PSI - Issue 13

Dan Eliezer et al. / Procedia Structural Integrity 13 (2018) 2233–2238 Eliezer et al/ Structural Integrity Procedia 00 (2018) 000 – 000

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amount of  -martensite is higher, Table 1. An additional interesting observation is the spectra intensity, Fig. 5, which indicates desorbed hydrogen content. The  -martensite phase significantly affects the amount of hydrogen absorption/desorption, due to its higher hydrogen diffusion constant [23]. SMSS presents the highest desorbed hydrogen content. Regarding these insights, it can be said that susceptibility to the hydrogen fracture mechanism is highly dependent on the γ -phase stability rather than the strength of the stainless steel (i.e high yield strength, and low ductility). 2.4. The effect of strain rate on the hydrogen trapping mechanism The effect of deformation levels on the hydrogen trapping mechanism in DSS was studied by comparing gas phase hydrogen charged samples at different states: non-loaded, quasi-static loaded and dynamic loaded at different loading pressures 2.33 GPa and 0.50 GPa, Fig. 5.

Fig. 5. (a) TDS spectra at a 2 o C/min heating rate of gas-phase hydrogen charged at different loading states: non-loaded, quasi-static-loaded and dynamic-loaded at different dynamic pressures (0.5 GPa and 2.33 GPa), and (b) determination of the activation energies by Lee et al model [24].

All samples presented about the same lower trapping energy values of ~20 kJ/mol and ~40 kJ/mol, which were related to elastic stress field and core dislocation or grain boundary at higher activation energies. The great differences are seen in the quasi-static loaded sample [24]. All of its trapping energies were lower than the value of 60 kJ/mol. The rest of the sample's higher energy levels (66-80 kJ/mol) were ascribed to the formation of σ -phase which diverge in its density. These results confirm that the susceptibility to hydrogen fracture mechanism-HELP depends on trapping states, and that reversible trapping contributes to the cracking phenomenon. Fig. 6 presents a plaucible schematic explenation of the applicability of HELP model in DSS. Hydrogen can promote cracking at low dynamic pressure and when the appearnce of hydrogen-induced second phases is limited. This proclaim was based on a development of analytical model for hydrogen trapping based on diffusion calculation in a large range of strain rates and is described in details in our works [17], [24]. Based on this behavior it was shown that hydrogen can escape from deeper potential trapping sites to less deeper trapping (low potential energy) sites during quasi-static deformation. This assumption was explained due to longer time for diffusion compared with dynamic deformation 240 sec and 10 -6 sec for the quasi-static and dynamic loaded samples, respectively. According to previous work of us on metals forming hydrides (Ti and Mg) [6], [25] it can be said that both mechanisms: the hydride embrittlement model and the second phases embrittlement model are dependent on the hydrogen diffusivity to the crack tip. Therefore, the severance of hydrogen embrittlement will be determined only by the hydrogen diffusivity in the second phases or in hydrides, i.e. hydrogen binding energy with it and the content of those phases.