PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2438–2443 Jesús Toribio / Procedia Structural Integrity 00 (2020) 000–000

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5. Micromechanics of hydrogen assisted cracking in the cold drawn material 5.1. Material anisotropy

In this section a simple composite approach is used to model the oriented microstructure of the cold drawn wire. Two approaches may be followed: a 1D-approach in the form of a fiber-reinforced composite to reproduce the alignment and a 2D-approach in the form of a laminate to account for the plated microstructure. Such a microstructural arrangement has consequences of both a mechanical and a chemical nature, namely: (i) Strength anisotropy (mechanical anisotropy) , i.e., fracture toughness K IC (or similar strength parameter) as a function of the orientation angle: K IC = K IC (  ), where  is the common fracture mechanics angle between the specific direction under consideration and the original plane of the macroscopic mode I crack which itself is perpendicular to the wire axis or main direction of the fiber alignment in the afore-mentioned composite approach. In particular, the fracture toughness values in direction perpendicular and parallel to the fibers are respectively: K IC  = K IC (  0º) for fibers fracture (transverse fracture) (1) K IC  = K IC (  0º) for longitudinal splitting (2) the former is the fracture toughness for breaking the fibers in the 1D model (fracture of the plates in the 2D model or the lamellae in the real material), whereas the latter is the fracture toughness for delamination or, generally speaking, debonding of any microstructural elements at any scale. Considering microstructural reasons, it may be assumed that K IC  is clearly higher than K IC  , cf. Fig. 1: K IC  >> K IC  (3) (ii) Chemical anisotropy , i.e., hydrogen diffusion coefficient D as a function of the orientation angle: D = D (  ),  having the same meaning as above. This coefficient is a key item in HAC processes in pearlitic steels where stress-assisted diffusion has been shown to be the main hydrogen transport mechanism (Toribio, 1996). In particular, the diffusion coefficients in direction perpendicular and parallel to the fibers are respectively: D  = D (  0º) diffusion in the transverse (crack) direction (4) D  = D (  90º) diffusion in the longitudinal direction (5) The former corresponds to diffusion in the crack direction, perpendicular to the fibers (or lamellae or plates, according to the modelling), and the latter is associated with diffusion parallel to the fibers. Considering again microstructural reasons, D  is clearly higher than D  , i.e. D  >> D  (6) which indicates that hydrogen tends to diffuse in the direction  =90, cf. Fig. 5a. 5.2. Micromechanics of HAC In the matter of HAC, the proposed mechanism is hydrogen-enhanced delamination (HEDE), cf. Fig. 5b. The lamellar structure of the steel (markedly oriented) which 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 (Fig. 5b).