Effect of welding current on microstructure and mechanical properties of Q690D high-strength steel welded joints

Welding experiments were conducted on 10 mm-thick Q690D high-strength steel using CO₂ gas shielded welding. The effects of welding current on the microstructure and properties of welded joints were investigated by means of optical microscopy, scanning electron microscopy, and SmartLab SE X-ray diffractometer. The results show that the microstructures of different regions in the welded joint vary significantly, where the weld metal zone is dominated by acicular ferrite, while the coarse-grained heat-affected zone and fine-grained heat-affected zone consist of lath martensite and bainite. Meanwhile, the partially recrystallized heat-affected zone exhibits significant softening, which is attributed to the ferrite-bainite mixed microstructure and low dislocation density. As the welding current increases, the recrystallization degree and dislocation density in the weld metal zone first increase and then decrease. At the optimal welding current of 260 A, the welded joint achieves the best mechanical properties. Although the microhardness distribution shows regional differences, the average hardness remains at a relatively high level. The tensile strength and elongation reach 724.50 MPa and 16.08%, respectively. The fracture surface displays a large number of dimples, but quasi-cleavage features are also observed locally, indicating that the joint fracture mode is a ductile-brittle mixed fracture.