PSI - Issue 2_A

3404 G Sudhakar Rao et al. / Procedia Structural Integrity 2 (2016) 3399–3406 G.S. Rao et al. / Structural Integrity Procedia 00 (2016) 000–000 (a) and 5 (a) revealed shear dimples and quasi-cleavage facets in combination with intergranular cracking along the prior austenite grain boundaries, respectively. The quasi-cleavage facets contained tear ridge marks, microvoids and secondary cracking, as reported by Nagao et al. (2012) confirming the role of hydrogen-induced micro-plasticity. It is important to mention that no quasi-cleavage features were observed at strong trap sites like MnS inclusions or oxides at both temperatures (in contrast to room temperature and high-temperature water tests, Roychowdhury (2016)), eventually due to the low-sulphur content of this steel (0.004 wt.%). Thus, the embrittlement observed in the steel is mainly due to the extensive localized plastic deformation enhanced by dislocation and hydrogen interactions that is further supported by the shear dominant macroscopic failure (45° inclination to the loading axis).

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Figure 4: Fracture morphology after hydrogen charging in 20 MnMoNi 5 5 tested at 250 °C and a strain rate 10 -2 s -1 : a) overall fracture surface, b) magnified view of region 1, c) magnified view region 2.

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Figure 5: Fracture morphology after hydrogen charging in 20 MnMoNi 5 5 tested at 288 °C and strain rate of 10 -2 s -1 : a) overall fracture surface, b) magnified view of region 1, c, d) magnified view of region 2. The fractographic appearance of the 22 NiMoCr 3 7 steel with low DSA susceptibility was very similar, whereas the amount of quasi-cleavage facets and intergranular cracking in presence of hydrogen was significantly lower and absent, respectively, in line with the observed higher reduction of area, Roychowdhury (2016). 4. Discussion Remarkable hydrogen softening and embrittling effects with a clear change in fracture mode/morphology were observed under temperatures (250 and 288 °C) and material (moderate yield stress levels around 400 MPa) conditions, where such effects usually are believed to be absent or negligible. The range and amplitude of hydrogen effects seem to be significantly amplified by a high DSA susceptibility in the DSA temperature-strain range, suggesting some synergies between DSA and hydrogen effects. The observed effects and fracture morphology may be qualitatively understood by the hydrogen effects on plasticity/dislocations and major hydrogen embrittlement processes for low-alloy RPV steels discussed in literature, Roychowdhury (2016): 1. Hydrogen-enhanced decohesion embrittlement (HEDE): According to this mechanism hydrogen atoms weaken the interatomic bonding leading to brittle or quasi-cleavage micro-cracks at strong traps like MnS/oxide inclusions or intergranular cracking at grain boundaries in combination with grain boundary carbides or metalloid segregation. 2. Hydrogen-enhanced local plasticity (HELP): According to this mechanism hydrogen reduces the elastic interactions forces between dislocations and dislocation-obstacles (shielding effect) and the stacking fault energy restricting cross slip and favoring shear localization, planar coarse slip and strain hardening in case of multiple