Evaluation of Hydrogen Embrittlement Type SCC in Duplex Stainless Steel Welded Joint and It's Mechanism

Abstract

The HESCC initiation time was predicted by numerical analysis method. In the analysis, the HESCC initiation condition was determined by the correlation between hydrostatic stress and hydrogen activity in ferritic phase near the tip of austenitic phase on specimen surface. It was confirmed that the criterion for HESCC initiation could be defined in low yield strength steel such as duplex stainless steels as same as high strength steels. In addition to that, the mechanism for HESCC in duplex stainless steels and its welded joints was studied by fractographic observation with facet pit method. As the results, it was found that ferritic phase fractured in quasi-cleavage mode and austenitic phase in ductile mode. From the morphologies of facet pits formed on ferritic phase, it was clarified that the fracture planes in ferritic phases were mainly constituted of {100} plane in lower stress intensity region, and {110}, {112} plane in higher one. Consequently, the mechanisms for HESCC in duplex stainless steels contained lattice decohesion at lower stress intensity and fracture on slip plane at high stress intensity. From the view point of mechanism for HESCC, it was clear that the criterion for HESCC initiation mentioned above is also applicable for a fracture under the low stress intensity region.