Fracture mechanical and microstructural characterization of narrow-gap safe-end dissimilar metal weld

Dissimilar metal welds (DMWs) have several different microstructural zones in the vicinity of the low-alloy steel - Ni-based weld metal fusion boundary that have an effect on the mechanical and fracture mechanical behaviour of the weld joint. These microstructural zones are especially small when modern narrow-gap (NG) gas tungsten arc welding (GTAW) technique is utilized. Determination of the fracture toughness properties of the microstructural zones of a DMW joint is of utmost importance for successful structural integrity and lifetime analyses. This paper presents the results from fracture resistance (J-R) tests, microhardness measurements and microstructural characterization performed for a DMW mock-up manufactured by Mitsubishi Heavy Industries (MHI). The studied material is fully representative to that of OL3 EPR pressurized water reactor safe-end weld. The results show that the DMW joint is tough at the SA 508 lowalloy steel - Alloy 52 weld metal interface, which typically is the weakest area of a DMW. Fracture surfaces of the J-R test specimens tested in T-L orientation show a wavy appearance that follows the weld bead boundaries. Crack propagation path was studied also using optical 3D profilometry. Microhardness results show that there is a steep hardness gradient at the fusion boundary where the carbon-depleted zone (CDZ) of the SA 508 exhibits the lowest hardness and the narrow zone exhibiting the highest hardness is located right next to the fusion boundary at the Alloy 52 side.