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.