The effect of mismatch on the mechanical properties of a dissimilar metal weld

Abstract

Dissimilar metal welds (DMW) can be found in joints where two different structural materials meet, e.g., the joint between the pressure vessel made of ferritic steel and the austenitc stainless steel piping in a nuclear power plant. DMWs have been found to be susceptible to premature failures; the environment of use and the mismatch in material properties between the two joined materials are two of the main reasons for this. The main goal for this thesis is to examine the strength mismatch state of a DMW.

The literature part of this thesis discusses the effect of strength mismatch on the fracture behaviour and welding metallurgy for DMW. The experimental test methods used for the examination of the mismatch state, especially for narrow local variations, are discussed.

A weld mock-up representing the DMW found in the "safe-end" of boiling water reactor (BWR) was manufactured for the experimental part of this study. The weld consisted of SA 508 and AISI 304 base materials, an Inconel 182 buttering layer, and Inconel 82 and 182 weld metals.

The microstructure of the weld mock-up was examined with optical and electron microscopy. Special attention was given to the fusion line between SA 508 base material and Inconel 182 buttering. The mismatch state of the weld was determined with tensile tests, which were performed on both miniature and standard-sized specimens. This arrangement enabled the measurement of the tensile properties of the narrow zones of the weld and the validation of the results. Fracture mechanical tests were performed to examine the effect of mismatch on the fracture behaviour.

A Type II boundary and a martensitic layer were observed at the weld interface (SA 508-lnc.182) in the microscopic examination. The results of the tensile tests showed that the weld metal was slightly under-matched compared to the SA 508 base material and that the highest mismatch was found between the SA 508 HAZ and buttering layer. The mismatch at the interface was reduced as a result of PWHT. The miniature-sized tensile and fracture mechanical test specimens were found to supply valid results.