Optimization of Ultrasonic Impact Treatment for Surface Finishing and Hardening of AISI O2 Tool Steel by Experimental Design

Advanced multi-pin ultrasonic impact treatment (UIT) technique is applied to improve the surface properties of AISI O2 tool steel. The main influential parameters, such as amplitude of the horn vibrations and peening intensity (UIT duration), were studied using a factorial design of experiments. The experimental conditions and response surface methodology designs were generated using the Design-Expert software. To optimize the UIT parameters for the manganese steel processing, statistical, diagnostic, and response surface methodology analyses were carried out. The effects of the UIT parameters on the surface crystallite/grain size, hardness, hardening depth, residual macro-stress, roughness, and waviness as response variables were evaluated. The surface topography and profiles of the untreated and UIT-processed specimens were also analyzed and compared. The linear/quadratic regression models were selected to predict the dependent output variables. The optimum UIT parameters were narrowed for the studied steel, setting maximum (hardness, hardening depth, and residual stress) and minimum (grain size, roughness, and waviness) limits. The results showed that the optimized UIT parameters lead to an increase in surface hardness by ~50% and a reduction in surface roughness by ~50%, providing both a specific surface texture and a nanostructure, and compressive residual stress in the subsurface layer.