PSI - Issue 13

A. Laureys et al. / Procedia Structural Integrity 13 (2018) 1330–1335 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Such curves allowed finding charging conditions resulting in similar hydrogen amounts for different applied charging current densities. Samples charged for 60 min at 0.8 mA/cm², 50 min at 5 mA/cm² and 20 min at 20 mA/cm² exhibited on average very similar hydrogen contents, i.e. around 5.5 wppm. These conditions all exhibited blisters on the surface of the cold deformed ULC steel. Fig. 2 (b) shows a typical blister appearance as found on the charged samples. The blister distribution on cold deformed ULC steel is also shown in Fig. 2 (a). The sample surfaces of the samples charged at lower current densities showed similar characteristics, but the number of surface blisters decreased with lower current densities, while the blister size increased. This trend was also observed in previous work by Laureys et al. (2017). They concluded that the amount of surface blisters can be considered proportional to the amount of internal hydrogen induced damage. Therefore, under the present conditions, samples with similar amounts of hydrogen and varying amounts of blisters were obtained. These samples were further evaluated by melt extraction and TDS analysis.

Figure 2: Blisters on samples charged for 20 min at 20 mA/cm². a) Overall blister distribution (small dots on sample surface are blisters). b) Detail of blister

3.2. Melt extraction Melt extraction did not only generate the total hydrogen content of the samples, but the hydrogen evolution in time during melting is also registered. These graphs revealed two hydrogen desorption peaks for all charged cold deformed ULC steel samples (cf. Fig. 3). The second peak was not observed for uncharged samples. However, the peak in the uncharged sample appeared at the same time as the first peak in the charged samples. Therefore, the source of the released hydrogen should be similar. This hydrogen could have entered the material during manufacturing or processing. Cold deformed material exhibits numerous strain induced defects, which can trap hydrogen from the environment at certain conditions. Additionally, surface related phenomena could also be the reason for the appearance of the first peak for all samples. Additional tests are needed for further clarification. The second peak originated from hydrogen introduced during charging and required more time to leave the sample upon melting. The latter hydrogen might originate from damage formed in the material during charging due to hydrogen accumulation at certain microstructural heterogeneities and the corresponding internal damage and blister formation in the materials. Blisters contain H 2 due to their formation process, which could take longer to diffuse and egress from the sample during melt extraction than hydrogen trapped at the surface or microstructural heterogeneities. The second peak height became also larger with applied charging time for a certain current density, which could indicate the growth of blisters with charging time and consequently its higher H 2 containing volume. However, for the three studied conditions no clear correlation could be made between the changing blister characteristics (amount, size, geometry) and the shape of the secondary peak. Additionally, the formation of blisters also affects the microstructural features near the blister, as such creating new potential hydrogen traps in the microstructure. Therefore, complementary TDS measurements were done to evaluate hydrogen trapping for the different charging conditions. 3.3. Thermal desorption spectroscopy TDS measurements (cf. Fig. 4) were performed for following hydrogen charging conditions in order to characterize the trapping behavior of deformed ULC steel all containing hydrogen induced damage: (i) 5 mA/cm² for 50 min, (ii) 20 mA/cm² for 20 min, (iii) and (iv) 5 mA/cm² for 50 min and 20 mA/cm² for 20 min followed by a waiting period of