Generation of Acicular Ferrite in the Laser‐Welded Weld of X100 Pipeline Steel Induced by CO2 Shielding Gas

Laser welding has the potential to be used for pipeline laying. Acicular ferrite (AF) is one of the ideal microstructures to improve the strength and toughness of welds. Herein, the effect of CO 2 shielding gas on the microstructure evolution in the laser‐welded weld of X100 pipeline steel is studied. The results show that CO 2 shielding gas can induce a large number of Ti 2 O 3 ‐Al 2 O 3 ‐MnO‐SiO 2 composite inclusions with a size of about 1 μm in the weld. The Ti 2 O 3 in the inclusions promotes the formation of the Mn‐depletion zone (MDZ), resulting in the generation of AF with a volume fraction of 90.6% in the weld. The sizes of AF grains surrounded by high‐angle grain boundaries range from 1 to 3 μm, while the microstructure of the Ar shielded weld is mainly granular bainite. Inclusion mainly contains Al 2 O 3 , which cannot promote the formation of MDZ in the Ar shielded weld. The study provides a new method to induce AF in the weld of X100 pipeline steel during laser welding, and it is also useful for steels containing Ti and Mn elements.