PSI - Issue 2_A

G Sudhakar Rao et al. / Procedia Structural Integrity 2 (2016) 3399–3406 G.S. Rao et al./ Structural Integrity Procedia 00 (2016) 000–000

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hydrogen charging at both temperatures, more prominently at 288 °C for all the strain rates. The maximum drop in reduction in area was observed at strain rates of 10 -2 s -1 and 10 -3 s -1 for 250 °C and 288 °C, respectively. Both the more pronounced effect at 288 °C and the maximum at lower strain rates at 288 °C are in conflict with our expectations. Both DSA and hydrogen effects and potential synergies among them in RPV steels were expected to be shifted to higher strain rates with increase in test temperature, Roychowdhury (2016). On the other hand, the higher temperature may facilitate the redistribution of hydrogen to susceptible locations with plastic strain localization and stress concentration and/or release more hydrogen from strong traps that would be available for hydrogen-dislocation interactions, which might partially compensate the other effects and explain the stronger effect at 288 °C. A comparison of the ultimate tensile strength and reduction in area of the 20 MnMoNi 5 5 and 22 NiMoCr 3 7 steels with different DSA susceptibilities for strain rates from 10-1 to 10-4 s-1 with and without hydrogen at 288 °C is shown in Fig. 3. Under identical hydrogen charging conditions, the reduction of ultimate tensile strength is a factor of 5 to 10 higher than in the steel with low DSA susceptibility. Furthermore, a maximum in softening was observed at strain rate of 10 -2 s -1 in the steel with high DSA with significant scatter in this region. A significantly larger drop in the reduction of area is observed in the 20 MnMoNi 5 5 steel with the higher DSA susceptibility. The very small reduction of area in the maximum embrittling region of 5 % is almost a factor 10 lower in the RPV steel with high DSA susceptibility. Furthermore, the hydrogen embrittling region is significantly extended over a much broader strain rate range at 288 °C. In the 22 NiMoCr 3 7 steel, the embrittling effect is strain rate dependent and restricted to a narrow strain rate range around 10 -2 s -1 .

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Figure 3: Comparison of ultimate tensile strength (a) and reduction of area (b) with and without hydrogen in 20 MnMoNi 5 5 and 22 NiMoCr 3 7 steel for different strain rates at 288 °C. These observations suggest some significant synergy between high DSA susceptibility and hydrogen effects. Materials with susceptibility to DSA exhibit inhomogeneous plastic deformation with plastic strain localization in the DSA range due to the negative strain rate sensitivity, an increase of planar deformation and dislocation density and in deformation bands that can act as regions for hydrogen accumulation, which may further amplify the localization of plastic deformation by hydrogen. This finally results in an earlier saturation of the capacity for uniform elongation and earlier initiation of necking and, in particular, a smaller reduction of area (see Section 4). 3.3. Fracture behavior Without hydrogen, a classical cup and cone fracture by pure microvoid coalescence in the centre of the specimen and final shear dominated ductile failure at the surfaces is observed. In presence of hydrogen, a shear dominated fracture is observed, where the fracture plane is inclined at a ~ 45 ° angle with respect to the loading axis. Effect of hydrogen charging on fracture behavior of 20MnMoNi 5 5 steel tested with a strain rate of 10 -2 s -1 at 250 °C and 288 °C are shown Figs. 4 and 5, respectively. In hydrogen containing samples, a mixed mode of fracture consisting on microvoid coalescence along with enhanced localized shear plastic deformation, quasi-cleavage facets and, to a lesser extent, intergranular cracking is observed. The magnified views of the regions 1 and 2 marked on the Fig. 4