PSI - Issue 28

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

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3. Pearlitic pseudocolonies affecting macro-structural integrity of cold drawn pearlitic steels The role of pearlitic pseudocolonies promoting anisotropic fracture and crack path deflection in heavily cold drawn pearlitic steel fractured in inert (air) and harsh environment (in particular hydrogen atmosphere) has been studied thoroughly by Toribio (2017b, 2017c, 2019). 3.1. Role of pearlitic pseudocolonies promoting anisotropic fracture in air As reported by Toribio (2019), hot rolled slightly drawn steels behave isotropically, i.e., cracking develops in mode I following the initial plane of fatigue crack propagation (Fig. 3a), while heavily drawn steels exhibit a clearly anisotropic fracture behaviour in the form of crack deflection after the fatigue precrack (and some mode I propagation in certain cases) with a deviation angle of almost 90º from the initial crack plane and further propagation in a direction close to the initial one (Fig. 3b).

Fig. 3. Crack paths (propagation profiles) produced by axial fracture in air environment in steels with 0 (a) and 6 (b) cold-drawing steps; f: fatigue crack growth; I: mode I propagation; II: mixed mode propagation (90º propagation step in heavily drawn steels); F: final fracture. Fig. 4 sketches the micromechanics of fracture in the case of heavily cold drawn pearlitic steel; as explained in previous paragraphs, the pseudocolonies represent weakest units promoting fracture and crack deflection appears when the macroscopic fracture path reaches a pearlitic pseudocolony, that itself fails by shear cracking of pearlite if not previously pre-cracked or pre-damaged as a consequence of the manufacture process by cold drawing.

Fig. 4. Microstructure-based model of fracture in the case of heavily cold drawn pearlitic steel (on the basis of the pearlitic pseudocolony as the weakest fracture unit promoting crack deflection).

3.2. Role of pearlitic pseudocolonies promoting anisotropic fracture in hydrogen assisted cracking (HAC) Toribio (2017c, 2019) studied the hydrogen assisted cracking (HAC) behaviour in cold drawn pearlitic steels. Fig. 5 summarizes the results regarding crack paths (propagation profiles), where a progressively anisotropic HAC behaviour (with increasing crack deflection) is observed.