Hydrogen-assisted cracking (HAC) has been intensively studied in the past. However, its governing mechanism is still under debate. Particularly, details of formation of specific cleavage-like or quasi-cleavage fracture surfaces related to hydrogen embrittled steels are unclear yet. Recently, it was found that the fracture surface of the hydrogen charged and tensile tested low-carbon steel exhibits quasi-cleavage facets having specific smoothly curved surface, which is substantially different from the commonly known flat cleavage facets. In the present contribution, we endeavor to shed light on the origin of such facets and their morphological features. For this purpose, the notched flat specimens of the commercial low carbon steel were annealed in vacuum and then tensile tested using ex- and in-situ hydrogen charging. It is found that in the ex-situ hydrogen charged specimens, HAC originates primarily at non-metallic inclusions inside the specimen bulk. The cracks expand radially from the origin to the specimen surface producing “fish-eyes” - round-shaped areas in which the surface is composed of curved quasi-cleavage facets. In contrast, during the tensile test with in-situ hydrogen charging, HAC initiates at the surface and extends to the bulk of the specimen. This process results in formation of a completely brittle fracture surface with the quasi-cleavage morphology - the same as in the fish-eyes. The examination of the side surface of the in-situ hydrogen charged specimens revealed straight and S-shaped sharp cracks which path is visually independent of the microstructure and grain crystallography but is strongly affected by the stress fields. It is concluded that growth of such cracks is responsible for the formation of fish-eyes and smoothly curved quasi-cleavage facets in hydrogen charged low-carbon steel.
Financial support from the Russian Foundation for Basic Research (grants-in-aid 17-08-01033) is gratefully acknowledged.