Examining the Effects of Severe Shot Peening on the Fatigue Strength of Ultra-High-Strength Steel Fabricated via Wire Arc Additive Manufacturing

Surface improvement methods such as severe shot peening (SSP) are crucial for enhancing the mechanical characteristics of materials. This research examines how SSP influences the fatigue strength of ultra-high-strength (UHS) steel produced through wire arc additive manufacturing (WAAM). To assess the effects of SSP on material properties, we performed a variety of tests, including microstructure analysis, hardness measurements, surface roughness assessments, tensile testing, and bending fatigue evaluations. Microstructural analysis reveals surface deformation and the presence of a deformed martensitic/bainitic microstructure, distinct from the base material. Kernel average misorientation (KAM) mapping highlights differences between surface and inner microstructures, indicating a higher local misorientation on the surface and the presence of retained austenite deeper below. Results indicate a significant enhancement in surface hardness by approximately 20%, with the highest hardness recorded near the surface. SSP led to a significant rise in surface roughness when compared to the machined version. Despite this, the enhanced hardness near the surface contributed positively to fatigue resistance and fatigue limit increased by a 6%. Although SSP led to a reduction in yield strength and elongation, it positively influenced fatigue resistance, primarily attributed to the enhanced surface properties. However, further exploration into residual stresses is recommended to comprehensively understand the mechanisms underlying the observed improvements. These results highlight SSP’s potential as an effective method for improving the fatigue resistance of WAAM UHS steel components.