Prevention of hydrogen embrittlement is one of the most important issues for the high strength steel structures used in corrosive environment and under gaseous hydrogen since strength grade of the steels has been increased recently which tends to enhances susceptibility to hydrogen cracking. In this paper, we try to describe the hydrogen embrittlement behavior under gaseous hydrogen, the intergranular cracking which is often observed in the high strength steels, from the multi-scale point of view. First, the process of hydrogen diffusion and accumulation in to the fracture point was analyzed by the numerical simulation based on the Fick's diffusion theory with the alpha multiplication method which multiplies the stress gradient induced terms. Hydrogen segregation into the grain boundary is the next step to the cracking. We conducted hydrogen micro printing analysis to visualize the hydrogen existence in the steel from the microscopic view point. A large scale molecular dynamic simulation was then conducted to investigate the atomistic behavior leading to the grain boundary cracking.
It is well understood that solute hydrogen in steel causes degradation of material properties, so called hydrogen embrittlement. Hydrogen has a strong effect on many aspects of mechanical properties such as deterioration of ductility and toughness (Candler, 1974; Wang, 2005; Wada, 2007). Acceleration of fatigue crack growth occurs under the pressurized hydrogen environment (Taketomi, 2008; Murakami, 2010; San Marchi, 2010; Matsuoka, 2011). Weld cold cracking is caused by the hydrogen introduced into the steel weld from the atmosphere and weld consumables (Suzuki, 1979). It is also known that steels with higher tensile strength are more sensitive to hydrogen embrittlement (Matsuyama, 1979). Fracture strength deteriorates, and many high strength steels and steel welds show intergranular fracture under the effect of hydrogen (Wang, 2005; Matsuda, 1996, Ishikawa, 2011; Ishikawa, 2015).
Several mechanisms have been proposed to explain the behavior of materials that leads to brittle or ductile fracture by hydrogen embrittlement, such as
the hydrogen enhanced decohesion mechanism, HEDE (Oriani, 1987; Yamaguchi, 2012),
the hydrogen enhanced localized plasticity, HELP (Birnbaum, 1994; Sofronis, 1996; Robertson, 2001) and
the hydrogen enhanced strain-induced vacancy model, HESIV. (Nagumo, 2001, 2004)
These mechanisms describe the interaction between hydrogen atoms and the grain boundary, dislocation and vacancies which enhances intergranular fracture, fatigue and ductile fracture. HEDE mechanism may well explain the intergranular fracture shown in high strength steels, which exhibits cracking along the prior austenite grain boundary. However, many investigations showed clear evidence of plastic deformation in the fractured surface (Wada, 2005; Martin, 2011; Nagao, 2012), and dislocation movement might be enhanced in the region close to the grain boundary. Interaction between dislocation movement and hydrogen may activate vacancy formation and stabilization, which may leads to micro voids and ductile fracture. (Nagumo, 2001, 2004)
