Effects of B on microstructure and hardenability of resistanceseam welded HSLA linepipe steel

Abstract

The effects of B on the microstructural and mechanical characteristicsof resistance seam welded high strength low alloy linepipe steel were investigatedat different regions, such as weld, heat affected zone (HAZ) andbase. Because most of the solute B atoms were already precipitated as ironborocarbides and iron borides in as welded steels, the improved hardenabilitywas not observed regardless of the observed regions. The amount of dissolvedB precipitates increased with austenitising temperature, and the pre-existingB precipitates were fully dissolved at 950°C for 180 s. The segregationof solute B atoms into austenite grain boundaries was more marked at higheraustenitising temperatures, and the segregated solute B atoms greatly increasedthe hardenability of the austenitised pipes, resulting in hard microstructureslike bainite and martensite. The B addition also effectively compensated forthe deteriorated hardness at the weld and HAZ of the tempered specimens, althoughB atoms were precipitated during tempering.