Research On The Disbonding Mechanism For SA508-3/EQ309L Dissimilar Joint

The disbonding crack in SA508-3/EQ309 L dissimilar joint can affect greatly on the service life and safety of the nuclear power plant,and is paid close attention widely.While there is no yet a unified understanding about the failure mode for the dissimilar joint,espetially the failure of the SA508-3/EQ309 L dissimilar during overlay welding.It is generally recongnized that residual stress and Type II grain boundaries are the key reasons for the failure of dissimilar joint,but there is no a reasonable explaintation why the dissimilar fail if the thickness of the welding overlay is more than a certain value for SA508-3/EQ309 L dissimilar joint.In addition,there has been no systematic study about the formation or extension of the disbonding crack,the factors that affect the disbonding crack,or how the performance of the dissimilar joint,Type II grain boundaries and the residual stress have influence on the disbonding crack.In this study,the disbonding mechanism for SA508-3/EQ309 L dissimilar joint is discussed in deep based on experiments and numerical simulation.Results show that the decreasing of the adhesive strength between the cladding layers and the base metal,and residual stresses are the main reasons of disbonding crack.Resutls show that Type-II grain boundary and martensite hardened microstructure can form in the dissimilar joint if the dilution ratio,heat input and preheat/interpass temperature were higher.Besides,the Type-II grain boundaries may migrate to the fusion line with increasing the width of the martensite hardened layer.The migration of Type-II grain boundaries and the concentration of diffusion hydrogen and carbides at the grain boundaries may increase the brittleness of the dissimilar joint and the adhesive strength between cladding layers and base metal.Results show that the adhesive strength between the cladding layers and base metal decreases by more than 30% if the above factors were being.This is the basic reason for the disbonding failure of SA508-3/EQ309 L dissimilar joint.The disbonding crack is caused by the weakened grain boundaries conbined with welding residual stresses.Finite element simulation shows that the residual stresses in the transition layer goes up with increasing the overlayers and concentrates at the triple point of the Type-II grain boundary.And the crack forms at the weakened Type-II grain boundary firstly.The residual stresses concentrating at the tip of the crack make the crack extend along the Type-II grain boundary,and the surfacing overlayers strip from the base metal at last.Tensile test shows that the simulations are agree with the test results,and the results can explain why the disbonding crack forms when the surfacing overlayers up to some thickness.