Microstructure And Corrosion Behaviors Of A Welding Fusion Boundary Region At Low Alloy Steel Side In A Safe End Of Nuclear Power Station

In nuclear power plants,metal dissimilar welding connects most of the components and the main pipes.The stress corrosion cracking of dissimilar metal weldments in the actual operation process poses a challenge to the safe operation of the nuclear power plant.Therefore,a systematic study on the microstructure and corrosion behavior of dissimilar metal weldments can deepen the understanding of dissimilar metal weldments,provide reference for the improvement of weldments' performance,and provide data support for the ability to resist stress corrosion crack growth of weldments.In this paper,the welding fusion boundary region between A508? low alloy steel and Alloy 82 at the safe end of pressure vessel in nuclear power plant is taken as the research object.The parts of weldment are analyzed by means of optical microscope,laser confocal microscope,scanning electron microscope and its energy spectrum,back scattering electron diffraction,transmission electron microscope and its energy spectrum,Raman spectrum,electrochemical potentiodynamic polarization curve,etc The microstructure,local corrosion and uniform corrosion at room temperature and oxidation in simulated nuclear power environment were studied.1.According to the sequence from the base metal of low alloy steel to the weld metal,the welding joints are divided into base metal of low alloy steel,fine grain heat affected zone,coarse grain heat affected zone,fusion boundary and weld metal.Among them,the low alloy steel has uniform bainite structure,the fine-grained heat affected zone has fine bainite shape,the coarse-grained heat affected zone has coarse bainite grain and a large number of granular bainite are distributed on its original austenite grain boundary.The weld metal grows perpendicular to the fusion boundary.There are two types of fusion boundaries-Martensitic fusion boundary and sharp fusion boundary,and the two fusion boundaries are alternately distributed.Among them,the martensitic fusion boundary and its adjacent structures are caused by the relatively gentle composition change perpendicular to the fusion boundary;in addition,the residual stress caused by welding also promotes the formation of martensite,while the sharp fusion boundary and its adjacent structures are caused by the relatively abrupt composition change perpendicular to the fusion boundary.Because of the existence of martensite,the hardness of martensite fusion boundary is much higher than that of sharp fusion boundary.2.Al2O3,MnS and Si rich inclusions in a508? low alloy steel are easy to pitting.The heat affected zone near the fusion boundary of a508? low alloy steel in weldment has higher corrosion rate than that of its base metal in 3.5%NaCl solution.The pit density and maximum depth near the martensitic fusion boundary are much higher than those near the sharp fusion boundary.Pitting did not initiate on martensitic phase.The results show that the number of pitting pits of a508? low alloy steel in 3.5%NaCl solution is small,the depth is low,and the diameter is large.3.The corrosion rate of low alloy steel increases with the increase of temperature in deaerated nuclear solution,aerated nuclear solution and aerated concentrated nuclear solution.The corrosion of low alloy steel in deaerated and concentrated solution is mainly controlled by H+ reduction.In the deaerated solution,the corrosion rate of low alloy steel increases linearly with the increase of temperature,but due to the limited content of H+,the trend of corrosion rate increase is slow.In the concentrated solution,sufficienet H+ leads to the increase of corrosion rate of low alloy steel with the increase of temperature.In the aerated solution,the corrosion of low alloy steel is controlled by the oxygen reduction process.The sufficient dissolved oxygen makes the corrosion rate of low alloy steel in the solution significantly higher than that in the deareted solution.The increase of temperature accelerates the oxygen diffusion process,resulting in the increase of corrosion rate of low alloy steel.In all cases,the corrosion rate of the low alloy steel close to the fusion boundary is significantly higher than that of the base metal of the low alloy steel,which is mainly caused by the inhomogeneity of the structure in the heat affected zone and the galvanic corrosion between it and the adjacent nickel base alloy weld metal.4.The oxide film formed in the dilution zone of weld metal changes with the type of fusion boundary.Among them,the oxide film formed in the dilution zone next to the martensite fusion boundary is a double-layer film with spinel type single crystal oxide as the inner layer and small spinel type oxide particles as the outer layer.The oxidation rate in this zone is relatively low.The formation of the oxide film is because of the low chromium content in the matrix,which results in a small amount of oxide nucleation in this area at the early stage of oxidation.In the subsequent process due to the galvanic corrosion effect from the adjacent martensite,the small amount of oxide slowly grows into single crystal as the inner layer of the oxide film.The reason for the low oxidation rate in this region is the protection of martensite and the stability of iron rich oxides in this environment.The oxide film formed in the dilution zone next to the sharp fusion boundary is a double-layer film with nano chromium rich oxide as the inner layer and large spinel oxide as the outer layer.