Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

Z. Que et al. / Procedia Structural Integrity 13 (2018) 926–931

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Z. Que et al. / Structural Integrity Procedia 00 (2018) 000–000

oxygenated HTW suggested that DSA effects might dominate over crevice chemistry effects in RPV steels with low sulphur and high DSA.

Figure 2: SICC crack initiation in 277 steel tested with 0.35 µm/min at 288  C in HWC (a) and 508 steel tested with 3.5 µm/min at 288  C in NWC (b); Fracture surface of 277 steel with 35 µm/min at 288  C in air (c & g), HTW with 2 ppm O 2 (d) and 2 ppm H 2 (e & f).

Figure 3: (a), (b) and (c), (d) are IPF (inversed pole figure) and GROD (grain reference orientation deviation) maps at crack tips of 508 material tested in air and HWC at 288 °C with 35  m/min, respectively. (b) and (d) show GROD mapping with a deviation angle range of 0-40°. The effects of loading rates and water chemistries were evaluated with EPFM tests on 508 specimens with high DSA susceptibility in various environments (Figure 1(b)) with loading rates of 0.35-350 µm/min at 288°C. Reduction in fracture initiation resistance was found in all three HTW environments and the environmental degradation was increased with decreasing loading rate (DSA effect). A stronger environmental reduction of fracture initiation resistance in HWC compared to NWC and nitrogenated HTW was observed, which confirmed the synergistic effects of hydrogen and DSA on the environmental degradation effects. The unexpected low fracture initiation resistance in NWC at 3.5  m/min was related to SICC (SICC cracks initiated without prior ductile crack growth and further propagated with high-sulphur EAC rates, as shown in Figure 2 (b)). Apparent high reduction of fracture resistance occurred at slow loading rates in NWC is related to the subcritical SICC and has to be strictly differentiated from a real toughness reduction. For studying the underlying mechanism of potential synergic effects of DSA and hydrogen, EBSD mappings were performed at crack tips on EPFM specimens of 508 material tested in air (J Q,AIR =380 kN/m) and HWC (J Q,HWC =316 kN/m) at 288 °C with 35  m/min, as shown in Figure 3. Stronger localized deformation at the crack tip in HWC was discovered, as depicted in GROD and KAM map where the local orientation deviation and kernel average misorientation were higher in the case of HWC compared to air. It is clear that at the region up to 20  m ahead of the crack tip, massive tiny grains with small orientation difference were observed in the specimen tested in HWC, which indicated a higher distortion and more localized plastic deformation at the crack tip region despite of lower J Q in the HWC environment. The EBSD mapping clearly revealed that the synergic effects of DSA and hydrogen might be due to the localization of plastic deformation by DSA (due to the negative strain rate sensitivity) which favours local hydrogen enrichment above the critical threshold concentration. Hydrogen transportation will be facilitated by mobile dislocations and stress-assisted diffusion to the process zone. The localized deformation bands