Cracking Process Related to Hydrogen Behavior in a Duplex Stainless Steel

Cracking process in hydrogen embrittlement (HE) of a duplex stainless steel (DSS) was investigated. Annealed DSS (SUS329J4L) specimens with phase volume fraction of about 50:50 were electrolytically hydrogen-charged and deformed in ambient atmosphere at a strain rate of 1.38x10(-4)s(-1). Microcracks were observed mostly to start and pass in the ferrite phase in the course of the deformation. In contrast, austenite phase acted as an obstacle against crack propagation. Delamination was also observed in accord with the authors' previous study, and the delamination crack also initiated and propagated in ferrite phase. Hydrogen microprint technique (HMPT) revealed that hydrogen atoms migrate mainly in ferrite phase over the distance of the sample thickness. Considering this long diffusion distance of hydrogen, dimpled area observed around the center of the fracture surface was attributable to the sharp increase in the strain rate because of the localized deformation arising from the major crack propagation. HMPT also revealed a marked effect of elastic stress on the acceleration of hydrogen diffusion. Thermal desorption spectroscopy (TDS) confirmed that some of the hydrogen diffuses out during keeping the specimen in the ambient atmosphere, which was accelerated by deformation during keeping presumably by the mechanism of hydrogen transport with gliding dislocations. Furthermore, TDS results demonstrated that the majority of the hydrogen migrated and was trapped by a site with lower binding energy during the deformation.