PSI - Issue 36

Ihor Dmytrakh et al. / Procedia Structural Integrity 36 (2022) 298–305 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2021) 000 – 000

301

4

The specimens after the cycles of charging-discharging and also after tensile test were examined by scanning electron microscopy (Scanning Electron Microscope Zeiss EVO-40XVP) to detect the possible difference in the microstructure of the steel. It should be noted that the SEM observations were made in the central area cross-section of the specimens with a diameter of 5 mm. For this purpose, the special disk-like mini samples were cut from the cylindrical tensile specimens (see Fig. 2). Thus, even possible surface damages can not affect on the data received from the bulk of the material. 3. Results and discussion The first stage of study consisted of the evaluation of the microstructural changes in the unloaded specimens ( σ = 0) after the different number of cycles of hydrogen charging-discharging N H . The specimens were preliminary hydrogen charged to the level С H  0.23 wppm and then electrochemically hydrogen discharged to value C H ≈ 0. Additionally, for the verification that the specimens are fully discharged, we also tested some portion of them by the standard thermal desorption analysis by using the hydrogen analyzer LECO ONH836 Instrument. The residual values of the hydrogen concentration in the specimens depending on the number of hydrogen charging-discharging cycles N H are given in Table 2. These data confirm the real absence of the hydrogen in the specimens before the further tensile test.

Table 2. Residual values of C H (wppm) in specimens depending on N H . N H , cycles 0 2 5

0.0163

C H , ppm

0.0052

0.0107

a

b

c

Fig. 3. The SEM images of the microstructure of the unloaded specimens ( σ = 0) after the different number of cycles of hydrogen charging discharging N H : a – 0; b – 2; c – 5 ( Ч 500). The area of observation was equal to 33516 μm 2 .