Local electropulse-induced gradient and hierarchical architecture of soft-hard phase in 35CrMo steel

To improve the comprehensive mechanical properties while avoiding the overall heat treatment, this study provided a method to prepare structural steel with soft-hard phases arranged in complex gradient, hierarchical structures via processing a homogenous material using local electropulsing treatment (LEPT). The results show that ultimate tensile strength, elongation-to-failure, and curves area of the structural steel plate are about 1400 MPa, 15.93%, and 19,100 MPa·%, respectively, which are 16.5%, 34.7%, and 47.5% higher than the conventional quenched and tempered steel. The excellent mechanical properties of LEPTed specimens are mainly due to the local strengthening caused by the effects of pulse current, and the synergistic effect of hard and soft phases. The designed hardened units produced by LEPT exhibit hierarchical structure and gradient variations in texture and grain size. There also exists gradient hardness distribution in hardened units. These structures are produced due to the multi-scale effects of current and the gradient current density distribution. Moreover, due to the introduction of hardened units at another scale, there were four stages in the strain-hardening behavior of LEPT based on differential Crussard-Jaoul (DC-J) analysis, which deviates from the three stages of two-phase or multi-phase steels.