PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 104–111 Jesús Toribio / Procedia Structural Integrity 00 (2024) 000 – 000

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3

Table 1. Chemical composition (wt %) of the steel. C Mn Si P S

Cr

Ni

Mo

0.74

0.70

0.20

0.016

0.023

0.01

0.01

0.001

Table 2. Mechanical properties of the steel. Young's Modulus E (GPa) Yield Strength  Y (MPa) UTS  R (MPa)

Fracture Toughness K IC (MPam 1/2 )

Ramberg-Osgood parameters*  p < 1.07  p > 1.07 P I (MPa) n I P II (MPa)

n II 17

195

725

1300

53

2120

5.8

2160

* P,n: Ramberg-Osgood Parameters  =  e +  p =  / E +(  / P ) n

There are two regions of potential in which the effect of the environment reduces the fracture load: the highest potentials correspond to the anodic regime, and the mechanism is localized anodic dissolution (LAD), whereas the lowest potentials are related to the cathodic regime, and the mechanism is hydrogen assisted cracking (HAC). Fig. 1 shows the experimental results in a plot representing failure load in solution Fc (divided by the reference value in air Fo) as a function of the maximum stress intensity factor (SIF) during the last stage of fatigue pre-cracking K max (divided by the fracture toughness K IC ). For both LAD and HAC, the susceptibility to the environment decreases (i.e., the fracture load increases) as the maximum fatigue load increases.

(a)

(b)

Fig. 1. Experimental results of the SSRT: (a) anodic regime of LAD ( – 400 mV vs. SCE); (b) cathodic regime of HAC ( – 1200 mV vs. SCE).

Fractographic analysis by scanning electron microscopy (SEM) showed the existence of a transition topography between the fatigue pre-crack and the cleavage-like (unstable) fracture mode. For the anodic regime of LAD, such a topography is a sort of damage area ahead of the crack tip (Fig. 2) and appears only in the most severe fatigue pre cracking programme (K max = 0.80 K IC ). For the cathodic regime of HAC it is the tearing topography surface or TTS (Thompson and Chesnutt, 1979; Costa and Thompson, 1982) in the vicinity of the crack tip (Fig. 2) that appears in all fatigue pre-cracking programmes, although its size is a decreasing function of the K max -level, as shown in Fig. 3. The TTS zone is associated with hydrogen-assisted micro-damage (HAMD) in pearlitic lamellar microstructures (Toribio et al., 1991a; 1991b; 1992; Toribio and Vasseur, 1997; Toribio, 1997; 2012) so that it represents the experimental evidence of hydrogen effects at the micro- and nano-levels (it is micro-tearing or even nano-tearing at the finest sub-micron level).