Effects of the addition of high nitrogen contents on the microstructure of 25%Cr-5%Ni ferritic-austenitic duplex stainless steel (DIN wr. nr. 1.4460)

In this work 0.87wt% nitrogen was added to a 25%Cr-5%Ni ferritic-austenitic duplex stainless steel. This addition caused significant modifications on its microstructure. These modifications were studied using several techniques such as optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, microhardness and magnetic measurements. It was observed that the addition of 0.87 wt% N in the 25%Cr-5%Ni steel, which had originally a ferritic-austenitic microstructure, rendered the steel fully austenitic. These high nitrogen contents lead to a peculiar precipitation behavior. Discontinuous and continuous precipitation of chromium nitride, followed by ferrite formation and also by sigma phase formation were observed. Ferrite formation was favored in the nitrogen-impoverished regions. Sigma phase occurred almost in the same areas where ferrite occurred. The kinetics of sigma phase formation was almost the same as in the low nitrogen duplex stainless steels, although, it occurred at higher temperatures. Martensite formation during cold work was also evaluated. Less martensite formation has been observed in the high nitrogen steel as compared to the duplex one.