Internal and surface damage after electrochemical hydrogen charging for ultra low carbon steel with various degrees of recrystallization

An ultra low carbon (ULC) steel was subjected to electrochemical hydrogen charging to provoke hydrogen induced damage in the material. The damage characteristics were analyzed for recrystallized, partially recrystallized, and cold deformed material. The goal of the study is to understand the effect of cold deformation on the hydrogen induced cracking behavior of a material which is subjected to cathodic hydrogen charging. Additionally, charging conditions, i.e. charging time and current density, were varied in order to identify correlations between, on the one hand, crack initiation and propagation, and, on the other hand, the charging conditions. The obtained hydrogen induced cracks were studied by optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Hydrogen induced cracks were observed to propagate transgranularly, independently of the state of the material. Deformed samples were considerably more sensitive to hydrogen induced cracking, which implies the important role of dislocations in hydrogen induced damage mechanisms.