Peening methods for AISI 4140 steel to induce stable compressive residual stress against cyclic axial loading

In this study, fine-particle peening (FPP) and shot peening (SP) were performed with a maximum compressive residual stress (CRS) of approximately − 650 MPa for AISI 4140 steels to increase the fatigue strength. The CRS stability during the fatigue process was analyzed using in situ X-ray stress measurements under axial compression-tension loading. The hardened layer formed via peening stabilized the CRS against cyclic axial loading, with fatigue strength at the 10⁷th cycle. This occurs because the increased local yield strength prevents localized compressive yielding caused by the superposition of the compressive loading and residual stresses. Although peening induced a CRS in a region deeper than the hardened layer, the CRS below the hardened layer was relaxed by a compressive stress loading lower than that on the surface, reducing the CRS stability in the hardened layer. This is because the compressive loading caused local compressive yielding below the hardened layer and resulted in stress redistribution. SP induced remarkable CRS deeper than the hardened layer, while FPP induced it mainly in the surface layer. Compared to SP, FPP increased the fatigue strength at the 10⁷th cycle by approximately 50 MPa and maintained a high CRS during fatigue testing with axial tension–compression loading and a stress amplitude of 700 MPa. The CRS stability and fatigue properties under axial loading were the most improved when the hardened layer with high yield strength was formed via FPP, and the peak CRS location was within the hardened layer.