USP technology.