The Strength and Fracture Mechanism of Unalloyed Medium-Carbon Steel with Ultrafine-Grained Structure under Single Loads

This work is devoted to the study of the tensile strength, static crack resistance (К_1С), and impact toughness of unalloyed medium-carbon (0.45% C) steel with ultrafine-grained (UFG) structure obtained by equal-channel angular pressing (ECAP) in comparison with coarse-grained (CG) steel after quenching and high tempering. The ECAP was carried out in the following regime: austenitization for 1 hour at 800°C + quenching in water + medium tempering (350°C) + ECAP at 350°C (Bc route, n = 6, φ = 120°). Our work shows that the K_1С of steel in the UFG state is 53 MPa m^1/2 and, in the CG state, 69 MPa m^1/2. The static failure of steel samples in the CG- and UFG states occurred under conditions of planar deformation. The relation between the depth of the plastic zone below the surface of the fractures in the region of crack propagation and the static crack resistance of steel in the CG- and UFG states is described by the equations h_y = 1/12π(K_1C/σ_0.2)² and h_y = 1/8π(K_1C/σ_0.2)². After ECAP, a narrowing of the ductile–brittle transition interval of steel and its shift towards low temperatures by 70–80°C can be observed. Moreover, the temperature dependence of the size of the shear lips at the surface of the fractures correlates with the temperature dependence of the impact toughness of the steel. The dominant fracture mechanism of CG steel in the lower and middle regions of the ductile–brittle transition is a cleavage, and that of UFG steels, a quasi-cleavage. In the upper region of the ductile–brittle transition, the steel under study fractures with the formation of a pit microrelief.