The effect of N addition on the micro-structure and corrosion resistance of modified 6Mo super austenitic stainless steels during isothermal aging treatment

Abstract

In this research, three types of super austenitic stainless steel (SASS) with nitrogen levels of 0.2, 0.28, and 0.38 wt.% were developed. By maintaining a relatively high manganese content of approximately 1.5 wt.% and nickel content of around 18 wt.%, nitrogen was fully incorporated into the austenite, resulting in alloy samples designated as 0.2N, 0.3N and 0.4N. The hot-rolled plates of these alloys underwent a solution treatment at 1180°C for 30 min, followed by aging at 950°C for durations of 30 min, 2 h and 6 h, respectively. The influence of nitrogen content on phase precipitation behaviour was examined through various micro-structure characterisation techniques. The corrosion resistance of the samples was further evaluated using potentiodynamic polarisation experiments, Electrochemical impedance spectroscopy (EIS) and double loop-electrochemical potentiokinetic reactivation (DL-EPR) method. The results indicated that with an increase in nitrogen (N) addition, the quantity of σ phases decreased while and the formation of Cr₂N phases increased during the same aging period. Furthermore, at the same aging duration, the 0.3N SASS exhibited the highest pitting resistance and the lowest degree of sensitization (DOS) value compared to all other steels, attributed to the minimal precipitates present along the grain boundaries.