Enhancing the hydrogen embrittlement resistance of medium-carbon high-strength steel by optimizing the tempering temperature

Hydrogen embrittlement (HE) and hydrogen diffusion in medium-carbon high-strength steel tempered at various temperatures were investigated by in situ slow strain rate testing and electrochemical hydrogen permeation, respectively. Synergistic correlations between the various microstructural components and reversible/irreversible hydrogen traps were established. Dislocations and the size, shape, distribution, and volume fraction of cementite were found to considerably influence the HE susceptibility. High dislocation density and interconnected needle-like cementite with a high aspect ratio, oriented along grain boundaries, were produced at a low tempering temperature (560 degrees C); this facilitated the decohesion of atoms at the cementite-matrix interface, enabling them to act as crack propagation sites and inducing brittle intergranular fracture. However, a significantly improved HE resistance associated with a quasi-cleavage-based fracture morphology was achieved at a high tempering temperature (680 degrees C) owing to the low dislocation density and the presence of uniformly distributed spheroidised cementite.