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

As a high-quality domestic offshore engineering steel,E690 steel is widely used in offshore equipment.Welding is one of the main connection methods of offshore equipment.Stress corrosion cracking(SCC)often occurred in welding heat affected zone(HAZ).Nb microalloying is widely used to improve the mechanical properties of marine steel,but the mechanism of its effect on SCC behavior is still unclear.In this work,SCC behavior and mechanism of Nb-microalloyed E690 steel and welded joint in simulated seawater were investigated.In order to effectively study the SCC behavior of HAZ,simulated HAZ microstructures were prepared by heat treatment method according to the real welded joint.The existing form of Nb in the HAZ and its influence on the mechanical properties of the welded joint were investigated.Then the influence of Nb on the corrosion behavior of HAZ was studied.After that,the SCC behaviors of real welded joints and simulated HAZ under open circuit potential and different cathodic protection potential were studied respectively.Furthermore,the influence of Nb in sulfur containing solution on the electrochemical behavior and SCC behavior of E690 welding heat affected zone was analyzed.The results show that:Compared with the Nb free steel,the micro-alloying of Nb results in the grain refinement of the HAZ of E690 steel,and the plastic toughness is improved.Nb in coarse grain HAZ(CGHAZ)exists in the matrix in the form of solid solution atoms,which will drag the grain boundary and hinder the movement of the grain boundary,thus refining the grain and strengthening the fine grain.Part of Nb in fine grain HAZ(FGHAZ)exists in the matrix in the form of solid solution,and the remaining part will combine with C to generate NbC nano precipitates.The solution Nb atoms will hinder the movement of grain boundaries.The nanosized NbC precipitates will pin the grain boundaries,inhibiting the growth of grains.The strength of steels was improved by fine-grained strengthening and precipitation strengthening.In intercritical HAZ(ICHAZ),almost no Nb can be dissolved in the primary austenite.The added Nb mainly exists in the form of NbC precipitates with an average size of no more than 10 nm.The grain boundaries can be pinned to refine the grains,and the strength can be improved by fine-grained strengthening and precipitation strengthening.Nb microalloying reduces the corrosion rate of the base metal(BM)and HAZs of E690 steel.Nb dissolved in the material or in the form of precipitates will not accelerate the corrosion process at the initial stage of corrosion.However,in the later stage of corrosion,Nb microalloying makes the rust layer more compact,thus improving the corrosion resistance of E690 steel in simulated seawater environment.In the tensile process at different cathodic potentials in simulated seawater,the SCC sensitivity was the lowest at-850 mV.Because the critical zone is a softening structure with the lowest yield strength,the fracture positions of the two real welded joints of E690 steel with or without Nb microalloying are in the critical zone.The yield strength of the critical area is the lowest.Under the external forces,plastic deformation takes priority.A large number of dislocations are generated at the necking position.Dislocations are easy to accumulate at the interface between M/A island and metal matrix.Especially,under a relatively negative cathodic potential,a large number of hydrogen atoms enter the material,and hydrogen gathers around the M/A island along with the movement of dislocations,accelerating the initiation and propagation of cracks.In addition,the SCC behavior of the HAZ of E690 steel is obviously affected by the microstructure.Nb microalloying reduces the SCC sensitivity by increasing the proportion of small angle grain boundaries,reducing grain size,and forming precipitation,but does not affect its SCC mechanism.Nb reduces the SCC sensitivity of E690 steel BM and welding HAZs in sulfurcontaining seawater.The SCC behavior of E690 steel in sulfur-containing seawater is controlled by the mixed mechanism of anodic dissolution and hydrogen embrittlement.The primary austenite grain boundary,bainite lath boundary and M/A island in the steel are the priority locations for crack initiation.The enrichment of SO42-in the corrosion products will promote the SCC crack initiation and propagation.NbC precipitates can inhibit the diffusion of hydrogen,reduce the local accumulation of hydrogen,and weaken the hydrogen embrittlement effect.Moreover,the solid-solution Nb and NbC precipitates can make the structure more uniform and reduce the internal stress,which inhibits the anodic dissolution process.These two effects together reduce the SCC sensitivity of the welding heat affected zone.