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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C H means the total hydrogen content C H(total) (see formula (2)). As it can be seen, the effect of the hydrogen content C H on the parameter W f is ambiguous and on the received dependence it is possible to mark three characteristic zones (Fig. 4).

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Fig. 4. “ W f – C H ” diagrams for the notched (a) and cracked (b) specimens.

For notched specimens (Fig. 4a), in zone I ( C H @ 0.01–0.2 ppm), the value of the parameter W f remains practically constant, although there is some tendency for its slight decrease. Hence there is no visible effect of hydrogen on the local strength of steel. In zone II ( C H @ 0.2–0.7 ppm) there is a sharp decrease of values W f to a certain minimum, and after that, there is increasing of W f up to hydrogen content equal to C H @ 0.7 ppm. Zone II can be interpreted as a transition zone, where two mechanisms of the hydrogen effect are realized. The decreasing value of W f is caused by the mechanism of conditional plasticization (enhanced deformability) of the material under the influence of a low content of diffusible hydrogen (Dmytrakh et al. (2015)). The following increase in values W f indicates a change in the mechanism of hydrogen influence on the material and the beginning of the manifestation of the phenomenon of hydrogen embrittlement of low-alloyed steel (Lynch (2003)). A further increase of the hydrogen content C H ≥ 0.7 ppm (zone III) leads to an increase in the degree of embrittlement of the material, which due to its increased brittleness loses the resistance to local fracture at the notch. This is confirmed by a sharp decrease in the parameter W f . The diagram W f = Φ( C H ) for specimens with a crack-like defect is presented in Fig. 4b. Here, as in the case of the notch, we can also identify three characteristic zones that correspond to the different features of the mechanisms of the hydrogen effect on the resistance to fracture of low-alloyed steel. It should be noted that there is a certain qualitative similarity between the results obtained with the case of the notch. However, here the parameter values W f are significantly lower in comparison to the notched specimens (see Fig. 4). In addition, the hydrogen content from which the mechanism of hydrogen embrittlement begins to dominate is also lower ( C H ≥ 0.6 ppm). Above-described tendencies we tried to explain by the consideration of the ratio between the diffusible and residual hydrogen in its total amount in low-alloyed steel under different test conditions. According to used experimental procedure (Dmytrakh et al. (2024)), quantitative data about the diffusible C H(dif) , residual C H(res) and total hydrogen content C H(total) in the steel were obtained and the plots C H(dif) = f ( C H(total) ) and C H(res) = f ( C H(total) ) are presented in Fig. 5.

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Fig. 5. Values of the diffusible and residual hydrogen contents for notched (a) and cracked (b) specimens depending on the total hydrogen content C H(total) : 1 – C H(dif) ; 2 – C H(res) .