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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Fig. 1. Stress-strain curves of the low-carbon steel before and after ECAP.

Reduced ductility due to the accumulation of dislocations in the steel structure as a result of the ECAP can negatively affect the impact toughness. According to research by Safarov et al. (2012), reducing the density of dislocations and their redistribution into the grains body increases the crack resistance of steel. To improve the resistance to impact fracture samples after 2 ECAP passes were subjected to the quenching and short-term tempering. Measurements on the impact coper were performed at room temperature (+20°C) and sub-zero temperature (–20°C). The measurements are shown in Table 2.

Table 2. Impact toughness at different temperatures, kJ/m 2 . Test temperature As-received

After 2 ECAP passes and quenching

+20°C –20°C

2580 2425

3326 3264

Dynamic destruction occurs at high loading speeds, the process of impact destruction of samples with an incision begins with the formation of a plastic zone at the top of the incision with the further development of a macro-crack. The impact toughness means for the resistance energy to fracture and increase of impact energy indicated that the material is more resistant to fracture, leading to the more difficult inducement and propagation of cracks as found by Guo et al. (2016). A decrease in the test temperature usually leads to a change in the fracture mechanism, increasing the friction stress of the grating, and consequently, the yield strength increases sharply as shown by Zrník et al. (2007), Marulanda et al. (2014), Strzelecki et al. (2019). At the same time, the ultimate stress is temperature independent; thus, if the stress reaches ultimate strength before loading, then brittle destruction will occur, but if yield limit is reached first, then plastic deformation will occur with further destruction as shown by Botvina et al. (2008). The morphologies of the fracture surfaces of the Charpy impact specimens of the steel in initial coarse-grained state tested at +20°C and –20 °C are shown in Figure 2. At small optical magnifications, the fracture surfaces at room and sub-zero temperatures look the same. The character of destruction is viscous. But at a higher magnification, it can be seen that at sub-zero temperature, zones with an intergrain brittle type of destruction were appeared (see Fig. 3). In general, the heterogeneity of the fracture surfaces is visible: smooth areas are interspersed with viscous zones. This is most likely due to the ferrite-perlite structure of the steel: zones with cementite carbide inclusions alternate with pure ferrite zones. In viscous materials, usually the main contribution to the destruction is done by the plastic zone, and the break in a small area serves only as a trigger for the promotion of the plastic zone along with the crack edge. Plastic deformation during loading plays a dual role. On the one hand, it creates stress concentrators in the crystal that contribute to the formation of germinal microcracks. On the other hand, microplastic deformation around concentrators reduces the stresses (their relaxation in neighboring micro volumes). By itself, the formation of the