PSI - Issue 30

M.Z. Borisova / Procedia Structural Integrity 30 (2020) 17–22 Borisova M. Z. / Structural Integrity Procedia 00 (2020) 000–000

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It can be assumed that ECAP shifts the threshold of brittleness in the direction of lower temperatures. This may be due to the formation of the SMC structure in the steel and a decrease in the contribution of plastic deformation mechanisms, such as sliding of dislocations. The greater the energy intensity of the crack development process, the greater the plastic deformation was realized during the crack development process. Then for normal separation fractures (equiaxed pits), more energy-intensive is considered with deeper pits. The size and number of break-off pits after ECAP are clearly larger than for the steel in its original state. This is what determines the increase in impact strength due to ECAP.

Fig. 4. SEM images of fracture surfaces of the low-carbon steel specimens received due ECAP and subsequent quenching after impact tests at +20°C: (a) x 100; (b) x 2000; (c) x 10000 and after impact tests at –20°C: (d) x 100; (e) x 2000; (f) x 10000.

Thus, it can be said that the fracture surfaces after ECAP and quenching is characterized by a high energy intensity of destruction in the comparison with initial state of the steel, which is associated with the formation of a more complex destruction surface. In addition, the destruction of perlite plates and the formation of globular inclusions of cementite led to an increase in the homogeneity of the steel structure and to an increase in the strength and impact toughness. 4. Conclusions Severe plastic deformation of the low-carbon steel leads to the formation of the SMC structure with grain size of ferrite 0.2~0.3 μ m and uniformly dispersed in ferrite matrix fine cementite particles. The yield strength and ultimate strength of the steel increase after ECAP about 2 and 1,5 times accordingly. The impact strength increased by 1.3 times compared to the initial state. The only mechanism that contributes to the simultaneous increase in strength and viscosity is the structural refinement. The microstructure changes in a steel after ECAP lead to a change in the mechanisms of fracture. The formation of more complex fracture surfaces of SMC material by impact test indicates that more energy is spent on destruction in comparison with coarse-grained material. The formation of a structure that provides the required set of properties is supposed to base on the choice of optimal combinations of chemical composition and thermomechanical processing technology. Acknowledgements I would like to show my gratitude to the Center for collective use of the Federal Research Centre «Yakut Scientific Centre of the Siberian Branch of the Russian Academy of Science» for conducting mechanical tests of this