Nitrogen Content Effects on Microstructural Evolution and Low-Temperature Impact Toughness in the Coarse-Grained Heat-Affected Zone of Welded X70 Pipeline Steel

The low-temperature toughness of a coarse-grained heat-affected zone (CGHAZ) is a critical factor governing the service safety of welded joints in X70 pipeline steel. This study systematically investigated the influence of nitrogen content (ranging from 0.0018 to 0.0120 wt%) on the microstructure and low-temperature impact toughness of the CGHAZ in X70 pipeline steel using welding thermal simulation tests with a heat input of 12.5 kJ/cm. The results indicate that the CGHAZ microstructure predominantly comprises lath bainite (LB) and minor martensite–austenite (M/A) constituents. With increasing nitrogen content, the austenite-to-ferrite transformation start temperature (Ar3) increased while the transformation finish temperature (Ar1) decreased, resulting in coarsening of the lath bainite packet structure. The M/A volume fraction rose from 2.11% to 5.23%, the average particle size grew from 0.17 to 0.71 μm, and the high-angle grain boundary (HAGB > 15°) fraction declined from 67.5% to 52.2%. These microstructural alterations collectively caused the Charpy impact energy of the CGHAZ to decrease from 269 J to 48 J. The deterioration in toughness is primarily attributed to blocky M-A constituents lowering the resistance to crack nucleation and the reduced HAGB fraction diminishing the resistance to crack propagation. This work provides a theoretical foundation for optimizing the performance of X70 pipeline steel welded joints, and it is recommended that the nitrogen content in the base metal be strictly maintained below 0.005 wt% to ensure superior CGHAZ toughness.