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

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 ? 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 sigma/gamma 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. (C) 2022 The Authors. Published by Elsevier B.V.

Automated summary

The precipitation behavior and transformation kinetics of secondary phases in super austenitic stainless steels with different nitrogen contents were investigated. The results showed that increasing the nitrogen content did not lead to the refinement of sigma precipitates. Instead, a lower nitrogen content resulted in more uniform precipitation. Furthermore, increasing the nitrogen content did not accelerate the precipitation of the Cr2N phase. The higher sigma transformation fraction in the alloy with 0.4wt.% N promoted the precipitation of nitrides. The data obtained from the experiments were in good agreement with the model used to describe the transformation kinetics.