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Nano-twins induced changes in the local grain orientation of the weld can promote dislocation slip, which leads to an increase in the fracture toughness of the weld. [Display omitted] • Microbands and grain boundary dislocation pileup hindered dislocation slip. • Nano-twins promoted dislocation slip and increased the fracture toughness. • Mn addition decreased valence electron concentration and reduced the stability of FCC structures. • Grain refinement with more grain boundaries inhibited crack nucleation and propagation. In this study, 316LN-Mn and 316LN plates were welded by gas tungsten arc welding. The fracture toughness of 316LN-Mn and 316LN steel welds at the cryogenic-temperature of 4.2 K was evaluated by focusing on the effects of microstructures and chemical elements. The results showed that both steel welds did not experience stress-induced martensite transformation during deformation. At 4.2 K, the microbands and dislocation pileup at grain boundaries blocked dislocations, thus reducing the fracture toughness of the welds. The embedded nano-twin clusters inhibited void initiation in the nano-grained matrix and formed a crack-bridging ligament behind the crack, which contributed to higher fracture toughness. It was found that microstructures with dominant equiaxed grains at an average grain size of 30 μm had the highest fracture toughness. While chemically, the reduction of manganese content blurred the outline of the electron density map, reduced the valence electron concentration, resulting in unstable austinites, which informed the significance of manganese content control in design and selection of weld metals. [ABSTRACT FROM AUTHOR]