PSI - Issue 13

Jean-gabriel Sezgin et al. / Procedia Structural Integrity 13 (2018) 1615–1619 Jean-Gabriel Sezgin/ Structural Integrity Procedia 00 (2018) 000 – 000

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Figure 2 – Nominal stress-strain curves resulting from the SSRT tests under various conditions

3.3 Effect of internal pressure on fracture surface morphology Fracture surfaces of the SSRT specimens for each condition have been observed by scanning electron microscopy (SEM) as shown in Figure 3. The reference state (Case I) showed an ordinary cup-and-cone failure. The non-charged specimen in 115-MPa nitrogen gas (Case IV) failed by a similar failure mode to the reference state; therefore, the external pressure did not affect the tensile ductility and fracture surface morphology. In contrast, the H-charged specimens (Cases (II) and (III)) presented some different failure characteristics: a strong reduction in dimple size was observable in the H charged specimens. However, the comparison of the fracture surfaces of Cases II and III did not present any notable difference neither in dimple size nor in dimple arrangement. This fact suggests that there was no evidence of an effect of internal pressure on the tensile ductility and fracture surface morphology of the H-charged specimens. The HE mechanism of H-charged Type-316L steel is then more likely to be based on other mechanisms which require no intervention of internal pressure, such as the hydrogen-enhanced localized plasticity (HELP) mechanism.

Figure 3 – Fracture surface morphologies by SEM for each condition

4. Conclusions

The present work has investigated the effect of internal pressure in pre-existing voids on tensile properties of H charged Type-316L stainless steels in SSRT test conditions. The modelling and experiments have jointly contributed to the following conclusions: • The internal pressure reached in the voids during the SSRT test was estimated to be on the order of 1 MPa, the maximal pressure reachable being 133 MPa. The simulations have shown that even in the conservative case ( e.g. dislocation pipe-diffusion) in which the maximal pressure is considered, the internal pressure could not corroborate the observed effect of hydrogen on void growth. • Based on the non-charged specimen in vacuum, the relative reduction in area (RRA) was 0.84 for the H-charged specimen in vacuum, 0.88 for the H-charged specimen in 115-MPa nitrogen gas, and 1.04 for the non-charged specimen in 115-MPa nitrogen gas. These experimental results suggest that there was no notable effect of internal pressure on the tensile ductility. • Reduction in dimple sizes was observed in the H-charged specimens, regardless of application of external pressure. This suggests that the internal pressure in voids had no effect on the void growth during the SSRT tests.