Exploring the influence of ultrasonic peening treatment at ambient and cryogenic conditions on the surface characteristics and fatigue life of austenitic stainless steel 304L

This study investigates the impact of Ultrasonic Peening Treatment (UPT) at room temperature (T room ) and cryogenic temperature (T cryo ) on phase transformation, surface morphology, and fatigue life of stainless steel 304L samples. Finite element simulations were developed to analyze the effects of different material models and the pin's vertical velocity on residual stress, martensite volume fraction (xi), and surface deformation. The numerical results of residual stress and xi were consistent with experimental measurements obtained via X-ray diffraction. Significant compressive residual stress and martensite volume fraction were induced in the subsurface of the specimens following UPT at both temperatures. Various UPT process parameters, including static load, the pin's horizontal velocity, the number of treatments, and lubrication conditions, were experimentally explored for their effects on surface morphology and hardness. Indentation hardness revealed values of 621 HV for samples treated at T- cryo and 489 HV for those treated at T room , compared to 286 HV for untreated specimens. Furthermore, UPT reduced surface roughness by approximately 88 % for a mechanically polished surface and over 93 % for a rough surface. Investigating conditions leading to potential damage and defects during the process was also undertaken. Fractography of the fracture surfaces and fatigue analysis revealed that the fatigue life of UPT-treated samples at T-room and T-cryo increased by 63 % and 45 %, respectively, compared to untreated specimens.