PSI - Issue 13

Elena Astafurova et al. / Procedia Structural Integrity 13 (2018) 1129–1134 Elena Astafurova et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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hardening. In this case, some factors can stimulate planar dislocation arrangement and inhibit neck formation: decrease in fraction of incoherent precipitates and increase in their size suppress Orowan-mechanism of the interaction of particles and moving dislocations; solid solution hardening by nitrogen promotes to short-range ordering. The characteristic fracture surfaces of the specimens tensile tested at different temperatures after both QT-regimes are shown in Figure 3. For the temperature interval of (200-673)K, the steel specimens fractured by transgranular ductile fracture mode (Figs. 3 a, b, e, f). A large number of round dimples are observed on fracture surfaces, which support a microvoid coalescence mechanism of fracture in this deformation regime. For steel specimens treated at QT=1200°C, dimples are smaller and shallower compared to that treated at QT=1100°C (Figs. 3b and f).

Fig. 3. The characteristic SEM images of fracture surfaces for CrMnVCN steel solution-treated (quenched) at 1100°C (a – d) and 1200°C (e – h) and tensile tested at 293K (a, b, e, f) and 77K (c, d, g, h). Brittle fracture components are shown by arrows on images (d) and (h).

After deformation at 77K, about 25% of the brittle component was observed on the fracture surfaces of specimens treated at QT=1200°C (Figs. 3 g, h). These components are associated with transgranular brittle fracture by cleavage micromechanism. Such cleavages are characteristic for low-temperature deformation of high-nitrogen V-free steels with low and high SFEs (Gavrilyuk and Berns, 1999; Tomota et al., 1998; Chumlyakov et.al., 1999; Wang et al., 2010; Liu et al., 2004). Brittle cracks arise on different stress concentrators and distribute in austenitic grains along boundaries of mechanical twinning, ε -phase, slip microbands (Tomota et al., 1998; Chumlyakov et.al., 1999; Mullner, 1997). The other part of fracture surface has viscous fracture characteristics with a large number of dimples and secondary cracks (Fig. 3g). Decrease in solution-treatment temperature down to QT=1100°C partially suppresses the formation of cleavages during deformation at 77K. The areas of the cleavage-like regions get smaller with decrease in QT, and fraction of brittle component decreases down to 8%. This effect is associated both with different solid-solution hardening and precipitate distributions in CrMnVCN steel after QT=1200°C and QT=1100°C.

4. Summary

Temperature dependence of tensile mechanical properties and fracture micromechanisms were investigated in a temperature range of 77K to 673K for specimens of high-nitrogen Fe-19Cr-21Mn-1.5V-0.3C-0.9N (wt. %) steel