PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2404–2409 Jesús Toribio / Procedia Structural Integ ity 00 (2020) 000–0 0

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Fig. 5. Crack paths (propagation profiles) produced by hydrogen assisted cracking (HAC) in steels with 0 (a), 3 (b) and 6 (c) cold-drawing steps; f: fatigue crack growth; I: mode I propagation; II: mixed mode propagation (propagation step in heavily drawn steels); F: final fracture. Fig. 6 sketches the micromechanics of HAC in the case of heavily cold drawn pearlitic steel. The lamellar structure of the steel (markedly oriented) produces anisotropy regarding fracture and hydrogen diffusion, so that hydrogen diffuses mainly in the direction of the plates and can weaken the bonds or interfaces between the ferrite and the cementite lamellae (which are the weakest links even before the hydrogen presence) thus contributing to the hydrogen-induced fracture by delamination or debonding between two similar microstructural units, i.e, at the ferrite/cementite interface or at the pearlitic colony boundaries, as sketched in Fig. 6. Thus the micromechanism of HAC is hydrogen-enhanced decohesion (HEDE) in the cold drawn wire (strongly anisotropic).

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(a) (b) Fig. 6. Micromechanism of HAC: (a) hydrogen diffusion in longitudinal and transverse directions, (b) fracture by hydrogen enhanced delamination or debonding (HEDE). 3.3. Role of pearlitic pseudocolonies promoting anisotropic fracture in localized anodic dissolution (LAD) Toribio (2017b, 2019) studied the localized anodic dissolution (LAD) behaviour in cold drawn pearlitic steels. Fig. 7 summarizes the results regarding crack paths (propagation profiles), where a progressively anisotropic LAD behaviour (with increasing crack deflection) is observed.

Fig. 7. Crack paths (propagation profiles) produced by localized anodic dissolution (LAD) in steels with 0 (a), 3 (b) and 6 (c) cold-drawing steps; f: fatigue crack growth; I: mode I propagation; II: mixed mode propagation (propagation step in heavily drawn steels); F: final fracture.