Analysis of the weakest regions and microstructural features influencing impact toughness in Q690 high-strength steel welded joints

To investigate the deterioration of impact toughness in Q690 high-strength steel welded joints, a systematic study was conducted using Gleeble thermal simulated tests combined with Charpy impact tests, microhardness measurements, and microstructural analysis. The research focused on the microstructural evolution in different zones of the welded joint and its influence on mechanical properties. The results indicate that the weld metal (WM) and coarse-grained heat-affected zone (CGHAZ) are the weakest regions of the joint. The CGHAZ exhibits significantly reduced impact toughness (72 J) due to grain coarsening and the formation of lath martensite/bainite, with fracture surfaces showing cleavage characteristics. The weld metal demonstrates inhomogeneous microstructures, leading to an average impact energy of only 79.2 J, much lower than that of the base metal (BM) and other sub-zones. In contrast, the fine-grained HAZ (FGHAZ) and intercritical HAZ (ICHAZ) exhibit improved impact toughness (up to 303 J), attributed to grain refinement and the dominance of granular bainite. Microhardness analysis further confirms that the increased hardness in the CGHAZ and WM is directly related to the formation of brittle and hard microstructures.