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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Fig 1. c-d present microstructural changes in the presence of 72 h cathodic charged hydrogen. The formation of hydrogen-induced ɛ -martensite phase is well seen in AUSS (Fig. 1c) and DSS (Fig. 1d). According to our previously published work [1], [8], as well as the work of others [9], internal stresses created during hydrogen absorption provide a significant driving force for the  martensite transition. The  -phase stability was calculated using the S stability equation (Eq. 1) following the works of Rozenak and Eliezer [10], [11], and others [12]. = + 0.68 + 0.55 + 0.45 + 27( + ) , (1) where S is the austenite stability factor and Ni, Cr, Mn, Si, C and N are the stabilizing elements in wt%. Table 1. Stability measurements and phase quantities of 72 h hydrogen charged: DSS, AUSS and SMSS, after one month at RT. Sample Phase concentration (wt%) stability factor ( S )    /  SMSS 30 70 8 17 SAF 2205 (DSS) 50 50 35 24 AUSS (316L) 95 5 7 32 Table 1 compares between the S stability values and phases' contents calculated using the Rietveld method [13]. Supported by our previous XRD work [1], [2], [14] and Fig 1. d-f, it was concluded that the sample with the highest ɛ -martensite phase content, i.e. with the lowest hydrogen-induced ɛ -phase stability, will show the least damage. An interesting observation is the hydrogen-induced-second phases during gas-gas phase hydrogen charging. The microstructure of the gas-phase hydrogen charged DSS, charged for 3 h at 60 MPa and 300 o C, as was already shown in some of our previously published works [1], [14] – [16], can be seen in Fig. 2. The gas-phase hydrogen charging revealed for the first time the appearance of needle‒shaped sigma (σ) phase, which is an intermetallic compound with the Fe(CrMo) composition [1], [14]. This phase was found to play an important role in the trapping mechanism.

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Fig. 2. (a) SEM micrograph of LDS after gas-phase charging for 3 h at 60 MPa and 300 o C, showing t he formation of needle shaped σ -phase after one month at RT, (b) bright- field TEM image, the inset shows the [100] zone axis from the σ -phase grain, and (c) dark-field TEM [6].

2.2. Quasi-Static and dynamic behavior in the presence of hydrogen

Quasi-static tensile testing was performed at RT on gas-phase hydrogen charged stainless steels. At this low strain rate (~10 -7 s -1 ) the role of hydrogen is clearly increasing tensile strength and decreasing elongation [1], [14], [17]. Fig. 3a summarizes yield strength values versus elongation of DSS, SMSS and AUSS. Supported by an extensive study of different research works [18] – [21] it can be proclaimed that high strength stainless steels are more susceptible to hydrogen embrittlement. A further investigation of DSS dynamic strength was applied due to its combination of great strength along with ductility. Dynamic loading effect was examined by comparing a non charged dynamic loaded DSS sample, Fig. 3b, to a gas-phase hydrogen charged DSS (0.5 GPa), Fig. 3c. The microstructural result of 0.5 GPa dynamic pressure shows a multiplication of cracks compared with the non-charged