Effect of ultrasonic nanocrystal surface modification on the fatigue behaviors of plasma-nitrided S45C steel

An ultrasonic nanocrystal surface modification (UNSM) technique with different striking numbers (34,000 mm− 2 and 69,000 mm− 2) was employed to modify the surface of plasma-nitrided S45C steel. After the UNSM treatment, the fatigue strength of the 8 h plasma-nitrided specimen (N8) was clearly improved, and the sub-surface cracks were nearly restrained, while the 48 h plasma-nitrided specimen (N48) showed no improvement of fatigue strength. The mechanical properties of the specimens before and after UNSM treatment were characterized, such as the microhardness, the surface roughness, and the surface compressive residual stress. A grain refinement layer was formed on the surface of the specimens due to the severe plastic deformation induced by the dense striking, and the grain distribution was analyzed by using X-ray diffraction. The grain refinement layer played an important role in the improved mechanical properties. The fatigue limit enhancement mechanism was examined by analyzing the stress intensity factor from fish-eye cracks. For the fish-eye cracks, the stress intensity factor in the facet area, △ Kfacet, was found to be a constant value of approximately 4.08 MPa·m1/2, regardless of either the UNSM treatment or the nitriding duration.