Effect of acicular ferrite on microstructure stability and low-temperature impact toughness of simulated CGHAZ in X65 grade submarine pipeline steel

Two X65 submarine pipeline steels with distinct microstructures, namely quasi-polygonal ferrite (QPF)-bainite dual-phase steel and acicular ferrite (AF)-dominant steel, were used to investigate the effects of microstructure on the microstructural stability and low-temperature impact toughness of simulated coarse-grained heat-affected zones (CGHAZs) after welding. The two base metals (BM) showed comparable strength, toughness and ductility. After exposure to welding thermal cycles with varied heat inputs, the low-temperature impact toughness of both steels degraded with increasing heat input. Compared with the QPF-bainite steel, the AF-dominant steel accommodates higher heat input welding, with its CGHAZ possessing more stable microstructure and better low-temperature toughness. Additionally, the AF-dominant steel exhibited higher CGHAZ microhardness than the dual-phase steel under identical heat input conditions. The superior combination of high microhardness and excellent low-temperature toughness of the AF-dominant steel stems from the synergistic Cu-Ni-V alloying design. This alloy system reduces C and Mn contents, lowers the carbon equivalent (Ceq) and crack susceptibility parameter (Pcm), mitigates matrix segregation, and promotes AF formation. At high heat input, bainite transformation in the AF-dominant steel CGHAZ occurs at a relatively low temperature. This optimizes crystallographic variant selection, enhances the formation of large-misorientation variant pairs, and suppresses sub-block and packet boundaries with misorientation angles below 15°. The resultant high-density high-angle grain boundaries (HAGBs) effectively hinder crack propagation, thereby improving the low-temperature impact toughness and hardness of the material.