Using magneto-plasticity to improve material properties assists with ultrasonic surface rolling process (USPR), is an effective strengthening method. The gradient grain refinement and wear resistance of 42CrMo machined surface with magnetic field assisted USRP (MF-USRP) were investigated. The formations of gradient nanostructured surface layers under the rolling parameters and MF intensities (maximum 0.12 T) were elucidated with the finite element (FE) simulation and cellular automaton (CA) modeling, together with TEM analysis. The relationships between gradient nanostructured surface layers and wear resistance were also analyzed. FE simulation results show that strain rate is increased significantly during MF-USRP. CA simulation results show the average size of surface nanostructures is refined to 54.62 nm, which can accurately reproduce the process of grain refinement. TEM results indicate that dislocation multiplication and migration are induced by the release of pinning effect under influence of MF. Refined grains with sharp boundaries are distributed at the grain boundaries, exhibiting a discontinuous dynamic recrystallization. The average friction coefficient and wear volume of machined surface processed by MF-USRP are reduced by 9.22% and 7.97%, respectively, comparing with that of single USRP. The machined surface processed by MF-USRP mainly exhibits abrasive wear. The improvements in wear resistance are attributed to the increased degree of surface nanocrystallization, reduced surface roughness and surface defects. The study confirms that the superposition of steady-state MF can enhance surface nanocrystallization of 42CrMo machined surface layer to a certain extent, which provides guidance for the development of MF-USRP technology.
 

ultrasonic surface rolling，gradient grain refinement，magnetic filed