HELP versus HEDE in progressively cold-drawn pearlitic steels: Between Donatello and Michelangelo

This paper reviews previous research by the author in the field of hydrogen effects on progressively cold-drawn pearlitic steels in terms of hydrogen degradation (HD), hydrogen embrittlement (HE) or, at the micro-level, hydrogen-assisted micro-damage (HAMD), thus affecting their microstructural integrity and compromising the (macro-)structural integrity of civil engineering structures such as prestressed concrete bridges. In particular, the two classical hydrogen-assisted deterioration micro-mechanisms are discussed, namely, hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE), together with their role in the embrittlement process as a function of the degree of cold-drawing undergone by each steel (associated with a different level of microstructural orientation of pearlitic colonies and lamellae). It is seen that hydrogen effects in pearlitic microstructure (either oriented or not) are produced at the finest micro-level by plastic tearing in the form, in general, of hydrogen damage topography (HDT) with different appearances depending of the cold drawing degree, evolving from the so-called tearing topography surface (TTS) in hot-rolled (not cold-drawn at all) or slightly cold-drawn pearlitic steels to a sort of enlarged and oriented TTS (EOTTS) in heavily drawn steels (the pronounced enlargement and marked orientation being along the wire axis or cold drawing direction). Whereas the pure TTS mode (null or low degree of cold drawing) resembles the Michelangelo stone sculpture texture (MSST), the EOTTS mode does the same in relation to the Donatello wooden sculpture texture (DWST).