Origin of tearing topography surface in hydrogen-charged pearlitic steel

In the present study, the formation mechanism of tearing topography surface (TTS), which is observed after slow strain-rate tensile tests of H-charged pearlitic specimens, was investigated in detail. The TTS always appeared at the subsurface of failed H-charged specimens, where H atoms were concentrated after room-temperature electrochemical H-charging. The TTS consisted of stepped flat surfaces and dimpled surfaces. The flat surface was caused by the coalescence of sharp micro-shear cracks, not micro-voids, of H-enriched cementite (θ) platelets in a pearlite colony, namely brittle transcolonial fracture. The regions surrounding the flat surface were H-depleted due to the migration of H atoms into neighboring micro-shear cracks during tensile deformation. As a result, the H-depleted regions were fractured by the typical coalescence of micro-voids, namely relatively ductile shear cracking, resulting in dimpled surfaces after tensile fracture. These results mean that the TTS region, strictly speaking, the flat surface region, which is generated by transcolonial fracture inside the TTS region, is an initial site of H-induced cracking of pearlitic steel. The ab-initio calculations of the energy value of H desorption from a C vacancy in θ supported the recent report that H atoms are concentrated inside the θ platelets after H charging.