PSI - Issue 60

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Gopal Sanyal et al. / Procedia Structural Integrity 60 (2024) 311–323 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Figure 6.SEM fractographs of the tensile samples tested in (a) Air (b) In-situ experiment with current density 40mA/cm 2 .

It is interesting to note that yield point phenomenon is not affected by the application of in-situ charging where a deforming sample is exposed to high fugacity hydrogen. This suggests that there is no appreciable increase in the interstitial content at the point where the deformation transitions from elastic to plastic regime in the Cr-Mo-V steel. The loss in tensile ductility then can be attributed to the rapid hydrogen ingress prior to necking during the in-situ test. It has been shown in previous ex-situ studies that a mere 2% pre-deformation in RAFM steel (Sagar, 2015) accelerated the degradation of tensile ductility. Similar loss of ductility is seen is ex-situ tested RAFM steels with up to 5 ppm of hydrogen (Sanyal, 2013). These results suggests that there is gradual build-up of interstitial H content in the matrix during the uniform deformation. At the formation of neck, the stress gradient under its developing non-uniform distribution results in the mobile hydrogen from interior and the ingressing H from the electrochemical charging to preferentially accumulate below the surface. The enhanced lattice concentration of hydrogen below surface promotes its embrittlement by de-cohesion. As a result, the localized lateral strains arelargely suppressed in the necking region due toHE led rapid loss of strength. In contrast, the presence of significant lateral strains in the necking region results in large reduction in area when tested in air where there is noHE led loss of strength. Since the range of current densities used in the work are rather high and the deformation rates are low, where it is believed thatmobile