Effects of Ni and Mn on brittle-to-ductile transition in ultralow-carbon steels

The temperature dependence of the effective stress indicated that both Ni and Mn induce solid solution softening at low temperatures. The activation energy for dislocation glide was obtained from the temperature dependence of the activation volume and effective shear stress. Either Ni or Mn decreases the activation energy for dislocation glide, which suggests that both Ni and Mn decrease the brittle-to-ductile transition (BDT) temperature. However, the temperature dependence of the absorbed energy for fracture showed that the transition temperature decreases with Ni but increases with Mn. Fracture surfaces tested at 100 K indicated transgranular fracture at 2 mass% Ni and intergranular fracture at 2 mass% Mn, which suggests a decrease in energy for grain boundary fracture with Mn. The mechanism behind the opposite effects of Ni and Mn on the transition temperature of ultralow-carbon steels was examined on the basis of dislocation shielding theory.