PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 131–136 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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3. Pearlite microstructure changes during cold drawing The pearlite microstructural evolution during drawing (cf. Fig. 2) was studied by Toribio and Ovejero (1997, 1998a, 1998b, 1998c), showing the following trends: (i) slenderizing of the colonies (Toribio and Ovejero, 1997); (ii) decrease of interlamellar spacing (Toribio and Ovejero, 1998a); (iii) orientation in the direction of cold drawing (wire axis) of both the colonies (Toribio and Ovejero, 1998b) and the lamellae (Toribio and Ovejero, 1998c), together with the appearance of pearlitic pseudocolonies (Toribio et al., 1997).

Fig. 2. Microstructures of progressively cold-drawn pearlitic steels (in longitudinal sections) for steels that have undergone 0 (left) and 6 (right) steps of cold drawing. Vertical side of the micrograph is always parallel to the wire axis or drawing direction. 4. Hydrogen effects on cold-drawn pearlitic steels Previous research by Toribio et al. (1991, 1992), Toribio and Vasseur (1997) and Toribio (1997, 2012) on HD/HE of hot-rolled pearlitic steels showed that degradation at the micro-scale (HAMD) in pearlite microstructures takes place in the form of a hydrogen damage topography (HDT). In hot-rolled and slightly drawn steels, it is a new fractographic mode: the tearing topography surface (TTS), as coined by Thompson and Chesnutt (1979) and Costa and Thompson (1982), consisting mainly of micro-tearing at the finest microscopic level, Fig. 3 (left).

Fig. 3. Hydrogen-assisted micro-damage (HAMD): (left) tearing topography surface (TTS) and HDT (4) in the form of EOTTS (right).

The extension of the TTS zone is an indicator of the HAMD, and therefore allows the clarification of the fact that the main mechanism of hydrogen transport in pearlitic steel takes place by diffusion, as demonstrated in earlier research works by Toribio (1992c, 1993, 1996), it being mainly driven by the gradient of hydrostatic stress, as formulated in the stress-assisted hydrogen-diffusion models proposed by Toribio and Kharin (1997, 2000).