Abstract
UNS N06686 is a Ni-base corrosion-resistant alloy containing relatively high concentrations of Cr, Mo, and W. Higher levels of these critical alloying elements generally lead to better resistance to both general and localized corrosion when referring to wrought microstructures in the proper annealed condition. However, due to elemental segregation during weld solidification, there is a wide variation in the local chemical composition of as-solidified weld metal. Components fabricated from Ni-base alloys are typically employed in the as-welded condition and the as-solidified weld metal is often the microstructural region most susceptible to corrosive attack. With this in mind, the purpose of this study was to gain a better understanding of the effects of chemical composition and elemental segregation on the properties of as-solidified N06686 weld metal. Corrosion test results of all-weld-metal samples showed that higher levels of Cr, Mo, and W led to inferior corrosion resistance in certain aqueous corrosion environments. These findings were correlated to the weld metal microstructure, which was established from experimental results and Scheil solidification calculations. The results suggest that higher levels of critical alloying elements within the N06686 composition range can be counterproductive to the corrosion resistance of its weld metal. Furthermore, declaring a particular filler metal as overmatching or overalloyed based on higher total concentration of critical alloying elements and/or Pitting Resistance Equivalent Number (PREN) may be an oversimplification with respect to highly alloyed Ni-Cr-Mo-W weld metal.
