Issue 51

M. Cauwels et alii, Frattura ed Integrità Strutturale, 51 (2020) 449-458; DOI: 10.3221/IGF-ESIS.51.33 455 Secondary cracks on the ND surface were further examined on tensile samples that were stopped at an intermediate elongation at which ultimate tensile strength was just reached. These cracks were primarily transgranular. Crack growth is reported to occur preferentially in ferrite [17, 18] and Zheng and Hardie [41] report that austenite acts as an obstacle to crack propagation since it is both more ductile and less susceptible to HE. For both HT 1190 and HT 1110, cracks across the austenite-ferrite interface were often present (Fig. 7a) as well as cracks that were seemingly unconnected to any ferrite grains (Fig. 7b). Oltra et al. [42] proposed a mechanism for hydrogen cracks in the austenitic phase of a DSS that cross a phase boundary. Cracking starts with brittle cleavage in ferrite, which induces a micro-crack in the adjacent austenite grain when it reaches the α/γ boundary. This allows a supply of hydrogen from the ferrite to the austenite at the crack tip, and particularly along slip planes. Local hydrogen accumulation increases dislocation mobility, as per the HELP mechanism, leading to pile-ups ahead of the enhanced plasticity zone [43]. At a critical stress, a new crack is formed at the pile-up and the hydrogen crack can begin to propagate from the initial micro-crack. The sequence can then start again at the new crack, and this process can be expected to lead to regular changes in crack plane. The crack in Fig. 7c, and some of the cracks in Fig. 7b are examples of cracks that most likely propagated via this proposed mechanism. Claeys [44] also noted that when charging in H 2 SO 4 , probably there is sufficient hydrogen accumulation in austenite to initiate cracks as an alternative starting point for this mechanism in DSS, and cleavage in ferrite is not strictly necessary. ‘Zigzag’ cracks were also observed in austenite. An example of such a crack in an austenite grain in HT 1190 is indicated on Fig. 8a with a white arrow. The shape of this crack can be a consequence of the micro-cracks alternating different slip planes while it propagates [42]. Alternatively, Koyama et al. [45] described hydrogen-assisted cracks in an austenitic steel occurring along primary and secondary twins also resulting in a zigzag pattern. Finally, Fig. 8b shows cracking around inclusions. The inclusions were identified as alumina by EDX, which can originate from the desoxidation step in the steel production process, when Al is added. Laureys et al. [46] found that hydrogen-induced crack initiation was linked to alumina particles in ultra-low carbon steel, and a mechanism for this phenomenon was proposed by Tiegel et al. [47]: as hydrogen accumulates at incoherent particle interfaces leading to interface debonding and vacancy stabilisation, leading to crack initiation. Figure 7 : Secondary cracks on the ND surface of a HT 1110 tensile sample Figure 8 : Secondary cracks on the ND surface of tensile samples: (a) zigzag crack (indicated by the white arrow) in an austenite grain in HT 1190 (b) Hydrogen cracks around alumina inclusions in HT 1110