NANOSTRUCTURAL CHARACTERIZATION OF NI-BASE ALLOY/LOW ALLOY STEEL DISSIMILAR METAL WELD (DMW) INTERFACES

Abstract

Dissimilar Metal Welds (DMWs) are generally applied to structural components such as reactor pressure vessel and pressurizer nozzles in nuclear power plants. A filler metal is used in the manufacture of DMWs in light water reactors (LWR) to join the low alloy steel pressure vessel nozzles and steam generator nozzles to nickel-based wrought alloy or austenitic stainless steel components. In recent years crackings have been observed in the welded joints of DMWs. Since there is the high susceptibility of heat affected zone (HAZ) and fusion zone (FZ) to stress corrosion cracking (SCC), a concern has been raised about the integrity and reliability in the joint transition zone. In this study, the dissimilar metal joints welded between Alloy 690, Ni-based alloy and Alloy 533 Gr. B (A533B), low alloy steel with Alloy 152 filler metal were investigated. Detail nano-structural and nano-chemical analysis were performed between Alloy 152 and A533B by using optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), secondary ion mass spectrometry (SIMS) and 3-dimensional atom probe tomography (3D APT). It was found that in the weld root region, the weld was divided into different regions including unmixed zone in Ni-alloy, fusion boundary (FB), and the HAZ in the low alloy steel. The result of TEM, EDS and 3D APT analyses showed the non-homogeneous distribution of elements with higher Fe but lower Mn, Ni and Cr in A533B compared with Alloy 152, and the precipitation of carbides near the FB.