Effects of B on microstructure and hardenability of resistance seam welded HSLA linepipe steel

The effects of B on the microstructural and mechanical characteristics of resistance seam welded high strength low alloy linepipe steel were investigated at different regions, such as weld, heat affected zone (HAZ) and base. Because most of the solute B atoms were already precipitated as iron borocarbides and iron borides in as welded steels, the improved hardenability was not observed regardless of the observed regions. The amount of dissolved B precipitates increased with austenitising temperature, and the pre-existing B precipitates were fully dissolved at 950°C for 180 s. The segregation of solute B atoms into austenite grain boundaries was more marked at higher austenitising temperatures, and the segregated solute B atoms greatly increased the hardenability of the austenitised pipes, resulting in hard microstructures like bainite and martensite. The B addition also effectively compensated for the deteriorated hardness at the weld and HAZ of the tempered specimens, although B atoms were precipitated during tempering.