Study Of Hydrogen Embrittlement Susceptibility Of AISI304 Stainless Steel Welded Joint

The microstructure of weld metal is usually distinct from that of base metal due to the thermal cycling condition,which affects the susceptibility of joint to hydrogen embrittlement.In this study,we investigated the effects of hydrogen on the tensile properties of weld metal and base metal in AISI304 stainless steel welded joint.We also researched the influence of hydrogen content and pre-strain on the failure mode and failure location of hydrogen-charged joint and discussed the failure mechanism.The results of the properties of hydrogen embrittlement of weld metal and base metal showed that hydrogen lead to a decrease in ductility and a increase in flow stress of the weld metal with a quasi-cleavage fracture surface in the brittle zone,which resulted from the fact that hydrogen induced local high density dislocation and enhanced mobility of dislocations.For base metal,hydrogen lead to a severer lose of macroscopic ductility and a slight decrease in flow stress and ultimate tensile strength.Depth of brittle zone increased and fracture surface in the brittle zone showed a cleavage fracture feature.Severe stress-induced??martensitic transformation occurred in base metal and phase boundary may extra provide the gathering sites of hydrogen and dislocation,resulting in more serious deterioration of tensile properties.The results of the effects of hydrogen content on the hydrogen embrittlement failure mechanism of joint showed that hydrogen exerted a dramatic influence on macroscopic ductility and ultimate tensile strength,and this influence depended on the hydrogen content.The hydrogen content in samples increased with increasing current density,which resulted in severe hydrogen embrittlement and higher flow stress.All samples fractured in the weld metal.The increasing current density from 10 mA/cm~2to 100 mA/cm~2 changed the failure mechanism in the brittle zone from quasi-cleavage or cleavage fracture to intergranular fracture.It is supposed that at low current density,hydrogen atoms uniformly distribute in the interior of grain;at high current density,stress-induced??martensite forms preferentially at the grain boundary which undergoes severe segregation of hydrogen atoms and intergranular fracture occurs due to severe strain localization at grain boundary.The results of the effects of pre-strain on the transition of hydrogen embrittlement failure location of joint showed as the pre-strain increased,the location of the fracture moved from the weld metal to the base metal and this was attributed to the combined influence of the hydrogen concentration and deformation microstructures during pre-straining,i.e.dislocations,??martensite,and deformation twins.At low pre-strain(<15%),the crack occurred at the site of dislocation pile-up,which indicated that the failure of the weld metal was dominated by the HELP mechanism.At high pre-strain(>15%),the base metal underwent severe stress-induced??martensitic transformation and cracking occurred at the sites of local enrichment of hydrogen atoms,i.e.phase boundaries or deformation twin boundaries,which indicated that the failure of the base metal was dominated by the HEDE mechanism.At a pre-strain<15%,strain hardening was dominant,and the flow stress and ultimate tensile strength of hydrogen-charged samples were higher than those of the hydrogen-free samples.At a pre-strain>15%,the HEDE mechanism was dominant and the flow stress and ultimate tensile strength of hydrogen-charged samples were lower than those of the hydrogen-free samples.