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

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These hydrogen discharged specimens were examined by scanning electron microscopy (SEM). We have detected void-like defects in the steel as the objects that are coloured in black and marked by white circles on the images (Fig. 3). It can be visually seen that the number of these defects depends on the number of cycles of hydrogen charging-discharging, namely: the defectiveness of the steel increases with increasing of the parameter N H . It was confirmed by the counting of the defects number and their total square (Table 3). Here, it should be noted that data for N H = 0 reflect the primary defectiveness of the steel that always exists in the engineering structural materials.

Table 3. Number of defects and their total square in the unloaded specimens ( σ = 0) on the number of cycles of hydrogen charging-discharging N H . The area of observation was equal to 33516 μm 2 . N H , cycles 0 2 5 Number of defects 45 50 73 Total square of defects S , μm 2 110 131 161

Therefore, we can state that the short-term presence of the low concentration diffusible hydrogen leads to the change of the material’s microstructure increasing its defectiveness , and consequently, some change of its macroscopic mechanical behaviour can be expected. It also is important to note that such kind of hydrogen can change the microstructural situation of the steel even in the absence of the applied stress ( σ = 0). The second stage of the study was devoted to the determination of the “stress σ – strain ε” dependencies for the steel under uniaxial loading. The specimens were also preliminary hydrogen charged to the level С H  0.23 wppm and then fully hydrogen discharged and the series of such specimens after the different number of cycles of hydrogen charging-discharging N H were obtained for further tensile test.

Fig. 4. Nominal and true stress-strain diagrams for specimens with the different number of cycles of hydrogen charging-discharging N H .

The nominal and true stress-strain diagrams were received for the specimens with the different values of N H (Fig. 4). It can be seen that the deformation ε , which corresponds to the fracture of the specimen, increases with increasing of N H . This fact indicates the facilitation of material deforming ability depending on the value of the parameter N H .