PSI - Issue 5

Jesús Toribio et al. / Procedia Structural Integrity 5 (2017) 1446–1453 Jesús Toribio / Structural Integrity Procedia 00 (2017) 000 – 000

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lamellae (Toribio and Ovejero, 1998c). In addition, a new (non-conventional) micro-unit, the pearlitic pseudocolony was identified, denoted and described by Toribio et al. (1997). It is a special colony in which the lamellae are not oriented along the wire axis or cold drawing direction, thus producing an anomalous (extremely high) interlamellar spacing, and making it the weakest zone or potential fracture initiation unit, able to produce fracture path deflection. Toribio et al. (2016) have recently published a systematic compendium on microstructural evolution in cold-drawn pearltic steels. Fig. 2 shows microstructures for different drawing degrees.

Fig. 2. Microstructures of progressively cold-drawn pearlitic steels (in longitudinal sections) for steels that have undergone 0 (left) 3 (middle) 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) on HD/HE of hot-rolled pèarlitic steels (not cold-drawn at all) showed that degradation at the micro-scale (HAMD) in pearlite-based microstructures takes place in the form of a hydrogen damage topography (HDT). In hot-rolled and slightly drawn steels, it is a non-conventional fractographic mode: the so-called tearing topography surface (TTS), as identified by Thompson and Chesnutt (1979) and Costa and Thompson (1982), consisting mainly of micro-tearing at the finest microscopic level, as shown in Fig. 3. Toribio and Vasseur (1997) went further in the analysis and analyzed the evolution of HAMD, describing another type of it (another sub-mode of cracking enhanced by hydrogen): a kind of microscopic mode of fracture resembling micro-void coalescence (MVC), a candidate to TTS (or ‘‘future’’ TTS zone) which does not reach s uch a condition because of an insufficient level of hydrogenation. Toribio (1997) performed a fracture mechanics approach to HAMD in pearlitic steel, demonstrating that the extension (progress) of this micro-damage in cracked specimens can be modeled as a macroscopic crack prolonging the original fatigue precrack and involving linear elastic fracture mechanics (LEFM) principles. In this case, the change from HAMD in the form of TTS to brittle cleavage-like topography takes place when a critical stress intensity factor ( K H ) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue precrack plus the TTS area). More recently, Toribio (2012) reviewed previous analyses and denoted the transition zone between the TTS and the conventional cleavage-like (fast brittle fracture without any need of environmental assistance) as quasi-MVC, describing the sequence of HAMD as TTS, quasi-MVC and final fracture by cleavage, cf. Fig. 4.