Effects of Surface-Hardened Layer Thickness Reduction on the Fatigue Characteristics of Shot-Peened AISI 4340 Steel

Numerous studies have focused on improving the fatigue strength and life of materials by introducing compressive residual stresses into the surface layer through surface treatment processes such as shot peening. This study analyzes the effects that partially removing the shot-peened surface-hardened layer through polishing has on the mechanical properties, surface characteristics, fatigue strength, and fatigue life of AISI 4340 steel. Surface roughness, hardness, and residual stress were measured experimentally. Mechanical properties were evaluated through tensile testing, and ultrasonic fatigue tests were performed using 20 kHz elastic waves. The results show that the fatigue limit decreased by approximately 9.4%, 26.3%, and 25% after a 10%, 15%, and 20% reduction in surface-hardened layer thickness, respectively. Furthermore, the fatigue strength at a life of 1 × 109 cycles (very-high-cycle fatigue) decreased by up to approximately 23.9% after 15% and 20% thickness reductions compared with that before surface layer removal. The findings of this study provide useful data for the optimal design of materials, components, and equipment requiring enhanced fatigue resistance through surface modification treatments.