Study On Mechanical Behavior And Microstructure Control Of Transition Zone In Dissimilar Steel Weld

A large number of dissimilar steel joints are involved in the manufacture of nuclear power equipment.Due to the difference in the structural properties between different materials,the heterogeneous steel joints have a large safety hazard at the joint interface,and in the severe case,the weld will be stripped apart along the interface,resulting in early failure of key components.This paper begined with the study of stripping failure in the dissimilar steel weld from actural nuclear component.Induced mechanical tests were adopted to analyze the the mechanical behavior of the transition zone,including the Type-11 boundary under different force state,confirming that the early failure occurred at the austenite region around the Type-11 boundary of stainless steel side.The feature of transition zone of SA508-3/EQ309L was analyze by optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),electron backscatter diffraction(EBSD).Nano indentation was applied to analyze the micro mechanical property of transition zone.A series of strain-stress curves for different part of transition zone has been obtained.The retain austenite region was weak,and easy to crack.The martensite was proposed to improve the properties of the transition zone.The comparative experiments were designed from three aspects,namely heat,force and composition differences.The effects of welding thermal cycle,stress strain and surfacing layer materials on the transition zone were analyzed.The influence of elements on growth of the martensite in transition zone was found through the inducing element adding welding experiment.The corresponding relationship between the diffusion range of Cr and growth range of martensite in transition zone was found.The growth behavior of martensite of the transition zone was studied by means of micro-analysis such as transmission electron microscopy(TEM)and high resolution transmission electron microscopy(HRTEM).The key factors affecting martensite growth of transition zone were analyzed.Based on Fick's law,the steady-like state equation was proposed,with the diffusion range of key element Cr as the reference,to descript the relationship between the width of the martensite layer of transition zone and high temperature dwelling time.Based on the MIG welding SA508-3/Ni52M welded joints,a process control equation for the transition zone was established,and its feasibility has been confirmed.The study shows that in the actual cracking process,the early failure occurrs at the austenite region around the Type-11 boundary of stainless steel side,and the end of the crack propagation can be captured by the martensite structure in the transition zone,and the crack cannot pass over the high?hardness martensite to the side of the base metal.The results of induced mechanical tests indicate that transition zone shows complicated mechanical behavior.The pair of fracture face can show different failure patterms.The Type-II boundary is not the main path of interfacial crack propagation,and it show a certain plastic deformation ability.Under the condition of shear strain,the hole in the grain boundary closes and the grain boundary performance is strengthened.Under normal stress conditions,Type-II boudnary exhibits work hardening and effectively stabilizes grain boundaries.The transition zone contains two parts,namely“white bright band" and austenite region,and further,"white bright band" consists of martensite region and white austenite region.Type-II boundary is not a characteristic grain boundary for the transition zone.Martensite is fine in transition zone and introduces a large number of ?3 grain boundaries,causing an increase in hardness in the transition region and ensuring the plastic deformation capacity of transition zone.The composition of the transition zone can not be predicted by the calculation of Jmat Pro,since there is no martensite+austenite mixed structure in the transition zone.The mechanical properties The elastic modulus(E),yield strength(?y)and work hardening index(n)for different locations of the transition zone were calculated by nanoindentation method.The results show that the "white bright band"generally has higher yield strength(>1500MPa)than the base metal and the surfacing layer.The martensite of“white bright band" has the characteristics of high yield strength and high work hardening index,shows good mechanical properties as overlay does,while the austenite region shows a decrease in performance that fails to fit the property of microstructure around.The failure of dissimilar steel joints is mainly due to sudden change in performance.The joints are located in the surfacing layer area near the type-11 grain boundary.From this point of view,increasing the proportion of martensite structure in the white bright band can alleviate the sudden change of joint properties.The transition zone is formed in a thermal cycle,including the martensitic structure in the "white bright band".When the temperature of the second or multiple thermal cycles is not higher than the austenite formation temperature or the holding time is short,the diffusion of the element in the transition zone is slightly promoted,and the width of the "white bright band" does not change significantly,mainly due to no secondary growth of martensite structure;when the secondary thermal cycle temperature is higher than the austenite formation temperature and the holding time is sufficient,significant elemental diffusion can be observed at the interface and the range of the martensite layer broaden.From the point of view of force influence,the transition zone,especially the austenite region at the front of the martensite region,is not stable.When the applied stress is large enough,the dislocation of the interface starts to grow forming a new martensite layer.From the point of view of compositional differences,when the surfacing materials with high Cr and Ni contents are used,the transition zone shows a distinct "white bright layer",and the martensite region is also relatively wide.In the study of the growth behavior of martensite of transition zone,it is found that the growth range is similar to the diffusion distance of Cr.In the growth process of martensite,the influence of mechanical driving force is much smaller than that of chemical driving force.Since the change of driving force,the growth of martensite in transition zone sequentially calls the adjustment mode from microscale to nanoscale to ensure the continuous phase transition.The diffusion behavior of the transition zone can be described by Fick's second law approximatively,but the diffusion width cannot be effectively calculated.According to the growth characteristics of transition zone,a steady-like state equation is established based on Fick's first law,then the diffusion range d and high temperature dwelling time to can be obtained.Based on the data from SA508-3/Ni52M MIG weld,the relationship between the high temperature dwelling time and the welding line energy is fitted,and the coefficients k and ? in the steady-like state equation are calibrated to obtain the process control equation for SA508-3/Ni52M MIG welding:Based on the process control equation,welds with the specific martensite width in the transition region were prepared.The calculated value was compared with the measured value.The error of high temperature dwelling time was below 0.15 s,and the error of martensite width in the transition zone was below 0.8?m.