PSI - Issue 59

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

105

2

1. Introduction Hydrogen transport in a metallic material takes place either by lattice diffusion or by dislocational dragging . With regard to lattice diffusion , it is usually assisted by the stress field in the material, and specifically by the trace of the stress tensor, i.e., by the hydrostatic stress term, so that it is properly stress-assisted diffusion of hydrogen (Van Leeuwen, 1974). When also the plastic strain distribution is supposed to affect hydrogen transport by diffusion, then it is named stress-and-strain assisted diffusion of hydrogen (Toribio and Kharin, 2015). In the matter of dilocational dragging , hydrogen transport by dislocations is another possible mechanism to be considered in metals and alloys, as described by Tien et al. (1976) and by Dadfarnia et al. (2015). Another representative macroscopic variable should be implemented if one considers hydrogen transport by dislocations. This process is evidently associated with the mean velocity of dislocations, which can be expressed in macroscopic terms through the plastic strain rate. The two transport modes — diffusional and dislocational — are essentially different. The first one is operative under both sustained and transient (time-dependent) stress-strain states, has the instantaneous plastic strain as one of the responsible variables, and evolves towards equilibrium hydrogen distributions. In contrast, the other one proceeds exclusively during continuing (dynamic) straining, has the plastic strain rate as the governing variable, and results in non-equilibrium hydrogen distributions (temporal localized over-saturations). They are fed by newly arriving dislocations that drive hydrogen into specific microstructural sites, from which hydrogen escapes by diffusion to restore thermodynamic equilibrium with local surroundings. Quickly after the end of straining, those hydrogen over-saturations completely relax to local equilibrium by short-range (local) diffusion. The efficiency of this mode of hydrogen accumulation in prospective damage sites results from competition between these two effects. Under sustained or quasi-static loading conditions, local over-saturations in damage sites created due to hydrogen supply by dislocations have time to be eliminated by short-range diffusion, and thus the significance of dislocational transport seems to be negligible in this case. Then the long-range diffusion driven by macroscopic stress-strain field is the main operative mode of hydrogen transport to fracture nuclei in metals. This transport step is frequently the slowest one among all the phases of hydrogen transportation, and thus the kinetics of hydrogen assisted cracking (HAC) is often diffusion controlled. This paper tries to elucidate the main hydrogen transport mechanism in pearlitic steels on the basis of the performance of stress corrosion cracking (SCC) tests on pre-cracked specimens in which a pre-crack must be produced by fatigue, thereby creating compressive residual stresses in the vicinity of the crack tip. These stresses are generated by strain compatibility in the subsequent plastic zone and may play a relevant role in the overall SCC process in general (and HAC in particular). The mechanical aspects of the phenomenon are analyzed and the extension of the plastic zone compared with the size of the environmentally-assisted micro-damage region. Thus the paper offers a combined micro -and macro approach to the phenomenon which is original and innovative from the materials science point of view, since microscopic modes of fracture associated with SCC or HAC ( micro-approach ) are related to stress state and plastic zone evolution ( macro-approach in the continuum mechanics sense) that are calculated from the Rices’s model. 2. Experimental programme This section analyzes the consequences of cyclic residual stresses in the development of SCC. Emphasis is placed on the effect of the maximum stress intensity factor (SIF) during the last stage of fatigue pre-cracking (K max ) on the posterior SCC behaviour of the material. To this end, four different pre-cracking schemes were designed with levels K max / K IC = 0.28, 0.45, 0.60 and 0.80, where K IC is the fracture toughness The SCC experiments were slow strain rate tests (SSRT) in aqueous solution (Toribio, 1993; Parkins, 1993) with samples of hot-rolled high-strength steel previously pre-cracked as described elsewhere (Toribio and Lancha, 1993; 1996). Tables 1 and 2 offer respectively the chemical composition and the mechanical properties of the steel. Susceptibility to SCC was measured through the ratio of the failure load in aggressive environment to that in air, as recommended by the ISO Standard (ISO 7539-6, 1989; ISO 7539-7, 1989).