Microstructures and high-temperature failure analyses of 25Cr-35Ni-Nb + MA/32Cr-20Ni-Nb dissimilar welded joint with different filler wires

Abstract

To address the issue of lacking scientific guidance for welding 25Cr-35Ni-Nb + MA/32Cr-20Ni-Nb joint in petrochemical plant applications, the joint microstructures were studied by adopting four types of filler wires. The high-temperature tensile tests and creep rupture tests were performed to investigate joint failure mechanisms. Cellular/columnar dendritic grains were formed at the fusion line region with similar orientations to adjacent un-melted grains. The fine-sized dendritic grains generated at the central weld seam presented a larger misorientation owing to a lowered cooling rate. The lesser alloying element in filler wire led to a larger area with cellular/columnar dendritic grains. The carbide precipitates mostly located at grain boundaries either in the form of particles or necklace structures depending upon the filler wire compositions. The heterogeneous microstructure was thought to be the main reason for the failure at weld seam and fusion line region with large hardness fluctuations when joints welded by the filler wire with a more diverse alloying element. The filler wire with the least introduction of alloying elements, i.e., ERNiCrCoMo-1, gave rise to a more uniform microstructure and hence higher joint property without failing at the weld seam, which was therefore considered as the most suitable filler wire candidate.