Effect of the stress-strain state on the path of quasi-cleavage hydrogen-assisted cracking in low-carbon steel

Сonflicting data on the effect of the stress-strain state on the shape and path of quasi-cleavage cracks in hydrogen-embrittled steels and iron have been reported in the literature recently. However, this issue is important for understanding the nature of hydrogen embrittlement (HE) and, particularly, the role of hydrogen in the mechanism of crack propagation. In this regard, in the present study, smooth and notched specimens of low-carbon steel were tensile tested with in-situ cathodic hydrogen charging. The side surface of the specimens fractured in this way was examined by scanning electron microscopy aiming at quantitative characterization of the length and curvature of secondary side surface cracks. In addition, fractographic analysis was performed. A great number of cracks having a characteristic S-shaped curved geometry are found on the side surface of the notched specimens. Moreover, the cracks can smoothly curve both on the scale of several grains as well as within the individual grain interior. In contrast, in the case of smooth specimens, the cracks generally exhibit relatively straight appearance. Quantitative analysis using a statistically-representative dataset has shown that, on average, the curvature of cracks in the notched specimens is significantly higher and is varied over a wider range of values than in the smooth specimens. Based on the data obtained, it is concluded that the path of quasi-cleavage cracks in hydrogen-embrittled ferritic and ferrite-pearlitic low-carbon steels is mainly determined by the characteristics of the stress-strain state, and not by the microstructure or crystallographic orientation of individual grains.

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1. Russian Science Foundation - 19-79-00188