Hydrogen-plasticity interactions in pearlitic steel: A fractographic and numerical study

A fractographic and numerical approach is presented to analyze hydrogen-plasticity interactions in pearlitic steel and to elucidate the main hydrogen transport mechanism in this material under triaxial stress states produced by notches. Fractographic analysis showed that the microdamage produced by the hydrogen was clearly detectable by scanning electron microscopy (SEM), through a specific microscopic topography associated with hydrogen effects (tearing topography surface or TTS). Numerical computations obtained by using an elastic-plastic finite element program gave the progressive spreading of the plastic zone, closely associated with the movement of dislocations. In the majority of cases, the plastic zone (PZ) clearly exceeds the hydrogen affected region (TTS) and has no relation with it. In some tests, however, the hydrogen-induced micro-damage surpasses the only region in which there is dislocation movement, and in this case the net macroscopic transport of hydrogen cannot be attributed to dislocation dragging, but only to a random-walk stress-assisted diffusion. Therefore, in spite of the fact that dislocational transport of hydrogen has been sufficiently demonstrated on a microscopic scale, it might not be a hydrogen embrittlement mechanism per se, detectable with loss of fracture load, on a macroscopic scale.