Microhardness and microstructure of austenite and ferrite in nitrogen alloyed duplex steels between 20 and 500°C

The plastic behavior of ferrite (α) and austenite (γ) in three nitrogen alloyed duplex steels containing 0.16, 0.22 and 0.34 wt.% N is investigated by Vickers microhardness measurements between 20 and 500°C. Nitrogen increases the microhardness of both ferrite and austenite. Some of the microhardness-temperature curves show peaks between 200 and 500°C indicating precipitation processes, dislocation-particles and dislocation-dislocation interactions. The microhardness of the austenite is represented as a function of its nitrogen content c^γ_N for each test temperature. Extrapolation to c^γ_N = 0 yields the microhardness of the nitrogen-free austenitic base material. Instead of a monotonic decrease of the austenitic base material's microhardness with increasing temperature, the analysis reveals a smooth maximum situated between 200 and 300°C. This indicates microstructural changes in the austenitic phase, which were examined in detail by transmission electron microscopy on the steel quenched from 200°C with the intermediate nitrogen content of 0.22 wt.%. Extremely fine lamellar structures possessing twin-like orientation relationships are present in this material. This may cause the plateau in the nitrogen-free austenites microhardness between 200 and 300°C. An electron microscopic study gives an overview on microstructural changes taking place in ferrite and austenite between room temperature and 500°C.