Mechanisms of Ti and B on improving weld metal toughness of a Nb-alloyed steel

Abstract

It is known that titanium (Ti) and boron (B) elements in the filler metal can effectively improve the toughness of high heat input welds of Nb-alloyed steels. This generally has been attributed to refinement of weld microstructure and modifications of different iron constituents in the optical microscopic length scale. To improve understanding of fundamental mechanisms and directly reveal the distribution and interactions of Ti, B, Nb and other alloying elements, in this study, atomic scale characterizations and microchemistry analysis are performed with atom probe tomography (APT) and transmission electron microscopy (TEM). Three types of welding consumables are evaluated on submerged arc welds of X70 steel, among which the Ti-B-Mo rich electrode produces the best toughness properties. In the weld from Ti-B-Mo rich electrode, significant amount of C, B and Ti is segregated along the prior austenite grain boundary (PAGB) while B is depleted adjacent to PAGB. Nb also decorates the PAGB but with a lower segregation level. This is potentially due to the lower affinity of Nb to C and B compared with Ti. In the vicinity of PAGB, Nb exists in the form of solid solution in the ferrite matrix. TEM analysis similarly shows that overall Nb is uniformly distributed throughout the weld without large scale segregation.