Understanding the thermal stability and friction behavior of ultrasonic shot peening-introduced gradient nanostructured M50 bearing steel

The M50 steel is a common material for main shaft bearings in aerospace applications. The thermal stability of gradient ultrafine grains/nanograins steels is of critical importance for engineering applications. Here, we fabricated gradient nanostructured M50 steel using ultrasonic shot peening and subsequent annealing. This study investigated the effect of annealing treatment on the microstructure evolution of gradient nanostructured M50 steel and the thermal stability of the USPed sample post-annealing. Furthermore, the friction behavior of gradient nanostructured M50 steel was also examined. After annealing at 350 °C for 30 min, ferrite phase at the depth span of 0–100 μm has better thermal stability, and formed a high-density dislocation network and cells at the top surface of the gradient layer. When annealed samples were continued annealed at 350 °C for 100 h, the average ferrite phase size at the depth span of 0–100 μm reduced to a fixed value (~434 nm). High-density dislocation cells with smaller and more dispersed ordered B2 phases have a stronger pinning effect of dislocation. Hetero-deformation induced hardening by a dislocation network of grain interior and dislocation wall (~200 nm thick) and precipitation hardening of dislocation cells with more dispersed ordered B2 phase is the main reason for the high microhardness of the samples annealing at 350 and 450 °C for 30 min. The USPed sample within the 100–200 μm depth span exhibits better friction properties attributed to the strengthened martensitic matrix, partial decomposition of spherical carbides, and the presence of high-density dislocations, which reduce the cracking of carbides and matrix, and carbide spalling. Although heat treatment results in a slight decrease in the friction properties of the USPed sample, they still surpass those of the tempered sample. This reduction in friction properties post-annealing primarily arises from the annihilation of high-density dislocations.