Effect of niobium on the microstructure and properties of submerged arc welds in HSLA steel

High-strength low-alloy or microalloyed steels are thermo-mechanically control processed (TMCP) to achieve desired microstructures and mechanical properties, and microalloy additions are critical for obtaining optimum properties. The steel of interest in this study is microalloyed with approximately 0.1 wt% Nb, levels that are typical in pipeline steels such as X70. Submerged arc welding (SAW) is a commonly utilized manufacturing method for constructing pipe segments via seam welding. There were two parts to this investigation. Part 1 analyzed controlled dilution levels consisting of commercially available SAW electrodes and Nb-bearing steel in proportions to simulate submerged arc weld metal. Part 2 of this investigation evaluated actual submerged arc welds on low-alloy steels. Bead-on-plate welds were deposited, and the Nb content of the weld metal was systematically increased by adding ferro-niobium directly to the flux. Two-run welds representative of line pipe production were also evaluated using the same ferro-niobium flux additions in order to increase the weld metal Nb content. The microstructural evolution and corresponding mechanical properties with increased Nb are evaluated based on this work and on the published literature.