Unravelling microstructure evolution induced mechanical responses in welded joints of QN1803 steel subjected to varied heat inputs

In this work, QN1803 steel plates were connected with butt welding. Electron beam welding (EBW) was utilized with different beam currents. The impact of beam current on the properties and microstructure of the joints was investigated completely. The results indicate that sound joints could be obtained with these beam currents. And the joints with 47 and 52 mA currents exhibited the optimal strengths. The joint with 52 mA exhibited the maximal impact work (287 J). The joints include transition zone (TZ) and fusion zone (FZ). These zones mainly are composed of TZ-columnar dendritic and FZ-equiaxed dendritic structure. These samples all exhibited finer grains and lower ferrite content compared to the base material (BM). As the heat input increased, the fine grain strengthening effect gradually became weak, leading to defects in the weld seam. Due to the fine grain strengthening and dislocation strengthening effects, these joints with 47 and 52 mA exhibited higher tensile strength than BM. However, for the joints with 42 and 57 mA, fractures occurred in the weld seam owing to defects. The dislocations and fine grain could make cracks extend into the BM. However, for the joint with 52 mA, the impact toughness could reach 281 J, which is still lower than that of BM (304 J).