The Grey-Taguchi method analysis for processing parameters optimization and experimental assessment of 42CrMo steel treated by ultrasonic surface rolling

Static force, ultrasonic amplitude, feed rate, and rolling spacing parameters of 42CrMo steel treated by the ultrasonic surface rolling process (USRP) were optimized by the Grey-Taguchi method. Nine sets of experiments based on an orthogonal array design and grey relational analysis were used to obtain the optimum combination of mechanical properties for USRP. The influence of processing parameters on mechanical properties was investigated based on the grey relational analysis. The results indicated that the feed rate is the dominant parameter affecting the mechanical properties, followed by rolling space, ultrasonic amplitude, and static force. The effects of plastic strain caused by USRP on the compressive residual stress and microhardness were investigated. The effects of the dislocation and cementite evolution on the mechanical properties were investigated to determine the origin of the best and worst mechanical properties. For the best mechanical property, the level and depth of distribution of compressive residual stress and tensile strain hardening capacity are the highest, which is benefited from the optimum synergistic of the elastic-plastic deformation, dislocation evolution, gradient distribution, and cementite evolution. For the worst mechanical property, a large amount of cementite is distributed within and around martensitic laths, increasing brittleness and the lowest comprehensive mechanical properties. These findings can provide a reference for accurately controlling the processing parameters of USRP and improving their mechanical properties.