PSI - Issue 2_A

3406 G Sudhakar Rao et al. / Procedia Structural Integrity 2 (2016) 3399–3406 G.S. Rao et al. / Structural Integrity Procedia 00 (2016) 000–000 hydrogen concentrations might thus be achieved in these regions to cause shielding effects over a significantly broader strain rate range. Due to the higher strains/stresses and trap center/vacancy density, the release of hydrogen is also significantly retarded with respect to steels with low DSA susceptibility. Shielding effects are significant, if hydrogen atmosphere at dislocation cores can be formed and the hydrogen diffusion rate (dependent on temperature) is comparable or higher (shielding) than the dislocation velocity (dependent on strain rate), which occurs for critical temperature-strain rate combinations only. At high temperatures, shielding effects disappear, since no hydrogen atmosphere at dislocation cores and strong trap centers/barriers can be formed and most of the hydrogen diffuses out of the steel or slip barriers can be more easily overcome by thermal activation. At low temperatures and high strain rates, hydrogen cannot follow the dislocations and shielding effects disappear. Due to the significantly higher diffusion rate of hydrogen with respect to C and N, the shielding effects already start at lower temperatures and persist to higher strain rates than DSA for given strain rates and temperatures, respectively, provided that hydrogen atmospheres can be formed. There is thus some overlap between DSA and H shielding effects. Furthermore, a competition between C & N and H for the lattice places in the core region of dislocations can be expected. The binding energy of C & N to dislocations cores of ~ 0.8 eV is higher than that of H of ~ 0.2 to 0.6 eV, the diffusion rates of hydrogen on the other hand are orders of magnitudes higher. The typical free C and N contents in RPV steels are usually higher than the bulk H content, but H can strongly accumulate at traps and in strained/stressed regions and the local content thus can be much higher. Depending on the C, N and H availability, microstructure (trap enters, carbo-nitride precipitates), strain (forest and mobile dislocation density, vacancy concentration), strain rate and temperature, this may result in a rather complex overall behavior with synergy or competition between DSA and the various H effects. Vacancies do not affect the diffusion of interstitials, but affect the plastic deformation behavior. The combined effects of HELP, HESIV and DSA thus appear to be quite complex and further investigations are necessary to clarify their specific role. 5. Conclusions The hydrogen embrittlement of two RPV steels with a low and high susceptibility to DSA was investigated by tensile tests in air at 250 and 288 °C and different strain rates. Remarkable hydrogen softening and embrittling effects with a clear change in fracture mode/morphology were observed under temperature (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, probably due to the localization of plastic deformation due to DSA and the shielding effect of hydrogen. In presence of hydrogen, shear dominated mixed mode fracture with varying amounts of ductile microvoid coalescence, quasi cleavage regions and secondary cracking along the prior austenite grain boundaries were observed. This suggests that HELP, HESIV and HEDE mechanism were simultaneously active under these specific conditions. Acknowledgements The financial support for carrying out this research under the SAFE-I & II programme from the Swiss Federal Nuclear Safety Inspectorate (ENSI) is gratefully acknowledged. References Birnbaum, H.K., Sofronis, P., 1994. Materials Science and Engineering A 176, 191-202. Hänninen, H., Seifert, H.-P., Yagodzinsky, Y., Ehrnstén, O., Tarasenko, O., Aaltonen, P., 2001. Effects of dynamic strain ageing on environment assisted cracking of low alloy pressure vessel and piping steels. In: 10th Int. Conf. on Environmental Degradation of Materials in Nuclear

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