Analysis of niobium-rich phases in the submerged arc welds of high strength low alloy steel

Niobium has been a well-recognized microalloying element to improve the strength and toughness for low carbon steels. However, fundamental understanding of effects of Nb on weld metal mechanical properties remain elusive. In this study, the distribution, morphology, crystallography and chemistry of Nb contained phases in a two pass submerged arc welded steel are analyzed through two atomic resolution characterization techniques, i.e. transmission electron microscope (TEM) and atom probe tomography (APT). In the first weld pass, which is reheated by the second weld pass, thin platelets Nb(CxN1-x) arranged in sheet-array structures are observed in the acicular ferrite (AF) region. Along the prior austenite grain boundary ferrite, long films of Nb2(CxN1-x) with orthorhombic crystal structure are identified, which is reported for the first time in the weld metal and plays a critical role in determining weld metal toughness.