Effect of nitrogen on short-range atomic order in the feritic δ phase of a duplex steel

The short-range atomic order in the δ-ferrite of three duplex steels SAF2205, SAF2507 and D2, containing 0.17, 0.25 and 0.62 wt.% N, respectively, was studied using conversion electron Mössbauer spectroscopy and internal friction and characterized in terms of the Cowley parameter α. It was established that an increase in the N content from 0.17 to 0.25% assists short-range atomic ordering of Cr and Mo atoms in the ferrite, i.e. prevalence of the Cr-Fe and Mo-Fe nearest neighbourhood. In contrast, clustering of Ni atoms is increased. Alloying with 0.62% N leads to clustering of Cr and Mo atoms and weakens clustering of Ni atoms. Measurements of the Snoek relaxation in the ferrite showed that, with increasing total N content, its concentration in the ferrite also increases, although most of the nitrogen is located in austenite. According to measurements of the Zener relaxation, the fraction of Cr(Mo)-Cr(Mo) pairs in the ferrite decreases with increasing N content, which can be attributed to formation of more complicated complexes of nitride-forming substitutional atoms with nitrogen atoms.