Stress Corrosion Cracking Behavior And Mechanism Of E690 Steel And Welded Joint In Simulated Seawater

In this work,stress corrosion cracking(SCC)behavior and mechanism of E690 steel and welded joint in simulated seawater were investigated.The SCC and crack growth behavior of E690 steel and welded joint were explored by slow strain rate tensile and cyclic load test.In order to effectively study the SCC behavior of heat affected zone(HAZ),HAZ-simulated microstructures were prepared by heat treatment method according to the real welded joint,then comparative studies of the SCC susceptibility and crack growth behaviors of base metal and HAZ simulated microstructures were conducted.After that,the effect of the cathodic potential on SCC behavior and crack growth behavior of base metal and HAZ simulated microstructures was investigated.Furthermore,the effects of pH value and plastic deformation on SCC behavior of E690 steel were analyzed.The main results are as follows:(1)In simulated seawater,the corrosion current density and SCC susceptibility of E690 welded joint were higher than those of base metal.The crack growth rate(GGR)of welded joint was higher than that of base metal,and the rolling direction impacted the CGR.During the crack growth process,the acidification and the concentration of Cl-happened at the crack tip.And,the pH value reached 4.1 and the concentration of Cl-was about 3.8mol/L at the crack tip of base metal.The crack grew along the low-angle boundary with transgranular fracture,and the crack growth behavior was controlled by hydrogen embrittlement(HE).(2)The SCC susceptibility and CGR of base metal and three kinds of simulated HAZ microstructures were different from each other.The SCC susceptibilities of base metal and simulated HAZ microstructures were different,and coarse grain microstructure exhibited the highest SCC susceptibility and fine grain microstructure showed the lowest SCC susceptibility.During the crack growth process,the acidification and the concentration of Cl-happened at the crack tip of three kind of simulated HAZ microstructures,with the pH value the concentration of Cl-the same to that of base metal.The simulated micro structures showed higher CGR than that of base metal.As the grain size decreased,the SCC mode during the crack growth mode was the combine of transgranular SCC and intergranular SCC.(3)With the negative shift of cathodic potential,E690 steel base metal and simulated HAZ microstructure showed different SCC susceptibilities.When the cathodic potential negatively shifted to-750 mV,SCC susceptibility of base metal and simulated HAZ microstructures increased.With negative shift of the cathodic potential to-850 mV,AD on the surface was inhibited,resulting in the decrease of SCC susceptibility of base metal,however,the SCC susceptibility of simulated HAZ microstructures increased.With the cathodic potential under-950 mV,HE was enhanced,resulting in the increase of the SCC susceptibility of base metal and simulated HAZ microstructures,and the SCC susceptibility of simulated HAZ microstructures was higher than that of base metal.The phenomenon revealed that the HE susceptibility of simulated HAZ microstructures was higher than that of base metal.(4)With the negative shift of cathodic potential,the variation of CGR of E690 steel and coarse grain microstructures was almost the same.As negatively decreasing cathodic potential,pH value of the solution at crack tip gradually increased;the CGR of E690 steel and coarse grain microstructure firstly increased and then decreased,and the CGR was the lowest at-750 mV.Under the cathodic potential,the CGR of coarse grain microstructure was higher than that of base metal,with the crack growth behavior controlled by HE.(5)The variation of simulated seawater pH value affected the crack tip pH value and CGR of E690 steel.When the simulated seawater pH value decreased from 8.2 to 4.0,the open circuit potential gradually decreased outside the crack,which increases the crack tip pH value,resulting in a decrease of CGR.When the simulated seawater pH value continuously decreased to 3.0,the open circuit potential suddenly decreased outside the crack,resulting in a decrease of the crack tip pH value,which further led to an increase of CGR.The crack tip pH value and the CGR were fundamentally affected by the potential outside the crack.(6)During the crack growth process,the dynamic strain was one of main reasons about the acidification at the crack tip of E690 steel in simulated seawater.In the simulated solution according to the solution at the crack tip,plastic deformation enhanced the absorption of hydrogen atoms,resulting in an increase of SCC susceptibility.Therefore,both the accumulation of dislocation and concentration of hydrogen atoms at the crack tip resulted in the crack growth.