PSI - Issue 68

Andriy Syrotyuk et al. / Procedia Structural Integrity 68 (2025) 880–886 Andriy Syrotyuk et al. / Structural Integrity Procedia 00 (2025) 000–000

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For notched specimens (Fig. 5a) the obtained dependences indicate the existence of three characteristic zones. At the total content of hydrogen in the material C H(total) ≤ 0.2 ppm (zone I), all hydrogen was practically diffusible ( C H(dif) ). Here, it should be noted that zone I refers to very low hydrogen content and we supposed that there is a specific situation, namely: all this hydrogen consists of the lattice hydrogen and/or the weakly trapped hydrogen (Bolzon et al. (2021); Lynch (2003)). Such type of hydrogen is easy for natural volatilization and consequently, there are no appropriate conditions for trapping in the ‘deep’ traps. Therefore, the content of residual hydrogen for zone I is about zero ( C H(res) » 0.010 ppm). In the range 0.2 ppm ≤ C H(total) ≤ 0.7 ppm (zone II), part of the hydrogen remains diffusible C H(dif) , but the portion of residual hydrogen C H(res) begins to increase. There is a specific point at C H(total) @ 0.7 ppm, namely: C H(dif) @ C H(res) . At the content C H ≥ 0.7 ppm (zone III) the diffusible hydrogen significantly decreases and the content of residual hydrogen C H(res) increases sharply simultaneously. As can be seen, the boundaries of the specific zones in Fig. 5a practically coincide with the boundaries of characteristic zones on the dependence W f = Φ( C H ) (see Fig. 4a). This fact can serve as evidence that the hydrogen content effect the fracture behaviour of the steel depends on the ratio of diffusible C H(dif) and residual hydrogen in the material C H(res) . Hence, for given steel three characteristic ranges of the hydrogen content exist, and for them, their mechanisms of hydrogen influence on the resistance to fracture are observed. As in the case of the notched specimens, we determined the values of diffusible C H(res) and residual hydrogen C H(dif) in its total amount of C H(total) in the pre-cracked specimens. The analysis of the plots C H(dif) = f ( C H(total) ) and C H(res) = f ( C H(total) ) showed that here three specific zones can be identified (Fig. 5b). The diffusible hydrogen C H(dif) dominates in zone I ( C H(total) ≤ 0.2 ppm). In zone II the diffusible hydrogen C H(dif) prevails within the range 0.2 ppm ≤ C H(total) ≤ 0.6 ppm. The value C H(total) @ 0.6 ppm is the point where C H(dif) @ C H(res) . At the content C H ≥ 0.6 ppm, the value of residual hydrogen C H(res) significantly increases (zone III). It can be noted that the boundaries of the specific zones in Fig. 4b and Fig. 5b also coincided as in the case of the notched specimens which is the confirmation of the existence of three different mechanisms of the hydrogen influence on the fracture behaviour of the studied steel.

Fig. 6. SEM images of the fracture surface of notched (a) and cracked (b) specimens depending on the hydrogen content C H in the metal ( ´ 500).