Influence of N on precipitation behavior and transformation kinetics of super austenitic stainless steels after isothermal ageing at 900 °C

Abstract

The precipitation behavior and transformation kinetics of secondary phases of super austenitic stainless steels 654SMO containing 0.4wt.% N and 0.5wt.% N were investigated following ageing at 900 °C for varied time. It was revealed that the increase of N did not lead to the refinement of sigma precipitates. Super austenitic stainless steel with 0.4wt.% N presented more uniform and homogeneous precipitation. This unexpected event was caused by a higher sigma nucleation rate than the growth rate of the 0.4wt.% N alloy. The area fraction of precipitates of 0.5wt.% N alloy was lower than that of 0.4wt.% N alloy, which was most likely due to the lesser number of particles nucleated overcompensating the fast growth of sigma particles caused by rising N concentration. Furthermore, increasing the N content did not speed up but slowed down the precipitation of the Cr2N phase. The Cr2N phases were found nucleated at the σ/γ interface, implying that the higher sigma transformation fraction of the 0.4wt.% N alloy promoted the precipitation of nitrides. The modified Johnson-Mehl-Avrami model was used to describe the transformation kinetics of precipitates in both alloys. There was a good agreement between experimental data and model.

Keywords

• austenitic stainless steel
• Nitrogen effect
• Precipitation behavior
• Transformation kinetics
• Thermodynamics calculation