The Relationship Between Microstructure and Hardness in the Heat-Affected Zone of Line Pipe Steels

Welding is an essential fabrication step during the construction of pipelines using high-strength low-alloy steels. The base metal experiences a rapid thermal excursion in the heat-affected zone, resulting in modification of the microstructure and mechanical properties. In particular, the coarse-grain heat-affected zone closest to the weld pool is of significance for the integrity of pipelines. In the present work, the microstructure and hardness were quantified from laboratory-simulated samples, obtained at different cooling rates for 12 steels, where their chemistry was systematically varied. Electron backscattered diffraction was used to quantify the bainitic microstructures based on the density of high-angle grain boundaries (HAGBs). It was observed that an increase of the cooling rate results in (1) a reduction of the transformation start temperature, (2) a larger density of HAGBs, and (3) an increase in hardness. A Hall–Petch-type relationship is proposed to link the hardness with the HAGB density.