Brittle to ductile transition in nickel free high nitrogen austenitic stainless steels

The brittle to ductile transition (BDT) in nickel free high nitrogen austenitic stainless steel was investigated. Falling weight impact tests at 176, 273 and 336 K revealed that Fe-25Cr-1·1N (wt-%) austenitic steel exhibits a sharp BDT in spite of being a face centred cubic alloy. The plastic deformation observed following the impact tests indicated that the BDT is induced by poor ductility at low temperatures, as is the case with ferritic steels. To measure the activation energy for the BDT, the strain rate dependence of the BDT temperature was examined using four-point bending tests. The BDT temperature was found to be weakly dependent on strain rate. Arrhenius plots of the BDT temperature against strain rate showed that the activation energy for the BDT of Fe-25Cr-1·1N steel is much higher than that of low carbon ferritic steels. The origins of this distinctive BDT and the large value for its activation energy in this high nitrogen steel are discussed in terms of the reduction in dislocation mobility at low temperatures because of the interactions between the glide dislocations and the solute nitrogen atoms.