Microstructure Of Dissimilar Metal Weldments And Its Effect On High Temperature Oxidation And Room Temperature Fracture Behavior

With the development of society,nuclear energy has become an important means to solve the global energy crisis.The dissimilar metal welding joint safe end is an important part of nuclear power plant cooling water loop,their safety is a key factor for the safety of nuclear power plant.Therefore,studying the microstructure of dissimilar metal weldments and their effects on the deformation behavior and high temperature oxidation mechanism are great significance for prolonging the life of welded joints and improving the safety of nuclear power.In this paper,the dissimilar welding welding safe end 316L stainless steel/Inconel 182 was used as the research object.The microstructure of the welding joint was characterized,the room temperature corrosion and high temperature oxidation behavior of welding were studied.The optical microscope(OM),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and transmission electron microscopy(TEM)were used to characterize the oxidation behavior and oxidation mechanism of DMWJ under high temperature water.The relationship between microstructure and oxidation mechanism of different parts of DMWJ under high temperature and high pressure water were also been proposed;meanwhile,the microstructure dependence of crack initiation and propagation was investigated by using in-situ techniques,the microstructure of welding joint during the tensile test were figured out,and relationship between microstructure and the mechanism of crack initiation and propagation were proposed.The main results obtained are listed as follows:The DMWJ exhibited a significant microstructure inhomogeneity.The microstructures of the base metal and the heat affected zone were constituted by the equiaxed austenite with twins.The microstructure of the Inconel 182 weld metal was dendrite austenite,and the growth direction of dendrite grains was perpendicular to the fusion boundary along the thermal gradient direction.There were two alternately distributed typical fusion boundaries,one was sharp fusion boundary(named Type-A fusion boundary),weld metal and base metal were separated by fusion boundary directly,another one was a broad boundary with an epitaxial growth structure(named Type-B fusion boundary),the weld metal and the base metal were separated by the epitaxial zone;The length of the type-A fusion boundaries possesses approximately 15%of the total length of the fusion boundary.The surface potential,residual strain,electrochemical performance and hardness distribution of dissimilar welded joints were nonuniform,and their distribution were related to the microstructure of DMWJ.For two kinds of fusion boundaries,the value of microhardness increased from base metal to weld metal,however,due to the existence of fine cell structure in the epitaxial growth zone,the hardening phenomenon appeared in the epitaxial growth area of Type-B fusion boundary.The surface potential test results shown that the value of surface potential change between base metal and weld metal of Type-A fusion boundary was bigger than that of Type-B fusion boundary,this phenomenon shown that the galvanic corrosion of Type-A fusion boundary was heavier than that of Type-B one.The residual strain in EBSD test shown that the residual strain value of base metal was high,while the weld material was low.Compared with the two kinds of fusion boundaries,the Type-A one had higher residual strain value than Type-B one.In general,when the weldment was put into corrosive solution,the Type-A fusion boundary will preferentially be corroded.Different microstructure of dissimilar welded joints had different effects on oxidation products and oxidation mechanism in high temperature and high pressure water environment.The outer oxide layer of the matrix metal were spinel oxide.The outer oxide layer products of the weld metal were Ni(OH)2 with preferred growth,and the growth direction was[0001].The oxide layer of 316L stainless steel base metal,the weld metal and the fusion zone of DMWJ exhibited double-layer structure.The 316L stainless steel side of the Type-A fusion boundary act as the anode during the corrosion,this led more metal ions released.And also,in the inner layer of the 316L stainless steel of Type-A fusion boundary,there was no Cr2O3 exist.Due to the spatial particularity of octahedral structure of spinel oxide,many holes are formed between the spinel oxides,thus,the corrosion medium and metal ions are diffused more easily,resulting in the aggravation of corrosion.The microstructure of dissimilar welded joints had different effects on the deformation behavior at room temperature.The twin boundaries in the matrix metal changed to large angle grain boundaries during tensile process,which had high residual strain and dislocation accumulation energy,leading to the initiation of cracks along those boundaries.The fracture mode of the matrix metal was ductile fracture.There was no twin exist in weld metal structure,crack initiated on the slip band which had high strain and dislocation energy,and the fracture mode was ductile fracture.For the two different kinds of the fusion boundaries,the crack both occur in the slip zone of the weld metal side.The fracture mode of the Type-A fusion boundary was ductile fracture,however,the fracture mode of the Type-B fusion boundary was the mixed brittleness and the ductile fracture.Due to the influence of grain orientation,the fracture mode of the epitaxial growth region was brittle fracture.