The Microstructure And Stress Corrosion Cracking Behavior Study Of Nuclear Plant 304 Stainless Steel Welded Joints

With the continuous development of society,the demand of energy is also increasing.The traditional energy such as coal,oil belong to nonrenewable resources,over exploitation and depletion will cause irreparable damage.And the combustion of traditional energy will emit a large amount of pollution gas,causing damage to the environment.As a new type of clean energy,nuclear energy has been paid more and more attention.However,nuclear power safety is a potential threat to human life and property safety.In many nuclear power accidents,stress corrosion cracking(SCC)is one of the main failure forms of nuclear power structures.Therefore,this paper studies the structure of 304 stainless steel(304SS)welded joints and their stress corrosion cracking behavior characteristic,as the basis of the mechanism research and protection.Firstly,the chemical composition,microstructure,phase,hardness,tensile properties and corrosion resistance of the materials were studied,pave the way for the stress corrosion cracking study behind.The experimental results show that the microstructure of the shielded metal arc welding(SMAW)welded joint is similar to that of the gas tungsten arc welding(GTAW)welded joint,but the grain size and distribution is different.XRD and energy spectra show that the weld zone does not generate harmful phase,and the elements are not shifted,which will not affect the SCC behavior.The hardness of the SMAW welded joint is higer than that of GTAW welded joint,but the tensile life of the SMAW welded joint in air is lower than that of the GTAW welded joint.The electrochemical experiment results show that the corrosion resistance of the GTAW welded joint is the worst and the 304 SS is the best.The SCC behavior in 4mol/L NaCl+0.01mol/L Na2S2O3(Cl-)and 0.5mol/L NaCl+1.5mol/L H2SO4(H+)solution was investigated by using electrochemical noise and acoustic emission.The results show that the SCC behavior of the two kings of welded joints is similar because of the similar microstructure.But the corrosion rate and SCC susceptibility are different due to the different grain size and distribution.In the Cl-solution,the SCC behavior include: in the early stretch,the passivation film rupture and repair;in the platic deformation,the pitting initiative;with stretch conduct,pitting extend to crack,and a large number of small cracks are connected to each other until fracture.In the H+ solution,the early behavior of SCC is similar to that of Cl-solution.But in the later stage of the tension,the corrosion type is changed from the local to the uniform.Electrochemical noise and acoustic emission can be used to characterize the whole process.Wavelet fractal dimension and acoustic emission clustering results is consistent on the corrosion morphology.The observation of the failure site by scanning electron microscope(SEM)and optical microscope show that the cracking form of the welded joints is related to the microstructure.The metallographic photos and the SEM photos can be a good correspondence,reflecting the relationship between the failure form and the microstructure.