Influence of temperature, cold deformation and a constant mechanical load on the microstructural stability of a nitrogen alloyed duplex stainless steel

The influence of temperature, cold deformation and constant mechanical load on the microstructural stability and the kinetics of phase decomposition of a nitrogen-alloyed duplex stainless steel (0.34 wt.% N) was investigated. Calculation of the phase equilibria was done with THERMOCALC using the steel database TCFE3 in order to predict the stability of the phases and to estimate the influence of temperature on the fraction and chemical composition of the phases. Various ageing treatments between 800 °C and 1300 °C were performed for different time intervals with controlled heating and cooling rates. In order to determine the influence of deformation, annealing at 800 °C after cold deformation as well as dilatometry experiments were performed under a constant mechanical compressive load at 800 °C and 900 °C. Microstructural characterization was carried out by means of light microscopy, electron microscopy and X-ray diffractometry. It was found that the microstructural evolution under a thermal load alone in the temperature range above 950 °C concerns mainly the transformation of austenite to ferrite, while below 950 °C ferrite decomposition and precipitation of nitrides occur. Since duplex stainless steels possess a microstructure consisting of paramagnetic austenite and ferromagnetic ferrite, the kinetics of ferrite decomposition can be determined easily by magnetic inductive measurements. The results of the microstructural investigations and the measurements of the saturation magnetization show that there is a satisfactory agreement with the theoretical predictions based on THERMOCALC. Ferrite decomposition is significantly accelerated by strain introduced during cold deformation. Furthermore, even under a small mechanical load the kinetics of phase decomposition behaviour at 900 °C is drastically changed. Whereas during short annealing times the microstructure remains nearly stable the same annealing conditions under a constant mechanical load accelerate the phase instability not only at the ferrite/austenite phase boundaries but also inside the ferrite grains.