Effects of nitrogen content on phase transformation behavior of nickel-free austenitic stainless steels

High nitrogen nickel-free austenitic stainless steels have wide application prospects in many fields. However, the effects of nitrogen mass fraction on phase transitions are still unclear yet. High nitrogen nickel-free austenitic stainless steels with nitrogen mass fraction from 0.02% to 1.20% were designed and melted. And the thermodynamic calculation of equilibrium phase transition of steels was carried out, the morphology of δ ferrite and Cr₂N was observed. The results showed that the maximum precipitation of δ ferrite was decreased with nitrogen mass fraction increase, but when nitrogen mass fraction beyond 1.05%, there was no δ ferrite found. The relations between the precipitation temperature of δ ferrite and nitrogen mass fraction during the steel heating process were obtained. With the increase of nitrogen mass fraction, the precipitation temperature of Cr₂N when steels were cooling increased gradually, and its quantitative relationship was obtained. In GN04 steel, the δ ferrite after 1 200 ℃ isothermal heat treatment in 2 h was mainly distributed along the trigeminal grain boundary. The precipitation of Cr₂N was formed firstly on grain boundary and then spread in crystal. It also increased with nitrogen mass fraction increase in the same isothermal heat treatment, but layer spacing decreased.