Microstructural and chemical dependences of fracture toughness in stainless steel welds at 4.2 K

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.