PSI - Issue 30

M.M. Kantor et al. / Procedia Structural Integrity 30 (2020) 45–52 M. М . Kantor et al. / Structural Integrity Procedia 00 (2020) 000–000

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Real microstructure was characterized at metallography specimens sampled from the 1/3 of the sheet thickness in RD-ND plane. SEM and EBSD measurements were performed using CrossBeam 1540 EsB (Carl Zeiss) equipped with Electron backscattered diffraction (EBSD) Nordlys S camera (Oxford Instruments). EBSD maps were acquired with 20 kV acceleration voltage, 60 µm aperture size in high current mode with a square grid with 200 nm step size using drift correction. Aztec 3.1 and hkl Channel 5 software packages were used for acquiring and post-processing of EBSD data. Parent austenite microstructure was characterized in etched condition after multiple treatment by heated saturated aqueous solution of picric acid with addition of natrium lauryl sulphate. 3. Results and discussion

Results of multiple impact toughness tests and fracture character assessment are represented in Fig. 2.

Fig. 2. DBT region impact fracture investigation: (a) KCU impact toughness temperature dependence; (b) macro fractography, KCU -80 min , OM; (c) micro fractography of fracture surface containing splitting, KCU -80 min , SEM.

Occurrence of remarkable impact toughness scattering pointed at the existence of DBT region in range from 0°C to - 100°C, Fig. 2 (a). Upper bound KCU values decreased monotonously with temperature decrement in DBT region. Uneven lower bound KCU values determining the DBT scattering range contained two zones. Minimal KCU values in range from 0°C to -40°C were almost equal 363-365 J/cm2. Significant KCU deterioration from 328 J/cm2 to 234 J/cm2 was observed in range from -60°C to -100°C. Macro fractographic analysis revealed the completely ductile fracture character at 20 °C. At lower temperatures, in addition to the formation of ductile fracture surfaces, there was also observed occurrence of splittings, Fig. 2 (b). The correlation relation between minimal DBT region impact toughness values, the volume fraction of projections of splittings areas and the quantity of splittings is represented in Fig. 3. The number and area of splittings projections increased with the decrease of minimal KCU values. Micro fractography of splitting region is showed in Fig. 2 (c). As well as formation of cleavage facets, the occurrence of dimples and localized tearing regions were observed.