MICROHARDNESS AND MICROSTRUCTURE OF AUSTENITE AND FERRITE IN NITROGENALLOYED DUPLEX STEELS BETWEEN 20-DEGREES-C AND 500-DEGREES-C

The plastic behavior of ferrite (alpha) and austenite (gamma) 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 5 00 degrees C. Nitrogen increases the microhardness of both ferrite and austenite. Some of the microhardness-temperature curves show peaks be tween 200 and 500 degrees C indicating precipitation processes, disloc ation-particles and dislocation-dislocation interactions. The microhar dness of the austenite is represented as a function of its nitrogen co ntent c(N)(gamma) for each test temperature. Extrapolation to c(N)(gam ma) = 0 yields the microhardness of the nitrogen-free austenitic base material. Instead of a monotonic decrease of the austenitic base mater ial's microhardness with increasing temperature, the analysis reveals a smooth maximum situated between 200 and 300 degrees C. This indicate s microstructural changes in the austenitic phase, which were examined in detail by transmission electron microscopy on the steel quenched f rom 200 degrees C with the intermediate nitrogen content of 0.22 wt.%. Extremely fine lamellar structures possessing twin-like orientation r elationships are present in this material. This may cause the plateau in the nitrogen-free austenites microhardness between 200 and 300 degr ees C. An electron microscopic study gives an overview on microstructu ral changes taking place in ferrite and austenite between room tempera ture and 500 degrees C. (C) 1998 Elsevier Science S.A. All rights rese rved.