Effect Of Residual Austenite In Micro/Nano-Structured Bainitic Steels On Electrochemical And Stress Corrosion Behavior

Micro-nano structured bainitic steels generally have a duplex structure consisting of a bainitic ferrite(BCC structure)matrix and residual austenite(FCC structure),in which residual austenite is a key factor in structural regulation and plays an important role in the mechanical properties and corrosion resistance of steels.In this thesis,based on alloy design of medium carbon,high silicon,and high manganese,through different heat treatment processes and pre-strain conditions(0.75/1/1.25σ_s),the quantity,morphology and stability of retained austenite in steel were regulated.The effects of retained austenite on the electrochemical and stress corrosion behavior of micro/nano structure bainitic steel were studied by using fine characterization methods(SEM/EDS,TEM,XRD,EBSD),electrochemical testing and slow strain rate stress corrosion test.The main results show that the two-step bainite process reduces the content of retained austenite,increases the carbon content of retained austenite,refines the structure,and improves the elongation and toughness of the test steel.The residual austenite in the pre-strained steel undergoes stress-induced martensite transformation,and the dislocation density,dislocation strengthening and transformation strengthening increase the strength of the pre-strained steel.In chloride environment,the corrosion current density and corrosion rate of two-step bainitic steel are lower than that of one-step bainitic steel.With the increase of pre-strain loading,the corrosion current density and corrosion rate of the test steel increase.After pre-straining,on the one hand,deformation induced martensite transformation occurs in austenite,which leads to stress concentration and increases the sensitivity of micro galvanic corrosion.On the other hand,the increase of dislocation density of the test steel increases the number of active site on the surface of the test steel and enhances the corrosion reaction.The SCC crack initiation of conventional steel in seawater environment is mainly related to anodic dissolution,which is mainly affected by micro galvanic corrosion and stress concentration caused by martensite transformation.When the pre-strain load is low,the stress concentration caused by phase transformation during the tensile process of pre-strain steel is less,the anodic dissolution is weakened,and the sensitivity to stress corrosion decreases.When the pre-strain load is high,the surface activity of the matrix with slip bands and dislocation accumulation is high,making it more prone to corrosion and becoming the source of crack initiation,increasing its stress corrosion sensitivity.