Microstructures and mechanical properties of the fusion zone of 316L-316LN stainless steel multi-pass gas tungsten arc welded joint

In this study, the microstructure and hardness of the fusion zone of 316L-316LN stainless steel multi-pass gas tungsten arc welding (GTAW) weld joint were systematically investigated. The results showed that there were two typical inhomogeneous intermediate mixed zones distributed in the weld metal along the 316L (filler metal)-316LN (base metal) stainless steel multi-pass GTAW weld joint fusion boundary, one was the dendritic structure region, the other was the cellular structure region. The dendritic structure region was duplex structure with ferrite in vermicular morphology in austenite. By contrast, the cellular structure region exhibited cellular austenite structure with a small amount of ferrite particles at the cellular boundary triple points. It was found that the cellular boundary was high-density tangled dislocation structure with Cr and Mo segregation. Obvious residual strain was found to be concentrated adjacent to cellular boundaries due to the thermal expansion/shrinkage during the rapid solidification process. Because of the high-density dislocations, grain refinement and solute micro-segregation at the cellular boundaries, the microhardness of the cellular structure region was 239 HV, higher than that of the dendritic structure region (227 HV), and the nano-hardness exhibited a same trend, which was 3.03 GPa and 2.74 GPa, respectively.